Machine Learning Models for Predicting Zirconocene Properties and Barriers.

Zr metallocenes have significant potential to be highly tunable polyethylene catalysts through modification of the aromatic ligand framework. Here we report the development of multiple machine learning models using a large library (>700 systems) of DFT-calculated zirconocene properties and barriers for ethylene polymerization. We show that very accurate machine learning models are possible for HOMO-LUMO gaps of precatalysts but the performance significantly depends on the machine learning algorithm and type of featurization, such as fingerprints, Coulomb matrices, smooth overlap of atomic positions, or persistence images. Surprisingly, the description of the bonding hapticity, the number of direct connections between Zr and the ligand aromatic carbons, only has a moderate influence on the performance of most models. Despite robust models for HOMO-LUMO gaps, these types of machine learning models based on structure connectivity type features perform poorly in predicting ethylene migratory insertion barrier heights. Therefore, we developed several relatively robust and accurate machine learning models for barrier heights that are based on quantum-chemical descriptors (QCDs). The quantitative accuracy of these models depends on which potential energy surface structure QCDs were harvested from. This revealed a Hammett-type principle to naturally emerge showing that QCDs from the π-coordination complexes provide much better descriptions of the transition states than other potential-energy structures. Feature importance analysis of the QCDs provides several fundamental principles that influence zirconocene catalyst reactivity.

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.

Experimental Demonstration and Density Functional Theory Mechanistic Analysis of Arene C-H Bond Oxidation and Product Protection by Osmium Tetroxide in a Strongly Basic/Nucleophilic Solvent.

Reactions that result in the oxy-functionalization of sp2 C-H bonds to give phenols are relatively rare. Here we report experiments and density functional theory (DFT) calculations that demonstrate selective C-H bond hydroxylation of nitroarenes to their corresponding mono-phenoxide as the exclusive product using OsO4 in a highly basic solvent mixture of water, hydroxide, and pyridine. DFT calculations using a mixed explicit/continuum solvent approach indicate that there is likely a mixture of OsO4-hydroxide/pyridine ground-state structures that have competitive reactivity and that the mechanism involves the nucleophilic addition of an anionic metal-oxo species to the arene followed by a hydride transfer process that is different from the standard [3 + 2] mechanism often invoked for the OsO4 oxidation of σ and π bonds. This work demonstrates the utility of using a strongly basic solvent for C-H bond oxidation reactions as this effectively converts any reactive phenolic product into the corresponding phenoxide, which is protected and essentially inert to further oxidation by the nucleophilic metal-oxo species.

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.

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.

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.

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.

Forging structural complexity: diastereoselective synthesis of densely substituted ß-lactams with dual functional handles for enhanced core modifications.

The preparation of the ß-lactam motif containing both C-Br and N-O bonds as functional handles remains an unmet synthetic challenge. Described herein is a novel and highly diastereoselective NBS-mediated cyclization of N-alkoxy α,ß-unsaturated silyl imino ethers to furnish nearly three dozen α-bromo N-alkoxy ß-lactams. The reaction gives rapid and convenient access to structurally diverse monocyclic, spirocyclic and fused ß-lactams in moderate to good yields. The two functional handles were shown to be useful for the further elaboration of the ß-lactam core.

Dynamic-dependent selectivity in a bisphosphine iron spin crossover C-H insertion/π-coordination reaction.

Reaction pathway selectivity is generally controlled by competitive transition states. Organometallic reactions are complicated by the possibility that electronic spin state changes rather than transition states can control the relative rates of pathways, which can be modeled as minimum energy crossing points (MECPs). Here we show that in the reaction between bisphosphine Fe and ethylene involving spin state crossover (singlet and triplet spin states) that neither transition states nor MECPs model pathway selectivity consistent with experiment. Instead, single spin state and mixed spin state quasiclassical trajectories demonstrate nonstatistical intermediates and that C-H insertion versus π-coordination pathway selectivity is determined by the dynamic motion during reactive collisions. This example of dynamic-dependent product outcome provides a new selectivity model for organometallic reactions with spin crossover.

Dinickel Catalyzed Vinylidene-Alkene Cyclization Reactions.

A dinickel catalyst promotes reductive cyclization reactions of 1,1-dichloroalkenes containing pendant olefins. The reactions can be conducted with a Zn reductant or electrocatalytically using a carbon working electrode. Mechanistic studies are consistent with the intermediacy of a Ni2(vinylidene) species, which adds to the alkene and generates a metallacyclic intermediate. ß-Hydride elimination followed by C-H reductive elimination forms the cyclization product. The proposed dinickel metallacycle is structurally characterized and its stoichiometric conversion to product is demonstrated. Spin polarized, unrestricted DFT calculations are used to further examine the cyclization mechanism. These computational models reveal that both nickel centers function cooperatively to mediate the key oxidative addition, migratory insertion, ß-hydride elimination, and reductive elimination steps.

Potent Anti-Inflammatory, Arylpyrazole-Based Glucocorticoid Receptor Agonists That Do Not Impair Insulin Secretion.

Glucocorticoids (GCs) are widely used in medicine for their role in the treatment of autoimmune-mediated conditions, certain cancers, and organ transplantation. The transcriptional activities GCs elicit include transrepression, postulated to be responsible for the anti-inflammatory activity, and transactivation, proposed to underlie the undesirable side effects associated with long-term use. A GC analogue that could elicit only transrepression and beneficial transactivation properties would be of great medicinal value and is highly sought after. In this study, a series of 1-(4-substituted phenyl)pyrazole-based GC analogues were synthesized, biologically screened, and evaluated for SARs leading to the desired activity. Activity observed in compounds bearing an electron deficient arylpyrazole moiety showed promise toward a dissociated steroid, displaying transrepression while having limited transactivation activity. In addition, compounds 11aa and 11ab were found to have anti-inflammatory efficacy comparable to that of dexamethasone at 10 nM, with minimal transactivation activity and no reduction of insulin secretion in cultured rat 832/13 beta cells.