Homoleptic ruthenium complexes with N-heterocyclic carbene ligands as photosensitizers in the photocatalytic generation of H2 from water

Abstract

There is great interest in the catalytic photoreduction of water to give hydrogen as a fuel to harness solar energy and a series of ruthenium complexes has been synthesized and tested as photosensitizers in this photoreduction process. There are very few precedents for N-heterocyclic carbene complexes in this field. The complexes obtained in this work were of the type [Ru(:CˆNˆC:)2](PF6)2 and [Ru(NˆC:)3](PF6)2 with N-heterocyclic carbene ligands derived from pyridine and imidazole heterocycles with methyl or benzyl substituents. The photophysical properties of the complexes were studied. Some complexes were luminescent and, although the quantum yields were rather low, the lifetimes were quite high (1.5–1.7 μs). The emissive complexes behave as photosensitizers in the generation of H2 using [Co(bpy)3]Cl2 (bpy = 2,2′-bipyridine) as catalyst and triethanolamine (TEOA) as the sacrificial reductant through an oxidative quenching mechanism. The amount of hydrogen obtained was higher for the benzyl derivative.