Electron Flow in Reaction Mechanisms-Revealed from First Principles

Theoretical insight of decatungstate photocatalyzed alkylation of N-tosylimine via hydrogen atom transfer and proton-coupled electron transfer

Decatungstate as a photocatalyst can activate various C(sp3)−H bond to successfully construct C(sp3)−C(sp2) bond with N-tosylimines. Herein density functional theory (DFT) calculations reveal the unique radical mechanism triggered by the...

A Free Aluminylene with Diverse σ-Donating and Doubly σ/π-Accepting Ligand Features for Transition Metals*

We report herein the synthesis, characterization, and coordination chemistry of a free N-aluminylene, namely a carbazolylaluminylene 2 b. This species is prepared via a reduction reaction of the corresponding carbazolyl aluminium diiodide. The coordination behavior of 2 b towards transition metal centers (W, Cr) is shown to afford a series of novel aluminylene complexes 3-6 with diverse coordination modes. We demonstrate that the tri-active ambiphilic Al center in 2 b can behave as: 1. a σ-donating and doubly π-accepting ligand; 2. a σ-donating, σ-accepting and π-accepting ligand; and 3. a σ-donating and doubly σ-accepting ligand. Additionally, we show ligand exchange at the aluminylene center providing access to the modulation of electronic properties of transition metals without changing the coordinated atoms. Investigations of 2 b with IDippCuCl (IDipp=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) show an unprecedented aluminylene-alumanyl transformation leading to a rare terminal Cu-alumanyl complex 8. The electronic structures of such complexes and the mechanism of the aluminylene-alumanyl transformation are investigated through density functional theory (DFT) calculations.

Origin and Regioselectivity of Direct Hydrogen Atom Transfer Mechanism of C(sp3)-H Arylation by [W10O32]4-/Ni Metallaphotoredox Catalysis

Polyoxometalates (POMs) have a broad array of applied platforms with well-characterized catalysis including photocatalysis to achieve aliphatic C(sp³)-H bond functionalization. However, the reaction mechanism of POMs in organic transformation remains unknown due to the complexity of POM structures. Here, a challenging [W₁₀O₃₂]^4-/Ni metallaphotoredox-catalyzed C(sp³)-H arylation of alkane has been investigated by density functional theory (DFT) calculations. The calculation revealed that the superficial active center located in bridged oxygen of *[W₁₀O₃₂]^4- is responsible for the abstraction of a foreign hydrogen atom and the activation of a C(sp³)-H bond. Furthermore, we discussed this activated process using the direct activation model of the C(sp³)-H σ-bond to deepen our mechanistic understanding of POM mediated C-H bond activation via the hydrogen atom transfer (HAT) pathway. Specifically, comparing three common mechanisms for nickel catalysis inducing by Ni⁰, Ni^I, and Ni^II to construct a C-C bond, the nickel catalytic cycle induced by the Ni^I active catalyst is profitable in kinetics and thermodynamics. Finally, a radical mechanism merging the ([W₁₀O₃₂]^4--*[W₁₀O₃₂]^4--[HW₁₀O₃₂]^4--[W₁₀O₃₂]^4-) decatungstate reductive quenching cycle, ([HW₁₀O₃₂]^4--[H₂W₁₀O₃₂]^4--[HW₁₀O₃₂]^4-) electron relay, and (Ni^I-Ni^II-Ni^I-Ni^III-Ni^I) nickel catalytic cycle is proposed to be favorable. We hope that this work would provide a better understanding of the unique catalytic activity of decatungstate anions for the direct functionalization of the C(sp³)-H bond.

Rearrangements of the Chrysanthenol Core: Application to a Formal Synthesis of Xishacorene B

Reported here are substrate-dictated rearrangements of chrysanthenol derivatives prepared from verbenone to access complex bicyclic frameworks. These rearrangements set the stage for a 10-step formal synthesis of the natural product xishacorene B. Key steps include an anionic allenol oxy-Cope rearrangement and a Suárez directed C-H functionalization. The success of this work was guided by extensive computational calculations which provided invaluable insight into the reactivity of the chrysanthenol-derived systems, especially in the key oxy-Cope rearrangement.

Carboboration-Driven Generation of a Silylium Ion for Vinylic C-F Bond Functionalization by B(C6 F5 )3 Catalysis

Strong main-group Lewis acids such as silylium ions are known to effectively promote heterolytic C(sp³ )-F bond cleavage. However, carrying out the C(sp² )-F bond transformation of vinylic C-F bonds has remained an unmet challenge. Herein, we describe our development of a new and simple strategy for vinylic C-F bond transformation of α-fluorostyrenes with silyl ketene acetals catalyzed by B(C₆ F₅ )₃ under mild conditions. Our theoretical calculations revealed that a stabilized silylium ion, which is generated from silyl ketene acetals by carboboration, cleaves the C-F bond of α-fluorostyrenes. A comparative study of α-chloro or bromostyrenes demonstrated that our reaction can be applied only to α-fluorostyrenes because the strong silicon-fluorine affinity facilitates an intramolecular interaction of silylium ions with fluorine atom to cleave the C-F bond. A broad range of α-fluorostyrenes as well as a range of silyl ketene acetals underwent this C-F bond transformation.

Second-Order CASSCF Algorithm with the Cholesky Decomposition of the Two-Electron Integrals

In this contribution, we present the implementation of a second-order complete active space–self-consistent field (CASSCF) algorithm in conjunction with the Cholesky decomposition of the two-electron repulsion integrals. The algorithm, called norm-extended optimization, guarantees convergence of the optimization, but it involves the full Hessian and is therefore computationally expensive. Coupling the second-order procedure with the Cholesky decomposition leads to a significant reduction in the computational cost, reduced memory requirements, and an improved parallel performance. As a result, CASSCF calculations of larger molecular systems become possible as a routine task. The performance of the new implementation is illustrated by means of benchmark calculations on molecules of increasing size, with up to about 3000 basis functions and 14 active orbitals.

One-to-One Correspondence between Reaction Pathways and Reactive Orbitals

The one-to-one correspondence between reaction pathways in the potential energy theory and reactive orbitals in the electronic theory for reactions is presented. In this study, the reactive orbital energy method is applied to the intrinsic reaction coordinates of the global reaction route map generated by an automated reaction path search method. The reactive orbital energy method specifies the pairs of occupied and unoccupied reactive orbitals driving chemical reactions and determines whether the reactions are electron transfer-driven or dynamics-driven. Surprisingly, it is found that the reactive orbital pairs are determined one by one for the electron transfer-driven reaction pathways from an identical molecule. The reactive orbital energy method is also found to provide the sophisticated interpretations of reactions for the electronic motions. This one-to-one correspondence is expected to trigger the unification of the potential energy theory and the electronic theory for reactions that have been independently developed.

Thermoneutral N-H Bond Activation of Ammonia by a Geometrically Constrained Phosphine

A geometrically constrained phosphine bearing a tridentate NNS pincer ligand is reported. The effect of the geometric constraint on the electronic structure was probed by theoretical calculations and derivatization reactions. Reactions with N−H bonds result in formation of cooperative addition products. The thermochemistry of these transformations is strongly dependent on the substrate, with ammonia activation being thermoneutral. This represents the first example of a molecular compound that reversibly activates ammonia via N−H bond scission in solution upon mild heating.

Analytical Gradient Theory for Resolvent-Fitted Second-Order Extended Multiconfiguration Perturbation Theory (XMCQDPT2)

We present the formulation and implementation of an analytical gradient algorithm for extended multiconfiguration quasidegenerate perturbation theory (XMCQDPT2) with the resolvent-fitting approximation by Granovsky. This algorithm is powerful when optimizing molecular configurations with a moderate-sized active space and many electronic states. First, we present the powerfulness and accuracy of resolvent-fitting approximations compared to canonical XMCQDPT2 theory. Then, we demonstrate the utility of the current algorithm in frequency analyses, optimizing the minimum energy conical intersection geometries of the retinal chromophore model RPSB6 and evaluating nuclear gradients when there are many electronic states. Furthermore, we parallelize the algorithm using the OpenMP/MPI hybrid approach. Additionally, we report the computational cost and parallel efficiency of the program.

Mechanistic Investigation of a Synthetic Route to Biaryls by the Sigmatropic Rearrangement of Arylsulfonium Species

A comprehensive mechanistic investigation was conducted on the coupling reaction of aryl sulfoxides with phenols by using trifluoroacetic anhydride to yield biaryls. NMR experiments revealed that our previously proposed mechanism, which consists of a cascade of an interrupted Pummerer reaction and a rate-determining [3,3] sigmatropic rearrangement, is reasonable. The electronic effects of the substrates were also evaluated to elucidate the nature of the rearrangement step. Based on experimental observations and theoretical calculations, we conclude that the rearrangement is highly asynchronous and stepwise rather than concerted when electron-rich phenols are employed for the reaction.

Two, Three, or Not to Be? Elucidating Multiple Bonding in d6 Pseudotetrahedral Oxo and Imide Complexes

Late-transition-metal oxo and imide complexes play an important role in the catalytic functionalization and activation of small molecules. An emerging theme in this area over the past few decades has been the use of lower coordination numbers, and pseudotetrahedral geometries in particular, to stabilize what would otherwise be highly reactive species. However, the bonding structure in d⁶ oxo and imide complexes in this geometry is ambiguous. These species are typically depicted with a triple bond; however, recent experimental evidence suggests significant empirical differences between these complexes and other triply bonded complexes with lower d counts. Here we use a suite of computational orbital localization methods and electron density analyses to probe the bonding structure of isoelectronic d⁶ Co^III oxo and imide complexes. These analyses suggest that a triple-bond description is inaccurate because of a dramatically weakened σ interaction. While the exact bond order in these cases is necessarily dependent on the model used, several metrics suggest that the strength of the metal-O/N bond is most similar to that of other formally doubly bonded complexes.

Transformation of various multicenter bondings within bicapped-square antiprismatic motifs: Z-rearrangement

Reported herein are mutual rearrangements in the whole series of seven bicapped-square antiprismatic closo-C₂B₈H₁₀ by means of high-quality computations that disprove the earlier postulated dsd (diamond-square-diamond) scheme for these isomerizations. The experimentally existing closo-1,2-C₂B₈H₁₀ was able to be converted to 1,6-, and 1,10-isomers by pyrolysis, and the dsd (diamond-square-diamond) mechanism was offered as an explanation of these processes. However, these computations disprove the postulated dsd scheme for these isomerizations that take place in the ten-vertex closo series. Experimentally observed thermal rearrangements, both in the parent and substituted closo-1,2-C₂B₈H₁₀, closo-1-CB₉H₁₀^-, and closo-B₁₀H₁₀^2-, indirectly support these refined computations. All these processes are based on the new concept of the so-called Z-mechanism, being consistent with a transition state of a boat shape with an open hexagonal belt that results from the initial breakage of three bonds. Such bond breakings and the consequent bond formations bring to mind the shape of the letter Z. In effect, the pattern of multicenter bonding shifts from reactant through a transition state to product. The molecular rearrangements that are available experimentally favour either the axial or equatorial isomers, and this ratio depends on temperature and the type of cluster and its substitution.

Ti-Catalyzed and -Mediated Oxidative Amination Reactions

Titanium is an attractive metal for catalytic reaction development: it is earth-abundant, inexpensive, and generally nontoxic. However-like most early transition metals-catalytic redox reactions with Ti are difficult because of the stability of the high-valent Ti^IV state. Understanding the fundamental mechanisms behind Ti redox processes is key for making progress toward potential catalytic applications. This Account details recent progress in Ti-catalyzed (and -mediated) oxidative amination reactions that proceed through formally Ti^II/Ti^IV catalytic cycles.This class of reactions is built on our initial discovery of Ti-catalyzed [2 + 2 + 1] pyrrole synthesis from alkynes and azobenzene, where detailed mechanistic studies have revealed important factors that allow for catalytic turnover despite the inherent difficulty of Ti redox. Two important conclusions from mechanistic studies are that (1) low-valent Ti intermediates in catalysis can be stabilized through coordination of π-acceptor substrates or products, where they can act as "redox-noninnocent" ligands through metal-to-ligand π back-donation, and (2) reductive elimination processes with Ti proceed through π-type electrocyclic (or pericyclic) reaction mechanisms rather than direct σ-bond coupling.The key reactive species in Ti-catalyzed oxidative amination reactions are Ti imidos (Ti≡NR), which can be generated from either aryl diazenes (RN═NR) or organic azides (RN₃). These Ti imidos can then undergo [2 + 2] cycloadditions with alkynes, resulting in intermediates that can be coupled to an array of other unsaturated functional groups, including alkynes, alkenes, nitriles, and nitrosos. This basic reactivity pattern has been extended into a broad range of catalytic and stoichiometric oxidative multicomponent coupling reactions of alkynes and other reactive small molecules, leading to multicomponent syntheses of various heterocycles and aminated building blocks.For example, catalytic oxidative coupling of Ti imidos with two different alkynes leads to pyrroles, while stoichiometric oxidative coupling with alkynes and nitriles leads to pyrazoles. These heterocycle syntheses often yield substitution patterns that are complementary to those of classical condensation routes and provide access to new electron-rich, highly substituted heteroaromatic scaffolds. Furthermore, catalytic oxidative alkyne carboamination reactions can be accomplished via reaction of Ti imidos with alkynes and alkenes, yielding α,β-unsaturated imine or cyclopropylimine building blocks. New catalytic and stoichiometric oxidative amination methods such as alkyne α-diimination, isocyanide imination, and ring-opening oxidative amination of strained alkenes are continuously emerging as a result of better mechanistic understanding of Ti redox catalysis.Ultimately, these Ti-catalyzed and -mediated oxidative amination methods demonstrate the importance of examining often-overlooked elements like the early transition metals through the lens of modern catalysis: rather than a lack of utility, these elements frequently have undiscovered potential for new transformations with orthogonal or complementary selectivity to their late transition metal counterparts.

Boraiminolithium: An Iminoborane-Transfer Reagent

BN/CC isosterism can give rise to attractive molecules with unique physical or chemical properties. We report here the synthesis, characterization, and reactivities of the boraiminolithium species 2, a room-temperature-stable crystalline solid accessible through a facile dehydrohalogenation/deprotonation reaction. This species, bearing a polarized B≡N triple bond and an anionic N center, is the first example of a BN analogue to the well-known alkynyllithium molecules (lithium acetylides). It has demonstrated a remarkable ability for iminoborane-transfer reactions, which allows for the isolation of a series of unprecedented N-functionalized iminoboranes as well as novel main-group heterocycles. Stable boraiminolithium reagents may become powerful tools in the exploration of new BN-containing building blocks for synthetic chemistry and materials science.

Comparison of RNC Coupling and CO Coupling Mediated by Cr-Cr Quintuple Bond and B-B Multiple Bonds: Main Group Metallomimetics

A theoretical analysis of reductive coupling of isocyanide and CO mediated by a Cr-Cr quintuple bonded complex and B-B multiple bonded complexes shows how the difference in donor-acceptor capability of isocyanide and CO ligands controls the product distributions. In the case of CO, the Cr-Cr quintuple bonded complex is unable to show C-C coupling due to the high π- back bonding possibility of CO and the reaction follows the singlet potential energy surface throughout, whereas, in the case of isocyanide, less π- back bonding possibility allows the reactions to undergo a spin transition and gives a series of products with different spin multiplicities. Similarly, reactions of B-B multiple bonded complexes with CO and isocyanides are also controlled by donor-acceptor capabilities of ligands, and the C-C coupling takes place by changing the oxidation state of the boron centers from +I to +II, in contrast to the classical main group mediated reactions where stable oxidation states are always preserved. This part of the main group chemistry which is dominated by donor-acceptor bonding interaction is more likely to follow transition metal behavior.

Combining Structural with Functional Model Properties in Iron Synthetic Analogue Complexes for the Active Site in Rabbit Lipoxygenase

Iron complexes that model the structural and functional properties of the active iron site in rabbit lipoxygenase are described. The ligand sphere of the mononuclear pseudo-octahedral cis-(carboxylato)(hydroxo)iron(III) complex, which is completed by a tetraazamacrocyclic ligand, reproduces the first coordination shell of the active site in the enzyme. In addition, two corresponding iron(II) complexes are presented that differ in the coordination of a water molecule. In their structural and electronic properties, both the (hydroxo)iron(III) and the (aqua)iron(II) complex reflect well the only two essential states found in the enzymatic mechanism of peroxidation of polyunsaturated fatty acids. Furthermore, the ferric complex is shown to undergo hydrogen atom abstraction reactions with O-H and C-H bonds of suitable substrates, and the bond dissociation free energy of the coordinated water ligand of the ferrous complex is determined to be 72.4 kcal·mol^-1. Theoretical investigations of the reactivity support a concerted proton-coupled electron transfer mechanism in close analogy to the initial step in the enzymatic mechanism. The propensity of the (hydroxo)iron(III) complex to undergo H atom abstraction reactions is the basis for its catalytic function in the aerobic peroxidation of 2,4,6-tri(tert-butyl)phenol and its role as a radical initiator in the reaction of dihydroanthracene with oxygen.

Controlled Regulation of the Nitrile Activation of a Peroxocobalt(III) Complex with Redox-Inactive Lewis Acidic Metals

Redox-inactive metal ions play vital roles in biological O₂ activation and oxidation reactions of various substrates. Recently, we showed a distinct reactivity of a peroxocobalt(III) complex bearing a tetradentate macrocyclic ligand, [Co^III(TBDAP)(O₂)]⁺ (1) (TBDAP = N,N'-di-tert-butyl-2,11-diaza[3.3](2,6)pyridinophane), toward nitriles that afforded a series of hydroximatocobalt(III) complexes, [Co^III(TBDAP)(R-C(═NO)O)]⁺ (R = Me (3), Et, and Ph). In this study, we report the effects of redox-inactive metal ions on nitrile activation of 1. In the presence of redox-inactive metal ions such as Zn²⁺, La³⁺, Lu³⁺, and Y³⁺, the reaction does not form the hydroximatocobalt(III) complex but instead gives peroxyimidatocobalt(III) complexes, [Co^III(TBDAP)(R-C(═NH)O₂)]²⁺ (R = Me (2) and Ph (2^Ph)). These new intermediates were characterized by various physicochemical methods including X-ray diffraction analysis. The rates of the formation of 2 are found to correlate with the Lewis acidity of the additive metal ions. Moreover, complex 2 was readily converted to 3 by the addition of a base. In the presence of Al³⁺, Sc³⁺, or H⁺, 1 is converted to [Co^III(TBDAP)(O₂H)(MeCN)]²⁺ (4), and further reaction with nitriles did not occur. These results reveal that the reactivity of the peroxocobalt(III) complex 1 in nitrile activation can be regulated by the redox-inactive metal ions and their Lewis acidity. DFT calculations show that the redox-inactive metal ions stabilize the peroxo character of end-on Co-η¹-O₂ intermediate through the charge reorganization from a Co^II-superoxo to a Co^III-peroxo intermediate. A complete mechanistic model explaining the role of the Lewis acid is presented.

Understanding the Nature and Properties of Hydrogen-Hydrogen Bonds: The Stability of a Bulky Phosphatetrahedrane as a Case Study

Recently, the first mixed C/P phosphatetrahedranes (^tBuC)₃P and (^tBuCP)₂ were reported. Unlike (^tBuCP)₂, (^tBuC)₃P exhibits remarkable thermal stability, which can be partially attributed to a network of nine hydrogen-hydrogen bonds (HHBs) localized between the tert-butyl substituents. The stabilizing contribution arising from this network of HHBs was obtained from local energy decomposition (LED) analysis calculated at the domain-based local pair natural orbital CCSD(T) (DLPNO-CCSD(T)) level of theory. These calculations suggest that each HHB contributes approximately -0.7 kcal/mol of stabilization; however, the net stabilization energy likely lies between -0.25 and -0.5 kcal/mol because of steric repulsion. Spatial analysis of the London dispersion energy via a dispersion interaction density (DID) plot reveals that the DID surface is localized at key C-H groups involved in HHBs, consistent with London dispersion interactions predominantly arising from HHBs. In addition, we present a computed mechanism that supports a phosphinidenoid species as a key reaction intermediate in the synthesis of (^tBuC)₃P.

Nuclear-electronic orbital methods: Foundations and prospects

The incorporation of nuclear quantum effects and non-Born-Oppenheimer behavior into quantum chemistry calculations and molecular dynamics simulations is a longstanding challenge. The nuclear-electronic orbital (NEO) approach treats specified nuclei, typically protons, quantum mechanically on the same level as the electrons with wave function and density functional theory methods. This approach inherently includes nuclear delocalization and zero-point energy in molecular energy calculations, geometry optimizations, reaction paths, and dynamics. It can also provide accurate descriptions of excited electronic, vibrational, and vibronic states as well as nuclear tunneling and nonadiabatic dynamics. Nonequilibrium nuclear-electronic dynamics simulations beyond the Born-Oppenheimer approximation can be used to investigate a wide range of excited state processes. This Perspective provides an overview of the foundational NEO methods and enumerates the prospects for using these methods as building blocks for future developments. The conceptual simplicity and computational efficiency of the NEO approach will enhance its accessibility and applicability to diverse chemical and biological systems.

Theoretical Assessment of Dinitrogen Fixation on Carbon Atom

Dinitrogen activation in non-metallic systems has received considerable attention in recent years. Herein, we report the theoretical feasibility of N₂ fixation using aminocarbenes (L) or their anionic derivatives. The molecular descriptors of L and anionic L^- , which affect the interaction of L and anionic L^- with N₂ , were identified through multiple linear regression analysis. Additionally, the electron flow during C-N bond formation was confirmed by performing intrinsic reaction coordination calculations with intrinsic bond orbital analysis for the reaction of anionic L^- with N₂ .

Insights into the nucleophilic substitution of pyridine at an unsaturated carbon center

Bimolecular nucleophilic substitution (S_N2) is a fundamental reaction that has been widely studied. So far, the nucleophiles are mainly anionic species in S_N2 reactions. In this study, we use density functional theory calculations to assess the mechanisms of substitution of carbonyl, imidoyl, and vinyl compounds with a neutral nucleophile, pyridine. Charge decomposition analysis is performed to explore the main components of the transition state's LUMO. For reactions of imidoyl or carbonyl compounds with pyridine or Cl⁻, the LUMOs of the transition states are composed of mixed orbitals originating from the nucleophile and the substrate. Considering the unique mixed nature of the orbitals, the reaction mode is termed S_Nm (m means mix). Moreover, the main components of the transition state's LUMO are pure σ*_C–Cl MO in the reactions of H₂C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> CHCl with pyridine or Cl⁻. Computations were also performed for RYCHX substrates with different X and Y groups (X= Cl⁻, Br⁻, or F⁻; Y = O, N, or C).

The nucleophilic substitution of carbonyl, imidoyl, and vinyl carbon centers with pyridine or halides is investigated in this paper.

Thermochemical Trends in Carbon Chain Molecules HC2kH/HC2k-1H (k = 1-6) Studied by Explicitly Correlated CCSD(T)-F12b Composite Methods

We present a composite procedure based on explicitly correlated CCSD(T)-F12 calculations for accurate energetic predictions for carbon chain molecules HC_nH encompassing both the even (HC_2kH) and odd series (HC_2k-1H), with the shorter members playing a key role in the evolution of cosmic carbon compounds in both circumstellar envelopes and interstellar medium. This approach considers the contributions of core-valence correlation, scalar relativistic effect, spin-orbit coupling, and zero-point vibrational energy in an additive manner. The computed ionization energies demonstrate outstanding agreement (±0.07 eV) up to a chain size of k = 6 and the literature heats of formation for k ≤ 2 are reproduced with "chemical accuracy" of 1 kcal mol^-1. Among the various corrections included, the importance of core-valence correlation effect has been highlighted in the thermochemical calculations for carbon chain growth. The thermochemical trend toward infinite length is also highlighted by extrapolation of ionization energy and triplet-singlet splitting at the CCSD(T) level for k up to 15. The correlation between the end-group effect and the even-odd parity effect observed for HC_nH chains has been established with the aid of intrinsic bond orbital localization.

Revisiting the Electronic Structure of Cobalt Porphyrin Nitrene and Carbene Radicals with NEVPT2-CASSCF Calculations: Doublet versus Quartet Ground States

Cobalt porphyrin complexes are established catalysts for carbene and nitrene radical group-transfer reactions. The key carbene and mono- and bisnitrene radical complexes coordinated to [Co(TPP)] (TPP = tetraphenylporphyrin) have previously been investigated with a variety of experimental techniques and supporting (single-reference) density functional theory (DFT) calculations that indicated doublet (S = 1/2) ground states for all three species. In this contribution, we revisit their electronic structures with multireference N-electron valence state perturbation theory (NEVPT2)-complete-active-space self-consistent-field (CASSCF) calculations to investigate possible multireference contributions to the ground-state wave functions. The carbene ([CoIII(TPP)(•CHCO2Et)]) and mononitrene ([CoIII(TPP)(•NNs)]) radical complexes were confirmed to have uncomplicated doublet ground states, although a higher carbene or nitrene radical character and a lower Co-C/N bond order was found in the NEVPT2-CASSCF calculations. Supported by electron paramagnetic resonance analysis and spin counting, paramagnetic molar susceptibility determination, and NEVPT2-CASSCF calculations, we report that the cobalt porphyrin bisnitrene complex ([CoIII(TPP•)(•NNs)2]) has a quartet (S = 3/2) spin ground state, with a thermally accesible multireference and multideterminant "broken-symmetry" doublet spin excited state. A spin flip on the porphyrin-centered unpaired electron allows for interconversion between the quartet and broken-symmetry doublet spin states, with an approximate 10-fold higher Boltzmann population of the quartet at room temperature.

Near-Exact CASSCF-Level Geometry Optimization with a Large Active Space using Adaptive Sampling Configuration Interaction Self-Consistent Field Corrected with Second-Order Perturbation Theory (ASCI-SCF-PT2)

An accurate description of electron correlation is one of the most challenging problems in quantum chemistry. The exact electron correlation can be obtained by means of full configuration interaction (FCI). A simple strategy for approximating FCI at a reduced computational cost is selected CI (SCI), which diagonalizes the Hamiltonian within only the chosen configuration space. Recovery of the contributions of the remaining configurations is possible with second-order perturbation theory. Here, we apply adaptive sampling configuration interaction (ASCI) combined with molecular orbital optimizations (ASCI-SCF) corrected with second-order perturbation theory (ASCI-SCF-PT2) for geometry optimization by implementing the analytical nuclear gradient algorithm for ASCI-PT2 with the Z-vector (Lagrangian) formalism. We demonstrate that for phenalenyl radicals and anthracene, optimized geometries and the number of unpaired electrons can be obtained at nearly the CASSCF accuracy by incorporating PT2 corrections and extrapolating them. We demonstrate the current algorithm's utility for optimizing the equilibrium geometries and electronic structures of six-ring-fused polycyclic aromatic hydrocarbons and 4-periacene.

A Ranked-Orbital Approach to Select Active Spaces for High-Throughput Multireference Computation

The past decade has seen a great increase in the application of high-throughput computation to a variety of important problems in chemistry. However, one area which has been resistant to the high-throughput approach is multireference wave function methods, in large part due to the technicalities of setting up these calculations and in particular the not always intuitive challenge of active space selection. As we look toward a future of applying high-throughput computation to all areas of chemistry, it is important to prepare these methods for large-scale automation. Here, we propose a ranked-orbital approach to select active spaces with the goal of standardizing multireference methods for high-throughput computation. This method allows for the meaningful comparison of different active space selection schemes and orbital localizations, and we demonstrate the utility of this approach across 1120 multireference calculations for the excitation energies of small molecules. Our results reveal that it is helpful to distinguish the method used to generate orbitals from the method of ranking orbitals in terms of importance for the active space. Additionally, we propose our own orbital ranking scheme that estimates the importance of an orbital for the active space through a pair-interaction framework from orbital energies and features of the Hartree-Fock exchange matrix. We call this new scheme the "approximate pair coefficient" (APC) method and we show that it performs quite well for the test systems presented.

Meisenheimer complexes as hidden intermediates in the aza-SNAr mechanism

In this work we report a computational study about the aza-SNAr mechanism in fluorine- and chlorine-containing azines with the aim to unravel the physical factors that determine the reactivity patterns in these heterocycles towards propylamine. The nature of the reaction intermediate was analyzed in terms of its electronic structure based on a topological analysis framework in some non-stationary points along the reaction coordinate. The mechanistic dichotomy of a concerted or a stepwise pathway is interpreted in terms of the qualitative Diabatic Model of Intermediate Stabilization (DMIS) approach, providing a general mechanistic picture for the SNAr process involving both activated benzenes and nitrogen-containing heterocycles. With the information collected, a unified vision of the Meisenheimer complexes as transition state, hidden intermediate or real intermediate was proposed.

Multi- and single-reference methods for the analysis of multi-state peroxidation of enolates

In spite of being spin-forbidden, some enzymes are capable of catalyzing the incorporation of O₂(Σg-3) to organic substrates without needing any cofactor. It has been established that the process followed by these enzymes starts with the deprotonation of the substrate forming an enolate. In a second stage, the peroxidation of the enolate formation occurs, a process in which the system changes its spin multiplicity from a triplet state to a singlet state. In this article, we study the addition of O₂ to enolates using state-of-the-art multi-reference and single-reference methods. Our results confirm that intersystem crossing is promoted by stabilization of the singlet state along the reaction path. When multi-reference methods are used, large active spaces are required, and in this situation, semistochastic heat-bath configuration interaction emerges as a powerful method to study these multi-configurational systems and is in good agreement with PNO-LCCSD(T) when the system is well-represented by a single-configuration.

One-Pot Oxidative C-H Activation/Aza-Prins-Type Reaction of Tertiary Alkynylamines: A Counter Ion-Induced Iminium Ion-Alkyne Cyclization

Herein, the design and development of a new one-pot and metal-free oxidative C-H activation/aza-Prins type cyclization of alkynylamines is reported. The scope of this method was demonstrated by the preparation of ten new pyrido[2,1-a]isoquinolines in moderate to high yields (38-92%). Furthermore, a mechanistic proposal for the alkyne aza-Prins cyclization is described based on DFT calculations.

An Androsterone-H2 @C60 hybrid: Synthesis, Properties and Molecular Docking Simulations with SARS-Cov-2

We report the synthesis and characterization of a fullerene‐steroid hybrid that contains H₂@C₆₀ and a dehydroepiandrosterone moiety synthesized by a cyclopropanation reaction with 76 % yield. Theoretical calculations at the DFT‐D3(BJ)/PBE 6‐311G(d,p) level predict the most stable conformation and that the saturation of a double bond is the main factor causing the upfield shielding of the signal appearing at −3.13 ppm, which corresponds to the H₂ located inside the fullerene cage. Relevant stereoelectronic parameters were also investigated and reinforce the idea that electronic interactions must be considered to develop studies on chemical‐biological interactions. A molecular docking simulation predicted that the binding energy values for the protease‐hybrid complexes were −9.9 kcal/mol and −13.5 kcal/mol for PL^pro and 3CL^pro respectively, indicating the potential use of the synthesized steroid‐H₂@C₆₀ as anti‐SARS‐Cov‐2 agent.

Planar Iron Hydride Nanoclusters: Combined Spectroscopic and Theoretical Insights into Structures and Building Principles

The controlled assembly of well-defined planar nanoclusters from molecular precursors is synthetically challenging and often plagued by the predominant formation of 3D-structures and nanoparticles. Herein, we report planar iron hydride nanoclusters from reactions of main group element hydrides with iron(II) bis(hexamethyldisilazide). The structures and properties of isolated Fe4 , Fe6 , and Fe7 nanoplatelets and calculated intermediates enable an unprecedented insight into the underlying building principle and growth mechanism of iron clusters, metal monolayers, and nanoparticles.

Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions

Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (7). In total, six new compounds (2–7) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (T_1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin (S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex (6a, S = 2) and a rare square-planar structure with an intermediate-spin state (6b, S = 1). SQUID measurements, ⁵⁷Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a. The complex that contains additional OMe donors (7) results in a six-coordinate (NNO)₂Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (T_1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal–ligand π-covalency.

A series of Fe(II) complexes with formazanate ligands are reported, and ligand substituent effects on structure and spin-crossover properties are examined. These ligand modifications allow isolation of compounds with tetrahedral geometries in both low- and high-spin ground states as well as an intermediate-spin square-planar complex. Steric properties, π-stacking interactions, and additional donor substituents lead to a wide range of spin-crossover temperatures (T_1/2) in this class of compounds.

Statistical analysis of C-H activation by oxo complexes supports diverse thermodynamic control over reactivity

Transition metal oxo species are key intermediates for the activation of strong C-H bonds. As such, there has been interest in understanding which structural or electronic parameters of metal oxo complexes determine their reactivity. Factors such as ground state thermodynamics, spin state, steric environment, oxygen radical character, and asynchronicity have all been cited as key contributors, yet there is no consensus on when each of these parameters is significant or the relative magnitude of their effects. Herein, we present a thorough statistical analysis of parameters that have been proposed to influence transition metal oxo mediated C-H activation. We used density functional theory (DFT) to compute parameters for transition metal oxo complexes and analyzed their ability to explain and predict an extensive data set of experimentally determined reaction barriers. We found that, in general, only thermodynamic parameters play a statistically significant role. Notably, however, there are independent and significant contributions from the oxidation potential and basicity of the oxo complexes which suggest a more complicated thermodynamic picture than what has been shown previously.

Dative versus electron-sharing bonding in the isoelectronic argon compounds ArR+ (R = CH3, NH2, OH, and F)

For the series of isoelectronic ArR⁺ (R = CH₃, NH₂, OH, and F) complexes, the nature of the bonding between Ar and R shifts from an Ar → R⁺ dative σ bond in ArCH₃⁺ and ArNH₂⁺ to an Ar⁺–R electron-sharing σ bond in ArOH⁺ and ArF⁺.

A Room-Temperature Stable Distonic Radical Cation

Distonic radical cations (DRCs) with spatially separated charge and radical sites have, so far, largely been observed by gas-phase mass spectrometry and/or matrix isolation spectroscopy work. Herein, we disclose the isolation of a crystalline dicarbondiphosphide-based β-distonic radical cation salt 3^.+ (BARF) (BARF=[B(3,5-(CF₃ )₂ C₆ H₃ )₄ )]^- ) stable at room temperature and formed by a one-electron-oxidation-induced intramolecular skeletal rearrangement reaction. Such a species has been validated by electron paramagnetic resonance (EPR) spectroscopy, single-crystal X-ray diffraction, UV/Vis spectroscopy and density functional theory (DFT) calculations. Compound 3^.+ (BARF) exhibits a large majority of spin density at a two-coordinate phosphorus atom (0.74 a.u.) and a cationic charge located predominantly at the four-coordinate phosphorus atom (1.53 a.u.), which are separated by one carbon atom. This species represents an isolable entity of a phosphorus radical cation that is the closest to a genuine phosphorus DRC to date.

Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Δ_O) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T_1/2 above room temperature (300–368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an “inverted” ligand field, with an approximate “two-over-three” splitting of the d-orbitals and a high degree of metal–ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C–Ar or N–Ar rings, which is ascribed to the opposing effect on metal–ligand σ- and π-bonding.

Localized Bond Orbital Analysis of the Bonds of O2

A localized bond orbital analysis of the bonding in dioxygen and related species provides a unique fundamental insight into its bonding characteristics. It reveals the coalescence of the molecular orbital and valence bond/Lewis approaches and clearly demonstrates that the often stated inability of valence bond theory to describe the bonding of O₂ is a myth. The analysis indicates that the σ-bond strength of ³O₂ is not weak as previously believed and accounts for much of its enhanced stability compared with hydroperoxyl. We attribute the stability and persistence of ³O₂ to a combination of this attribute and favorable maximization of exchange coupling between the valence electrons.

The Transition Metal Catalyzed [π2s + π2s + σ2s + σ2s] Pericyclic Reaction: Woodward-Hoffmann Rules, Aromaticity, and Electron Flow

We have shown that the fundamental step responsible for enantioinduction in the inner-sphere asymmetric Tsuji allylic alkylation is C-C bond formation through a seven-membered pericyclic transition state. We employ an extensive series of quantum mechanics (QM) calculations to delineate how the electronic structure of the Pd-catalyzed C-C bond forming process controls the reaction. Phase inversion introduced by d orbitals renders the Pd-catalyzed [π2s + π2s + σ2s + σ2s] reaction symmetry-allowed in the ground state, proceeding through a transition state with Craig-Möbius-like σ-aromaticity. Lastly, we connect QM to fundamental valence bonding concepts by deriving an ab initio "arrow-pushing" mechanism that describes the flow of electron density through the reaction.