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

© 2016 The Royal Society of Chemistry. Four heterotrinuclear complexes containing the ligands 3,5-bis(2-pyridyl)pyrazolate (bpp-) and 2,2′:6′,2′′-terpyridine (trpy) of the general formula {[RuII(trpy)]2(μ-[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 = CoII, 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, 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 = CoII: Ru2Co-(H2O)4; M = MnII: 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 (TOFi = 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.