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) as the 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.