Enhanced photocatalytic hydrogen production by introducing the carboxylic acid group into cobaloxime catalysts

A series of cobaloxime complexes, [Co(iii)(dmgH)2(py-m-X)Cl] (dmgH = dimethylglyoxime, py-m-X = meta-substituted pyridine, X = COOH (2), COOCH3 (3), CH2CH2COOH (6), and CH2CH2COOCH3 (7)), and [Co(iii)(dmgH)2(py-p-X)Cl], (py-p-X = para-substituted pyridine, X = COOH (4) and COOCH3 (5)), were synthesized and their photocatalytic H2 production activities were compared in an artificial photosynthesis system containing a xanthene dye Eosin Y as the photosensitizer (PS) and triethanolamine (TEOA) as the sacrificial reductant (SR) in CH3CN/H2O (1 : 1, pH = 7.5). Irrespective of substitution by an electron-donating or electron withdrawing group, the photocatalytic H2 production activities of 2-7 are all higher than that of [Co(iii)(dmgH)2(py)Cl] (1). Importantly, meta-substitution is more efficient than para-substitution, and COOH is more efficient than COOCH3, in enhancing the photocatalytic activities. 6 showed the highest activity among the examined complexes. The -CH2CH2- chain linking COOH and pyridine might play a role in the promising performance of 6, which makes the proton relay via interaction between COOH and dmgH possible. This work may open new avenues for developing more efficient cobaloxime-based H2 evolution catalysts (HERs).