Effectual electrocatalytic proton and water reduction by CuII terpyridine scaffolds

In this paper, three Cu(II) complexes [{(OAc)(2)Cu(3py-tpy)}(2)Cu(OAc)(2)(H2O)(2)] (1a ), {[Cu(4py-tpy)(OAc)]Cl}(n) (2a) and [Cu(Ph-tpy)(OAc)(2)] (3a) have been successfully employed for electrochemical hydrogen production in both organic and acidic aqueous medium (3py-tpy = 4'-(pyridin-3-yl)-2,2':6',2 ''-terpyridine; 4py-tpy = 4'-(pyridin-4-yl)-2,2':6',2 ''-terpyridine; Ph-tpy = 4'-phenyl-2,2' :6' ,2 ''-terpyridine). All the complexes exhibit efficient catalytic activity for proton reduction in 95:5 (v/v) DMF/H2O using acetic acid as a proton source. Among all the three complexes, 1a shows the highest TOF value of 1473 s(-1). The complexes show similar acid-base equilibria, and pK(a) for all the complexes are found to be 4.8, 4.6, and 4.3 respectively, for 1a , 2a , and 3a . The catalysts generate the aqua complex, through the substitution of the axial ligand. The aqua complex undergoes deprotonation to generate the corresponding hydroxo complex, i.e., [CuL(OAc)(H2O)](+) reversible arrow [CuL(OAc)(OH)] + H+ (where L indicates 3py-tpy, or 4py-tpy or Ph-tpy). The complexes remain stable in acidic conditions at low pH and exhibit very high catalytic activity. Among all these complexes 3a shows the higher catalytic activity for water reduction and TOF value of 810 mol of H-2 h(-1) (mole of catalyst)(-1). The presence of PCET process was noticed in case of proton reduction, which generates [(CuL)-L-0(OAc)(OH2)] from [(CuL)-L-II(OAc)(OH)], followed by protonation to generate the Cu-II-H intermediate species. The Cu-II-H in presence of H2O revert into [CuL(OAc)(OH)]. During water reduction in an acidic aqueous medium of pH 1.62, the [(CuL)-L-II(OAc)(H2O)](+) undergoes 2e-reduction to generate [(CuL)-L-0(OAc)(OH2)](-). The [(CuL)-L-0(OAc)(OH2)] interacts with H+ to generate Cu-II-H intermediate species. The Cu-II-H in the presence of H3O+ evolves H-2 and revert to [(CuL)-L-II(OAc)(H2O)](+). (C) 2020 Elsevier Ltd. All rights reserved.