N-substituted 2-pyridinecarbothioamides and polypyridyl mixed-ligand cobalt(III)-containing complexes for photocatalytic hydrogen generation

Abstract Two mixed-ligand cobalt(III) complexes containing 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen), and N-(3,5-bis(trifluoromethyl)phenyl)pyridine-2-carbothioamide (PCA-(CF3)2) as ligands were synthesized, and characterized by a variety of spectroscopic techniques and elemental analyses. Crystals of the PCA-(CF3)2 molecule are of the monoclinic, C2/c crystal system and space group as determined via X-ray crystallography. The voltammetric properties of six cobalt(III) complexes, [Co(bpy)2Cl2]Cl 1, [Co(phen)2Cl2]Cl 2, [Co(bpy)2(PCA-(CF3)2)](PF6)2·H2O 3, [Co(phen)2(PCA-(CF3)2)](PF6)2·1.25H2O 4, [Co(bpy)3](PF6)3 5, and [Co(phen)3](PF6)3 6, were compared to assess the potential influence of the PCA-(CF3)2 ligand on the effectiveness of cobalt in the catalytic hydrogen evolution reaction (HER) under electrochemical and photochemical conditions. The PCA-(CF3)2 moiety caused an anodic shift in the reduction potential of CoII/I redox couple of complexes 3 and 4, in comparison to the other complexes. Complexes 3 and 4 demonstrated catalytic HER in the presence of p-cyanoanilinium tetrafluoroborate in CH3CN, with overpotentials for the HER of 730 and 630 mV for complexes 3 and 4, respectively. The reduction potentials suggest that the HER was most likely facilitated by a homolytic pathway. Complexes 3 and 4 also demonstrated photocatalytic HER in the presence of [Ru(bpy)3](PF6)2 as a photosensitizer and triethanolamine as a sacrificial reductant in DMF. Complexes 3 and 4 attained rates of 2700 mmol H2.mol−1 CAT min−1 and 2600 mmol H2 mol−1 CAT min−1, respectively, compared to a standard complex [Co(dmgH)2(py)Cl] which had a rate of 4500 mmol H2 mol−1 CAT min−1, under similar conditions. Turnover numbers (TON) of 140 were observed for [Co(dmgH)2(py)Cl] and complex 4, compared to 91 for complex 3, over a 3 h period.