Substituent Effects on Physical Properties and Catalytic Activities toward Water Oxidation in Mononuclear Ruthenium Complexes

Abstract

A series of monomeric Ru II complexes [Ru(bda)(pyR) 2 ] (H 2 bda = 2,2Ј-bipyridine-6,6Ј-dicarboxylic acid, pyR = pyridine with a substituent R at the 4-position) were prepared, and the effects of substituent groups on the pyridine ligands of the complexes on catalytic activities toward water oxidation, as well as on photophysical and electrochemical properties, were examined. An increase in the electron-withdrawing ability of the substituent results in a redshift of the absorption maximum assigned to the transition from the d orbital of Ru to the π* orbitals of the pyridine ligands, and an increase in the oxidation potential of the complex. These results are reasonably explained in terms of the variation of the energy level of molecular orbitals induced by the substituent group, which is supported by density functional theory [a]

Off‐Set Interactions of Ruthenium–bda Type Catalysts for Promoting Water‐Splitting Performance

“…ESI, Figure S2 and Table S1). Interestingly, a good Pearson's r value could be obtained correlating σ m with the Ru III /Ru II oxidation, as previously also observed by Murata and co‐workers for catalysts bearing a para ‐substituted pyridine [12] . As the Ru II ‐species is not involved in the catalytic cycle, the two other redox couples were investigated as well, however, no clear correlation could be observed between their oxidation potentials and σ m (Figure 4).…”

“…From their studies it is apparent that introduction of electron withdrawing groups on the axial ligands improves the catalyst activity towards water oxidation, whereas the introduction of electron‐donating functionalities deterred catalysis [11] . However, the presence or absence of electronic effects yet remains unclear as Murata and co‐workers clearly show that electron donating groups can present an advantage to catalysis as well [12] . One aspect that arises from most of the ligand studies is that secondary effects facilitate the I2M pathway by minimizing the energy required to bring two catalytic units together, e .g ., hydrophobic, π‐π‐stacking, dispersive and electrostatic interactions [14]…”

Artificial photosynthesis: opportunities and challenges of molecular catalysts

“…However, this cannot indicate that the catalyst is not excellent or not promising for an applicable AP device, because the performance of a PEC cell or an electrolyzer is determined by a number of factors, such as catalyst loading amount, loading method, electrolyte, device design for mass transfer and proton transfer. For example, the Ru-bda WOC exhibited an activity similar to the Nature's OEC when it is evaluated with Ce 4+ as the oxidant under acidic conditions; [146][147][148][149] however, it exhibited very moderate performance under the three-component photocatalytic system. 145,616,617 In particular, when the Ru-bda WOC was employed for the first time to construct a DSPEC cell as shown in Fig.…”

“…144 An electrochemical study of 5 indicates that it needs an overpotential of only 180 mV to initiate water oxidation. 145 Because of the outstanding activity, Ru-bda-type WOCs have received extensive research attention worldwide and have been widely investigated by the Sun group among many other groups; researchers have studied the effect of axial ligands, [146][147][148][149] catalytic mechanisms, 80,81,[150][151][152][153][154][155][156] development of Ru-bda analogues [157][158][159][160] and applications of Ru-bda in AP devices. [161][162][163][164][165][166] The studies on the effect of axial ligands of Ru-bda-type WOCs were very fruitful.…”

Impact of substituents on molecular properties and catalytic activities of trinuclear Ru macrocycles in water oxidation

“…[10][11][12][13] Among those, ruthenium-based catalysts 12,13 particularly Ru(bda) WOCs (bda: 2,2 0 -bipyridine-6,6 0 -dicarboxylic acid) 14,15 have attracted much attention as some of these catalysts exhibit activities comparable to those of the oxygen-evolving complex of photosystem II. 16,17 During the last decade, some insightful studies have been performed on the effect of substituents at axial [18][19][20][21][22][23][24][25] and equatorial [26][27][28] ligands on the catalytic performance of monomeric Ru(bda) WOCs. Hereby, uncomplicated manipulations of the axial ligands have enabled the synthesis of a plethora of catalysts within the family of mononuclear Ru(bda) WOCs.…”