Gold–allenylidenes – an experimental and theoretical study

Abstract

Herein we describe the isolation and characterisation of the first “gold–allenylidene” complexes [AuCCCR₂]⁺ X⁻ (R = N(CH₂)_n, OMe; n = 3, 4; X⁻ = OTf⁻) and present a bonding model for these species based on an experimental and theoretical analysis. Heteroatom stabilisation (oxygen and nitrogen donors) overpowers the ligand effect on gold and has a large impact on the distribution of electron density in the allenylidene fragment. The description of a gold-stabilised propargylic cation rather than a gold–vinylidene complex is favoured, clearly indicated by experimental and theoretical work. Removing this heteroatom stabilisation by means of a theoretical analysis shows that the choice of ligand can fine-tune the electronics ranging from a gold-stabilised carbocation towards a Au–allenylidene. This observation is in excellent agreement with the recent gold “carbene vs. carbocation” discussion and expands the understanding of Au–carbocation interactions to include gold–cumulene systems.