Chemical And Electrochemical Water Oxidation Mediated By Bis(Pyrazol-1-Ylmethyl)Pyridine-Ligated Cu(I) Complexes

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)(4)PF6 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 degrees 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.