Mechanistic Investigations on E–N Bond-Breaking and Ring Expansion for <i>N</i>‑Heterocyclic Carbene Analogues Containing the Group 14 Elements (E)

The potential-energy surfaces for the ring-expansion reactions ^<i>i</i>PrN­(CH)₂N­(^<i>i</i>Pr)­E:(<b>Rea</b>–<b>E</b>) + SiH₂Ph₂ → six-membered ring heterocyclcic product (E = C, Si, Ge, Sn, and Pb) and ^<i>i</i>PrN­(CH)₂N­(^<i>i</i>Pr)­C:(<b>Rea</b>–<b>C</b>) + EH₂Ph₂ → six-membered ring heterocyclcic product are studied at the M06-2X/Def2-TZVP level of theory. These theoretical investigations suggest that for a given SiH₂Ph₂, the relative reactivity of <b>Rea</b>–<b>E</b> toward the ring–ring expansion reaction decreases as the atomic weight of the central atom E increases, that is, in the order <b>Rea</b>–<b>C</b> ≫ <b>Rea</b>–<b>Si</b> > <b>Rea</b>–<b>Ge</b> > <b>Rea</b>–<b>Sn</b> > <b>Rea</b>–<b>Pb</b>. However, for a given <b>Rea</b>–<b>C</b>, these theoretical observations demonstrate that the reactivity of the EH₂Ph₂ molecule that undergoes the ring-expansion reaction decreases in the order SiH₂Ph₂ ≈ GeH₂Ph₂ ≈ SnH₂Ph₂ > PbH₂Ph₂ ≫ CH₂Ph₂. This theoretical study indicates that both the electronic structure and steric effects play a crucial role in determining their reactivities. The model conclusions are consistent with available experimental findings. Furthermore, a valence bond state correlation diagram model can be used to rationalize the computational results. These results allow a number of predictions to be made

Mechanistic Investigations of the Photochemical Isomerizations of [(CO)₅MC(Me)(OMe)] (M = Cr, Mo, and W) Complexes

The mechanisms for the photochemical isomerization reactions are determined theoretically using group 6 Fischer carbene complexes (CO)₅MC­(Me)­(OMe) (M = Cr, Mo, and W) and the complete-active-space self-consistent field (CASSCF) (10-orbital/8-electron active space) and second-order Møller–Plesset perturbation (MP2-CAS) methods with the Def2-SVPD basis set. The structures and energies of the singlet/singlet conical intersections and the triplet/singlet intersystem crossings, which play a decisive role in these photoisomerizations, are determined. The former is applied to the chromium and molybdenum systems because their photoproducts are essentially from the singlet excited states. The latter is applied to the tungsten complex because its photoproducts are formed from a low-lying triplet excited state. Two reaction pathways are examined in this work: photocarbonylation (path I) and CO-photoextrusion (path II). The model studies strongly indicate that in the photochemistry of Cr and Mo Fischer carbene systems, the formation of metallaketene intermediates may occur at higher excitation wavenumbers, whereas the five-coordinated complexes that are attached by a solvent molecule are obtained at lower excitation wavenumbers. However, in the W analogue, because the activation barriers for path I are greater than that for path II and path I has more reaction steps than path II, the quantum yields for the metallaketene intermediate should be smaller than those for the five-coordinated species, which is also attached by a solvent molecule. These theoretical studies also suggest that the conical intersection and the spin crossover mechanisms that are identified in this work explain the process well and support the experimental observations

Excited-State Photolytic Mechanism of Cyclopentene Containing a Group 14 Element: An MP2-CAS//CASSCF Study

The potential energy surfaces corresponding to the photolytic reactions of 1,2-dimethyl-cyclopentene, 3,4-dimethyl-silacyclopent-3-ene, and 3,4-dimethyl-germacyclopent-3-ene were investigated by employing the CAS­(6,6)/6-311G­(d) and MP2-CAS-(6,6)/6-311++G­(3df,3pd)//CAS­(6,6)/6-311G­(d) methods. Also, six kinds of substituted germacyclopent-3-ene were used as model reactants by way of the CASSCF and MP2-CAS methods to study their photolytic mechanisms. The theoretical findings indicate that the photolysis of the above reactants all adopt the same reaction path as follows: reactant → Franck–Condon region → conical intersection → germylene and 1,3-butadiene. However, the theoretical results demonstrate that no photolysis (¹(π →π*)) can be observed in the 1,2-dimethyl-cyclopentene system. Above all, the theoretical investigations strongly suggest that both steric effects, originating from the bulky substituents, and the atomic radius of the group 14 element (C, Si, and Ge) play a crucial role in determining the cis/trans selectivity of the conformation of 1,3-butadiene during their photolytic reactions

Mechanistic Investigations on the Photoisomerization Reactions of Five-Membered Ring Heterocyclic Molecules Containing Sulfur and Selenium Atoms

The restricted active space self-consistent field method in the 26-electron/27-orbital active space and the 6-311­(d) basis set has been used to investigate the mechanisms of the photochemical isomerization reactions concerning the model systems of 1,2,3-thiadiazole and 1,2,3-selenadiazole. The computational works suggest that the preferred reaction paths for both 1,2,3-thiadiazole and 1,2,3-selenadiazole are as follows: reactant → Franck–Condon region → conical intersection → intermediate → transition states → photoproducts. As a result, the structures of the conical intersections, which play a decisive role in these photoisomerization reactions, are obtained. In particular, the present theoretical evidences demonstrate that the potential energy surfaces for the formation of 1,3-diradicals are quite flat. This may explain why their experimental detections are so difficult

Mechanistic Analysis of an Isoxazole–Oxazole Photoisomerization Reaction Using a Conical Intersection

The mechanisms of the three reaction pathways for the photochemical transformation of 3,5-dimethylisoxazole (<b>1</b>) in its first singlet excited state (π→ π*) have been determined using the CASSCF (11-orbital/14-electron active space) and MP2-CAS methods with the 6-311G­(d) basis set. These three reaction pathways are denoted as (i) the internal cyclization-isomerization path (path A), (ii) the ring contraction-ring expansion path (path B), and (iii) the direct path (path C). This work provides the first theoretical examinations of mechanisms for such photochemical rearrangements. The present theoretical findings suggest that the photoisomerization of <b>1</b> via path C should be much more favorable then either path A or path B. Nevertheless, the theoretical observations reveal that path B, which consists of a sequence of small geometric rearrangements, should be energetically feasible as well. Accordingly, the fleeting intermediate, acetyl nitrile ylide (<b>4</b>), which arises from the mechanism of path B, can be detected experimentally

Mechanistic Study of the Photochemical Isomerization Reactions of Silabenzene

The mechanisms for photochemical isomerization reactions were examined theoretically using a model system of a parent silabenzene with CAS­(6,6)/6-311G­(d) and MP2-CAS-(6,6)/6-311++G­(3df,3pd)//CAS­(6,6)/6-311G­(d) methods. Five reaction pathways leading to five types of photoisomers have been investigated. The theoretical computations indicate that conical intersections play a prominent role in the photoisomerization of silabenzenes. The model investigations reveal that the preferred reaction route for silabenzene should result in the corresponding silabenzvalene, rather than the Dewar silabenzene isomer. Moreover, the theoretical computations suggest that all of the photochemical mechanisms of silabenzene should proceed as follows: reactant → Franck–Condon region → conical intersection → photoproduct. In other words, the photochemical mechanism for silabenzene should be a barrierless, single-step process. The computational results agree well with available experimental observations

Theoretical Designs for Fullerene Carbenes, C₆₀–E–C₆₀ and C₇₀–E–C₇₀ (E = Group 14 Elements): A Target for Experimental Studies

A density functional study of singlet and triplet state fullerene carbenes is performed, using the M06-2X/CRENBL ECP, M06-2X/Def2-SV­(P), and M06-2X/LANL2DZ levels of theory. The structures of two model carbenes (C₆₀–E–C₆₀ and C₇₀–E–C₇₀; E = C, Si, Ge, Sn, and Pb) in their closed-shell singlet and open-shell triplet states are obtained. The theoretical computations suggest that the formation of C₆₀–E–C₆₀ should be energetically more feasible than that of C₇₀-E-C₇₀ (E = C, Si, and Ge). In particular, the C₇₀–E–C₇₀ species with tin and lead should be relatively difficult to produce, from the viewpoint of bonding dissociation energy. The theoretical evidence also suggests that the bulky substituents that occupy the inner side of C₆₀–E–C₆₀ and C₇₀–E–C₇₀ should make the triplet ground state easily achievable. These theoretical findings show that both the relativistic effect and steric effect play an essential role in determining the electronic states of fullerene carbenes

Reactivity Analysis of the [2 + 2] Cycloaddition between Group‑6 Group-14 Triple-Bonded Complexes and Acetylene: Insights from Theoretical Studies

Theoretical examinations of reactivity for the formal [2 + 2] cycloaddition of Me–CC–Ph to Group-6(G6)Group-14(G14) triple-bonded organometallic complexes have been carried out using the M06-2X-D3/def2-TZVP level of theory. Our theoretical findings suggest that Me–CC–Ph can undergo adduct formation with all G6Si complexes, resulting in the generation of four-membered ring structures. However, among the WGroup-14 complex reactants, only WSi-based, WGe-based, and WSn-based organometallic molecules are capable of undergoing a [2 + 2] cycloaddition reaction with Me–CC–Ph. Based on energy decomposition analysis, our theoretical investigations demonstrate that the bonding mechanism in such [2 + 2] cycloaddition reactions involves the creation of two dative bonds between singlet fragments (the donor–acceptor model), as opposed to two electron-sharing bonds between triplet fragments. In addition, the examinations based on the activation strain model indicate that the activation barrier of the [2 + 2] cycloaddition reaction is predominantly governed by the geometric deformation energy of the two reactants (G6G14-Rea and Me–CC–Ph). Our research using the M06-2X method shows that the barrier heights of [2 + 2] cycloaddition reactions between Me–CC–Ph and G6Si-Rea are dependent on the geometric changes occurring in both fragments during the transition states, consistent with Hammond’s postulate

Theoretical Study of Reaction Mechanisms of Carbon Dioxide with E–CH₂–Z-Type Frustrated Lewis Pairs (E = C–Pb; Z = N–Bi)

Carbon dioxide (CO2) emission poses several environmental challenges, such as global warming and harm to living creatures. Therefore, developing efficient CO2-fixing methods under mild conditions is particularly urgent and essential. In this study, a metal-free CO2 binding reaction using E (= C, Si, Ge, Sn, and Pb) Lewis acid (E/P-based) and a Z (= N, P, As, Sb, and Bi) Lewis base (Sn/Z-based) frustrated Lewis pairs (FLPs) as model reactants was theoretically investigated using density functional theory calculations. The theoretical results suggested that in both E/P-based and Sn/Z-based FLPs, a five-membered heterocyclic adduct was produced only from CH2-bridged Si/P-Rea and Sn/P-Rea (Rea = reactant) that can bind CO2, both kinetically and thermodynamically. An energy decomposition analysis–natural orbitals for chemical valence analysis revealed that the bonding interactions between E/P-based and Sn/Z-based with CO2 are better described in terms of the highest occupied molecular orbital (HOMO) (Z) → lowest unoccupied molecular orbital (LUMO) (CO2) interaction, which is the FLP-to-CO2 forward bonding. However, the LUMO(E) ← HOMO (CO2) interaction, which is the CO2-to-FLP back-bonding, plays a minor role in such CO2 activation reactions. According to the activation strain model, it was found that the origin of the reaction barrier could be due to the atomic radius of either the E or Z elements. That is, obtaining a better orbital overlap between the E/P-Rea and Sn/Z-Rea FLP-type compounds and CO2 influences the barrier heights through the atomic radius of E and Z, respectively

Insights into the Reactivity of the Ring-Opening Reaction of Tetrahydrofuran by Intramolecular Group-13/P- and Al/Group-15-Based Frustrated Lewis Pairs

A theoretical study concerning key factors affecting activation energies for ring-opening reactions of tetrahydrofuran (THF) by G13/P-based (G13 = B, Al, Ga, In, and Tl) and Al/G15-based (G15 = N, P, As, Sb, and Bi) frustrated Lewis pairs (FLPs) featuring the dimethylxanthene scaffold was performed using density functional theory. Our theoretical findings indicate that only dimethylxanthene backbone Al/P-Rea (Rea = reactant) FLP-type molecules can be energetically favorable to undergo the ring-opening reaction with THF. Our theoretical evidence reveals that the shorter the separating distance between Lewis acidic (LA) and Lewis basic (LB) centers of the dimethylxanthene backbone FLP-type molecules, the greater the orbital overlaps between the FLP and THF and the lower the activation barrier for such a ring-opening reaction. Energy decomposition analysis (EDA) evidence suggests that the bonding interaction for such a ring-opening reaction is predominated by the donor–acceptor interaction (singlet–singlet interaction) compared to the electron-sharing interaction (triplet–triplet interaction). In addition, the natural orbitals for chemical valence (NOCV) evidence demonstrate that the bonding situations of such ring-opening reactions can be best described as FLP-to-THF forward bonding (the lone pair (G15) → the empty σ*(C–O)) and THF-to-FLP back bonding (the empty σ*(G13) ← filled p-π(O)). The EDA-NOCV observations show that the former plays a predominant role and the latter plays a minor role in such bonding conditions. The activation strain model reveals that the deformation energy of THF is the key factor in determining the activation energy of their ring-opening reactions. Comparing the geometrical structures of the transition states with their corresponding reactants, a linear relationship between them can be rationally explained by the Hammond postulate combined with the respective activation barriers calculated in this work