Ruthenium Water Oxidation Catalysts based on Pentapyridyl Ligands

Abstract

Ruthenium complexes containing the pentapyridyl ligand 6,6′′‐(methoxy(pyridin‐2‐yl)methylene)di‐2,2′‐bipyridine (L‐OMe) of general formula trans‐[RuII(X)(L‐OMe‐κ‐N5)]n+ (X=Cl, n=1, trans‐1+; X=H2O, n=2, trans‐22+) have been isolated and characterized in solution (by NMR and UV/Vis spectroscopy) and in the solid state by XRD. Both complexes undergo a series of substitution reactions at oxidation state RuII and RuIII when dissolved in aqueous triflic acid–trifluoroethanol solutions as monitored by UV/Vis spectroscopy, and the corresponding rate constants were determined. In particular, aqueous solutions of the RuIII‐Cl complex trans‐[RuIII(Cl)(L‐OMe‐κ‐N5)]2+ (trans‐12+) generates a family of Ru aquo complexes, namely trans‐[RuIII(H2O)(L‐OMe‐κ‐N5)]3+ (trans‐23+), [RuIII(H2O)2(L‐OMe‐κ‐N4)]3+ (trans‐33+), and [RuIII(Cl)(H2O)(L‐OMe‐κ‐N4)]2+ (trans‐42+). Although complex trans‐42+ is a powerful water oxidation catalyst, complex trans‐23+ has only a moderate activity and trans‐33+ shows no activity. A parallel study with related complexes containing the methyl‐substituted ligand 6,6′′‐(1‐pyridin‐2‐yl)ethane‐1,1‐diyl)di‐2,2′‐bipyridine (L‐Me) was carried out. The behavior of all of these catalysts has been rationalized based on substitution kinetics, oxygen evolution kinetics, electrochemical properties, and density functional theory calculations. The best catalyst, trans‐42+, reaches turnover frequencies of 0.71 s−1 using CeIV as a sacrificial oxidant, with oxidative efficiencies above 95 %.

Synthesis and Physicochemical Properties of Ruthenium(II) Complexes Having Pentadentate Scaffolds: Water Oxidation Activity and Deactivation Pathway

“…A similar finding was reported by Zhao et al where the replacement of one of the pyridine group of tris(2‐pyridylmethyl)amine (TPA) ligand with bipyridine improves the catalytic activity of [Ru(DPA‐Bpy)(H 2 O)] 2+ complex (DPA‐Bpy = N , N ‐Bis(2‐pyridinylmethyl)‐2,2′‐bipyridine‐6‐methanamine). [17a] The catalytic performance of 2 Aq is comparable to the reported ones with similar structures; however the activity is much lower than the most efficient mononuclear Ru complex having pentapyridyl ligand scaffold …”

Quantum Chemical Study of the Mechanism of Water Oxidation Catalyzed by a Heterotrinuclear Ru2Mn Complex

“…The lower oxidation state transitions from A to B and B to C are not observed experimentally probably owing to the low quality of the electrochemical response for this complex. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations...…”

“…[41] It is worth mentioning here that the active species, derived from A,t hat are responsible for the OÀOb ond formation, reach formal the oxidation state Ru1 IV Mn V Ru2 IV .T his is in good agreement with the OEC-PSII,w here af ormal Mn V is also neededi nt he S 4 state. [66][67][68] OÀObondformation OÀOb ond formation in water oxidationb yR u [17][18][19][20][76][77][78][79][80] and Mn [62,[69][70][71][72][81][82][83][84][85][86] catalysts have been widelys tudied by quantum chemicalc alculations.T he two popularp athways, namely direct coupling (DC)o ft wo adjacent oxygen units andw ater nucleophilic attack (WNA), were studied herein.…”

“…[1][2][3][4][5] During the past few decades, extraordinary efforts have been dedicated to the synthesis of manganese-based molecular complexes to mimic the structure and function of the OEC. [6][7][8][9][10] Other transition metals were also used for the synthesis of water oxidation catalysts, including Ru, [11][12][13][14][15][16][17][18][19][20][21][22][23][24] Ir, [25,26] Fe, [27][28][29][30][31] Co, [32][33][34] Ni, [35][36][37] and Cu. [38][39][40] Recently,the Llobet group disclosed the synthesisofaheterotrinuclear Ru 2 Mn complex A 0 {[Ru II (tpy)] 2 (m-[Mn II (bpp) 2 ]) (OAc) 2 } 2 + [tpy = 2,2':6',2''-terpyridine, bpp = 3,5-bis(2-pyridyl)pyrazolate].…”

Complete active space analysis of a reaction pathway: Investigation of the oxygen–oxygen bond formation

“…Water splitting, which encompasses water oxidation and reduction, has been investigated intensively over the past few decades as a potential solution for sustainable energy storage and conversion. Various molecular structures, for example, manganese clusters, [ 1–3 ] cobalt cubanes, [ 4–11 ] and ruthenium polypyridyl complexes [ 12–17 ] (see e.g., reviews in refs. [11, 18–22]), have been proposed as potential water oxidation catalysts.…”