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)21 (1) and [Mn(bdmpza)2] (2) (Pockaj et al. 2015 [1]) are afforded by employing a `scorpionate' type precursor [bdmpza =bis(3,5-di-methylpyrazoll-yl)acetate]. In complex 1, the central Cu(II) atom is surrounded by four N-pyrazoi atoms from two units of bis(pyrazol-l-yl)acetate and the axial position is occupied by O-acetate 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 complexl a structural variation by lowering the temperature and a change in the EPR signals from a S=1/2 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 H-2 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 x 104 M-1 s(-1) and an onset potential 0.5V less negative than blank TFA [Trifluoroacetic acid] solution without 1. Hence, high rate of H-2 evolution using this cheap and earth abundant Cu complex demonstrates a promising strategy to design a potential heterogeneous catalyst for H-2 evolution from water splitting under electrochemical or by photocatalysis. (C) 2017 Elsevier B.V. All rights reserved.