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.