Structural and mechanistic studies of co-ordination compounds. Part III. Steric effects in octahedral aquation of cobalt(III) complexes containing different quadridentate macrocyclic secondary amines

Abstract

Kinetics of the aquation and radioactive chloride exchange reactions of trans-[Co(L)Cl2]+, (L =meso- or, L′=rac-5,7,7,12,14,14-hexamethyl-1,4,8,11-tetra-azacyclotetradecane), have been investigated over a range of temperature. The studies confirm that the two co-ordinated chlorides in the L complex are kinetically equivalent with ΔH‡= 25·6 ± 0·7 kcal mol–1 and ΔS‡= 13 ± 2 cal deg–1 mol–1, whereas they are kinetically different in the L′ complex with ΔH1‡= 28·3 ± 1 kcal mol–1, ΔS1‡= 25 ± 5 cal deg–1 mol–1, and ΔH2‡= 39·5 ± 1 kcal mol–1, ΔS2‡= 58 ± 5 cal deg–1 mol–1. The observation that k1(L′) > k(L) > k2(L′) > k(1,4,8,11-tetra-azacyclotetradecane), where k represents the appropriate first-order rate constant for the release of the co-ordinated chloride, is satisfactorily explained in terms of the steric acceleration of unimolecular reactions of these complexes. The relatively large values of ΔS‡ found for these systems are taken to imply a relaxation of the sterically crowded complexes in the transition state.