The Reactivity of Hexa(N‐methylideneimine)CoIII Complexes towards Nucleophiles: Structure and Mechanism

Abstract

AbstractThe reactivity of the hexa(N‐methylideneimine)CoIII complex [Co‐hmi‐(taci)2]3+ (taci = 1,3,5‐triamino‐1,3,5‐trideoxy‐cis‐inositol) towards the nucleophiles ammonia, aniline and cyanide was investigated and the main products were identified by spectroscopic methods and by a crystal structure analysis. The relative energies of the complexes formed and those of possible stereo isomers were analysed by means of molecular mechanics calculations. In the reaction with NH3, two condensation products, [Co(La)]3+ and [Co(Lb)]3+, were identified with the two taci moieties linked by one or two N=CH–N(H)‐CH2–NH bridges, respectively. The reaction with aniline yielded the complex [Co(Lc)(Ld)]3+ (where Lc is a mono‐N‐methylated and Ld a phenylformamidine‐substituted taci), whereas the reaction with cyanide resulted in the formation of [Co(Le)]3+ having an NH–CH2–C(NH2)=N bridge between the two taci subunits. A mechanism that accounts for the different types of products can be suggested as follows: In [Co(La)]3+, [Co(Lb)]3+ and [Co(Lc)(Ld)]3+, N‐methylation was observed and this can be interpreted in terms of an intramolecular Cannizzaro type hydride shift. For [Co(La)]3+ and [Co(Lb)]3+ two subsequent addition steps of the nucleophile to adjacent imino groups must take place prior to the hydride shift, whereas for [Co(Lc)(Ld)]3+ the hydride shift appears to have already occurred after the first addition step of aniline to an imino group. Formation of [Co(Le)]3+ can be explained in terms of a nucleophilic attack of CN– to an imino group and a subsequent attack of a deprotonated amino group (which must have previously been formed by hydrolysis of a methylideneimino group) to the nitrile C‐atom. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)