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 = CoII or NiII; 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 (Ia) 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): 10–5kex. = 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 NiII, for CoII 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.