Quantum chemical approaches today are a powerful tool to study the properties and reactivity of metalloenzymes. In the field of solar fuels research these involve predominantly photosystem II and ...hydrogenases, which catalyze water oxidation and hydrogen evolution, as well as related biomimetic and bio-inspired models. Theoretical methods are extensively used to better comprehend the nature of catalytic intermediates, establish important structure-function and structure-property correlations, elucidate functional principles, and uncover the catalytic activity of these complex systems by unravelling the key steps of their reaction mechanism. Computations in the field of water oxidation and hydrogen evolution are used as predictive tools to elucidate structures, explain and synthesize complex experimental observations from advanced spectroscopic techniques, rationalize reactivity on the basis of atomistic models and electronic structure, and guide the design of new synthetic targets. This feature article covers recent advances in the application of quantum chemical methods for understanding the nature of catalytic intermediates and the mechanism by which photosystem II and hydrogenases achieve their function, and points at essential questions that remain only partly answered and at challenges that will have to be met by future advances and applications of quantum and computational chemistry.
Overview of the rich and diverse contributions of quantum chemistry to understanding the structure and function of the biological archetypes for solar fuel research, photosystem II and hydrogenases.
Molybdenum sulfides are very attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) from water. The atomic structure and identity of the catalytically active sites ...have been well established for crystalline molybdenum disulfide (c-MoS2) but not for amorphous molybdenum sulfide (a-MoSx), which exhibits significantly higher HER activity compared to its crystalline counterpart. Here we show that HER-active a-MoSx, prepared either as nanoparticles or as films, is a molecular-based coordination polymer consisting of discrete Mo3S13(2-) building blocks. Of the three terminal disulfide (S2(2-)) ligands within these clusters, two are shared to form the polymer chain. The third one remains free and generates molybdenum hydride moieties as the active site under H2 evolution conditions. Such a molecular structure therefore provides a basis for revisiting the mechanism of a-MoSx catalytic activity, as well as explaining some of its special properties such as reductive activation and corrosion. Our findings open up new avenues for the rational optimization of this HER electrocatalyst as an alternative to platinum.
The synthesis and characterization of a dinuclear bis(thiosemicarbazone) cobalt complex Co2L2(NCS)2 is reported. This complex exhibits significant catalytic activity for hydrogen production in DMF by ...using triethylammonium (Et3NHBF4) as the proton source. Cyclic voltammetry data allowed a maximum turnover frequency of 130 s−1 for 1 m proton concentration to be determined. The catalytic nature of the process and the production of dihydrogen were confirmed by gas analysis during controlled potential electrolysis experiments. Quantum chemical calculations show that the complex displays a ligand‐assisted metal‐centered reactivity and supports a catalytic mechanism involving ligand‐based reduction and protonation steps followed by metal‐centered processes.
Center of attraction: A dinuclear cobalt complex based on a thiosemicarbazone ligand is an efficient electrocatalyst for hydrogen evolution reactions. The catalytic mechanism involves ligand‐based reductions alternating with protonation steps consistent with an unusual ligand‐assisted metal‐centered reactivity (see scheme).
Copper or nickel? The depicted CuII salen complex, an active‐site model of galactose oxidase (GO), is a localized radical in the solid state with a quinoid distribution of bond lengths in one ring. ...While the ligand‐radical structure is not metal‐dependent, the composition of the radical SOMO is. This feature may explain the much lower reactivity of the Ni complex and nature's preference for a CuII center in GO.
We report herein the synthesis and characterization of a new mononuclear nickel complex based on a thiosemicarbazone ligand that exhibits an electrocatalytic behavior for H2 evolution in DMF using ...trifluoroacetic acid (TFA) as the proton source. Catalysis is observed at quite small overpotential values and a maximum turnover frequency (TOFmax) of 3080 s−1 was extrapolated for 1 m proton concentration using the foot‐of‐the wave analysis of cyclic voltammetry data. Gas analysis during controlled potential electrolysis experiments confirmed the catalytic nature of the process and production of dihydrogen with 80 % faradaic yield. Quantum chemical calculations indicate that the catalytic mechanism involves first ligand‐based reduction and protonation steps followed by metal‐centered processes.
Nickel‐centered: A mononuclear nickel complex based on a thiosemicarbazone ligand ranks among the best molecular H2‐evolving electrocatalysts in DMF in the catalytic Tafel plot. The catalytic mechanism involves ligand‐based reduction and protonation steps followed by metal‐centered processes.
Spin-state energetics of exchange-coupled copper complexes pose a persistent challenge for applied quantum chemistry. Here, we provide a comprehensive comparison of all available theoretical ...approaches to the problem of exchange coupling in two antiferromagnetically coupled bis-μ-hydroxo Cu(II) dimers. The evaluated methods include multireference methods based on the density matrix renormalization group (DMRG), multireference methods that incorporate dynamic electron correlation either perturbatively, such as the
N
-electron valence state perturbation theory, or variationally, such as the difference-dedicated configuration interaction. In addition, we contrast the multireference results with those obtained using broken-symmetry approaches that utilize either density functional theory or, as demonstrated here for the first time in such systems, a local implementation of coupled cluster theory. The results show that the spin-state energetics of these copper dimers are dominated by dynamic electron correlation and represent an impossible challenge for multireference methods that rely on brute-force expansion of the active space to recover correlation energy. Therefore, DMRG-based methods even at the limit of their applicability cannot describe quantitatively the antiferromagnetic exchange coupling in these dimers, in contrast to dinuclear complexes of earlier transition metal ions. The convergence of the broken-symmetry coupled cluster approach is studied and shown to be a limiting factor for the practical application of the method. The advantages and disadvantages of all approaches are discussed, and recommendations are made for future developments.
A key intermediate in the electroconversion of carbon dioxide to carbon monoxide, catalyzed by a manganese tris(carbonyl) complex, is characterized. Different catalytic pathways and their potential ...reaction mechanisms are investigated using a large range of experimental and computational techniques. Sophisticated spectroscopic methods including UV/Vis absorption and pulsed‐EPR techniques (2P‐ESEEM and HYSCORE) were combined together with DFT calculations to successfully identify a key intermediate in the catalytic cycle of CO2 reduction. The results directly show the formation of a metal–carboxylic acid–CO2 adduct after oxidative addition of CO2 and H+ to a Mn0 carbonyl dimer, an unexpected intermediate.
New insights into the catalytic mechanism pathway for selective electrocatalytic reduction of carbon dioxide to carbon monoxide by a manganese(I) carbonyl catalyst are given by using pulsed‐EPR spectroscopy combined with DFT calculations. The results directly show the formation of a metal(II)–carboxylic acid–CO2 adduct (see picture) after oxidative addition of CO2 and H+ to a Mn0 carbonyl dimer.
Despite the report of several structural and functional models of the NiFe-hydrogenases, it is still unclear how the succession of electron and proton transfers during H2 production catalysis are ...controlled in terms of both sequence (order of the chemical or redox steps) and sites (metal and/or ligand). To address this issue, the structure of the previously described bioinspired NiFe-hydrogenase complex LN2S2NiIIFeIICp(CO)+ (LNiIIFeIICp, with LN2S2 = 2,2′-(2,2′-bipyridine-6,6′-diyl)bis(1,1′-diphenylethanethiolate) and Cp = cyclopentadienyl) has been fine-tuned by modifying exclusively the Fe site. In LN2S2NiIIFeIICp*(CO)+ (LNiIIFeIICp*, with Cp* = pentamethylcyclopentadienyl), the Cp– ligand has been replaced by Cp*– to change both the redox and structural properties of the overall complex as a consequence of the steric hindrance of Cp*–. The LNiIIFeIICp* complex acts as an efficient electrocatalyst to produce H2. Density functional theory (DFT) calculations support a CEEC cycle, following an initial reduction. The initial protonation leads to the cleavage of one thiolate–iron bond and the next reduction to the generation of a bridging Fe-based hydride moiety. Interestingly, the second protonation step generates a species containing a terminal Ni-based thiol and a bridging hydride. In the presence of CO, the electrocatalytic activity of LNiIIFeIICp* for H2 production is markedly inhibited (about 90% of loss), while only a partial inhibition (about 30% of loss) is observed in the case of LNiIIFeIICp. DFT calculations rationalized this effect by predicting that interactions of the one- and two-electron-reduced species for LNiIIFeIICp* with CO are thermodynamically more favorable in comparison to those for LNiIIFeIICp.
This work reports on the synthesis and characterization of a series of mononuclear thiosemicarbazone nickel complexes that display significant catalytic activity for hydrogen production in DMF using ...trifluoroacetic acid as the proton source. The ligand framework was chemically modified by varying the electron-donating abilities of the
para
substituents on the phenyl rings, which was expected to impact the capability of the resulting complexes to reduce protons into hydrogen. Over the four nickel complexes that were obtained, the one with the thiomethyl substituent,
NiSCH
3
, was found to overtake the catalytic performances of the parent complex
NiOCH
3
featuring lower overpotential values and similar maximum turnover frequencies. These results confirm the electronic effects of the ligand on HER when using thiosemicarbazone nickel complexes and support that chemical modifications can tune the catalytic performances of such systems.
This work reports on the synthesis and characterization of a series of mononuclear thiosemicarbazone nickel complexes that display significant catalytic activity for hydrogen production in DMF using trifluoroacetic acid as the proton source.
Nitroxides are potent tools for studying biological systems by electron paramagnetic resonance (EPR). Whatever the application, a certain stability is necessary for successful detection. Since ...conventional tetramethyl-substituted cyclic nitroxides have insufficient in vivo stability, efforts have recently been made to synthesize more stable, tetraethyl-substituted nitroxides. In our previous study on piperidine nitroxides, the introduction of steric hindrance around the nitroxide moiety successfully increased the resistance to reduction into hydroxylamine. However, it also rendered the carbon backbone susceptible to modifications by xenobiotic metabolism due to increased lipophilicity. Here, we focus on a new series of three nitroxide candidates with tetraethyl substitution, namely with pyrrolidine, pyrroline, and isoindoline cores, to identify which structural features afford increased stability for future probe design and application in in vivo EPR imaging. In the presence of rat liver microsomes, pyrrolidine and pyrroline tetraethyl nitroxides exhibited a higher stability than isoindoline nitroxide, which was studied in detail by HPLC-HRMS. Multiple metabolites suggest that the aerobic transformation of tetraethyl isoindoline nitroxide is initiated by hydrogen abstraction by P450-Fe
= O from one of the ethyl groups, followed by rearrangement and further modifications by cytochrome P450, as supported by DFT calculations. Under anaerobic conditions, only reduction by rat liver microsomes was observed with involvement of P450-Fe
.
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