Dual homogeneous and heterogeneous pathways in photo- and electrocatalytic hydrogen evolution with nickel(II) catalysts bearing tetradentate macrocyclic ligands

A series of nickel(II) complexes bearing tetradentate macrocyclic N₄, N₃S, and N₃P ligands were synthesized, and their photocatalytic activity toward proton reduction has been investigated by using [Ir(dF(CF₃)ppy)₂(dmbpy)]PF₆ (dF(CF₃)ppy =2-(2,4-difluorophenyl)-5-trifluoromethylpyridine and dmbpy = 4,4′-dimethyl-2,2′-dipyridyl) as the photosensitizer and triethylamine (TEA) asthe sacrificial reductant. The complex [Ni(L4)]²⁺ (L4 =2,12-dimethyl-7-phenyl-3,11,17-triaza-7-phospha-bicyclo[11,3,1]heptadeca-1(17),13,15-triene), which bears a phosphorus donor atom, shows the highest efficiency with TON up to 5000 under optimized conditions, while the tetraaza macrocyclic nickel complexes [Ni(L1)]²⁺ and [Ni(L2)]²⁺ (L1 = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo[11.3.l]heptadeca-1(17),2,11,13,15-pentaene; L2 = 2,12-dimethyl-3,7,11,17-tetra-azabicyclo[11.3.l]heptadeca-1(17),13,15-triene)show lower photocatalytic activities. Transient UV-vis absorption and spectroelectrochemical experiments show that Ni(II) is reduced to Ni(I) under photocatalytic conditions. However, dynamic light scattering and mercury poisoning experiments suggest that the Ni(I) is further reduced to Ni(0) nanoparticles which are the real catalysts for H₂ production. Electrocatalytic proton reduction by [Ni(L4)]²⁺ has also been investigated. In this case, the electrochemical behavior is consistent with a homogeneous pathway, and no Ni nanoparticles were observed on the electrode surface during the first few hours of electrolysis. However, on prolonged electrolysis for >17 h, nickel-based nanoparticles were observed on the electrode surface, which are active catalysts for H₂ production. Copyright © 2014 American Chemical Society.