Crystal structures, dynamic behaviour and electrochemical properties of the copper-(I) and -(II) complexes of a new N2S2 ligand

Abstract

The complexing properties of the new quadridentate N₂S₂ ligand N,N′-bis{[2-(phenylmethylthio)phenyl]-methylene}ethane-1,2-diamine, L, with Cu^I and Cu^II have been investigated. The perchlorate salt of [CuL]⁺ occurs as orange crystals in which mirror image forms, [(±)-(R,*R*S*)], of the highly distorted tetrahedral cation coexist. The CuN₂–CuS₂ dihedral angle = 60.0(1)°, bond lengths and angles are Cu–S 2.285(1) and 2.258(1)Å, Cu–N 2.029(3) and 2.069(3)Å, N–Cu–N 82.7(1)°, S–Cu–S 123.56(4)°. In nitromethane solution, however, variable-temperature ¹³C NMR spectroscopy indicates that [CuL]⁺ exists predominantly as an equilibrium mixture of the two other diastereomers, [(±)-(R*,R*,R*)] and [(±)-(R*,S*,S*)] for which the interconversion barrier corresponds to ΔG₂₈₀^‡= 59.8 ± 0.6 kJ mol^–1. Exposure of an ethanolic solution of [CuL]ClO₄ to the atmosphere leads to slow deposition of brown crystals of [CuL(H₂O)][ClO₄]₂. Crystal structure determination revealed the Cu^II cation to be square-pyramidal with the water molecule occupying the apical position: Cu–S 2.323(6) and 2.318(5)Å; Cu–N 1.94(1) and 2.00(2)Å; Cu–O (H₂O) 2.29(1)Å. The crystals are achiral and racemic, arising through coexistence of the [(±)-(R*,R*)] enantiomeric forms of the complex. Cyclic voltammetry indicates a quasi-reversible system for which E_½= 0.28 V vs. saturated calomel electrode.