A novel ruthenium(ii)–cobaloxime supramolecular complex for photocatalytic H2 evolution: synthesis, characterisation and mechanistic studies

Abstract

We report the synthesis and characterization of novel mixed-metal binuclear ruthenium(II)–cobalt(II) photocatalysts for hydrogen evolution in acidic acetonitrile. First, 2-(2′-pyridyl)benzothiazole (pbt), 1, was reacted with RuCl₃·xH₂O to produce [Ru(pbt)₂Cl₂]·0.25CH₃COCH₃, 2, which was then reacted with 1,10-phenanthroline-5,6-dione (phendione), 3, in order to produce [Ru(pbt)₂(phendione)](PF₆)₂·4H₂O, 4. Compound 4 was then reacted with 4-pyridinecarboxaldehyde in order to produce [Ru(pbt)₂(L-pyr)](PF₆)₂·9.5H₂O, 5 (where L-pyr = (4-pyridine)oxazolo[4,5-f]phenanthroline). Compound 5 was then reacted with [Co(dmgBF₂)₂(H₂O)₂] (where dmgBF₂ = difluoroboryldimethylglyoximato) in order to produce the mixed-metal binuclear complex, [Ru(pbt)₂(L-pyr)Co(dmgBF₂)₂(H₂O)](PF₆)₂·11H₂O·1.5CH₃COCH₃, 6. [Ru(Me₂bpy)₂(L-pyr)Co(dmgBF₂)₂(OH₂)](PF₆)₂, 7 (where Me₂bpy = 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine) and [Ru(phen)₂(L-pyr)Co(dmgBF₂)₂(OH₂)](PF₆)₂, 8 were also synthesised. All complexes were characterized by elemental analysis, ESI MS, HRMS, UV-visible absorption, ¹¹B, ¹⁹F, and ⁵⁹Co NMR, ESR spectroscopy, and cyclic voltammetry, where appropriate. Photocatalytic studies carried out in acidified acetonitrile demonstrated constant hydrogen generation longer than a 42 hour period as detected by gas chromatography. Time resolved spectroscopic measurements were performed on compound 6, which proved an intramolecular electron transfer from an excited Ru(II) metal centre to the Co(II) metal centre via the bridging L-pyr ligand. This resulted in the formation of a cobalt(I)-containing species that is essential for the production of H₂ gas in the presence of H⁺ ions. A proposed mechanism for the generation of hydrogen is presented.