Carbon-carbon coupling in dinuclear cycloaurated complexes containing bridging 2-(diphenylphosphino)phenyl or 2-(diethylphosphino)phenyl. Role of the axial ligand and the fine balance between gold(II)-gold(II) and gold(I)-gold(III)

Reactions of the homovalent bis(benzoato)digold(II) complexes [Au-2(II)(O2CPh)(2)(mu -2-C6H4PR2)(2)] (R = Ph (1a), Et (1b)) with the anions CH3-, C6F5-, and SCN- have been investigated. Dimethylmagnesium gives the stable heterovalent complexes [(AuCu)-Cu-I-2-C6H4PR2)(2)Au-III(CH3)(2)] (R = Ph (2a), Et (2b)), whereas (pentafluorophenyl)lithium gives the homovalent complexes [Au-2(II)(C6F5)(2)(mu -2-C6H4PR2)(2)] (R = Ph (3a), Et (3b)). On prolonged heating in toluene, the latter rearrange to give predominantly dinuclear bis(pentafluorophenyl)digold(I) complexes of the corresponding (2,2'-biphenylyl)bis(tertiary phosphines) [Au-2(I)(C6F5)(2)(mu -2,2'-R2PC6H4C6H4PR2)] (R = Ph (4a), Et (4b)), resulting from coupling of the C6H4PR2 units. Minor products of these rearrangements are the zwitterionic heterovalent complexes [(C6F5)(2)Au-III(mu -2-C6H4PR2)(2)Au-I] (R = Ph (5a), Et (5b)). The structure of 5a differs from that of 2a by rotation of one of the bridging 2-C6H4PPh2 groups through 180 degrees. Precursors to 4a/5a and 4b/5b have been detected by P-31 NMR spectroscopy and tentatively assigned the heterovalent structures (C6F5)Au-I(mu -2-R2PC6H4)Au-III(C6F5)(eta (2)-2-C6H4PR2)] (R = Ph (6a), Et (6b)), in which one C6H4PR2 group bridges the two gold atoms while the other binds as a bidentate chelate ligand to gold(III). The S-bonded bis(thiocyanato) complexes [Au-2(II)(SCN)(2)(mu -2-C6H4PR2)(2)] (R = Ph (8a), Et (8b)) formed from 1a or 1b and KSCN undergo C-C coupling more rapidly than the C6F5 compounds to give [Au-2(I)(SCN)(2)(mu -2,2'-RaPC6H4C6H4PR2)] (R = Ph (10a), Et (10b)) via the detectable heterovalent intermediates so or 9b, which are analogues of 6a and 6b. The X-ray structures of complexes 2a, 3a, 4a, 4b, 5a, and 10b have been determined. The gold-gold separations in 2a, 3a, and 5a are respectively 2.8874(4), 2.6139(4), and 2.931(1)/2.921(1) Angstrom (for independent molecules), the shorter distance in 3a corresponding to a covalent metal-metal bond. In 4a and 10b the torsion about the central C-C bond of the biphenyl backbone results in a syn configuration for the Au-X (X = C6F5, SCN) fragments and a close intramolecular aurophilic contact between the gold atoms (r(Au . . . Au) = 3.0688(8) Angstrom (4a), 3.0853(9) Angstrom (10b)), whereas in 4b the Au-C6F5 units adopt an anti configuration relative to the biphenyl fragment, leading to a nonbonding gold-gold separation of 5.3469(7) Angstrom. The differences in behavior of digold(II) complexes [Au2X2(mu -2-C6H4PR2)(2)] as the axial anionic ligand X is varied and the pathway of the intramolecular C-C coupling reaction are discussed.