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 2,20:60,20 0- terpyridine (trpy) of the general formula {[RuII(trpy)]2(m-[M(X)2(bpp)2])}(PF6)2, where M ¼ CoII, MnII and X ¼ Cl , AcO (M ¼ CoII, X ¼ Cl : Ru2Co-Cl2; M ¼ MnII, X ¼ Cl : Ru2Mn-Cl2; M ¼ CoII, X ¼ AcO : Ru2Co- OAc2; M ¼ MnII, X ¼ 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 ¼ 7 generating the active catalysts, {[Ru(H2O)(trpy)]2(m-[M(H2O)2(bpp)2])}4+ (M ¼ CoII: Ru2Co-(H2O)4; M ¼ 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 ¼ 7. The Co containing complex generates a TON of 50 in about 10 minutes (TOFi ¼ 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 ¼ 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.