Synthesis, electrochemical and e.p.r. spectral studies of binuclear copper(II) complexes with new pentadentate L-proline-based binucleating ligands

Abstract

The synthesis, reduction, optical and e.p.r. spectral properties of a series of new binuclear copper(II) complexes, containing bridging moieties (OH−, MeCO2−, NO2−, and N3−), with new proline-based binuclear pentadentate Mannich base ligands is described. The ligands are: 2,6-bis[(prolin-1-yl)methyl]4-bromophenol [H3L1], 2,6-bis[(prolin-1-yl)methyl]4-t-butylphenol [H3L2] and 2,6-bis[(prolin-1-yl)methyl]4-methoxyphenol [H3L3]. The exogenous bridging complexes thus prepared were hydroxo: [Cu2L1(OH)(H2O)2] · H2O (1a), [Cu2L2(OH)(H2O)2] · H2O (1b), [Cu2L3(OH)(H2O)2] · H2O (1c), acetato [Cu2L1(OAc)] · H2O (2a), [Cu2L2(OAc)] · H2O (2b), [Cu2L3(OAc)] · H2O (2c), nitrito [Cu2L1(NO2)(H2O)2] · H2O (3a), [Cu2L2(NO2)(H2O)2] · H2O (3b), [Cu2L3(NO2)(H2O)2] · H2O (3c) and azido [Cu2L1(N3)(H2O)2] · H2O (4a), [Cu2L2(N3)(H2O)2] · H2O (4b) and [Cu2L3(N3)(H2O)2] · H2O (4c). The complexes were characterized by elemental analysis and by spectroscopy. They exhibit resolved copper hyperfine e.p.r. spectra at room temperature, indicating the presence of weak antiferromagnetic coupling between the copper atoms. The strength of the antiferromagnetic coupling lies in the order: NO2 ≅ N3 ≥ OH ≥ OAc. Cyclic voltammetry revealed the presence of two redox couples CuIICuII ⇌ CuIICuI ⇌ CuICuI. The conproportionality constant Kcon for the mixed valent CuIICuI species for all the complexes have been determined electrochemically.