Structural and mechanistic studies of co-ordination compounds. Part X. Steric effects in aquation of some octahedral cobalt(III) complexes of 1,4,8,11-tetra-azacyclotetradecane and meso-1,4,8,11-tetra-aza-5,5,7,12,12,14-hexamethylcyclotetradecane

Abstract

The preparation of a series of cations of the type trans-[CoL′(A)Cl]⁺ are described (I; L′=meso-1,4,8,11-tetra-aza-5,5,7,12,12,14-hexamethylcyclotetradecane; A = NO₂, N₃, NCS, and CN). The acid hydrolysis of these cations has been studied over a range of temperature. The lability of the relatively inert cations trans-[CoL″(A)Cl]⁺(II; L″= 1,4,8,11-tetra-azacyclotetradecane; A = NCS and N₃) at 25·0 °C has been reassessed by following the thiocyanate-substitution reactions over a rather extended range of temperature. A comparison of the reaction rates of (I) with those of the corresponding complexes (II) shows that the former are more reactive by a factor of ca. 10³ at 25·0 °C, depending only slightly on the different electronic-displacement properties of A. These results are consistent with a dissociative mechanism with basically a tetragonalpyramidal intermediate. In the sterically congested complexes (I), a slight distortion of the intermediate from tetragonal-pyramidal geometry is evident. A linear Hammett relation with unit gradient exists over five logarithmic units between the reactions of these two series of complexes. This observation strongly indicates that the sensitivity of acid hydrolysis of these complexes to changes in the electronic structure of non-labile orienting ligands is independent of steric effects. This further suggests, together with the nearly constant rate increment as described previously, that electronic and steric effects independently affect the lability of cobalt(III) amine complexes.