Wenzhen Lai

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Because of the significance of energy-related small molecule activation, this review covers recent progress in hydrogen evolution, oxygen evolution, and oxygen reduction reactions catalyzed by porphyrins and corroles.

Catalytic water oxidation at single metal sites

Nature utilizes solar energy to extract electrons and release protons from water, a process called photosynthetic water oxidation or oxygen evolution. This sunlight-driven reaction is vital to the planet because it directly produces dioxygen and couples with photosystem I to generate the reducing equivalents for the reduction of carbon dioxide to carbohydrates (also known as CO2 fixation). Inspired by this natural process, people are intensely interested in water splitting using sunlight to convert and store solar energy into chemical energy, which is believed to be able to ultimately solve the energy problem that we are facing. Water splitting can be separated into two half reactions, namely water oxidation and water reduction, and they can be studied individually. Catalysts are very helpful in both reactions. Recent progress in finding new highly efficient water oxidation catalysts (WOCs) has shed light on this complicated four-electron/four-proton reaction and made it possible to catalyze water oxidation using mononuclear metal complexes. This article focuses on molecular catalysts that are able to perform catalytic water oxidation at single metal sites. Different series of catalysts (or precatalysts) made of ruthenium, iridium and earth abundant elements (iron, cobalt, and manganese) that can be applied in chemical, electrochemical and photochemical (light-driven) water oxidation are summarized, and their catalytic mechanisms are discussed in detail. Finally, the future outlook and perspective to design and develop catalysts that are efficient, cheap and stable are presented.

The valence bond way: reactivity patterns of cytochrome P450 enzymes and synthetic analogs.

The preceding decade has witnessed an immense surge of activity in the bioinorganic chemistry of transition metal enzymes and synthetic analogs that model their operation. The wide range of research covers both experimental and theoretical investigations of structure and reactivity patterns. Theory, and especially density functional theory (DFT), has become a very useful tool, an important partner of experiment in resolving structural and mechanistic issues. This flare of activity has generated a great deal of knowledge on intermediates, transition states, barriers, rate constants, rate-equilibrium relationships, stereoselectivity, and so forth. This abundance of acquired knowledge has created the need for establishing order, namely, the outlining of broad generalizations, as well as the creation of a more-intuitive interface between experimental and theoretical data. The valence bond (VB) diagram model, originally developed for organic reactions, is such a theoretical framework that has the potential to guide the requisite generalizations in the field of bioinorganic chemical reactivity. In this Account, we briefly describe the principles of construction of VB diagrams for bioinorganic reactions, detailing applications in the booming research area of heme enzyme (specifically cytochrome P450) reactivity, and particularly two archetypal reactions of these enzymes, alkane hydroxylation and thioether sulfoxidation. For congruence with the lingua franca of bioinorganic chemistry, the VB model is formulated to create bridges to (i) the molecular orbital (MO) description, (ii) the oxidation state formulation of transition metal complexes, and (iii) widely used concepts such as the Bell-Evans-Polanyi (BEP) principle. The VB diagram model reveals the origins of the barrier, describes the formation of transition states and reaction intermediates, and allows the prediction of barrier heights and structure-reactivity relationships. Thus, from the VB diagram model, we can rationalize the mechanistic selection during alkane hydroxylation compared with thioether sulfoxidation, as well as the different behaviors of the spin states during the reactions with the active species of P450, the high-valent iron oxo species called compound I (Cpd I). Furthermore, the VB model leads to expressions that enable us to estimate barrier heights from easily accessible reactant properties, such as bond energies, ionization potential, and electron affinities. We further show that the model is not limited to these archetypal processes: its applicability is wider and more general. Accordingly, we outline the potential applications of these principles to other reactions of P450 (such as olefin epoxidation and arene hydroxylation) and to similar reactions of nonheme enzymes and synthetic models. The VB diagram model leads to a unified understanding of complex bioinorganic transformations, creates order in the data, and provides an important framework for making useful predictions.

Electrocatalytic Water Oxidation by a Water-Soluble Nickel Porphyrin Complex at Neutral pH with Low Overpotential

The water-soluble cationic nickel(II) complex of meso-tetrakis(4-N-methylpyridyl)porphyrin (1) can electrocatalyze water oxidation to O2 in neutral aqueous solution (pH 7.0) with the onset of the catalytic wave appearing at ∼1.0 V (vs NHE). The homogeneous catalysis with 1 was verified. Catalyst 1 exhibited water oxidation activity in a pH range 2.0-8.0 and had a strict linear dependence of catalytic current on its concentration. After 10 h of constant potential electrolysis at 1.32 V (vs NHE), a negligible difference of the solution was observed by UV-vis. In addition, inspection of the working electrode by electrochemistry, scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDX) showed no sign of deposition of NiOx films. These results strongly argued that 1 is a real molecular electrocatalyst for water oxidation. The turnover frequency (TOF) for this process was 0.67 s(-1) at 20 °C. On the basis of results from the kinetic isotope effect (KIE) and inhibition experiments, electrochemical studies in various buffer solutions with different anions and pHs, and DFT calculations, a catalytic cycle of 1 for water oxidation via a formally Ni(IV) species was proposed.

Enhanced Reactivities of Iron(IV)‐Oxo Porphyrin π‐Cation Radicals in Oxygenation Reactions by Electron‐Donating Axial Ligands

The proximal axial ligand in heme iron enzymes plays an important role in tuning the reactivities of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions. The present study reports the effects of axial ligands in olefin epoxidation, aromatic hydroxylation, alcohol oxidation, and alkane hydroxylation, by [(tmp)(+*) Fe(IV)(O)(p-Y-PyO)](+) (1-Y) (tmp = meso-tetramesitylporphyrin, p-Y-PyO = para-substituted pyridine N-oxides, and Y = OCH(3), CH(3), H, Cl). In all of the oxidation reactions, the reactivities of 1-Y are found to follow the order 1-OCH(3) > 1-CH(3) > 1-H > 1-Cl; negative Hammett rho values of -1.4 to -2.7 were obtained by plotting the reaction rates against the sigma(p) values of the substituents of p-Y-PyO. These results, as well as previous ones on the effect of anionic nucleophiles, show that iron(IV)-oxo porphyrin pi-cation radicals bearing electron-donating axial ligands are more reactive in oxo-transfer and hydrogen-atom abstraction reactions. These results are counterintuitive since iron(IV)-oxo porphyrin pi-cation radicals are electrophilic species. Theoretical calculations of anionic and neutral ligands reproduced the counterintuitive experimental findings and elucidated the root cause of the axial ligand effects. Thus, in the case of anionic ligands, as the ligand becomes a better electron donor, it strengthens the FeO-H bond and thereby enhances its H-abstraction activity. In addition, it weakens the Fe=O bond and encourages oxo-transfer reactivity. Both are Bell-Evans-Polanyi effects, however, in a series of neutral ligands like p-Y-PyO, there is a relatively weak trend that appears to originate in two-state reactivity (TSR). This combination of experiment and theory enabled us to elucidate the factors that control the reactivity patterns of iron(IV)-oxo porphyrin pi-cation radicals in oxidation reactions and to resolve an enigmatic and fundamental problem.

Multireference and Multiconfiguration Ab Initio Methods in Heme-Related Systems: What Have We Learned So Far?

This work reviews the recent applications of ab initio multireference/multiconfiguration (MR/MC) electronic structure methods to heme-related systems, involving tetra-, penta-, and hexa-coordinate species, as well as the high-valent iron-oxo species. The current accuracy of these methods in the various systems is discussed, with special attention to potential sources of systematic errors. Thus, the review summarizes and tries to rationalize the key elements of MR/MC calculations, namely, the choice of the employed active space, especially the so-called double-shell effect that has already been recognized to be important in transition-metal-containing systems, and the impact of these elements on the spin-state energetics of heme species, as well as on the bonding mechanism of small molecules to the heme. It is shown that expansion of the MC wave function into one based on localized orbitals provides a compact and insightful view on some otherwise complex electronic structures. The effects of protein environment on the MR/MC results are summarized for the few available quantum mechanical/molecular mechanical (QM/MM) studies. Comparisons with corresponding DFT results are also made wherever available. Potential future directions are proposed.

Multiple Low-Lying States for Compound I of P450cam and Chloroperoxidase Revealed from Multireference Ab Initio QM/MM Calculations.

The hybrid CASPT2/MM approach is employed to systematically study the ground and low-lying excited states of the ultimate active species of the enzymes P450cam and chloroperoxidase (CPO): the oxoiron(IV)-porphyrin cation-radical Por(•+)Fe(IV)═O(Cys) species, the so-called Compound I (Cpd I). The results underscore the fact that the B3LYP/MM method is quite accurate on the most part. However, the CASPT2/MM energies for the ferryl-pentaradicaloid quartet state and the perferryl Fe(V)O doublet and quartet states are significantly lower than the B3LYP/MM results. Thus, while the present CASPT2/MM may still overestimate the stability of these states, nevertheless, taken at its face value, the result raises the question whether these states actually contribute to the reactivity of Cpd I. Our paper tries to grapple with this question in view of (a) the recent speculations that the perferryl Fe(V)O states may be involved in unusual reactivities of Cpd I species (Pan, Z. Z.; Wang, Q.; Sheng, X.; Horner, J. H.; Newcomb, M. J. Am. Chem. Soc. 2009, 131, 2621-2628) and (b) the DFT/MM results which show that the pentaradicaloid states have intrinsically low barriers for H-abstraction (Altun, A.; Shaik, S.; Thiel, W. J. Am. Chem. Soc. 2007, 129, 8978-8987). The application of CASPT2/MM to high valent transition metal states like the perferryl are far from being trivial, and the experience and insight gained in this study are expected to be helpful for future successful application of this type of method to resolve key issues in P450 reactivity.

Dioxygen Activation by a Non-Heme Iron(II) Complex: Theoretical Study toward Understanding Ferric-Superoxo Complexes.

We present a systematic study using density functional theory (DFT) and coupled cluster (CCSD(T)) computations with an aim of characterizing a non-heme ferric-superoxo complex [(TMC)Fe(O2)](2+) (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) that was proposed to perform allylic C-H activation of cyclohexene (Lee, Y.-M. et al. J. Am. Chem. Soc.2010, 132, 10668). As such, we investigated a series of iron-O2 species without and with a sixth ligand bound to the iron ion in different O2 coordination modes (end-on and side-on) and different spin states. Most of the iron-O2 complexes were found to be iron(III)-superoxo species, Fe(III)(O2(-)), with high-spin (S = 5/2) or intermediate-spin (S = 3/2) ferric centers coupled ferromagnetically or antiferromagnetically to the superoxide anion radical. One iron(IV)-peroxo state, Fe(IV)(O2(2-)), was also examined. The preference for ferromagnetic or antiferromagnetic coupling modes between the superoxo and ferric radicals was found to depend on the FeOO angle, where a side-on tilt favors ferromagnetic coupling whereas the end-on tilt favors antiferromagnetic states. Experimental findings, e.g., the effects of solvent, spin state, and redox potential of non-heme Fe(II) complexes on O2 activation, were corroborated in this work. Solvent effects were found to disfavor O2 binding, relative to the unbound ferrous ion and O2. The potential H-abstraction reactivity of the iron(III)-superoxo species was considered in light of the recently proposed exchange-enhanced reactivity principle (Shaik, S.; Chen, H.; Janardanan, D. Nat. Chem.2011, 3, 19). It is concluded that localization and/or decoupling of an unpaired electron in the d-block of high-spin Fe(III) center in the S = 2 and 3 ferric-superoxo complexes during H abstractions enhances exchange stabilization and may be the root cause of the observed reactivity of [(TMC)Fe(O2)](2+).

Singly versus Doubly Reduced Nickel Porphyrins for Proton Reduction: Experimental and Theoretical Evidence for a Homolytic Hydrogen-Evolution Reaction

Abstract A nickel(II) porphyrin Ni‐P (P=porphyrin) bearing four meso‐C6F5 groups to improve solubility and activity was used to explore different hydrogen‐evolution‐reaction (HER) mechanisms. Doubly reduced Ni‐P ([Ni‐P]2−) was involved in H2 production from acetic acid, whereas a singly reduced species ([Ni‐P]−) initiated HER with stronger trifluoroacetic acid (TFA). High activity and stability of Ni‐P were observed in catalysis, with a remarkable i c/i p value of 77 with TFA at a scan rate of 100 mV s−1 and 20 °C. Electrochemical, stopped‐flow, and theoretical studies indicated that a hydride species [H‐Ni‐P] is formed by oxidative protonation of [Ni‐P]−. Subsequent rapid bimetallic homolysis to give H2 and Ni‐P is probably involved in the catalytic cycle. HER cycling through this one‐electron‐reduction and homolysis mechanism has been proposed previously but rarely validated. The present results could thus have broad implications for the design of new exquisite cycles for H2 generation.

Why Is Cobalt the Best Transition Metal in Transition-Metal Hangman Corroles for O-O Bond Formation during Water Oxidation?

O-O bond formation catalyzed by a variety of β-octafluoro hangman corrole metal complexes was investigated using density functional theory methods. Five transition metal elements, Co, Fe, Mn, Ru, and Ir, that are known to lead to water oxidation were examined. Our calculations clearly show that the formal Co(V) catalyst has a Co(IV)-corrole(•+) character and is the most efficient water oxidant among all eight transition-metal complexes. The O-O bond formation barriers were found to change in the following order: Co(V) ≪ Fe(V) < Mn(V) < Ir(V) < Co(IV) < Ru(V) < Ir(IV) < Mn(IV). The efficiency of water oxidation is discussed by analysis of the O-O bond formation step. Thus, the global trend is determined by the ability of the ligand d-block to accept two electrons from the nascent OH(-), as well as by the OH(•) affinity of the TM(IV)═O species of the corresponding TM(V)═O·H2O complex. Exchange-enhanced reactivity (EER) is responsible for the high catalytic activity of the Co(V) species in its S = 1 state.