Indexed on: 04 Sep '09Published on: 04 Sep '09Published in: Dalton Transactions
Treatment of [AuXL] (X = Br, L = AsPh(3); 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(mu-carbanion)2] (carbanion = 2-C6F4PPh2 2, C6H3-5-F-2-PPh2 3, C6H3-6-F-2-PPh2 4) 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(mu-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)(mu-P,C-carbanion)(kappa2-P,C-carbanion)Au(III)X] (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(mu-2-C6F4PPh2)2] (Y = OAc 14, ONO2 15, OBz 16, O2CCF3 17 and OTf 20), which do not isomerise to the corresponding gold(I)-gold(III) complexes [YAu(mu-2-C6F4PPh2)(kappa2-2-C6F4PPh2)AuY] on heating, though the latter [Y = OAc 35, ONO2 36, OBz 37, O2CCF3 38] 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)(mu-2-C6F4PPh2)2Au(III)(CH3)2] 19, in which both 2-C6F4PPh2 groups are bridging. In contrast, the corresponding reaction of 16 with C6F5Li gives a digold(II) complex [Au(II)2(C6F5)2(mu-2-C6F4PPh2)2] 18, which on heating isomerises to a gold(I)-gold(III) complex, [(C6F5)Au(I)(mu-2-C6F4PPh2)(kappa2-2-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(mu-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(mu-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(mu-2-C6F4PPh2)2] (X = Cl 39, Br 40) and [X3Au(mu-2-C6F4PPh2)AuX(kappa2-2-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) A 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 A suggestive of aurophilic interactions, but these are probably absent in the digold(III) compounds (Au...Au separation ca. 5.8 A). Attempted recrystallisation of complex 10 gave a trinuclear gold(II)-gold(II)-gold(I) complex, [Au3Br2(mu-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 sigma-carbon bonded (C6H3-6-F-2-PPh2)Au(I)Br fragment.