Synthesis, structure, and reactions of a binuclear gold(I)-gold(III) complex containing bridging and bidentate (2-diphenylphosphino-6-methyl)phenyl groups

Reaction of the organolithium reagents (C6H3-2-PPh2-6-Me)Li or (C6H3-2-PPh2-5-Me)Li with [AuBr(PEt3)] gives the corresponding cyclometalated digold(I) complexes [Au-2(mu -C6H3-2-PPh2-n-Me)(2)] (n = 6, la; n = 5, Ib), the metal-metal distance in la, 2.861(2) A, being similar to that in the unsubstituted compound [Au-2(mu -2-C6H4PPh2)(2)] Both complexes oxidatively add halogens to give metal-metal bonded digold(II) complexes [Au2X2(mu -C6H3-2-PPh2-n-Me)(2)] [n = 6, X = Cl (2a), Br (3a), I(4a); n = 5, X = C1 (2b), Br (3b); I(4b)], and Ib also adds dibenzoyl peroxide to give the bis(benzoato)digold(II) complex [Au-2(O2CPh)(2)(mu -C6H3-2-PPh2-5-Me)(2)] (5b), whereas la is unreactive. The behavior of the 6- and 5-methyl-substituted digold(II) complexes in solution is very different. Complexes 2a-4a isomerize in solution above -20 degreesC to give gold(I)-gold(III) complexes [XAuI(mu -2-Ph2PC6H3-6-Me)(AuX)-X-III{eta (2)-(C6H3-2-PPh2-6-Me)}] [X = Cl (6a), Br (7a), I(8a)], Whereas complexes 2b-4b isomerize more slowly in solution by C-C coupling to give digold(I) complexes [Au2X2{mu -2, 2'-Ph2P(5,5'-Me2C6H3C6H3)PPh2}] [X = Cl (6b), Br (7b), I (8b)] containing (5,5'-dimethyl-2,2'-biphenylyl)bis(diphenylphosphine) (12b); the:structures of 8a and 8b have been determined by X-ray crystallography. Complex 8a contains linearly coordinated gold(I) and planar coordinated gold(III) atoms. separated by 3.4692(7) Angstrom one arylphosphine group bridges the metal atoms, the other acts as a bidentate chelate ligand to gold(III). Complexes 6a-8a can be oxidized further by halogens to give either gold(I) complexes of(2-halo-3-methylphenyl)diphenylphosphine, [AuX{Ph2P(C6H3-2-X-3-Me)] [X = Br (10a); I (11a)], arising from electrophilic cleavage of the Au-C bonds, or a binuclear digold(III) complex [Cl3Au(mu -C6H3-2-PPh2-6-Me)AuCl{eta (2)-(C6B3-2-PPh2-6-Me)}] (9a) in which the gold(III)-carbon bonds are retained. The differences in oxidative addition behavior between the 6- and 5-methyl-substituted series of compounds are believed to be caused mainly by the steric effect of the 6-methyl group adjacent to the gold-gold axis.