Structural and mechanistic studies of co-ordination compounds. Part 20. Preparation and base hydrolysis of some cobalt(III) complexes of quadridentate macrocyclic amines

Abstract

The complexes trans-[CoBr(L¹)(N₃)]⁺(L¹= 1,4,8,11-tetra-azacyclotetradecane) and trans-(RS)-[CoA(L²)X]⁺(L²= 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradeca-7,14-diene; A = CN, X = NO₂, NCS, or N₃; A = NO₂. X = Cl or Br) have been prepared and characterized. The kinetics of base hydrolysis of an extensive set of complexes of the type trans-[CoA(L)X]⁺[L = L¹, L², and L³(= C-meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane)] have been studied. All the complexes react by second-order kinetics with complete release of the leaving group X and complete retention of configuration. An S_N1(cb) mechanism operating under three different conditions (normal, intermediate, and limiting) has been assigned on the basis of the magnitudes of the second-order rate constants, k_OH, enthalpies (ΔH‡) and entropies (ΔS‡) of activation, and the effects of the orientating and leaving groups on k_OH.