Dinuclear Cycloaurated Complexes Containing Bridging (2-Diphenylphosphino)phenylphosphine and (2-Diethylphosphino)phenylphosphine, C6H4PR2 (R = Ph, Et). Carbon−Carbon Bond Formation by Reductive Elimination at a Gold(II)−Gold(II) Center

The digold(I) complexes Au2(μ-C6H4PR2)2 [R = Ph (1a), Et (1b)] obtained by treatment of AuBr(PEt3) with o-LiC6H4PR2 undergo addition with halogens or benzoyl peroxide to give metal−metal bonded digold(II) complexes Au2X2(μ-C6H4PR2)2 [R = Ph, Et; X = I (2a, 2b), Br (3a, 3b), Cl (4a, 4b), O2CPh (5a, 5b)], which are structurally similar to the bis(ylide) complexes Au2X2{μ-(CH2)2PR2}2. The benzoate ligands in 5b are monodentate and the gold−gold bond length [2.5243(7) Å] is significantly less than that in the diiodide (2a) [2.5898(6) Å, 2.5960 (Å) for independent molecules], reflecting the trans influences of the axial anionic ligands. The corresponding complexes Au2X2(μ-C6H4PR2)2 [R = Ph, Et; X = O2CMe (6a, 6b), ONO2 (7a, 7b)] are made from 2−4 and the appropriate silver salt. The axial anionic ligands undergo immediate scrambling when solutions of Au2X2(μ-C6H4PR2)2 and Au2Y2(μ-C6H4PR2)2 are mixed. The bridging C6H4PR2 units also scramble rapidly on mixing solutions of Au2X2(μ-C6H4PPh2)2 [X = I (2a), Br (3a)] and Au2X2(μ-C6H4PEt2)2 [X = I (2b), Br (3b)], but this occurs only slowly for X = Cl and not at all for X = O2CPh, O2CMe, or ONO2. Solutions of the diiodo complexes 2a, 2b and the dibromo complexes 3a, 3b isomerize cleanly to the digold(I) complexes Au2X2(μ-R2PC6H4C6H4PR2) [R = Ph, Et; X = I (8a, 8b), Br (9a, 9b)] containing 2,2‘-biphenylyl(diphenylphosphine) or 2,2‘-biphenylyl(diethylphosphine), respectively, as a consequence of a reductive elimination in which a C−C bond is formed at the expense of two Au−C bonds. In 8b the Au−Au separation is 3.167(1) Å and the phenyl rings of the biphenyl unit are almost orthogonal. Qualitatively, the rates of isomerization of Au2X2(μ-C6H4PR2)2 to Au2X2(μ-R2PC6H4C6H4PR2) are in the order R = Ph > Et; X = I > Br >> Cl; isomerization does not occur for X = O2CPh, O2CMe, or ONO2. The rates of thermal isomerization of 2a and 3a are first order in complex, only slightly sensitive to solvent polarity, and, for 2a, inhibited by iodide ion. It is suggested that reversible loss of halide ion initiates aryl group transfer between the gold atoms, thus allowing reductive elimination of Au−C bonds to take place at one center. Treatment of 2a or 3a with an excess of iodine or bromine gives initially digold(III) complexes cis,trans-Au2X4(μ-C6H4PPh2)2 [X = I (14), Br (15)], which are in equilibrium with monomers AuX2(C6H4PPh2) [X = I (16), Br (17)], as shown by 31P NMR spectroscopy. These species isomerize at room temperature by int