Benzimidazole Schiff base copper(II) complexes as catalysts for environmental and energy applications: VOC oxidation, oxygen reduction and water splitting reactions

The new copper(II) complexes [Cu(μ-1κO:2κONN′-HL1)(μ-1κO:2κO′-NO3)]2.[Cu(μ-1κO:2κONN′-HL1)(CH3OH)]2(NO3)2 (1) and [Cu(κONN′-HL2)(μ-1κOO':2κO′-NO3)]n (2), derived from the new pro-ligands H2L1 = 2-(5,6-dihydroindolo[1,2-c]quinazolin-6-yl)-5-methylphenol and H2L2 = 2-(5,6-dihydroindolo[1,2-c]quinazolin-6-yl)-4-nitrophenol, were synthesized and characterized by elemental analysis, FT-IR, ESI-MS, and their structural features were unveiled by single-crystal X-ray diffraction analysis. This discloses a dimeric structure for 1 and a polymeric infinite 1D metal-organic chain for 2. The complexes were evaluated as catalysts for the oxidation of toluene, a volatile organic compound (VOC), and for oxygen reduction and water splitting reactions. 1 exhibits a higher activity for the peroxidative conversion of toluene to oxygenated products (total yields up to 38%), whereas 2 demonstrates a superior performance for electrochemical energy conversion applications, i.e., for oxygen reduction (ORR), oxygen evolution (OER) and hydrogen evolution (HER) reactions in an alkaline medium in terms of higher ORR current densities, lower Tafel slope (73 mV dec−1) and higher number of electrons exchanged (3.9), comparable to that of commercial Pt/C. Complex 2 also shows a better performance with lower onset potential and higher current densities for both OER and HER when studied as electrocatalyst for water splitting.