Synthesis, structure, spectroscopy and reactivity of new heterotrinuclear water oxidation catalysts. Issue 5 (16th February 2016) - Mognon, Lorenzo
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 have been prepared for the first time. 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 − :Ru2 Co–Cl2 ; M = Mn II, X = Cl − :Ru2 Mn–Cl2 ; M = Co II, X = AcO − :Ru2 Co–OAc2 ; M = Mn II, X = AcO − :Ru2 Mn–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 theRu2 Mn–Cl2 andRu2 Mn–OAc2 complexes. The cyclic voltammograms (CV) for all four complexes in organic solvent (CH3 CN or CH2 Cl2 ) display three successive reversible oxidative waves corresponding to one-electron oxidations of each of the three metal centers. The oxidized forms of the complexesRu2 Co–OAc2 andRu2 Mn–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 complexesRu2 Co–OAc2 andRu2 Mn–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(H2 O)(trpy)]2 (μ-[M(H2 O)2 (bpp)2 ])} 4+ (M = Co II :Ru2 Co–(H2 O)4 ; M = Mn II :Ru2 Mn–(H2 O)4 ). The photochemical water oxidation reaction is studied using [Ru(bpy)3 ] 2+ as the photosensitizer and Na2 S2 O8 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 ofRu2 Co–(H2 O)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.