Structural and mechanistic studies of co-ordination compounds. Part 22. Preparation and ligand-substitution kinetics of some trans-di-halogeno- and trans-halogenoisothiocyanato-cobalt(III) complexes of 2,12-dimethyl- and 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]-heptadeca-1 (17),2,11,13,15-pentaene

Abstract

Complexes trans-[CoL¹(NCS)X]⁺{L¹= 2,7,12-trimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]heptadeca-1 (17),-2,11,13,15-pentaene; X = NCS, Cl, or Br} have been prepared and characterized. The kinetics of thiocyanate-substitution reactions of trans-[CoLX₂]⁺{L = L¹ or L²(2,12-dimethyl-3,7,11,17-tetra-azabicyclo[11.3.1]hepta-deca-1 (17),2,11,13,15-pentaene); X = Cl or Br} have been studied over a range of temperature. The rate constants for the replacement of the first chloride from the dichloro-complexes are virtually independent of the nature of L [k_Cl= 1.1 × 10^–2(L¹ and 2.9 × 10^–2 s^–1 at 25.0 °C (L²)] whereas those for the replacement of the second halide are much faster for L²(k_Cl= 7.6 × 10^–4 and k_Br= 9.1 × 10^–4 s^–1 at 25.0 °C) than for L¹(k_Cl= 2.8 × 10^–6 and k_Br= 4.5 × 10^–6 s^–1 at 25.0 °C) by two orders of magnitude. This observation seems to indicate that the first step of substitution involves the release of the halide farther away from the N–R group of the L¹(R = Me) and L²(R = H) macrocycles. The role played by the amine proton in promoting the rates of substitution of the second halide in the L² complexes is discussed. The hydrolysis of trans-[CoL¹(NCS)X]⁺ involves concurrent release of both isothiocyanate and halide. This, coupled with a relatively low value of k_Br/k_Cl, is taken to indicate that the cobalt(III) ion in these L¹ and L² complexes is much softer than that in complexes of saturated macrocyclic amines.