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.

Exploration of the Two-Electron Excitation Space with Data-Driven Coupled Cluster.

Computational cost limits the applicability of post-Hartree-Fock methods such as coupled-cluster on larger molecular systems. The data-driven coupled-cluster (DDCC) method applies machine learning to predict the coupled-cluster two-electron amplitudes (t2) using data from second-order perturbation theory (MP2). One major limitation of the DDCC models is the size of training sets that increases exponentially with the system size. Effective sampling of the amplitude space can resolve this issue. Five different amplitude selection techniques that reduce the amount of data used for training were evaluated, an approach that also prevents model overfitting and increases the portability of data-driven coupled-cluster singles and doubles to more complex molecules or larger basis sets. In combination with a localized orbital formalism to predict the CCSD t2 amplitudes, we have achieved a 10-fold error reduction for energy calculations.

Three-Center M-H-B Bonds Are Strong Field Interactions. Synthesis and Characterization of M(CH2NMe2BH3)3 Complexes of Titanium, Chromium, and Cobalt.

We describe new compounds of stoichiometry M(CH2NMe2BH3)3 (M = Ti, Cr, and Co), each of which contains three chelating boranatodimethylaminomethyl (BDAM) ligands. In all three compounds, the BDAM anion, which is isoelectronic and isostructural with the neopentyl group, is bound to the metal center at one end by a metal-carbon σ bond and at the other by one three-center M-H-B interaction. The crystal structures show that the d1 titanium(III) compound is trigonal prismatic (or eight-coordinate, if two longer-ranged M···H interactions with the BH3 groups are included), whereas the d3 chromium(III) compound and the d6 cobalt(III) compounds are both fac-octahedral. The Cr and Co compounds exhibit two rapid dynamic processes in solution: exchange between the Δ and Λ enantiomers and exchange of the terminal and bridging hydrogen atoms on boron. For the Co complex, the barrier for Δ/Λ exchange (ΔG⧧298 = 10.1 kcal mol-1) is significantly smaller than those seen in other octahedral cobalt(III) compounds; DFT calculations suggest that Bailar twist and dissociative pathways for Δ/Λ exchange are both possible mechanisms. The UV-vis absorption spectra of the cobalt(III) and chromium(III) species show that the ligand field splittings Δo caused by the M-H-B interactions are unexpectedly large, thus placing them high on the spectrochemical series (near ammonia and alkyl groups); their nephelauxetic effect is also large. The DFT calculations suggest that these properties of M-H-B interactions are in part a consequence of their three-center nature, which delocalizes electron density away from the metal center and reduces electron-electron repulsions.

Computational investigation of functionalized carbenes on dinitrogen activation.

Activation of the dinitrogen triple bond is a crucial step in the overall fixation of atmospheric nitrogen into usable forms for industrial and biological applications. Current synthetic catalysts incorporate metal ions to facilitate the activation and cleavage of dinitrogen. The high price of metal-based catalysts and the challenge of catalyst recovery during industrial catalytic processes has led to increasing interest in metal-free catalysts. One step toward metal-free catalysis is the use of frustrated Lewis pairs (FLPs). In this study, we have examined 18 functionalized carbenes as FLPs to elucidate the influence of steric and electronic effects on the activation of dinitrogen. To test the effects of functionalization on dinitrogen activation, we have performed density functional theory (DFT), multireference, non and extended transition state-natural orbital for chemical valence (ETS-NOCV) calculations. Our results suggest that functional groups which introduce strong electron-withdrawing effects and/or engage in extended π/π* systems lead to the lowering of the dissociation energy of the dinitrogen bond, which further contributes to greater nitrogen activation. We conjecture that these effects are due to enhanced back-bonding capability of the p orbital of the carbene carbon atoms to the adjacent nitrogen atoms (increasing Lewis basicity of the carbene carbon atom) and enhanced stability of dissociated products. Our concluding remarks include opportunities to extend this activation study to explore the entire catalytic cycle with promising functionalized carbenes for experimental evaluation.

Accurate Interaction Energies of CO2 with the 20 Naturally Occurring Amino Acids.

We have performed a series of highly accurate calculations between CO2 and the 20 naturally occurring amino acids for the investigation of the attractive noncovalent interactions. Different nucleophilic groups present in the amino acid structures were considered (α-NH2 , COOH, side groups), and the stronger binding sites were identified. A database of accurate reference interactions energies was compiled as computed by explicitly-correlated coupled-cluster singles-and-doubles, together with perturbative triples extrapolated to the complete-basis-set limit. The CCSD(F12)(T)/CBS reference values were used for comparing a variety of popular density functionals with different basis sets. Our results show that most density functionals with the triple-zeta basis set def2-TZVPP align with the CCSD(F12)(T)/CBS reference values, but errors range from 0.1 kcal/mol up to 1.0 kcal/mol.

Theoretical Investigation of Actinide Ligation in Aqueous and Organic Phase for Nuclear Waste Treatment.

The generation of radioactive waste has a prominent negative impact on the use of nuclear energy due to potential health concerns and cost of waste storage. This potential impact continues to rise as the quantity of waste increases due to the increasing growth in energy demand. One of the leading contributions to the radioactivity of this waste is due to the presence of actinides. The removal of these actinides by ligand-based solvent extraction methodologies provides an invaluable process necessary for the promotion of nuclear energy. By evaluating different ligands that are currently applied for actinide solvent extractions, more effective ligands could be proposed and synthesized for the successful separation of actinides from nuclear waste. Here, density functional theory (DFT) calculations for a variety of ligands and actinides are reported to explain the extraction process. Different solvation and ligation effects were evaluated for the computation of stability constants. The ratio of water and nitrate ligands in the coordination environment of actinides (Ac(III), Th(IV), Am(III), and Cm(III)) was first examined. Results from this step provided reliable initial conditions for the extraction of these actinides in both the aqueous (343HOPO) and the organic phase (BTBP). We also report a DFT benchmarking study as well as a modified BTBP ligand performance evaluation.

Electrocatalytic Hydrogen Evolution Using A Molecular Antimony Complex under Aqueous Conditions: An Experimental and Computational Study on Main-Group Element Catalysis.

Electrocatalytic hydrogen gas production is considered a potential pathway towards carbon-neutral energy sources. However, the development of this technology is hindered by the lack of efficient, cost-effective, and environmentally benign catalysts. In this study, a main-group-element-based electrocatalyst, SbSalen, is reported to catalyze the hydrogen evolution reaction (HER) in an aqueous medium. The heterogenized molecular system achieved a Faradaic efficiency of 100 % at -1.4 V vs. NHE with a maximum current density of -30.7 mA/cm2 . X-ray photoelectron spectroscopy of the catalyst-bound working electrode before and after electrolysis confirmed the molecular stability during catalysis. The turnover frequency was calculated as 43.4 s-1 using redox-peak integration. The kinetic and mechanistic aspects of the electrocatalytic reaction were further examined by computational methods. This study provides mechanistic insights into main-group-element electrocatalysts for heterogeneous small-molecule conversion.

A PEGylated Tin Porphyrin Complex for Electrocatalytic Proton Reduction: Mechanistic Insights into Main-Group-Element Catalysis.

Electrocatalytic proton reduction to form dihydrogen (H2 ) is an effective way to store energy in the form of chemical bonds. In this study, we validate the applicability of a main-group-element-based tin porphyrin complex as an effective molecular electrocatalyst for proton reduction. A PEGylated Sn porphyrin complex (SnPEGP) displayed high activity (-4.6 mA cm-2 at -1.7 V vs. Fc/Fc+ ) and high selectivity (H2 Faradaic efficiency of 94 % at -1.7 V vs. Fc/Fc+ ) in acetonitrile (MeCN) with trifluoroacetic acid (TFA) as the proton source. The maximum turnover frequency (TOFmax ) for H2 production was obtained as 1099 s-1 . Spectroelectrochemical analysis, in conjunction with quantum chemical calculations, suggest that proton reduction occurs via an electron-chemical-electron-chemical (ECEC) pathway. This study reveals that the tin porphyrin catalyst serves as a novel platform for investigating molecular electrocatalytic reactions and provides new mechanistic insights into proton reduction.

Nature of the Short Rh-Li Contact between Lithium and the Rhodium ω-Alkenyl Complex [Rh(CH2CMe2CH2CH═CH2)2].

We describe the preparation of the cis-bis(η1,η2-2,2-dimethylpent-4-en-1-yl)rhodate(I) anion, cis-[Rh(CH2CMe2CH2CH═CH2)2]-, and the interaction of this species with Li+ both in solution and in the solid state. For the lithium(diethyl ether) salt [Li(Et2O)][Rh(CH2CMe2CH2CH═CH2)2], VT-NMR and 1H{7Li} NOE NMR studies in toluene-d8 show that the Li+ cation is in close proximity to the dz2 orbital of rhodium. In the solid-state structure of the lithium(12-crown-4) salt [Li(12-crown-4)2][Li{Rh(CH2CMe2CH2CH═CH2)2}2], one lithium atom is surrounded by two [Rh(CH2CMe2CH2CH═CH2)2]- anions, and in this assembly there are two unusually short Rh-Li distances of 2.48 Å. DFT calculations, natural energy decomposition, and ETS-NOCV analysis suggest that there is a weak dative interaction between the 4dz2 orbitals on the Rh centers and the 2pz orbital of the Li+ cation. The charge-transfer term between Rh and Li+ contributes only about the 1/5 of the total interaction energy, however, and the principal driving force for the proximity of Rh and Li in compounds 1 and 2 is that Li+ is electrostatically attracted to negative charges on the dialkylrhodiate anions.

Electrocatalytic Dechlorination of Dichloromethane in Water Using a Heterogenized Molecular Copper Complex.

The remediation of organohalides from water is a challenging process in environment protection and water treatment. Herein, we report a molecular copper(I) complex with two triazole units, CuT2, in a heterogeneous aqueous system that is capable of dechlorinating dichloromethane (CH2Cl2) to afford hydrocarbons (methane, ethane, and ethylene). The catalytic performance is evaluated in water and presented high Faradaic efficiency (average 70% CH4) across a range of potentials (-1.1 to -1.6 V vs Ag/AgCl) and high activity (maximum -25.1 mA/cm2 at -1.6 V vs Ag/AgCl) with a turnover number of 2.0 × 107. The CuT2 catalyst also showed excellent stability for 14 h of constant exposure to CH2Cl2 and 10 h of CH2Cl2 exposure cycling. The control compound, a copper-free triazole unit (T1), was also investigated under the same condition and showed inferior catalytic activity, indicating the importance of the copper center. Plausible catalytic mechanisms are proposed for the formation of C1 and C2 products via radical intermediates. Computational studies provided additional insight into the reaction mechanism and the selectivity toward the CH4 formation. The findings in this study demonstrate that complex CuT2 is an efficient and stable catalyst for the dehalogenation of CH2Cl2 and could potentially be used for the exploration of the removal of halogenated species from aqueous systems.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities.

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

σ-Donation and π-Backdonation Effects in Dative Bonds of Main-Group Elements.

The nature of donor-acceptor interactions is important for the understanding of dative bonding and can provide vital insights into many chemical processes. Here, we have performed a computational study to elucidate substantial differences between different types of dative interactions. For this purpose, a data set of 20 molecular complexes stabilized by dative bonds was developed (DAT20). A benchmark study that considers many popular density functionals with respect to accurate quantum chemical interaction energies and geometries revealed two different trends between the complexes of DAT20. This behavior was further explored by means of frontier molecular orbitals, extended-transition-state natural orbitals for chemical valence (ETS-NOCV), and natural energy decomposition analysis (NEDA). These methods revealed the extent of the forward and backdonation between the donor and acceptor molecules and how they influence the total interaction energies and molecular geometries. A new classification of dative bonds is suggested.

Redox states of dinitrogen coordinated to a molybdenum atom.

Chemical structures bearing a molybdenum atom have been suggested for the catalytic reduction of N2 at ambient conditions. Previous computational studies on gas-phase MoN and MoN2 species have focused only on neutral structures. Here, an ab initio electronic structure study on the redox states of small clusters composed of nitrogen and molybdenum is presented. The complete-active space self-consistent field method and its extension via second-order perturbative complement have been applied on [MoN]n and [MoN2]n species (n = 0, 1±, 2±). Three different coordination modes (end-on, side-on, and linear NMoN) have been considered for the triatomic [MoN2]n. Our results demonstrate that the reduced states of such systems lead to a greater degree of N2 activation, which can be the starting point of different reaction channels.

Selecting Quantum-Chemical Methods for Lanthanide-Containing Molecules: A Balance between Accuracy and Efficiency.

An analysis of how different density functionals, basis sets, and relativistic approximations affect the computed properties of lanthanide-containing molecules allows one to determine which method provides the highest accuracy. Historically, many different density functional methods have been employed to perform calculations on lanthanide complexes and so herein is a detailed analysis of how different methodological combinations change the computed properties of three different families of lanthanide-bearing species: lanthanide diatomic molecules (fluorides and oxides) and their dissociation energies; larger, molecular complexes and their geometries; and lanthanide bis(2-ethylhexyl)phosphate structures and their separation free energies among the lanthanide series. The B3LYP/Sapporo/Douglas-Kroll-Hess (DKH) method was shown to most accurately reproduce dissociation energies calculated at the CCSDT(Q) level of theory with a mean absolute deviation of 1.3 kcal/mol. For the calculations of larger, molecular complexes, the TPSSh/Sapporo/DKH method led to the smallest deviation from experimentally refined crystal structures. Finally, this same method led to calculated separation factors for lanthanide bis(2-ethylhexyl)phosphate structures that matched very closely with experimental values.

CO2 Capture on Functionalized Calixarenes: A Computational Study.

High carbon emissions have shown a strong correlation with rising global temperatures as the world's climate undergoes a dramatic shift. Work to mitigate the potential damage using materials such as metal-organic frameworks (MOFs), covalent organic frameworks (COFs), and polymer membranes (PMs) has proven successful in small-scale approaches; however, research is still being performed to enhance the capabilities of these materials for use at an industrial scale. One strategy for increasing performance is to embed these materials with CO2-philic molecules, which enhance selective binding over other gases. Calixarenes are promising candidates due to their large chalice shape, which allows for the possibility to bind multiple CO2 molecules per site. In this study, a dataset including 40 functionalized calixarene structures and one unfunctionalized (bare) calixarene was constructed with an automated, high-throughput structure generation through directed modifications to a molecular scaffold. A conformational search based on molecular mechanics allowed the faster determination of optimal binding energies for a vast array of chemical functional groups with less computational effort. Density functional theory and symmetry-adapted perturbation theory calculations were performed for the exploration of their interactions with CO2. Our work has identified new organic cages with increased CO2-philicity. In four cases, CO2 binding is stronger than 9.0 kcal/mol and very close to the targets set by previous studies. The nature of the noncovalent interactions for these cases is analyzed and discussed. Conclusions from this study can aid synthetic efforts for the next generation of functional materials.

Understanding the Nature of Weak Interactions between Functionalized Boranes and N2/O2, Promising Functional Groups for Gas Separations.

The separation of nitrogen and oxygen gases is considered as a very challenging process, since both O2 and N2 are nonpolar molecules with similar kinetic diameters. Electronic structure theory can provide a fundamental understanding of effects that can lead to selective binding of nitrogen or oxygen gas for the development of novel separation processes. Boranes can bind dinitrogen through a dative bond, where the boron acts as a σ acceptor and back-donates through π orbitals. To better understand these interactions, we have performed highly accurate CCSD(F12)(T) and CCSDT(Q) computations for the BH3-N2 and BH3-O2 complexes. The coupled-cluster binding energies were used as reference for benchmarking different density functionals, and larger functionalized boranes were examined at the M05/def2-TZVPPD level. Symmetry adapted perturbation theory (SAPT) calculations were performed for the elucidation of the nature of the interaction between nitrogen and substituted boranes and how direct or distal functionalizations affect the strength of the weak dative bonds. By use of these methods, several boranes were found to bind N2 over O2. These molecular species are promising functional groups for incorporation into the next generation of advanced materials for efficient N2/O2 separations.

Unprecedented Five-Coordinate Iron(IV) Imides Generate Divergent Spin States Based on the Imide R-Groups.

Three five-coordinate iron(IV) imide complexes have been synthesized and characterized. These novel structures have disparate spin states on the iron as a function of the R-group attached to the imide, with alkyl groups leading to low-spin diamagnetic (S=0) complexes and an aryl group leading to an intermediate-spin (S=1) complex. The different spin states lead to significant differences in the bonding about the iron center as well as the spectroscopic properties of these complexes. Mössbauer spectroscopy confirmed that all three imide complexes are in the iron(IV) oxidation state. The combination of diamagnetism and 15 N labeling allowed for the first 15 N NMR resonance recorded on an iron imide. Multi-reference calculations corroborate the experimental structural findings and suggest how the bonding is distinctly different on the imide ligand between the two spin states.

Selective Catalytic Chemistry at Rhodium(II) Nodes in Bimetallic Metal-Organic Frameworks.

We report the first study of a gas-phase reaction catalyzed by highly dispersed sites at the metal nodes of a crystalline metal-organic framework (MOF). Specifically, CuRhBTC (BTC3- =benzenetricarboxylate) exhibited hydrogenation activity, while other isostructural monometallic and bimetallic MOFs did not. Our multi-technique characterization identifies the oxidation state of Rh in CuRhBTC as +2, which is a Rh oxidation state that has not previously been observed for crystalline MOF metal nodes. These Rh2+ sites are active for the catalytic hydrogenation of propylene to propane at room temperature, and the MOF structure stabilizes the Rh2+ oxidation state under reaction conditions. Density functional theory calculations suggest a mechanism in which hydrogen dissociation and propylene adsorption occur at the Rh2+ sites. The ability to tailor the geometry and ensemble size of the metal nodes in MOFs allows for unprecedented control of the active sites and could lead to significant advances in rational catalyst design.

Ligand field effects on the ground and excited states of reactive FeO2+ species.

High-valent Fe(iv)-oxo species have been found to be key oxidizing intermediates in the mechanisms of mononuclear iron heme and non-heme enzymes that can functionalize strong C-H bonds. Biomimetic Fe(iv)-oxo molecular complexes have been successfully synthesized and characterized, but their catalytic reactivity is typically lower than that of the enzymatic analogues. The C-H activation step proceeds through two competitive mechanisms, named σ- and π-channels. We have performed high-level wave function theory calculations on bare FeO2+ and a series of non-heme Fe(iv)-oxo model complexes in order to elucidate the electronic properties and the ligand field effects on those channels. Our results suggest that a coordination environment formed by a weak field gives access to both competitive channels, yielding more reactive Fe(iv)-oxo sites. In contrast, a strong ligand environment stabilizes only the σ-channel. Our concluding remarks will aid the derivation of new structure-reactivity descriptors that can contribute to the development of the next generation of functional catalysts.

Gas-Phase Ion Chemistry of Metalloporphyrin Anions with Molecular Oxygen: Probing the Influence of the Oxidation and Spin State of the Central Transition Metal by Experiment and Theory.

We performed a comprehensive gas-phase experimental and quantum-chemical study of the binding properties of molecular oxygen to iron and manganese porphyrin anions. Temperature-dependent ion-molecule reaction kinetics as probed in a Fourier-transform ion-cyclotron resonance mass spectrometer reveal that molecular oxygen is bound by, respectively, 40.8 ± 1.4 and 67.4 ± 2.2 kJ mol-1 to the FeII or MnII centers of isolated tetra(4-sulfonatophenyl)metalloporphyrin tetraanions. In contrast, FeIII and MnIII trianion homologues were found to be much less reactive-indicating an upper bound to their dioxygen binding energies of 34 kJ mol-1. We modeled the corresponding O2 adsorbates at the density functional theory and CASPT2 levels. These quantum-chemical calculations verified the stronger O2 binding on the FeII or MnII centers and suggested that O2 binds as a superoxide anion.