Rong-Zhen Liao

Electrocatalytic Water Oxidation by a Dinuclear Copper Complex in a Neutral Aqueous Solution

Electrocatalytic water oxidation using the oxidatively robust 2,7-[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine ligand (BPMAN)-based dinuclear copper(II) complex, [Cu2(BPMAN)(μ-OH)](3+), has been investigated. This catalyst exhibits high reactivity and stability towards water oxidation in neutral aqueous solutions. DFT calculations suggest that the O-O bond formation takes place by an intramolecular direct coupling mechanism rather than by a nucleophilic attack of water on the high-oxidation-state Cu(IV)=O moiety.

A Dinuclear Ruthenium-Based Water Oxidation Catalyst: Use of Non-Innocent Ligand Frameworks for Promoting Multi-Electron Reactions

Insight into how H2O is oxidized to O2 is envisioned to facilitate the rational design of artificial water oxidation catalysts, which is a vital component in solar-to-fuel conversion schemes. Herein, we report on the mechanistic features associated with a dinuclear Ru-based water oxidation catalyst. The catalytic action of the designed Ru complex was studied by the combined use of high-resolution mass spectrometry, electrochemistry, and quantum chemical calculations. Based on the obtained results, it is suggested that the designed ligand scaffold in Ru complex 1 has a non-innocent behavior, in which metal–ligand cooperation is an important part during the four-electron oxidation of H2O. This feature is vital for the observed catalytic efficiency and highlights that the preparation of catalysts housing non-innocent molecular frameworks could be a general strategy for accessing efficient catalysts for activation of H2O.

Water oxidation catalyzed by molecular di- and nonanuclear Fe complexes: importance of a proper ligand framework

The synthesis of two molecular iron complexes, a dinuclear iron(iii,iii) complex and a nonanuclear iron complex, based on the dinucleating ligand 2,2-(2-hydroxy-5-methyl-1,3-phenylene)bis(1H-benzo[d]imidazole-4-carboxylic acid) is described. The two iron complexes were found to drive the oxidation of water by the one-electron oxidant [Ru(bpy)3](3+).

Molecular ruthenium water oxidation catalysts carrying non-innocent ligands: mechanistic insight through structure–activity relationships and quantum chemical calculations

Robust catalysts that mediate H2O oxidation are of fundamental importance for the development of novel carbon-neutral energy technologies. Herein we report the synthesis of a group of single-site Ru complexes. Structure–activity studies revealed that the individual steps in the oxidation of H2O depended differently on the electronic properties of the introduced ligand substituents. The mechanistic details associated with these complexes were investigated experimentally along with quantum chemical calculations. It was found that O–O bond formation for the developed Ru complexes proceeds via high-valent RuVI species, where the capability of accessing this species is derived from the non-innocent ligand architecture. This cooperative catalytic involvement and the ability of accessing RuVI are intriguing and distinguish these Ru catalysts from a majority of previously reported complexes, and might generate unexplored reaction pathways for activation of small molecules such as H2O.

Mechanism of Water Oxidation Catalyzed by a Mononuclear Manganese Complex

The design and synthesis of biomimetic Mn complexes to catalyze oxygen evolution is a very appealing goal because water oxidation in nature employs a Mn complex. Recently, the mononuclear Mn complex [LMnII (H2 O)2 ]2+ [1, L=Py2 N(tBu)2 , Py=pyridyl] was reported to catalyze water oxidation electrochemically at an applied potential of 1.23u2005V at pHu200512.2 in aqueous solution. Density functional calculations were performed to elucidate the mechanism of water oxidation promoted by this catalyst. The calculations showed that 1 can lose two protons and one electron readily to produce [LMnIII (OH)2 ]+ (2), which then undergoes two sequential proton-coupled electron-transfer processes to afford [LMnV OO]+ (4). The O-O bond formation can occur through direct coupling of the two oxido ligands or through nucleophilic attack of water. These two mechanisms have similar barriers of approximately 17u2005kcalu2009mol-1 . The further oxidation of 4 to generate [LMnVI OO]2+ (5), which enables O-O bond formation, has a much higher barrier. In addition, ligand degradation by C-H activation has a similar barrier to that for the O-O bond formation, and this explains the relatively low turnover number of this catalyst.

On the mechanism of water oxidation catalyzed by a dinuclear ruthenium complex: a quantum chemical study

The development of efficient and robust catalysts for H2O oxidation is an essential element in solar water splitting. The reaction mechanism for a previously reported dinuclear Ru water oxidation catalyst (1) has been investigated in detail through quantum chemical calculations. The predicted mechanism starts from a Ru2III,III complex with two aqua ligands. After three sequential oxidations, O–O bond formation occurs at a formal Ru2IV,V state via the direct coupling of two adjacent oxo moieties while the water nucleophilic attack mechanism was found to be associated with a higher energy barrier. Two H2O molecules are then inserted with subsequent release of O2, which was found to be the rate-limiting step with a barrier of 22.7 kcal mol−1. In a previous work, it was revealed that the ligand scaffold in the studied Ru complex has a non-innocent function. Here, we further highlight this behavior, where the ligand was shown to mediate proton transfer events and accept/donate electrons during the catalytic cycle, which can significantly decrease the redox potentials and facilitate the access to high-valent redox states. This study provides further insight into the H2O oxidation mechanism and principles for designing improved catalysts for activation of small molecules, such as H2O.

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

The design of efficient and robust water oxidation catalysts has proven challenging in the development of artificial photosynthetic systems for solar energy harnessing and storage. Tremendous progress has been made in the development of homogeneous transition-metal complexes capable of mediating water oxidation. To improve the efficiency of the catalyst and to design new catalysts, a detailed mechanistic understanding is necessary. Quantum chemical modeling calculations have been successfully used to complement the experimental techniques to suggest a catalytic mechanism and identify all stationary points, including transition states for both O-O bond formation and O2 release. In this review, recent progress in the applications of quantum chemical methods for the modeling of homogeneous water oxidation catalysis, covering various transition metals, including manganese, iron, cobalt, nickel, copper, ruthenium, and iridium, is discussed.

Chemical and Photochemical Water Oxidation Mediated by an Efficient Single-Site Ruthenium Catalyst

Abstract Water oxidation is a fundamental step in artificial photosynthesis for solar fuels production. In this study, we report a single‐site Ru‐based water oxidation catalyst, housing a dicarboxylate‐benzimidazole ligand, that mediates both chemical and light‐driven oxidation of water efficiently under neutral conditions. The importance of the incorporation of the negatively charged ligand framework is manifested in the low redox potentials of the developed complex, which allows water oxidation to be driven by the mild one‐electron oxidant [Ru(bpy)3]3+ (bpy=2,2’‐bipyridine). Furthermore, combined experimental and DFT studies provide insight into the mechanistic details of the catalytic cycle.

Unraveling the Mechanism and Regioselectivity of the B12-Dependent Reductive Dehalogenase PceA

PceA is a cobalamin-dependent reductive dehalogenase that catalyzes the dechlorination of perchloroethylene to trichloroethylene and then to cis-dichloroethylene as the sole final product. The reaction mechanism and the regioselectivity of this enzyme are investigated by using density functional calculations. Four different substrates, namely, perchloroethylene, trichloroethylene, cis-dichloroethylene, and chlorotheylene, have been considered and were found to follow the same reaction mechanism pattern. The reaction starts with the reduction of Co(II) to Co(I) through a proton-coupled electron transfer process, with the proton delivered to a Tyr246 anion. This is followed by concerted C-Cl bond heterolytic cleavage and proton transfer from Tyr246 to the substrate carbon atom, generating a Co(III) -Cl intermediate. Subsequently, a one-electron transfer leads to the formation of the Co(II) -Cl product, from which the chloride and the dehalogenated product can be released from the active site. The substrate reactivity follows the trend perchloroethylene>trichloroethylene≫cis-dichloroethylene≫chlorotheylene. The barriers for the latter two substrates are significantly higher compared with those for perchloroethylene and trichloroethylene, implying that PceA does not catalyze their degradation. In addition, the formation of cis-dichloroethylene has a lower barrier by 3.8u2005kcalu2009mol(-1) than the formation of trans-dichloroethylene and 1,1-dichloroethylene, reproducing the regioselectivity. These results agree quite well with the experimental findings, which show cis-dichloroethylene as the sole product in the PceA-catalyzed dechlorination of perchloethylene and trichloroethylene.

Mechanism of Water Oxidation Catalyzed by a Mononuclear Iron Complex with a Square Polypyridine Ligand: A DFT Study

The mononuclear [Cl-FeIII(dpa)-Cl]+ (1Cl) complex containing a square planar tetradentate polypyridine ligand has been reported to catalyze water oxidation in pH = 1 aqueous medium with ceric ammonium nitrate (CAN) as a chemical oxidant. The reaction mechanism of the oxygen evolution driven by this catalyst was investigated by means of density functional calculations. The results showed that one chloride ligand of 1Cl has to exchange with a water molecule to generate 1, [Cl-FeIII(dpa)-OH2]2+, as the starting species of the catalytic cycle. The initial one-electron oxidation of 1 is coupled with the release of two protons, generating [Cl-FeIV(dpa)═O]+ (2). Another one-electron transfer from 2 leads to the formation of an FeV═O complex [Cl-FeV(dpa)═O]2+ (3), which triggers the critical O-O bond formation. The electronic structure of 3 was found to be very similar to that of the high-valent heme-iron center of P450 enzymes, termed Compound I, in which a π-cation radical ligand is believed to support a formal iron(IV)-oxo core. More importantly, 3 and Compound I share the same tendency toward electrophilic reactions. Two competing pathways were suggested for the O-O bond formation based on the present calculations. One is the nitrate nucleophilic attack on the iron(V)-oxo moiety with a total barrier of 12.3 kcal mol-1. In this case, nitrate functions as a co-catalyst for the dioxygen formation. The other is the water nucleophilic attack on iron(V)-oxo with a greater barrier of 16.5 kcal mol-1. In addition, ligand degradation via methyl hydrogen abstraction was found to have a barrier similar to that of the O-O bond formation, while the aromatic carbon hydroxylation has a higher barrier.