Synthesis, Structure, Spectroscopy and Reactivity of New Heterotrinuclear Water Oxidation Catalysts

Synthesis, Structure, Spectroscopy and Reactivity of New Heterotrinuclear Water Oxidation Catalysts

Lorenzo Mognon, Sukanta Mandal, Carmen E Castillo, Jérôme Fortage, Florian Molton, Guillem Aromi, Jordi Benet-Buchholz, Carole Duboc, Marie-Noëlle Collomb and Antoni Llobet
Chem. Sci., 2016, Accepted Manuscript
DOI: 10.1039/C5SC04672F

Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bpp-) and 2,2’:6’,2’’-terpyridine (trpy) of general formula {[RuII(trpy)]2µ-[M(X)2(bpp)2])}(PF6)2, where M = CoII, MnII and X = Cl-, AcO- (M = CoII, X = Cl-: Ru2Co-Cl2; M = MnII, X = Cl-: Ru2Mn-Cl2; M = Co, X = AcO-: Ru2Co-OAc2; M = MnII, X = AcO-: Ru2Mn-OAc2) have been prepared for the first time. The complexes have been characterized using different spectroscopic techniques such as UV-vis and IR, and mass spectrometry. X-Ray diffraction analyses have been used to characterize the Ru2Mn-Cl2 and Ru2Mn-OAc2 complexes. 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 complexes Ru2Co-OAc2 and Ru2Mn-OAc2 are further characterized by EPR and UV-vis spectroscopy. The magnetic susceptibility measurements of all complexes at temperature range 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 pre catalysts for 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 = CoII: Ru2Co-(H2O)4; M = MnII: Ru2Mn-(H2O)4). The photochemical water oxidation reaction is studied using [Ru(bpy)3]2+ as photosensitizer and Na2S2O8 as sacrificial electron acceptor at pH = 7. The Co containing complex generates 50 TON in about 10 minutes (TOFi = 0.21 s-1) whereas the Mn containing one only generates 8 TON. The water oxidation reaction of Ru2Co-(H2O)4 is further investigated using Oxone as 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.