Formation of Gold(III) Alkyls from Gold Alkoxide Complexes

The
gold­(III) methoxide complex (C∧N∧C)­AuOMe (1) reacts with tris­(p-tolyl)­phosphine
in benzene at room temperature under O abstraction to give the methylgold
product (C∧N∧C)­AuMe (2) together with OP­(p-tol)3 ((C∧N∧C) = [2,6-(C6H3tBu-4)2pyridine]2–). Calculations show that this reaction is energetically favorable
(ΔG = −32.3 kcal mol–1). The side products in this reaction, the Au­(II) complex [Au­(C∧N∧C)]2 (3) and the phosphorane (p-tol)3P­(OMe)2, suggest that at least two reaction pathways may operate,
including one involving (C∧N∧C)­Au• radicals. Attempts to model the reaction by DFT methods
showed that PPh3 can approach 1 to give a
near-linear Au–O–P arrangement, without phosphine coordination
to gold. The analogous reaction of (C∧N∧C)­AuOEt, on the other hand, gives exclusively a mixture of 3 and (p-tol)3P­(OEt)2. Whereas the reaction of (C∧N∧C)­AuOR (R = But, p-C6H4F) with P­(p-tol)3 proceeds over
a period of hours, compounds with R = CH2CF3, CH­(CF3)2 react almost instantaneously, to
give 3 and OP­(p-tol)3. In chlorinated solvents, treatment of the alkoxides (C∧N∧C)­AuOR with phosphines generates [(C∧N∧C)­Au­(PR3)]­Cl, via Cl abstraction from
the solvent. Attempts to extend the synthesis of gold­(III) alkoxides
to allyl alcohols were unsuccessful; the reaction of (C∧N∧C)­AuOH with an excess of CH2CHCH2OH in toluene led instead to allyl alcohol isomerization to
give a mixture of gold alkyls, (C∧N∧C)­AuR′ (R′ = −CH2CH2CHO
(10), −CH2CH­(CH2OH)­OCH2CHCH2 (11)), while 2-methallyl
alcohol affords R′ = CH2CH­(Me)­CHO (12). The crystal structure of 11 was determined. The formation
of Au–C instead of the expected Au–O products is in
line with the trend in metal–ligand bond dissociation energies
for Au­(III): M–H > M–C > M–O.