Synthesis, structures and reactions of cyclometallated gold complexes containing (2-diphenylarsino-n-methyl)phenyl (n = 5, 6)

Reaction of (C6H3-2-AsPh2-n-Me)Li (n = 5 or 6) with AuBr(AsPh3)] at -78 degrees C gives the corresponding cyclometallated gold(I) complexes Au2(mu-C6H3-n-Me)AsPh2]2] n = 5, (1); n = 6, (9)]. 1 undergoes oxidative addition with halogens and with dibenzoyl peroxide to give digold(II) complexes Au2X2(mu-C6H3-5-Me)AsPh2]2] X = Cl (2a), Br (2b), I (2c) and O2CPh (3)] containing a metal-metal bond between the 5d9 metal centres. Reaction of 2a with AgO2CMe or of 3 with C6F5Li gives the corresponding digold(II) complexes in which X = O2CMe (4) and C6F5 (6), respectively. The Au-Au distances increase in the order 4 < 2a < 2b < 2c < 6, following the covalent binding tendency of the axial ligand. Like the analogous phosphine complexes, 2a-2c and 6 in solution rearrange to form C-C coupled digold(I) complexes Au2X2mu-2,2-Ph2As(5,5-Me2C6H3C6H3)AsPh2]] X = Cl (5a), X = Br (5b), X = I (5c) and C6F5 (7)] in which the gold atoms are linearly coordinated by As and X. In contrast, the products of oxidative additions to 9 depend markedly on the halogens. Reaction of 9 with chlorine gives the gold(I)-gold(III) complex, ClAumu-2-Ph2As(C6H3-6-Me)]AuCl(6-MeC6H3)-2-AsPh2]-kappa2As,C] (10), which contains a four-membered chelate ring, Ph2As(C6H3-6-Me), in the coordination sphere of the gold(III) atom. When 10 is heated, the ring is cleaved, the product being the digold(I) complex ClAumu-2-Ph2As(C6H3-6-Me)]AuAsPh2(2-Cl-3-Me-C6H3)]] (11). Reaction of 9 with bromine at 50 degrees C gives a monobromo digold(I) complex (12), which is similar to 11 except that the 2-position of the substituted aromatic ring bears hydrogen instead halogen. Reaction of 9 with iodine gives a mixture of a free tertiary arsine, (2-I-3-MeC6H3)AsPh2 (13), a digold diiodo compound (14) analogous to 11, and a gold(I)-gold(III) zwitterionic complex I2Au(III)(mu-C6H3-2-AsPh2-6-Me)]2Au(I)] (15) in which the bridging units are arranged head-to-head between the metal atoms. The structures of 2a-2c and 4-15 have been determined by single-crystal X-ray diffraction analysis. The different behaviour of 1 and 9 toward halogens mirrors that of their phosphine analogues; the 6-methyl substituent blocks C-C coupling of the aryl residues in the initially formed oxidative addition product. In the case of 9, the greater lability of the Au-As bond in the initial oxidative addition product may account for the more complex behaviour of this system compared with that of its phosphine analogue.