Chemical and electrochemical water oxidation mediated by bis(pyrazol-1-ylmethyl)pyridine-ligated Cu(i) complexes

Abstract

Herein a series of novel bis(pyrazol-1-ylmethyl)pyridine-ligated Cu(I) complexes, C1–C4, bearing different donating groups [[H(C1), Me(C2), t-Bu(C3), Ph(C4)])] on the pyrazole rings, were synthesized and investigated as pre-catalysts in chemical and electrocatalytic water oxidation reactions. Ligands, 2,6-bis((1H-pyrazol-1-yl)methyl)pyridine (L1), 2,6-bis((1H-pyrazol-1-yl)methyl)pyridine (L2), 2,6-bis((3,5-di-tert-butyl-1H-pyrazol-1-yl)methyl)pyridine (L3), and 2,6-bis((3,5-diphenyl-1H-pyrazol-1-yl)methyl)pyridine (L4) were reacted with Cu(MeCN)₄PF₆ to form complexes C1–C4 respectively. Cerium ammonium nitrate (CAN), sodium m-periodate, and sodium persulfate were investigated as chemical oxidants in chemical water oxidation. Complexes C1–C4 showed catalytic activity towards chemical water oxidation in the presence of CAN as the primary oxidant at 25 °C. Complex C2 was the most active with a turnover number (TON) of 4.6 and a turnover frequency (TOF) of 0.31 s⁻¹. The least active catalyst was complex C4, with a TON of 2.3 and a TOF of 0.0086 s⁻¹. This observed difference in catalytic activity between the complexes illustrated the key role that electronic effects play during catalysis. Other oxidants evaluated with C2 were sodium m-periodate (TON, 3.77; TOF 0.14 s⁻¹) and sodium persulfate (TON, 4.02; TOF 0.044 s⁻¹) however, CAN exhibited the greatest activity. Complexes C1–C4 were investigated in electrocatalytic water oxidation at a neutral pH of 6.5. Complex C2 was the most active in electrocatalytic water oxidation as well, exhibiting an overpotential of 674 mV and TOF of 9.77 s⁻¹ (at 1.7 V vs. NHE), which is better than most reported copper(II) complexes. These Cu(I) complexes C1–C4 show potential as efficient chemical and electrocatalytic water oxidation catalysts, which can be achieved by fine-tuning the steric and electronic properties of the catalysts.