Equilibrium studies and kinetics of solvent exchange with five- and six-co-ordinate mono- and bis-(solvent)(1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane)nickel(II) perchlorate by oxygen-17 nuclear magnetic resonance spectroscopy

Abstract

Natural-abundance ¹⁷O n.m.r. spectroscopy at 54.24 MHz has been used to measure the rates of D₂O and N,N-dimethylformamide (dmf) exchange with [NiLS_n]²⁺ ions (S = solvent: L =R,S,R,S- and R,S,S,R-1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane with n= 1 and 2 respectively). The rate constants and activation parameters for D₂O and dmf exchange with the five-co-ordinate trans-(I) complexes are 10^–7k_ex^I/s^–1= 1.59 ± 0.11 and 1.51 ± 0.08, ΔH^‡/kJ mol^–1= 24.7 ± 1.4 and 30.1 ± 0.7, and ΔS^‡/J K^–1 mol^–1=–24.0 ± 5.2 and –6.4 ± 2.9 respectively; for the six-co-ordinate trans-(III) species, 10^–7k_ex^I/s^–1= 15.8 ± 1.8 and 32.8 ± 4.1, ΔH^‡/kJ mol^–1= 37.4 ± 2.4 and 38.3 ± 1.4, and ΔS^‡/JK^–1 mol^–1=+ 37.6 ± 9.0 and + 46.7 ± 5.6 for D₂O and dmf respectively. These rates are the largest ever observed for solvent exchange at Ni²⁺, and show the pronounced labilising effect of a tetra-aza macrocyclic ligand. The activation parameters are interpreted in terms of an associative-interchange (l_a) mechanism for the five-co-ordinate species, and a dissociative-interchange (l_d) mechanism for the six-co-ordinate complexes. Equilibrium constants and associated thermodynamic parameters were determined, either by spectrophotometry or by ¹H or ¹⁷O n.m.r. shifts, for equilibria between diamagnetic four-co-ordinate [NiL]²⁺ and the corresponding five- or six-co-ordinate paramagnetic solvates. The rate of dmf exchange with [Ni(dmf)₆]²⁺ was redetermined by natural-abundance ¹⁷O n.m.r.