Allyl- and propadienyl-cobaloximes: character and reactions with tetracyanoethylene

Abstract

The temperature-dependent ¹H n.m.r. spectra of allylcobaloximes show characteristics of a dynamic process in which interchange between the two possible σ-allylcobaloxime complexes takes place. This is ascribed to two processes: allylaq uabis(dioximato)cobalt(III) complexes undergo a unimolecular reaction which involves a transient η-allylcobaloxime formed after loss of the axial aqua-ligand; allylbis(dioximato) pyridineco balt (III) complexes undergo a bimolecu lar reaction in which an S_H2′ attack of a cobaloxime(II) complex (present as impurity) attacks the δ-carbon of the allyl group and displaces cobaloxime(II) from the α-carbon. The latter mechanism may also operate with the aIlylaquabis(dioximato)cobaIt(III) complexes. Under conditions where the η-allylcobal-oximes are not formed, i.e. in the presence of an appropriate axial ligand, the σ-allylcobaloximes react with C₂(CN)₄ to give 3,3,4,4-tetracyanocyclopentylcobaloximes. The formation of trans-bis(dimethylglyoximato)imidazole-(3,3,4,4-tetracyano-2-phenylcyclopentyl)cobalt(III) from the reaction of trans-cinnamylbis(dimethylglyoximato)-imidazolecobalt(III) with C₂(CN)₄ indicates that the cycloaddition involves a σ-π-σantarafacial migration of the cobalt from the α- to the β-carbon of the allyl group.