Coordination chemistry of a calix[4]arene-based NHC ligand: dinuclear complexes and comparison to IⁱPr₂Me₂

Abstract

The preparation and coordination chemistry of 5,17-bis(3-methyl-1-imidazol-2-ylidene)-25,26,27,28-tetrapropoxycalix[4]arene (1) is described. Starting from the bis(imidazolium) pro-ligand 1·2HI, the free carbene 1 was readily generated in solution through deprotonation using K[O^tBu] and its reactivity with rhodium(I) dimers [Rh(COD)Cl]₂ (COD = 1,5-cyclooctadiene) and [Rh(CO)₂Cl]₂ investigated. Dinuclear complexes were isolated in both cases, where the calix[4]arene-based NHC ligand adopts a bridging μ²-coordination mode, and in one case characterised in the solid-state by X-ray diffraction. Using instead an isolated and well-defined (mononuclear) silver transfer agent, generated by reaction of 1·2HI with Ag₂O in the presence of a halide extractor, reactions with [Rh(COD)Cl]₂ and [Rh(CO)₂Cl]₂ produced cationic dinuclear complexes bearing μ²-1 and μ²-Cl bridging ligands. The structural formulation of the novel dinuclear adducts of 1 was aided through spectroscopic congruence with model complexes, containing monodentate 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene (IⁱPr₂Me₂).

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Introduction

In addition to desirable donor properties, the structural versatility of N-heterocyclic carbenes (NHCs) has proven to be a major contributing factor in the widespread application of these ligands throughout organometallic chemistry and catalysis.¹ The root of this diversity lies in the numerous and generally straightforward preparative procedures available for the construction of the respective NHC pro-ligands; synthetic methodologies that have enabled access not only a plethora of monodenate ligands, but also an extensive range of polydentate variants.^2,3

As polydentate ligand scaffolds, calix[4]arenes are well suited building blocks with scope for functionalisation across the upper and/or lower rims of these ‘bowl’ shaped macromolecules.⁴ Despite extensive investigation of heteroatom-based donor systems, the coordination chemistry of NHC-functionalized calix[4]arenes is not well developed.⁵ 5,17-Difunctionalised examples, bearing NHC donors on opposite faces of the upper rim, are of particular interest for positioning bound metal centres across the macrocycle cavity. Well-defined complexes of these ligands are, however, limited to imidazol-2-ylidene based A–D (Chart 1). Complexes A illustrate this principle well, positioning square planar palladium centres directly over the macrocycle cavity.⁶ The incorporation of a methylene spacer between the calix[4]arene rim and the NHC donor group in B results instead in unfavourable twisting of the donor groups that skews the bound metal centres to one side of the ligand, while in C and D, the cavity is pinched closed as a consequence of the cis-coordination geometry of the metal centres.⁷ Closely related to A, phosphine-based systems E have also been described, where the ligand backbone enforces trans-coordination.^8,9

Chart 1 Complexes of upper rim functionalised calix[4]arene NHC and phosphine ligands.

In this report the preparation and coordination chemistry of an upper rim functionalised calix[4]arene pro-ligand 1·2HI are described (Chart 2). Through generation of the free NHC ligand (1) and use of transmetallation methodology, reactions with rhodium(I) dimers [Rh(COD)Cl]₂ (COD = 1,5-cyclooctadiene) and [Rh(CO)₂Cl]₂ lead to a range of dinuclear complexes containing either discrete or chloro-bridged metal fragments depending on the reagents employed. To help understand the influence of the calix[4]arene scaffold and corroborate the structural formulations of these species, the spectroscopic characteristics of complexes of 1 are contrasted with analogues containing the monodentate NHC ligand IⁱPr₂Me₂.¹⁰

Chart 2 NHC systems of interest.

Results and discussion

The synthesis of 5,17-(3-methyl-1-imidazole)-functionalised pro-ligand 1·2HI was achieved through straightforward adaption of literature protocols for related N-butyl and N-isopropyl analogues, involving alkylation of F with methyl iodide and isolated in 84% yield (Scheme 1).¹¹ The known precursor F was obtained following a five step synthesis, starting from commercially available 4-tert-butylcalix[4]arene (ca. 2 weeks, 8% yield in our hands), giving an overall yield of 7% for the six step procedure.¹² The formation of the new bis(imidazolium) salt was fully corroborated through a combination of NMR spectroscopy, ESI-MS, combustion analysis and X-ray crystallography. In solution the pre-carbenic centre is characterized by ¹H and ¹³C resonances at δ 9.20 and δ 134.9 (CD₂Cl₂), respectively, while the mass spectrum showed a strong dication signal centred at 377.2220 m/z (calcd 377.2224) with half-integer spacing. The solid-state structure features two independent but structurally similar dications that adopt distinctive ‘pinched-cone’ conformations, reflecting intra-charge repulsion between the imidazolium groups (C2⋯C8, 13.43(2) Å/C102⋯C108, 13.09(2) Å; Fig. 1).¹³ In order to quantify this distortion, the angles between the least squares planes (Mpln) of opposing aryloxy groups Θ_CALIX(R), where R = the aryl para substituent and the sign reflects the relative disposition of the R substituents (+ve, outwards; −ve, inwards), have been measured. Using this approach distortion from an ideal C_4v symmetric ‘cone’ to C_2v symmetric ‘pinched cone’ conformation of the calix[4]arene skeleton was found to be most pronounced in the independent molecule shown in Fig. 1 with Θ_CALIX(NHC·H⁺)/Θ_CALIX(H) = +80.8(2)/-19.3(2)° (ΔΘ_CALIX = 100.1(4)°); the other is characterised by Θ_CALIX(NHC·H⁺)/Θ_CALIX(H) = +86.4(2)/−2.8(2)° (ΔΘ_CALIX = 89.2(4)°).

Scheme 1 Preparation and deprotonation of pro-ligand 1·2HI.

Fig. 1 Solid-state structure of 1·2HI. Thermal ellipsoids for selected atoms drawn at the 30% probability level; only one of the two unique molecules shown (Z′ = 2); hydrogen atoms and anions are omitted for clarity. Selected bond lengths (Å) and angles (°): C2⋯C8, 13.43(2); ∠Mpln(C13–C18,O41)–Mpln(C27–32,O49), 80.8(2); ∠Mpln(C20–C25,O45)–Mpln(C34–C39,O53), 19.3(2); C102⋯C108, 13.09(2); ∠Mpln(C113–C118,O141)–Mpln(C127–132,O149), 86.4(2); ∠Mpln(C120–C125,O145)–Mpln(C134–C139,O153), 2.8(2).

Deprotonation of 1·2HI with strong hindered base K[O^tBu] in C₆D₆ or d₈-THF at 293 K resulted in quantitative formation of the free carbene 1, which was thoroughly characterised in situ using NMR spectroscopy. The deprotonation was corroborated by the absence of the ¹H NCHN resonance and characteristically high frequency ¹³C resonance at δ 213.2 (C₆D₆)/216.0 (d₈-THF) of the carbenic centre.¹⁴ For comparison, the equivalent signal in IⁱPr₂Me₂ is located at δ 212.7 (C₆D₆).¹⁰ Multiple attempts at isolating 1 from solution proved unsuccessful and instead it was generated in situ in subsequent studies.

Reactions of 1, generated as described above, with rhodium(I) dimers [Rh(COD)Cl]₂ and [Rh(CO)₂Cl]₂ were explored in THF, using excess KI (ca. 10 equiv.) to avoid formation of mixed halide products. Using either 0.5 or 1 equiv. of rhodium precursor per ligand, the only well-defined species that could be identified were dinuclear complexes 2a and 3a, where 1 adopts a bridging μ²-coordination mode (Scheme 2). In this manner, rhodium-diene-based 2a was obtained in good isolated yield of 53% following purification over alumina. Isolation of 3a, however, proved to be very low yielding (ca. 17%) and in order to acquire a more meaningful quantity of 3a, alterative preparation from 2avia diene displacement with CO (1 atm) in CH₂Cl₂ solution was required (quantitative conversion by ¹H NMR spectroscopy, 37% isolated yield). Mononuclear analogues of these new complexes bearing IⁱPr₂Me₂, 2b and 3b, were readily obtained through direct adaptions of the aforementioned procedures (Scheme 2).

Scheme 2 Reactions of 1 and IⁱPr₂Me₂ with rhodium dimers.

For both NHC ligands, the rhodium-diene complexes 2 were characterised in the solid-state by X-ray crystallography (Fig. 2). The conformation of the calix[4]arene backbone in 2a is notable for a dramatic conformational inversion in comparison to the pro-ligand 1·2HI, that appears to be driven by a favourable (off-centre) π-stacking interaction between the imidazolylidene rings (Cnt-Cnt = 3.608(5) Å; ΔΘ_CALIX = −98.41(12)°, 2a; 100.1(4)/89.2(4)°, 1·2HI), and pseudo C₂ symmetry. As expected for d⁸-metal complexes of this type, the rhodium centres adopt square planar coordination geometries in 2 and the associated metal–ligand bonding metrics are in line with related literature precedents.¹⁵ The similarity of the metal-NHC bonding characteristics in the new systems is apparent in both the solid-state (Rh–C = 2.021(3)/2.024(3) Å, 2a; 2.021(3) Å, 2b) and in solution by ¹³C NMR spectroscopy [δ 180.4 (¹J_RhC = 49 Hz), 2a; 178.4 (¹J_RhC = 49 Hz), 2b]. The structures were fully corroborated in solution by ¹H and ¹³C NMR spectroscopy, with intact coordination of the diene ligands readily established by presence of alkene ¹³C signals at ca. δ 95 (¹J_RhC = 7 Hz) and 72 (¹J_RhC = 14 Hz) displaying coupling to ¹⁰³Rh. Loss of C_s symmetry in the {Rh(COD)I} fragment and C_2v symmetry of the ligand 1 is observed on complexation, with C₂ symmetry evident on inspection of the ¹H and ¹³C{¹H} NMR spectra of 2a. Moreover, a large difference in chemical shift is observed for the now inequivalent ¹H resonances of the imidazolylidene functionalised aryl ring (Δδ = 2.03), with the high frequency signal at δ 7.95 presumably a consequence of the close proximity of the halogen atom (cf. C̲–H⋯I̲ = 4.139(3)/4.295(3) Å observed in the solid-state and δ 6.61 for 1·2HI), as previously noted in a related calix[4]arene-based system.^5b Combined, these NMR data of 2a are consistent with retention of solid-state structure in solution.

Fig. 2 Solid-state structures of 2a and 2b. Thermal ellipsoids for selected atoms drawn at the 50% probability level; minor disordered components (two OⁿPr groups in 2a), and hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (°): 2a: Rh1–I3, 2.6905(3); Rh1–Cnt(C5,C6), 2.089(3); Rh1–Cnt(C9,C10), 2.001(3); Rh1–C21, 2.021(3); Rh2–I4, 2.6934(3); Rh2–Cnt(C13,C14), 2.098(3); Rh1–Cnt(C17,C18), 1.990(4); Rh2–C27, 2.024(3); Rh1⋯Rh2, 8.5698(3); Cnt(C21–N25)–Cnt(C27–N31), 3.608(5); ∠Mpln(C33–C38,O61)–Mpln(C47–C52,O69), 22.35(6); ∠Mpln(C40–C45,O65)–Mpln(C54–C59,O73), 76.06(6); 2b: Rh1–I2, 2.6729(2); Rh1–Cnt(C3,C4), 2.109(2); Rh1–Cnt(C7,C8), 2.00(3); Rh1–C11, 2.021(3).

Although, we have so far been unable to grow suitable samples of 3 for structural elucidation by X-ray crystallography, the solution data points strongly to equivalent structural formations as 2. For instance, the relative cis-configuration of the carbonyl ligands was established through observation of two bands in the respective IR spectra (υ(CO) = 2071, 2002, 3a; 2072, 1998 cm⁻¹, 3b) and two distinct high frequency doublet resonances in the ¹³C{¹H} NMR spectra [δ 188.4 (¹J_RhC = 54 Hz), 181.6 (¹J_RhC = 78 Hz), 3a; 188.7 (¹J_RhC = 53 Hz), 182.7 (¹J_RhC = 78 Hz), 3b]. ¹H and ¹³C{¹H} NMR spectra of 3a also exhibit C₂ symmetry. In comparison to 2, the carbenic resonances are notable for a shift to lower frequency by ca. 10 ppm and there is a reduction in the magnitude of the coupling to ¹⁰³Rh [δ 171.5 (¹J_RhC = 44 Hz), 3a; 166.8 (¹J_RhC = 41 Hz), 3b].

To further explore the coordination chemistry of 1, we sought to utilise widely employed transmetallation methodology based on silver transfer agents.^1c,16 To this end, 1·2HI was reacted with a slight excess of Ag₂O in the presence of Na[BAr^F₄] (Ar^F = 3,5-C₆H₃(CF₃)₂), as a halide extractor, resulting in chelation of 1 and subsequent isolation of 4a in 73% yield (Scheme 3). The structural formulation of the new silver(I) complex was readily established in solution though a combination of ¹H and ¹³C NMR spectroscopy and ESI-MS. The ¹H NMR spectrum of isolated 4a shows C_2v symmetry, with the N-methyl signal located to lower frequency than found in the pro-ligand (δ 3.80 vs. 4.17). A strong parent ion is observed by ESI-MS in positive ion mode at 859.3340 m/z (calcd 859.3347 m/z) with a correct isotope pattern. Moreover the cation : [BAr^F₄] ratio was verified by integration of ¹H NMR data and elemental analysis. The ¹³C{¹H} NMR spectrum is notable for the carbenic signal at δ 180.7 that shows coupling to both ¹⁰⁹Ag (¹J_AgC = 211 Hz) and ¹⁰⁷Ag (¹J_AgC = 183 Hz); the chemical shift and relative magnitudes of these coupling constants are in good agreement with literature values.^16a Interrogation of single crystals of 4a by X-ray diffraction provides further evidence to the mononuclear formulation of 4a in solution, revealing a distorted linear geometry of the silver cation [C2–Ag1–C8 = 171.00(14)°] and essentially equivalent Ag–C bond lengths (Ag1–C2, 2.085(4); Ag1–C8, 2.083(4); Fig. 3). The biscarbene silver complex of IⁱPr₂Me₂ bearing the tetrafluoroborate counter anion [Ag(IⁱPr₂Me₂)₂][BF₄] G has previously been prepared,¹⁷ however, for comparison the analogue bearing the [BAr^F₄]⁻ counter anion 4b was generated in situ through reaction of Ag[OTf], Na[BAr^F₄] and 2 equiv. IⁱPr₂Me₂ in 1,2-C₆H₄F₂. The solid-state structure of 4b is notable for a significantly more orthogonal orientation of the NHC ligands in comparison to 4a and G [e.g. |N6–C2–C8–N12| = 3.7(5)°, 4a; |N3–C2–C15–N19| = 77.0(5)°, 4b; |N–C–C–N| = 20.6(6)°, G]. The near coplanar disposition of the NHC donors in 4a is fully in line with expectation, and readily attributed to the calix[4]arene scaffold [ΔΘ_CALIX = −96.4(2)°], while the conformational differences in 4b/G are presumably attributed to crystal packing effects, induced by disparate anion sizes, and enabled through low-energy Ag–NHC bond rotation. The Ag–C bond lengths are all within experimental error [2.085(4)/2.083(4) Å, 4a; 2.093(4)/2.092(4) Å, 4b; 2.078(11) Å, G], although we note an apparent trend of bond length contraction for 4a in comparison to 4b and that this parameter was not determined with high precision for G.

Scheme 3 Preparation and transmetallation reactions of silver NHC complexes.

Fig. 3 Solid-state structures of 4a and 4b. Thermal ellipsoids for selected atoms drawn at the 30% probability level; minor disordered components (four OⁿPr groups in 4a), hydrogen atoms and anions omitted for clarity. Selected bond lengths (Å) and angles (°): 4a: Ag1–C2, 2.085(4); Ag1–C8, 2.083(4); C2–Ag1–C8, 171.00(14); ∠Mpln(C14–C19,O42)–Mpln(C28–C33,O50), 1.01(8); ∠Mpln(C21–C26,O46)–Mpln(C35–C40,O54), 97.44(10); 4b: Ag1–C2, 2.093(4); Ag1–C15, 2.092(4); C2–Ag1–C15, 176.81(15).

Silver complex 4a acts as an effective carbene transfer agent, resulting in the formation of dinuclear complexes 5a and 6a bearing both μ²-1 and μ²-Cl ligands on reaction with 1 equiv. of [Rh(COD)Cl]₂ and [Rh(CO)₂Cl]₂, respectively, in 1,2-C₆H₄F₂ – established by NMR spectroscopy and ESI-MS. The reaction stoichiometries were maintained when using 0.5 equiv. of rhodium dimer instead, indicating selective formation 5a and 6a. Both dinuclear products were obtained in moderate isolated yields (63% and 59% respectively) and were difficult to fully purify, due to limited solution stability. Aided by comparison to 2a/3a and supported through preparation of direct IⁱPr₂Me₂ analogues (vide infra), the formulation of the structures of 5a/6a was achieved through a combination of ¹H, ¹³C NMR and IR spectroscopy, ESI-MS and in the case of 5b a low resolution X-ray structure (Fig. 4). The ESI-MS in particular evidenced formation of these dinuclear monocationic species, with parent cation signals at 1209.3986 (calcd 1209.3973) and 1105.1680 (calcd 1105.1891) m/z, respectively for 5a and 6a, with correct isotope patterns and integer mass spacing. Moreover the cation : [BAr^F₄] ratio was verified by integration of ¹H NMR data. In both cases, single carbene resonances [δ 176.9 (¹J_RhC = 50 Hz), 5a; 168.1 (¹J_RhC = 43 Hz), 6a] with very similar chemical shifts and ¹J_RhC coupling constants to the respective neutral analogues [δ 180.4 (¹J_RhC = 49 Hz), 2a; 171.5 (¹J_RhC = 44 Hz), 3a], alongside ¹⁰³Rh coupled alkene and carbonyl signals, were observed by ¹³C NMR spectroscopy. For 6a both carbonyl stretching bands are perturbed (as expected) to higher frequency relative to neutral 3a [υ(CO) = 2083, 2016, 6a; 2071, 2002 cm⁻¹, 3a]. Bond connectivity and conformational features of 6a were established through X-ray diffraction, using a poor quality crystalline sample [R₁ = 0.2744, I ≥ 2σ(I)]. The data nevertheless can affirm the structural formulation, with a reduced pinching of calix[4]arene core [ΔΘ_CALIX = −83.1(7)°] relative to 2a [ΔΘ_CALIX = −98.41(12)°] and 4a [ΔΘ_CALIX = −96.4(2)°].

Fig. 4 Solid-state structure of 6a (ball and stick, poor quality data). Minor disordered component (CO), hydrogen atoms and anion omitted for clarity. Selected bond lengths (Å) and angles(°): Rh1–Cl3, 2.353(5); Rh2–Cl3, 2.360(5); Rh1⋯Rh2, 3.861(3); Rh1–Cl3–Rh2, 110.0(3); ∠Mpln(C24–C29,O52)–Mpln(C38–C43,O60), 9.7(3); ∠Mpln(C31–C36,O56)–Mpln(C45–C50,O64), 92.8(4).

Starting from the known precursors [Rh(IⁱPr₂Me₂)(COD)Cl] and [Rh(IⁱPr₂Me₂)(CO)₂Cl], the new μ²-Cl bridged dinuclear complexes 5b and 6b were readily prepared by partial halide extraction using 0.5 equiv. of Na[BAr^F₄] in CH₂Cl₂ (Scheme 4). These structurally simple and fully characterised species (solution and solid-state, Fig. 5) help verify the formulation of 5a and 6a, with excellent agreement of the related spectroscopic data e.g. carbene ¹³C signals of 5 [δ176.9 (¹J_RhC = 50 Hz), 5a; 175.6 (¹J_RhC = 50 Hz), 5b] and carbonyl stretching bands of 6 [υ(CO) = 2083, 2016, 6a; 2090, 2019 cm⁻¹, 6b] – see Experimental section for full details. Although iridium NHC and rhodium phosphine examples have been reported, there are no proceeding crystallographically characterised examples of rhodium NHC complexes featuring a single otherwise unsupported bridging halogen atom to the best of our knowledge.¹⁸ Conformational differences, associated with the relative orientation of the NHC ligands about the Rh–Cl–Rh bridge, are evident when comparing the solid-state structures of 6a (anti) 5b (syn) 6b (anti); ¹H and ¹³C NMR data for 5a and 6a are most consistent with C₂ symmetry and anti-configurations in solution. Contrasting reactions of 4a and 4b, the calix[4]arene scaffold promotes the selective formation of dinuclear complexes. For instance, when 4b is reacted with 1 equiv. of [Rh(COD)Cl]₂ or [Rh(CO)₂Cl]₂ in 1,2-C₆H₄F₂ mixtures of 5b/cis-[Rh(IⁱPr₂Me₂)₂(COD)][BAr^F₄] 7 (2 : 1) and 6b/cis-[Rh(IⁱPr₂Me₂)₂(CO)₂][BAr^F₄] 8 (5 : 2) are the organometallic species produced as determined by ¹H NMR spectroscopy (Scheme 3) – with the structures of 7 and 8 verified by independent synthesis and full characterisation, including in the solid-state by X-ray diffraction (see experimental for full details).

Scheme 4 Preparation of dinuclear complexes 5b and 6b.

Fig. 5 Solid-state structures of 5b and 6b. Thermal ellipsoids drawn at the 50% probability level; minor disordered components (two ⁱPr groups in 6b), hydrogen atoms, solvent (6a), and anions omitted for clarity. Selected bond lengths (Å) and angles (°): 5b: Rh1–Cl3, 2.3988(9); Rh1–Cnt(C4,C5), 2.092(5); Rh1–Cnt(C8,C9), 1.974(4); Rh1–C20, 2.025(3); Rh2–Cl3, 2.4067(9); Rh2–Cnt(C12,C13), 2.101(4); Rh2–Cnt(C16,C17), 1.972(4); Rh2–C33, 2.022(3); Rh1–Cl3–Rh2, 144.13(4); 6b: Rh1–Cl3, 2.4045(9); Rh1–C4, 1.915(4); Rh1–C6, 1.833(4); Rh1–C22, 2.069(3); Rh2–Cl3, 2.3987(9); Rh2–C8, 1.920(4); Rh2–C10, 1.817(4); Rh2–C25, 2.069(3); Rh1–Cl3–Rh2, 122.26(4).

Summary

Using both free carbene and transmetallation methodologies the coordination chemistry of bis(imidazolium) pro-ligand 1·2HI with rhodium(I) dimers [Rh(COD)Cl]₂ and [Rh(CO)₂Cl]₂ has been investigated, culminating in the isolation of neutral and cationic dinuclear complexes where the calix[4]arene based-NHC is bound in a μ²-coordination mode. These dinuclear complexes have been well characterised in solution using NMR spectroscopy, IR spectroscopy (CO derivatives) and ESI-MS (cationic complexes), and their structural formulation corroborated through excellent agreement of these data with that associated with simpler model complexes containing instead IⁱPr₂Me₂. Structures determined though X-ray diffraction highlight the flexibility of the calix[4]arene ligand scaffold, which undergoes significant conformational change driven by charge repulsion (bis-cationic 1·2HI), π-stacking of the imidazolylidene rings (2a), trans-coordination to silver (4a), or presence of a bridging μ²-Cl ligand (6a).

Experimental

General considerations

All manipulations were performed under an atmosphere of argon, using Schlenk and glove box techniques unless otherwise stated. Glassware was oven dried at 150 °C overnight and flamed under vacuum prior to use. Anhydrous CH₂Cl₂, Et₂O, and pentane (<0.005% H₂O) were purchased from ACROS or Aldrich and freeze–pump–thaw degassed three times before being placed under argon. THF was dried over sodium/benzophenone, vacuum distilled, and freeze–pump–thaw degassed three times before being placed under argon. 1,2-C₆H₄F₂ was stirred over neutral alumina, filtered, dried over CaH₂, vacuum distilled, and freeze–pump–thaw degassed three times before being placed under argon over 3 Å molecular sieves. CD₂Cl₂ was dried over CaH₂, vacuum distilled, and freeze–pump–thaw degassed three times before being placed under argon. C₆D₆ was dried over Na, vacuum distilled, and freeze–pump–thaw degassed three times before being placed under argon. d₈-THF was dried over 3 Å molecular sieves and freeze–pump–thaw degassed three times before being placed under argon. 5,17-bis(imidazolium)-25,26,27,28-tetrapropoxycalix[4]arene (F),¹² IⁱPr₂Me₂,¹⁰ [Rh(COD)Cl]₂,¹⁹ [Rh(CO)₂Cl]₂,²⁰ Na[BAr^F₄],²¹ [Rh(IⁱPr₂Me₂)(COD)Cl],²² and cis-[Rh(IⁱPr₂Me₂)(CO)₂Cl]²² were synthesised using literature protocols. All other solvents and reagents are commercial products and were used as received. NMR spectra were recorded on Bruker AV spectrometers at 298 K unless otherwise stated. ¹H NMR spectra recorded in 1,2-C₆H₄F₂ were referenced using the highest intensity peak of the highest (δ 6.865) frequency fluoroarene multiplet. ¹³C{¹H} NMR spectra recorded in 1,2-C₆H₄F₂ were referenced using an internal sealed capillary of C₆D₆. Chemical shifts are quoted in ppm and coupling constants in Hz. IR spectra were recorded on a PerkinElmer Spectrum One FT-IR spectrometer at 293 K. ESI-MS analyses were recorded on Bruker Maxis Impact instrument. Microanalyses were performed by Stephen Boyer at London Metropolitan University.

Preparation of 1·2HI

A solution of F (1.00 g, 1.38 mmol) was stirred with iodomethane (0.84 mL, 13.8 mmol) in THF (30 mL) at 70 °C for 14 hours. The product was isolated by filtration following addition of Et₂O (ca. 20 mL), washed with Et₂O (2 × 20 mL) and dried in vacuo. Yield = 1.17 g (84%, fine off-white powder).

¹ H NMR (CD₂Cl₂, 400 MHz): δ 9.20 (br, 2H, NCHN), 8.04 (t, ³J_HH = 1.8, 2H, imid.), 7.23 (d, ³J_HH = 7.5, 4H, Ar), 7.05 (t, ³J_HH = 7.5, 2H, Ar), 6.72 (t, ³J_HH = 1.8, 2H, imid.), 6.61 (s, 4H, Ar), 4.54 (d, ²J_HH = 13.5, 4H, ArCH̲₂Ar), 4.17 (s, 6H, NCH₃), 4.03–4.09 (m, 4H, OCH₂), 3.74 (t, ³J_HH = 6.8, 4H, OCH₂), 3.28 (d, ²J_HH = 13.5, 4H, ArCH̲₂Ar), 1.87–2.06 (m, 8H, CH̲₂CH₃), 1.12 (t, ³J_HH = 7.4, 6H, CH₂CH̲₃), 0.93 (t, ³J_HH = 7.5, 6H, CH₂CH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 101 MHz): δ 157.5 (s, C̲OCH₂), 157.3 (s, C̲OCH₂), 137.7 (s, Ar{C̲CH₂}), 135.7 (s, Ar{C̲CH₂}), 134.9 (s, NCHN), 130.2 (s, Ar), 129.2 (s, Ar{CN}), 125.6 (s, imid.), 124.4 (s, Ar), 120.8 (s, Ar), 119.9 (s, imid.), 78.1 (s, OCH₂), 77.4 (s, OCH₂), 37.8 (s, NCH₃), 31.4 (s, ArC̲H₂Ar), 23.9 (s, C̲H₂CH₃), 23.5 (s, C̲H₂CH₃), 10.9 (s, CH₂C̲H₃), 10.2 (s, CH₂C̲H₃). ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 377.2220 m/z, [M]²⁺ (calcd 377.2224 m/z). Anal. Calcd For C₄₈H₅₈I₂N₄O₄ (1008.81 g mol⁻¹): C, 57.13; H, 5.80; N, 5.55. Found: C, 56.88; H, 6.00; N, 5.55.

Deprotonation of 1·2HI

A suspension of 1·2HI (10.2 mg, 0.0101 mmol) and K[O^tBu] (3.1 mg, 0.0276 mmol) in d₈-THF (0.5 mL) was agitated and sonicated for several minutes inside a sealed J. Young's NMR tube. Analysis by NMR spectroscopy indicated quantitative formation of 1 with the concomitant formation of ^tBuOH (δ_H 5.33, OH; 1.15, CH₃; δ_C 68.1, C̲CH₃; 31.6, CH₃). The reaction was repeated in C₆D₆ with the same outcome. The ¹H NMR spectra remained unchanged on standing at 293 K for 24 h in both cases.

¹ H NMR (d₈-THF, 500 MHz): δ 7.41 (s, 4H, Ar), 7.24 (d, ³J_HH = 1.6, 2H, imid.), 6.94 (d, ³J_HH = 1.6, 2H, imid.), 6.40 (d, ³J_HH = 7.5, 4H, Ar), 6.31 (app t, 2H, J = 8, Ar), 4.51 (d, ²J_HH = 13.1, 4H, ArCH̲₂Ar), 4.00–4.04 (m, 4H, OCH₂), 3.76–3.79 (m, 4H, OCH₂), 3.76 (s, 6H, NCH₃), 3.20 (d, ²J_HH = 13.1, 4H, ArCH̲₂Ar), 1.99–2.07 (m, 4H, CH̲₂CH₃), 1.92–1.99 (m, 4H, CH̲₂CH₃), 1.10 (t, ³J_HH = 7.4, 6H, CH₂CH̲₃), 0.98 (t, ³J_HH = 7.5, 6H, CH₂CH̲₃). ¹³C{¹H} NMR (d₈-THF, 126 MHz): δ 216.0 (s, NCN), 156.7 (s, C̲OCH₂), 156.3 (s, C̲OCH₂), 137.9 (s, Ar{CN}), 137.6 (s, Ar{C̲CH₂}), 134.5 (s, Ar{C̲CH₂}),128.9 (s, Ar), 123.1 (s, Ar), 121.6 (s, Ar), 121.1 (s, imid.), 118.1 (s, imid.), 77.9 (s, OCH₂), 77.7 (s, OCH₂), 38.3 (s, NCH₃), 32.0 (s, ArC̲H₂Ar), 24.5 (s, C̲H₂CH₃), 24.2 (s, C̲H₂CH₃), 11.2 (s, CH₂C̲H₃), 10.7 (s, CH₂C̲H₃). ¹H NMR (C₆D₆, 500 MHz): δ 7.40 (s, 4H, Ar), 6.74 (d, ³J_HH = 7.5, 4H, Ar), 6.71 (br, 2H, imid.), 6.61 (t, ³J_HH = 7.5, 2H, Ar), 6.18 (br, 2H, imid.), 4.53 (d, ²J_HH = 13.2, 4H, ArCH̲₂Ar), 3.87 (t, ³J_HH = 7.6, 4H, OCH₂), 3.76 (t, ³J_HH = 7.4, 4H, OCH₂), 3.42 (s, 6H, NCH₃), 3.17 (d, ²J_HH = 13.3, 4H, ArCH̲₂Ar), 1.84–1.95 (m, 8H, CH̲₂CH₃), 0.93 (app. t, J = 7, 12H, CH₂CH̲₃). ¹³C{¹H} NMR (C₆D₆, 126 MHz): δ 213.2 (s, NCN), 156.7 (s, C̲OCH₂), 155.5 (s, C̲OCH₂), 137.1 (s, Ar{CN}), 136.4 (s, Ar{C̲CH₂}), 134.7 (s, Ar{C̲CH₂}), 128.8 (s, Ar), 123.0 (s, Ar), 121.5 (s, Ar), 120.1 (s, imid.), 117.8 (s, imid.), 77.1 (s, OCH₂), 77.0 (s, OCH₂), 37.8 (s, NCH₃), 31.6 (s, ArC̲H₂Ar), 23.7 (s, C̲H₂CH₃), 23.6 (s, C̲H₂CH₃), 10.6 (s, CH₂C̲H₃), 10.5 (s, CH₂C̲H₃).

Preparation of 2a

A solution of 1·2HI (99.7 mg, 0.0987 mmol) and K[O^tBu] (27.8 mg, 0.248 mmol) was stirred in THF (10 mL) for 1 hour then added to a flask charged with [Rh(COD)Cl]₂ (51.6 mg, 0.105 mmol) and KI (165.1 mg, 1.42 mmol). After a further hour the volatiles were removed in vacuo. The product was extracted with CH₂Cl₂ (2 × 10 mL) and purified over alumina (1 : 1 EtOAc/CH₂Cl₂, Air). Yield = 75.5 mg (53%, yellow powder).

¹ H NMR (CD₂Cl₂, 400 MHz): δ 7.95 (d, ⁴J_HH = 2.8, 2H, Ar), 7.29–7.39 (m, 2H, Ar), 7.04–7.14 (m, 2H, Ar), 6.97 (t, ³J_HH = 7.4, 2H, Ar), 6.44 (d, ³J_HH = 2.0, 2H, imid.), 6.19 (d, ³J_HH = 2.0, 2H, imid.), 5.92 (d, ⁴J_HH = 2.8, 2H, Ar), 5.24–5.29 (m, 2H, COD{CH}), 4.92–5.09 (m, 2H, COD{CH}), 4.62 (d, ²J_HH = 13.3, 2H, ArCH̲₂Ar), 4.50 (d, ²J_HH = 13.5, 2H, ArCH̲₂Ar), 3.99–4.18 (m, 4H, OCH₂), 3.85 (s, 6H, NCH₃), 3.65–3.82 (m, 4H, OCH₂), 3.38 (d, ²J_HH = 13.4, 2H, ArCH̲₂Ar), 3.21–3.26 (m, 2H, COD{CH}), 3.19 (d, ²J_HH = 13.7, 2H, ArCH̲₂Ar), 2.29–2.39 (m, 2H, COD{CH}), 2.09–2.29 (m, 6H, COD{CH₂}), 1.97–2.09 (m, 4H, CH̲₂CH₃), 1.83–1.97 (m, 6H, CH̲₂CH₃ + COD{CH₂}), 1.58–1.79 (m, 4H, COD{CH₂}), 1.27–1.46 (m, 4H, COD{CH₂}), 1.15 (t, ³J_HH = 7.4, 6H, CH₂CH̲₃), 0.95 (t, ³J_HH = 7.5, 6H, CH₂CH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 101 MHz): δ 180.4 (d, ¹J_RhC = 49, NCN), 158.2 (s, C̲OCH₂), 155.0 (s, C̲OCH₂), 137.0 (s, Ar{C̲CH₂}), 136.9 (s, Ar{C̲CH₂}), 134.9 (s, Ar{C}), 134.7 (s, Ar{C}), 134.3 (s, Ar{C}), 131.5 (s, Ar), 129.6 (s, Ar), 123.6 (s, Ar), 123.4 (s, imid.), 122.9 (s, Ar), 122.5 (s, imid.), 122.5 (s, Ar), 95.0 (d, ¹J_RhC = 7, COD{CH}), 95.0 (d, ¹J_RhC = 7, COD{CH}), 77.7 (s, OCH₂), 77.3 (s, OCH₂), 71.4 (d, ¹J_RhC = 14, 2 × COD{CH}), 39.1 (s, NC̲H₃), 33.9 (s, COD{CH₂}), 31.7 (s, CH₂), 31.4 (s, CH₂), 31.0 (s, CH₂), 30.7 (s, CH₂), 29.2 (s, COD{CH₂}), 23.8 (s, C̲H₂CH₃), 23.4 (s, C̲H₂CH₃), 11.3 (s, CH₂C̲H₃), 10.3 (s, CH₂C̲H₃). Anal. Calcd For C₆₄H₈₀I₂N₄O₄ (1428.24 g mol⁻¹): C, 53.79; H, 5.64; N, 3.92. Found: C, 53.71; H, 5.75; N, 4.04.

Preparation of 2b

A solution of IⁱPr₂Me₂ (61.2 mg, 0.336 mmol), [Rh(COD)Cl]₂ (166.1 mg, 0.337 mmol) and KI (1072.2 mg, 6.46 mmol) in THF (10 mL) was stirred for 1 hour. The volatiles were removed in vacuo and the product obtained after purification over silica (CH₂Cl₂, Air) and subsequent recrystallisation from CH₂Cl₂/pentane. Yield = 55.0 mg (31%, yellow crystals).

¹ H NMR (CD₂Cl₂, 500 MHz): δ 5.97 (sept, ³J_HH = 7.1, 2H, NCH), 5.03–5.09 (m, 2H, COD{CH}), 3.50–3.57 (m, 2H, COD{CH}), 2.26–2.36 (m, 4H, COD{CH₂}), 2.16 (s, 6H, CCH̲₃), 1.87–1.98 (m, 2H, COD{CH₂}), 1.73–1.83 (m, 2H, COD{CH₂}), 1.56 (d, ³J_HH = 7.1, 6H, CHCH̲₃), 1.50 (d, ³J_HH = 7.1, 6H, CHCH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 126 MHz): δ 178.4 (d, ¹J_RhC = 49, NCN), 126.0 (s, C̲CH₃), 95.2 (d, ¹J_RhC = 7, COD{CH}), 71.5 (d, ¹J_RhC = 14, COD{CH}), 53.7 (s, NCH), 32.8 (s, COD{CH₂}), 30.0 (s, COD{CH₂}), 22.4 (s, CHC̲H₃), 21.4 (s, CHC̲H₃), 10.7 (s, CC̲H₃). Anal. Calcd for C₁₉H₃₂I₂N₂Rh (518.29 g mol⁻¹): C, 44.03; H, 6.22; N, 5.41. Found: C, 44.11; H, 6.14; N, 5.47.

Preparation of 3a

Method A: A solution of 1·2HI (102.1 mg, 0.101 mmol) and K[O^tBu] (28.9 mg, 0.258 mmol) was stirred in THF (10 mL) for 1 hour then added to a flask charged with [Rh(CO)₂Cl]₂ (40.7 mg, 0.105 mmol) and KI (165.4 mg, 0.996 mmol). After a further hour the volatiles were removed in vacuo. The product was extracted with CH₂Cl₂ (2 × 10 mL) and purified over alumina (1 : 1 Et₂O/CH₂Cl₂, Air). Yield = 22.4 mg (17%, yellow powder). Method B: A solution of 2a (121.2 mg, 0.0848 mmol) in CH₂Cl₂ (10 mL) was stirred under CO (1 atm) for 12 hours, resulting in a colour change from bright to pale yellow. The mixture was dried in vacuo and washed with pentane (3 × 5 mL). Yield = 42.1 mg (37%, yellow powder).

¹ H NMR (CD₂Cl₂, 500 MHz): δ7.26 (s, 2H, Ar), 7.02 (br, 4H, Ar), 6.82 (t, ³J_HH = 7.4, 2H, Ar), 6.77 (s, 2H, imid.), 6.46 (s, 2H, imid.), 6.41 (s, 2H, Ar), 4.51 (d, ²J_HH = 13.3, 2H, ArCH̲₂Ar), 4.48 (d, ²J_HH = 13.6, 2H, ArCH̲₂Ar), 3.80–4.09 (m, 8H, OCH₂), 3.77 (s, 6H, NCH₃), 3.24 (d, ²J_HH = 14.1, 2H, ArCH̲₂Ar), 3.22 (d, ²J_HH = 14.1, 2H, ArCH̲₂Ar), 1.79–2.16 (m, 8H, CH̲₂CH₃), 1.03–1.13 (m, 6H, CH₂CH̲₃), 0.92–1.03 (m, 6H, CH₂CH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 126 MHz): δ 188.4 (d, ¹J_RhC = 54, CO), 181.6 (d, ¹J_RhC = 78, CO), 171.5 (d, ¹J_RhC = 44, NCN), 157.5 (s, C̲OCH₂), 156.3 (s, C̲OCH₂), 136.2, 136.1, 135.9, 135.6, 130.6, 129.4, 123.8, 123.4, 123.2, 122.8, 77.8 (OCH₂), 77.4(OCH₂), 40.0 (s, NCH₃), 31.5 (br, ArC̲H₂Ar), 24.0 (C̲H₂CH₃), 23.7 (C̲H₂CH₃), 11.0 (s, CH₂C̲H₃), 10.4 (s, CH₂C̲H₃). Not all signals were unambiguously identified. IR (CH₂Cl₂, cm⁻¹): ν(CO) 2071, 2002. Anal. Calcd for C₅₂H₅₆I₂N₄O₈Rh₂ (1324.02 g mol⁻¹): C, 47.15; H, 4.26; N, 4.23. Found: C, 46.91; H, 4.12; N, 4.25.

Preparation of 3b

A solution of 2b (40.1 mg, 0.0774 mmol) in CH₂Cl₂ (10 mL) was placed under an atmosphere of CO (1 atm) for 2 hours. The volatiles were removed in vacuo to afford the product, which was washed with pentane (3 × 10 mL) and dried thoroughly under vacuum. Yield = 21.4 mg (59%, fine yellow powder).

¹ H NMR (CD₂Cl₂, 500 MHz): δ 5.26 (sept, ³J_HH = 7.1, 2H, NCH), 2.22 (s, 6H, CCH₃), 1.501 (d, ³J_HH = 7.1, 6H, CHCH̲₃), 1.496 (d, ³J_HH = 2.7, 6H, CHCH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 126 MHz): δ 188.7 (d, ¹J_RhC = 53, CO), 182.7 (d, ¹J_RhC = 78, CO), 166.8 (d, ¹J_RhC = 41, NCN), 127.2 (s, C̲CH₃), 54.3 (s, C̲HCH₃), 22.2 (s, CHC̲H₃), 21.1 (s, CHC̲H₃), 10.7 (s, CC̲H₃). IR (CH₂Cl₂, cm⁻¹): ν(CO) 2072, 1998. Anal. Calcd for C₁₃H₂₀I₂N₂O₂Rh (466.12 g mol⁻¹): C, 33.50; H, 4.33; N, 6.01. Found: C, 33.41; H, 4.25; N, 6.03.

Preparation of 4a

A suspension of 1·2HI (79.5 mg, 0.0788 mmol), Ag₂O (20.0 mg, 0.0862 mmol) and Na[BAr^F₄] (69.0 mg, 0.0778 mmol) in CH₂Cl₂ (20 mL) was sonicated periodically over two hours. The solution was filtered and the solvent was removed in vacuo to afford the product. Yield = 100.0 mg (73%, white crystals).

¹ H NMR (400 MHz, CD₂Cl₂): δ 7.72–7.76 (m, 8H, Ar^F), 7.57 (br, 4H, Ar^F), 7.10 (d, ³J_HH = 7.4, 4H, Ar), 6.92 (d, ³J_HH = 1.7, 2H, imid.), 6.85 (t, ³J_HH = 7.5, 2H, Ar), 6.81 (d, ³J_HH = 1.7, 2H, imid.), 6.40 (s, 4H, Ar), 4.57 (d, ²J_HH = 12.8, 4H, ArCH̲₂Ar), 4.10–4.22 (m, 4H, OCH₂), 3.80 (s, 6H, NCH₃), 3.74 (t, ³J_HH = 7.1, 4H, OCH₂), 3.24 (d, ²J_HH = 12.8, 4H, ArCH̲₂Ar), 2.10–2.21 (m, 4H, CH̲₂CH₃), 1.99 (app. sex., J = 7, 4H, CH̲₂CH₃), 1.12 (t, ³J_HH = 7.4, 6H, CH₂CH̲₃), 0.98 (t, ³J_HH = 7.5, 6H, CH₂CH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 101 MHz): δ 180.7 (two coincident d, ¹J_109AgC = 211, ¹J_107AgC = 183, NCN), 162.3 (q, ¹J_BC = 50.5, Ar^F), 157.8 (C̲OCH₂), 156.7 (s, C̲OCH₂), 136.5 (s, 2 × Ar{C}), 135.4 (s, Ar^F), 134.3 (s, Ar{C}), 129.4 (qq, ²J_FC = 32, ³J_BC = 3, Ar^F), 129.3 (s, Ar), 118.0 (sept, ³J_FC = 4, Ar^F), 126.4 (s, Ar), 125.2 (q, ¹J_FC = 272, Ar^F), 124.7 (d, ³J_AgC = 6, imid.), 123.4 (s, Ar), 121.9 (d, ³J_AgC = 6, imid.), 118.0 (sept, ³J_FC = 4, Ar^F), 78.7 (s, OCH₂), 77.2 (s, OCH₂), 38.8 (d, ³J_AgC = 3, NCH₃), 31.4 (s, ArC̲H₂Ar), 24.0 (s, C̲H₂CH₃), 23.5 (s, C̲H₂CH₃), 11.0 (s, CH₂C̲H₃), 10.1 (s, CH₂C̲H₃). ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 859.3340 m/z, [M]⁺ (calcd 859.3347 m/z). Anal. Calcd For C₈₀H₆₈AgBF₂₄N₄O₄ (1724.09 g mol⁻¹): C, 55.73; H, 3.98; N, 3.25. Found: C, 55.85; H, 4.08; N, 3.33.

In situ reactions of 4b

A suspension of IⁱPr₂Me₂ (9.9 mg, 0.0549 mmol), Ag[OTf] (7.2 mg, 0.0277 mmol) and Na[BAr^F₄] (24.0 mg, 0.0271 mmol) in difluorobenzene (0.5 mL) was agitated for several minutes. Analysis in situ by NMR spectroscopy indicated quantitative formation of 4b (data below). Solutions of 4b, prepared in this way, where then filtered onto either [Rh(COD)Cl]₂ (13.6 mg, 0.0276 mmol) or [Rh(CO)₂Cl]₂ (10.8 mg, 0.0278 mmol), resulting in the formation of 5b : 7 (2 : 1) and 6b/8 (5 : 2).

¹ H NMR (1,2-C₆H₄F₂, 500 MHz): δ 8.11–8.13 (m, 8H, Ar^F), 7.49 (br, 4H, Ar^F), 4.20 (sept, ³J_HH = 6.8, 4H, NCH), 1.87 (s, 12H, CCH₃), 1.31 (d, ³J_HH = 6.8, 24H, CHCH̲₃). ¹³C{¹H} NMR (1,2-C₆H₄F₂, 126 MHz): δ162.5 (q, ¹J_BC = 50, Ar^F), 135.1 (s, Ar^F), 129.7 (qq, ²J_FC = 32, ³J_BC = 3, Ar^F), 124.9 (q, ¹J_FC = 272, Ar^F), 124 (obscured, C̲CH₃), 117.6 (sept, ³J_FC = 4, Ar^F), 50.1 (s, NCH), 23.3 (s, CHCH̲₃), 8.1 (s, CC̲H₃). The carbene resonance was not located. ¹⁹F{¹H} NMR (1,2-C₆H₄F₂, 282 MHz): δ-62.25 (Ar^F). No signal for [OTf]⁻ detected. ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 467.2297 m/z, [M]⁺ (calcd 467.2298 m/z).

Preparation of 5a

A solution of 4a (29.9 mg, 0.0173 mmol) and [Rh(COD)Cl]₂ (9.4 mg, 0.0191 mmol) in 1,2-C₆H₄F₂ (0.5 mL) was stirred for 1 hour with the exclusion of light. The solvent was removed in vacuo and the resulting crude material washed with pentane and dried in vacuo. Yield = 22.3 mg (63%, yellow powder). The product was characterised in situ, although has limited stability in solution.

¹ H NMR (1,2-C₆H₄F₂/C₆D₆, 500 MHz): δ 8.09–8.15 (m, 8H, Ar^F), 7.49 (br, 4H, Ar^F) 5.44 (br, 2H, COD{CH}), 5.38 (br, 2H, COD{CH}), 4.58 (d, ²J_HH = 13.6, 2H, ArCH̲₂Ar), 4.49 (d, ²J_HH = 13.6, 2H, ArCH̲₂Ar), 4.20 (s, 6H, NCH₃), 3.96–4.24 (m), 3.67–3.76 (m), 3.55–3.63 (m), 3.49–3.55 (m), 3.46 (d, ²J_HH = 13.6, 2H, ArCH̲₂Ar), 3.23–3.33 (m), 3.09 (d, ²J_HH = 13.6, 2H, ArCH̲₂Ar), 1.35–2.50 (m, CH₂), 1.03 (t, ³J_HH = 6.9, 6H, CH₂CH̲₃), 0.85 (t, ³J_HH = 6.9, 6H, CH₂CH̲₃), 0.7–1.1 (m, COD{CH₂}). Not all signals unambiguously assigned and some signals obscured by solvent peak. ¹³C{¹H} NMR (1,2-C₆H₄F₂/C₆D₆, 126 MHz, selected signals only): δ 176.9 (d, ¹J_RhC = 50, NCN), 100.3 (br, COD{CH}), 96.4 (br, COD{CH}), 78.5 (d, ¹J_RhC = 14, COD{CH}), 71.8 (d, ¹J_RhC = 13, COD{CH}), 97.7 (d, ¹J_RhC = 7, COD{CH}), 70.2 (d, ¹J_RhC = 16, COD{CH}), 37.8 (s, NCH₃). ESI-MS (CH₃CN, 180 °C, 3 kV): positive ion: 1209.3986 m/z, [M]⁺ (calcd 1209.3973 m/z).

Preparation of 5b

A solution of [Rh(IⁱPr₂Me₂)(COD)Cl] (23.6 mg, 0.0553 mmol) and Na[BAr^F₄] (24.5 mg, 0.0275 mmol) in CH₂Cl₂ (10 mL) was stirred for 1 hour. The solution was then filtered and layered with pentane to afford the product on diffusion. Yield = 32.0 mg (69%, yellow crystals).

¹ H NMR (CD₂Cl₂, 500 MHz): δ 7.70–7.74 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 5.91 (sept, ³J_HH = 7.1, 4H, NCH), 4.75 (s, 4H, COD{CH}), 3.51 (s, 4H, COD{CH}), 2.22–2.36 (m, 8H, COD{CH₂}), 2.11 (s, 12H, CCH₃), 1.81–1.93 (m, 8H, COD{CH₂}), 1.57 (d, ³J_HH = 7.0, 12H, CHCH̲₃), 1.36 (d, ³J_HH = 7.0, 12H, CHCH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 126 MHz): δ 175.6 (d, ¹J_RhC = 50, NCN), 162.3 (q, ¹J_BC = 51, Ar^F), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 31, ³J_BC = 3, Ar^F), 126.3 (s, C̲CH₃), 125.2 (q, ¹J_FC = 272, Ar^F), 118.0 (sept, ³J_FC = 4, Ar^F), 97.7 (d, ¹J_RhC = 7, COD{CH}), 70.2 (d, ¹J_RhC = 16, COD{CH}), 54.3 (s, NCH), 33.1 (s, COD{CH₂}), 29.1 (s, COD{CH₂}), 22.5 (s, CHC̲H₃), 22.2 (s, CHC̲H₃), 10.5 (s, CC̲H₃). ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 817.2872 m/z, [M]⁺ (calcd 817.2924 m/z). Anal. Calcd For C₇₀H₇₆BF₂₄N₄Rh₂Cl (1434.39 g mol⁻¹): C, 49.97; H, 4.56; N, 3.33. Found: C, 50.18; H, 4.52; N, 3.23.

Preparation of 6a

A solution of 4a (32.0 mg, 0.0186 mmol) and [Rh(CO)₂Cl]₂ (7.8 mg, 0.0201 mmol) in 1,2-C₆H₄F₂ (10 mL) was stirred for one hour with the exclusion of light. The cloudy yellow solution was filtered, reduced to dryness and the resulting residue washed with pentane. Yield = 21.4 mg (31%, yellow powder). The product was characterised in situ, although has limited stability in solution.

¹ H NMR (1,2-C₆H₄F₂/C₆D₆, 500 MHz): δ 8.11–8.15 (m, 8H, Ar^F), 7.49 (br, 4H, Ar^F), 4.52 (d, ²J_HH = 12.6, 2H, ArCH̲₂Ar), 4.51 (d, ²J_HH = 12.6, 2H, ArCH̲₂Ar), 4.05–4.19 (m, 2H, OCH₂), 3.88–3.99 (m, 2H, OCH₂), 3.84 (s, 6H, NCH₃), 3.51–3.65 (m, 4H, OCH₂), 3.18 (d, ²J_HH = 12.6, 2H, ArCH̲₂Ar), 3.15 (d, ²J_HH = 12.6, 2H, ArCH̲₂Ar), 2.11–2.23 (m, 4H, CH̲₂CH₃), 1.79–1.92 (m, 4H, CH̲₂CH₃), 0.94 (t, ³J_HH = 7.5, 6H, CH₂CH̲₃), 0.93 (t, ³J_HH = 7.5, 6H, CH₂CH̲₃). Some signals obscured by solvent peak. ¹³C{¹H} NMR (1,2-C₆H₄F₂/C₆D₆, 126 MHz, selected signals only): δ 182.7 (d, ¹J_RhC = 53, CO), 181.0 (d, ¹J_RhC = 80, CO), 168.1 (d, ¹J_RhC = 43, NCN), 36.3 (s, NCH₃). ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 1105.1680 m/z, [M]⁺ (calcd 1105.1891 m/z). IR (CH₂Cl₂, cm⁻¹): ν(CO) 2083, 2016.

Preparation of 6b

A solution of [Rh(IⁱPr₂Me₂)(CO)₂Cl] (26.5 mg, 0.709 mmol) and Na[BAr^F₄] (32.6 mg, 0.368 mmol) in CH₂Cl₂ (5 mL) was stirred for one hour. The solution was then filtered and layered with pentane to afford the product on diffusion. Yield = 37.1 mg (66%, yellow crystals).

¹ H NMR (CD₂Cl₂, 400 MHz): δ 7.67–7.77 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 5.20 (sept, ³J_HH = 7.1, 4H, NCH), 2.18 (s, 12H CCH̲₃), 1.55 (d, ³J_HH = 7.1, 12H, CHCH̲₃), 1.50 (d, ³J_HH = 7.0, 12H, CHCH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 101 MHz): δ 185.4 (d, ¹J_RhC = 53, CO), 181.0 (d, ¹J_RhC = 84, CO), 162.3 (q, ¹J_BC = 50, Ar^F), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_BC = 3, Ar^F), 128.0 (s, CCH₃), 125.2 (q, ¹J_FC = 272, Ar^F), 54.8 (s, NCH), 22.8 (s, CHC̲H₃)₂, 22.4 (s, CHC̲H₃)₂, (s, 10.6 CC̲H₃). The carbene resonance was not located. ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 713.0829 m/z, [M]⁺ (calcd 713.0843 m/z). IR (CH₂Cl₂, cm⁻¹): ν(CO) 2090, 2019. Anal. Calcd For C₅₈H₅₂BClF₂₄N₄O₄Rh₂Cl (1577.11 g mol⁻¹): C, 44.17; H, 3.32; N, 3.55. Found: C, 44.51; H, 3.57; N, 3.49.

Preparation of 7

A solution of IⁱPr₂Me₂ (50.8 mg, 0.282 mmol), [Rh(COD)Cl]₂ (34.3 mg, 0.0696 mmol) and Na[BAr^F₄] (123.4 mg, 0.139 mmol) was stirred in 1,2-C₆H₄F₂ (5 mL) for 2.5 h. The solution was then filtered and layered with pentane to afford the product on diffusion. Yield = 114.0 mg (56%, yellow crystals).

¹ H NMR (CD₂Cl₂, 500 MHz): δ 7.71–7.75 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 5.56 (sept, ³J_HH = 7.1, 4H, NCH), 4.27 (s, 4H, COD{CH}), 2.31–2.40 (m, 4H, COD{CH₂}), 2.16 (s, 12H, CCH₃), 2.01–2.11 (m, 4H, COD{CH₂}), 1.55 (d, ³J_HH = 7.1, 12H, CHCH̲₃), 1.22 (d, ³J_HH = 7.1, 12H, CHCH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 126 MHz): δ 178.1 (d, ¹J_RhC = 55.8, NCN), 162.3 (q, ¹J_BC = 50, Ar^F), 135.4 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_BC = 3, Ar^F), 127.1 (s, C̲CH₃), 125.2 (q, ¹J_FC = 272, Ar^F), 118.0 (sept, ³J_FC = 4, Ar^F), 87.2 (d, ¹J_RhC = 8, COD{CH}), 54.5 (s, NCH), 31.8 (s, COD{CH}), 22.9 (s, CHC̲H₃), 21.6 (s, CHC̲H₃), 10.9 (s, CC̲H₃). ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 571.3215 m/z [M]⁺ (calcd 571.3242 m/z). Anal. Calcd For C₆₂H₆₄BF₂₄N₄Rh (1434.39 g mol⁻¹): C, 51.96; H, 4.49; N, 3.90. Found: C, 52.05; H, 4.59; N, 3.93.

Preparation of 8

A solution of 7 (113.9 mg, 0.0794 mmol) in 1,2-C₆H₄F₂ (5 mL) was stirred under an atmosphere of CO (1 atm) for 24 hours. The solvent was removed in vacuo and the resulting crude material washed with pentane and recrystallised from CH₂Cl₂/pentane. Yield = 72.4 mg (66%, orange powder).

¹ H NMR (CD₂Cl₂, 500 MHz): δ 7.71–7.75 (m, 8H, Ar^F), 7.56 (br, 4H, Ar^F), 5.05 (sept, ³J_HH = 7.1, 4H, NCH), 2.20 (s, 12H, CCH₃), 1.46 (d, ³J_HH = 7.1, 12H, CHCH̲₃), 1.19 (d, ³J_HH = 7.1, 12H, CHCH̲₃). ¹³C{¹H} NMR (CD₂Cl₂, 126 MHz): δ 186.9 (d, ¹J_RhC = 57, CO), 168.2 (d, ¹J_RhC = 47, NCN), 162.3 (q, ¹J_BC = 50, Ar^F), 135.3 (s, Ar^F), 129.4 (qq, ²J_FC = 32, ³J_BC = 3, Ar^F), 128.2 (s, C̲CH₃), 125.2 (q, ¹J_FC = 272, Ar^F), 118.0 (sept, ³J_FC = 4, Ar^F), 55.3 (s, NCH), 22.0 (s, CHC̲H₃), 21.1 (s, CHC̲H₃), 10.9 (s, CC̲H₃). ESI-MS (CH₃CN, 180 °C, 3 kV) positive ion: 519.2249 m/z [M]⁺ (calcd 520.2201 m/z). IR (CH₂Cl₂, cm⁻¹): ν(CO) 2077, 2018. Anal. Calcd For C₅₆H₅₂BF₂₄N₄Rh (1382.29 g mol⁻¹): C, 48.64; H, 3.79; N, 4.05. Measured: C, 48.51; H, 3.63; N, 4.15.

Crystallography

Full details about the collection, solution and refinement are documented in the CIF, which have been deposited with the Cambridge Crystallographic Data Centre under CCDC 1448987–1448996.

Acknowledgements

We thank the Royal Society (A. B. C.), EPSRC (R. P.) and University of Warwick for financial support. Crystallographic and high-resolution mass-spectrometry data were collected using instruments purchased through support from Advantage West Midlands and the European Regional Development Fund.

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR, IR and ESI-MS spectra of new complexes. CCDC 1448987–1448996. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt01001f

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