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

Abstract

Altres ajuts: EU COST actions CM1202 i CM1205; Labex Arcane (ANR-11-LABX-0003-01)

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)](μ-[M(X)(bpp)])}(PF) have been prepared for the first time. 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)](μ-[M(X)(bpp)])}(PF), where M = Co II, Mn II and X = Cl -, AcO - (M = Co II, X = Cl - : RuCo-Cl ; M = Mn II, X = Cl - : RuMn-Cl ; M = Co II, X = AcO - : RuCo-OAc ; M = Mn II, X = AcO - : RuMn-OAc), 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 RuMn-Cl and RuMn-OAc complexes. The cyclic voltammograms (CV) for all four complexes in organic solvent (CHCN or CHCl) display three successive reversible oxidative waves corresponding to one-electron oxidations of each of the three metal centers. The oxidized forms of the complexes RuCo-OAc and RuMn-OAc 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() and Mn() centers. The complexes RuCo-OAc and RuMn-OAc 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(HO)(trpy)](μ-[M(HO)(bpp)])} 4+ (M = Co II : RuCo-(HO) ; M = Mn II : RuMn-(HO)). The photochemical water oxidation reaction is studied using [Ru(bpy)] 2+ as the photosensitizer and NaSO as a sacrificial electron acceptor at pH = 7. The Co containing complex generates a TON of 50 in about 10 minutes (TOF = 0.21 s -1), whereas the Mn containing complex only generates a TON of 8. The water oxidation reaction of RuCo-(HO) 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.