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 [kCl= 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(kCl= 7.6 × 10–4 and kBr= 9.1 × 10–4 s–1 at 25.0 °C) than for L1(kCl= 2.8 × 10–6 and kBr= 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 kBr/kCl, 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.