Performance of nickel diselenolene complexes in photocatalytic hydrogen evolution

Two nickel-diselenolene complexes (NBu 4 )[Ni(bds) 2 ] ( 1 ), (NBu 4 )[Ni(tds) 2 ] ( 2 ) (bds 2− = benzene-1,2-diselenolate and tds 2− = toluene-3,4-diselenolate), along with their dithiolene analog (NBu 4 )[Ni(bdt) 2 ] ( 3 ) (bdt 2− = benzene-1,2-dithiolate) were employed for the first time as catalysts for the photocatalytic hydrogen evolution reaction in a homogeneous system with Ru(bpy) 3 2+ photosensitizer and ascorbate buffer (pH = 4.2) as electron donor in mixed DMF-aqueous media. All three complexes were catalytically active, achieving up to 108 TON cat after 24 h of irradiation with visible light (white LEDs). Electrocatalytic experiments indicated a CE initiation of mechanism for hydrogen evolution, whereas Hg poisoning experiments suggested a dominant homogeneous path. We also report the crystal structure of 2 for the first time, observing both cis - and trans - conformations of the ligand in the solid state. DFT calculations revealed that the two isomers are isoenergetic, suggesting that both coexist in solution. Although the position of the methyl substituent does not affect their structural properties, it reduces their catalytic activity, in contrast to what has been observed in their dithiolene analogs.