Selective conversion of methane to methanol facilitated by molecular metal-methoxy complexes a self-correcting chemical cycle.

The controlled oxidation of methane to methanol has been an area of intense research over the past decades. Despite the efforts, the identification of an efficient catalyst with high selectivity is still elusive. Here we propose a thoroughly different strategy employing catalysts containing a metal methoxy unit.

Potential of Molecular Catalysts with Electron-Rich Transition Metal Centers for Addressing Long-Standing Chemistry Enigmas.

Molecular complexes with electron-rich metal centers are highlighted as potential catalysts for the following five important chemical transformations: selective conversion of methane to methanol, capture and utilization of carbon dioxide, fixation of molecular nitrogen, water splitting, and recycling of perfluorochemicals. Our initial focus lies on negatively charged metal centers and ligands that can stabilize anionic metal atoms. Catalysts with electron-rich metal atoms (CERMAs) can sustain catalytic cycles with a "ping-pong" mechanism, where one or more electrons are transferred from the metal center to the substrate and back.

Electronic Structure and Ligand Effects on the Activation and Cleavage of N on a Molybdenum Center.

Dinitrogen fixation under ambient conditions remains a challenge in the field of catalytic chemistry due to the inertness of N. Nitrogenases and heterogeneous solid catalysts have displayed remarkable performance in the catalytic conversion of dinitrogen to ammonia. By introduction of molybdenum centers in molecular complexes, one of the most azophilic metals of the transitional metal series, moderate ammonia yields have been attained.

Computational Analysis of the Superoxide Dismutase Mimicry Exhibited by a Zinc(II) Complex with a Redox-Active Organic Ligand.

Previously, we found that a Zn(II) complex with the redox-active ligand -(2,5-dihydroxybenzyl)-,','-tris(2-pyridinylmethyl)-1,2-ethanediamine (Hqp1) was able to act as a functional mimic of superoxide dismutase, despite its lack of a redox-active transition metal. As the complex catalyzes the dismutation of superoxide to form O and HO, the quinol in the ligand is believed to cycle between three oxidation states: quinol, quinoxyl radical, and -quinone. Although the metal is not the redox partner, it nonetheless is essential to the reactivity since the free ligand by itself is inactive as a catalyst.

Methane against Methanol: The Tortoise and the Hare of the Oxidation Race.

The selective partial oxidation of methane to methanol has been a major chemistry challenge over the past several decades. The reason for this is that the weaker C-H bond of the desired product (methanol) is readily activated by the same catalyst used to activate the stronger C-H bond of methane. Quantum chemical calculations reveal how hydrogen-bonding interactions with the catalyst as well as other electronic and geometric effects slow the unwanted methanol oxidation reaction.

Asymmetric Syntheses of ()- or ()-β,γ-Unsaturated Ketones via Silane-Controlled Enantiodivergent Catalysis.

Cu-catalyzed highly stereoselective and enantiodivergent syntheses of ()- or ()-β,γ-unsaturated ketones from 1,3-butadienyl silanes are developed. The nature of the silyl group of the dienes has a significant impact on the stereo- and enantioselectivity of the reactions. Under the developed catalytic systems, the reactions of acyl fluorides with phenyldiemthylsilyl-substituted 1,3-diene gave ()-β,γ-unsaturated ketones bearing an α-tertiary stereogenic center with excellent enantioselectivities and high -selectivities, where the reactions with triisopropylsilyl-substituted 1,3-butadiene formed ()-β,γ-unsaturated ketones with high optical purities and excellent -selectivities.

A fresh perspective on metal ammonia molecular complexes and expanded metals: opportunities in catalysis and quantum information.

Recent advances in our comprehension of the electronic structure of metal ammonia complexes have opened avenues for novel materials with diffuse electrons. These complexes in their ground state can host peripheral "Rydberg" electrons which populate a hydrogenic-type shell model imitating atoms. Aggregates of such complexes form the so-called expanded or liquid metals.

Ab Initio Calculations on the Ground and Excited Electronic States of Thorium-Ammonia, Thorium-Aza-Crown, and Thorium-Crown Ether Complexes.

Positively charged metal-ammonia complexes are known to host peripheral, diffuse electrons around their molecular skeleton. The resulting neutral species form materials known as expanded or liquid metals. Alkali, alkaline earth, and transition metals have been investigated previously in experimental and theoretical studies of both the gas and condensed phase.

Solvated dielectrons from optical excitation: An effective source of low-energy electrons.

Low-energy electrons dissolved in liquid ammonia or aqueous media are powerful reducing agents that promote challenging reduction reactions but can also cause radiation damage to biological tissue. Knowledge of the underlying mechanistic processes remains incomplete, particularly with respect to the details and energetics of the electron transfer steps. In this work, we show how ultraviolet (UV) photoexcitation of metal-ammonia clusters could be used to generate tunable low-energy electrons in situ.

Near-thermo-neutral electron recombination of titanium oxide ions.

While the dissociative recombination (DR) of ground-state molecular ions with low-energy free electrons is generally known to be exothermic, it has been predicted to be endothermic for a class of transition-metal oxide ions. To understand this unusual case, the electron recombination of titanium oxide ions (TiO) with electrons has been experimentally investigated using the Cryogenic Storage Ring. In its low radiation field, the TiO ions relax internally to low rotational excitation (≲100 K).

Transition metal oxide complexes as molecular catalysts for selective methane to methanol transformation: any prospects or time to retire?

Transition metal oxides have been extensively used in the literature for the conversion of methane to methanol. Despite the progress made over the past decades, no method with satisfactory performance or economic viability has been detected. The main bottleneck is that the produced methanol oxidizes further due to its weaker C-H bond than that of methane.

Being negative can be positive: metal oxide anions promise more selective methane to methanol conversion.

Computational studies are performed to show that metal oxide anionic complexes promote the CH + NO → CHOH + N reaction with low activation barriers for the C-H activation and the formation of the CH-OH bond. The energy for the release of the produced methanol is minimal, reducing the residence time of methanol around the catalytic center and preventing its overoxidation.

[FeS] cubane in sulfite reductases: new insights into bonding properties and reactivity.

The dissimilatory sulfite reductase enzyme has very characteristic active site where the substrate binds to an iron site, ligated by a siroheme macrocycle and a thiol directly connected to a [FeS] cluster. This arrangement gives the enzyme remarkable efficiency in reducing sulfite and nitrite all the way to hydrogen sulfide and ammonia. For the first time we present a theoretical study where substrate binding modalities and activation are elucidated using active site models containing proton supply side chains and the [FeS] cluster.

The nature of supermolecular bonds: Investigating hydrocarbon linked beryllium solvated electron precursors.

Beryllium ammonia complexes Be(NH) are known to bear two diffuse electrons in the periphery of a Be(NH) skeleton. The replacement of one ammonia with a methyl group forms CHBe(NH) with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH). Two CHBe(NH) monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH)Be(CH)Be(NH), n = 1-6, with one electron around each beryllium ammonia center.

Electronic structure of the ground and excited states of neutral and charged silicon hydrides, SiH, = 1-4.

The ground and excited electronic states of the titled species are investigated with multi-reference configuration interaction and diffuse basis sets. We found that in addition to the valence orbitals, the inclusion of the 4s, 4p, and especially 3d orbitals (although with minimal population) of silicon in the active space of the reference complete active space self-consistent field wavefunction are necessary for the proper convergence of the calculations. We also demonstrate that the aug-cc-pVTZ basis set provides quite accurate results compared to both larger basis sets and basis set limit results at a lower computational cost.

Molecular-level electrocatalytic CO reduction reaction mediated by single platinum atoms.

The activation and transformation of HO and CO mediated by electrons and single Pt atoms is demonstrated at the molecular level. The reaction mechanism is revealed by the synergy of mass spectrometry, photoelectron spectroscopy, and quantum chemical calculations. Specifically, a Pt atom captures an electron and activates HO to form a H-Pt-OH complex.

Simultaneous CO capture and functionalization: solvated electron precursors as novel catalysts.

Metal complexes with diffuse solvated electrons (solvated electron precursors) are proposed as alternative catalysts for the simultaneous CO capture and utilization. Quantum chemical calculations were used to study the reaction of CO with H and CH to produce formic acid, methyldiol and δ-lactone. Mechanisms of a complete reaction pathway are found and activation barriers are reasonably low.

Electronic Structure of RhO, Its Ammoniated Complexes (NH)RhO, and Mechanistic Exploration of CH Activation by Them.

High-level electronic structure calculations are initially performed to investigate the electronic structure of RhO. The construction of potential energy curves for the ground and low-lying excited states allowed the calculation of spectroscopic constants, including harmonic and anharmonic vibrational frequencies, bond lengths, spin-orbit constants, and excitation energies. The equilibrium electronic configurations were used for the interpretation of the chemical bonding.

Electronic structure of the dicationic first row transition metal oxides.

Multi-reference electronic structure calculations combined with large basis sets are performed to investigate the electronic structure of the ground and low-lying electronic states of the MO diatomic species with M = Ti-Cu. These systems have shown high efficiency in the activation of the C-H of saturated hydrocarbons. This study is the first systematic and accurate work for these systems and our results and discussion provides insights into the reactivity and stability of MO units.

Ground and excited states analysis of alkali metal ethylenediamine and crown ether complexes.

High-level electronic structure calculations are carried out to obtain optimized geometries and excitation energies of neutral lithium, sodium, and potassium complexes with two ethylenediamine and one or two crown ether molecules. Three different sizes of crowns are employed (12-crown-4, 15-crown-5, 18-crown-6). The ground state of all complexes contains an electron in an s-type orbital.

Electronic and geometric structure of cationic and neutral chromium and molybdenum ammonia complexes.

High level quantum chemical approaches are used to study the geometric and electronic structures of M(NH) and M(NH) (M = Cr, Mo for n = 1-6). These complexes possess a dual shell electronic structure of the inner metal (3d or 4d) orbitals and the outer diffuse orbitals surrounding the periphery of the complex. Electronic excitations reveal these two shells to be virtually independent of the other.

Methane to Methanol Conversion Facilitated by Anionic Transition Metal Centers: The Case of Fe, Ni, Pd, and Pt.

Density functional theory and high-level ab initio electronic structure calculations are performed to study the mechanism of the partial oxidation of methane to methanol facilitated by the titled anionic transition metal atoms. The energy landscape for the overall reaction M + NO + CH → M + N + CHOH (M = Fe, Ni, Pd, Pt) is constructed for different reaction pathways for all four metals. The comparison with earlier experimental and theoretical results for cationic centers demonstrates the better performance of the metal anions.

Infrared spectroscopy of RG-Co(HO) complexes (RG = Ar, Ne, He): The role of rare gas "tag" atoms.

RG-Co(HO) cation complexes (RG = Ar, Ne, He) are generated in a supersonic expansion by pulsed laser vaporization. Complexes are mass-selected using a time-of-flight spectrometer and studied with infrared laser photodissociation spectroscopy, measuring the respective mass channels corresponding to the elimination of the rare gas "tag" atom. Spectral patterns and theory indicate that the structures of the ions with a single rare gas atom have this bound to the cobalt cation opposite the water moiety in a near-C arrangement.

Radical abstraction oxidative addition mechanisms for the activation of the SH, OH, and CH bonds using early transition metal oxides.

Quantum chemical calculations are performed to study the S-H, O-H, and C-H bond activation of H2S, H2O, and CH4 by bare and ligated ZrO+ and NbO+ units. These representative oxides bear low energy oxo and higher energy oxyl units. S-H and C-H bonds are readily activated by metal oxyl states (radical mechanism), but the O-H bond is harder to activate with either the oxyl or oxo states.

Simultaneous Functionalization of Methane and Carbon Dioxide Mediated by Single Platinum Atomic Anions.

Mass spectrometric analysis of the anionic products of interaction among Pt, methane, and carbon dioxide shows that the methane activation complex, HC-Pt-H, reacts with CO to form [HC-Pt-H(CO)]. Two hydrogenation and one C-C bond coupling products are identified as isomers of [HC-Pt-H(CO)] by a synergy between anion photoelectron spectroscopy and quantum chemical calculations. Mechanistic study reveals that both CH and CO are activated by the anionic Pt atom and that the successive depletion of the negative charge on Pt drives the CO insertion into the Pt-H and Pt-C bonds of HC-Pt-H.