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 SN1(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, kOH, enthalpies (ΔH‡) and entropies (ΔS‡) of activation, and the effects of the orientating and leaving groups on kOH.