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

The reactivity of the h exa( N ‐ m ethylidene i mine)Co III 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 NH 3 , two condensation products, [Co(L a )] 3+ and [Co(L b )] 3+ , were identified with the two taci moieties linked by one or two N=CH–N(H)‐CH 2 –NH bridges, respectively. The reaction with aniline yielded the complex [Co(L c )(L d )] 3+ (where L c is a mono‐ N ‐methylated and L d a phenylformamidine‐substituted taci), whereas the reaction with cyanide resulted in the formation of [Co(L e )] 3+ having an NH–CH 2 –C(NH 2 )=N bridge between the two taci subunits. A mechanism that accounts for the different types of products can be suggested as follows: In [Co(L a )] 3+ , [Co(L b )] 3+ and [Co(L c )(L d )] 3+ , N ‐methylation was observed and this can be interpreted in terms of an intramolecular Cannizzaro type hydride shift. For [Co(L a )] 3+ and [Co(L b )] 3+ two subsequent addition steps of the nucleophile to adjacent imino groups must take place prior to the hydride shift, whereas for [Co(L c )(L d )] 3+ the hydride shift appears to have already occurred after the first addition step of aniline to an imino group. Formation of [Co(L e )] 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)