Synthesis, structure, and hydrogen evolution studies of a heteroleptic Co(III) complex

Abstract

[Co(tpy)(phen)Cl](PF6)2•0.25CH3CN (where tpy = 2,2′;6′,2″-terpyridine and phen = 1,10-phenanthroline) was prepared from a one pot mixture involving stoichiometric quantities of tpy and phen. The structure of [Co(tpy)(phen)Cl](PF6)2•0.25CH3CN was confirmed by elemental analysis, high resolution mass spectroscopy (HRMS), various spectroscopic analyses, and X-ray crystallography. Density functional theory calculations were also carried out. The crystal structure of [Co(tpy)(phen)Cl](PF6)2•0.25CH3CN, which was grown from acetonitrile, revealed a monoclinic crystal system with a C2/c space group. The cyclic voltammogram which was acquired in acetonitrile revealed reversible CoIII/II, CoII/I, and CoI/0 mixed with ligand-based redox couples at E½ = +0.35, –0.81, and –1.37 V (vs Ag/AgCl), respectively. In the presence of p-cyanoanillinium tetrafluoroborate with acetonitrile as the solvent, [Co(tpy)(phen)Cl](PF6)2•0.25CH3CN displayed electrocatalytic hydrogen evolution activity at a 830 mV overpotential, as evidenced by a catalytic wave which was observed in the voltammogram, and by the detection of hydrogen in the headspace of the reaction vessel of a controlled potential electrolysis experiment. Photocatalytic hydrogen evolution studies with [Co(tpy)(phen)Cl](PF6)2•0.25CH3CN produced a turnover frequency (TOF) of 3300 mmol H2 mol−1CAT min−1 when compared to [Co(dmgH)2(py)Cl] (where dmgH = dimethylglyoximato), which had a TOF of 4500 mmol H2.mol−1CAT min−1 under the same conditions. [Co(tpy)(phen)Cl](PF6)2•0.25CH3CN produced a turnover number (TON) of 79 when compared to 141 for [Co(dmgH)2Cl(py)] in DMF in ca 3 h.