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

Abstract

Reaction of (C₆H₃-2-AsPh₂-n-Me)Li (n = 5 or 6) with [AuBr(AsPh₃)] at −78 °C gives the corresponding cyclometallated gold(I) complexes [Au₂{(µ-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{(µ-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 [Au₂X₂{µ-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, [ClAu{µ-2-Ph₂As(C₆H₃-6-Me)}AuCl{(6-MeC₆H₃)-2-AsPh₂}-κ²As,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 [ClAu{µ-2-Ph₂As(C₆H₃-6-Me)}Au{AsPh₂(2-Cl-3-Me-C₆H₃)}] (11). Reaction of 9 with bromine at 50 °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{(µ-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.