Synthesis and interconversions of digold(i), tetragold(i), digold(ii), gold(i)–gold(iii) and digold(iii) complexes of fluorine-substituted aryl carbanions

Abstract

Treatment of [AuXL] (X = Br, L = AsPh₃; X = Cl, L = tht) with the lithium or trimethyltin derivatives of the carbanions [2-C₆F₄PPh₂]⁻ and [C₆H₃-n-F-2-PPh₂]⁻ (n = 5, 6) gives digold(I) complexes [Au₂(μ-carbanion)₂] (carbanion = 2-C₆F₄PPh₂2, C₆H₃-5-F-2-PPh₂3, C₆H₃-6-F-2-PPh₂4) which, like their 2-C₆H₄PPh₂ counterpart, undergo oxidative addition with halogens X₂ (X = Cl, Br, I) to give the corresponding, metal-metal bonded digold(II) complexes [Au₂X₂(μ-carbanion)₂] (carbanion = 2-C₆F₄PPh₂, X = Cl 5, Br 8, I 11; carbanion = C₆H₃-5-F-2-PPh₂, X = Cl 6, Br 9, I 12; carbanion = C₆H₃-6-F-2-PPh₂, X = Cl 7, Br 10, I 13). In the case of 2-C₆F₄PPh₂ and C₆H₃-6-F-2-PPh₂, the dihalodigold(II) complexes rearrange on heating to isomeric gold(I)–gold(III) complexes [XAu^I(μ-P,C-carbanion)(κ²-P,C-carbanion)Au^IIIX] (carbanion = 2-C₆F₄PPh₂, X = Cl 25, Br 26, I 27; carbanion = C₆H₃-6-F-2-PPh₂, X = Cl 28, Br 29, I 30), in which one of the carbanions chelates to the gold(III) atom. This isomerisation is similar to, but occurs more slowly than, that in the corresponding C₆H₃-6-Me-2-PPh₂ system. The Au₂X₂ complexes 6, 9 and 12, on the other hand, rearrange on heating via C–C coupling to give digold(I) complexes of the corresponding 2,2′-biphenyldiylbis(diphenylphosphine), [Au₂X₂(2,2′-Ph₂P-5-F-C₆H₃C₆H₃-5-F-PPh₂)] (X = Cl 32, Br 33, I 34), this behaviour resembling that of the 2-C₆H₄PPh₂ and C₆H₃-5-Me-2-PPh₂ systems. Since the C–C coupling probably occurs via undetected gold(I)–gold(III) intermediates, the presence of a 6-fluoro substituent is evidently sufficient to suppress the reductive eliminations, possibly because of an electronic effect that strengthens the gold(III)-aryl bond. Anation of 5 or 8 gives the bis(oxyanion)digold(II) complexes [Au₂Y₂(μ-2-C₆F₄PPh₂)₂] (Y = OAc 14, ONO₂15, OBz 16, O₂CCF₃17 and OTf 20), which do not isomerise to the corresponding gold(I)–gold(III) complexes [YAu(μ-2-C₆F₄PPh₂)(κ²-2-C₆F₄PPh₂)AuY] on heating, though the latter [Y = OAc 35, ONO₂36, OBz 37, O₂CCF₃38] can be made by anation of 25–27. Reaction of the bis(benzoato)digold(II) complex 16 with dimethylzinc gives a dimethyl gold(I)–gold(III) complex, [Au^I(μ-2-C₆F₄PPh₂)₂Au^III(CH₃)₂] 19, in which both 2-C₆F₄PPh₂groups are bridging. In contrast, the corresponding reaction of 16 with C₆F₅Li gives a digold(II) complex [Au^II₂(C₆F₅)₂(μ-2-C₆F₄PPh₂)₂] 18, which on heating isomerises to a gold(I)–gold(III) complex, [(C₆F₅)Au^I(μ-2-C₆F₄PPh₂)(κ²-2-C₆F₄PPh₂)Au^III(C₆F₅)] 31, analogous to 25–27. The bis(triflato)digold(II) complex 20 is reduced by methanol or cyclohexanol in CH₂Cl₂ to a tetranuclear gold(I) complex [Au₄(μ-2-C₆F₄PPh₂)₄] 21 in which the four carbanions bridge a square array of metal atoms, as shown by a single-crystal X-ray diffraction study. The corresponding tetramers [Au₄(μ-C₆H₃-n-F-2-PPh₂)₄] (n = 5 22, 6 23) are formed as minor by-products in the preparation of dimers 3 and 4; the tetramers do not interconvert readily with, and are not in equilibrium with, the corresponding dimers 2–4. Addition of an excess of chlorine or bromine (X₂) to the digold(II) complexes 5 and 8, and to their gold(I)–gold(III) isomers 25 and 26, gives isomeric digold(III) complexes [Au₂X₄(μ-2-C₆F₄PPh₂)₂] (X = Cl 39, Br 40) and [X₃Au(μ-2-C₆F₄PPh₂)AuX(κ²-2-C₆F₄PPh₂)] (X = Cl 41, Br 42), respectively. The structures of the digold(I) complexes 2, 4 and 32, the digold(II) complexes 5–11 and 14–18, the gold(I)–gold(III) complexes 19, 25, 35 and 38, the tetragold(I) complexes 21 and 22, and the digold(III) complexes 41 and 42, have been determined by single-crystal X-ray diffraction. In the digold(II) (5d⁹–5d⁹) series, there is a systematic lengthening, and presumably weakening, of the Au–Au distance in the range 2.5012(4)–2.5885(2) Å with increasing trans-influence of the axial ligand, in the order X = ONO₂ < O₂CCF₃ < OBz < Cl < Br < I < C₆F₅. The strength of the Au–Au interaction is probably the main factor that determines whether the digold(II) compounds isomerise to gold(I)–gold(III). The gold-gold separations in the digold(I) and gold(I)–gold(III) complexes are in the range 2.8–3.6 Å suggestive of aurophilic interactions, but these are probably absent in the digold(III) compounds (Au⋯Au separation ca. 5.8 Å). Attempted recrystallisation of complex 10 gave a trinuclear gold(II)–gold(II)–gold(I) complex, [Au₃Br₂(μ-C₆H₃-6-F-2-PPh₂)₃] 24, which consists of the expected digold(II) framework in which one of the axial bromide ligands has been replaced by a σ-carbon bonded (C₆H₃-6-F-2-PPh₂)Au^IBr fragment.