Ligand substitution processes at five coordinate copper(II) centres in hydrophilic and hydrophobic environments

Abstract

In [Cu(Me₆tren)H₂O]²⁺[where Me₆tren is 2,2′,2″-(N,N-dimethylamine]triethylamine] the aqualigand occupies a hydrophobic environment resembling the sites of aqua ligands in some metalloenzymes, whereas in [Cu(tren)H₂O]²⁺(where tren is 2,2′,2″-triaminotriethylamine) the aqua ligand occupies a hydrophilic site. The lability of the aqua ligand in [Cu(Me₆tren)H₂O]²⁺ towards ligand substitution (e.g. for substitution by azide through an I_d mechanism: k₂₃(298 K)= 34.6 ± 1.3 s^–1; ΔH^#= 54.8 ± 1.0 kJ mol^–1; ΔS^#=–31.7 ± 3.5 J K^–1 mol^–1) is several orders of magnitude less than that for the aqua ligand in [Cu(tren)H₂O]²⁺. These differences in lability are discussed in relation to the possible influences of the local environment at the site of substitution.