Relative labilities of five-co-ordinate mono(acetonitrile)(1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane)-nickel(II) and -cobalt(II) ions towards associative solvent exchange

Abstract

A comparison of the rates of solvent exchange and anation of the paramagnetic five-co-ordinate complex ions [ML(S)]²⁺(M = Co^II or Ni^II; L = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane, S = NCMe or OH₂) shows that the nickel(II) complexes are significantly more labile than the cobalt(II) analogues. For acetonitrile solvent exchange, ΔS^‡ values are large and negative and ΔH^‡ values are low for both the nickel(II) and cobalt(II) complexes, in line with an associative interchange (I_a) mechanism. Previous ¹³C n.m.r. line-broadening results obtained for the [NiL(NCMe)]²⁺ ion are confirmed by ¹⁵N n.m.r., and an analogous ¹³C and ¹⁵N line-broadening study of the high-spin [CoL(NCMe)]²⁺ ion gives (at 298.2 K): 10^–5k_ex. = 5.65 ± 0.50 s^–1, ΔH^‡= 19.5 ± 0.8 kJ mol^–1, and ΔS^‡=–69.5 ± 3.5 J K^–1 mol^–1. Comparison with the parent hexasolvated metal ions shows that the quadridentate macrocyclic ligand (L) significantly lowers ΔH^‡ for acetonitrile exchange by 44 and 30 kJ mol^–1 for nickel(II) and cobalt(II) ions respectively, but whereas this results in a marked rate increase (>10³) for Ni^II, for Co^II the rate increases by less than a factor of two because of the more negative ΔS^‡ value. The results are compared with previous studies of the effects of amine ligands on the rates of acetonitrile and water exchange on cobalt(II) and nickel(II) ions.