Understanding and Quantifying London Dispersion Effects in Organometallic Complexes

Mechanism of Fuel Gas Denitration on the KOH-Activated Biochar Surface

Previous experimental studies have found that biochar after KOH activation can significantly improve the efficiency of NO removal, but its mechanism is still unclear. To investigate the reaction mechanism of this denitration reaction, in this study, the aromatic benzene ring structure was used to simulate the surface of biochar, and the reaction process was calculated by density functional theory (DFT). The reaction process on the pristine biochar was simulated for comparison. The results indicated that there were two potential mechanisms for NO removal and had the identical rate-determining step, with an activation energy of 161.5 kJ/mol. Second, the influence by K coadsorbates on the NO reduction mechanism was studied. The adsorption by K atoms does not alter the last reaction step, but it was found to reduce the activation energy of this rate-determining step (to 129.3 kJ/mol). A third type of reaction mechanism was theoretically studied for the situation with both K and OH coadsorbates on the biochar surface. The reaction mechanism changed with an increase in the overall reaction rate by increasing the pre-exponential factor. In summary, the rate-determining activation energy for the heterogeneous NO reduction was found to decrease in the following order: 161.5 kJ/mol (pristine biochar) → 129.3 kJ/mol (activation by K adsorbates) → 125.8 kJ/mol (activation by both K and OH adsorbates); the pre-exponential factor was found to change in the following order: 6.23 × 10¹⁴ s^-1 (pristine biochar) → 4.86 × 10¹⁴ s^-1 (activation by K adsorbates) → 8.89 × 10¹⁴ s^-1 (activation by K and OH adsorbates). Hence, the role by K adsorbates is primarily to reduce the rate-determining activation energy, while the OH group adsorbate increases the number of active sites on the surface of biochar.

Cobalt-Catalyzed Enantioselective C-H Arylation of Indoles

Atropoisomeric (hetero)biaryls are scaffolds with increasing importance in the pharmaceutical and agrochemical industries. Although it is the most obvious disconnection to construct such compounds, the direct enantioselective C-H arylation through the concomitant induction of the chiral information remains extremely challenging and uncommon. Herein, the unprecedented earth-abundant 3d-metal-catalyzed atroposelective direct arylation is reported, furnishing rare atropoisomeric C2-arylated indoles. Kinetic studies and DFT computation revealed an uncommon mechanism for this asymmetric transformation, with the oxidative addition being the rate- and enantio-determining step. Excellent stereoselectivities were reached (up to 96% ee), while using an unusual N-heterocyclic carbene ligand bearing an essential remote substituent. Attractive dispersion interactions along with positive C-H---π interactions exerted by the ligand were identified as key factors to guarantee the excellent enantioselection.

An open-source framework for fast-yet-accurate calculation of quantum mechanical features

We present the open-source framework kallisto that enables the efficient and robust calculation of quantum mechanical features for atoms and molecules. For a benchmark set of 49 experimental molecular polarizabilities,...

Designing a Solution-Stable Distannene: The Decisive Role of London Dispersion Effects in the Structure and Properties of {Sn(C6H2-2,4,6-Cy3)2}2 (Cy = Cyclohexyl)

The reaction of 1 equiv of the dimeric lithium salt of a new London dispersion effect donor ligand {Li(C₆H₂-2,4,6-Cy₃)·OEt₂}₂ (Cy = cyclohexyl) with SnCl₂ afforded the distannene {Sn(C₆H₂-2,4,6-Cy₃)₂}₂ (1). The distannene remains dimeric in solution, as indicated by its room-temperature ¹¹⁹Sn NMR signal (δ = 361.3 ppm) and its electronic spectrum, which is invariant over the temperature range of -10 to 100 °C. The formation of the distannene, which has a short Sn-Sn distance of 2.7005(7) Å and greatly enhanced stability in solution compared to that of other distannenes, is due to increased interligand London dispersion (LD) attraction arising from multiple close approaches of ligand C-H moieties across the Sn-Sn bond. DFT-D4 calculations revealed a dispersion stabilization of dimer 1 of 38 kcal mol^-1 and a dimerization free energy of ΔG_dimer = -6 kcal mol^-1. In contrast, the reaction of 2 equiv of the similarly shaped but less bulky, less hydrogen-rich Li(C₆H₂-2,4,6-Ph₃)·(OEt₂)₂ with SnCl₂ yielded the monomeric stannylene Sn(C₆H₂-2,4,6-Ph₃)₂ (2), which is unstable in solution at ambient temperature.

London Dispersion Helps Refine Steric A-Values: Dispersion Energy Donor Scales

We suggest a scale of dispersion energy donors (DEDs) that allows for direct comparisons with steric effects. This scale is based on the classic A-values and allows groups to reorient to minimize strain, thereby providing an advantage over raw group polarizabilities. The A-value can no longer be considered purely a steric factor. Even for groups that do not participate in charge transfer or electrostatic interactions, the A-value includes Pauli repulsion (steric hindrance) and attractive London dispersion (LD) interactions. Although the common assumption is that, at the distances found in monosubstituted cyclohexanes, steric demands are the key factors influencing conformer preferences, we show in this computational study that there is a non-negligible LD part. We use this system to build a DED scale and a complementary steric scale. These scales are quantitatively comparable, as they are based on the same system, and allow for comparison of the two competing interactions in experimentally relevant settings. In addition, we show that LD interactions can be used to explain puzzling data regarding relative group sizes.

Noncovalent Interactions in Organometallic Chemistry: From Cohesion to Reactivity, a New Chapter

Noncovalent interactions (NCIs) have long interested a vast community of chemists who investigated their "canonical categories" derived from descriptive crystallography, e.g., H-bonds, π-π interactions, halogen/chalcogen/tetrel bonds, cation-π and C-H-π interactions, metallophilic interactions in the broad sense, etc. Recent developments in theoretical chemistry have enabled the treatment of noncovalent interactions under new auspices: dispersion-force-inclusive density functionals have emerged, which are reliable for modeling small to large molecular systems. It is possible to perform the full analysis of the contributions of London, Debye, and Keesom forces, i.e., the main components of van der Waals forces, by the DFT-D and ab initio methods at a reasonable computational cost. Our research has been focusing for now 15 years on the role of NCIs in the cohesion of organometallic complexes. NCIs are not only effective in Werner's secondary coordination sphere but also in the metal's primary one. The stabilization of electron-unsaturated transition metal complexes by hemichelation, metal-metal donor-acceptor complexes, and self-aggregation of cationic Rh(I) chromophores have indeed outlined the significance of the London dispersion force as an attractive force operating throughout the whole molecule or molecular assembly. The recent outburst of interest in C-H bond functionalization led us to address the broader question of reaction and catalyst engineering: although one can now satisfactorily analyze bonding and molecular cohesion in transition-metal-based organometallic systems, can modern theoretical methods guide reactivity exploration and the engineering of novel catalytic systems? We addressed this question by investigating the ambiphilic metal-ligand activation/concerted metalation-deprotonation mechanism involved in transition-metal-catalyzed directed C-H bond functionalization. This endeavor was initiated having in scope the construction of a rationale for the transposition of 4-5d metal chemistry to earth-abundant 3d metals. In this base-assisted mechanism of C-H bond metalation, agostic interactions are necessary but not sufficient because C-H bond breaking actually relies on the attractive NCI coding of a proton-transfer step and the minimization of metal-H repulsion. This Account introduces the recent shift of our research toward the construction of an NCI-inclusive paradigm of chemical reactivity engineering based on experimental efforts propped up by state-of-the-art theoretical tools.

Hydrogenation of Secondary Amides using Phosphane Oxide and Frustrated Lewis Pair Catalysis

The metal‐free catalytic hydrogenation of secondary carboxylic acid amides is developed. The reduction is realized by two new catalytic reactions. First, the amide is converted into the imidoyl chloride by triphosgene (CO(OCCl₃)₂) using novel phosphorus(V) catalysts. Second, the in situ generated imidoyl chlorides are hydrogenated in high yields by an FLP‐catalyst. Mechanistic and quantum mechanical calculations support an autoinduced catalytic cycle for the hydrogenation with chloride acting as unusual Lewis base for FLP‐mediated H₂‐activation.

Assessing Density Functional Theory for Chemically Relevant Open-Shell Transition Metal Reactions

Due to the principle lack of systematic improvement possibilities of density functional theory, careful assessment of the performance of density functional approximations (DFAs) on well-designed benchmark sets, for example, for reaction energies and barrier heights, is crucial. While main-group chemistry is well covered by several available sets, benchmark data for transition metal chemistry is sparse. This is especially the case for larger, chemically relevant molecules. Addressing this issue, we recently introduced the MOR41 benchmark which covers chemically relevant reactions of closed-shell complexes. In this work, we extend these efforts to single-reference open-shell systems and introduce the "reactions of open-shell single-reference transition metal complexes" (ROST61) benchmark set. ROST61 includes accurate coupled-cluster reference values for 61 reaction energies with a mean reaction energy of -42.8 kcal mol^-1. Complexes with 13-93 atoms covering 20 d-block elements are included, but due to the restriction to single-reference open-shell systems, important elements such as iron or platinum could not be taken into account, or only to a small extent. We assess the performance of 31 DFAs in combination with three London dispersion (LD) correction schemes. Further, DFT-based composite methods, MP2, and a few semiempirical quantum chemical methods are evaluated. Consistent with the results for the MOR41 closed-shell benchmark, we find that the ordering of DFAs according to Jacob's ladder is preserved and that adding an LD correction is crucial, clearly improving almost all tested methods. The recently introduced r²SCAN-3c composite method stands out with a remarkable mean absolute deviation (MAD) of only 2.9 kcal mol^-1, which is surpassed only by hybrid DFAs with low amounts of Fock exchange (e.g., 2.3 kcal mol^-1 for TPSS0-D4/def2-QZVPP) and double-hybrid (DH) DFAs but at a significantly higher computational cost. The lowest MAD of only 1.6 kcal mol^-1 is obtained with the DH DFA PWPB95-D4 in the def2-QZVPP basis set approaching the estimated accuracy of the reference method. Overall, the ROST61 set adds important reference data to a sparsely sampled but practically relevant area of chemistry. At this point, it provides valuable orientation for the application and development of new DFAs and electronic structure methods in general.

DFT Functionals for Modeling of Polyethylene Chains Cross-Linked by Metal Atoms. DLPNO-CCSD(T) Benchmark Calculations

Density functional theory (DFT) functionals for calculations of binding energies (BEs) of the polyethylene (PE) chains cross-linked by selected metal atoms (M) are benchmarked against DLPNO-CCSD(T) and DLPNO-CCSD(T1) data. PEX-M-PEX complexes as compared with plain parallel PEX···PEX chains with X = 3-9 carbon atoms are model species characterized by a cooperative effect of covalent C-M-C bonds and interchain dispersion interactions. The accuracy of DLPNO-CC methods was assessed by a comparison of BEs with the canonical CCSD(T) results for small PE3-M-PE3 complexes. Functionals for PEX···PEX and closed-shell PEX-M-PEX complexes (M = Be, Mg, Zn) were benchmarked against DLPNO-CCSD(T) BEs; open-shell complexes (M = Li, Ag, Au) were benchmarked against the DLPNO-CCSD(T1) method with iterative triples. Three dispersion corrections were combined with 25 DFT functionals for calculations of BEs with respect to PEX-M and PEX fragments employing def2-TZVPP and def2-QZVPP basis sets. Accuracy to within 5% for the closed-shell PEX-M-PEX complexes was achieved with five functionals. Less accurate are functionals for the open-shell PEX-M-PEX complexes; only two functionals deviate by less than 15% from DLPNO-CCSD(T1). Particularly problematic were PEX-Li-PEX complexes. A reasonable overall performance across all complexes in terms of the mean absolute percentage error is found for the range-separated hybrid functionals ωB97X-D3 and CAM-B3LYP/D3(BJ)-ABC.

N-Heterocyclic Carbene/Carboxylic Acid Co-Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading

We report the discovery that simple carboxylic acids, such as benzoic acid, boost the activity of N-heterocyclic carbene (NHC) catalysts in the oxidative esterification of aldehydes. A simple and efficient protocol for the transformation of a wide range of sterically hindered α- and β-substituted aliphatic aldehydes/enals, catalyzed by a novel and readily accessible N-Mes-/N-2,4,6-trichlorophenyl 1,2,4-triazolium salt, and benzoic acid as co-catalyst, was developed. A whole series of α/β-substituted aliphatic aldehydes/enals hitherto not amenable to NHC-catalyzed esterification could be reacted at typical catalyst loadings of 0.02-1.0 mol %. For benzaldehyde, even 0.005 mol % of NHC catalyst proved sufficient: the lowest value ever achieved in NHC catalysis. Preliminary studies point to carboxylic acid-induced acceleration of acyl transfer from azolium enolate intermediates as the mechanistic basis of the observed effect.

Could London Dispersion Force Control Regioselective (2 + 2) Cyclodimerizations of Benzynes? YES: Application to the Synthesis of Helical Biphenylenes

In recent years, London dispersion interactions, which are the attractive component of the van der Waals potential, have been found to play an important role in controlling the regio- and/or stereoselectivity of various reactions. Particularly, the dispersion interactions between substrates and catalysts (or ligands) are dominant in various selective catalyzes. In contrast, repulsive steric interactions, rather than the attractive dispersion interactions, between bulky substituents are predominant in most of the noncatalytic reactions. Herein, we demonstrate the first example of London dispersion-controlled noncatalytic (2 + 2) cyclodimerization of substituted benzynes to selectively afford proximal biphenylenes in high yields and regioselectivities, depending on the extent of dispersion interactions in the substituents. This method can be applied for the synthesis of novel helical biphenylenes, which would be fascinating for chemists as these compounds are potential skeletons for ligands, catalysts, and medicines.

Robust and Efficient Implicit Solvation Model for Fast Semiempirical Methods

We present a robust and efficient method to implicitly account for solvation effects in modern semiempirical quantum mechanics and force fields. A computationally efficient yet accurate solvation model based on the analytical linearized Poisson-Boltzmann (ALPB) model is parameterized for the extended tight binding (xTB) and density functional tight binding (DFTB) methods as well as for the recently proposed GFN-FF general force field. The proposed methods perform well over a broad range of systems and applications, from conformational energies over transition-metal complexes to large supramolecular association reactions of charged species. For hydration free energies of small molecules, GFN1-xTB(ALPB) is reaching the accuracy of sophisticated explicitly solvated approaches, with a mean absolute deviation of only 1.4 kcal/mol compared to the experiment. Logarithmic octanol-water partition coefficients (log K_ow) are computed with a mean absolute deviation of about 0.65 using GFN2-xTB(ALPB) compared to experimental values indicating a consistent description of differential solvent effects. Overall, more than twenty solvents for each of the six semiempirical methods are parameterized and tested. They are readily available in the xtb and dftb+ programs for diverse computational applications.

Comment on "The Nature of Chalcogen-Bonding-Type Tellurium-Nitrogen Interactions": Fixing the Description of Finite-Temperature Effects Restores the Agreement Between Experiment and Theory

Mitzel and co-workers recently presented an intriguing molecule displaying a tellurium-nitrogen interaction. Structural data obtained in the solid and in gas phase indicated a large increase of the Te-N equilibrium distance re from 2.64 to 2.92 Å, respectively. Although some DFT calculations appear to support the large re in gas phase, we argue that the lions share of the increase is due to an incomplete description of finite-temperature effects in the back-corrected experimental data. This hypothesis is based on high-level coupled-cluster (CC) and periodic DFT calculations, which consistently point towards a much smaller re in the isolated molecule. Further support comes through MD simulations with a tuned GFN2-xTB Hamiltonian: Calibrated against a CC reference, these show a six-times larger influence of temperature than with the originally used GFN1-xTB. Taking this into account, the back-corrected re in gas phase becomes 2.67±0.08 Å, in good agreement with high-level CC theory and most DFT methods.

Is the Electrophilicity of the Metal Nitrene the Sole Predictor of Metal-Mediated Nitrene Transfer to Olefins? Secondary Contributing Factors as Revealed by a Library of High-Spin Co(II) Reagents

Recent research has highlighted the key role played by the electron affinity of the active metal-nitrene/imido oxidant as the driving force in nitrene additions to olefins to afford valuable aziridines. The present work showcases a library of Co(II) reagents that, unlike the previously examined Mn(II) and Fe(II) analogues, demonstrate reactivity trends in olefin aziridinations that cannot be solely explained by the electron affinity criterion. A family of Co(II) catalysts (17 members) has been synthesized with the assistance of a trisphenylamido-amine scaffold decorated by various alkyl, aryl, and acyl groups attached to the equatorial amidos. Single-crystal X-ray diffraction analysis, cyclic voltammetry and EPR data reveal that the high-spin Co(II) sites (S = 3/2) feature a minimal [N3N] coordination and span a range of 1.4 V in redox potentials. Surprisingly, the Co(II)-mediated aziridination of styrene demonstrates reactivity patterns that deviate from those anticipated by the relevant electrophilicities of the putative metal nitrenes. The representative L4Co catalyst (-COCMe3 arm) is operating faster than the L8Co analogue (-COCF3 arm), in spite of diminished metal-nitrene electrophilicity. Mechanistic data (Hammett plots, KIE, stereocontrol studies) reveal that although both reagents follow a two-step reactivity path (turnover-limiting metal-nitrene addition to the C b atom of styrene, followed by product-determining ring-closure), the L4Co catalyst is associated with lower energy barriers in both steps. DFT calculations indicate that the putative [L4Co]NTs and [L8Co]NTs species are electronically distinct, inasmuch as the former exhibits a single-electron oxidized ligand arm. In addition, DFT calculations suggest that including London dispersion corrections for L4Co (due to the polarizability of the tert-Bu substituent) can provide significant stabilization of the turnover-limiting transition state. This study highlights how small ligand modifications can generate stereoelectronic variants that in certain cases are even capable of overriding the preponderance of the metal-nitrene electrophilicity as a driving force.

Dispersion forces in chirality recognition - a density functional and wave function theory study of diols

In the discussion of chirality recognition, steric considerations and strongly directed interactions such as hydrogen bonds are primarily discussed. However, given the sheer size of biomolecules, it is expected that dispersion forces could also play a determining role for aggregate formation and associated chirality recognition. With the example of diol molecules, we explore different factors in the formation of homo- and hetero-dimers as well as their relative stability. By comparing density functional results with the analysis of local correlation methods, we infer the impact of dispersion not only on the energies but also on the structures of such chiral aggregates. A local orbital based scheme is used to calculate wave function dispersion-free gradients and compare to uncorrected density functional structures.

Benchmarking London dispersion corrected density functional theory for noncovalent ion-π interactions

The strongly attractive noncovalent interactions of charged atoms or molecules with π-systems are important binding motifs in many chemical and biological systems. These so-called ion-π interactions play a major role in enzymes, molecular recognition, and for the structure of proteins. In this work, a molecular test set termed IONPI19 is compiled for inter- and intramolecular ion-π interactions, which is well balanced between anionic and cationic systems. The IONPI19 set includes interaction energies of significantly larger molecules (up to 133 atoms) than in other ion-π test sets and covers a broad range of binding motifs. Accurate (local) coupled cluster values are provided as reference. Overall, 19 density functional approximations, including seven (meta-)GGAs, eight hybrid functionals, and four double-hybrid functionals combined with three different London dispersion corrections, are benchmarked for interaction energies. DFT results are further compared to wave function based methods such as MP2 and dispersion corrected Hartree-Fock. Also, the performance of semiempirical QM methods such as the GFNn-xTB and PMx family of methods is tested. It is shown that dispersion-uncorrected DFT underestimates ion-π interactions significantly, even though electrostatic interactions dominate the overall binding. Accordingly, the new charge dependent D4 dispersion model is found to be consistently better than the standard D3 correction. Furthermore, the functional performance trend along Jacob's ladder is generally obeyed and the reduction of the self-interaction error leads to an improvement of (double) hybrid functionals over (meta-)GGAs, even though the effect of the SIE is smaller than expected. Overall, the double-hybrids PWPB95-D4/QZ and revDSD-PBEP86-D4/QZ turned out to be the most reliable among all assessed methods for the description of ion-π interactions, which opens up new perspectives for systems where coupled cluster calculations are no longer computationally feasible.

Mechanisms of metal-dependent non-redox decarboxylases from quantum chemical calculations

Quantum chemical calculations are today an extremely valuable tool for studying enzymatic reaction mechanisms. In this mini-review, we summarize our recent work on several metal-dependent decarboxylases, where we used the so-called cluster approach to decipher the details of the reaction mechanisms, including elucidation of the identity of the metal cofactors and the origins of substrate specificity.

Decarboxylases are of growing potential for biocatalytic applications, as they can be used in the synthesis of novel compounds of, e.g., pharmaceutical interest. They can also be employed in the reverse direction, providing a strategy to synthesize value‐added chemicals by CO₂ fixation. A number of non-redox metal-dependent decarboxylases from the amidohydrolase superfamily have been demonstrated to have promiscuous carboxylation activities and have attracted great attention in the recent years. The computational mechanistic studies provide insights that are important for the further modification and utilization of these enzymes in industrial processes. The discussed enzymes are: 5‐carboxyvanillate decarboxylase, γ‐resorcylate decarboxylase, 2,3‐dihydroxybenzoic acid decarboxylase, and iso-orotate decarboxylase.

Quantification of Noncovalent Interactions in Azide-Pnictogen, -Chalcogen, and -Halogen Contacts

The noncovalent interactions between azides and oxygen‐containing moieties are investigated through a computational study based on experimental findings. The targeted synthesis of organic compounds with close intramolecular azide–oxygen contacts yielded six new representatives, for which X‐ray structures were determined. Two of those compounds were investigated with respect to their potential conformations in the gas phase and a possible significantly shorter azide–oxygen contact. Furthermore, a set of 44 high‐quality, gas‐phase computational model systems with intermolecular azide–pnictogen (N, P, As, Sb), –chalcogen (O, S, Se, Te), and –halogen (F, Cl, Br, I) contacts are compiled and investigated through semiempirical quantum mechanical methods, density functional approximations, and wave function theory. A local energy decomposition (LED) analysis is applied to study the nature of the noncovalent interaction. The special role of electrostatic and London dispersion interactions is discussed in detail. London dispersion is identified as a dominant factor of the azide–donor interaction with mean London dispersion energy‐interaction energy ratios of 1.3. Electrostatic contributions enhance the azide–donor coordination motif. The association energies range from −1.00 to −5.5 kcal mol⁻¹.

Interplay between non-covalent interactions in 1D supramolecular polymers based on 1,4-bis(iodoethynyl)benzene

A study of the solution-phase, solid-state structures of halogen-bonded co-crystals of 1,4-bis(iodoethynyl)benzene (p-BIB) with three salts, namely, decyltrimethylammonium bromide (DTMABr), tetrapropylammonium bromide (TPABr), and tetrabutylammonium bromide (TBABr), has been carried out, along with theoretical calculations. Isothermal titration calorimetry (ITC) showed that the binding constant of bromide with p-BIB in THF is not strongly dependent on the cation, and that the entropic term clearly dominates the enthalpic one in the free energy of binding. In the three crystal structures, the bromide anion acts as a doubly connected node for halogen bonding interactions, which results in linear or angular open chains. The intrachain angles (IBr^-I) of the 1D supramolecular polymers based on p-BIB depend on the geometry and size of the cation and vary from 180° for DMTABr to 75° for TBABr. Non-covalent interaction (NCI) analysis of selected motifs and optimized crystals demonstrates that the balance between halogen bonds, hydrogen bonds, and van der Waals interactions, especially type-I halogenhalogen contacts, determines the crystal structures.

Theoretical study on conformational energies of transition metal complexes

Conformational energies are an important chemical property for which a performance assessment of theoretical methods is mandatory. Existing benchmark sets are often limited to biochemical or main group element containing molecules, while organometallic systems are generally less studied. A key problem herein is to routinely generate conformers for these molecules due to their complexity and manifold of possible coordination patterns. In this study we used our recently published CREST protocol [Pracht et al., Phys. Chem. Chem. Phys., 2020, 22, 7169-7192] to generate conformer ensembles for a variety of 40 challenging transition metal containing molecules, which were then used to form a comprehensive conformational energy benchmark set termed TMCONF40. Several low-cost semiempirical, density functional theory (DFT) and force-field methods were compared to high level DLPNO-CCSD(T1) and double-hybrid DFT reference values. Close attention was paid to the energetic ordering of the conformers in the statistical evaluation. With respect to the double-hybrid references, both tested low-cost composite DFT methods produce high Pearson correlation coefficients of rp,mean,B97-3c//B97-3c = 0.922 and rp,mean,PBEh-3c//B97-3c = 0.890, with mean absolute deviations close to or below 1 kcal mol-1. This good performance also holds for a comparison to DLPNO-CCSD(T1) reference energies for a smaller subset termed TMCONF5. Based on DFT geometries, the GFNn-xTB methods yield reasonable Pearson correlation coefficients of rp,mean,GFN1-xTB//B97-3c = 0.617 (MADmean = 2.15 kcal mol-1) and rp,mean,GFN2-xTB//B97-3c = 0.567 (MADmean = 2.68 kcal mol-1), outperforming the widely used PMx methods on the TMCONF40 test set. Employing the low-cost composite DFT method B97-3c on GFN2-xTB geometries yields an slightly improved correlation of rp,mean,B97-3c//GFN2-xTB = 0.632. Furthermore, for 68% of the investigated complexes at least one low-energy conformer was found that is more stable than the respective crystal structure conformation, which signals the importance of conformational studies. General recommendations for the application of the CREST protocol and DFT methods for transition metal conformational energies are given.

Strong metal-metal Pauli repulsion leads to repulsive metallophilicity in closed-shell d8 and d10 organometallic complexes

Metallophilicity is defined as the interaction among closed-shell metal centers, the origin of which remains controversial, particularly for the roles of spd orbital hybridization (mixing of the spd atomic orbitals of the metal atom in the molecular orbitals of metal complex) and the relativistic effect. Our studies reveal that at close M-M' distances in the X-ray crystal structures of d8 and d10 organometallic complexes, M-M' closed-shell interactions are repulsive in nature due to strong M-M' Pauli repulsion. The relativistic effect facilitates (n + 1)s-nd and (n + 1)p-nd orbital hybridization of the metal atom, where (n + 1)s-nd hybridization induces strong M-M' Pauli repulsion and repulsive M-M' orbital interaction, and (n + 1)p-nd hybridization suppresses M-M' Pauli repulsion. This model is validated by both DFT (density functional theory) and high-level coupled-cluster singles and doubles with perturbative triples computations and is used to account for the fact that the intermolecular or intramolecular Ag-Ag' distance is shorter than the Au-Au' distance, where a weaker Ag-Ag' Pauli repulsion plays an important role. The experimental studies verify the importance of ligands in intermolecular interactions. Although the M-M' interaction is repulsive in nature, the linear coordination geometry of the d10 metal complex suppresses the L-L' (ligand-ligand) Pauli repulsion while retaining the strength of the attractive L-L' dispersion, leading to a close unsupported M-M' distance that is shorter than the sum of the van der Waals radius (rvdw) of the metal atoms.

Comprehensive Benchmark Study on the Calculation of 29Si NMR Chemical Shifts

A comprehensive and diverse benchmark set for the calculation of ²⁹Si NMR chemical shifts is presented. The SiS146 set includes 100 silicon containing compounds with 146 experimentally determined reference ²⁹Si NMR chemical shifts measured in nine different solvents in a range from -400 to +828 ppm. Silicon atoms bound to main group elements as well as transition metals with coordination numbers of 2-6 in various bonding patterns including multiple bonds and coordinative and aromatic bonding are represented. The performance of various common and specialized density functional approximations including (meta-)GGA, hybrid, and double-hybrid functionals in combination with different AO basis sets and for differently optimized geometries is evaluated. The role of scalar-relativistic effects is further investigated by inclusion of the zeroth order regular approximation (ZORA) method into the calculations. GGA density functional approximations (DFAs) are found to outperform hybrid DFAs with B97-D3 performing best with an MAD of 7.2 ppm for the subset including only light atoms (Z < 18), while TPSSh is the best tested hybrid functional with an MAD of 10.3 ppm. For ²⁹Si cores in the vicinity of heavier atoms, the application of ZORA proved indispensable. Inclusion of spin-orbit effects into the ²⁹Si NMR chemical shift calculation decreases the mean absolute deviations by up to 74% compared to calculations applying effective core potentials.

Allenes for Versatile Iron-Catalyzed C-H Activation by Weak O-Coordination: Mechanistic Insights by Kinetics, Intermediate Isolation, and Computation

The iron-catalyzed hydroarylation of allenes was accomplished by weak phenone assistance. The C-H activation proceeded with excellent efficacy and high ortho-regioselectivity in proximity to the weakly coordinating carbonyl group for a range of substituted phenones and allenes. Detailed mechanistic studies, including the isolation of key intermediates, the structural characterization of an iron-metallacycle, and kinetic analysis, allowed the sound elucidation of a plausible catalytic working mode. This mechanistic rationale is supported by detailed computational density functional theory studies, which fully address multi-spin-state reactivity. Furthermore, in operando nuclear magnetic resonance monitoring of the catalytic reaction provided detailed insights into the mode of action of the iron-catalyzed C-H alkylation with allenes.

Which Stereoinductor Is Better for Asymmetric Functionalization of α-Amino Acids in a Nickel(II) Coordination Environment? Experimental and DFT Considerations

The results of extended comparative investigation of nickel(II) Schiff base complexes (containing various auxiliary chiral moieties) commonly used as a methodological platform for the asymmetric synthesis of tailor-made α-amino acids are provided. The following issues are addressed: 1) redox activity (determining the possibility for electrochemically induced reactions); 2) quantitative estimation of the reactivity of deprotonated complexes towards electrophiles; and 3) quantum-chemical estimation of noncovalent interactions in the metal coordination environment (which shed light on the origin of the stereochemical outcome observed for different stereoinductors). Possible mechanisms that determine the relationship between the stereochemical configuration of a molecule and its electronic structure are discussed. The DFT-calculated HOMO-LUMO energies and localization, as well as relative energies for the (S)- and (R)-alanine derivatives, that determine the stereoinduction efficiency in thermodynamically controlled reactions in nickel(II) coordination are provided. The computational data are supported by experimental results on the monobenzylation of glycine derivatives.

Quantum Chemical Calculation of Molecular and Periodic Peptide and Protein Structures

Special-purpose classical force fields (FFs) provide good accuracy at very low computational cost, but their application is limited to systems for which potential energy functions are available. This excludes most metal-containing proteins or those containing cofactors. In contrast, the GFN2-xTB semiempirical quantum chemical method is parametrized for almost the entire periodic table. The accuracy of GFN2-xTB is assessed for protein structures with respect to experimental X-ray data. Furthermore, the results are compared with those of two special-purpose FFs, HF-3c, PM6-D3H4X, and PM7. The test sets include proteins without any prosthetic groups as well as metalloproteins. Crystal packing effects are examined for a set of smaller proteins to validate the molecular approach. For the proteins without prosthetic groups, the special purpose FF OPLS-2005 yields the smallest overall RMSD to the X-ray data but GFN2-xTB provides similarly good structures with even better bond-length distributions. For the metalloproteins with up to 5000 atoms, a good overall structural agreement is obtained with GFN2-xTB. The full geometry optimizations of protein structures with on average 1000 atoms in wall-times below 1 day establishes the GFN2-xTB method as a versatile tool for the computational treatment of various biomolecules with a good accuracy/computational cost ratio.

DFTB+, a software package for efficient approximate density functional theory based atomistic simulations

DFTB+ is a versatile community developed open source software package offering fast and efficient methods for carrying out atomistic quantum mechanical simulations. By implementing various methods approximating density functional theory (DFT), such as the density functional based tight binding (DFTB) and the extended tight binding method, it enables simulations of large systems and long timescales with reasonable accuracy while being considerably faster for typical simulations than the respective ab initio methods. Based on the DFTB framework, it additionally offers approximated versions of various DFT extensions including hybrid functionals, time dependent formalism for treating excited systems, electron transport using non-equilibrium Green's functions, and many more. DFTB+ can be used as a user-friendly standalone application in addition to being embedded into other software packages as a library or acting as a calculation-server accessed by socket communication. We give an overview of the recently developed capabilities of the DFTB+ code, demonstrating with a few use case examples, discuss the strengths and weaknesses of the various features, and also discuss on-going developments and possible future perspectives.

Enantiospecific Synthesis of Nepetalactones by One-Step Oxidative NHC Catalysis

An efficient oxidative NHC-catalyzed one-step transformation of (S)- or (R)-8-oxocitronellal to nepetalactone (NL) in enantio- and diastereomerically pure form has been developed. Several new and "easy to make" N-Mes- or N-Dipp-substituted 1,2,4-triazolium salts carrying nitroaromatic groups on N1 were synthesized and evaluated as precatalysts in combination with base and stoichiometric organic oxidant. Under optimized conditions, NLs are accessible in very good yields and diastereomerically pure under mild conditions. The oxidant used could be recovered and recycled under operationally simple conditions.

Extension and evaluation of the D4 London-dispersion model for periodic systems

We present an extension of the DFT-D4 model [J. Chem. Phys., 2019, 150, 154122] for periodic systems. The main new ingredients are additional reference polarizabilities for highly-coordinated group 1-5 elements derived from pseudo-periodic electrostatically-embedded cluster calculations. To illustrate the performance of the updated method, several test cases are considered, for which we compare D4 to its predecessor D3(BJ), as well as to a comprehensive set of other dispersion-corrected methods. The largest improvements are observed for solid-state polarizabilities of 16 inorganic salts, where the D4 model achieves an unprecedented accuracy, surpassing its predecessor as well as other, computationally much more demanding approaches. For cell volumes and lattice energies of two sets of chemically diverse molecular crystals, the accuracy gain is less pronounced compared to the already excellently performing D3(BJ) method. For the challenging adsorption energies of small organic molecules on metallic as well as on ionic surfaces, DFT-D4 provides values in good agreement with experimental and/or high-level references. These results suggest the application of the proposed D4 model as a physically improved yet computationally efficient dispersion correction for standard DFT calculations as well as low-cost approaches like semi-empirical or even force-field models.

A theoretical review for novel Lewis base amine/imine-catalyzed reactions

Recent advances in computational investigations of Lewis base amine/imine-catalyzed reactions have been systematically summarized and reviewed for the first time.

Synthesis and Photodimerization of 2- and 2,3-Disubstituted Anthracenes: Influence of Steric Interactions and London Dispersion on Diastereoselectivity

There is increased evidence that the effect of bulky groups in organic, organometallic, and inorganic chemistry is not only repulsive but can be attractive because of London dispersion interactions. The influence of the size of primary alkyl substituents in 2- and 2,3-positions of anthracenes on the diastereoselectivity (anti vs syn dimer) of the [π_4s + π_4s] photoinduced dimerization is investigated. The synthesis of the anthracene derivatives was achieved by Suzuki-Miyaura reaction of 2,3-dibromoanthracene with alkylboronic acids as well as by reduction of anthraquinones that were obtained from 2,3-disubstituted 1,3-butadienes and naphthoquinone followed by dehydrogenation. The mixtures of dianthracene isomers were analyzed with respect to the anti/syn-ratio of the products by X-ray crystallography and nuclear Overhauser effect spectroscopy. While for the 2,3-dimethylanthracene the anti and syn isomers were formed in equal amounts, the anti dimers are the major products in all other cases. A linear correlation (R² = 0.98) between the steric size (Charton parameter) and the isomeric ratio suggests that the selectivity is dominated by classical repulsive steric effects. An exception is the iso-butyl substituent that produces an increased amount of the syn isomer. It is suggested that this is due to an exalted effect of London dispersion interactions.

Structure Optimisation of Large Transition-Metal Complexes with Extended Tight-Binding Methods

Large transition-metal complexes are used in numerous areas of chemistry. Computer-aided theoretical investigations of such complexes are limited by the sheer size of real systems often consisting of hundreds to thousands of atoms. Accordingly, the development and thorough evaluation of fast semi-empirical quantum chemistry methods that are universally applicable to a large part of the periodic table is indispensable. Herein, we report on the capability of the recently developed GFNn-xTB method family for full quantum-mechanical geometry optimisation of medium to very large transition-metal complexes and organometallic supramolecular structures. The results for a specially compiled benchmark set of 145 diverse closed-shell transition-metal complex structures for all metals up to Hg are presented. Further the GFNn-xTB methods are tested on three established benchmark sets regarding reaction energies and barrier heights of organometallic reactions.

Assessment of DFT Methods for Transition Metals with the TMC151 Compilation of Data Sets and Comparison with Accuracies for Main-Group Chemistry

In the present study, we have gathered a collection (that we term TMC151) of accurate reference data for transition-metal reactions for the assessment of quantum chemistry methods. It comprises diatomic dissociation energies and reaction energies and barriers for prototypical transition-metal reactions. Our assessment of a diverse range of different types of DFT methods shows that the most accurate functionals include ωB97M-V, ωB97X-V, MN15, and B97M-rV. Notably, they have also been previously validated to be highly robust for main-group chemistry. Nevertheless, even these methods show substantially worse accuracies for transition metals than for main-group chemistry. For less accurate methods, there is not a good correlation between their accuracies for main-group and transition-metal chemistries. Thus, in the development of new DFT, it is important to assess the accuracies for both types of data. In this regard, we have formulated the TMC34 model for efficient assessment of the performance for transition metals, which complements our previously developed MG8 model for main-group chemistry. Together, they provide a cost-effective means for initial assessment of new methodologies.