Electronic Structure and Ligand Effects on the Activation and Cleavage of N2 on a Molybdenum Center.

Dinitrogen fixation under ambient conditions remains a challenge in the field of catalytic chemistry due to the inertness of N2. 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. Here, we present a combined multiconfigurational/density functional theory study that addresses how ligand fields of different strengths affect the binding and activation of dinitrogen on molybdenum atoms. First, we explored with MRCI computations the diatomic Mo-N and triatomic Mo-N2 molecular systems. Then, we performed a systematic examination on the stabilization effects introduced by external NH3 ligands, before we explore model neutral and charged complexes with different types of ligands (H2O, NH3, and PH3) and their consequences on the N2 binding and activation.

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 N-(2,5-dihydroxybenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (H2qp1) 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 O2 and H2O2, the quinol in the ligand is believed to cycle between three oxidation states: quinol, quinoxyl radical, and para-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. In the present work, we primarily use calculations to probe the mechanism. The calculations support the inner-sphere decomposition of superoxide, suggest that the quinol/quinoxyl radical couple accounts for most of the catalysis, and elucidate the many roles that proton transfer between the zinc complexes and buffer has in the reactivity. Acid/base reactions involving the nonmetal-coordinating hydroxyl group on the quinol are predicted to be key to lowering the energy of the intermediates. We prepared a Zn(II) complex with N-(2-hydroxybenzyl)-N,N',N'-tris(2-pyridinylmethyl)-1,2-ethanediamine (Hpp1) that lacks this functional group and found that it could not catalyze the dismutation of superoxide; this confirms the importance of the second, distal hydroxyl group of the quinol.

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. Thus, the oxidation of methane (the tortoise in Aesop's fable) becomes faster than methanol (Aesop's hare), increasing the selectivity toward the desired product. Activation barriers are calculated for two different mechanisms (2+2 and radical), and reaction rates for the oxidation of the two molecules are obtained using semiclassical instanton theory to include tunneling effects for the proton transfers. The tunneling effects are shown to accelerate all reactions substantially but do not dramatically affect the selectivity.

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. Expanded metals composed of d- and f-block metal ammonia complexes offer properties, such as magnetic moments and larger numbers of diffuse electrons, not present for alkali and alkaline earth (s-block) metals. In addition, tethering metal ammonia complexes via hydrocarbon chains (replacement of ammonia ligands with diamines) yields materials that can be used for redox catalysis and quantum computing, sensing, and optics. This perspective summarizes the recent findings for gas-phase isolated metal ammonia complexes and projects the obtained knowledge to the condensed phase regime. Possible applications for the newly introduced expanded metals and linked solvated electrons precursors are discussed and future directions are proposed.

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

Cu-catalyzed highly stereoselective and enantiodivergent syntheses of (Z)- or (E)-ß,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 (Z)-ß,γ-unsaturated ketones bearing an α-tertiary stereogenic center with excellent enantioselectivities and high Z-selectivities, where the reactions with triisopropylsilyl-substituted 1,3-butadiene formed (E)-ß,γ-unsaturated ketones with high optical purities and excellent E-selectivities. The products generated from the reactions contain three functional groups with orthogonal chemical reactivities, which can undergo a variety of transformations to afford synthetically valuable intermediates.

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. This work is the first ab initio exploration of an f-block metal-ammonia complex. The ground and excited states are calculated for Th0-3+ complexes with ammonia, crown ethers, and aza-crown ethers. For Th3+ complexes, the one valence electron Th populates the metal's 6d or 7f orbitals. For Th0-2+, the additional electrons prefer occupation of the outer s- and p-type orbitals of the complex, except Th(NH3)10, which uniquely places all four electrons in outer orbitals of the complex. Although thorium coordinates up to ten ammonia ligands, octa-coordinated complexes are more stable. Crown ether complexes have a similar electronic spectrum to ammonia complexes, but excitations of electrons in the outer orbitals of the complex are higher in energy. Aza-crown ethers disfavor the orbitals perpendicular to the crowns, attributed to the N-H bonds pointing along the plane of the crowns.

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. Specifically, we identified UV light-induced generation of spin-paired solvated dielectrons and their subsequent relaxation by an unconventional electron transfer-mediated decay as an efficient, low-energy electron source. The process is robust and straightforward to induce with the prospect of improving our understanding of radiation damage and fostering mechanistic studies of solvated electron reduction reactions.

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). Under controlled collision energies down to ∼2 meV within the merged electron and ion beam configuration, fragment imaging has been applied to determine the kinetic energy released to Ti and O neutral reaction products. Detailed analysis of the fragment imaging data considering the reactant and product excitation channels reveals an endothermicity for the TiO+ dissociative electron recombination of (+4 ± 10) meV. This result improves the accuracy of the energy balance by a factor of 7 compared to that found indirectly from hitherto known molecular properties. Conversely, the present endothermicity yields improved dissociation energy values for D0(TiO) = (6.824 ± 0.010) eV and D0(TiO+) = (6.832 ± 0.010) eV. All thermochemistry values were compared to new coupled-cluster calculations and found to be in good agreement. Moreover, absolute rate coefficients for the electron recombination of rotationally relaxed ions have been measured, yielding an upper limit of 1 × 10-7 cm3 s-1 for typical conditions of cold astrophysical media. Strong variation of the DR rate with the TiO+ internal excitation is predicted. Furthermore, potential energy curves for TiO+ and TiO have been calculated using a multi-reference configuration interaction method to constrain quantum-dynamical paths driving the observed TiO+ electron recombination.

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. Every improvement in the efficiency of a catalyst to activate methane leads to reduction of the selectivity towards methanol. Is it therefore prudent to keep studying (both theoretically and experimentally) metal oxides as catalysts for the quantitative conversion of methane to methanol? This perspective focuses on molecular metal oxide complexes and suggests strategies to bypass the current bottlenecks with higher weight on the computational chemistry side. We first discuss the electronic structure of metal oxides, followed by assessing the role of the ligands in the reactivity of the catalysts. For better selectivity, we propose that metal oxide anionic complexes should be explored further, while hydrophylic cavities in the vicinity of the metal oxide can perturb the transition-state structure for methanol increasing appreciably the activation barrier for methanol. We also emphasize that computational studies should target the activation reaction of methanol (and not only methane), the study of complete catalytic cycles (including the recombination and oxidation steps), and the use of molecular oxygen as an oxidant. The titled chemical conversion is an excellent challenge for theory and we believe that computational studies should lead the field in the future. It is finally shown that bottom-up approaches offer a systematic way for exploration of the chemical space and should still be applied in parallel with the recently popular machine learning techniques. To answer the question of the title, we believe that metal oxides should still be considered provided that we change our focus and perform more systematic investigations on the activation of methanol.

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 CH4 + N2O → CH3OH + N2 reaction with low activation barriers for the C-H activation and the formation of the CH3-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.

[Fe4S4] 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 [Fe4S4] 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 [Fe4S4] cluster. Density functional theory (DFT) was deployed in conjunction with the energy decomposition scheme (as implemented in AMS), the quantum theory of atoms in molecules (QTAIM), and conceptual DFT (cDFT) descriptors. We quantified the role of the electrostatic interactions inside the active site created by the side chains as well as the influence of the [Fe4S4] cluster on the substrate binding. Furthermore, using conceptual DFT results we shed light of the activation process, thus, laying foundation for further mechanistic studies. We found that the bonding of the ligands to the iron complex is dominated by electrostatic interactions, but the presence of the [Fe4S4] cubane leads to substantial changes in electronic interaction. The spin state of the cubane, however, affects the binding energy only marginally. The conceptual DFT results show that the presence of the [Fe4S4] cubane affects the reactivity of the active site as it is involved in electron transfer. This is corroborated by an increase in the electrophilicity index, thus making the active site more prone to react with the ligands. The interaction energies between the ligand and the siroheme group are also increased upon the presence of the cubane group, thus, suggesting that the siroheme group is not an innocent spectator but plays an active role in the reactivity of the dSIR active site.

Electronic structure of the ground and excited states of neutral and charged silicon hydrides, SiHx0/+/-, x = 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. The excited states involve excitations within the 3s and 3p orbitals of silicon (especially for the mono- and di-hydrides), followed by excitations from the Si-H bonding orbitals to either silicon valence or Rydberg (4s, 4p) orbitals. The number of electronic states per energy unit decrease as we add hydrogen atoms, and the first excited state of SiH4 is at 9.0 eV and leads to SiH3 + H. All species have stable ground state structures with all hydrogen atoms bound to silicon, except for SiH4+ and SiH4-. The former dissociates to SiH2+ + H2, while the latter loses an electron or can dissociate forming H2 as well.

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

Beryllium ammonia complexes Be(NH3)4 are known to bear two diffuse electrons in the periphery of a Be(NH3)4 2+ skeleton. The replacement of one ammonia with a methyl group forms CH3Be(NH3)3 with one peripheral electron, which is shown to maintain the hydrogenic-type shell model observed for Li(NH3)4. Two CH3Be(NH3)3 monomers are together linked by aliphatic chains to form strongly bound beryllium ammonia complexes, (NH3)3Be(CH2)nBe(NH3)3, n = 1-6, with one electron around each beryllium ammonia center. In the case of a linear carbon chain, this system can be seen as the analog of two hydrogen atoms approaching each other at specific distances (determined by n). We show that the two electrons occupy diffuse s-type orbitals and can couple exactly as in H2 in either a triplet or singlet state. For long hydrocarbon chains, the singlet is an open-shell singlet nearly degenerate with the triplet spin state, which transforms to a closed-shell singlet for n = 1 imitating the σ-covalent bond of H2. The biradical character of the system is analyzed, and the singlet-triplet splitting is estimated as a function of n based on multi-reference calculations. Finally, we consider the case of bent hydrocarbon chains, which allows the closer proximity of the two diffuse electrons for larger chains and the formation of a direct covalent bond between the two diffuse electrons, which happens for two Li(NH3)4 complexes converting the open-shell to closed-shell singlets. The energy cost for bending the hydrocarbon chain is nearly compensated by the formation of the weak covalent bond rendering bent and linear structures nearly isoenergetic.

Molecular-level electrocatalytic CO2 reduction reaction mediated by single platinum atoms.

The activation and transformation of H2O and CO2 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 H2O to form a H-Pt-OH- complex. This complex reacts with CO2via two different pathways to form formate, where CO2 is hydrogenated, or to form bicarbonate, where CO2 is carbonated. The overall formula of this reaction is identical to a typical electrochemical CO2 reduction reaction on a Pt electrode. Since the reactants are electrons and isolated, single atoms and molecules, we term this reaction a molecular-level electrochemical CO2 reduction reaction. Mechanistic analysis reveals that the negative charge distribution on the Pt-H and the -OH moieties in H-Pt-OH- is critical for the hydrogenation and carbonation of CO2. The realization of the molecular-level CO2 reduction reaction provides insights into the design of novel catalysts for the electrochemical conversion of CO2.

Simultaneous CO2 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 CO2 capture and utilization. Quantum chemical calculations were used to study the reaction of CO2 with H2 and C2H4 to produce formic acid, methyldiol and δ-lactone. Mechanisms of a complete reaction pathway are found and activation barriers are reasonably low. The metal ligand complex readily reduces CO2 and significantly stabilizes CO2˙-. Ligand identity minimally influences the reaction. Additional reactions and future strategies are proposed with the goal of inducing experimental interest.

Electronic Structure of RhO2+, Its Ammoniated Complexes (NH3)1-5RhO2+, and Mechanistic Exploration of CH4 Activation by Them.

High-level electronic structure calculations are initially performed to investigate the electronic structure of RhO2+. 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. We further monitored how the Rh-O bonding scheme changes with the gradual addition of ammonia ligands. The nature of this bond remains unaffected up to four ammonia ligands but adopts a different electronic configuration in the pseudo-octahedral geometry of (NH3)5RhO2+. This has consequences in the activation mechanism of the C-H bond of methane by these complexes, especially (NH3)4RhO2+. We show that the [2 + 2] mechanism in the (NH3)4RhO2+ case has a very low energy barrier comparable to that of a radical mechanism. We also demonstrate that methane can coordinate to the metal in a similar fashion to ammonia and that knowledge of the electronic structure of the pure ammonia complexes provides qualitative insights into the optimal reaction mechanism.

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. For the mono-crown ether complexes, this orbital is the polarized valence s-orbital of the metal, but for the other systems this orbital is a peripheral diffuse orbital. The nature of the low-lying electronic states is found to be different for each of these species. Specifically, the metal ethylenediamine complexes follow the previously discovered shell model of metal ammonia complexes (1s, 1p, 1d, 2s, 1f), but both mono- and sandwich di-crown ether complexes bear a different shell model partially due to their lower (cylindrical) symmetry and the stabilization of the 2s-type orbital. Li(15-crown-5) is the only complex with the metal in the middle of the crown ether and adopts closely the shell model of metal ammonia complexes. Our findings suggest that the electronic band structure of electrides (metal crown ether sandwich aggregates) and expanded metals (metal ammonia aggregates) should be different despite the similar nature of these systems (bearing diffuse electrons around a metal complex).

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 MO2+ 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 MO2+ units. We find that they can be divided in three groups. The early transition metals (Ti, V, Cr) have very stable and well separated oxo (M4+O2-) character ground states, the middle transition metals (Mn, Fe) have oxyl (M3+O˙-) ground states with low-lying oxo excited states, and the late transition metals (Co, Ni, Cu) have well separated oxyl states. The reported spectroscopic constants will aid future experimental investigations, which are sparse in the literature. Periodic trends for the bond lengths, energetics, excitation energies, and wavefunction composition are discussed in detail. Complete basis set limit results indicate the high accuracy of the quintuple-ζ basis sets.

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(NH3)n and M(NH3)n + (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. Molybdenum and chromium ammonia complexes are found to differ significantly in geometry with the former adopting an octahedral geometry and the latter a Jahn-Teller distorted octahedral structure where only the axial distortion is stable. The hexa-coordinated complexes and the tetra-coordinated complexes with two ammonia molecules in the second solvation shell are found to be energetically competitive. Electronic excitation energies and computed IR spectra are provided to allow the two isomers to be experimentally distinguished. This work is a component of an ongoing effort to study the periodic trends of transition metal solvated electron precursors.

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- + N2O + CH4 → M- + N2 + CH3OH (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. The main advantage is that anionic centers interact weakly with the produced methanol. This fact facilitates the fast removal of methanol from the catalytic center and prevents the overoxidation of methane. Moreover, a moderate or high energy barrier for the M- + CH4 → HMCH3- reaction step is observed, which protects the metal center from deactivation. Future work should focus on the identification of proper ligands, which stabilize the negative charge on the metal (electronic factors) and prevent the formation of the global CH3MOH- minimum (steric factors). Finally, a composite electronic structure method (combining size extensive coupled clusters approaches and accurate multireference configuration interaction) is proposed for computationally demanding systems and is applied to Fe-.