Synthesis, structure, spectroscopy and reactivity of new heterotrinuclear water oxidation catalysts

Abstract

Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bpp⁻) and 2,2′:6′,2′′-terpyridine (trpy) of the general formula {[Ru^II(trpy)]₂(μ-[M(X)₂(bpp)₂])}(PF₆)₂, where M = Co^II, Mn^II and X = Cl⁻, AcO⁻ (M = Co^II, X = Cl⁻: Ru₂Co–Cl₂; M = Mn^II, X = Cl⁻: Ru₂Mn–Cl₂; M = Co^II, X = AcO⁻: Ru₂Co–OAc₂; M = Mn^II, X = AcO⁻: Ru₂Mn–OAc₂), have been prepared for the first time. The complexes have been characterized using different spectroscopic techniques such as UV-vis, IR, and mass spectrometry. X-Ray diffraction analyses have been used to characterize the Ru₂Mn–Cl₂ and Ru₂Mn–OAc₂ complexes. The cyclic voltammograms (CV) for all four complexes in organic solvent (CH₃CN or CH₂Cl₂) display three successive reversible oxidative waves corresponding to one-electron oxidations of each of the three metal centers. The oxidized forms of the complexes Ru₂Co–OAc₂ and Ru₂Mn–OAc₂ are further characterized by EPR and UV-vis spectroscopy. The magnetic susceptibility measurements of all complexes in the temperature range of 2–300 K reveal paramagnetic properties due to the presence of high spin Co(II) and Mn(II) centers. The complexes Ru₂Co–OAc₂ and Ru₂Mn–OAc₂ act as precatalysts for the water oxidation reaction, since the acetato groups are easily replaced by water at pH = 7 generating the active catalysts, {[Ru(H₂O)(trpy)]₂(μ-[M(H₂O)₂(bpp)₂])}⁴⁺ (M = Co^II: Ru₂Co–(H₂O)₄; M = Mn^II: Ru₂Mn–(H₂O)₄). The photochemical water oxidation reaction is studied using [Ru(bpy)₃]²⁺ as the photosensitizer and Na₂S₂O₈ as a sacrificial electron acceptor at pH = 7. The Co containing complex generates a TON of 50 in about 10 minutes (TOF_i = 0.21 s⁻¹), whereas the Mn containing complex only generates a TON of 8. The water oxidation reaction of Ru₂Co–(H₂O)₄ is further investigated using oxone as a sacrificial chemical oxidant at pH = 7. Labelled water oxidation experiments suggest that a nucleophilic attack mechanism is occurring at the Co site of the trinuclear complex with cooperative involvement of the two Ru sites, via electronic coupling through the bpp⁻ bridging ligand and via neighboring hydrogen bonding.