A novel ruthenium(II)–cobaloxime supramolecular complex for photocatalytic H_2 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 RuCl_(3)·xH_(2)O to produce [Ru(pbt)_(2)Cl_2]·0.25CH_(3)COCH_3, 2, which was then reacted with 1,10-phenanthroline-5,6-dione (phendione), 3, in order to produce [Ru(pbt)_(2)(phendione)](PF_(6))_2·4H_(2)O, 4. Compound 4 was then reacted with 4-pyridinecarboxaldehyde in order to produce [Ru(pbt)_(2)(L-pyr)](PF_6)_(2)·9.5H_(2)O, 5 (where L-pyr = (4-pyridine)oxazolo[4,5-f]phenanthroline). Compound 5 was then reacted with [Co(dmgBF_2)_(2)(H_(2)O)_2] (where dmgBF_(2) = difluoroboryldimethylglyoximato) in order to produce the mixed-metal binuclear complex, [Ru(pbt)_(2)(L-pyr)Co(dmgBF_(2))_(2)(H_(2)O)](PF_(6))_2·11H_(2)O·1.5CH_(3)COCH_3, 6. [Ru(Me_(2)bpy)_2(L-pyr)Co(dmgBF_2)_(2)(OH_2)](PF_6)_(2), 7 (where Me_(2)bpy = 1,10-phenanthroline, 4,4′-dimethyl-2,2′-bipyridine) and [Ru(phen)_(2)(L-pyr)Co(dmgBF_2)_(2)(OH_2)](PF_(6))_2, 8 were also synthesised. All complexes were characterized by elemental analysis, ESI MS, HRMS, UV-visible absorption, ^(11)B, ^(19)F, and ^(59)Co 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 H_2 gas in the presence of H^+ ions. A proposed mechanism for the generation of hydrogen is presented.

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