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 [AuII2(O2CPh)2(μ-2-C6H4PR2)2] (R = Ph (1a), Et (1b)) with the anions CH3-, C6F5-, and SCN- have been investigated.
Dimethylmagnesium gives the stable heterovalent complexes [AuI(μ-2-C6H4PR2)2AuIII(CH3)2]
(R = Ph (2a), Et (2b)), whereas (pentafluorophenyl)lithium gives the homovalent complexes
[AuII2(C6F5)2(μ-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) [AuI2(C6F5)2(μ-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)2AuIII(μ-2-C6H4PR2)2AuI] (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°. Precursors to 4a/5a and 4b/5b have
been detected by 31P NMR spectroscopy and tentatively assigned the heterovalent structures
(C6F5)AuI(μ-2-R2PC6H4)AuIII(C6F5)(η2-2-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 [AuII2(SCN)2(μ-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 [AuI2(SCN)2(μ-2,2‘-R2PC6H4C6H4PR2)] (R = Ph (10a), Et (10b)) 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 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) Å (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) Å
(4a), 3.0853(9) Å (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) Å. The
differences in behavior of digold(II) complexes [Au2X2(μ-2-C6H4PR2)2] as the axial anionic
ligand X is varied and the pathway of the intramolecular C−C coupling reaction are
discussed.