Catalytic Oxidation of Water to Dioxygen by Mononuclear Ru Complexes Bearing a 2,6‐Pyridinedicarboxylato Ligand

Abstract

The synthesis, purification, and isolation of mononuclear Ru complexes containing the tridentate dianionic meridional ligand pyridyl‐2,6‐dicarboxylato (pdc2−) of general formula [RuIII(pdc‐κ3‐N1O2)(bpy)Cl] (1III) and [RuII(pdc‐κ2‐N1O1)(bpy)2] (2II) (bpy is 2,2′‐bipyridine) is reported. These two complexes and their derivatives were thoroughly characterized through spectroscopic (UV/Vis, NMR) and electrochemical (cyclic voltammetry, differential pulse voltammetry, and coulometry) analyses, and three of the complexes were analyzed by single‐crystal X‐ray diffraction techniques. Under a high anodic applied potential, both complexes evolve towards the formation of Ru‐aquo/oxo derivative species, namely, [RuIII(pdc‐κ3‐N1O2)(bpy)(OH2)]+ (1‐O) and [RuIV(O)(pdc‐κ2‐N1O1)(bpy)2] (2‐O). These two complexes are active catalysts for the oxidation of water to dioxygen and their catalytic activity was analyzed through electrochemical techniques. A maximum turnover frequency (TOFmax)=2.4–3.4×103 s−1 was calculated for 2‐O.

Synthesis, Characterization, and Water Oxidation Activity of Isomeric Ru Complexes

“…For this reason, the study of new molecular water oxidation catalysts (WOC) has become one of the main research areas in the field, achieving tremendous progress in the past 10 years. Among the most studied catalysts are those based on ruthenium, starting from the first well characterized blue dimer cis , cis -[(bpy) 2 (H 2 O)­Ru III ORu III (OH 2 )­(bpy) 2 ] 4+ , where bpy is 2,2′-bipyridine to the recently reported fastest molecular WOC based on mononuclear Ru complexes [Ru­(bda)­(isoq) 2 ], [Ru­(tda)­(py) 2 ], [Ru­(pdc)­(bpy) 2 ], [Ru­(mcbp)­(py) 2 ], and [Ru II (H 3 tPa-κ-N 3 O)­(py) 2 ] + , (bda 2– is (2,2′-bipyridine)-6,6′-dicarboxylate, tda 2– is 2,2′:6′,2″-terpyridine-6,6″-dicarboxylate, pdc 2– is pyridine-2,6-dicarboxylate, mcbp 2– is 2,6-bis­(1-methyl-4-(carboxylate)­benzimidazol-2-yl)­pyridine, H 3 tPa is 2,2′:6′,2″-terpyridine-6,6″- diphosphonic acid, isoq is isoquinoline, and py is pyridine). Further, these catalyst precursors have also been successfully used in water oxidation reactions triggered by light. − …”

Second Coordination Sphere Effects in an Evolved Ru Complex Based on Highly Adaptable Ligand Results in Rapid Water Oxidation Catalysis

“…The addition of two equivalents of Ce­(IV) generates the corresponding seven coordinated Ru­(IV) complex [Ru IV (tPa-κ-N 3 O 2 )­(py) 2 ], 3 , where the tPa 4– ligand now acts in a pentadentate manner and the metal center possesses a pentagonal bipyramidal geometry as can be observed in the computed structure shown in Figure . This complex is diamagnetic at room temperature (RT) with a high field low spin d 4 electronic configuration, ( e 1 ′′ ) 4 ( e 2 ′ ) 0 ( a 1 ′ ) 0 , analogous to related Ru­(IV) complexes. ,, Its 1 H, 13 C, and 31 P NMR spectra are presented Figures S27–29.…”

“…This complex is diamagnetic at room temperature (RT) with a high field low spin d 4 electronic configuration, (e 1 ′′) 4 (e 2 ′) 0 (a 1 ′) 0 , analogous to related Ru(IV) complexes. 19,40,41 Its 1 H, 13 C, and 31 2). The dynamic behavior is associated with the synchronic coordination and decoordination of the dangling phosphonate that has an activation energy of 13.3 kcal/mol based on Eyring plots generated from VT-NMR (Scheme S1).…”

“…This complex is diamagnetic at room temperature (RT) with a high field low spin d 4 electronic configuration, (e1 " ) 4 (e2 ' ) 0 (a1 ' ) 0 , analogous to related Ru(IV) complexes. 19,40,41 .…”

Bio-Inspired Molecular Catalysts for Water Oxidation

“…There is a family of mononuclear ruthenium complexes, [Ru II (pdc-κ 3 -N 1 O 2 )(bipy)(H 2 O)] ( 31) and [Ru II (pdc-κ 2 -N 1 O 1 )(bipy) 2 ] (32), containing the tridentate dianionic meridional pyridyl-2,6-dicarboxylato (pdc 2− ) ligand that have been studied for their electrochemical activity towards water oxidation [78,103]. Complex 31 has been shown to electrochemically catalyze water oxidation with a low overpotential of 240 mV under acidic conditions (pH of 1), due to the presence of two carboxylate groups on the pdc 2− ligand.…”