On the chemistry of p-cymene ruthenium iodide complexes: entry into octahedral phenylated ruthenium(II) complexes supported by chelating bidentate N,N′-donor ligands

Abstract

This study investigates the synthesis and reactivity of (η⁶-p-cymene)ruthenium(II) iodide complexes supported by the phosphite ligand P(OCH₂)₃CEt, aiming to better understand the behavior of the Ru–I bond in the context of synthesizing ruthenium(II) complexes featuring bidentate nitrogen-donor ligands. The complex (η⁶-p-cymene)RuI₂(P{OCH₂}₃CEt) (2) was synthesized and phenylated to produce (η⁶-p-cymene)RuPh(I)(P{OCH₂}₃CEt) (6). Both compounds were subjected to halide abstraction reactions with silver tetrakis[3,5-((trifluoromethyl)phenyl)borate], affording their acetonitrile-coordinated, cationic species [(η⁶-p-cymene)RuI(NCMe)(P{OCH₂}₃CEt)][BArF′] (4) and [(η⁶-p-cymene)RuPh(NCMe)(P{OCH₂}₃CEt)][BArF′] (7) (BArF′ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, [B(C₆H₃-3,5-(CF₃)₂)₄]⁻), respectively. Complex 4 dimerizes when heated in the absence of acetonitrile to form [(η⁶-p-cymene)Ru(μ-I)(P{OCH₂}₃CEt)]₂[BArF′]₂ (5), while complex 7 activates chloroform to produce the isoelectronic chloro analogue of 5, [(η⁶-p-cymene)Ru(μ-Cl)(P{OCH₂}₃CEt)]₂[BArF′]₂ (8). Heating 6 in acetonitrile affords the tetra-acetonitrile complex [(NCMe)₄RuPh(P{OCH₂}₃CEt)][I] (9), whose iodide counterion can be exchanged with triflate or BArF′ anions to yield complexes 10 and 11, respectively. The tetra(acetonitrile)ruthenium complexes (9, 10, and 11) exhibit differentiated lability among its acetonitrile ligands, enabling selective substitution with bidentate N,N′-donor ligands to give cationic species of the type [(κ²-N,N-L)RuPh(P{OCH₂}₃CEt)(NCMe)₂]⁺, where L = bis-(2,6-diisopropylphenyl)ethane-1,2-diimine (DAB, 12) or 4,4′-tert-butyl-2,2′-bipyridine (t-bipy, 14). The DAB ligand of complex 12 is highly labile when heated in acetonitrile, while the t-bipy analogue maintains coordination to the metal center at elevated temperatures. Heating complex 14 in benzene under pressurized ethylene resulted in stoichiometric formation of styrene, most likely via olefin insertion followed by β-hydride elimination. Cyclic voltammetry revealed a Ru(III/II) redox potential of +0.53 V for 14, suggesting that the complex may be too electron-rich to serve as an efficient olefin hydroarylation catalyst. All complexes were characterized by multinuclear NMR spectroscopic methods (¹H, ¹³C, ³¹P, ¹⁹F), and several were structurally confirmed by single-crystal X-ray diffraction (2, 5, 6, 9, 11, and 12). The structure of 14 was assigned using advanced 2D NMR techniques (COSY, NOESY, HSQC).

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1. Introduction

Efforts to integrate metal-mediated C–H activation of arenes into catalytic functionalization schemes for the synthesis of alkylarenes and alkenylarenes have garnered significant interest. This approach presents a promising alternative to traditional methods, such as the Friedel–Crafts alkylation, for synthesizing alkylarenes, which often proceeds via electrophilic aromatic substitution (EAS), causing issues with poor regioselectivity, carbocation rearrangements, and limited functional group tolerance.^1–3 Although several powerful catalytic tools for synthesizing alkenylarenes via oxidative coupling/dehydrogenative^4–6 coupling have been developed, these approaches often require prior functionalization of the arene, the alkene, or both.^4–6 Over the past two decades, technologies leveraging metal-mediated C–H bond activation with subsequent functionalization have emerged as promising solutions to address the impediments associated with current methods for arene alkylations/alkenylations.⁷ While numerous examples of directed metal-mediated C–H activation of arenes are well-documented in the literature,^8–17 reports of activating non-activated C–H bonds are comparatively less common due to their inherent inertness.^18,19 The platinum-group metals have been well-studied in this vein of research, including palladium,^18,19 iridium,^20,21 rhodium,^22,23 and platinum.^22,23 This is consistent with their propensity for two-electron redox processes, rendering their reactivity highly predictable via well-defined oxidative addition and reductive elimination pathways. There are rare reports of ruthenium undergoing Ru(0)/(II) redox changes in Suzuki–Miyaura-type cross-coupling reactions;^24,25 however, these reactions do not follow C–H activation pathways and require pre-functionalized substrates. Ruthenium has redox flexibility, but one-electron changes (e.g., Ru(II)/Ru(III)) are more common,^26,27 making classical two-electron processes less straightforward than for Pd, Ir, Rh, or Pt. Nevertheless, several ruthenium complexes have been shown to be effective catalysts while maintaining a +2 oxidation state.^28–31 Although various competing pathways can complicate olefin hydroarylation and oxidative hydroarylation, several key mechanistic steps are central to the catalytic cycle (Fig. 1). The cycle is initiated by the formation of a catalytically active, 16-electron unsaturated Ru(II) species, typically generated through the dissociation of a labile ligand. Subsequent coordination of the olefin to the metal center facilitates migratory insertion into the Ru–Ph bond, yielding a 16-electron, unsaturated ruthenium(II) phenylethyl intermediate. This intermediate may follow one of two principal pathways: C–H activation of benzene occurs via an agostic interaction that proceeds through α-bond metathesis, culminating in alkylarene formation and regeneration of the catalytically active 16-electron Ru(II) complex, or β-hydride elimination, which affords an alkenylarene product and a Ru–H species.

Fig. 1 Mechanistic pathways in olefin hydroarylation and oxidative hydroarylation with ruthenium(II) catalysts.

The most successful ruthenium(II) olefin hydroarylation catalysts have been supported by tridentate, facially coordinating, poly(pyrazolyl) ligands;^32–36 however, successful platinum(II) olefin hydroarylation catalysts are often supported by bidentate ligands.^7,22,23,37 To the best of our knowledge, a ruthenium(II) catalyst for olefin hydroarylation supported by a single bidentate ligand and a phenyl substituent has not yet been systematically explored. The Ru–Ph bond introduces a reactive handle directly involved in catalysis, while the remaining coordination sites can be strategically occupied by ancillary and labile ligands. In such a system, the bidentate ligand would serve as a stable chelating anchor to support the metal center, preserving structural integrity while occupying only two coordination sites and leaving three positions available for further modification (see Fig. 2). The choice of the ancillary ligands can be used to modulate the electron density at the metal center, influencing catalytic activity and selectivity, whereas labile ligands—by virtue of their dynamic binding—facilitate substrate coordination and product release. Notably, the number and type of ancillary ligands can be adjusted, ranging from zero to two, offering an additional level of control over the geometry and electronic profile of the complex. This architectural flexibility enables precise tuning of catalyst reactivity and performance across a range of substrates and conditions. Altogether, this modular, tunable framework offers a versatile platform for the rational design and optimization of efficient and selective olefin hydroarylation catalysts.

Fig. 2 Generic ruthenium complex supported by a bidentate N,N′-donor ligand showing flexibility in number and type of ancillary ligand(s) (L_an) and labile ligand(s) (L_lab).

In pursuit of this catalyst design, we focused on a well-established ruthenium precursor—the dichloro η⁶-p-cymene ruthenium(II) dimer, [(η⁶-p-cymene)RuCl₂]₂—which offers both structural versatility and synthetic accessibility. This complex has long served as a reliable starting material in organometallic synthesis and has been widely utilized in the development of bioorganometallic anticancer agents.^38,39 While the dichloro dimer offers a convenient entry point into organometallic synthesis, modifications to the halide ligands can significantly influence the reactivity of the resulting complexes. Due to the weaker Ru–Br bond dissociation energy, which facilitates key steps such as transmetalation and nucleophilic substitution, the dibromo dimer [(η⁶-p-cymene)RuBr₂]₂ ⁴⁰ has emerged as the preferred precursor for the synthesis of ruthenium-based olefin hydroarylation catalysts.^28–31 Relative to its lighter congeners, the diiodide dimer, [(η⁶-p-cymene)RuI(μ-I)]₂, has received comparatively less attention.³⁹ Due to its larger size and lower electronegativity, iodide forms a weaker halogen–metal bond than its lighter congeners, and as a stable anion, it serves as a good leaving group—both properties making it useful in halide abstractions and transmetalation reactions.⁴¹ The highly polarizable nature of iodide can also stabilize softer metals due to soft–soft interactions.⁴¹ To further explore the reactivity of the Ru–I bond in (η⁶-p-cymene) ruthenium(II) complexes, a series of reactions was conducted, especially as an entry point into preparing a ruthenium(II) olefin hydroarylation catalyst supported by a single bidentate ligand. We report our initial efforts to investigate these systems using the strained phosphite ligand (P(OCH₂)₃CEt), which has proven effective in ruthenium(II)-catalyzed olefin hydroarylation reactions.

2. Experimental

2.1 General experimental

All commercially available reagents were used as received unless otherwise noted. All reactions were performed inside a nitrogen-filled glovebox (Vigor®) or on a Schlenk double manifold system. Sodium⁴² and silver⁴³tetrakis[((3,5-trifluoromethyl)phenyl)borate] (Na[BArF′] and Ag[BArF′], respectively) and were prepared by literature methods. [(η⁶-p-Cymene)RuI(μ-I)]₂ was prepared by a modified literature protocol.³⁹ All solvents were purified by standard methods or collected from an Inert Pure Solv® solvent purification system and stored over molecular sieves under a nitrogen atmosphere. Pressure reactions with ethylene were conducted in a Parr Series 5500 HP compact reactor equipped with 50 mL glass inserts and mechanical stirring. All ¹H, ¹³C{¹H}, ³¹P{¹H} and ¹⁹F{¹H}NMR spectra were recorded on a JEOL JNM-ECP300 FT 300 MHz NMR. All ¹H and ¹³C{¹H} spectra are referenced against residual proton signals (¹H NMR) or the ¹³C resonances of the deuterated solvent (¹³C{¹H}) NMR, and chemical shifts are recorded in ppm (δ). ³¹P{¹H} and ¹⁹F{¹H} NMR spectra were recorded with an internal standard of phosphoric acid and trifluoroacetic acid in D₂O, respectively. Coupling constants (J_HH values) are reported in Hz. A BAS Epsilon potentiostat was used to perform electrochemical experiments under a nitrogen atmosphere in acetonitrile (NCMe) using a standard three-electrode cell over a potential range of −1700 to +1700 mV at a variable scan rate from 50–150 mV s⁻¹. A glassy carbon working electrode was employed, with tetrabutylammonium hexafluorophosphate (n-Bu₄NPF₆) serving as the supporting electrolyte. All potentials are reported versus normal hydrogen electrode (NHE) using ferrocene or cobaltocene as the internal standard. Elemental analyses were performed at Atlantic Microlab, Inc., Norcross, GA (note: efforts to retrieve appropriate EA analysis on tetra-acetonitrile ruthenium complexes repeatedly failed). Single-crystal X-ray diffraction measurements were made on a Rigaku XtaLAB mini diffractometer or a Rigaku XtaLab Synergy-i diffractometer equipped with a Cu micro-focused source at 1.5418 A and a HyPix Bantam detector.

2.2 Syntheses

2.2.1. Synthesis of [(η⁶-p-cymene)RuI(μ-I)]₂ (1). This compound has been previously reported. To a chloroform solution (50 mL) of di-μ-chloro-bis(η⁶-p-cymene)chlororuthenium (5.00 g, 8.16 mmol), an aqueous solution (50 mL) of potassium iodide (13.55 g, 81.6 mmol) was added. The reaction was stirred at room temperature for 18 hours. The contents were transferred to a separatory funnel and extracted with chloroform three times (50 mL each). The extracts were combined and then dried over anhydrous magnesium sulfate. The solution was vacuum filtered through a column of Celite. The Celite was washed with additional chloroform (20 mL). The filtrate was collected, and the solution was reduced to dryness under a vacuum with heat to yield the product as a burgundy powder (6.96 g, 14.23 mmol, 87%). ¹H NMR (300 MHz, CDCl₃) δ: 1.25 (d, 6H, –CH(CH₃)₂, ³J_HH = 6 Hz), 2.33 (s, 3H, cymene-CH₃), 3.01 (sept., 1H, –CH(CH₃)₂, ³J_HH = 6 Hz), 5.44 (d, 2H, cymene Ar–H, ³J_HH = 6 Hz), 5.54 (d, 2H, cymene Ar–H, ³J_HH = 6 Hz).

2.2.2. Synthesis of (η⁶-p-cymene)RuI₂(P{OCH₂}₃CEt) (2). Compound 1 (9.62 g, 19.67 mmol) and P(OCH₂)₃CEt (3.98 g, 24.57 mmol) were added to a 250 mL round-bottom flask and dissolved in chloroform (75 mL). The reaction was stirred at room temperature for 18 hours. The solvent was removed under reduced pressure with heating. The residue was dissolved in a minimum amount of chloroform, and the solution was pipetted into hexanes (175 mL) to induce precipitation. The precipitate was collected on a fine-porosity frit by vacuum filtration (12.73 g, 19.581 mmol, 99%). Alternative synthesis: compound 3 (η⁶-p-cymene)RuCl₂(POCH₂)₃CEt (0.875 g, 1.878 mmol) was dissolved in chloroform (30 mL) and added to a 250 mL round-bottom flask. Potassium iodide (9.357 g, 56.37 mmol) was dissolved in water (50 mL) and transferred to a round-bottom flask containing compound 3. The reaction was refluxed at 70 °C with stirring for 7 days. The reaction was monitored by taking ¹H NMR spectra of aliquots of the reaction solution. The workup was performed as previously described in the preferred method (0.482 g, 0.8638 mmol, 40%). ¹H NMR (300 MHz, CDCl₃) δ: 0.82 (t, 3H, –CH₂CH₃, ³J_HH = 6 Hz), 1.26 (m, 8H, overlapping –CH₂CH₃, –CH(CH₃)₂), 2.45 (s, 3H, –CH₂CH₃), 3.19 (sept., 1H, –CH(CH₃)₂, ³J_HH = 6 Hz), 4.33 (d, 6H, (–OCH₂)₃CEt, ³J_HP = 6 Hz), 5.48 (d, 2H, cymene Ar–H, ³J_HH = 6 Hz), 5.64 (d, 2H, cymene Ar–H, ³J_HH = 6.0 Hz); ¹³C NMR (75.57 MHz, CDCl₃) δ: 7.34 (–CH₂CH₃), 20.58 (cymene-CH₃), 22.71 (–CH₂CH₃), 23.50 (–CH(CH₃)₂), 31.74 (–CH(CH₃)₂), 35.83 (d, –(OCH₂)₃CEt, ³J_CP = 33 Hz), 75.85 (d, –(OCH₂)₃CEt, ²J_CP = 8 Hz), 89.5 (d, cymene Ar–C, ²J_HP = 8 Hz), 89.92 (d, cymene Ar–C, ²J_HP = 8 Hz), 104.95 (d, ipso-cymene, ²J_HP = 4 Hz), 113.23 (ipso-cymene). ³¹P NMR (121.65 MHz, CDCl₃) δ: 114.50. EA: for C₁₅H₂₂I₂O₃PRu (636.19 g mol⁻¹) theo% (found%), C, 29.51(29.67); H, 3.87(3.71).

2.2.3. Synthesis of (η⁶-p-cymene)RuCl₂(P{OCH₂}₃CEt) (3). Compound 3 has been previously reported;⁴⁴ however, a more thorough characterization is provided. [(η⁶-p-Cymene)RuCl(μ-Cl)]₂ (0.500 g, 0.816 mmol) was dissolved in dichloromethane (25 mL), and P(OCH₂)₃CEt (0.373 g, 2.3 mmol) was added. The reaction was stirred at room temperature for 30 minutes. Hexane (50 mL) was added to the reaction to induce precipitation. The slurry was filtered on a medium frit, and the orange solution was collected and dried under reduced pressure to yield an orange powder (0.646 g, 1.379 mmol, 84%). ¹H NMR (300 MHz, CDCl₃) δ: 0.85 (t, 3H, –CH₂CH₃, J_HH = 6 Hz), 1.20–1.27 (m, 8H, overlapping –CH₂CH₃ and –CH(CH₃)₂), 2.16 (s, 3H, cymene-CH₃), 2.88 (sept., 1H, –CH(CH₃)₂, ³J_HH = 7 Hz), 4.36 (d, 6H, –(OCH₂)₃CEt, ³J_HP = 5 Hz), 5.49 (d, 2H, cymene Ar–H, ³J_HH = 6 Hz), 5.62 (d, 2H, cymene Ar–H, ³J_HH = 6 Hz). ¹³C NMR (75.57 MHz, CDCl₃) δ: 7.18 (s, –CH₂CH₃), 18.44 (CH₂CH₃), 21.97 (cymene-CH₃), 23.36 (–CH(CH₃)₂), 30.38 (–CH(CH₃)₂), 35.56 (d, –(OCH₂)₃CEt, ³J_CP = 32 Hz), 75.45 (d, –(OCH₂)₃CEt, ²J_CP = 8 Hz), 89.1 (cymene Ar–C), 89.9 (cymene Ar–C), 103.1 (ipso-cymene), 108.9 (s, ipso-cymene). ³¹P NMR (121.66 MHz, CDCl₃) δ: 111.34 (s). EA: for C₁₅H₂₂I₂O₃PRu (636.19 g mol⁻¹) theo% (found%), C, 29.51(29.67); H, 3.87(3.71).

2.2.4. Synthesis of [(η⁶-p-cymene)RuI(NCMe)(P{OCH₂}₃CEt)][BArF′] (4). Compound 2 (50 mg, 0.0769 mmol) was dissolved in acetonitrile (3 mL) and added to a vial equipped with a stir bar. Ag[BArF′] (156 mg, 0.160 mmol) was added to the solution and stirred for 18 hours. The solution was filtered through Celite, and the filtrate was dried under a vacuum to afford a yellow solid (0.103 g, 0.0721 mmol, 94%). ¹H NMR (300 MHz, CDCl₃) δ: 0.74 (t, 3H, –CH₂CH₃, ³J_HH = 8 Hz), 1.21–1.28 (m, 8H, overlapping –CH(CH₃)₂ and –CH₂CH₃), 2.19 (s, 3H, cymene-CH₃), 2.32 (s, 3H, NCCH₃), 2.75 (sept., 1H, –CH(CH₃)₂, ³J_HH = 8 Hz), 4.34 (d, 6H, –(OCH₂)₃CEt, ³J_HH = 6 Hz), 5.62 (d, 1H, cymene Ar–H, ³J_HH = 6 Hz), 5.75–5.82 (m, 2H, cymene Ar–H), 5.89 (d, 1H, cymene Ar–H, ³J_HH = 6 Hz), 7.55 (s, 4H, p-BArF′), 7.71 (s, 8H, o-BArF′). ¹³C NMR (75.57 MHz, CDCl₃) δ: 3.80 (NCCH₃), 6.78 (–CH₂CH₃), 19.39 (cymene-CH₃), 21.69 and 22.19 (–CH(CH₃)₂), 23.00 (–CH₂CH₃), 31.78 (–CH(CH₃)₂), 36.23 (d, –(OCH₂)₃CEt, ³J_CP = 34 Hz), 76.49 (d, –(OCH₂)₃CEt, ²J_CP = 8 Hz), 90.80 (d, cymene Ar–C, ²J_CP = 6 Hz), 91.21 (d, cymene Ar–C, ²J_CP = 6 Hz), 92.06 (d, cymene Ar–C, ²J_CP = 5 Hz), 92.93 (d, cymene Ar–C, ²J_CP = 5 Hz), 106.08 (d, ipso-cymene, ²J_CP = 2 Hz), 114.78 (d, ipso-cymene, ²J_CP = 3 Hz), 117.61 (bs, p-BArF′), 124.62 (q, –CF₃, ¹J_CF = 271 Hz), 126.81 (CH₃CN), 128.98 (q, m-BArF′, ²J_CF = 32 Hz), 134.87 (o-BArF′), 162.08 (four line pattern, B–C BArF′, ¹J_CB = 50 Hz). ³¹P NMR (121.65 MHz, CDCl₃) δ: 117.36. ¹⁹F NMR (282.78 MHz, NCMe-d₃) δ: −63.14.

2.2.5 Synthesis of [(η⁶-p-cymene)Ru(μ-I)(P{OCH₂}₃CEt)]₂[BArF′]₂ (5). Compound 2 (50 mg, 0.077 mmol) was dissolved in methylene chloride (3 mL) and added to a vial equipped with a stir bar. Ag[BArF′] (82 mg, 0.084 mmol) was added to the solution and stirred for 18 hours. The solution was filtered through Celite and dried under vacuum to yield a dark purple powder (0.089 g, 0.056 mmol, 84%). ¹H NMR (300 MHz, NCMe-d₃) δ: 0.9 (t, 3H, –CH₂CH₃, ³J_HH = 6.0 Hz), 1.08 (d, 6H, –CH(CH₃)₂, ³J_HH = 6 Hz), 1.42 (q, 2H, –CH₂CH₃, ³J_HH = 6 Hz), 2.23 (s, 3H, cymene-CH₃), 2.68 (sept, 1H, –CH(CH₃)₂, ³J_HH = 6 Hz), 4.67 (d, 6H, –(OCH₂)₃CEt, ³J_HP = 5 Hz), 5.87 (m, 4H, cymene Ar–H), 7.64–7.72 (m, 12H, BArF′). ³¹C NMR (75.57 MHz, NCMe-d₃) δ: 7.401 (s, –CH₂CH₃), 19.71 (s, cymene-CH₃), 22.26 (s, –CH(CH₃)₂), 23.63 (s, –CH₂CH₃), 33.00 (s, –CH(CH₃)₂), 37.29 (d, –(OCH₂)₃CEt, ³J_CP = 33 Hz), 77.91 (d, –(OCH₂)₃CEt, ³J_CP = 33 Hz), 91.49 (m, cymene Ar–C), 92.95 (m, cymene Ar–C), 107.23 (ipso-cymene), 112.53 (s, ipso-cymene), 117.75 (s, p-BArF′), 125.43 (q, –CF₃, ¹J_CF = 271 Hz), 129.83 (q, m-BArF′, ¹J_CF = 31 Hz), 135.62 (s, o-BArF′), 162.49 (four line pattern, B–C BArF′, ¹J_CB = 50 Hz). ³¹P NMR (121.65 MHz, NCMe-d₃) δ: 119.71. ¹⁹F NMR (282.78 MHz, NCMe-d₃) δ: −63.25. EA: for C₄₈H₃₇BF₂₄IO₃PRu (1387.54 g mol⁻¹, monomer), theo. (found): C, 41.55(41.53); H, 2.69(2.73).

2.2.6. Synthesis of (η⁶-p-cymene)RuI(Ph)(P{OCH₂}₃CEt) (6). Compound 2 (0.50 g, 0.767 mmol) was dissolved in methylene chloride (10 mL) and added to a 15 mL round-bottom flask equipped with a stir bar. Phenyl magnesium bromide (1.00 mL, 3.00 mmol, 3 M in ether) was added to the solution. The reaction was stirred at room temperature overnight. The solution was filtered, the filtrate collected, and all solvent was removed under rotary evaporation. Diethyl ether was added to the residue, causing the remaining magnesium salts to precipitate. The solution was filtered over Celite on a frit. The filtrate was collected and reduced to dryness to afford a red-orange colored powder (0.456 g, 0.759 mmol, 99%). ¹H NMR (300 MHz, CDCl₃) δ: 0.81 (t, 3H, –CH₂CH₃, ³J_HH = 9 Hz), 1.13–1.23 (m, 8H, overlapping –CH₂CH₃ and –CH(CH₃)₂), 1.81 (s, 3H, –CH₃), 2.74 (sept., 1H, –CH(CH₃)₂, ³J_HH = 6 Hz), 4.22 (d, 6H, –(OCH₂)₃CEt, ³J_HP = 6 Hz), 5.04 (d, 1H, cymene Ar–H, ³J_HH = 6 Hz), 5.43 (d, 1H, cymene Ar–H, ³J_HH = 6 Hz), 5.55 (m, 2H, overlapping cymene Ar–H), 6.79–6.91 (m, 3H, overlapping m/p-Ph), 7.73 (d, 2H, o-Ph, ³J_HH = 9 Hz). ¹³C NMR (75.57 MHz, CDCl₃) δ: 7.32 (–CH₂CH₃), 18.76 (cymene-CH₃), 22.29 (–CH₂CH₃), 23.46 (–CH(CH₃)₂), 31.26 (–CH(CH₃)₂), 35.31 (d, –(OCH₂)₃CEt, ³J_CP = 32 Hz), 75 (d, –(OCH₂)₃CEt, ³J_CP = 7 Hz), 88.74 (d, cymene Ar–C, ³J_CP = 2 Hz), 90.80 (d, cymene Ar–C, ²J_CP = 3 Hz), 93.8 (d, cymene Ar–C, ²J_CP = 4 Hz), 110.87 (d, ipso-cymene, ²J_CP = 5 Hz), 116.68 (d, ipso-cymene, ²J_CP = 4 Hz), 121.84 (p-Ph), 126.6 (m-Ph), 145.14 (d, o-Ph, ³J_CP = 5 Hz), 153.49 (d, ipso-Ph, ²J_CP = 29 Hz). ³¹P NMR (122 MHz, CDCl₃) δ: 126.05. EA: for C₂₂H₃₀IO₃PRu (601.43 g mol⁻¹): theo% (found%), C, 43.86(43.99); H, 5.19(5.11).

2.2.7. Synthesis of [(η⁶-p-cymene)Ru(Ph)(NCMe)(P{OCH₂}₃CEt)][BArF′] (7). Ag[BArF′] (0.444 g, 0.458 mmol) was added to a solution of 6 (0.200 g, 0.458 mmol) dissolved in methylene chloride (15 mL). Acetonitrile (25 μL, 0.458 mmol) was added to the solution via syringe. The reaction flask was allowed to stir overnight. The solution became pale yellow with a grey precipitate (AgI). The reaction mixture was then filtered through Celite to yield a yellow solution. All solvent was removed under reduced pressure to yield a glassy, pale yellow solid (0.362 g, 0.263 mmol, 79%). ¹H NMR (300 MHz, CDCl₃) δ: 0.73 (t, 3H, –CH₂CH₃, ³J_HH = 6 Hz), 1.07–1.17 (m, 8H, coincidental overlap, –CH₂CH₃, –CH(CH₃)₂), 1.72 (s, 3H, cymene-CH₃), 2.27 (s, 3H, NCCH₃), 2.57 (sept., 1H, –CH(CH₃)₂, ³J_HH = 6 Hz) 4.22 (d, 6H, –(OCH₂)₃CEt, ³J_HH = 5 Hz), 5.11, 5.51, 5.66, 5.70 (all d, 4H, cymene Ar–H, ³J_HH = 6 Hz), 6.86–7.07 (m, 3H, overlapping m/p-Ph), 7.28 (d, 2H, o-Ph, ³J_HH = 7 Hz), 7.66 (s, 12H, BArF′). ¹³C NMR (300 MHz, CDCl₃) δ: 3.99 (CH₃CN), 6.98 (–CH₂CH₃), 18.43 (cymene-CH₃), 21.98 (–CH₂CH₃), 23.19 (–CH(CH₃)₂), 31.433 (cymene-CH(CH₃)₂), 35.90 (d, ³J_CP = 33 Hz, –(OCH₂)₃CEt), 75.53 (d, (OCH₂)₃CEt, ²J_CP = 8 Hz), 89.08 (cymene Ar–C), 92.03 (d, cymene Ar–C, ²J_CP = 3 Hz), 93.38 (d, cymene Ar–C, ²J_CP = 9 Hz), 94.63 (d, cymene Ar–C, ²J_CP = 3 Hz), 114.65 (ipso-cymene), 117.58 (m, p-BArF′), 119.28 (CH₃CN), 121.73 (d, ipso-cymene, ³J_CP = 3 Hz), 123.89 (p-Ph), 124.68 (q, –CF₃, ¹J_CF = 272 Hz), 128.05 (m-phenyl), 128.95 (q, m-BArF′, ²J_CF = 33 Hz), 134.93 (s, o-BArF′), 140.35 (d, o-Ph, ³J_CP = 4 Hz), 148.27 (d, ipso-Ph, ³J_CP = 24 Hz), 161.77 (four line pattern, B–C BArF′, ¹J_CB = 50 Hz). ³¹P NMR (121 MHz, CDCl₃)δ: 126.12. ¹⁹F NMR (282.78 MHz, CDCl₃) δ: −62.22 (s). EA: for C₂₂H₃₁O₃PIRu (1378.80 g mol⁻¹) theo. (found): C 43.86(43.99), H 5.19(5.11).

2.2.8. Synthesis of [(NCMe)₄RuPh(P{OCH₂}₃CEt)][I] (9). Compound 6 (0.456 g, 0.759 mmol) was dissolved in acetonitrile (5 mL) and added to a pressure tube equipped with a stir bar. The pressure tube was capped and stirred overnight in a 70 °C oil bath. The reaction was cooled to room temperature and brought into the glove box. All the reaction solvent was removed under reduced pressure and then reconstituted in a minimum amount of acetonitrile (∼5 mL). The product was triturated into stirring diethyl ether (50 mL) with stirring. The off-white powder was collected by vacuum filtration and dried under reduced pressure (0.460 g, 0.879 mmol, 90%). ¹H NMR (300 MHz, NCMe-d₃) δ: 0.78 (t, 3H, –CH₂CH₃, ³J_HH = 8 Hz), 1.21 (q, 2H, –CH₂CH₃, ³J_HH = 8 Hz), 2.17 (s, 6H, NCCH₃), 2.31 (s, 6H, NCCH₃), 4.20 (d, 6H, –(OCH₂)₃CEt, ³J_HP = 5 Hz), 6.75 (t, 1H, p-Ph, ³J_HH = 7 Hz), 6.85 (t, 2H, m-Ph, ³J_HH = 7 Hz), 7.50 (d, 1H, o-Ph, ³J_HH = 7 Hz). ¹³C NMR (75.57 MHz, NCMe-d₃) δ: 4.45 (NCCH₃), 7.32 (–CH₂CH₃), 22.80 (–CH₂CH₃), 36.02 (d, –(OCH₂)₃CEt, ³J_CP = 32 Hz), 74.90 (d, –(OCH₂)₃CEt, ²J_CP = 7 Hz), 121.49 (p-Ph), 122.06 (NCCH₃), 126.29 (m-Ph), 141.65 (o-Ph), 159.56 (d, ipso-Ph, ²J_CP = 18 Hz). ³¹P NMR (122 MHz, CD₃CN): 132.23.

2.2.9 Synthesis of [Ru(NCMe)₄Ph(P{OCH₂}₃CEt)][OTF] (10). Compound 9 (102 mg, 0.162 mmol) and silver triflate (40 mg, 0.154 mmol) were added to a vial equipped with a stir bar and dissolved in acetonitrile. The vial was capped tightly and stirred overnight. All the reaction solvent was removed by rotary evaporation to dryness. The residue was reconstituted in dichloromethane and filtered over a fine frit. The solution was collected and reduced to dryness under vacuum to yield a white powder (84 mg, 84%). ¹H NMR (300 MHz, NCMe-d₃) δ: 0.78 (t, 3H, –CH₂CH₃, ³J_HH = 8 Hz), 1.21 (q, 2H, –CH₂CH₃, ³J_HH = 8 Hz), 2.17 (s, 6H, NCCH₃), 2.31 (s, 6H, NCCH₃), 4.20 (d, 6H, –(OCH₂)₃CEt, ³J_CP = 5 Hz), 6.75 (t, 1H, p-Ph, ³J_HH = 7 Hz), 6.85 (t, 2H, m-Ph, ³J_HH = 7 Hz), 7.72 (d, 2H, o-Ph, ³J_HH = 7 Hz). ¹³C NMR (75.57 MHz, NCMe-d₃) δ: 4.25 (NCCH₃), 7.32 (–CH₂CH₃), 23.65 (–CH₂CH₃), 36.03 (d, –(OCH₂)₃CEt, ³J_CP = 32 Hz), 74.90 (d, –(OCH₂)₃CEt, ³J_CP = 7 Hz), 121.49 (p-Ph), 122.91 (q, triflate SO₄CF₃, ³J_CP = 323 Hz), 123.01 (NCCH₃), 126.26 (m-Ph), 141.65 (o-Ph), 160.53 (d, ipso-Ph, ²J_CP = 18 Hz). ³¹P NMR (122 MHz, NCMe-d₃) δ: 132.24 (s) ¹⁹F NMR (282.78 MHz, NCMe-d₃) δ: −79.06 (s).

2.2.10 Synthesis of [Ru(NCMe)₄Ph(P{OCH₂}₃CEt)][BArF′] (11). Compound 9 (200 mg, 0.317 mmol) was dissolved in acetonitrile (4 mL) in a vial equipped with a stir bar. Na[BArF′] (298 mg, 0.337 mmol) was dissolved in acetonitrile 4 mL and transferred to the solution of 9. The vial was capped and stirred for 2 hours. All solvent was removed in vacuo with heat. The residue was taken up in dichloromethane to produce a slurry, which was filtered through a pipette of Celite and collected in a tared vial. All solvent was removed under a vacuum to yield a frothy off-white solid (412 mg, 0.301 mmol, 95%). ¹H NMR (300 MHz, CDCl₃) δ: 0.79 (t, 3H, –CH₂CH₃, ³J_HH = 8 Hz), 1.18 (t, 2H, –CH₂CH₃, ³J_HH = 8 Hz), 2.10 (s, 3H, NCCH₃), 2.16 (s, 3H, NCCH₃), 2.26 (s, 6H, NCCH₃), 4.18 (d, 6H, –(OCH₂)₃CEt, ³J_HP = 5 Hz), 6.92 (t, 1H, p-Ph, ³J_HH = 8 Hz), 7.02 (t, 2H, m-Ph, ³J_HH = 8 Hz), 7.40 (d, 2H, o-Ph, ³J_HH = 8 Hz), 7.55 (s, 4H, p-BArF′), 7.73 (s, 8H, o-BArF′). ¹³C NMR: (75.57 MHz, CDCl₃) δ: 3.06 (CH₃CN). 3.30 (CH₃CN), 4.23 (CH₃CN), 7.05 (–CH₂CH₃), 23.43 (s, –CH₂CH₃), 35.42 (d, –(OCH₂)₃CEt, ³J_CP = 32 Hz), 74.4 (d, –(OCH₂)₃CEt, ²J_CP = 7 Hz), 117.6 (bs, p-BArF′), 121.06 (CH₃CN), 121.78 (p-Ph), 124.67 (q, CF₃-BArF′, ¹J_CF = 272 Hz), 126.32 (s, m-Ph), 129.05 (q, m-BArF′, ²J_CF = 31 Hz), 134.82 (s, o-BArF′), 140.12 (s, o-Ph), 157.63 (d, ipso-Ph, ²J_CP = 18 Hz), 162.33 (four line pattern, B–C BArF′, ¹J_CB = 50 Hz). ³¹P NMR (122 MHz, CDCl₃), δ: 131.89. ¹⁹F NMR: (282.78 MHz, CDCl₃) δ: −62.28 (s).

2.2.11. Synthesis of [(DAB)Ru(NCMe)₂(Ph)(P{OCH₂}₃CEt)][BArF′] (DAB = bis(2,6-diisopropylphenyl)ethane-1,2-diimine) (12). Compound 11 (130 mg, 0.095 mmol) was added to a vial, equipped with a stir bar, and dissolved in dichloromethane. Bis-(2,6-diisopropylphenyl)ethane-1,2-diimine (DAB) (39 mg, 0.105 mmol) was placed in a separate vial and dissolved in dichloromethane, then transferred to the solution of 11. The reaction was stirred for 1 hour. All solvent was removed by vacuum to yield a dark green material (149 mg, 94%). The complex was crystallized from dichloromethane by slow vapor diffusion of pentane at −27 °C. ¹H NMR (300 MHz, NCMe-d₃) δ: 0.65 (t, 3H, –CH₂CH₃, ³J_HH = 7 Hz), 0.83 (d, 6H, –CH(CH₃)₂, ³J_HH = 7 Hz), 0.98–1.09 (m, 8H, overlapping –CH(CH₃)₂ and –CH₂CH₃), 1.15 (d, 6H, –CH(CH₃)₂, ³J_HH = 7 Hz), 1.33 (d, 6H, –CH(CH₃)₂, ³J_HH = 7 Hz), 2.29 (s, 6H, NCCH₃), 2.62 (sept, 1H, –CH(CH₃)₂, ³J_HH = 7 Hz), 2.96 (sept. 6 H, –CH(CH₃)₂, ³J_HH = 7 Hz), 3.87 (d, 6H, –(OCH₂)₃CEt, ²J_CP = 6 Hz), 6.52–6.82 (m, 3H, m- and p-Ph), 6.90–6.93 (m, 2H, o-Ph), 7.15 (d, 2H, DAB-Ar, ³J_HH = 8 Hz), 7.27 (d, 1H, DAB-Ar, ³J_HH = 8 Hz), 7.31 (bs, 3H, DAB-Ar), 7.67–7.70 (m, 12H, BArF′), 8.15–8.55 (m, 2H, HC=N). ¹³C NMR (75.57 MHz, CDCl₃) δ: 4.28 (CH₃CN), 6.97 (–CH₂CH₃), 22.32 and 22.59 (–CH(CH₃)₂₎, 23.17 (–CH₂CH₃), 25.74 and 25.83 (–CH(CH₃)₂), 28.31 and 28.58 (–CH(CH₃)₂), 35.42 (d, –(OCH₂)₃CEt, ³J_CP = 31 Hz), 74.23 (d, (OCH₂)₃CEt, ²J_CP = 7 Hz), 117.58 (bs, p-BArF′), 121.03 (p-Ph), 121.03 and 121.92 (DAB-ArCCH(CH₃)₂), 123.53 and 123.77 (m-DAB), 124.69 (q, CF₃-BArF′, ¹J_CF = 272 Hz), 126.02 (m-Ph), 127.76 (NCCH₃), 128.57 (p-DAB), 128.99 (q, C-CF₃, ²J_CF = 31 Hz), 134.92 (s, o-BArF′), 138.75 (DAB-CCH(CH₃)₂), 139.06 (o-Ph), 139.98 (DAB-CCH(CH₃)₂), 145.84 (DAB-Ar–C–C=N), 149.49 (DAB Ar–C–C=N), 158.46 (d, ipso-Ph, ²J_CP = 18 Hz), 161.81 (four line pattern, B–C BArF′, ¹J_CB = 50 Hz) 163.5 and 165.57 (C=N). ¹⁹F NMR (282.78 MHz, NCMe-d₃) δ: −63.1729 (s). ³¹P NMR (121.65 MHz, NCMe-d₃) δ: 129.8042 (s). EA: for C₇₄H₇₀BF₂₄N₄O₃PRu (1662.21 g mol⁻¹), theo. (found): C, 53.47(53.28); H, 4.24(4.18); N, 3.37(3.37).

2.2.12 Synthesis of [(t-bipy)Ru(NCMe)₂(Ph)(P{OCH₂}₃CEt)][I] (13). A dichloromethane solution (10 mL) of t-bipy (233 mg, 0.867 mmol, 1.1 equiv.) was added to a dichloromethane solution (25 mL) of compound 11 (500 mg, 0.791 mmol). The solutions were mixed and transferred to a round-bottom flask. The reaction was stirred overnight at room temperature. All solvent was removed under reduced pressure to yield an orange residue. The material was dissolved in a minimum of dichloromethane and triturated into pentane. The solution was removed under vacuum to produce an orange, glassy material (505 mg, 78%). ¹H NMR (300 MHz, NCMe-d₃) δ: 0.82 (t, 3H, –CH₂CH₃, ³J_HH = 8 Hz), 1.25 (2H, q, –CH₂CH₃, ³J_HH = 8 Hz), 1.41 and 1.47 (s, each 9H, –C(CH₃)₃), 2.03 (6H, s, NCCH₃), 4.26 (d, 6H, –(OCH₂)₃CEt, ³J_HP = 5 Hz), 6.88 (dt, 1H, p-Ph, ³J_HH = 7 Hz, ⁴J_HH < 1 Hz), 6.99 (dt, 2H, m-Ph, ³J_HH = 7 Hz, ⁴J_HH < 1 Hz), 7.46 (ddd, 1H, 5-bipy-trans-PR₃, ³J_HH = 6 Hz, ⁴J_HP = 2 Hz, ⁴J_HH = 1 Hz), 7.55 (dd, 2H, o-Ph, ³J_HH = 8 Hz, ⁴J_HH < 1 Hz), 7.63 (dd, 1H, 5-bipy-trans-Ph, ³J_HH = 6 Hz, ⁴J_HP = 2 Hz), 8.31 (dd, 1H, 6-bipy-trans-PR₃, ³J_HH = 6 Hz, ⁴J_HP = 4 Hz), 8.36 (bs, 2H, 3/3′-bipy), 9.53 (d, 1H, 6-bipy-trans-Ph, ³J_HH = 6 Hz). ¹³C NMR (75.57 MHz, NCMe-d₃) δ: 4.06 (s, CH₃CN), 7.36 (s, –CH₂CH₃), 23.82 (s, –CH₂CH₃), 30.44 (s, C(CH₃)₃), 30.51 (s, –C(CH₃)₃), 35.78 (s, –C(CH₃)₃), 36.09 (d, –(OCH₂)₃CEt, ³J_CP = 8 Hz), 36.19 (s, C(CH₃)₃), 74.65 (d, –(OCH₂)₃CEt, ³J_CP = 7 Hz), 120.61 (d, ³J_HH = 3 Hz), 120.72 (bipy), 121.58 (bipy), 122.42 (bipy), 123.73 (d, bipy, ³J_CP = 4 Hz), 124.08 (CH₃CN), 126.66 (m-Ph), 141.31 (s, o-Ph), 151.73 (bipy), 155.33 (s, p-Ph), 156.72 (d, 6-bipy, ³J_CP = 2 Hz), 157.59 (d, 6-bipy ³J_CP = ∼1 Hz), 162.86 (bipy), 163.01 (bipy), 163.99 (bipy), 166.30 (d, ipso-Ph,³J_HH = 22 Hz). ³¹P NMR (121.65 MHz, NCMe-d₃) δ: 131.57. ¹⁹F NMR (282.78 MHz) δ: −62.23 (s). EA: for C₃₄H₄₆IN₄O₃PRu (817.72) theo. (found): C 49.94(48.41), H 5.67(5.85), N 6.85(6.41).

2.2.13 Synthesis of [(t-bipy)Ru(NCMe)₂(Ph)(P{OCH₂}₃CEt)][BArF′] (14). Compound 13 (300 mg, 0.3732 mmol) was dissolved in dichloromethane in a round-bottom flask. Na[BArF′] (1.1 equivalence; 0.363 g, 0.4096 mmol) was added. The reaction was stirred overnight. The solution was vacuum filtered over a fine frit covered with Celite. The filtrate was collected and dried in vacuo to yield a dark orange solid (0.495 g, 77%). Alternate procedure: compound 11 (250 mg, 0.181 mmol) was placed in a vial equipped with a stir bar and dissolved in chloroform, and t-bipy (54 mg, 0.192 mmol) was added. The reaction was stirred for one hour. All solvent was removed to give an orange solid. The solid was dissolved in dichloromethane and passed through a column of silica prewashed with hexane. The column was washed with hexane followed by diethyl ether to remove excess t-bipy. The orange ban was collected and dried under vacuum (261 mg, 0.166 mmol, 92%). ¹H NMR (300 MHz, CDCl₃)δ: 0.81 (3H, t, –CH₂CH₃, ³J_HH = 8 Hz), 1.22 (q, 2H, CH₂CH₃, ³J_HH = 8 Hz), 1.41 and1.46 (s, each 9H, –C(CH₃)₃), 2.15 (s, 6H, (NCCH₃)₂), 425 (d, 6H, –(OCH₂)₃CEt, ³J_HH = 5 Hz), 6.99 (t, 2H, m-Ph, ³J_HH = 7 Hz), 7.46 (d, 1H, bipy overlapping with o-Ph, ³J_HH = 7 Hz), 7.56 (d, 1H, o-Ph, ³J_HH = 6 Hz), 7.61 (d, 1H, 5′-bipy, ³J_HH = 6 Hz), 8.32 (t, 1H, p-Ph, ³J_HH = 7 Hz), 8.36 (s, 2H, 3/3′-bipy), 9.53 (2H, d, 6′-bipy, ³J_HH = 6 Hz). ¹³C NMR (75.57 MHz, NCMe-d₃)δ: 4.06 (CH₃CN), 7.36 (–CH₂CH₃), 23.82 (–CH₂CH₃), 30.44 (–C(CH₃)₃), 30.51 (–C(CH₃)₃), 35.78 (–C(CH₃)₃), 36.09 (d, –(OCH₂)₃CEt, ³J_CP = 8 Hz), 36.19 (–C(CH₃)₃), 74.65 (d, –(OCH₂)₃CEt, ³J_CP = 7 Hz), 120.61 (d, ³J_HH = 3 Hz), 120.72 (bipy), 121.58 (bipy), 122.42 (bipy), 123.73 (d, bipy, ³J_CP = 4 Hz), 124.08 (bipy), 124.52 (q, CF₃-BArF′, ¹J_CF = 271 Hz), 126.66 (m-Ph), 129.00 (q, –CCF₃, ²J_CF = 31 Hz), 134.71 (s, o-BArF′), 141.31 (s, o-Ph), 151.73 (bipy), 155.33 (s, p-Ph), 156.72 (d, 6-bipy, ³J_CP = 2 Hz), 157.59 (d, 6-bipy, ³J_CP = ∼1 Hz), 161.99 (four line pattern, B–C BArF′, ¹J_CB = 50 Hz), 162.86, 163.01, 163.99, 166.30 (d, i-Ph, ³J_HH = 22 Hz) (note: para-BArF′ obscured by acetonitrile-d₃). ³¹P NMR (131 MHz, CDCl₃) δ: 131.81. ¹⁹F NMR (282.78 MHz, CDCl₃) δ: −62.2. EA: for C₆₆H₅₈BF₂₄N₄O₃PRu (1584.10) theo. (found): C 51.01(49.70), H 3.76(3.67), N 3.81(3.61).

2.3 Olefin hydroarylation catalytic studies

In general, complex 14 (5 mg) was dissolved in benzene (10 mL) and added to a pressure reactor glass insert. The insert was added to a high-pressure reaction vessel, and the vessel was fastened. The pressure reactor was connected to a double manifold gas system equipped with ethylene and nitrogen. All lines were purged and vacuumed with nitrogen. The reactor was then purged with ethylene for 30 seconds at the desired pressure, and then the system was closed. The reaction mixture was stirred using an overhead stirrer and heated at the designated temperature for the specified duration. Upon completion of the reaction, a 100 μL aliquot of the reaction solution was withdrawn, diluted to 0.5 mL with dichloromethane, and transferred to a GC-MS vial for analysis. Product identification was performed via mass spectral matching against a reference library. Quantification of styrene and ethylbenzene was carried out using a standard calibration curve generated from authentic samples.

3. Results and discussion

3.1. Preparation of (η⁶-p-cymene)RuI₂(P{OCH₂}₃CEt) (2) and its halide abstraction chemistry

(η⁶-p-Cymene)RuI₂(P{OCH₂}₃CEt) (2) can be prepared in near quantitative yield (99%) (Scheme 1; reaction (a)) by reaction of the iodide cymene ruthenium dimer, [(η⁶-p-cymene)RuI₂]₂ (1),³⁹ with P(OCH₂)₃CEt. The bromo^29,30 and chloro⁴⁴ analogs of 2 have been prepared similarly. Alternatively, compound 2 can also be prepared from its dichloro congener, (η⁶-p-cymene)RuCl₂(P{OCH₂}₃CEt) (3),⁴⁵via halide exchange with potassium iodide (30 equiv.) in a biphasic water/chloroform system at 70 °C (Scheme 1; reaction (b)). However, this route is less favorable due to prolonged reaction times (3–7 days) and lower yields (40%). To evaluate the lability of the iodide ligand, compound 2 was subjected to halide abstraction conditions by treatment with excess silver tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (Ag[BArF′]) in acetonitrile, affording the cationic species [(η⁶-p-cymene)RuI(P{OCH₂}₃CEt)(NCMe)][BArF′] (4) in good yield (86%) (Scheme 1; reaction (c)). Despite the use of excess (2.1 equivalents) halide abstraction reagent, no evidence was found for the formation of the dicationic, bis-acetonitrile, ruthenium(II) complex [(η⁶-p-cymene)Ru(P{OCH₂}₃CEt)(NCMe)₂][BArF′]_2. The ¹H NMR spectrum of complex 4 provides clear evidence of its formation, featuring a diagnostic singlet at 2.24 ppm corresponding to the methyl group of the coordinated acetonitrile ligand. This signal appears slightly more downfield than free acetonitrile (1.94 ppm) and that observed for the similar chloro analog, [(η⁶-p-cymene)RuCl(P{OCH₂}₃CEt)(NCMe)][BArF′] (2.12 ppm).⁴⁴ Complex 4 displays a single resonance signal in its ³¹P NMR spectrum at 117.5 ppm, appearing slightly more downfield than its chloride congener (115.1 ppm). Despite multiple attempts, the elemental analyses of compound 4 did not provide acceptable or reliable values.

Scheme 1 Synthesis of ruthenium(II) complexes 2–8. ^a P(OCH₂)₃CEt (1.1 equiv.), CH₂Cl₂, rt., 30 min; ^b KI (30 equiv.), reflux, CHCl₃/H₂O, 7 d; ^c Ag[BArF′] (2.1 equiv.), NCMe, rt, 18 h; ^d Ag[BArF′] (1.1 equiv., CH₂Cl₂, rt, 18 h); ^e 2 equiv. NCMe, 12 h, CHCl₃; ^f CH₂Cl₂, rt, 3 d; ^g PhMgBr, CH₂Cl₂, rt, 1 h or THF, −78 °C, 1 h; ^h Ag[BArF′], NCMe (1 equiv.), CH₂Cl₂, 1 d; ⁱ CDCl₃, 3 d.

Encouraged by the successful preparation and characterization of the monocationic acetonitrile complex 4, the halide abstraction chemistry of compound 2 was examined under noncoordinating conditions. The reaction of 2 with excess (2.1 equiv.) Ag[BArF′] in dichloromethane affords the dimeric complex [(η⁶-p-cymene)Ru(μ-I)(P{OCH₂}₃CEt)]₂[BArF′]₂ (5) (Scheme 1; reaction (d)), which can be converted quantitatively back to 4 by heating in acetonitrile (Scheme 1; reaction (f)), highlighting the weak intermolecular forces associated with the dimeric structure. Conversely, heating 4 in dichloromethane produced 5 in quantitative yield (Scheme 1; reaction (e)), illustrating their reversible interconversion driven by solvent choice. A comparison of the ¹H NMR spectra of complexes 4 and 5 reveals pronounced differences in their respective cymene regions. Whereas compound 4 displays four well-separated doublets for the cymene ligand, the spectrum of 5 has a single, broad, complicated multiplet due to signal overlap. To gain further insight into the structural differences underlying these spectroscopic features, large purple-red crystals of complex 5 were grown from chloroform and analyzed by single-crystal X-ray diffraction. The structure of 5 is shown in Fig. 3b. Given the limited number of reported ruthenium cymene complexes containing Ru–I bonds, crystals of complex 2 were also obtained and analyzed for comparison (Fig. 3a). The structure of complex 5 features a planar, rhomboidal Ru₂I₂ core bonding motif. The distance between the diagonal ruthenium centers (3.963 Å) is slightly longer than that between the diagonal iodine atoms (3.592 Å). The I(1)–Ru(1)–I(1) bond angles (84.38°) are notably more acute than the Ru(1)–I(1)–Ru(1) bond angles (95.62°). The Ru–I bonds of 5 are statistically equivalent (2.671(2) Å and 2.678(1) Å) and are shorter than both Ru–I bonds in 1 (bridging = 2.736(1) Å and terminal = 2.726(1) Å)⁴⁰ or those in 2 (2.715(1) Å and 2.705(2) Å).

Fig. 3 (a) Single crystal X-ray structures of (a) (η⁶-p-cymene)RuI₂(P{OCH₂}₃CEtCEt) (2) and (b) [(η⁶-p-cymene)Ru(P{OCH₂}₃CEtCEt)(μ-I)]₂[BArF′]₂ (5) (35% probability with hydrogen atoms and BArF′ ligand removed for 5 to improve clarity). Selected bond lengths (Å) for 2: Ru(1)–I(1) = 2.715(1), Ru(1)–I(2) = 2.705(2); Ru(1)–P(1) = 2.237(3). Selected bond angles (°) for 2: I(2)–Ru(1)–I(1) = 88.99(4); P(1)–Ru(1)–I(2) = 86.26(8); P(1)–Ru(1)–I(2) = 89.22(8). Selected bond lengths (Å) for 5: Ru(1)–I(1) = 2.671(2); Ru(1)–P(1) = 2.264(2). Selected bond angles (°) for 5: Ru(1)–I(1)–Ru(1) = 95.62(2); I(1)–Ru(1)–I(1) = 84.39(5); I(1)–Ru(1)–P(1) = 88.56(1); I(1)–Ru(1)–I(1) = 84.38(2).

3.2. Preparation of (η⁶-p-cymene)RuPh(I)(P{OCH₂}₃CEt) (6) and its halide abstraction chemistry

Organometallic ruthenium complexes bearing σ-phenyl–ruthenium bonds (Ru–Ph) have been implicated in catalytic cycles involving C–H activation,^32,46 cross-coupling,^32,47 insertion chemistry,^32,47 and redox chemistry.⁴⁸ Phenylation of (η⁶-p-cymene)ruthenium complexes bearing organophosphorus ligands (η⁶-p-cymene)RuX₂(PR₃) (X = Cl⁴⁷ and Br;^28,29,31 PR₃ = phosphine or phosphite) has been previously reported with chloride and bromide ligands; however, their preparations pose challenges, such as requiring extremely low temperatures (−78 °C)⁴⁷ or the use of diphenylmagnesium, a non-commercially available reagent that is somewhat difficult to prepare.⁴⁹ Complex 2 can be cleanly phenylated with commercially available phenylmagnesium bromide (3 M in THF) in methylene chloride (CH₂Cl₂) at room temperature to yield (η⁶-p-cymene)RuPh(I)(P{OCH₂}₃CEt) (6) in 86% yield (Scheme 1, reaction (g)). The fact that the reaction of complex 2 with PhMgBr in CH₂Cl₂ proceeds cleanly, without notable decomposition or side product formation, underscores the unique reactivity of the Ru–I bond in salt metathesis reactions. With the recent ban on methylene chloride, it was found that good yields of 6 can also be acquired by performing the reaction in THF at −78 °C; however, performing the reaction at room temperature led to a mixture of products. It was uncertain whether this was due to over-phenylation, as has been reported with the phenylation of (η⁶-p-cymene)RuCl₂(PPh₃) to produce a suspected diphenylated Ru(II) species.⁴⁷ Compound 6 is air-stable and highly soluble in common organic solvents such as diethyl ether, with moderate solubility in hexane. This favorable solubility profile facilitated detailed spectroscopic and structural characterization. The ¹H NMR spectrum of 6 shows two complicated multiplets for the phenyl ligand (6.79–6.91 ppm (3H) and 7.75 ppm (2H)) and four well-resolved doublets for the cymene ligand, indicating asymmetry. X-ray quality crystals of 6 were grown from a methylene chloride/hexane solution and analyzed by single-crystal X-ray crystallography (Fig. 4), further confirming its structure. Complex 6 is isostructural to its previously reported bromide³⁶ and chloride⁴⁷ congeners.

Fig. 4 Single crystal X-ray structure of η⁶-(p-cymene)Ru(I)(Ph)(P{OCH₂}₃CEt) (6) (35% probability with hydrogen atoms omitted). Selected bond lengths (Å): Ru(1)–C(1) = 2.104(3); Ru(1)–I(1) = 2.7119(4); Ru(1)–P(1) = 2.2157(8); selected bond angles (°): P(1)–Ru(1)–I(1) = 87.75(2); C(1)Ru(1)–I(1) = 94.40(8); P(1)–Ru(1)–C(1) = 86.44(8).

Replacing the halides in (η⁶-p-cymene)ruthenium complexes with labile ligands like acetonitrile is an effective strategy to enhance the metal's reactivity toward substrates.^50–52 Indeed, the reaction of 6 with Ag[BArF′] in the presence of one equivalent of acetonitrile in CH₂Cl₂ at room temperature effectively replaces the iodide ligand with acetonitrile to produce the cationic ruthenium(II) complex, [(η⁶-p-cymene)RuPh(NCMe)(P{OCH₂}₃CEt)][BArF′] (7) (Scheme 1, reaction (h)). During the reaction, the dark-red colored solution representing 6 gradually turns yellow, the color of 7, serving as a built-in indicator for reaction completion. The ¹H NMR spectrum of 7 provides a glimpse into its structural properties and an opportunity to compare with that of complex 4. Similar to 4, the ¹H NMR spectrum of 7 displays a singlet at 2.27 ppm for the methyl group of the coordinated acetonitrile ligand, and for comparison purposes, is statistically equivalent to 4 (2.24 ppm). This suggests the cationic state of the ruthenium complex has a greater effect on the chemical shift of the coordinated acetonitrile methyl group than the surrounding ligands. Unpredictably, it was discovered that storing an NMR sample of complex 7 in chloroform-d over several days at room temperature produced purple colored crystals of the dicationic, chloro-bridged dimer, [(η⁶-p-cymene)Ru(μ-Cl)(P(OCH₂)₃CEt)]₂[BArF′]₂ (8) (Scheme 1, reaction (i)), instead of expected complex (7).⁴⁴ The isolation of 8 proves compound 7 is unstable towards chloroform, causing the exchange of the phenyl group for a chloride ligand.^53,54 The structure of 8 has been previously reported and is isostructural to compound 5.⁴⁴ The only significant difference between compounds 5 and 8 is the separation between the two ruthenium centers. In compound 5, the Ru⋯Ru separation distance is 3.96 Å, while in compound 8, it is comparably shorter at 3.662 Å, likely due to the smaller ionic radius of the chloride ligands.

3.3 Preparation of Ru(II) tetra-acetonitrile complexes

It has been reported that heating (η⁶-p-cymene)Ru(Br)(Ph)(P{OCH₂}₃CEt) in acetonitrile results in the formation of (NCMe)₃RuPh(Br)(P{OCH₂}₃CEt), a tris-acetonitrile species with the bromide remaining intact.³¹ On the contrary, heating 6 in acetonitrile at 70 °C produced a cationic, tetra-acetonitrile ruthenium(II) complex, [(NCMe)₄RuPh(P{OCH₂}₃CEt)][I] (9), with the iodide ligand displaced as a counter anion (Scheme 2, reaction (a)).

Scheme 2 Scheme showing the formation of a tetra-acetonitrile ruthenium complex [(NCMe)₄RuPh(P{OCH₂}₃CEt)][I] (9) and halide exchange reactions with AgOTf and Na[BArF′] to produce 10 and 11, respectively. ^a Acetonitrile, 70 °C, 1 d; ^b AgOTf, NCMe, rt, 18 h; ^c Na[BArF′], 1 h, rt.

Compound 9 is sparingly soluble in most organic solvents but is readily dissolved in acetonitrile. Recording its ¹H NMR spectrum in NCMe-d₃ causes immediate protio/deutero-acetonitrile exchange, making it difficult to identify whether 9 possessed three or four acetonitrile ligands. Notably, the ¹H NMR resonances for the remaining intact acetonitrile ligands appear as one singlet at 2.39 ppm (6H), suggesting they are symmetry related. This indicates the non-symmetry related acetonitrile ligands (those that are trans to the phenyl and phosphite ligands) are displaced when introduced to acetonitrile-d₃. The differing labilities of the acetonitrile ligands are likely due to the trans-influence of the phosphorus and phenyl ligands.⁵⁵ This phenomenon is similar to that observed with acetonitrile exchange studies with the cationic, tris-acetonitrile ruthenium(II) complexes supported by bis(pyrazolyl)acetate.⁵² Recording the ¹H NMR spectrum of 9 in acetone-d₆ allowed the observance of all acetonitrile resonances (12H); however, only two of the three expected acetonitrile methyl resonances were observed (2.1 and 2.03 ppm) due to signal overlap. To verify the structure of 9, crystals of the complex were grown by slow evaporation of acetonitrile at −36 °C. Its structure (Fig. 5) reveals four acetonitrile ligands with an iodide anion in the outer coordination sphere. The Ru–C(1) bond distance (2.076(5) Å) is slightly shorter than that in 6 (2.109(4) Å), and the Ru–N bond distances for the acetonitrile ligands trans to the phenyl and phosphite ligands (Ru–N2 = 2.127(4) Å; Ru–N4 = 2.163(4) Å, respectively) are significantly longer than the other two (Ru–N1 = 2.027(3) Å; Ru–N3 = 2.021(3) Å), reflecting the strong trans influence of the phenyl and phosphite donors, and the source of their enhanced lability.

Fig. 5 Single crystal X-ray structure of [(NCMe)₄Ru(Ph)(P{OCH₂}₃CEt) (9)][I] (35% probability with hydrogen atoms omitted). The asymmetric unit contains two independent molecules; however, only one is shown for clarity. Selected bond lengths (Å): Ru(1)–C(1) = 2.075(4); Ru(1)–N(1) = 2.027(3); Ru(1)–N(2) = 2.128(4); Ru(1)–N(3) = 2.022(3); Ru(1)–N(4) = 2.162(4), Ru(1)–P(1) = 2.196(1). Selected bond angles (°): C(1)–Ru(1)–N(1) = 89.0(1); C(1)–Ru(1)–N(2) = 86.6(1); C(1)–Ru(1)–N(3) = 89.0(1); C(1)–Ru(1)–N(4) = 175.4(1); C(1)–Ru(1)–P(1) = 92.6(1); N(1)–Ru(1)–N(2) = 87.0(1); N(1)–Ru(1)–N(3) = 176.3(1); 89.01(10); N(1)–Ru(1)–N(4) = 89.01(10); N(1)–Ru(1)–P(1) = 91.7(1).

Due to the poor solubility of compound 9 in less polar solvents such as THF, diethyl ether, and benzene, the iodide counterion was exchanged with triflate and BArF′ by treatment with silver triflate (AgOTf) or sodium tetrakis(3,5-bis(trifluoromethyl)phenyl)borate (Ag[BArF′]), affording [(NCMe)₄RuPh(P{OCH₂}₃CEt)][OTf] (10) and [(NCMe)₄RuPh(P{OCH₂}₃CEt)][BArF′] (11), respectively (Scheme 2 reactions (b) and (c)). While both compounds exhibit improved solubility as compared to 9, compound 10 is significantly more sensitive to air, moisture, and heat, whereas compound 11 remains shelf-stable for months in an enclosed container. The single-crystal X-ray structure of compound 11 was determined; however, due to its close structural similarity to compound 9 (Fig. 5), the crystallographic data are not shown here but are provided in the Fig. S38. Compounds 9–11 were characterized by elemental analysis (C, H, N); however, reliable elemental analyses could not be obtained, as the results were consistently less than half of the theoretical values. This discrepancy likely reflects significant decomposition during shipment (Atlantic Microlab, Inc., Atlanta, GA) and/or analyses, which may be attributed to the enhanced lability of the coordinated acetonitrile ligands.

3.4 Reaction of tetra-acetonitrile ruthenium complexes with bidentate N′N-donor ligands

The purported complex (NCMe)₃Ru(Ph)(Br)(P{OCH₂}₃CEt) has been reported to react with tridentate poly(pyrazolyl)methane ligands such as tris(pyrazolyl)methane^29,30 and tris(triazolyl)methanol³¹ to produce facial coordinated, cationic, octahedral complexes with the general formula of [(fac-κ³-L)Ru(NCMe)(Ph)(P{OCH₂}₃CEt)][Br]. The differing lability of the acetonitrile ligands for the tetra-acetonitrile ruthenium complexes (9–11) presented an opportunity to explore the possibility of coordinating a single bidentate ligand to the ruthenium center, therefore leaving two acetonitrile ligands that could be used as labile ligands in catalytic olefin hydroarylation reactions. The main difficulty lay in avoiding full replacement of the acetonitrile ligands with two bidentate donors, as has been previously observed.⁵⁶ To mitigate this risk, the N,N′-bis(2,6-diisopropylphenyl)ethane-1,2-diimine (DAB) ligand was chosen for its steric bulk, which would likely deter double ligand coordination.⁵⁷ The reaction of 11 with DAB causes an immediate color change from a faint greenish-brown color to a blackish-green colored solution (Scheme 3). After workup, [(κ²-N,N-DAB)Ru(NCMe)₂(Ph)(P{OCH₂}₃CEt)][BArF′] (12) was isolated in excellent yield (94%). Heating the complex 12 in acetonitrile-d₃ with ¹H NMR monitoring revealed dissociation of the DAB ligand over two days, as evidenced by the appearance of signals corresponding to the free ligand (Scheme 3), thereby limiting its evaluation as a potential olefin hydroarylation catalyst.

Scheme 3 Reaction scheme showing the reaction of DAB with 11 in dichloromethane to form [(κ²-N,N-DAB)Ru(NCMe)₂(Ph)(P{OCH₂}₃CEt)][BArF′] (12) and DAB ligand displacement upon heating 12 in acetonitrile.

The ¹H NMR spectrum of 12 reveals a C_s-symmetric complex, as indicated by two septets for the isopropyl groups and four doublets for the isopropyl methyl groups. The resonance for imine protons appears far downfield as an overlapping virtual triplet at 8.53 ppm. The intense color of compound 12 prompted an investigation of its absorption properties. Its UV-vis spectrum displays a sharp, intense π → π* transition at 284 nm,⁵⁸ a strong metal-to-ligand charge transfer (MLCT) band at 490 nm,⁵⁹ and a d → d* transition at 589 nm.^58,60 The structure of compound 12 (Fig. 6) was confirmed by single-crystal X-ray diffraction, revealing an octahedral ruthenium center with the basal plane comprising the DAB, phosphorus, and phenyl ligands, while the two acetonitrile ligands are positioned above and below the basal plane. The Ru–N bond lengths of the DAB ligand (2.188(4) Å and 2.187(5) Å) are approximately 0.17 Å longer than those of the coordinated acetonitrile ligands (2.016(6) Å and 2.019(6) Å), indicating that the DAB ligand is weaklier bound. The acetonitrile ligands are bowed away from the bulky DAB ligand, displaying nonlinear Ru–N≡C and N≡C–CH₃ bond angles; Ru–N≡C bond angles; Ru1–N4–C35 = 168.2(6)° and Ru1–N4–C35 = 173.6(6)°; and N≡C–CH₃ bond angles; N4–C35–C36 = 176.1(9)° and N3–C33–C34 = 177.0(9)°. This geometric distortion highlights the significant steric encumbrance in complex 12 and suggests that such congestion may contribute to the observed lability of the DAB ligand.

Fig. 6 Single crystal X-ray structure of [(κ²-N,N-DAB)Ru(NCMe)₂(Ph)(P{OCH₂}₃CEt)][BArF′] 12. The BArF′ counter anion and hydrogen atoms have been removed for clarity. The isopropyl groups of the DAB ligand are depicted as sticks to lessen distracting overlaps. Selected bond lengths (Å): Ru(1)–C(27) = 2.077(7); Ru(1)–P(1) = 2.203(2); Ru(1)–N(1) = 2.188(4); Ru(1)–N(2) = 2.187(5); Ru(1)–N(3) = 2.016(6); Ru(1)–N(4) = 2.019(6); C(1)–C(2) = 1.44. Selected bond angles (°): P(1)–Ru(1)–N(1) = 174.9(1); P(1)–Ru(1)–N(2) = 99.6(1); P(1)–Ru(1)–N(3) = 92.5(1); P(1)–Ru(1)–N(4) = 85.2(2); P(1)–Ru(1)–C(27) = 89.0(2); N(1)–Ru(1)–N(2) = 75.9(2); N(1)–Ru(1)–C(27) = 95.6(2).

Recognizing the weak binding affinity of the DAB ligand in these systems, 4,4′-tert-butyl-2,2′-bipyridine (t-bipy) was investigated due to its narrower steric profile, enhanced nucleophilicity, and well-documented coordination chemistry.^60,61 Compound 9 reacts cleanly with t-bipy to produce [κ²-N,N-(t-bipy)Ru(Ph)(NCMe)₂(Ph)(P{OCH₂}₃CEt)][I] (13) in good yield (Scheme 4).

Scheme 4 Scheme showing the reaction of 9 with t-bipy to produce [κ²-N,N-(t-bipy)Ru(Ph)(NCMe)₂(Ph)(P{OCH₂}₃CEt)][I] (13) and swapping the iodo ligand for BArF′ to produce [κ²-N,N-(t-bipy)Ru(Ph)(NCMe)₂(P{OCH₂}₃CEt)][BArF′] (14).

In contrast to the dark-blackish green color of compound 12, complex 13 appears as an orange material. Its UV-vis spectrum is comparatively simpler than that of compound 12, showing a strong π → π* transition at 288 nm, which is likely due to the BArF′ anion and bipy conjugated system (λ_max = 284 nm),⁶⁰ and weaker shoulders at 383 nm and 463 nm due to metal-to-ligand charge transfer^58–60 or possibly d → d* transitions, respectively, which can be overlap in these types of complexes.⁵⁸ X-ray quality crystals of 13 could not be grown to examine its structure using single-crystal X-ray diffraction studies. In lieu of a structure, advanced NMR techniques were used to definitively assign the structure of 13. The ¹H NMR spectrum of 13 suggests a D_2h symmetric complex with a plane of symmetry splitting laterally through the bipyridine ligand plane. This is clearly evidenced by the two t-butyl resonances (1.47 ppm and 1.41 ppm). Only one singlet at 2.03 ppm is observed for the two acetonitrile ligands (6H), reflecting their chemical equivalence. The literature provides little precedent for accurate ¹H NMR assignments of the aromatic region for the t-bipy system of asymmetric ruthenium coordination complexes. gCOSY and phase-sensitive nuclear Overhauser effect correlation spectroscopy (PS-NOESY) (see Fig. S48 and S50) of 13 were recorded to better assign its ¹H NMR aromatic resonances (Fig. 7). Based on these data, the downfield doublet at 9.53 ppm, which is coupled to the doublet-of-doublets at 7.63 ppm, suggested these resonances belong to the ortho and meta protons of one of the chemically inequivalent pyridine rings. The corresponding resonances of the other pyridine rings appeared at 8.31 ppm (1H, dd) and 7.46 ppm (1H, ddd). A PS-NOESY spectrum was used to distinguish the absolute regiochemical position of these protons. The PS-NOESY suggests the downfield doublet at 9.53 ppm belongs to the ortho-proton of the pyridyl ligand that is trans to the phenyl ligand (closest to the phosphite ligand), while the resonance at 8.31 ppm (1H, dd) belongs to the corresponding position of the other pyridyl ring. These data provide two important insights: (1) the ortho positions are more deshielded than the meta position, which is likely due to its proximity to the metal, and (2) the pyridyl system trans to the phenyl ligand is more deshielded than that trans to the phosphite ligand. This correlates well with the structural trans-effects of these ligands and previous reports of similar complexes.^55,56

Fig. 7 Assigned ¹H NMR spectrum of the aromatic region of complex 13 following COSY and PS-NOESY analysis. Proton environments are labeled according to the structure, with phenyl (Ph) and bipyridine ligand protons (H^a–H^f) identified.

Preliminary catalytic olefin hydroarylation studies with complex 13, using ethylene (25 psi) and benzene at 100 °C, did not yield ethylbenzene; instead, iodobenzene was formed in near-quantitative yield. The formation of iodobenzene is consistent with iodide anion coordination to the metal center, followed by reductive elimination with the phenyl ligand. These results suggest that the iodide anion is not innocent and must be replaced with a non-nucleophilic counterion to be useful in catalytic olefin hydroarylation studies. To reduce this undesirable reactivity, two approaches were used to replace the iodide ligand of complex 13 with the non-coordinating BArF′ anion. The preferred route involves reacting complex 11 with one equivalent of t-bipy to produce [κ²-N,N-(t-bipy)Ru(Ph)(NCMe)₂(P{OCH₂}₃CEt)][BArF′] (14) in near quantitative yield (Scheme 4). This complex can also be made, albeit with somewhat lower yields, by reacting complex 13 with Na[BArF′] at room temperature (Scheme 5). The latter method forms an unidentified byproduct, which can be removed from 14 by column chromatography. The resonances in the ¹H NMR spectrum (Fig. 8, lower image) of impure 14 suggest the impurity is structurally related, mainly based on the additional sets of resonances that have been assigned to the phosphite (∼4.2 ppm), phenyl (∼6.5 ppm), and acetonitrile ligands (2.5 ppm) (resonances of impurity are signified by the green stars in Fig. 8). Heating the mixture of impure 14 for 1 hour causes the resonances identified as the impurity to transform into those of 14 (∼85% complete, based on integration of phosphite CH₂ doublets at 4.25 ppm; see top spectrum in Fig. 8). These observations suggest that the impurity is either an isomer of 14 or arises from an intermediate interaction between complex 13 and Na[BArF′] before complete halide abstraction.^62,63 Despite repeated efforts, the impurity could not be isolated for full characterization; in all cases, only pure 14 or a mixture was obtained.

Scheme 5 Olefin hydroarylation studies with 14 showing the formation of styrene and trace ethylbenzene.

Fig. 8 Stacked ¹H NMR of the crude reaction mixture of 14. The corresponding resonances for the impurity are marked with green stars. The bottom spectrum shows the unpurified sample, while the top spectrum demonstrates a decrease in impurity resonances after heating the sample in acetonitrile at 70 °C for one hour.

3.5 Olefin hydroarylation studies

Compound 14 was selected as a potential olefin hydrophenylation catalyst because the tightly bound t-bipy ligand maintains the coordination geometry at the metal center. Initial reactions carried out in benzene under 25 psi of ethylene showed no significant formation of ethylbenzene; instead, styrene was the primary product, with less than one turnover and only trace amounts of ethylbenzene detected (Scheme 5).

The formation of styrene is attributed to the facile insertion of ethylene into the Ru–Ph bond, generating a Ru-CH₂CH₂Ph species that subsequently undergoes β-hydride elimination to yield styrene and a ruthenium hydride complex. The reaction was performed on the NMR scale with 14 in benzene-d₆ under 15 psi of ethylene in a J-Young tube and heated to 90 °C, with progress monitored by ¹H NMR spectroscopy. After 24 hours of reactivity, characteristic resonances for styrene appeared at 5.08 ppm (d, 1H, ³J_HH = 8 Hz), 5.61 ppm (d, 1H, ³J_HH = 18 Hz), and 6.59 ppm (dd, 1H, ³J_HH = 18 Hz, ³J_HH = 8 Hz). A minor resonance at −2.22 ppm (m) was also observed and assigned to a Ru–H intermediate. Copper was then added in an attempt to regenerate the catalyst via hydride abstraction from the metal center.⁶⁴ However, the copper species investigated—copper(II) 2-propylethanoate and copper(II) acetoacetate—resulted in complete decomposition of the complex 14 and no production of styrene. The lack of a full turnover of styrene suggests that complex 14 decomposes under the reaction conditions. Increasing the reaction temperature only accelerated decomposition and changing the ethylene pressure (10–100 psi) did not improve styrene production. Neither did extending the reaction time to 72 improve catalytic performance. These results indicate that complex 14 undergoes facile insertion chemistry; however, the rate of β-hydride elimination supersedes metal-mediated C–H activation of benzene. To place these results in context to cationic ruthenium complexes supported by facially coordinating tridentate ligands, the complex [(κ³-HC(pz⁵)₃)Ru(NCMe)Ph(PR₃)]⁺ (HC(pz⁵)₃ = tris(5-methyl-pyrazolyl)methane),²⁹ provided the highest turnovers of ethylbenzene (565 TON; 131 hours). However, the formation of styrene observed for 14, suggests its mechanism is likely more similar in reactivity to the cationic complex [(κ³-MeOTTM)Ru(NCMe)Ph(PR₃)]⁺ (MeOTTM = (4,4′,4′′-(methoxymethanetriyl)-tris(l-benzyl-1H-1,2,3-triazole))),³¹ which produced 53 TON of styrene.

3.6 Cyclic voltammetry studies

The inability of 14 to effectively catalyze the formation of ethylbenzene or styrene prompted an investigation into its poor reactivity. Evaluation of the relative electron density in ruthenium(II) olefin hydroarylation catalysts has proven to be a useful indicator of their ability to undergo key steps such as C–H activation and olefin insertion.⁶⁵ It is generally assumed that C–H activation is facilitated by electron-rich metal centers, while electron-poor centers are more favorable for insertion chemistry. To better assess the electron density at the ruthenium center in complex 14, its redox potential was measured using cyclic voltammetry. The voltammogram of complex 14, shown in Fig. 9, reveals a quasi-reversible redox couple at +0.53 V, assigned to the Ru(II/III) redox process. Previously reported Ru(III/II) potentials for successful hydroarylation catalysts supported by tridentate, facially coordinating ligands range from +0.54 to +0.86 V, with more positive potentials generally correlating with higher catalytic activity. Despite being cationic, complex 14 exhibits an electronic character more akin to the neutral Tp-complex (κ³-N,N,N-Tp)Ru(NCMe)(P{OCH₂}₃CEt) (Tp = hydridotris(pyrazolyl)borate),²⁸ which has a Ru(II/III) redox potential of +0.55 V and achieves a paltry 20 turnovers of ethylbenzene. In contrast, complex 14 is significantly more electron-rich than the charge-similar cationic catalysts [HC(pz⁵)₃Ru(NCMe)Ph(P{OCH₂}₃CEt)]⁺ and [(MeOTTM)Ru(NCMe)Ph(P{OCH₂}₃CEt)]⁺ (HC(pz⁵)₃ = (tris(5-methylpyrazolyl)methane)³⁰ and MeOTTM = 4,4′,4′′-(methoxymethanetriyl)-tris(1-benzyl-1H-1,2,3-triazole)),²⁹ which have Ru(II/III) redox potentials of +0.82 V and +0.86 V, respectively. This suggests that despite only having two nitrogen donors, the bipyridine-based ligand system is more electron-donating than the tris-(pyrazolyl) or tris-triazole methane ligand motif. This can likely be accounted for by using the pK_a values of their conjugate acids (pyridine (5.25),⁶⁶ pyrazole (2.48),⁶⁷ or triazoles (1.17)),⁶⁸ wherein higher pK_a values indicate greater basicity and, thus, stronger electron-donating ability.

Fig. 9 Voltammogram of complex 14 against a cobaltocenium standard with a scan rate of 150 mV s⁻¹.

4. Summary and conclusions

This study highlights the versatile coordination chemistry and reactivity of the phosphite-containing ruthenium complex (η⁶-p-cymene)RuI₂(P{OCH₂}₃CEt). The Ru–I bond proved highly effective in halide abstraction and phenylation reactions, enabling the isolation of a range of unique species, including cationic dimeric complexes. Notably, the characterization and structural identification of the cationic tetra-acetonitrile complex [(NCMe)₄RuPh(P{OCH₂}₃CEt)][I] (9), in contrast to the previously reported tris-acetonitrile complex (NCMe)₃RuPh(Br)(P{OCH₂}₃CEt), underscores the distinctive reactivity of the iodide ligand and highlighting the role of halide identity in directing complex speciation. The lability of the acetonitrile ligands in the tetra-acetonitrile complex facilitates selective substitution with bidentate nitrogen-donor ligands, providing access to a series of cationic ruthenium(II) species. Comparative studies revealed that the electronic and steric properties of these ligands—particularly DAB and t-bipy—significantly impact complex stability and reactivity. Catalytic olefin hydroarylation studies with the t-bipy complex (14) proved to be non-catalytic; but instead showed stoichiometric formation of styrene with scant production of ethylbenzene, attributed to efficient β-hydride elimination. Electrochemical analysis indicates that the bipy ligand is a much stronger electron-donor than the tris(pyrazolyl)- or tris(1,2,3-triazolyl)-methane frameworks. These results suggest that fine-tuning ligand electronics, particularly using less electron-donating ligands, may be a method to promote catalysis. While electronic properties are critical, steric factors and overall molecular geometry must also be considered in the rational design of olefin hydroarylation catalysts.

Author contributions

The manuscript was written through contributions of all authors. Conceptualization [B. Q.], formal analysis [B. Q., A. M., K. M., G. D., J. B., A. F., D. Z.], funding acquisition [B. Q.], investigation [B. Q, A. M., K. M., G. D., J. B., A. F., D. Z., C. W. P.], project administration [B. Q.], supervision [B. Q.], validation [B. Q., A. M., K. M., G. D., J. B., A. F., D. Z.], visualization [B. Q.], writing – original draft [B. Q.], writing – review & editing [B. Q., A. M., K. M., G. D., J. B., A. F., D. Z., C. W. P.]. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

Supplementary information: Representative NMR, UV-vis, X-ray, and IR data spectra are available. See DOI: https://doi.org/10.1039/d5dt01437a.

CCDC 2463395 (2), 2463400 (5), 2463401 (6), 2463398 (9), 2463399 (11) and 2463402 (12) contain the supplementary crystallographic data for this paper.^69a–f

Acknowledgements

The work was supported or partially supported by the American Chemical Society Petroleum Research Fund (PRF# 53848-UNI3), the National Science Foundation, Chemical Synthesis – Research in Undergraduate Institutions Grant (NSF-RUI# Chem Syn-2154822), and Major Research Instrumentation funds (NSF-MRI# 2215812) for SCXRD analyses.

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