Carbon-carbon coupling in dinuclear cycloaurated complexes containing bridging 2-(diphenylphosphino)phenyl or 2-(diethylphosphino)phenyl. Role of theaxial ligand and the fine balance between gold(II)-gold(II) and gold(I)-gold(III)
Ma. Bennett et al., Carbon-carbon coupling in dinuclear cycloaurated complexes containing bridging 2-(diphenylphosphino)phenyl or 2-(diethylphosphino)phenyl. Role of theaxial ligand and the fine balance between gold(II)-gold(II) and gold(I)-gold(III), ORGANOMETAL, 20(1), 2001, pp. 79-87
Reactions of the homovalent bis(benzoato)digold(II) complexes [Au-2(II)(O2C
Ph)(2)(mu -2-C6H4PR2)(2)] (R = Ph (1a), Et (1b)) with the anions CH3-, C6F5
-, and SCN- have been investigated. Dimethylmagnesium gives the stable hete
rovalent 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 b
is(pentafluorophenyl)digold(I) complexes of the corresponding (2,2'-bipheny
lyl)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 p
roducts of these rearrangements are the zwitterionic heterovalent complexes
[(C6F5)(2)Au-III(mu -2-C6H4PR2)(2)Au-I] (R = Ph (5a), Et (5b)). The struct
ure of 5a differs from that of 2a by rotation of one of the bridging 2-C6H4
PPh2 groups through 180 degrees. Precursors to 4a/5a and 4b/5b have been de
tected by P-31 NMR spectroscopy and tentatively assigned the heterovalent s
tructures (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 whil
e 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)/2.92
1(1) Angstrom (for independent molecules), the shorter distance in 3a corre
sponding to a covalent metal-metal bond. In 4a and 10b the torsion about th
e 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 aurophili
c 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 c
onfiguration relative to the biphenyl fragment, leading to a nonbonding gol
d-gold separation of 5.3469(7) Angstrom. The differences in behavior of dig
old(II) complexes [Au2X2(mu -2-C6H4PR2)(2)] as the axial anionic ligand X i
s varied and the pathway of the intramolecular C-C coupling reaction are di
scussed.