Carbocations as Secondary Coordination Sphere Z-Type Ligands – Anion-Induced Au(I)/Au(III) Oxidation and Impact on Reactivity

We describe our efforts towards the design of phosphine-based ligands featuring an electrophilic carbocationic moiety positioned to enter the secondary coordination sphere of the phosphine-bound metal atom. Using simple synthetic protocols, we have been able to obtain and fully characterize the complexes [(o-Ph₂P(C₆H₄)Acr)AuCl]⁺ ([3]⁺, Acr = 9-N-methylacridinium) and [(o-Ph₂P(C₆H₄)Xan)AuCl]⁺ ([4]⁺, Xan = 9-xanthylium). While only weak interactions occur between the gold atom and the methylium centers of these complexes, the more Lewis acidic [4]⁺ readily reacts with chloride to afford a trivalent phosphine gold dichloride derivative (7) in which the metal atom is covalently bound to the former methylium center. This anion-induced Au(I)/Au(III) oxidation is accompanied by a conversion of the Lewis acidic methylium center in [4]⁺ into an X-type ligand in 7. This transformation leads us to conclude that the methylium center acts as a latent Z-type ligand poised to increase the Lewis acidity of the gold center, a notion supported by the carbophilic reactivity of these complexes.