Structural and mechanistic studies of co-ordination compounds. Part XIII. Preparation and acid hydrolysis of some trans-azidochloro- and azidobromo-cobalt(III) complexes containing macrocyclic quadridentate amine ligands having different degrees of unsaturation

Abstract

The preparation and characterization of trans-[CoX(L)(N₃)]ⁿ⁺[L = 2,3-dimethyl-1,4,8,11-tetra-azacyclotetra-deca-1,3-diene (L¹), X = Cl^– and Br^– L = 2,3,9,10-tetramethyl-1,4,8,11 -tetra-azacyclotetradeca-1,3,8,10-tetraene (L²), X = Cl^–, Br^–, N₃^–, and H₂O] cations are described. The acid hydrolysis of these azidohalogeno-cations and of trans-[CoBr(L³)(N₃)]⁺(L³= 1,4,8,11-terra-azacyclotetradecane, cyclam) has been studied over a range of temperature. The rate constants of corresponding complexes increase in the following order of L: L³ < L¹ < L²(1 : 440 : 2 400 for the chloro- and 1 : 58 : 200 for the bromo-series at 25.0 °C respectively), This, coupled with the observation that the quotient of the rate constant with bromide to that with chloride as leaving group. k_Br/k_Cl, at the same temperature decreases in the same order of L (11 : 1.4 : 1.1), strongly supports an earlier proposition that the kinetic lability of these macrocyclic amine complexes increases with the ‘softness’ of the central cobalt(III) ion, as brought about by the increasing extent of electronic delocalization between the metal ion and the encircling macrocycle.