Establishing Structure–Activity Relationships in Photocatalytic Systems by Using Nickel Bis(dithiolene) Complexes as Proton Reduction Catalysts

Herein, we present an artificial photocatalytic system for H₂ evolution based on a series of proton reduction catalysts, namely [Ni{S₂C₂(Ph)₂}₂] (1), [Ni{S₂C₂(Ph)₂}₂](NEt₄) (1^–), [Ni{S₂C₂(Ph)(Ph–OCH₃-4)}₂] (2), [Ni{S₂C₂(Ph)(Ph–OCH₃-4)}₂](NEt₄) (2^–), [Ni{S₂C₂(Ph–OCH₃-4)₂}₂] (3) and [Ni{S₂C₂(Ph–OCH₃-4)₂}₂](NEt₄) (3^–), [Ni(mnt)₂] (NBu₄) (4^–), [Ni(mnt)(S₂C₂(Ph)₂)](NBu₄) (5^–). These complexes are different of both in charge and the substituents on dithiolene ligand and represent a group of active catalysts for the reduction of protons with high TONs. A series of Re^I complexes, [ReBr(CO)₃ (bpy)], [ReCl(CO)₃(bpy)], [Re(NCS)(CO₃)(bpy)], [ReBr(CO)₃(amphen)] and [ReBr(CO)₃ (pq)] were used as photosensitizers, in combination with triethanolamine as a sacrificial electron donor and acetic acid as a proton source. The differences of the ligands in the electron donating properties and reactivity are discussed in the light of the experimental data. We also present the physicochemical characterization of a previously reported heteroleptic monoanionic complex (5^–), by UV/Vis spectroscopy, FTIR spectroscopy, cyclic voltammetry and single-crystal X-ray diffraction studies.