Mechanism of formation of (methoxy)(amino) — and bis(amino) carbene complexes by nucleophilic attack of methoxide ion and amines on platinum(II)-coordinated isocyanide in anhydrous methanol

Abstract

The isocyanide complexes [Pt(CNC6H11)(R)(dppe)]ClO4 (R = C4H3N2-C2, 2-pyz; C5H4N-C4, 4-py) undergo nucleophilic attack on the coordinated isocyanide by methoxide ion in acid-base equilibrium with triethylamine in anhydrous methanol, leading to the (methoxy)(amino)carbene species [Pt{C(NHC6H11)OMe}(R)(dppe)]+ by a bimolecular, second-order path. When a primary or secondary amine is used instead of triethylamine, concomitant formation of (methoxy)(amino)- and bis(amino)carbene complexes takes place via parallel, second-order paths involving nucleophilic attack by both methoxide and amine on the isocyanide. A marked solvent effect depressing the rates in comparison to those of the corresponding reactions of amines in chlorinated solvents is observed. No substantial difference in reactivity between the two metal substrates can be detected, indicating the lack of anchimeric assistance to proton transfer from the entering amine to the isocyanide nitrogen, possibly replaced by a solvent mediated pathway. The methoxide ion proves a much better nucleophile than amines in this type of reaction. The interconversion of (methoxy)(amino)- with bis(amino)carberie species via methoxy—amine exchange is extremely slow.