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-(C₆H₃ Bu-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 = Bu^t, 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.