Controlling alkyne dimerization and trimerization with ruthenium(II) arene isocyanide catalysts

Abstract

Ruthenium(II) arene complexes represent a renowned platform to develop effective catalysts for a variety of organic transformations, including C–C bonding processes. On the other hand, isocyanides are overlooked ligands in the design of transition metal catalysts. Herein a panel of ruthenium(II) arene isocyanide complexes were found to be versatile catalytic precursors for the dimerization/trimerization of aryl alkynes in aqueous medium. Fifteen compounds of general formula [RuX₂(CNR)(η⁶-arene)] (X = Cl, I; R = alkyl or aryl; arene = C₆H₆, p-cymene, C₆Me₆) were prepared from the corresponding halido-bridged Ru dimers and the selected isocyanide according to optimized procedures, including examples with the simplest alkyl isocyanide (MeNC) and arene (C₆H₆). Next, two acetylide complexes of the type [RuCl(CCPh)(CNR)(η⁶-C₆Me₆)] were obtained by reaction of the corresponding dichlorido complexes with phenylacetylene and NaOH. In addition, a protocol for the thermally promoted p-cymene/MeCN substitution was optimized, giving access to hexacoordinate complexes with isocyanide and acetonitrile ligands, [RuCl₂(MeCN)₃(CNR)] (two examples). The Ru(II) compounds, fourteen of which are unprecedented, were characterized by CHNS analyses, IR and NMR spectroscopy and X-ray diffraction in eight cases. The catalytic activity of the complexes was assessed, highlighting the role of the solvent, base, Ru loading, energy source and, more importantly, isocyanide/arene ligands to control the selectivity between dimerization and trimerization of phenylacetylene. Pointing to a sustainable process, a catalytic protocol involving Na₂CO₃ as a base, water as a solvent and a low Ru loading (1%) was applied for the dimerization/trimerization of a range of terminal alkynes, with [RuCl₂(CNCy)(η⁶-p-cymene)] emerging as the best performing pre-catalyst. Combined IR, NMR and MS data were instrumental in the elucidation of the reactivity of the isocyanide–arene complexes with PhCCH/Na₂CO₃ and the formulation of a possible mechanism of pre-catalyst activation.

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1. Introduction

Ruthenium occupies a prominent role in catalysis, both on academic and industrial levels. Over the past 20 years, remarkable catalytic performances of several classes of ruthenium complexes have been highlighted.¹ Among them, half-sandwich ruthenium(II) arene complexes rose to prominence for (asymmetric) transfer hydrogenation reactions (Noyori–Ikariya catalysts)² and have found widespread use in terms of substrates³ and H₂-donors,⁴ as well as commercial availability.⁵ Meanwhile, this class of compounds has been extensively investigated for their catalytic properties in dehydrogenation/hydrogen borrowing processes,^6,7 nitrile hydration⁸ and C–C bond formation reactions such as arylation of C(sp²)–H bonds⁹ and alkene metathesis.¹⁰ In terms of synthetic design, the {Ru(η⁶-arene)}²⁺ moiety allows to coordinate a wide range of ligands that can be introduced by substitution reactions under mild conditions from the halide-bridged dimers [RuX₂(η⁶-arene)]₂ (X = halide).¹¹ These benchmark diruthenium precursors can be prepared on a multigram scale and almost quantitative yield from commercial RuCl₃ hydrate.^7,12,13 Interestingly, only a few among thousands half-sandwich complexes of the general formula [RuX₂(L)(η⁶-arene)] (L = monodentate ligand)¹¹ contain isocyanides (L = CNR).^14,15 For instance, the simplest alkyl isocyanide (MeNC) has not been coordinated to the ruthenium(η⁶-arene) scaffold and only one isocyanide complex featuring η⁶-benzene has been reported. Isocyanides (CNR) represent valuable and versatile ligands for transition metal complexes.¹⁶ A large number of isocyanides are commercially available or can be prepared from the corresponding primary amine by established methodologies.¹⁷ Their σ-donor and π-acceptor character may provide a robust metal–carbon bond, adapting to diverse metal centers. The organic substituent (R) regulates the steric and electronic properties of isocyanides and may also allow for introduction of additional functional groups. The intense IR absorption for the C≡N stretching is diagnostic of the M–C–N bonding and the electron density at the metal center. Isocyanide coordination to metal centers may promote subsequent reactivity giving access to various, functionalized organometallic fragments.¹⁸ Despite these useful features and their cost-effectiveness, isocyanides have been overlooked in catalyst development with respect to more popular monodentate ligands (e.g. NHC carbenes).^19,20 In this respect, we recently reported the optimized synthesis of six [RuX₂(CNR)(η⁶-p-cymene)] derivatives [X = Cl, R = m-xylyl (Xyl), 2-naphthyl, benzyl, cyclohexyl (Cy), ^tBu; X = I, R = ^tBu] and their catalytic activity in the transfer hydrogenation of ethyl levulinate to γ-valerolactone.²¹

Creating new carbon–carbon bonds is essential in several industries, including those responsible for producing polymeric materials, pharmaceuticals, agricultural chemicals, and natural products.^22,23,24

Terminal alkynes are valuable building blocks, which can engage in a variety of C–C bond forming processes among which homocoupling, dimerization and trimerization (Scheme 1). The homocoupling is a dehydrogenation reaction affording a 1,3-diyne, the dimerization involves a formal addition of a C–H group of an alkyne across a C≡C bond of another alkyne, affording a conjugated enyne (sp–sp²), while the trimerization is a [2 + 2 + 2] cycloaddition reaction providing a benzene ring (sp²–sp²). Controlling the selectivity between these processes is essential.

Scheme 1 General scheme for the homocoupling, dimerization and trimerization of terminal alkynes.

Although successful, several transition metal-based systems for dimerization, trimerization and coupling of terminal alkynes suffer from the same problems.²⁵ To achieve high conversion and selectivity, non-environmentally friendly protocols are usually required, including organic solvents (toluene, DMA and DMF, for example) and organic bases (usually in high concentration), and a relatively high catalyst loading (>5 mol%).

Given our interest in the development of sustainable catalytic systems based on ruthenium(II) η⁶-arene complexes employing simple, easily available ligands,^6f,7 in this work we extended the family of [RuX₂(CNR)(η⁶-arene)] (X = Cl, I) derivatives, starting from seven isocyanides featuring various (hetero)alkyl or (hetero)aryl substituents. These compounds were screened as pre-catalysts for the dimerization or trimerization of terminal alkynes. The reaction conditions (solvent, base, Ru loading, temperature/time), including the energy source, were optimized with phenylacetylene as a model substrate, and then a range of aryl alkynes were tested using [RuCl₂(CNCy)(η⁶-p-cymene)] as the best catalyst. The role of isocyanide and arene ligands in the catalytic process was investigated also taking advantage of the synthesis of the derivatives [RuCl(CCPh)(CNR)(η⁶-arene)] and [RuCl₂(MeCN)₃(CNR)].

2. Results and discussion

2.1. Synthesis and characterization of Ru compounds

2.1.1. Bis-halide arene isocyanide complexes. The reaction of [RuCl₂(η⁶-arene)]₂ (arene = p-cymene, C₆Me₆) with a stoichiometric amount of the selected isocyanide (CNR), in CH₂Cl₂ or MeCN at room temperature, gave the corresponding [RuCl₂(CNR)(η⁶-arene)] complex (arene = p-cymene, R = CH₃, 1a; R = Cy, 1b; (S)-CHMePh, 1c; CH₂PO(OEt)₂, 1d; CHCO₂Et, 1e; Xyl, 1f; 4-C₆H₄OMe, 1g; arene = C₆Me₆, R = CH₃, 2a; Cy, 2b; (S)-CHMePh, 2c; CH₂PO(OEt)₂, 2d; Xyl, 2f) (Scheme 2a). [RuI₂(CNCy)(η⁶-p-cymene)], 1b-I, was prepared from [RuI₂(η⁶-p-cymene)]₂ and CyNC in refluxing CH₂Cl₂ (Scheme 2b). The isocyanide coordination is generally complete within 1 hour, as checked by IR spectroscopy. All the reactions are straightforward except that of methyl isocyanide and [RuCl₂(η⁶-p-cymene)]₂. In this case, a slight excess of isocyanide led to an inseparable mixture of 1a and trans-[RuCl₂(MeNC)₄] (ca. 6%) while a sub-stoichiometric amount of isocyanide resulted in the formation of unidentified quinoline complexes in small quantity (quinoline is an impurity of freshly prepared methyl isocyanide). Purification via silica chromatography afforded 1a as an orange powder in 75% yield. In all the other cases, filtration of the final reaction mixture over a celite pad allowed the removal of a brown impurity and the desired complex was isolated as an orange (1b–g, 2a–d, and 2f) or violet-red (1b-I) powder in 84–98% yield.

Scheme 2 Synthesis of [RuX₂(isocyanide)(η⁶-arene)] complexes from the corresponding halide-bridged η⁶-arene Ru(II) dimers [RuX₂(η⁶-arene)] (X = Cl, I; arene = p-cymene, C₆Me₆, C₆H₆). All reactions were carried out under N₂; isolated yields in parentheses. The preparation of 1b and 1f is described in the literature.²¹

The preparation of the benzene analogues, [RuCl₂(CNR)(η⁶-C₆H₆)]₂ (R = Cy, 3b; CH₂PO(OEt)₂, 3d; Xyl, 3f), was challenging. The reaction of [RuCl₂(η⁶-C₆H₆)]₂ with a stoichiometric amount of cyclohexyl or xylyl isocyanide resulted in the formation of a significant amount of trans-[RuCl₂(RNC)₄] (R = Xyl, Cy), together with the desired 3b, f (Scheme 2c). These octahedral complexes, together with the phenyl and tolyl analogues, were previously obtained by the reaction of [RuCl₂(η⁶-arene)]₂ (arene = C₆H₆, p-cymene) with excess isocyanide in refluxing benzene or toluene (yields not given).^15a,26 However, the very low solubility of [RuCl₂(C₆H₆)]₂ in CH₂Cl₂ results in a large excess of isocyanide in solution even under stoichiometric conditions. These issues were circumvented by adopting a two-step procedure with a more soluble intermediate containing a labile monodentate ligand.²⁷ We selected [RuCl₂(Py)(η⁶-C₆H₆)] and [RuCl₂(SMe₂)(η⁶-C₆H₆)] for this purpose, which were isolated in 92–93% yield from [RuCl₂(C₆H₆)]₂ (up to 600 mg scale) according to an optimized procedure (Scheme 2d). Next, the substitution of either pyridine or dimethylsulfide with the selected isocyanide in CH₂Cl₂ proceeded with discrete selectivity (Scheme 2e) to give 3b, d, f. The impurities (by-products) in the crude reaction mixture were removed by trituration in Et₂O/THF mixtures, affording the desired compounds as orange powders in 82–90% yield.

Compounds 1c, 1e and 2b were previously obtained in lower yield and/or using harsher reaction conditions (e.g. a large excess of isocyanide, longer reaction time, higher temperature) and a more complex purification (chromatography and/or re-crystallization), while 3b was not isolated as a pure compound.^15a,b,g Instead, 1b, f were prepared according to the same optimized procedure herein described.²¹ To the best of our knowledge, 1a, 1d, 1g, 2a, 2c, 2d, 2f, 3b, and 3f are unprecedented. Moreover, 3d, f add to the very few examples of [RuX₂(C-donor)(C₆H₆)] complexes (X = halide).²⁸

All the ruthenium isocyanide complexes 1–3 are air- and moisture-stable in the solid state, except for 3d. The benzene complexes 3 are generally less soluble in chlorinated solvents than their p-cymene or C₆Me₆ counterparts and they are more sensitive to decomposition in solution under air, as shown by the partial release of benzene occurring at room temperature in CDCl₃ (6–35% after 14 h).

All compounds were characterized by elemental (CHN) analyses, IR and NMR spectroscopy (Fig. S1–S53). The isocyanide ligand gives rise to a strong IR absorption (CH₂Cl₂ solution) in the 2140–2220 cm⁻¹ range and a relatively weak ¹³C NMR signal (CDCl₃ solution) around 135–155 ppm. Minimal differences (≤5 cm⁻¹) in the isocyanide stretching absorption were generally observed between IR spectra of the solids and their CH₂Cl₂ solutions. Table S1 shows IR and ¹³C NMR data for 1–3, some previously reported complexes of the same type, and the corresponding isocyanides. The C≡N stretching band in 1–3 is shifted to higher wavenumbers (+20–50 cm⁻¹) with respect to the free isocyanide, indicating a scarce π-backdonation from the Ru(II) center.^15b,g,j Specifically, the coordination-induced IR shift (Δν_CN/cm⁻¹ = ν_Ru-CNR − ν_CNR) reflects the donor/acceptor properties of the isocyanide substituent (aliphatic > aromatic) and the arene and halide co-ligands (C₆H₆/Cl₂ > p-cymene/Cl₂ > p-cymene/I₂ ≈ C₆Me₆/I₂; Fig. 1). Similarly, the chemical shift of the ¹³C NMR resonance of isocyanide carbon decreases in the order: CNR > [RuCl₂(CNR)(η⁶-C₆Me₆)] > [RuCl₂(CNR)(η⁶-p-cymene)] > [RuCl₂(CNR)(η⁶-C₆H₆)]. The P=O stretching in 1–3d gives rise to a strong absorption around 1250–1270 cm⁻¹ in the solid-state IR spectra.

Fig. 1 Wavenumber of the C≡N stretching band for selected isocyanides and their corresponding {RuX₂(η⁶-arene)} adducts (X = Cl, I; arene = p-cymene, C₆Me₆, C₆H₆). IR data refer to CH₂Cl₂ solutions except [RuI₂(CNXyl)(η⁶-p-cymene)]^15j (solid state).

The crystal structures of 1a, 1c, 1e, 1g, 2c and 1b-I were elucidated by single-crystal X-ray diffraction (Fig. 2, S54–S56 and Table S2). The complexes display a three-legged piano-stool geometry, as found in related compounds where Ru is bonded to an η⁶-arene, two chlorides and one isocyanide ligand.^15f,j,h,21 The C(1)–N(1) [1.209(15), 1.150(8), 1.172(10), 1.156(11), and 1.142(5) Å for 1a, 1c, 1e, 1g, 2c and 1b-I, respectively] and the Ru–C(1) contacts [1.981(10), 1.969(6), 1.960(5), 1.954(8), 1.961(3), and 1.962(4) Å for 1a, 1c, 1e, 1g, 2c and 1b-I, respectively], as well as the Ru1–C1–N1 angle [165.7(12), 178.3(7), 172.7(8), 177.2(8), 176.1(3), and 176.2(4)° for 1a, 1c, 1e, 1g, 2c and 1b-I, respectively], are in agreement with a σ-bonded isocyanide with little π-backdonation from the metal center.¹⁶ With the exception of 1a, very small differences in Ru–C–N bond distances and angles are observed in this series of compounds, even when changing the arene (p-cymene in 1c vs. C₆Me₆ in 2c) or the halide (chloride in 1b (ref. 21) vs. iodide in 1b-I) co-ligands, in contrast to the infrared scenario.

Fig. 2 Molecular structures of 1a (left), 2c·H₂O (center) and 1b-I (right). Displacement ellipsoids are at the 50% probability level.

2.1.2. Acetylide–isocyanide and acetonitrile–isocyanide complexes. The reactivity of [RuCl₂(isocyanide)(η⁶-arene)] complexes is almost completely unexplored in the literature.^15b,h We investigated two processes, namely the activation of terminal alkynes,²⁹ and the isocyanide-promoted thermal displacement of the arene ligand in coordinating solvents,²¹ which could be potentially relevant in view of the catalytic application. Cyclohexyl and xylyl isocyanide, phenylacetylene and acetonitrile were selected as model isocyanides/alkyne/coordinating solvent, respectively.

The acetylide compounds [RuCl(CCPh)(CNR)(η⁶-C₆Me₆)] (R = Cy, 4b; Xyl, 4f) were obtained from the reaction of the corresponding bis-chloride complexes 2b, f with a slight excess of phenylacetylene and NaOH as a Brønsted base in MeOH at room temperature (4b) or 45 °C (4f). A two-step procedure starting from [RuCl₂(CNR)(η⁶-C₆Me₆)]₂ was optimized (Scheme 3), thus 4b, f were isolated as ochre-yellow solids in 57–64% yields. The synthetic steps and the work-up were carried out under N₂ since both compounds are moderately air-sensitive. Conversely, all reactions of 1b, f, phenylacetylene and various Brønsted bases resulted in mixtures of non-arene ruthenium complexes, some of which probably contain an acetylide ligand (IR data). The expected p-cymene derivatives [RuCl(CCPh)(CNR)(η⁶-p-cymene)] (R = Cy, Xyl) were never observed.

Scheme 3 Two-step synthesis of [RuCl(CCPh)(CNR)(η⁶-C₆Me₆)] from [RuCl₂(η⁶-C₆Me₆)], and the corresponding isocyanide, phenylacetylene and NaOH. Reactions were carried out under N₂; isolated yields in parentheses.

Complex 4b is unprecedented while 4f was previously obtained in lower yield using a large excess of phenylacetylene, and limited spectroscopic characterization was provided.^15c In general, relatively few Ru(η⁶-arene) acetylide complexes are known^29,30 and Ru(isocyanide)(acetylide) complexes are also comparatively rare.³¹

Compounds 4b, f were characterized by elemental analyses (CHN), IR and NMR spectroscopy (Fig. S57–S64). The Ru–C≡C(Ph) fragment is identified by a strong absorption at 2100 cm⁻¹ in the IR spectrum (CH₂Cl₂) and by ¹³C NMR signals at ca. 105 and 110 ppm (CDCl₃). By comparison, phenylacetylene shows a weak IR absorption at 2110 cm⁻¹ (pure liquid) and ¹³C NMR signals at 77 and 83 ppm (CDCl₃) for the alkynyl carbons.³² A strong IR absorption in the 2130–2160 cm⁻¹ range (CH₂Cl₂) is related to the isocyanide ligand in 4b, f. The formal replacement of Cl⁻ with PhCC⁻ on the {RuCl(CNR)(η⁶-C₆Me₆)}⁺ scaffold leads to a decrease of about 15 cm⁻¹ in the wavenumber of the isocyanide (C≡N) stretching band (Fig. S58 and S62), reflecting an increased electron density at the metal center.

The crystal structures of 4b and 4f were elucidated by single-crystal X-ray diffraction (Fig. 3 and Table S3). The compounds display a three-legged piano-stool geometry as 2c where one chloride has been replaced by an acetylide. The structures of 4b and 4f may be compared to that of the previously reported [RuCl(CCPh)(PPh₃)(η⁶-C₆Me₆)]:²⁹ the replacement of PPh₃ with CNR (R = Cy, Xyl) causes a slight elongation of the Ru1–C11 bond [2.020(3) Å in 4b; 2.025(4) Å in 4f; compared to 1.952(9) Å for the phosphane derivative]. These bonding parameters are in agreement with a Ru–C(sp) single bond, whereas C11–C12 [1.208(4) Å in 4b; 1.170(6) Å in 4f] is a typical triple bond,²⁹ as also corroborated by the fact that Ru1–C11–C12 [171.1(3)° in 4b; 176.9(4)° in 4f] and C11–C12–C13 [172.6(3)° in 4b; 176.6(5)° in 4f] are close to linearity.

Fig. 3 Molecular structures of 4b (left) and 4f (right). Displacement ellipsoids are at the 50% probability level.

Heating acetonitrile solutions of 1b, f at reflux resulted in p-cymene displacement and acetonitrile coordination. The process takes several hours to reach complete conversion, while the solution progressively changes from red to light yellow. The XylNC complex 1f is more reactive than the CyNC complex 1f (14 h vs. 24 h as the optimal reaction time, respectively), analogously to what previously observed for a related p-cymene/DMSO exchange.²¹ A one-pot procedure starting from [RuCl₂(η⁶-p-cymene)]₂ was optimized and the resulting tris-acetonitrile complexes [RuCl₂(MeCN)₃(CNR)] (R = Cy, 5b; Xyl, 5f) were isolated as sand yellow powders in almost quantitative yield (Scheme 4). To the best of our knowledge, trans,trans,trans-[RuCl₂(CNR)₂(NCR′)₂] (R′ = Me, Ph; R = ^tBu, Cy, Xyl)³³ complexes are the only prior examples of Ru complexes containing an isocyanide, a nitrile and chloride as ligands.

Scheme 4 One-pot, two step synthesis of [RuCl₂(MeCN)₃(CNR)] from [RuCl₂(η⁶-p-cymene)] and the corresponding isocyanide in MeCN as a solvent. Reactions were carried out under N₂; isolated yields in parentheses. Wavy bonds denote fac/mer and cis/trans isomerism.

Compounds 5b, f were characterized by elemental analyses (CHN), molar conductivity, IR and NMR spectroscopy (Fig. S65–S74). The IR spectra of 5b, f in CH₂Cl₂ show a strong absorption at 2104 or 2137 cm⁻¹, with a shoulder about 30 cm⁻¹ higher, accounting for xylyl and cyclohexyl isocyanide ligands, respectively. The main isocyanide stretching band shows a negative coordination shift (−7 cm⁻¹ for CyNC → 5b and −19 cm⁻¹ for XylNC → 5f; Fig. S66 and S71), indicative of increased backdonation from the Ru(II) center as compared to the p-cymene precursors 1b, f. The relative intensity of these bands is reversed in the solid-state IR spectra. A weak absorption around 2290 cm⁻¹ (CH₂Cl₂) or 2280 cm⁻¹ (solid state) is ascribable to coordinated acetonitrile.³⁴ The ¹³C NMR spectra (CD₃CN) of 5b, f show four sets of signals of comparable intensity for isocyanide ligands. Several resonances for acetonitrile ligands are present around 4 and 125 ppm in the ¹³C NMR spectra and around 2.4 ppm in the ¹H NMR spectra. There is some free MeCN in the ¹H NMR spectra, which, if included in the integrals, returns a MeCN/isocyanide ratio of 3 : 1. Overall, 5b, f are presumably mixtures of fac, mer,cis and mer,trans isomers of [RuCl₂(MeCN)₃(CNR)], while a fourth species may form in solution by water/acetonitrile replacement.³⁵ A similar case of lability towards water is documented for the well-known cis,fac-[RuCl₂(κS-DMSO)₃(κO-DMSO)].³⁶ The four species are not clearly discernible in the ¹H NMR spectra of 5b, f due to the extensive overlap of signals. Exchange of the coordinated acetonitrile with the solvent (CD₃CN) is minimal over several days at room temperature while it is almost complete after 24 h at 50 °C. The IR spectrum of the solid recovered from these solutions shows a weak band at 2300 cm⁻¹ attributed to nitrile stretching (+15 cm⁻¹ shift due to H/D exchange; Fig. S65 and S70).³⁴

2.2. Catalytic studies

The ruthenium(II) isocyanide complexes reported in this work were screened for dimerization/trimerization of phenylacetylene as a benchmark substrate. The reaction conditions were optimized using 1b as a model compound, featuring an isocyanide ligand with a simple alkyl substituent, p-cymene and chloride ligands. Several parameters were investigated to identify the key factors for designing an efficient and sustainable catalytic system, including temperature and energy source, solvent effects, catalyst concentration and choice of the base. The remaining complexes were screened under the optimized conditions to assess the influence of different isocyanide substituents, arene and halide ligands on the catalytic performances. Additionally, the activation steps of the Ru pre-catalysts were studied to elucidate the mechanism of the reaction.

Overall, the reactions afforded a maximum of five coupling products: three dimers (cis and trans dimers plus the product of homocoupling) and two trimers (Scheme 5). It was essential to assess the selectivity between dimerization and trimerization, rather than focusing on individual products. However, it was determined from combined GC–MS (Fig. S75) and ¹H NMR data (section 4.7) that the major dimer was the trans (E) enyne.

Scheme 5 Phenylacetylene dimerization/cyclotrimerization reaction and products.

2.2.1. Solvent and base screening. The coupling of terminal alkynes requires a base to facilitate proton removal. Organic bases are usually preferred due to their high activity and solubility in organic solvents; however, several of them are environmentally hazardous and pose increased risk in the overall process due to safety concerns such as flammability.³⁷ We focused our catalytic investigation on Na₂CO₃ as a base, which is cheap, non-toxic and environmentally benign.

In general, the choice of solvent is one of the most critical parameters in homogeneous catalysis, as it may face significant environmental issues and shape the activity of the catalyst.³⁸ Organic solvents such as DMF and toluene are often used in coupling reactions of terminal alkynes, negatively impacting the environment. The search for greener alternative organic or water systems is highly needed.

Table 1 comprises the results obtained for the coupling reaction of phenylacetylene using 1b as a catalytic precursor and Na₂CO₃ as a base in various solvents at 80 °C. As expected, the solvent exerted a significant influence on the catalytic activity, from both conversion and selectivity standpoints. Acetonitrile, water and DMF provided the highest conversions of phenylacetylene, with the major products being the dimers. Interestingly, in ethanol, hexane, or toluene the selectivity of the reaction shifted towards the trimers. It is noteworthy that the catalytic system is effective despite the near insolubility of Na₂CO₃ in most of these solvents. Water as a solvent created a biphasic (liquid/liquid) mixture, wherein the organic phase consisted of phenylacetylene, mesitylene and 1b, while the aqueous phase completely dissolved Na₂CO₃. This system presented the second highest conversion and selectivity. Using water as a solvent allows for a more environmentally friendly process, in which the products can be easily recovered through solvent extraction.

Table 1 Catalytic results for the coupling of phenylacetylene with 1b as a catalytic precursor and Na₂CO₃ as a base in different solvents

Entry^a	Solvent	Conversion (%)	Dimer selectivity (%)	Trimer selectivity (%)
a Reaction conditions: solvent (3.0 mL), phenylacetylene (1.0 mmol), Na2CO3 (100 μmol; 10 mol%), 80 °C, 24 h, 1b (25 μmol, 2.5 mol%). Conversion and selectivity values were determined by GC–MS after extraction with ethyl acetate (2 mL), using mesitylene as an internal standard.
1	Ethanol	58	38	62
2	Hexane	39	21	79
3	Toluene	63	34	66
4	Acetonitrile	73	66	34
5	Water	85	81	19
6	Acetone	31	81	19
7	DMF	96	85	15
8	THF	45	61	39

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Next, different organic (pyridine, triethylamine and N,N-diisopropylethylamine (DIPEA)) and inorganic (sodium, potassium and cesium carbonate) bases were tested in water, each one at 10 mol% loading. Fig. 4a and 4b present the conversion of phenylacetylene and dimer/trimer selectivity, respectively. The catalytic performance of 1b remains high with all these bases except triethylamine, determining a substantial decrease in substrate conversion. Pyridine and DIPEA gave a 10–15% increased conversion with respect to Na₂CO₃, with the selectivity to the dimer production remaining high (91 and 75%, respectively). These results do not correlate with the basicity (pK_a of conjugate acids in water: 5.2 for pyridine, 10.3 for carbonate, ≈11 for DIPEA and Et₃N). Despite the slight increase in conversion and selectivity in the case of pyridine, Na₂CO₃ is preferable as a more sustainable reagent. Phenylacetylene conversion tends to increase with the size of the carbonate cation according to the sequence Na⁺ < K⁺ < Cs⁺ (85% < 88% < 91%) while the selectivity towards the dimers follows the opposite trend, with a complementary increase in trimer production. Thus Cs₂CO₃ displayed a dimerization selectivity of 65% that contrasts with the value obtained for Na₂CO₃ (81%).

Fig. 4 Catalytic results for the coupling of phenylacetylene with 1b as a catalytic precursor and different bases in water as solvent: a) phenylacetylene conversion and b) selectivity for phenylacetylene dimers or trimers. Reaction conditions: water (3.0 mL), phenylacetylene (1.0 mmol), base (10 mol%), 1b (2.5 mol%), 80 °C, 24 h.

2.2.2. Ru catalyst and Na₂CO₃ loading. The metal catalyst loading represents one of the most significant economic impacts when designing a catalytic system. Several catalytic systems designed for C–C coupling reactions applied high catalytic loadings (up to 10 mol%) and/or precious metals such as palladium and platinum.³⁹

Table 2 shows the catalytic results of the coupling of phenylacetylene in water at 80 °C after 24 h using variable loadings of 1b and Na₂CO₃. A high conversion (85%) was achieved with 2.5 mol% of 1b and 10 mol% of Na₂CO₃, accompanied by high selectivity towards the dimerization process (81%). When the loading of 1b was increased to 3 mol%, no variation in the conversion was observed, although a slightly higher selectivity toward dimerization was obtained. In contrast, a decrease in conversion and, more importantly, a decrease in selectivity were observed with a 1 mol% Ru loading. A decline in conversion is anticipated due to the reduction of the ruthenium complex available for the reaction. Still, the change in selectivity represents an important aspect of the system, as it becomes possible to control the product distribution by combining the solvent effect with the catalyst loading.

Table 2 Catalytic results obtained for the coupling of phenylacetylene with different loadings of 1b and Na₂CO₃ in water using different catalyst loads

Entry^a	1b (mol%)	Na2CO3 (mol%)	Conversion (%)	Dimer selectivity (%)	Total TON
a Reaction conditions: water (3.0 mL), phenylacetylene (1.0 mmol), Na2CO3, 80 °C, 24 h. Conversion and selectivity values were determined by GC–MS after extraction with ethyl acetate (2 mL), using mesitylene as an internal standard.
1	1.0	10	46	65	46
2	2.5	10	85	81	33
3	3.0	10	84	85	28
4	2.5	5	60	62	24
5	2.5	15	72	69	26
6	0	10	0	—	—
7	2.5	0	0	—	—

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Next, the amount of Na₂CO₃ was screened to 5 and 15 mol% while keeping 1b at 2.5 mol%. Decreasing the base resulted in a decreased conversion of phenylacetylene to 60% while also reducing the selectivity toward dimerization to 62%. On the other hand, the increase in base leads to a drop of the conversion to 72% and the selectivity to 69% with respect to the results with 10 mol%.

Blank experiments were also performed, one without the Ru catalyst and one without Na₂CO₃, and in both cases no conversion or product formation was observed.

2.2.3. Temperature screening and time-course conversion/selectivity. Fig. 5 illustrates the variation of conversion with temperature. Compound 1b showed activity even at room temperature, converting 22% of phenylacetylene. The conversion increased to 50% with a slight rise to 50 °C. The highest conversion was obtained at 80 °C. Importantly, temperature did not significantly affect the product distribution, with only a slight increase in selectivity from 73 to 81%.

Fig. 5 Conversion of phenylacetylene at different reaction temperatures. Reaction conditions: water (3.0 mL), phenylacetylene (1.0 mmol), Na₂CO₃ (10 mol%), 1b (2.5 mol%), 24 h.

To better understand the evolution of the reaction, experiments were carried out at different times, and the results are summarized in Table 3. The reaction follows a very fast initial rate, reaching 21% conversion after one hour, although with low selectivity toward dimerization. As time progresses, phenylacetylene conversion continues to increase, with the most significant rise occurring between 4 and 8 h, and continuing up to 24 h. The gradual increase in yield is accompanied by an increase in dimerization selectivity, reaching its maximum after 24 h.

Table 3 Catalytic results for the coupling of phenylacetylene mediated by 1b at different reaction times

Entry^a	Time (hours)	Conversion (%)	Dimers selectivity (%)	Total TOF (h^−1)
a Reaction conditions: water (3.0 mL), phenylacetylene (1.0 mmol), Na2CO3 (10 mol%), 1b (2.5 mol%), 80 °C. Conversion and selectivity values were determined by GC–MS after extraction with ethyl acetate (2 mL) using mesitylene as an internal standard.
1	1	21	60	8.4
2	2	25	63	5.0
3	4	30	68	3.0
4	8	61	76	3.1
5	24	85	81	1.4

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2.2.4. Isocyanide and arene effect. Complexes 1–3 were tested under the optimized conditions (2.5 mol% Ru, 10 mol% Na₂CO₃, water, 80 °C, 24 h) to investigate the influence of the different isocyanide and arene ligands on the catalytic performances. During optimization, it was found that the loading of 1b could switch the reaction's selectivity. Therefore, each Ru pre-catalyst was tested at 1.0 and 2.5 mol%. Results are compiled in Table 4. The change in Ru loading has mixed effects on the conversion and selectivity of the catalytic process that depend on the specific arene/isocyanide combination. Overall, 1b stands out among the investigated complexes with an 85% phenylacetylene conversion and 81% selectivity for dimer formation at 2.5 mol% loading. In the other cases, phenylacetylene conversion varies in relatively limited ranges, 45–69% and 51–76% with 1.0 and 2.5 mol% Ru, respectively. Increasing the amount of Ru pre-catalyst results in a marked increase in conversion with 1–2a, 1–3b, 1b-I, 2c, 1f, and 1g (+15% to + 40%) while negligible changes or a slight decrease (down to −5%) are observed with 1c, 1–2d, 1e, and 2–3f.

Table 4 Catalytic results for the coupling of phenylacetylene with different ruthenium(II) arene complexes of general formula [RuX₂(L)(η⁶-arene)] (X = Cl except for 1b-I; L = isocyanide or CO), [RuCl(OAc)(η⁶-p-cymene)] and their halide-bridged precursors [RuCl₂(η⁶-arene)]₂

Isocyanide or other ligand	η^6-arene ligand
a Reaction conditions: water (3.0 mL), phenylacetylene (1.0 mmol), Na2CO3 (10 mol%), Ru pre-catalyst (1.0 mol%), 80 °C, 24 h. Conversion values (selectivity to dimer in parentheses) were determined by GC–MS after extraction with ethyl acetate (2 mL), using mesitylene as an internal standard.b Reaction performed with 2.5 mol% of catalyst, corresponding to 5% Ru loading for [RuCl2(η^6-arene)]2.
CN–	1a	2a	—
	50% (44%)^a	54% (69%)^a
	67% (47%)^b	70% (74%)^b
	1b	2b	3b
	46% (65%)^a	45% (66%)^a	56% (64%)^a
	85% (81%)^b	65% (67%)^b	71% (69%)^b
	1b-I (X = I)
	40% (72%)^a
	76% (66%)^b
	1c	2c	—
	62% (28%)^a	45% (70%)^a
	61% (57%)^b	65% (75%)^b
	1d	2d	3d
	54% (53%)^a	55% (38%)^a	69% (23%)^a
	59% (68%)^b	51% (64%)^b	57% (18%)^b
	1e	—	—
	62% (48%)^a
	58% (44%)^b
	1f	2f	3f
	55% (45%)^a	62% (60%)^a	67% (42%)^a
	71% (52%)^b	62% (54%)^b	66% (43%)^b
	1g	—	—
	65% (34%)^a
	77% (58%)^b
CO	27% (31%)^a	19% (27%)^a	—
	66% (35%)^b	68% (36%)^b
CH3CO2^−	[RuCl(OAc)(η^6-p-cymene)]	—	—
	29% (33%)^a
	46% (35%)^b
—	[RuCl2(η^6-p-cymene)]2	[RuCl2(η^6-C6Me6)]2	[RuCl2(η^6-C6H6)]2
	66% (41%)^b	55% (62%)^b	48% (43%)^b

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The catalytic activity of the Ru complexes is more discriminated by the selectivity for dimer or trimer formation. All cyclohexyl isocyanide complexes (1–3b and 1b-I) as well as 1d, 2a, and 2c show a net preference (selectivity values 64–75%; excluding 1b, 81%) for dimer formation at both catalytic loadings. Instead, the xylyl isocyanide complexes 1–3f as well as 1a, e equally promote dimerization and trimerization processes (dimer selectivity 44–54% except 2f at 1 mol%) at both catalytic loadings.

In most cases, it is possible to observe a shift in selectivity with increasing Ru loading, favoring the formation of dimers over trimers. The selectivity increase is moderate (+3–5%) with 1–2a, 3b, 2c, and 1f while being more pronounced (+15–25%) with 1b, 1c, 1g, and 2b–d. Compounds 1c, 1g and 2d are remarkable as they tend to favor trimer formation at low catalyst concentration (dimer selectivity 28–38%), whereas dimerization becomes the dominant pathway at the higher concentration (57–64%). Negligible changes (2b and 3f) or a slight decrease (−4–6%; 1b-I, 3d, 1e, and 2f) in selectivity for dimer formation are observed with the remaining compound. The benzene complex 3d shows the highest tendency for trimer formation among the investigated complexes, further increasing at higher catalyst amount (dimer selectivity 18–23%) while the conversion decreases substantially (−12%). A slight decrease in conversion was observed on comparing 1b with the bis-iodide analogue 1b-I at 1 mol%, with minor changes in selectivity. A more pronounced decrease in conversion (−9%) and selectivity (−15%) occurred at 2.5 mol%. On considering the greater lability of Ru–Cl bonds compared to Ru–I bonds,⁴⁰ these results indicate that halide dissociation is relevant for the catalytic mechanism.

Overall, the different arene, isocyanide and halide ligands have variable effects on the conversion and selectivity and respond differently to changes in the catalytic loading. This flexibility, combined with the factors discussed above, demonstrates the versatility of these ruthenium(II) isocyanide complexes.

Next, reference ruthenium(II) complexes were tested for comparative purposes (Table 4). The chloride-bridged precursors [RuCl₂(η⁶-arene)]₂ (arene = C₆H₆, p-cymene, C₆Me₆) performed worse in terms of both conversion and selectivity compared to most of their piano-stool isocyanide derivatives. It should also be considered that a 2.5 mol% loading corresponds to 5 mol% of Ru in these cases.

Complexes [RuCl₂(CO)(η⁶-arene)] (arene = p-cymene, C₆Me₆) provide an interesting comparison of isoelectronic carbonyl versus isocyanide ligands.¹⁹ The carbonyl complexes performed poorly in terms of both phenylacetylene conversion (20–30%) and dimerization selectivity (≈30%), using 1% Ru loading. At 2.5% loading, the conversion approached a level comparable to some isocyanide analogues but dimer selectivity only marginally increased (≈35%).

The acetate derivative [RuCl(OAc)(η⁶-p-cymene)], known to catalyze alkyne dimerization in acetic acid (see section 2.4),⁴¹ showed significantly lower conversion and selectivity compared to the p-cymene isocyanide complexes investigated in this work at both catalytic loadings (1 and 2.5%). Overall, these results highlight the beneficial effects of introducing isocyanide ligands in the design of ruthenium(II) arene catalysts.

2.2.5. Scope of substituted terminal alkynes. Table 5 illustrates the conversion and dimer selectivity for the various terminal alkynes studied under optimized conditions (2.5 mol% of Ru, 10% of Na₂CO₃ in water, 80 °C, 24 h) with the best catalytic precursor 1b. Overall, reactions with substituted terminal alkynes achieved lower conversions compared to phenylacetylene. This can be attributed to several factors, including solubility (some substrates were solid and poorly soluble in water), steric hindrance that may interfere with coordination to ruthenium and electronic effects of the substituents determining activation or deactivation of the alkyne. Nevertheless, some interesting trends were observed. For instance, the three ethynylanisoles showed differences in both conversion and selectivity. ortho- and para-Ethynylanisoles strongly favored dimerization over trimerization, while meta-ethynylanisole gave a less selective outcome, with only 57% dimer formation. Ethynyltoluenes revealed a similar behavior. However, in this case the least selective toward dimerization was the ortho substituted derivative (58% dimer). Notably, changing the methyl group to an ethyl (1-ethyl-4-ethynylbenzene) resulted in a 50% decrease in conversion relative to para-ethynyltoluene, while maintaining similar dimer selectivity. As expected, an electronegative substituent deactivates the alkyne, leading to lower conversion, as observed for 1-chloro-4-ethynylbenzene. Finally, the combination of the methyl group in the ortho position and a methoxy group in the para position exhibited a mixed effect: conversion decreased relative to para-ethynylanisole (similar to ortho-ethynyltoluene), but dimer selectivity increased slightly (by 4%).

Table 5 Catalytic results for different substituted terminal alkynes using 1b as a pre-catalyst

Reaction conditions: water (3.0 mL), terminal alkyne (1.0 mmol), Na2CO3 (10 mol%), 1b (2.5 mol%), 80 °C, 24 h. Conversion and selectivity toward dimerization (in parentheses) values determined by GC–MS after extraction with ethyl acetate (2 mL), using mesitylene as an internal standard.
58% (74%)	42% (89%)	40% (57%)
22% (69%)	18% (72%)	32% (50%)
37% (75%)	31% (58%)	50% (79%)
38% (59%)	34% (62%)	85% (81%)

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2.2.6. Alternative energy sources. Metal complexes can respond differently to various energy sources, and this characteristic can be exploited in the design of a catalytic system. Fig. 6a shows phenylacetylene conversion promoted by 1b and Na₂CO₃ (2.5 and 10 mol%, respectively) under different energy sources, while Fig. 6b presents the corresponding dimer selectivities.

Fig. 6 Conversion of phenylacetylene (a) and dimerization selectivity (b) under different energy sources. Reaction conditions: water (3.0 mL) except ball milling (solventless), phenylacetylene (1.0 mmol), Na₂CO₃ (10 mol%), 1b (2.5 mol%), 80 °C, 1 h.

After 1 h of reaction, the catalytic system is more responsive to alternative energy sources than to thermal heating. The highest conversion (34%) was obtained under microwave irradiation, likely due to the high dielectric constant of water, which efficiently absorbs microwave energy and leads to greater system activation. The performance of the ultrasonic bath strongly depends on the reaction temperature. At room temperature (purple color), the conversion was very low (12.5%). When the reaction was performed at 80 °C, the synergistic effect between ultrasonic waves and thermal activation enhanced the conversion of phenylacetylene (21% under thermal heating vs. 28% under ultrasound at 80 °C). Ball-milling also proved to be an interesting method, as it enables the reaction to be performed without solvent and provided a conversion similar to thermal heating.

Regarding selectivity, Fig. 6b shows that ultrasonic irradiation favors trimer formation. Even at 80 °C, the reaction predominantly produced trimers, demonstrating another parameter that can modulate the flexibility of the system. Thermal heating and ball-milling catalysis yielded similar selectivities for dimer formation, while the microwave reaction provided the highest dimer selectivity (77%).

2.3. Reactivity and mechanistic studies

To gain insights into the catalytic mechanism, the reactivity of ruthenium(II) arene isocyanide complexes with phenylacetylene and sodium carbonate was investigated. Compounds 1–3b and 1–3f, combining an aliphatic (cyclohexyl) or aromatic (xylyl) isocyanide with the three investigated arene ligands (benzene, p-cymene, hexamethylbenzene), were selected. Acetonitrile as a solvent is convenient for this purpose due to complete solubilization of both the Ru precursors and phenylacetylene, its coordinative ability and the possibility of monitoring the reaction by IR. A first set of experiments was carried out with each Ru compound, Na₂CO₃ and PhCCH in 1 : 1 : 10 molar ratio. After 2 h at 80 °C, the solutions were analyzed by IR and ¹H NMR (Table S4 and Fig. S76–S81). Two reactivity trends emerged based on the consumption of the starting material, which increased in the order C₆Me₆ < p-cymene < C₆H₆ and XylNC < CyNC. The displacement of the η⁶-arene ligand is minor for 2b, f (≈10%, C₆Me₆), substantial for 1b, f (50–60%, p-cymene) and quantitative for 3b, f (C₆H₆). In parallel, both hexamethylbenzene complexes 2b, f partially converted into the corresponding phenylacetylide derivatives 4b, f. In detail, 4b was the predominant species in the final solution, while the conversion of 2f into 4f was much lower. Compound 1f partially transformed in a chiral p-cymene species, while 1b was the only p-cymene complex identified in the final reaction mixture. On the other hand, no traces of 3b, 3f or any other benzene complex were detected. The Ru compounds resulting from the release of p-cymene and benzene ligands are probably associated with the intense bands appearing in the 2140–2150, 2120–2100, and 1950–1960 cm⁻¹ regions of the IR spectra, attributed to CyNC, XylNC and CO ligands, respectively.

Additional experiments were carried out with 1b, 2b and 2f in MeCN under more catalytically relevant conditions, involving a lower Ru concentration and a 1 : 10 : 100 mol ratio with Na₂CO₃ and PhCCH, respectively. Both 80 °C and lower temperatures were tested to better follow the transformations. Aliquots of each reaction mixture were analyzed by IR at different times, allowing to trace a qualitative profile of Ru speciation (see Tables S5–S9 and Fig. S82–S86). In addition, at specified times, volatiles were removed under vacuum; the residue was washed with Et₂O to remove organic compounds, and was then analyzed by IR (CH₂Cl₂), ¹H and ¹H–¹³C HMBC (CDCl₃) and ESI-MS (MeOH). In agreement with previous results, the new tests confirmed the overall reactivity trend 1b ≫ 2b > 2f. Both hexamethylbenzene complexes 2b, f initially converted into the corresponding phenylacetylide derivatives 4b, f. This reaction was relatively rapid and straightforward for 2b, which was no longer present after 1.5 h at 80 °C or after 4 h at 60 °C, while 4b became the main species in solution (IR). The less reactive 2f required 3 h at 80 °C for its complete consumption, while 4f already appeared after 1.5 h (Fig. 7). Afterwards, the intensity of the IR bands of 4b, f decreased over time while new IR absorptions appeared around 2137–2120, 2110 and 2100–2080 cm⁻¹, due to CyNC, XylNC and PhCC⁻ ligands, respectively (Fig. S85–S86). Moreover, IR bands in the 2180–2200 cm⁻¹ region were assigned to the C≡C stretching of an (ethynyl)alkenyl ligand derived from the coupling of phenylacetylide and an alkyne or vinylidene ligand.^41,42 The associated processes required several hours to reach high conversion for both 4b and 4f at 80 °C and were almost completely inhibited for 4b at 60 °C over 12 h. NMR analyses indicated the formation of a mixture of Ru complexes with some (2f/XylNC) or all (2b/CyNC) lacking the C₆Me₆ ligand.

Fig. 7 Solvent-subtracted IR spectra (CH₂Cl₂, 1850–2250 cm⁻¹) of aliquots of the mixture of 2f, Na₂CO₃ and PhCCH (1 : 10 : 100 molar ratio) in MeCN heated at 80 °C at different times, normalized for the intensity at 2127 cm⁻¹ (isocyanide stretching band of 4f). The red spectrum is the initial spectrum, corresponding to 2f, while the dashed purple line shows the spectrum of 4f. Inset shows the MS profile of 4f observed after 3 h of reaction.

The p-cymene compound 1b confirmed to be more reactive than the related hexamethylbenzene complexes 2b, f. After 1 h at 80 °C, the isocyanide stretching band of 1b completely disappeared and new isocyanide (2165, 2144 cm⁻¹) and acetylide (2080 cm⁻¹) bands emerged. The conversion of 1b slowed down at 55 °C, being about 50% after 4.5 h, and was accompanied by the growth of a similar profile of IR bands. The formation of these isocyanide/acetylide complexes is likely associated with the loss of the p-cymene ligand since 1b is the only piano-stool species observed in the ¹H NMR spectrum. The putative [RuCl(CCPh)(CyNC)(p-cymene)] (expected C≡N stretching ≈2180 cm⁻¹) is bypassed or is short-lived.

For each compound (1b, 2b, and 2f) two IR bands around 1970 and 1955 cm⁻¹ slowly increased over time at 80 °C, reaching a comparable intensity to that of isocyanide stretchings after 3–4 h for 1b, 7.5 h for 2b and 22 h for 4f. These absorptions were assigned to non-arene carbonyl complexes,⁴³ which may arise from the hydrolysis or oxidative cleavage of phenylacetylide or related ligands (vinylidene, carbene, and vinyl).^29,44

Control experiments indicated that 1b, 2b, and 2f are fairly inert in MeCN with Na₂CO₃ and without PhCCH (Fig. S87–S89). Complex 2f was practically unchanged after 3 h at 80 °C while 1b and 2b probably underwent a partial chloride/carbonate exchange after 2 h at 55 °C or 2.5 h at 80 °C, respectively, as suggested by a broad IR absorption emerging at 1450 cm⁻¹.⁴⁵ IR bands related to the tris-acetonitrile isocyanide complexes 5b and f were not observed, confirming that the interaction of 1b, 2b, and 2f with PhCCH/Na₂CO₃ is much more rapid than arene/acetonitrile substitution. In fact, 1b, f required 14–24 h in refluxing acetonitrile to fully convert into 5b, f (Scheme 4) and is foreseeable that the displacement of C₆Me₆ from 2b, f would be even more difficult.^12,13 This outcome confirms that acetylide coordination (formal Cl⁻/PhCC⁻ exchange in 2 → 4) occurs first and likely facilitates the subsequent displacement of the arene ligand (Scheme 6).

Scheme 6 Overview of the investigated reactivity of ruthenium(II) arene isocyanide complexes with excess PhCCH and Na₂CO₃ in MeCN (L = Cl⁻, solvent or other ligands).

Related experiments carried with 1b/PhCCH/Na₂CO₃ in ethanol (80 °C) or water/mesitylene (55 °C) showed the rapid (1–2 h) formation of new isocyanide–acetylide species, as occurred in MeCN (Fig. S90–S91, Table S10). A reaction carried out with 2f in water/mesitylene at 80 °C pointed out the initial formation of 4f, with a similar kinetics to that in acetonitrile as a solvent (comparing Table S11/Fig. S92 vs. Table S9/Fig. S86). At variance to MeCN, the scenario in both EtOH and water/mesitylene is complicated by an enhanced tendency to carbonyl formation. In this regard, the higher solubility of Na₂CO₃ probably accelerates the hydrolysis of Ru–C bonds.

To confirm the mechanistic hypotheses, catalytic tests were carried out with 1, 2, 4, 5b and 1, 2, 4, 5f in acetonitrile or water at 80 °C at different reaction times (1 or 24 h). Results are collected in Table 6. First, the catalytic activity of the tris-acetonitrile complexes 5b, f was compared with the p-cymene precursors 1b, f (entries 1–8). The significant differences in conversion and selectivity both at 1 h and 24 h support the hypothesis that 5b, f are not formed in situ from 1b, f under catalytic conditions. A similar comparison of catalytic activity involved 2b and 2f, and 4b and 4f, in both water and acetonitrile (entries 9–16 and 21–28). The catalytic activity was similar at both short (1 h) and long (24 h) reaction times, suggesting that the first step of the activation of the dichloride pre-catalysts 2 involved the formation of their acetylide derivatives 4. The catalytic performances of each Ru compound in water vs. in MeCN are not remarkably different at both reaction times (1 h and 24 h), with a few exceptions (namely, 4b at 1 h; 1f at 1 h and 24 h), despite the enhanced tendency for carbonyl formation in the aqueous medium.

Table 6 Results obtained for the coupling of phenylacetylene with 1b, 1f, 2b, 2f, 4b, 4f and 5b, and 5f as pre-catalysts

Entry^a	Catalyst	Time (hours)	Solvent	Conversion (%)	Dimer selectivity (%)
a Reaction conditions: solvent (3.0 mL), phenylacetylene (1.0 mmol), Na2CO3 (10 mol%), Ru pre-catalyst (2.5 mol%), 80 °C. Percentage of conversion and selectivity values determined by GC–MS after extraction with ethyl acetate (2 mL), using mesitylene as an internal standard.
1	1b	1	MeCN	20	53
2	1b	24	MeCN	73	65
3	5b	1	MeCN	8	65
4	5b	24	MeCN	73	74
5	1f	1	MeCN	13	95
6	1f	24	MeCN	63	80
7	5f	1	MeCN	8	62
8	5f	24	MeCN	82	81
9	2b	1	MeCN	17	68
10	2b	24	MeCN	75	74
11	4b	1	MeCN	15	54
12	4b	24	MeCN	64	72
13	2f	1	MeCN	21	64
14	2f	24	MeCN	63	71
15	4f	1	MeCN	14	56
16	4f	24	MeCN	63	69
17	1b	1	Water	21	60
18	1b	24	Water	85	81
19	1f	1	Water	38	52
20	1f	24	Water	71	52
21	2b	1	Water	18	86
22	2b	24	Water	65	68
23	4b	1	Water	12	82
24	4b	24	Water	72	72
25	2f	1	Water	14	65
26	2f	24	Water	62	54
27	4f	1	Water	12	60
28	4f	24	Water	62	69

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In conclusion, key steps of the activation of the arene–isocyanide pre-catalysts 1–3 with PhCCH/Na₂CO₃ are phenylacetylide coordination and arene displacement, leading to octahedral Ru(II) isocyanide/acetylide species (Scheme 6). This is a multi-step process for hexamethylbenzene complexes 2 that begins with the formation of [RuCl(CCPh)(CNR)(C₆Me₆)], 4, via formal Cl⁻/PhCC⁻ exchange. The release of C₆Me₆ from 4 or related complexes becomes significant after several hours, while p-cymene and benzene dissociation from 1 and 3 presumably occurs concerted with phenylacetylene activation. The thermally-induced displacement of the η⁶-arene (often p-cymene) from ruthenium(II) complexes has been invoked in several cases as a key step to generate catalytically-active species.^7,9c,10,46 Based on literature findings,⁴⁷ a generic catalytic cycle for terminal alkyne dimerization may involve the stepwise coordination of two alkyne molecules as acetylide and vinylidene ligands. Nucleophilic (intramolecular) attack of the acetylide on the electrophilic vinylidene carbon followed by protonation of the resulting η³-butenynyl or η¹-(ethynyl)alkenyl complex by another alkyne molecule liberates the enyne product and regenerates the initial species (Scheme 7). In principle, both piano-stool and octahedral Ru(II) complexes may participate in such a catalytic cycle. However, based on collected evidence, η⁶-arene species are short-lived for 1 and 3 while they might become important in the early stages of the process mediated by 2. Moreover, the selectivity for the dimerization process increases over time with 1b as a pre-catalyst (Table 3). Therefore, it could be hypothesized that the observed switch in selectivity (dimerization vs. trimerization) reflects a different pathway of activation of the pre-catalyst, which depends on the arene and isocyanide ligands and the solvent.

Scheme 7 Proposed general catalytic cycle for phenylacetylene dimerization based on the literature (L = Cl⁻, solvent or other ligands).

2.4. Comparison with the literature

There is an extensive catalogue of ruthenium complexes applied as catalysts for the coupling/dimerization/trimerization of phenylacetylene and other terminal alkynes.^25d,48,49 but only a few of these feature an η⁶-arene ligand. The halido-bridged dimers [RuX₂(η⁶-arene)]₂ (X = Cl; arene = C₆H₆, p-cymene, 1,3,5-C₆H₃Me₃, C₆Me₆, X = I, arene = p-cymene; Fig. 8a)^50,51 promote the dimerization of terminal aryl alkynes in acetic acid or acetic acid/water 1 : 1 as a solvent at room temperature with 5 mol% of catalyst load, corresponding to 10 mol% Ru. Specifically, [RuCl₂(η⁶-arene)]₂ (arene = p-cymene, C₆Me₆) achieved 60–86% yield for the dimerization of phenylacetylene and a range of substituted alkynes after 24 h, and consistently maintained high selectivity (usually > 90%) towards the E (trans) dimer. The in situ formed [RuCl(κ²O-O₂CCH₃)(η⁶-arene)]^41,51 (arene = p-cymene, C₆Me₆; Fig. 8b) were identified as the catalytically active species. Under our conditions (Table 4), [RuCl₂(p-cymene)]₂, [RuCl₂(C₆H₆)]₂ and [RuCl(O₂CCH₃)(p-cymene)] (2.5 mol%) produced a modest PhCCH conversion and favored phenylacetylene trimerization rather than dimerization. [RuCl₂(η⁶-C₆Me₆)]₂ was more active and more selective for the dimerization process but still less effective than most of the isocyanide derivatives.

Fig. 8 Previously-investigated η⁶-arene ruthenium(II) complexes for the catalytic dimerization or trimerization of alkynes: halide-bridged arene dimers (X = Cl, I; arene = C₆H₆, p-cymene, 1,3,5-C₆H₃Me₃, C₆Me₆) (a) and related mononuclear acetate as active species (b); benzophenone imine derivative with a 2-hydroxyethyl functionalized arene ligand (c); abnormal N-heterocyclic carbene complex (d); phosphane derivatives (R = Ph, Cy, ⁱPr, formed in situ from [RuCl₂(p-cymene)]₂ + PR₃) (e).

A few piano-stool ruthenium(II) η⁶-arene complexes bearing different monodentate ligands such as NHC carbenes,^52,53 phosphanes^54,55 or an imine⁵⁶ (Fig. 8c–e) have been investigated for dimerization/trimerization of terminal alkynes. The protocols involved the use of organic solvents (toluene,^52,54,56 acetonitrile,⁵³ and 1,2-dichloroethane⁵⁵), high (5–10%) Ru loading,^53–55 high amounts (20–25%) of base or other additives (Et₃N,^53,56 NH₄PF₆,⁵⁵ and phosphanes⁵⁴) and/or inert atmosphere.^54–56 These conditions represent an increase in the environmental impact, costs and operational aspects compared to the methodology reported here (water, Na₂CO₃ 10 mol%, Ru 2.5 mol%, and in air). The N-heterocyclic carbene complexes⁵³ (one example of which is shown in Fig. 8d) predominantly favored trimer formation (58–81% selectivity) in MeCN at 70 °C with 25 mol% Et₃N, whereas 1b herein reported preferentially dimerizes phenylacetylene in both acetonitrile as a solvent (Table 1) or in water with triethylamine (Fig. 4). Özgün et al. applied [RuCl₂(p-cymene)]₂ (4 mol% corresponding to 8 mol% Ru) for the selective cyclotrimerization of a range of terminal alkynes (80–99% yields) in toluene at 80 °C and studied the switchability between dimerization and cyclotrimerization of phenylacetylene using [RuCl₂(p-cymene)]₂ by adding PCy₃ or PⁱPr₃.⁵⁴ By progressively increasing the phosphine to [RuCl₂(p-cymene)]₂ ratio up to 40 : 1, the reaction switched from cyclotrimerization to dimerization (90% selectivity). These results were related to the initial formation of [RuCl₂(PR₃)(p-cymene)] (Fig. 8e) followed by p-cymene displacement and the formation of the vinylidene derivative [RuCl₂(PR₃)₂{=C=C(H)Ph}].

3. Conclusions

Ruthenium(II) η⁶-arene isocyanide complexes are a versatile class of compounds offering structural variability in terms of arene, halide and isocyanide ligands, a simple and straightforward synthesis, air-stability, good solubility in organic solvents and a peculiar reactivity, which makes them effective catalysts for the dimerization/trimerization of terminal alkynes.

Herein, a panel of fifteen [RuX₂(CNR)(arene)] (X = Cl, I) derivatives were prepared in 80–98% yields from the readily available isocyanide and halide-bridged arene precursors. New, effective synthetic procedures were developed for preparing the elusive benzene derivatives. Aiming to investigate the reactivity of this class of underexplored ruthenium compounds, two protocols were optimized for the preparation of rare acetylide derivatives of the type [RuCl(CCPh)(CNR)(C₆Me₆)] (60% yield) and unprecedented acetonitrile complexes of the type [RuCl₂(MeCN)₃(CNR)] (> 98% yield).

The bis-halide isocyanide complexes behave as effective catalytic precursors for the dimerization or trimerization of phenylacetylene. Remarkably, the selectivity of the catalytic process can be controlled by several parameters such as solvent, base, Ru loading, energy source and the arene/isocyanide combination. The crucial role played by isocyanide ligands was corroborated by control catalytic experiments with the diruthenium precursors [RuCl₂(arene)]₂ and the carbonyl analogues [RuCl₂(CO)(arene)]. The best catalytic performances for the formation of enynes were achieved using [RuCl₂(CNCy)(p-cymene)] as a pre-catalyst. A low catalyst loading (1 mol%) and Na₂CO₃ as a base were efficient in aqueous solution, thus providing a significant step forward in process sustainability with respect to the literature. The activation of PhCCH through coordination as an alkynyl ligand and the subsequent thermal dissociation of the arene were identified as key steps in the generation of the catalytically active species. Overall, the results reported in this work encourages further investigations on this easily available and tunable class of compounds in homogeneous catalysis.

4. Experimental section

4.1. General experimental details

All reagents and solvents were obtained from Alfa Aesar, Merck, Carlo Erba or TCI Europe and were used without further purification. Isocyanides were stored at 4 °C or −20 °C; contaminated labware was treated with HCl/EtOH. Methyl isocyanide,⁵⁷ [RuCl₂(η⁶-arene)]₂ (arene = p-cymene, C₆Me₆, C₆H₆),^7,12,13 [RuI₂(η⁶-p-cymene)]₂ (ref. 40) and [RuCl₂(CNR)(η⁶-p-cymene)] (R = Cy, 1b; Xyl, 1f)²¹ were prepared according to the literature. The purity of methyl isocyanide was assessed by ¹H NMR (CDCl₃) using Me₂SO₂ as an internal standard.⁵⁸ The synthesis of [RuCl₂(CO)(η⁶-arene)] (arene = p-cymene, C₆Me₆)⁵⁹ and the optimized preparation of [RuCl(κ²O-O₂CCH₃)(η⁶-p-cymene)],⁶⁰ [RuCl₂(Py)(η⁶-C₆H₆)]⁶¹ and [RuCl₂(SMe₂)(η⁶-C₆H₆)]⁶² are described in the SI. All preparations were carried out under N₂ using standard Schlenk techniques. Anhydrous CH₂Cl₂ and THF were obtained from an SPS 5 solvent purifier (MBraun) and were stored over 4 Å MS; MeCN and MeOH were deaerated by bubbling Ar for 30 min. All the other operations, including work-up procedures (except for 3d and 4b), were carried out in air with non-anhydrous solvents. Filtrations were carried out on G3 or G4 sintered-glass filters; Celite Standard Super Cel® (Alfa Aesar) was used when indicated. All the isolated complexes are free-flowing powders relatively inert to air and moisture (except 3d), which were kept under N₂ for long term storage as a precaution. Percent reaction yields are calculated on a molar basis with respect to the precursors [RuX₂(η⁶-arene)]₂ and are referred to the isolated, powdered material. Carbon, hydrogen, nitrogen and sulfur (CHNS) analyses were performed on a Vario MICRO cube instrument (Elementar). Infrared spectra of solid samples (650–4000 cm⁻¹ range) were recorded on a Perkin Elmer Spectrum Two spectrometer equipped with a diamond-ATR sampling accessory. IR spectra of solutions were recorded using a CaF₂ liquid transmission cell (1500–2300 cm⁻¹) on a Perkin Elmer Spectrum 100 FT-IR spectrometer. IR spectra were processed with Spectragryph.⁶³ NMR spectra were recorded on a YH JNM-ECZ400S instrument (JEOL) equipped with a Royal HFX Broadband probe. CDCl₃ stored in the dark over Na₂CO₃ was used for NMR analysis. ¹³C{¹H} NMR spectra were recorded with 6 or 8 s relaxation time. Chemical shifts are referenced to the residual solvent peaks (¹H, ¹³C) or to the external standard (³¹P to 85% H₃PO₄).⁶⁴ NMR resonances were assigned with the support of ¹H–¹³C gs-HSQC and gs-HMBC correlation experiments. Conductivity measurements were carried using an XS COND 8 instrument (cell constant = 1.0 cm⁻¹) equipped with an NT 55 temperature probe (measurements automatically adjusted to 25 °C) and calibrated using standard KCl solutions in ultrapure water. ESI-Q/TOF flow injection analyses (FIA) were carried out using a 1200 Infinity HPLC coupled to a Jet Stream ESI interface with a quadrupole-time of flight tandem mass spectrometer 6530 Infinity Q-TOF (all from Agilent Technologies). HPLC-MS grade acetonitrile was used as the mobile phase (flow rate 0.2 mL min⁻¹, total run time 3 min). Samples were weighed, dissolved in HPLC-MS grade methanol and diluted to 10 ppm prior to injection (injection volume: 0.1 μL). ESI operating conditions: drying gas (N₂, purity >98%): 350 °C and 10 L min⁻¹; capillary voltage 4.5 kV; nozzle voltage: 1 kV; nebulizer gas 35 psig; sheath gas (N₂, purity >98%): 375 °C and 11 L min⁻¹. The fragmentor was kept at 50 V, the skimmer at 65 V and the OCT 1 RF at 750 V. High resolution MS spectra were obtained in positive mode in the range 100–1700 m/z; the mass axis was calibrated using the Agilent tuning mix HP0321 (Agilent Technologies) prepared in acetonitrile and water.

4.2. Synthesis and characterization of p-cymene and hexamethylbenzene isocyanide complexes

Procedure A. In a 25 mL Schlenk tube under N₂, a solution of [RuCl₂(η⁶-arene)]₂ (arene = p-cymene, C₆Me₆) (80–150 mg) in anhydrous CH₂Cl₂ (6–8 mL) was treated with the selected isocyanide (1.0–1.1 eq.). The brick red/orange mixture was stirred for 1–3 h at room temperature. The conversion was checked by IR (CH₂Cl₂) then the resulting orange-red suspension was filtered over celite. The filtrate was taken to dryness under vacuum, and the residue was triturated with a Et₂O/hexane 1 : 1 v/v mixture (2 : 1 v/v for 1d and 2d) and stirred at room temperature for some time. Next, the suspension was filtered and the resulting orange (1b–f, 2a–d, 2f, 3b, 3d, and 3f) or orange-brownish (1g) solid was washed with Et₂O/hexane 1 : 1 v/v and hexane and then dried under vacuum (40 °C). Minor variations were adopted for 1a (specified below). The filtration step is fundamental to remove a dark brown solid (by-product); otherwise samples have a brownish shade and a lower CHN content, despite showing no impurities in ¹H and ¹³C NMR spectra.

Procedure B. The reactions were carried out as described above using deaerated MeCN as a solvent (ca. 8–10 mL) at room temperature, unless otherwise specified.

Compounds [RuCl₂(CNR)(η⁶-p-cymene)] (R = Cy, 1b; R = Xyl, 1f) were prepared on a 300–500 mg scale (10 mL solvent) according to both procedures, as previously reported.²¹

[RuCl₂(CNMe)(η⁶-p-cymene)], 1a (Chart 1).

Chart 1 Structure of 1a (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-p-cymene)]₂, methyl isocyanide (1.0 eq. vs. Ru) and CH₂Cl₂; reaction time: 24 h. The selective formation of the title compound faces challenges related to the quinoline impurity in methyl isocyanide⁵⁸ and the consequent difficulty in adding a stoichiometric amount of the reactant. Reactions carried out with a sub-stoichiometric amount of isocyanide lead to the formation of ruthenium–quinoline by-products, while those carried out with an excess of isocyanide lead to the partial formation of trans-[RuCl₂(MeNC)₄].^15a Only the former impurity can be effectively removed by silica chromatography, while washing the solid products with Et₂O, toluene, etc. is ineffective in any case. The two reaction types are described below.
Reaction #1. [RuCl₂(η⁶-p-cymene)]₂ (263 mg, 0.859 mmol Ru), methyl isocyanide (60 μL, 1.00 mmol if pure) and CH₂Cl₂ (15 mL). The reaction mixture was transferred on top of a silica column (h 4 cm, d 2.3 cm). Impurities were eluted with CH₂Cl₂, and then a brown-orange band was eluted with acetone. Volatiles were removed under vacuum. The residue was dissolved in CH₂Cl₂ and filtered over celite. The filtrate was taken to dryness under vacuum, and the resulting orange-red solid was triturated with Et₂O. The suspension was filtered and the solid was washed with Et₂O and hexane and was dried under vacuum. Yield: 225 mg, 75%.
Reaction #2. [RuCl₂(η⁶-p-cymene)]₂ (80 mg, 0.261 mmol Ru), methyl isocyanide (60 μL, 0.25 mmol if pure) and CH₂Cl₂ (10 mL) resulted in a bright red reaction mixture. A subsequent workup, as per procedure A, gave an orange-pink solid. Yield: 82 mg of a 1a:trans-[RuCl₂(MeNC)₄] ≈ 15 mol mol⁻¹ mixture; ¹H NMR (CDCl₃). Compound 1a is soluble in CH₂Cl₂, insoluble in Et₂O, toluene, and hexane. X-ray quality crystals of 1a were obtained from a CH₂Cl₂ solution layered with diethyl ether and settled aside at −20 °C. Anal. calcd. for C₁₂H₁₇Cl₂NRu: C: 41.51; H: 4.93; N: 4.03. Found: C: 41.58; H: 5.11; N: 4.20. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3058w-sh, 3041w, 3032w, 2969w, 2962w, 2937w, 2872w, 2213s (C≡N), 1532w, 1509w-sh, 1470m, 1443m, 1414–1409m, 1385m, 1374m, 1330w, 1314w, 1299w, 1284w, 1262w, 1202w, 1164w, 1142w, 1086m, 1069m, 1058m, 1034m, 961w, 925w, 914w-sh, 872m, 853m, 802m, 787m, 698w, 671w. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2218 s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 5.61 (d, ³J_HH = 5.9 Hz, 2H, C⁴H), 5.43 (d, ³J_HH = 5.9 Hz, 2H, C³H), 3.62 (s, 3H, C⁹H), 2.86 (hept, ³J_HH = 6.9, C⁶H), 2.28 (s, 3H, C¹H), 1.30 (d, ³J_HH = 6.9 Hz, C⁷H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 139.2 (br., C⁸); 107.5, 106.7 (C² + C⁵); 87.8, 87.6 (C³ + C⁴); 31.3 (C⁶), 30.9 (C⁹), 22.6 (C⁷), 19.0 (C¹).

trans-[RuCl₂(MeNC)₄] (impurity in 1a). IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 2178s-sh (C≡N). IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2186 m (C≡N). ¹H NMR (CDCl₃): δ/ppm = 3.54 (s, CH₃). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 30.2 (CH₃).

[RuCl₂{S-CNCH(Me)Ph}(η⁶-p-cymene)], 1c (Chart 2).

Chart 2 Structure of 1c (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-p-cymene)]₂ (149 mg, 0.480 mmol Ru) and (S)-(−)-α-methylbenzyl isocyanide (70 μL, 0.52 mmol); reaction time: 1 h; yield: 210 mg, 98%. The previously-reported preparation involves the use of excess isocyanide (3.1 eq.) in refluxing hexane (70 °C) for 12 h followed by recrystallization from CH₂Cl₂/petroleum ether (89% yield).^15g Soluble in CH₂Cl₂, poorly soluble in Et₂O, and insoluble in hexane. X-ray quality crystals of 1c were obtained from a CH₂Cl₂ solution layered with diethyl ether and settled aside at −20 °C. Anal. calcd. for C₁₉H₂₃Cl₂NRu: C, 52.18; H, 5.30; N, 3.20. Found: C, 51.49; H, 5.26; N, 3.16. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3087w, 3060w, 3052w, 3030w, 2979w, 2966w, 2959w, 2928w, 2904w, 2867w, 2183s (C≡N), 1602w, 1538w, 1513w, 1495w, 1469m-sh, 1456m, 1449m, 1390w, 1377w, 1365w, 1350m, 1328w, 1308w, 1295w, 1281w, 1201w, 1159w, 1112w, 1093m, 1075m, 1058w-sh, 1029w, 993m, 972w, 922w, 907w, 879m, 809w, 763s, 738w, 697s, 676m-sh. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2189s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 7.44–7.38 (m, 4H, C¹²H + C¹³H); 7.36–7.32 (m, C¹⁴H); 5.58 (d, ³J_HH = 6.1 Hz), 5.56 (d, ³J_HH = 6.1 Hz), 5.43 (d, ³J_HH = 6.0 Hz), 5.41 (d, ³J_HH = 5.9 Hz), 5.20 (q, ³J_HH = 6.7 Hz, C⁹H), 2.77 (hept, ³J_HH = 6.9 Hz, C⁶H), 2.27 (s, 3H, C¹H), 1.77 (d, ³J_HH = 6.8 Hz, C¹⁰H); 1.24, 1.23 (d, ³J_HH = 6.9 Hz, 6H, C⁷H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 141.2 (br, C⁸), 138.1 (C¹¹), 129.3 (C¹²), 128.8 (C¹⁴), 125.7 (C¹³), 107.7 (C⁵), 107.3 (C²); 88.1, 88.0, 87.7, 87.6 (C³ + C⁴); 57.2 (C⁹), 31.4 (C⁶), 25.0 (C¹⁰); 22.58, 22.54 (C⁷); 18.9 (C¹).

[RuCl₂{CNCH₂PO(OEt)₂}(η⁶-p-cymene)], 1d (Chart 3).

Chart 3 Structure of 1d (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-p-cymene)]₂ (82 mg, 0.27 mmol Ru) and diethyl isocyanomethyl phosphonate (44 μL, 0.27 mmol); reaction time: 3 h; yield: 110 mg, 85%. Alternatively prepared from [RuCl₂(η⁶-p-cymene)]₂ (106 mg, 0.346 mmol Ru) and diethyl isocyanomethyl phosphonate (58 μL, 0.36 mmol) according to procedure B; reaction time: 3 h; yield: 143 mg, 86%. Soluble in CH₂Cl₂ and MeCN, poorly soluble in Et₂O, and insoluble in hexane. Anal. calcd. for C₁₆H₂₆Cl₂NO₃PRu: C, 39.76; H, 5.42; N, 2.90. Found: C, 38.45; H, 5.39; N, 2.86. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3054w, 3033w, 2971w, 2960w, 2909w, 2872w-sh, 2207s (C≡N), 2131w-sh, 1539w, 1498w, 1469m, 1442w, 1402w, 1388m, 1326w, 1272s (P=O), 1212w, 1202w, 1162m, 1095m-sh, 1050s-sh, 1026s, 976s, 967s-sh, 880m, 845w, 784m, 706w, 693w. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2198 (C≡N). IR (MeCN): [small nu, Greek, tilde]/cm⁻¹ = 2199 (C≡N). ¹H NMR (CDCl₃): δ/ppm = 5.65 (d, ³J_HH = 6.1 Hz, 2H, C⁴H), 5.46 (d, ³J_HH = 6.1 Hz, 2H, C³H), 4.31–4.24 (m, 4H, C¹⁰H), 4.22 (d, ²J_HP = 15.1 Hz, 2H, C⁹H), 2.90 (hept, ³J_HH = 6.8 Hz, 1H, C⁶H), 2.28 (s, 3H, C¹H), 1.39 (t, ³J_HH = 7.0 Hz, C¹¹H), 1.29 (d, ³J_HH = 6.9 Hz, C⁷H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 144.6 (br., C⁸), 108.6 (C⁵), 107.6 (C²), 88.5 (C⁴), 88.3 (C³), 64.5 (d, ²J_CP = 6.6 Hz, C¹⁰), 40.7 (d, ¹J_CP = 153.7 Hz, C⁹), 31.3 (C⁶), 22.6 (C⁷), 18.9 (C¹), 16.6 (d, ³J_CP = 5.7 Hz, C¹¹). ³¹P{¹H} NMR (CDCl₃): δ/ppm = 13.8.

[RuCl₂{CNCH₂CO₂Et}(η⁶-p-cymene)], 1e (Chart 4).

Chart 4 Structure of 1e (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-p-cymene)]₂ (83 mg, 0.27 mmol Ru) and ethyl isocyanoacetate (32 μL, 0.29 mmol); reaction time: 1 h; yield: 104 mg, 91%. Alternatively prepared from [RuCl₂(η⁶-p-cymene)]₂ (104 mg, 0.340 mmol Ru) and ethyl isocyanoacetate (38 μL, 0.35 mmol) according to procedure B; reaction time: 3 h; yield: 124 mg, 87%. The previously-reported preparation involves a prolonged reaction time (20 h) and purification via silica gel chromatography followed by crystallization (28% yield).^15b Soluble in CH₂Cl₂ and MeCN and insoluble in Et₂O and hexane. X-ray quality crystals of 1e·CH₂Cl₂ were obtained from a CH₂Cl₂ solution layered with hexane and settled aside at −20 °C. Anal. calcd. for C₁₅H₂₁Cl₂NO₂Ru: C: 42.97; H: 5.05; N: 3.34. Found: C: 42.05; H: 4.99; N: 3.30. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3057w, 2066w, 2893w, 2831w, 2196s (C≡N), 1753s (C=O), 1472m-sh, 1500w, 1467w, 1444w, 1427w, 1388w-sh, 1376m, 1351w, 1295w, 1280w, 1253w, 1206s-br, 1166m-sh, 1118w, 1096w, 1060w, 1041m, 1003w, 948w, 928w, 895w, 957m, 795w, 704w. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2205 (C≡N); 1759s, 1750sh (C=O). IR (MeCN): [small nu, Greek, tilde]/cm⁻¹ = 2206 (C≡N); 1761s, 1750s (C=O). ¹H NMR (CDCl₃): δ/ppm = 5.68 (d, ³J_HH = 6.1 Hz, 2H, C⁴H), 5.48 (d, ³J_HH = 6.0 Hz, 2H, C³H), 4.63 (s, 2H, C⁹H), 4.32 (q, ³J_HH = 7.2 Hz, 2H, C¹¹H), 2.96 (hept, ³J_HH = 6.9 Hz, 1H, C⁶H), 2.32 (s, 3H, C¹H), 1.35 (t, ³J_HH = 7.2 Hz, 3H, C¹²H), 1.31 (d, ³J_HH = 6.9 Hz, 6H, C⁷H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 164.3 (C¹⁰), 146.4 (br, C⁸), 109.1 (C⁵), 107.7 (C²); 88.7, 88.3 (C³ + C⁴); 63.3 (C¹¹), 46.6 (C⁹), 31.3 (C⁶), 22.6 (C⁷), 19.0 (C¹), 14.2 (C¹²).

[RuCl₂{CN(4-C₆H₄OMe)}(η⁶-p-cymene)], 1g (Chart 5).

Chart 5 Structure of 1g (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-p-cymene)]₂ (73 mg, 0.24 mmol Ru) and 4-methoxyphenyl isocyanide (33 mg, 0.25 mmol); reaction time: 1 h; yield: 95 mg, 91%. Soluble in CH₂Cl₂, poorly soluble in Et₂O, and insoluble in hexane. Anal. calcd. for C₁₈H₂₁Cl₂NORu: C, 49.21; H, 4.82; N, 3.19. Found: C, 49.08; H, 4.80; N, 3.11. X-ray quality crystals of 1g·CHCl₃ were obtained from a CHCl₃ solution layered with hexane and settled aside at −20 °C. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3068w, 3037w, 3006w, 2959s, 2926w, 2872w, 2840w, 2167s (C≡N), 1601s, 1581w, 1503s, 1465m, 1443m, 1242w, 1405w, 1386m, 1377m, 1323w, 1302m, 1253s, 1201m, 1182m, 1161m, 1117w, 1107m, 1090m, 1058m, 1028s, 922w, 878w, 849m, 831s, 810m, 798m-sh, 726w, 706w, 694w, 670w.

IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2168 (C≡N). ¹H NMR (CDCl₃): δ/ppm = 7.38 (d, ³J_HH = 8.9 Hz, 1H), 6.88 (d, ³J_HH = 8.9 Hz, 1H), 5.69 (d, ³J_HH = 6.0 Hz, 1H), 5.51 (d, ³J_HH = 6.0 Hz, 1H), 3.83 (s, 3H), 2.91 (hept, ³J_HH = 7.1 Hz, 1H), 2.35 (s, 3H), 1.34 (d, ³J_HH = 6.9 Hz, 6H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 160.5 (C¹²O), 147.4 (br, C⁸N), 128.3 (C¹⁰), 120.3 (C⁹), 114.8 (C¹¹), 108.6 (C⁵), 108.0 (C²); 88.53, 88.49 (C³ + C⁴); 55.8 (C¹³O), 31.5 (C⁶), 22.7 (C⁷), 19.1 (C¹).

[RuCl₂(CNMe)(η⁶-C₆Me₆)], 2a (Chart 6).

Chart 6 Structure of 2a (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-C₆Me₆)]₂ (87 mg, 0.26 mmol Ru) and methyl isocyanide (70 μL, ca. 0.3 mmol);⁵⁸ reaction time: 1 h; yield: 92 mg, 94%. Differently from 1a, a reaction carried out with a slight excess of isocyanide in refluxing CH₂Cl₂ selectively afforded 2a, without any [RuCl₂(MeNC)₄] by-product. Soluble in CH₂Cl₂ and insoluble in Et₂O/hexane mixtures and hexane. Anal. calcd. for C₁₈H₂₁Cl₂NORu: C, 44.80; H, 5.64; N, 3.73. Found: C, 43.10; H, 5.43; N, 3.72. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3033–2993w, 2939w, 2198s (C≡N), 1446m, 1416m, 1386m, 1066m, 1014m, 960w, 817w, 785w. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2200 s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 3.61 (s, 3H, C⁴H), 2.14 (s, 18H, C¹H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 144.3 (C³), 98.1 (C²), 30.9 (C⁴), 16.1 (C¹).

[RuCl₂(CNCy)(η⁶-C₆Me₆)], 2b (Chart 7).

Chart 7 Structure of 2b (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-C₆Me₆)]₂ (82 mg, 0.25 mmol Ru) and cyclohexyl isocyanide (33 μL, 0.27 mmol); reaction time: 2 h; yield: 99 mg, 91%. Alternatively prepared from [RuCl₂(η⁶-C₆Me₆)]₂ (275 mg, 0.822 mmol Ru) and cyclohexyl isocyanide (102 μL, 0.820 mmol) according to procedure B; reaction time: 3 h; yield: 326 mg, 90%. The previously-reported preparation involves excess isocyanide (10 eq.), prolonged reaction time (20 h) and purification via silica gel chromatography (59% yield).^15b Compound 2b is soluble in CH₂Cl₂ and insoluble in hexane. Anal. calcd. for C₁₉H₂₉Cl₂NRu: C, 51.47; H, 6.59; N, 3.16. Found: C, 50.75; H, 6.62; N, 3.12. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 2960w, 2939w, 2917m, 2857m, 2159s (C≡N), 1523w, 1540m, 1437m, 1379m, 1360m-sh, 1353m, 1314m, 1269w, 1240w, 1154w, 1128w, 1067m, 1025m, 1007m, 933w, 866w, 658m. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2175 (C≡N). ¹H NMR (CDCl₃): δ/ppm = 3.94 (td, ³J_HH = 8.5, 4.0 Hz, 1H, C⁴H); 2.12 (s, 18H, C¹H); 2.06–1.97 (m, 2H), 1.84–1.70 (m, 4H), 1.58–1.46 (m, 1H), 1.44–1.32 (m, 3H) (C⁵H + C⁶H + C⁷H); no changes were observed in the ¹H NMR spectrum after several days at −20 °C. ¹³C{¹H} NMR (CDCl₃): δ/ppm = 143.4 (br., C³), 97.8 (C²), 55.2 (C⁴), 33.3 (C⁵), 24.9 (C⁷), 23.3 (C⁶), 16.0 (C¹).

[RuCl₂{S-CNCH(Me)Ph}(η⁶-C₆Me₆)], 2c (Chart 8).

Chart 8 Structure of 2c (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-C₆Me₆)]₂ (81 mg, 0.24 mmol Ru) and (S)-(−)-α-methylbenzyl isocyanide (35 μL, 0.26 mmol); reaction time: 1.5 h; yield: 98 mg, 86%. Soluble in CH₂Cl₂ and insoluble in Et₂O and hexane. Needle-shaped X-ray quality crystals of 2c·H₂O were obtained from a CH₂Cl₂ solution layered with hexane and settled aside at −20 °C. The water molecule in the crystal structure probably originates from the residual water content of the organic solvents used for crystallization since the isolated powder of 2c does not contain water (CHN analyses) and is not hygroscopic. Anal. calcd. for C₂₁H₂₇Cl₂NRu: C, 54.19; H, 5.85; N, 3.01. Found: C, 53.77; H, 5.75; N, 2.94. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3056w, 2997w, 2985w, 2935w, 2890w, 2872w, 2167s (C≡N), 1601w, 1496w, 1449m, 1380m, 1338m, 1315w-sh, 1228w, 1195w, 1156w, 1097w-sh, 1068m, 1031m, 1009m, 917w, 850w, 778s, 740s, 700s. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2169 s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 7.47–7.32 (m, 5H, Ph), 5.17 (q, ³J_HH = 6.7 Hz, 1H, C⁴H), 2.06 (s, 18H, C¹H), 1.77 (d, ³J_HH = 6.8 Hz, 3H, C⁵H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 146.3 (br. C³), 138.9 (C⁶), 129.2 (C⁷), 128.8 (C⁹), 125.9 (C⁸), 98.3 (C²), 56.9 (C⁴), 24.7 (C⁵), 16.0 (C¹).

[RuCl₂{CNCH₂PO(OEt)₂}(η⁶-C₆Me₆)], 2d (Chart 9).

Chart 9 Structure of 2d (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-C₆Me₆)]₂ (52 mg, 0.16 mmol Ru) and diethyl isocyanomethyl phosphonate (30 μL, 0.19 mmol); reaction time: 3 h; yield: 70 mg, 88%. Alternatively prepared from [RuCl₂(η⁶-C₆Me₆)]₂ (81 mg, 0.24 mmol Ru) and diethyl isocyanomethyl phosphonate (46 μL, 0.29 mmol) according to procedure B; reaction time: 2.5 h; yield: 109 mg, 88%. Soluble in CH₂Cl₂ and MeCN and insoluble in Et₂O and hexane. Anal. calcd. for C₁₈H₃₀Cl₂NO₃PRu: C, 42.28; H, 5.91; N, 2.74. Found: C, 41.56; H, 5.86; N, 2.69. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 2995w, 2983w, 2974w, 2954w, 2934w, 2920w, 2182s (C≡N), 1484w, 1456w, 1441w, 1398m, 1390m, 1371m-sh, 1291m-sh, 1271s (P=O), 1247w-sh, 1216w, 1160w, 1150w, 1102w, 1070w-sh, 1042s-sh, 1020s, 982s, 964s, 852m, 820w-sh, 792s, 774m, 719w. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2180s (C≡N). IR (MeCN): [small nu, Greek, tilde]/cm⁻¹ = 2181 s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 4.31–4.22 (m, 4H, C⁵H), 4.16 (d, ¹J_HP = 15 Hz, 2H, C⁴H), 2.14 (s, 18H, C¹H), 1.39 (t, ³J_HH = 7.1 Hz, C⁶H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 149.6 (br, C³), 99.1 (s, C²), 64.3 (d, ²J_CP = 7 Hz, C⁵), 40.6 (d, ¹J_CP = 154 Hz, C⁴), 16.6 (d, ³J_CP = 6 Hz, C⁶), 16.0 (s, C¹). ³¹P{¹H} NMR (CDCl₃): δ/ppm = 14.7.

[RuCl₂(CNXyl)(η⁶-C₆Me₆)], 2f (Chart 10).

Chart 10 Structure of 2f (numbering refers to carbon atoms).

Prepared according to procedure A using [RuCl₂(η⁶-C₆Me₆)]₂ (62 mg, 0.19 mmol Ru) and xylyl isocyanide (25 mg, 0.19 mmol); reaction time: 3 h; yield: 73 mg, 84%. Alternatively prepared according to procedure B using [RuCl₂(η⁶-C₆Me₆)]₂ (105 mg, 0.314 mmol Ru) and xylyl isocyanide (43 mg, 0.32 mmol); reaction temperature/time: 45 °C, 3.5 h. Yield: 124 mg, 85%. The reaction in MeCN at room temperature is very slow. Compound 2f is soluble in CH₂Cl₂ and insoluble in Et₂O and hexane. Anal. calcd. for C₂₁H₂₇Cl₂NRu: C, 54.19; H, 5.85; N, 3.01. Found: C, 53.65; H, 5.54; N, 3.01. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 2981w, 2914w, 2128s (C≡N), 1526-1488w, 1462m, 1438m, 1377m, 1264w, 1185w, 1162w, 1091w, 1067m, 1029w, 1007m, 975w, 921w, 897w, 814w, 777s, 738m, 718m, 667m. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2141 (C≡N). ¹H NMR (CDCl₃): δ/ppm = 7.14 (dd, ³J_HH = 8.6, 6.4 Hz, 1H, C⁸H); 7.07 (d, ³J_HH = 7.4 Hz, 2H, C⁷H); 2.45 (s, 6H, C⁶H); 2.19 (s, 18H, C¹H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 157.3 (br., C³), 135.6 (C⁵), 128.7 (C⁸), 127.9 (C⁷), 127.8 (sh., C⁴), 99.2 (C²), 19.3 (C¹), 16.2 (C⁶).

[RuI₂{CN(C₆H₁₁)}(η⁶-p-cymene)], 1b-I (Chart 11).

Chart 11 Structure of 1b-I (numbering refers to carbon atoms).

In a 25 mL Schlenk tube under N₂, a violet-red solution of [RuI₂(η⁶-p-cymene)]₂ (102 mg, 0.209 mmol Ru) and cyclohexyl isocyanide (28 μL, 0.23 mmol) in anhydrous CH₂Cl₂ (4 mL) was stirred for 1 h at reflux temperature. The conversion was checked by IR (CH₂Cl₂) and then the resulting violet-red suspension was filtered over celite. The filtrate was taken to dryness under vacuum and the residue was triturated in hexane and stirred at room temperature for some time. Next, the suspension was filtered and the resulting purple red-violet solid was washed with hexane and dried under vacuum (40 °C). Yield: 114 mg, 91%. Soluble in MeCN and CH₂Cl₂, poorly soluble in Et₂O, and insoluble in hexane. X-ray quality crystals of 1b-I were obtained from an acetone solution layered with a diethyl ether/hexane mixture and settled aside at −20 °C. Anal. calcd. for C₁₇H₂₅I₂NRu: C, 34.19; H, 4.21; N, 2.34. Found: C, 33.97; H, 4.18; N, 2.31. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3056w, 3036w, 2959w, 2933m, 2922m, 2855w, 2184s (C≡N), 1536w, 1495w, 1467w, 1444w, 1434w, 1381m, 1363m, 1352m, 1324s, 1276w, 1262w, 1241w, 1201w, 1153w, 1140w, 1128w, 1115w, 1088w, 1055m, 1021m, 998w-sh, 931w-sh, 921w, 897w, 886w, 863m, 822w, 799w, 671w, 659m. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2177s (C≡N). IR (MeCN): [small nu, Greek, tilde]/cm⁻¹ = 2177 s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 5.61 (d, ³J_HH = 6.1 Hz, 2H, C⁴H), 5.49 (d, ³J_HH = 6.0 Hz, 2H, C³H), 4.09 (m, ³J_HH = 7.3, 3.6 Hz, C⁹H), 2.97 (hept, ³J_HH = 6.9 Hz, C⁶H), 2.55 (s, 3H, C¹H), 1.95–1.73 (m, 6H), 1.47–1.36 (m, 4H) (C¹⁰H + C¹¹H + C¹²H); 1.31 (d, ³J_HH = 6.9 Hz, 6H, C⁷H); no changes were observed in the ¹H NMR spectrum after several weeks at −20 °C. ¹³C{¹H} NMR (CDCl₃): δ/ppm = 136.3 (br, C⁸), 110.0 (C⁵), 105.9 (C²); 89.1, 87.8 (C³ + C⁴); 55.2 (C⁹); 32.9, 32.1 (C⁶ + C¹⁰); 25.0 (C¹²); 23.1, 22.8 (C⁷ + C¹¹); 20.6 (C¹).

4.3. Synthesis and characterization of benzene isocyanide complexes

Procedure C. In a 25 mL Schlenk tube under N₂, an orange-brown suspension of [RuCl₂(SMe₂)(η⁶-C₆H₆)] (90–200 mg) in anhydrous CH₂Cl₂ (8–10 mL) was treated with the appropriate isocyanide (1.0–1.1 eq.) and was stirred at reflux for 1–2.5 h. The conversion was checked by IR (CH₂Cl₂) and then the resulting mixture (red solution + orange solid) was cooled to room temperature and filtered over a celite pad (3–4 cm), to remove a dark brown residue. The red/brick orange filtrate was taken to dryness under vacuum and the residue was triturated with a Et₂O/THF 1 : 1 v/v mixture under stirring at room temperature for a few hours. Next, the resulting suspension (orange solid + pale yellow-greenish solution) was filtered. The orange solid was washed with Et₂O/THF 1 : 1 v/v, Et₂O, and hexane and then dried under vacuum (40 °C). The workup procedure for 3d is slightly different (described below).

The filtration step is fundamental to remove a dark brown solid (by-product). The final solid/liquid extraction with the Et₂O/THF mixture is crucial to remove several minor by-products; in this respect, toluene, Et₂O or their mixtures with hexane are ineffective.

Procedure D. The procedure was carried out as described above, using [RuCl₂(Py)(η⁶-C₆H₆)] as a precursor and running the reaction at room temperature for 2 h.

Procedure E. In a 25 mL Schlenk tube under N₂, a brick-red suspension of [RuCl₂(η⁶-C₆H₆)] (75 mg, 0.30 mmol Ru) in anhydrous CH₂Cl₂ (6 mL) was treated with the appropriate isocyanide (1.0 eq.) and stirred at reflux for 4 h. The conversion was checked by IR (CH₂Cl₂) and then the resulting mixture (red solution + a small amount of an orange-red solid) was cooled to room temperature and filtered over celite. The filtrate was taken to dryness under vacuum. The resulting orange solid was washed with a Et₂O/hexane 2 : 1 v/v mixture and hexane and dried under vacuum (40 °C).

[RuCl₂(CNCy)(η⁶-C₆H₆)], 3b (Chart 12).

Chart 12 Structure of 3b (numbering refers to carbon atoms).

The title compound was previously isolated as a ‘salmon pink’ solid in unknown yield from the reaction of [RuCl₂(η⁶-C₆H₆)]₂ and cyclohexyl isocyanide (10 eq.) in refluxing benzene; elemental analyses (CHNCl) and partial ¹H NMR (CDCl₃) and IR (solid state) data were given.^15a

Prepared according to procedure C using [RuCl₂(SMe₂)(η⁶-C₆H₆)] (189 mg, 0.604 mmol) and cyclohexyl isocyanide (75 μL, 0.60 mmol); reaction time: 1 h, yield: 199 mg, 91%. Alternatively prepared from [RuCl₂(Py)(η⁶-C₆H₆)] (104 mg, 0.32 mmol) and cyclohexyl isocyanide (40 μL, 0.32 mmol) according to procedure D; yield: 93 mg, 82%. Soluble in CH₂Cl₂, poorly soluble in THF and acetone, and insoluble in toluene, Et₂O, hexane, and ⁱPrOH. Anal. calcd. for C₁₃H₁₇Cl₂NRu: C, 43.46; H, 4.77; N, 3.90. Found: C, 42.0; H, 4.51; N, 3.81. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3077w, 3051w, 2957w-sh, 2937m, 2858w, 2219s (C≡N), 2115w, 1460w, 1445m-sh, 1429s, 1364w, 1354w, 1326m, 1272w, 1237w, 1152m, 1137w, 1127w, 1114w, 1032w, 1012w, 980w, 964s-sh, 927w, 863w, 824s, 796-761w, 732w, 697w, 663m. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2203 s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 5.81 (s, 6H, C₆H₆), 4.10–3.99 (m, 1H, C²H); 2.02–1.89 (m, 2H, C³H), 1.89–1.73 (m, 4H, C³H + C⁴H), 1.53–1.34 (m, 4H, C⁴H + C⁵H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 136.1 (br., C¹), 89.3 (C₆H₆), 55.6 (C²), 32.6 (C³), 24.9 (C⁵), 22.8 (C⁴). Release of benzene is observed in the CDCl₃ solution of 3b stored at room temperature (ca. 6% after 14 h). ¹H NMR (CH₃OD): δ/ppm = 5.87 (s, 6H, C₆H₆), 4.22–4.14 (m, 1H, C²H); 1.98–1.88, 1.86–1.75, 1.52–1.39 (m, 10H, C³H + C⁴H + C⁵H).

The reaction of [RuCl₂(η⁶-C₆H₆)]₂ and cyclohexyl isocyanide, according to procedure E, gave a peach orange solid (94 mg) consisting of 3b [IR (CH₂Cl₂): [small nu, Greek, tilde] = 2203 cm⁻¹], trans-[RuCl₂(CNCy)₄] [IR (CH₂Cl₂): [small nu, Greek, tilde] = 2154 cm⁻¹]^15a and unknown isocyanide complexes [IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2114, 2061].

[RuCl₂{CNCH₂PO(OEt)₂}(η⁶-C₆H₆)], 3d (Chart 13).

Chart 13 Structure of 3d (numbering refers to carbon atoms).

Prepared according to procedure C using [RuCl₂(SMe₂)(η⁶-C₆H₆)] (94 mg, 0.30 mmol) and diethyl isocyanomethyl phosphonate (54 μL, 0.34 mmol); reaction time: 2.5 h. The resulting dark red turbid solution was taken to dryness under vacuum. All the subsequent operations were carried out under N₂ using anhydrous solvents. The residue was triturated with Et₂O (20 mL) and the solution was discarded. Next, the solid was washed twice with a Et₂O/THF 4 : 1 v/v mixture (20 mL), as described above, and then suspended in Et₂O. The mixture was filtered (G4) and the resulting orange solid was washed with Et₂O and hexane, dried under vacuum (room temperature) and stored under N₂. Yield: 110 mg, 85%. A workup in air using non-anhydrous solvents led to extensive decomposition of the compound. Soluble in CH₂Cl₂ and CHCl₃, scarcely soluble in Et₂O and Et₂O/THF 5 : 1 v/v, and insoluble in hexane. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3074w, 3049w, 2976w, 2958w, 2905w; 2215s, 2191s, 2127m-sh (C≡N); 1430w, 1395w, 1368w, 1280w, 1250s (P=O), 1221w, 1160w, 1098w, 1046s-sh, 1012s, 978s, 948s-sh, 857w, 823m, 794m, 776m, 715w. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2208 s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 5.87 (s, 6H, C₆H₆); 4.33–4.25 (m, 4H, C³H), 4.21 (d, ²J_HP = 15.2 Hz, 2H, C²H), 1.41 (t, ³J_HH = 7.04 Hz, 6H, C⁴H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 142.2 (C¹), 90.8 (+ sh.*, C₆H₆), 65.1 (+ sh.*, C³), 42.8 (br.*, C²), 16.6 (C⁴); *C₆H₆ and C³ resonances show line broadening and a shoulder peak, the C² resonance is very broad and almost hidden over the baseline. ³¹P{¹H} NMR (CDCl₃): δ/ppm = 13.6. Release of benzene (ca. 35% after 14 h) and the formation of Ru(C₆H₆) by-product(s) (δ_H = 5.69 ppm, δ_C = 81.2, 64.7 ppm, δ_P = 14.9 ppm) are observed in the CDCl₃ solution of 3d stored at room temperature.

[RuCl₂(CNXyl)(η⁶-C₆H₆)], 3f (Chart 14).

Chart 14 Structure of 3f (numbering refers to carbon atoms).

Prepared according to procedure C using [RuCl₂(SMe₂)(η⁶-C₆H₆)] (204 mg, 0.653 mmol) and xylyl isocyanide (87 mg, 0.66 mmol); reaction time: 1 h, yield: 203 mg, 82%. Alternatively prepared from [RuCl₂(Py)(η⁶-C₆H₆)] (54 mg, 0.16 mmol) and xylyl isocyanide (28 mg, 0.18 mmol) according to procedure D; yield: 56 mg, 90%. Soluble in CH₂Cl₂, less soluble in CHCl₃ and acetone, scarcely soluble in THF, and insoluble in Et₂O and hexane. Anal. calcd. for C₁₅H₁₅Cl₂NRu: C, 47.25; H, 3.97; N, 3.67. Found: C, 46.3; H, 3.65; N, 3.62. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3070w, 3058w-sh, 2980w, 2947w, 2158s (C≡N), 1600w, 1590w, 1471m, 1435m, 1382m, 1370w-sh, 1264w, 1237w, 1214w, 1180w, 1158w, 1084w, 1067w, 1036w, 1016w, 1006w, 990w, 971w, 900w, 829m-sh, 815s, 786s, 762w-sh, 718w, 692w, 665m. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2172s (C≡N). ¹H NMR (CDCl₃): δ/ppm = 7.22–7.17 (m, 1H, C⁶H), 7.11 (d, ³J_HH = 7.6 Hz, 2H, C⁵H), 5.93 (s, 6H, C₆H₆), 2.48 (s, 6H, C⁴H). ¹³C{¹H} NMR (CDCl₃): δ/ppm = 149.5 (br., C¹), 135.9 (C³), 129.6 (C⁶), 128.1 (C⁵), 126.9 (br., C²), 90.2 (C₆H₆), 19.0 (C⁴). Release of benzene is observed in the CDCl₃ solution of 3f stored at room temperature (ca. 14% after 14 h).

The reaction of [RuCl₂(η⁶-C₆H₆)]₂ and xylyl isocyanide, according to procedure E, gave an orange solid consisting of 3f [IR (CH₂Cl₂): [small nu, Greek, tilde] = 2172 cm⁻¹], presumably trans-[RuCl₂(CNXyl)₄] [IR (CH₂Cl₂): [small nu, Greek, tilde] = 2135 cm⁻¹]²⁶ and unknown isocyanide complexes [IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2082, 2020].

4.4. Synthesis and characterization of acetylide–isocyanide and acetonitrile–isocyanide complexes

[RuCl(CCPh)(CNCy)(η⁶-C₆Me₆)], 4b (Chart 15).

Chart 15 Structure of 4b (numbering refers to carbon atoms).

In a 25 mL Schlenk tube under N₂, a solution of [RuCl₂(η⁶-C₆Me₆)]₂ (60 mg, 0.18 mmol Ru) in anhydrous CH₂Cl₂ (3 mL) was treated with CyNC (22 μL, 0.18 mmol) and was stirred at room temperature for 1 h. IR analysis of the bright orange-red solution confirmed the quantitative formation of 2b. Volatiles were removed under vacuum; all the subsequent operations (including the workup) were carried out under N₂ using deaerated solvents. The residue was re-dissolved with MeOH (3 mL) and treated with PhCCH (22 μL, 0.20 mmol, 1.1 eq.) and NaOH (1.56 mol L⁻¹ solution in MeOH; 0.13 mL, 0.20 mmol, 1.1 eq.). The orange solution was stirred at room temperature for ca. 2.5 h, affording an orange-amber solution. The formation of 4b was checked by IR. The MeOH solution was extracted with hexane (2 × 5 mL), and then dried under vacuum. The residue was triturated with hexane/CH₂Cl₂ 2 : 1 v/v. The suspension was moved into a column and filtered over a celite pad. The procedure was repeated until complete extraction (colorless solution) of the title product from the yellow residue of sodium salts. The orange-amber filtrate solution was taken to dryness under vacuum. The residue was triturated with cold (−20 °C) pentane and the mixture was rapidly filtered in air (the pale-yellow filtrate rapidly darkened in air). The resulting ochre solid was washed further with pentane, dried under vacuum (RT) and stored under N₂. Yield: 58 mg, 64%. The formation of 4b is quite selective, as shown by IR analysis, but solutions/suspensions of 4b in Et₂O or hexane are unstable when exposed to air. A progressive browning is observed when the workup is carried out in air, together with the appearance of carbonyl stretching bands in the IR spectrum; eventually, an impure brown 4b is obtained in a low yield (down to ca. 25%). However, solid 4b stored under N₂ is stable for months (IR analysis).

Soluble in CH₂Cl₂ and CHCl₃, less soluble in Et₂O and Et₂O/hexane mixtures, and poorly soluble in hexane and pentane. X-ray quality crystals of 4b were obtained from a CH₂Cl₂ solution layered with hexane and settled aside at −20 °C. Anal. calcd. for C₂₇H₃₄ClNRu: C, 63.70; H, 6.73; N, 2.75. Found: C, 59.8; H, 6.39; N, 2.71. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 2928 m, 2855w, 2144 s (C≡N), 2088s (C≡C), 1939w, 1902w-sh, 1591m, 1482m, 1447m, 1382m, 1363m, 1351m, 1325m, 1260w, 1240w, 1208w, 1171w, 1155w, 1129w, 1067m, 1024m, 1012m, 933w, 891w, 863w, 765s, 753s-sh, 696s, 659m. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2161s (C≡N), 2098s (C≡C), 1594w, 1485w, 1451w, 1441w, 1421w. ¹H NMR (CDCl₃): δ/ppm = 7.36 (d, ³J_HH = 7.6 Hz, 2H, C¹¹H), 7.14 (t, ³J_HH = 7.6 Hz, 2H, C¹²H), 7.02 (t, ³J_HH = 7.4 Hz, 1H,C¹³H), 3.91 (tt, ³J_HH = 7.5, 3.6 Hz, 1H, C⁴H), 2.17 (s, 18H, C¹H), 1.99–1.90 (m, 2H), 1.87–1.69 (m, 4H), 1.51–1.43 (m, 4H), (C⁵H + C⁶H + C⁷H); no changes were observed in the ¹H NMR spectrum after 24 h at room temperature. ¹³C{¹H} NMR (CDCl₃): δ/ppm = 147.8 (br., C³), 131.7 (C¹¹), 129.4 (C¹⁰), 127.6 (C¹²), 124.4 (C¹³); 110.6 (C⁸); 104.1 (C⁹); 101.9 (C²), 54.9 (C⁴); 33.5, 33.4 (C⁵); 25.1, 23.0 (C⁶); 16.2 (C¹).

[RuCl(CCPh)(CNXyl)(η⁶-C₆Me₆)], 4f (Chart 16).

Chart 16 Structure of 4f (numbering refers to carbon atoms).

The synthetic procedure was optimized with respect to the literature,^15c wherein the reaction of 2f, PhCCH (≈12 eq.) and KOH (1 eq.) in MeOH at room temperature for 4 h resulted in 43% yield of 4f. A comprehensive characterization of the title compound is also provided, while only ¹H NMR, UV-vis and a few IR data were given in the literature.^15c

In a 25 mL Schlenk tube under N₂, a solution of [RuCl₂(η⁶-C₆Me₆)]₂ (61 mg, 0.18 mmol Ru) and XylNC (24 mg, 0.19 mmol) in anhydrous CH₂Cl₂ (5 mL) was stirred at room temperature for 1 h. An IR analysis of the bright red solution confirmed the quantitative formation of 2f. Volatiles were removed under vacuum, and the residue was re-dissolved with deaerated MeOH (6 mL) under N₂ and treated with PhCCH (24 μL, 0.22 mmol, 1.2 eq.) and NaOH (1.48 mol L⁻¹ solution in MeOH; 0.15 mL, 0.22 mmol, 1.2 eq.). The red solution was stirred at 45 °C for 3 h, affording a yellow-ochre solid and a red-brown solution. The mixture was cooled to −20 °C and filtered. The filtrate, containing a mixture of unidentified Ru complexes, hexamethylbenzene and only a trace amount of 4f, was discarded. The resulting yellow-ochre solid was washed with cold MeOH (−20 °C, ca. 2 mL) Et₂O, and hexane and was dried under vacuum (RT) and stored under N₂. Yield: 55 mg, 57%. A procedure in which the second step was carried out in air led to the isolation of 4f in lower yield (43%) while, contrary to the expectations,^15c no reaction occurred in another test carried out at room temperature. Soluble in CH₂Cl₂ and CHCl₃, poorly soluble in MeOH, Et₂O and their mixtures, and insoluble in hexane. X-ray quality crystals of 4f were obtained from a CH₂Cl₂ solution layered with hexane and settled aside at −20 °C. Anal. calcd for C₂₉H₃₂ClNRu: C, 65.58; H, 6.07; N, 2.64. Found: C, 62.20; H, 6.02; N, 2.48. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3065w, 3015w, 2975w, 2944w, 2915w, 2110s (C≡N); 2091s (C≡C); 1594w, 1554w, 1483m, 1465w, 1437w, 1377w, 1210w, 1193w, 1171w, 1162w, 1138w, 1066w, 1025w, 1006w, 904w, 842w, 818w, 773s, 762s, 719m, 695s, 673m. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2127 s (C≡N); 2099s (C≡C); 1594w, 1485w, 1467w, 1385w. ¹H NMR (CDCl₃): δ/ppm = 7.34 (d, 2H, ³J_HH = 7.2 Hz, C¹²H), 7.15 (t, ³J_HH = 7.6 Hz, 2H, C¹³H), 7.12–6.99 (m, 4H, C¹⁴H + C⁷H + C⁸H), 2.49 (s, 6H, C⁶H), 2.24 (s, 18H, C¹H); no changes were observed in the ¹H NMR spectrum after 14 h at room temperature. ¹³C{¹H} NMR (CDCl₃): δ/ppm = 161.4 (br., C³), 135.3 (C⁵), 131.6 (C¹²), 129.2 (C¹¹), 128.7 (C⁴), 128.0 (C⁸), 127.8 (C¹³), 127.7 (C⁷), 124.6 (C¹⁴), 109.5 (C⁹), 104.3 (C¹⁰), 103.3, 19.3, 16.4 (C⁶).

Attempts to synthesize [RuCl(CCPh)(CNR)(η⁶-p-cymene)] (R = Cy, Xyl). All reactions of 1b and PhCCH with various bases/solvents (Et₃N/EtOH; ^tBuOK/THF; Na₂CO₃/MeCN) gave brown solutions, which did not contain the expected acetylide compound. Likewise, a reaction of 1f, PhCCH and NaOH/MeOH carried out as described for 4f gave a brown solution after 1.5 h at room temperature, which contained a mixture of unidentified Ru complexes and a minor amount of unreacted 1f (IR and ¹H NMR). In each case, the IR band for the isocyanide stretching of the title compounds (estimated at ca. 2180 cm⁻¹ for R = Cy and 2150 cm⁻¹ for R = Xyl) was not detected. A typical IR profile of the products obtained by these reactions is given as follows. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2165s, 2146s, 2081m-w for R = Cy; 2096s, 2070s, 2013m for R = Xyl.

[RuCl₂(MeCN)₃(CNCy)], 5b (Chart 17). ³⁵

Chart 17 Structure of 5b (numbering refers to carbon atoms; wavy bonds represent fac, mer,cis and mer,trans isomers).

In a 50 mL Schlenk tube under N₂, a solution of [RuCl₂(η⁶-p-cymene)]₂ (205 mg, 0.672 mmol Ru) and CyNC (84 μL, 0.68 mmol) in deaerated MeCN (10 mL) was stirred at room temperature for 3 h. An IR analysis of the red solution confirmed the quantitative formation of 1b. The mixture was heated at reflux (T_bath = 90 °C) for 24 h, affording a light-yellow solution. The quantitative formation of 5b was checked by IR. The solution was cooled to room temperature and was extracted with hexane (15 mL × 4). The acetonitrile solution was taken to dryness under vacuum. The yellow residue was triturated in a Et₂O/hexane 1 : 2 v/v mixture and the suspension was filtered. The resulting sand yellow solid was thoroughly washed with Et₂O/hexane 1 : 1 v/v and hexane and was dried under vacuum (room temperature). Yield: 270 mg, 99%. A long reaction time is essential to ensure quantitative conversion of 1b; after 14 h a bright yellow solution is obtained, which contains traces of unreacted 1b. The MeCN/hexane extraction is fundamental to remove p-cymene (normal boiling point 177 °C); otherwise a sticky yellow solid is obtained. Soluble in MeCN, CH₂Cl₂, and CHCl₃, moderately soluble in acetone, and insoluble in Et₂O and hexane. Anal. calcd. for [RuCl₂(MeCN)₃(CNCy)] = C₁₃H₂₀Cl₂N₄Ru: C, 38.62; H, 4.99; N, 13.86; anal. calcd. for [RuCl₂(MeCN)_2.5(CNCy)(H₂O)_0.5]⁶⁵ = C₁₂H_19.5Cl₂N_3.5O_0.5Ru: C, 36.69; H, 5.00; N, 12.48. Found: C, 36.64; H, 5.28; N, 12.45. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3460m-br (H₂O),⁶⁵ 2926m, 2855w-sh; 2281w (MeC≡N); 2154m-sh, 2118s, 2059m-sh (C≡NCy); 1363w, 1449m, 1420m, 1364m, 1351m-sh, 1321m, 1268w, 1239w, 1150w, 1127w, 1032m, 1026m, 944m, 930w, 892w, 862w, 657s. Λ_m (MeCN, 5.7 × 10⁻³ mol L⁻¹) = 24 S cm² mol⁻¹. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2288w (MeC≡N); 2165m-br, 2137s (C≡NCy); 1452w, 1365w, 1325w. IR (MeCN): [small nu, Greek, tilde]/cm⁻¹ = 2161m-sh, 2132s (C≡NCy). ¹H NMR (CDCl₃): δ/ppm = 4.24, 4.02 (m, 1H, C²H); 2.58, 2.40 (s, 9H, CH₃CN) 1.96 (s, MeCN) (9H); 2.15–1.97 (m, 2H), 1.93–1.71 (m, 4H), 1.59–1.29 (m, 4H) (C³H + C⁴H + C⁵H). ¹H NMR (CD₃CN): δ/ppm = 4.17, 4.10 (m, 1H, C²H); 2.44, 2.44, 2.43, 2.40, 2.39 (s, 8.1H, Ru–NCMe), 1.96 (s, 0.9H, free MeCN) (tot. 9H); 1.95–1.88 (m, 2H), 1.87–1.66 (m, 4H), 1.56–1.40 (m, 4H) (C³H + C⁴H + C⁵H). ¹³C{¹H} NMR (CD₃CN): δ/ppm = 149.4 (br., C¹), 125.5, 125.1, 124.9, 123.8, 123.4, 123.3, 121.8 (Ru–NC̲Me); 118.2* (free MeCN); 56.1, 55.9, 55.7, 55.5 (C²); 34.3, 34.2, 33.6, 33.5 (C³); 25.89, 25.86, 25.69, 25.66 (C⁵); 23.1, 23.0 (br., C⁴); 4.38, 4.31, 4.27, 4.25 (M̲e̲CN–Ru); *from ¹H–¹³C HMBC; relative intensity of the four sets of signals 38 : 30 : 19 : 13. A change in the relative intensity of C²H and CH₃CN signals in the ¹H NMR spectrum occurred over 14 h at room temperature, accompanied by an increase of the peak due to free CH₃CN (corresponding to ca. 0.07 eq. after 30 min; 0.3 eq. after 14 h); no further changes occurred over the next 10 days. Complete CH₃CN/CD₃CN exchange, except for the signal at 2.44 ppm (residual relative integral 1.9H vs. CyNC), occurred after heating the solution at 50 °C for 24 h (Fig. S68). IR (solid from CD₃CN solution, differences with respect to the original sample): [small nu, Greek, tilde]/cm⁻¹ = 3494m-br, 3410m-br (H₂O); 2299w (CD₃C≡N); 2226w (CD₃ + C–C); 2121s-sh, 2095s, 2017w-sh (C≡NCy); 1633m (H₂O).

[RuCl₂(MeCN)₃(CNXyl)], 5f (Chart 18). ³⁵

Chart 18 Structure of 5f (numbering refers to carbon atoms; wavy bonds represent fac, mer,cis and mer,trans isomers).

In a 50 mL Schlenk tube under N₂, a solution of [RuCl₂(η⁶-p-cymene)]₂ (200 mg, 0.654 mmol Ru) and XylNC (86 mg, 0.66 mmol) in deaerated MeCN (10 mL) was stirred at room temperature for 3 h, affording an orange suspension (1f is poorly soluble in MeCN). An aliquot of the solution was analyzed by IR and then the mixture was heated at reflux (T_bath = 90 °C). After 14 h, a yellow solution was obtained. IR (MeCN) and ¹H NMR (CD₃CN) spectra indicated the quantitative formation of 5f and the release of p-cymene.‡ The solution was cooled to room temperature and was extracted with hexane (4 × 15 mL). Next, the acetonitrile solution was taken to dryness under vacuum. The yellow residue was triturated in a Et₂O/hexane 1 : 1 v/v mixture and the suspension was filtered. The resulting sand yellow solid was thoroughly washed with Et₂O/hexane 1 : 1 v/v and hexane and was dried under vacuum (room temperature). Yield: 270 mg, 97%. Soluble in MeCN and CH₂Cl₂, moderately soluble in acetone, and insoluble in Et₂O/hexane 1 : 1 v/v and hexane. Anal. calcd for [RuCl₂(MeCN)₃(CNXyl)] = C₁₅H₁₈Cl₂N₄Ru: C, 42.26; H, 4.26; N, 13.14; anal. calcd for [RuCl₂(MeCN)_2.5(CNXyl)(H₂O)_0.5]⁶⁵ = C₁₄H_17.5Cl₂N_3.5O_0.5Ru: C, 40.54; H, 4.25; N, 11.82. Found: C, 40.21; H, 4.46; N, 11.98. IR (solid state): [small nu, Greek, tilde]/cm⁻¹ = 3460w-br (H₂O),⁶⁵ 2958w, 2908w; 2283w (MeC≡N); 2072s, 2016s-sh (C≡NXyl); 1589w, 1466m, 1420m, 1375w, 1261w, 1193w, 1165w, 1091w, 1032m, 944w, 820w, 771m, 721m, 674m. Λ_m (MeCN, 3.4 × 10⁻³ mol L⁻¹) = 17 S cm² mol⁻¹. IR (CH₂Cl₂): [small nu, Greek, tilde]/cm⁻¹ = 2290w (MeC≡N); 2133m-sh, 2104s, 2018w (C≡NXyl); 1606w, 1469w, 1416w. IR (MeCN): [small nu, Greek, tilde]/cm⁻¹ = 2130m-sh; 2100s, 2015w (C≡NXyl). ¹H NMR (CDCl₃): δ/ppm = 7.16, 7.08, 7.06 (m, 3H, C⁵H + C⁶H); 2.62 (s-br, 1.6H, Ru–NCMe); 2.52, 2.51, 2.48 (s, 6H, C⁴H); 2.43, 2.42, 2.41 (s, 6H, Ru–NCMe), 2.12 (s-br, 1.4H, Ru–NCMe). ¹H NMR (CD₃CN): [small nu, Greek, tilde]/cm⁻¹ = 7.26–7.11 (m, 3H, C⁵H + C⁶H), 2.49 (s, 6H, C⁴H); 2.45, 2.446, 2.442, 2.43, 2.42, 2.40 (s, 7.6H, Ru–NCMe), 1.96 (s, 1.4H, free MeCN) (tot. 9H). ¹³C{¹H} NMR (CD₃CN): δ/ppm = 167.7, 163.4 (C¹), 136.13, 136.08, 135.6, 135.6 (C³); 130.64, 130.56 (C²); 129.5, 129.4, 129.0, 128.9, 128.73, 128.67, 128.2, 128.0 (C⁵ + C⁶); 125.7, 125.4, 125.3, 123.8 (Ru–NC̲Me); 118.2* (free MeCN); 19.1, 19.0, 18.9, 18.8 (C⁴); 4.36, 4.29, 4.27, 4.24 (M̲e̲CN–Ru); *from ¹H–¹³C HMBC; relative intensity of the four sets of signals: 45 : 23 : 18 : 14.

Exchange with the deuterated solvent lowered the relative integral of Ru–NCMe in the ¹H NMR spectrum from 7.6H (freshly prepared solution) to 7.2H (14 h at room temperature) to 7.0H after several days. Complete CH₃CN/CD₃CN exchange, except for the signal at 2.446 ppm (residual relative integral 1.3H vs. CNXyl), occurred after heating the solution at 50 °C for 24 h (Fig. S73). IR (solid from CD₃CN solution, differences with respect to the original sample): [small nu, Greek, tilde]/cm⁻¹ = 3494m-br, 3395m-br (H₂O); 2298w (CD₃C≡N); 2226w (CD₃ + C–C); 2140s, 2097m-sh (C≡NCy); 1627m (H₂O).

4.5. X-ray crystallography

Crystal data and collection details for 1a, 1c, 1e·½CH₂Cl₂, 1g·CHCl₃, 1e, 2c·H₂O, 1b-I, 4b and 4f are reported in Table 7. Data were recorded on a Bruker APEX II diffractometer equipped with a PHOTON2 detector using Mo-Kα radiation. The structures were solved by direct methods and refined by full-matrix least-squares based on all data using F².⁶⁶ C-bonded hydrogen atoms were fixed at calculated positions and refined using a riding model. O-bonded hydrogen atoms of 2c·H₂O were located in the Fourier difference map and refined isotropically using a riding model.

Table 7 Crystal data and measurement details for 1a, 1c, 1e·½CH₂Cl₂, 1g·CHCl₃, 1e, 2c·H₂O, 1b-I, 4b and 4f

Compound	1a	1c	1e·½CH2Cl2	1g·CHCl3	2c·H2O	1b-I	4b	4f
a Goodness on fit on F^2 = [∑w(F^2O − F^2C)^2/(Nref − Nparam)]^1/2, where w = 1/[σ^2(F^2O) + (aP)^2 + bP], where P = (F^2O + 2F^2C)/3; Nref = number of reflections used in the refinement; Nparam = number of refined parameters.b R1 = ∑||FO| − |FC||/∑|FO|.c wR2 = [∑w(F^2O − F^2C)^2/∑w(F^2O)^2]^1/2, where w = 1/[σ^2(F^2O) + (aP)^2 + bP], where P = (F^2O + 2F^2C)/3.
Formula	C12H17Cl2NRu	C19H23Cl2NRu	C15.5H22Cl3NO2Ru	C19H22Cl5NORu	C21H29Cl2NORu	C17H25I2NRu	C27H34ClNRu	C29H32ClNRu
FW	347.23	437.35	461.76	558.69	483.42	598.25	509.07	531.07
T, K	100(2)	100(2)	100(2)	100(2)	100(2)	100(2)	100(2)	100(2)
λ, Å	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073	0.71073
Crystal system	Monoclinic	Orthorhombic	Monoclinic	Triclinic	Orthorhombic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	P212121	C2/c	P1̄	P212121	P21/c	P21/n	P21/c
a, Å	9.745(2)	9.1021(6)	25.2987(9)	10.3192(14)	7.3074(4)	9.7509(6)	7.5945(4)	8.2888(9)
b, Å	10.958(3)	11.0511(8)	17.9124(6)	12.9409(19)	12.8439(7)	16.0618(10)	12.2445(6)	12.6570(13)
c, Å	12.730(3)	18.3886(13)	8.3465(3)	18.335(2)	22.1781(13)	12.8319(8)	25.2295(12)	23.549(3)
α, °	90	90	90	71.028(4)	90	90	90	90
β, °	98.973(5)	90	94.2520(10)	89.935(4)	90	99.133(2)	91.324(2)	98.116(4)
γ, °	90	90	90	78.471(3)	90	90	90	90
Cell volume, Å^3	1342.7(5)	1849.7(2)	3771.9(2)	2263.4(5)	2081.5(2)	1984.2(2)	2345.5(2)	2445.8(4)
Z	4	4	8	4	4	4	4	4
Dc, g cm^−3	1.718	1.571	1.626	1.640	1.543	2.003	1.442	1.442
μ, mm^−1	1.538	1.135	1.262	1.293	1.020	3.897	0.796	0.767
F(000)	696	888	1864	1120	991	1136	1056	1096
Crystal size, mm	0.12 × 0.10 × 0.06	0.13 × 0.11 × 0.08	0.18 × 0.15 × 0.12	0.24 × 0.21 × 0.18	0.19 × 0.17 × 0.13	0.21 × 0.18 × 0.11	0.15 × 0.13 × 0.10	0.14 × 0.11 × 0.08
θ limits, °	2.116–25.090	2.150–26.995	1.614–25.680	1.702–26.024	1.832–27.997	2.047–27.000	1.615–27.996	1.747–27.000
Reflections collected	11 631	22 191	25 475	24 777	32 825	27 260	28 581	35 819
Independent reflections	2353 [Rint = 0.0732]	4024 [Rint = 0.0537]	3563 [Rint = 0.0296]	8611 [Rint = 0.0761]	5026 [Rint = 0.0286]	4310 [Rint = 0.0418]	5623 [Rint = 0.0482]	5346 [Rint = 0.0554]
Data/restraints/parameters	23 536/202/197	4024/0/212	3563/169/248	8611/198/496	5026/4/248	4310/12/193	5623/3/277	5346/0/297
Goodness on fit on F^2^a	1.200	1.240	1.274	1.216	1.145	1.303	1.113	1.193
R1 (I > 2σ(I))^b	0.0692	0.0426	0.0459	0.0669	0.0206	0.0300	0.0430	0.0535
wR2 (all data)^c	0.1681	0.0979	0.1137	0.1665	0.0531	0.0677	0.0955	0.1069
Largest diff. peak and hole, e Å^−3	2.127/−1.416	1.247/−1.127	1.221/−0.984	1.593/−1.183	0.512/−0.621	1.185/−0.625	1.319/−1.238	1.034/−1.626

---

4.6. Catalytic studies

Catalytic experiments were performed in a reaction station (electrothermal STEM) glass tube where 3 mL of the desired solvent, one mmol of the terminal alkyne, one mmol of mesitylene as an internal standard, the base and the Ru pre-catalyst (1 to 2.5 mol%) were added. The reaction was then heated to the desired temperature and left to react for up to 24 hours. All reactions were performed in closed vessels with a cooling coil of water. An extraction was performed with ethyl acetate to a final organic volume of 4 mL. Samples were centrifuged and analyzed by GC–MS (Shimadzu QP201OSE) using mesitylene as an internal standard. Conversion and selectivity were calculated using a calibration curve using the internal standard method. The alternative energy methods were performed using the mono-waver CEM Discovery, ball-mill PM100, and ultrasonic bath ATU series ATM.

4.7. Reactivity with phenylacetylene and Na₂CO₃ under catalytically relevant conditions

In MeCN with a higher Ru load and concentration. The selected Ru compound (1b, 1f, 2b, 2f, 3b, or 3f; 7–8 mg, 0.018 mmol, 1 eq.) was added to a 5 mL test tube together with Na₂CO₃ (20 mg, 0.19 mmol, 1 eq.), MeCN (2.0 mL) and PhCCH (20 μL, 0.18 mmol, 10 eq.). The mixture (orange solution + colorless solid; c⁰_Ru = 9 × 10⁻³ mol L⁻¹) was stirred while immersed into a pre-heated oil bath at 80 °C. After 2 h, the brown (1b, 1f, 3b, and 3f) or orange (2b and f) suspension was cooled to room temperature and was analyzed by IR (MeCN). Next, volatiles were removed under vacuum, and the residue was analyzed by ¹H NMR (CDCl₃) and IR (CH₂Cl₂). In each case, both Z and E diphenyl-but-3-en-1-yne were identified from the characteristic doublets,⁶⁷ with prevalence of the latter. IR spectra are shown in Fig. S76–S81 while results of the IR/NMR analyses are compiled in Table S4.

In EtOH or MeCN. The selected Ru compound (1b, 2b or 2f; 10 mg, 0.021–0.024 mmol) was added to a 50 mL test tube together with Na₂CO₃ (22–25 mg, 0.21–0.24 mmol, 10 eq.), the desired solvent (MeCN or EtOH; 10 mL) and PhCCH (231–264 μL, 2.10–2.40 mmol, 100 eq.). The mixture (yellow solution + colorless solid; c⁰_Ru = 2.1–2.4 × 10⁻³ mol L⁻¹) was stirred while immersed into a pre-heated oil bath at 55, 60 or 80 °C for 4–24 h and progressively turned to dark brown over time. At different reaction times, an aliquot (0.50 mL) of the solution was taken to dryness under vacuum, re-dissolved in a small volume of CH₂Cl₂ and analyzed by IR [note: direct IR or Raman analyses of the reaction mixture were not suitable due to the low Ru concentration and the overwhelming absorptions of phenylacetylene and organic products]. Subsequently, CH₂Cl₂ was removed under vacuum, and the residue was dissolved in MeCN (0.50 mL) and then re-added to the reaction mixture. The final reaction mixture was filtered over a celite pad to remove most of the sodium salts and then volatiles were removed under vacuum. The residue was re-dissolved in CH₂Cl₂, filtered over a celite pad and taken to dryness under vacuum. The oily residue was triturated in hexane or hexane/Et₂O mixtures to solubilize the remaining PhCCH and the related organic products. The suspension was filtered and the resulting brown solid was washed with hexane, dried under vacuum and analyzed by ¹H, ¹H–¹³C HMBC NMR (CDCl₃), ESI-MS and MS/MS (MeOH). Control experiments without PhCCH in MeCN were carried out as described above and samples were analyzed by IR (CH₂Cl₂). Experimental details, IR spectra and spectroscopic/spectrometric data are reported in the SI (Tables S5–S10 and Fig. S82–S90).

In water/mesitylene. The selected Ru compound (1b or 2f; 10 mg, 0.021–0.024 mmol) was added to a 50 mL test tube together with Na₂CO₃ (22–25 mg, 0.21–0.24 mmol, 10 eq.), H₂O (10 mL), mesitylene (3.4 mL) and PhCCH (231–264 μL, 2.10–2.40 mmol, 100 eq.). The biphasic mixture (colorless aqueous phase/orange organic phase) was stirred while immersed into a pre-heated oil bath at the desired temperature (≈25, 55 or 80 °C) for 1–4 h. Both phases progressively turned brown over time. Afterwards, the organic phase was separated, taken to dryness under vacuum (40 °C) and analyzed by IR (CH₂Cl₂). The aqueous phase was extracted with CH₂Cl₂ and the organic solution was taken to dryness under vacuum. The brown residue was analyzed by IR (CH₂Cl₂). Experimental details, IR spectra and data are reported in the SI (Table S11, Fig. S91–S92).

Conflicts of interest

The authors declare no competing financial interest.

Data availability

Supplementary information (SI): preparation and characterization of [RuCl₂(L)(η⁶-C₆H₆)] (L = Py, SMe₂) and [RuCl₂(CO)(η⁶-arene)] (arene = p-cymene, C₆Me₆); IR and NMR spectra of ruthenium bis-halide–isocyanide, acetylide–isocyanide and acetonitrile–isocyanide complexes; comparison of selected spectroscopic data for isocyanide ligands; catalytic studies: GC-MS spectra reactivity with phenylacetylene and Na₂CO₃ under catalytically relevant conditions.

CCDC 2504593 (1a), 2504594 (1c), 2504595 (1e·½CH₂Cl₂), 2504596 (1g·CHCl₃), 2504597 (2c·H₂O), 2504598 (1b-I), 2504599 (4b) and 2504600 (4f) contain the supplementary crystallographic data for the X-ray studies reported in this paper.^68a–h

Acknowledgements

L. B. and F. M. thank the University of Pisa for financial support under the “PRA 2022-2023 – Progetti di Ricerca di Ateneo” (Institutional Research Grants) – Project no. PRA_2022_12 “New challenges of transition metal and lanthanide complexes in the perspective of green chemistry”. This research was funded in whole or in part by the Fundação para a Ciência e a Tecnologia, I. P. (FCT, https://ror.org/00snfqn58 [1]) under the projects UIDB/00100/2020 (https://doi.org/10.54499/UIDB/00100/2020) and UIDP/00100/2020 (https://doi.org/10.54499/UIDP/00100/2020) of Centro de Química Estrutural, LA/P/0056/2020 (https://doi.org/10.54499/LA/P/0056/2020) of Institute of Molecular Sciences, and 2021.04926.BD, (https://doi.org/10.54499/2021.04926.BD) Ph.D. grant of H. M. L., and by the Instituto Politécnico de Lisboa through the IPL/IDI&CA2023/SMARTCAT_ISEL project. For the purpose of Open Access, the author applied a CC BY public copyright license to any Author Accepted Manuscript (AAM) version arising from this submission. L. B. thanks Prof. Ilaria Degano (University of Pisa) for ESI-MS analyses.

Notes and references

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68. (a) CCDC 2504593: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27bd; (b) CCDC 2504594: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27cf; (c) CCDC 2504595: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27dg; (d) CCDC 2504596: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27fh; (e) CCDC 2504597: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27gj; (f) CCDC 2504598: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27hk; (g) CCDC 2504599: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27jl; (h) CCDC 2504600: Experimental Crystal Structure Determination, 2025,  DOI:10.5517/ccdc.csd.cc2q27km.

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Footnotes

† Current address: Department of Energy Conversion and Storage, Technical University of Denmark, Fysikvej, 2800, Kgs. Lyngby, Denmark.

‡ p-cymene. ¹H NMR (CD₃CN): [small nu, Greek, tilde]/cm⁻¹ = 7.11 (app. q, ³J_HH = 8.3 Hz, 4H), 2.86 (hept, ³J_HH = 6.8 Hz), 2.43 (s, 3H), 1.20 (d, ³J_HH = 6.9 Hz, 6H).

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