EPR interpretation and electrocatalytic H2 evolution study of bis(3,5-di-methylpyrazol-1-yl)acetate anchored Cu(II) and Mn(II) complexes

Two mononuclear Cu(II) and Mn(II) complexes, [Cu(bdmpza)2] (1) and [Mn(bdmpza)2] (2) (Počkaj et al. 2015 [1]) are afforded by employing a ‘scorpionate†type precursor [bdmpza = bis(3,5-di-methylpyrazol-1-yl)acetate]. In complex 1, the central Cu(II) atom is surrounded by four Npyrazol atoms from two units of bis(pyrazol-1-yl)acetate and the axial position is occupied by Oacetate atom, thus exhibiting a square-pyramidal geometry with N4O coordination environment. Both the complexes, 1 and 2 are thoroughly characterized by IR and UV–vis spectroscopy. Detailed temperature and solvent dependent EPR spectra for both complexes 1 and 2 are also carried out to get a profoundly insight on magnetic coupling. For the solid complex 1 a structural variation by lowering the temperature and a change in the EPR signals from a S = ½ to a S = 1 state, in which two Cu(II) magnetically interact, is observed. In organic solution an equilibrium between the penta-coordinated with only one axial COO− group and the hexa-coordinated with two axial COO− groups is revealed for compound 1, whereas 2 maintains the octahedral structure. The highly activity for H2 evolution from proton reduction of complex 1 is reported here. Based on electrochemical investigations (cyclic voltammetry, linear sweep voltammetry, and bulk electrolysis) and gas chromatography experiment, complex 1 afforded an observed rate constant (kobs or TOF) [TOF = Turnover number] value about 2900 s−1, 2nd order catalytic rate constant of 9.7 × 104 M−1 s−1 and an onset potential 0.5 V less negative than blank TFA [Trifluoroacetic acid] solution without 1. Hence, high rate of H2 evolution using this cheap and earth abundant Cu complex demonstrates a promising strategy to design a potential heterogeneous catalyst for H2 evolution from water splitting under electrochemical or by photocatalysis.