Dinuclear Ruthenium Complexes Containing the Hpbl Ligand: Synthesis, Characterization, Linkage Isomerism, and Epoxidation Catalysis

Catalytic water oxidation by an in situ generated ruthenium nitrosyl complex bearing a bipyridine-bis(alkoxide) ligand

Oxidative degradation and transformation of catalysts are commonly observed in water oxidation by molecular catalysts, especially when a highly oxidizing reagent such as (NH₄)₂[Ce(NO₃)₆] [Ce(IV)] is used. We report herein the synthesis of a ruthenium(III) complex bearing an oxidative resistant bipyridine-bis(alkoxide) ligand, [Ru(bdalk)(pic)₂]⁺ (1, H₂bdalk = 2,2'-([2,2'-bipyridine]-6,6'-diyl)bis(propan-2-ol), pic = 4-picoline) as a water oxidation catalyst (WOC). A ruthenium(II) nitrosyl complex [Ru(Hbdalk)(NO)(pic)₂]²⁺ (3) was also formed during the water oxidation process by 1/Ce(IV), and was isolated and structurally characterized. Complex 3 was found to be an active WOC, with the nitrosyl group remaining intact during water oxidation.

Electrochemically Driven Water Oxidation by a Highly Active Ruthenium-Based Catalyst

The highly active ruthenium-based water oxidation catalyst [Ru^X (mcbp)(OH_n )(py)₂ ] [mcbp^2- =2,6-bis(1-methyl-4-(carboxylate)benzimidazol-2-yl)pyridine; n=2, 1, and 0 for X=II, III, and IV, respectively], can be generated in a mixture of Ru^III and Ru^IV states from either [Ru^II (mcbp)(py)₂ ] or [Ru^III (Hmcbp)(py)₂ ]²⁺ precursors. The precursor complexes are isolated and characterized by single-crystal X-ray analysis, NMR, UV/Vis, EPR, and FTIR spectroscopy, ESI-HRMS, and elemental analysis, and their redox properties are studied in detail by electrochemical and spectroscopic methods. Unlike the parent catalyst [Ru(tda) (py)₂ ] (tda^2- =[2,2':6',2''-terpyridine]-6,6''-dicarboxylate), for which full transformation into the catalytically active species [Ru^IV (tda)(O)(py)₂ ] could not be carried out, stoichiometric generation of the catalytically active Ru-aqua complex [Ru^X (mcbp)(OH_n )(py)₂ ] from the Ru^II precursor was achieved under mild conditions (pH 7.0) and short reaction times. The redox properties of the catalyst were studied and its activity for electrocatalytic water oxidation was evaluated, reaching a maximum turnover frequency (TOF_max ) of around 40 000 s^-1 at pH 9.0 (from foot-of-the-wave analysis), which is comparable to the activity of the state-of-the-art catalyst [Ru^IV (tda)(O)(py)₂ ].

Electrochemical properties and C-H bond oxidation activity of [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)]. Dalton Trans 2018;47:9701-9708. [PMID: 29978176 DOI: 10.1039/c8dt02260g]

[Ru(tpy)(pyalk)Cl]Cl (pyalk = 2-(2'-pyridyl)-2-propanol) was synthesized and characterized crystallographically and electrochemically. Upon dissolution in water and acetonitrile, [Ru(tpy)(pyalk)Cl]Cl was found to form [Ru(tpy)(pyalk)Cl]+ and [Ru(tpy)(pyalk)(OH)]+, respectively. The Ru(ii/iii) couple of [Ru(tpy)(pyalk)Cl]+ was found to be relatively low compared to that of other Ru complexes in acetonitrile, but the Ru(iii/iv) couple was not significantly different than other Ru complexes bearing anionic ligands. Pourbaix diagrams were generated for [Ru(tpy)(phpy)(OH2)]+ (phpy = 2-phenylpyridine) and [Ru(tpy)(pyalk)(OH)]+ in water, and it was found that [Ru(tpy)(pyalk)(OH)]+ has a lower Ru(ii/iii) potential than [Ru(tpy)(phpy)(OH2)]+ under neutral to alkaline pH. [Ru(tpy)(pyalk)(OH)]+ was found to catalyze C-H bond hydroxylation of secondary alkanes and epoxidation of alkenes using cerium(iv) ammonium nitrate as the primary oxidant.

Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic 2,2'-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(ii) complex by PhI(OAc)2 generates the Ru(iv)[double bond, length as m-dash]O species, which is very sluggish for olefin epoxidation. When Ru(bpy)2Cl2 was tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclooctane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation of Ru(ii) by PhI(OAc)2 and the reduction of Ru(iv)[double bond, length as m-dash]O by olefin are kinetically slow. However, adding non-redox metal ions such as Al(iii) can sharply improve the oxygen transfer efficiency of the catalyst to 100% conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic characterizations, an adduct of Ru(iv)[double bond, length as m-dash]O with Al(iii), Ru(iv)[double bond, length as m-dash]O/Al(iii), was proposed to serve as the active species for epoxidation, which in turn generated a Ru(iii)-O-Ru(iii) dimer as the reduced form. In particular, both the oxygen transfer from Ru(iv)[double bond, length as m-dash]O/Al(iii) to olefin and the oxidation of Ru(iii)-O-Ru(iii) back to the active Ru(iv)[double bond, length as m-dash]O/Al(iii) species in the catalytic cycle can be remarkably accelerated by adding a non-redox metal, such as Al(iii). These results have important implications for the role played by non-redox metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox mechanism of ruthenium catalysts in the oxygen atom transfer reaction.

Intramolecular Proton Transfer Boosts Water Oxidation Catalyzed by a Ru Complex

We introduce a new family of complexes with the general formula [Ru(n)(tda)(py)2](m+) (n = 2, m = 0, 1; n = 3, m = 1, 2(+); n = 4, m = 2, 3(2+)), with tda(2-) being [2,2':6',2″-terpyridine]-6,6″-dicarboxylate, including complex [Ru(IV)(OH)(tda-κ-N(3)O)(py)2](+), 4H(+), which we find to be an impressive water oxidation catalyst, formed by hydroxo coordination to 3(2+) under basic conditions. The complexes are synthesized, isolated, and thoroughly characterized by analytical, spectroscopic (UV-vis, nuclear magnetic resonance, electron paramagnetic resonance), computational, and electrochemical techniques (cyclic voltammetry, differential pulse voltammetry, coulometry), including solid-state monocrystal X-ray diffraction analysis. In oxidation state IV, the Ru center is seven-coordinated and diamagnetic, whereas in oxidation state II, the complex has an unbonded dangling carboxylate and is six-coordinated while still diamagnetic. With oxidation state III, the coordination number is halfway between the coordination of oxidation states II and IV. Species generated in situ have also been characterized by spectroscopic, computational, and electrochemical techniques, together with the related species derived from a different degree of protonation and oxidation states. 4H(+) can be generated potentiometrically, or voltammetrically, from 3(2+), and both coexist in solution. While complex 3(2+) is not catalytically active, the catalytic performance of complex 4H(+) is characterized by the foot of the wave analysis, giving an impressive turnover frequency record of 8000 s(-1) at pH 7.0 and 50 000 s(-1) at pH 10.0. Density functional theory calculations provide a complete description of the water oxidation catalytic cycle of 4H(+), manifesting the key functional role of the dangling carboxylate in lowering the activation free energies that lead to O-O bond formation.