Manganese-Schiff base complexes as catalysts for water photolysis

Abstract

Four manganese(III)-Schiff base complexes (1–4) of formula [MnLⁿ(H₂O)₂]₂(ClO₄)₂·mH₂O (n = 1–4; m = 0, 1) have been prepared. The multidentate H₂Lⁿ Schiff base ligands consist of 3R,5R-substituted N,N′-bis(salicylidene)-1,2-diimino-2,2-dimethylethane, where R = OEt, OMe, Br or Cl. The complexes have been thoroughly characterized by elemental analysis, mass spectrometry, magnetic susceptibility measurements, IR, UV, paramagnetic ¹H NMR and EPR spectroscopies. Other properties, including redox studies and molar conductivity measurements, have also been assessed. The crystal structure of 1 was solved by X-ray diffraction, which revealed the dimeric nature of the compound through μ-aqua bridges. The ability of these complexes to split water has been studied by water photolysis experiments, with the oxygen evolution measured in aqueous media in the presence of a hydrogen acceptor (p-benzoquinone), the reduction of which was followed by UV-spectroscopy. The discussion of the photolytic behaviour includes advances in the knowledge of the structural motifs and the chemical activity of this type of complex, as revealed by the development of several characterization techniques in the last decade. Parallel-mode Mn^III EPR shows that complexes 1–4 not only mimic reactivity but also share some structural characteristics from partially assembled natural OEC clusters.