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).

Abstract

Reactions of the homovalent bis(benzoato)digqld(II) complexes [Aun2(O2CPh)2(//-2-C6H4PR2)2] (R = Ph (la), Et (lb)) with the anions CH3-, C6F5~, and SON'- have been investigated. Dimethylmagnesium gives the stable heterovalent complexes [AuI(H-2-C6H4PR2)2AuIII(CH3)2]. (R = Ph (2a), Et (2b)), whereas (pentafluorophenyl)lithium gives the homovalent complexes " [Aun2(C6F5)2(-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) [AuCeFsizO'-'PCeHiCeHr PR2)1 (R = Ph (4a), Et (4b)), resulting from coupling of the C6H4PR2 units. Minor products of these rearrangements are the zwitterionic heterovalent complexes [(CeFsV-CeH PR2)2Au1] (R = Ph (5a), Et (5b)). The structure of 5a differs from that of 2a by rotation of one of the bridging 2-CeH4PPh2 groups through 180°. Precursors to 4a/5a and 4b/5b have been detected by 31P NMR spectroscopy and tentatively assigned the heterovalent structures (C6F5)AuI(//-2-R2PC6H4)AuIII(C6F5)(?;2-2-C6H4PR2)] (R = Fh (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 [Aun2(SCN)2(//-2-C6H4PR2)2] (R = Ph (8a), Et (8b)). formed from la or lb and KSCN undergo C-C coupling more'rapidly than the , CGF5 compounds to give [AuI2(SCN)2(-2,2'-R2PC6H4C6H4PR2)] (R = Ph (10a), Et (lOb)) via the detectable heterovalent intermediates 9a or 9b, which are analogues of 6a and 6b. The X-ray structures of complexes 2a, 3a, 4a, 4b, 5a, and lOb have been determined. The gold- ? gold separations in 2a, 3a, and 5a are respectively 2.8874(4), 2.6139(4), and 2.931(1)72.921(1) A (for independent molecules), the shorter distance in 3a corresponding to a covalent metal-metal bond. In 4a and lOb the torsion about the central C-C. bond of the biphenyl backbone results in a syn configuration for the Au-X (X = CeFs, SON) fragments and a close intramolecular aurophilic contact between the gold atoms (r(Au-Au) =,3.0688(8) A (4a), 3.0853(9) a(lOb)), whereas in 4b the Au-CeFs units adopt an anti configuration relative to the biphenyl fragment, leading to a nonbonding gold-gold separation of 5.3469(7) A. The differences in behavior of digold(II) complexes [Au2X2(-2-C6H4PR2)2] as the axial anionic ligand X is varied and the pathway of the intramolecular C-C coupling reaction are discussed.