Mononuclear nickel(ii) complexes as electrocatalysts in hydrogen evolution reactions: effects of alkyl side chain lengths

Abstract

We report three mononuclear Ni(II) complexes, namely, [Ni(L¹)₂] (1), [Ni(L²)₂] (2) and [Ni(L³)₂] (3), where HL¹ = 1-((4-hydroxybutylimino)methyl)naphthalen-2-ol, HL² = 1-((5-hydroxypentylimino)methyl)naphthalen-2-ol and HL³ = 1-((6-hydroxyhexylimino)methyl)naphthalen-2-ol, as electrocatalysts for hydrogen evolution reactions (HERs). Complexes 1, 2 and 3 were characterized by various standard analytical methods. Single-crystal X-ray structure analysis reveals that nickel is in square planar geometry in all the complexes. These complexes act as efficient electrocatalysts in HERs using acetic acid (AA) and trifluoroacetic acid (TFA) as the proton source in DMF. Controlled-potential electrolysis experiments show that these complexes are capable of reducing protons of AA and TFA to produce H₂. Control experiments show that the complexes are essential for improved production of hydrogen. Theoretical calculations were performed to support the mechanism of HER and to check the effect of chain lengths on the catalytic activity. The catalytic activity runs in the order of complex 1 > 2 > 3.