Rates and activation parameters for acetonitrile exchange with five- and six-co-ordinate mono- and bis-(acetonitrile)(1,4,8,11-tetramethyl-1,4,8,1 1-tetra-azacyclotetradecane)nickel(II) perchlorate by carbon-13 paramagnetic nuclear magnetic resonance line broadening

Abstract

Two isomers of [NiL][ClO₄]₂(L = 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane) have been investigated in acetonitrile solution. One isomer has all four N-methyl groups on the same side of the macrocyclic ligand plane and forms a paramagnetic five-co-ordinate complex ion, [NiL(NCMe)]²⁺(A), in which the nickel is probably trigonal bipyramidal in solution. The second isomer has adjacent pairs of N-methyl groups on opposite sides of the macrocyclic plane and forms an octahedral complex ion, [NiL(NCMe)₂]²⁺(B). The rates of acetonitrile exchange with (A) and (B) have been determined by ¹³C n.m.r. line broadening, and although at 298.2 K the rate constants are very similar [10^–6k_ex= 5.2 and 14.7 s^–1 for the exchange of a single solvent molecule from (A) and (B) respectively] the activation parameters differ markedly: (A), ΔH^‡= 19.8 ± 0.6 kJ mol^–1 and ΔS^‡=–50 ± 3 J K^–1mol^–1; (B), ΔH^‡= 41.5 ± 1.1 kJ^–1 mol^–1and ΔS^‡= 32 ± 5 J K^–1mol^–1. This difference is interpreted in terms of an associative mechanism for the five-co-ordinate complex and a dissociative mechanism for the six-co-ordinate complex.