Effects of Ligand, Metal, and Solvation on the Structure and Stability of Contact Ion Pairs Relevant to Olefin Polymerization Catalyzed by Rare-Earth-Metal Complexes: A DFT Study

A full account of theoretical analyses at the DFT level has been reported, focusing on the formation and reactivity of a family of cationic [R-(CH₂)_<i>n</i>-Py-Sc­(CH₂SiMe₃)]⁺ catalysts and the effects of counterion and solvation. Two sets of model systems have been considered: (a) structures having identical bridging unit (<i>n</i> = 1) but having varying cyclopentadienyl groups (R = Cp′ (<b>1</b>), R = Ind (<b>2</b>), and R = Flu (<b>3</b>)) and (b) systems with the identical cyclopentadienyl moiety (Flu) but with varying bridging groups (<i>n</i> = 1 (<b>3</b>), <i>n</i> = 0 (<b>4</b>), and <i>n</i> = 2 (<b>5</b>)). For complex <b>3</b>, various metal ions (Sc, Y, La, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu) were considered to investigate the effect of central metals on the contact ion pairs (CIP). The formation and separation of CIP were found to be influenced by the steric hindrance of the ligand, the electron-donating ability of the cyclopentadienyl group, and the rare-earth-metal ion radius. The separation enthalpy of the CIPs decreases with increasing dielectronic constant of the solvent. The solvation hardly affects the energy barrier for styrene insertion into the Sc–C₁₇H₁₉ bond of the CIP [(Flu-CH₂-Py)­Sc-(C₁₇H₁₉)]­[B­(C₆F₅)₄]. A bulkier and more electron donating ancillary ligand, a smaller ion radius of the rare-earth metal, and a greater polarity of the solvent are more beneficial to the separation of CIP and thus to the monomer coordination, which could contribute to the improvement of polymerization activity

Mechanistic Insights into the Copper-Cocatalyzed Sonogashira Cross-Coupling Reaction: Key Role of an Anion

The Sonogashira cross-coupling reaction is one of the most important and widely used sp²–sp carbon–carbon bond formation reactions in organic synthesis. Up to now, the exact mechanism of the palladium/copper-catalyzed Sonogashira reaction is far from being fully understood, mainly due to the difficulties in clarifying the combination behavior of the two metal catalysts. In this study, DFT calculations have been performed to elucidate the mechanism of the copper-cocatalyzed Sonogashira cross-coupling reaction, where bis­(triphenylphosphino)palladium was used as a catalyst and Cs₂CO₃ was applied as a base. In an agreement between theory and experiment, the Cu cycle could favorably generate an I^–-coordinated copper acetylide as the catalytically active species rather than the generally considered neutral copper acetylide. In addition, the transmetalation is calculated to be the rate-determining step. The results reported herein are expected to have broad mechanistic implications for other bimetal-catalyzed reactions employing metal salts as additives

Effects of Ligand, Metal, and Solvation on the Structure and Stability of Contact Ion Pairs Relevant to Olefin Polymerization Catalyzed by Rare-Earth-Metal Complexes: A DFT Study

A full account of theoretical analyses at the DFT level has been reported, focusing on the formation and reactivity of a family of cationic [R-(CH₂)_<i>n</i>-Py-Sc­(CH₂SiMe₃)]⁺ catalysts and the effects of counterion and solvation. Two sets of model systems have been considered: (a) structures having identical bridging unit (<i>n</i> = 1) but having varying cyclopentadienyl groups (R = Cp′ (<b>1</b>), R = Ind (<b>2</b>), and R = Flu (<b>3</b>)) and (b) systems with the identical cyclopentadienyl moiety (Flu) but with varying bridging groups (<i>n</i> = 1 (<b>3</b>), <i>n</i> = 0 (<b>4</b>), and <i>n</i> = 2 (<b>5</b>)). For complex <b>3</b>, various metal ions (Sc, Y, La, Pr, Nd, Gd, Tb, Dy, Ho, Er, Tm, and Lu) were considered to investigate the effect of central metals on the contact ion pairs (CIP). The formation and separation of CIP were found to be influenced by the steric hindrance of the ligand, the electron-donating ability of the cyclopentadienyl group, and the rare-earth-metal ion radius. The separation enthalpy of the CIPs decreases with increasing dielectronic constant of the solvent. The solvation hardly affects the energy barrier for styrene insertion into the Sc–C₁₇H₁₉ bond of the CIP [(Flu-CH₂-Py)­Sc-(C₁₇H₁₉)]­[B­(C₆F₅)₄]. A bulkier and more electron donating ancillary ligand, a smaller ion radius of the rare-earth metal, and a greater polarity of the solvent are more beneficial to the separation of CIP and thus to the monomer coordination, which could contribute to the improvement of polymerization activity

Theoretical Investigations of Isoprene Polymerization Catalyzed by Cationic Half-Sandwich Scandium Complexes Bearing a Coordinative Side Arm

Density functional theory studies have been conducted for isoprene polymerization catalyzed by the cationic half-sandwich scandium alkyl species containing a methoxy side arm [(C₅Me₄C₆H₄OMe-<i>o</i>)­Sc­(CH₂SiMe₃)]⁺ (<b>1</b>) and that containing a phosphine oxide side arm [{C₅Me₄SiMe₂CH₂P­(O)­Ph₂}­Sc­(CH₂SiMe₃)]⁺ (<b>2</b>). It has been found that <i>trans</i>-1,4-polymerization of isoprene by species <b>1</b> prefers an insertion–isomerization mechanism: (i) an insertion of <i>cis</i>-isoprene into the metal–alkyl bond to give η³-π-<i>anti</i>-form, (ii) <i>anti</i>-<i>syn</i> isomerization of the resulting 1,2-disubstituted allyl complex to yield a <i>syn</i>-allyl form, (iii) repetitive insertion of <i>cis</i>-isoprene into the metal–<i>syn</i>-allyl bond and subsequent <i>anti</i>–<i>syn</i> isomerization. The resulting η³-π-<i>syn</i>-allyl species is suitable for more kinetically favorable <i>cis</i>-monomer insertion. The stability of the key transition state involved in the most feasible pathway could be ascribed to the smaller deformation of <i>cis</i>-isoprene and stronger interaction between the <i>cis</i>-isoprene moiety and the remaining metal complex. The origin of experimentally observed inertness of <b>2</b> toward isoprene polymerization is that the steric hindrance derived from the crowding of η³-π-<i>syn</i>-allyl species hampers the insertion of the incoming isoprene monomer. The modeling of <b>2</b>-mediated chain propagation also has a high energy barrier and is endergonic. To corroborate the steric effect on the kinetic and thermodynamic aspects, various analogue complexes with smaller hindrance have been computationally modeled on the basis of <b>2</b>. Expectedly, lower energy barrier and favorable thermodynamics are found for the monomer insertion mediated by these complexes with less steric hindrance around the metal center

DFT Studies on Styrene Polymerization Catalyzed by Cationic Rare-Earth-Metal Complexes: Origin of Ligand-Dependent Activities

The mechanism of styrene polymerization catalyzed by five analogous cationic rare-earth-metal complexes [(RCH₂–Py)­Y­(CH₂SiMe₃)]⁺ (R = C₅Me₄ (Cp′), <b>1</b>^<b>+</b>; R = C₉H₆ (Ind), <b>2</b>^<b>+</b>; R = C₁₃H₈ (Flu), <b>3</b>^<b>+</b>), [(Flu–Py)­Y­(CH₂SiMe₃)]⁺ (<b>4</b>^<b>+</b>), and [(Flu–CH₂CH₂–NHC)­Y­(CH₂SiMe₃)]⁺ (<b>5</b>^<b>+</b>) has been studied through DFT calculations. Having achieved an agreement between theory and experiment in the activity discrepancy and selectivity, it is found that styrene polymerization kinetically prefers 2,1-insertion to 1,2-insertion. The free energy profiles for the insertion of a second monomer molecule have been computed for both migratory and stationary insertion manners, and the former resulting in a syndiotactic enchainment indicates obvious kinetic preference. The current results suggest that the coordination of styrene to the active metal center could play an important role in the observed activity difference. Interestingly, the charge on central metal of the cationic species accounts for the activities of <b>1</b>^<b>+</b>, <b>2</b>^<b>+</b>, and <b>3</b>^<b>+</b>: the higher the charge on the central metal, the higher the activity. The coordination of a THF molecule to the central metal and more difficult generation of the active species could be responsible for the low activity of <b>4</b>^<b>+</b>. For species <b>5</b>^<b>+</b>, the resulting product of the first styrene insertion is quite stable, and the ancillary ligand and styryl group hamper the insertion of the incoming styrene molecule. This could be responsible for the absolute inertness of <b>5</b>^<b>+</b> toward styrene polymerization. The calculated results also suggest that a longer alkyl chain of the side arm of the ancillary ligand could deter monomer coordination and thus decrease the polymerization activity

Computational Toxicological Investigation on the Mechanism and Pathways of Xenobiotics Metabolized by Cytochrome P450: A Case of BDE-47

Understanding the transformation mechanism and products of xenobiotics catalyzed by cytochrome P450 enzymes (CYPs) is vital to risk assessment. By density functional theory computation with the B3LYP functional, we simulated the reaction of 2,2′,4,4′-tetrabromodiphenyl ether (BDE-47) catalyzed by the active species of CYPs (Compound I). The enzymatic and aqueous environments were simulated by the polarizable continuum model. The results reveal that the addition of Compound I to BDE-47 is the rate-determining step. The addition of Compound I to the ipso and nonsubstituted C atoms forms tetrahedral σ-adducts that further transform into epoxides. Hydroxylation of the epoxides leads to hydroxylated polybrominated diphenyl ethers and 2,4-dibromophenol. The addition to the Br-substituted C2 and C4 atoms has a higher barrier than addition to the nonsubstituted C atoms, forming phenoxide and cyclohexadienone which subsequently undergo debromination/hydroxylation. A novel mechanism was identified in which the approach of Compound I to C2 led to formation of a phenoxide and an expelled Br^– ion. The predicted products were consistent with the metabolites identified by others. As a first attempt to simulate the enzymatic transformation of a polycyclic compound, this study may enlighten a computational method to predict the biotransformation of xenobiotics catalyzed by CYPs

Alkyl Effects on the Chain Initiation Efficiency of Olefin Polymerization by Cationic Half-Sandwich Scandium Catalysts: A DFT Study

The effect of alkyls on the chain initiation efficiency of ethylene, propene, 1-hexene, styrene, butadiene, and isoprene polymerizations catalyzed by the half-sandwich cationic rare-earth-metal alkyl complexes [(η⁵-C₅Me₅)­ScR]⁺ (R = CH₂SiMe₃, <b>1</b>; R = <i>o</i>-CH₂C₆H₄NMe₂, <b>2</b>; R = η³-C₃H₅, <b>3</b>) has been studied by using a DFT approach. It has been found that <b>2</b> with the largest sterically demanding aminobenzyl group results in the lowest initiation efficiency and thus longest induction period among the three catalysts investigated. In contrast, <b>1</b> with CH₂SiMe₃ displays the best chain initiation ability, and <b>3</b> with η³-allyl gives moderate chain initiation activity, mainly due to the most stable resulting coordination complex. Species <b>1</b> and <b>3</b> have better regioselectivity in the chain initiation of styrene polymerization than species <b>2</b>. In addition, species <b>1</b>′ ([(η⁵-C₅Me₅)­Sc­(CH₂SiMe₃)­THF]⁺) with a THF ligand has better chain initiation efficiency in styrene and isoprene polymerizations than species <b>2</b> but is reasonably worse than the analogue <b>1</b> without a THF ligand

Nature of the Entire Range of Rare Earth Metal-Based Cationic Catalysts for Highly Active and Syndioselective Styrene Polymerization

Because of the steric bulkiness and the η⁵/κ¹-constrained-geometry-configuration (CGC) geometry, the entire range of pyridyl-methylene-fluorenyl-stabilized rare earth metal bisalkyl complexes, (Flu-CH₂-Py)­Ln­(CH₂SiMe₃)₂(THF)<i>_x</i> (Flu = fluorenyl; Py = pyridyl; for <b>1</b>, Ln = Sc and <i>x</i> = 0; for <b>2–11</b>, Ln = Lu, Tm, Er, Ho, Y, Dy, Tb, Gd, Nd, or Pr and <i>x</i> = 1), and monoalkyl complex, (Flu-CH₂-Py)₂La­(CH₂SiMe₃) (THF) (<b>12</b>), has been successfully achieved for the first time via the sequential salt metathesis reactions. Activated by [Ph₃C]­[B­(C₆F₅)₄] and Al^<i>i</i>Bu₃, complexes <b>1–9</b> showed high activity and perfect syndioselectivity for styrene polymerization, while the large Nd- and Pr-attached precursors <b>10</b> and <b>11</b> exhibited slightly decreased syndioselectivity but rather low activity; the monoalkyl La precursor <b>12</b> was completely inert. The activity increased with the decrease in the rare earth metal size, in striking contrast to the literature that has shown that a large metal facilitates a high activity, which was also not a result of an enthalpic effect (Δ<i>H</i>^⧧) or an entropic effect (Δ<i>S</i>^⧧) according to Eyring plots. The types of organoborates and the aluminum alkyls, the electron donors, and the polarity of the reaction medium, which affected the coordination of styrene to the active species, aroused significantly different catalytic activity, indicating that styrene coordination played the key role in the polymerization process. On the basis of this, the density functional theory calculation of the active species in the model of [(Flu-CH₂-Py)­Ln-<i>n</i>C₁₇H₁₉]⁺ revealed whenever the orbitals of the pyridyl-methylene fluorenyl ligand overlapped with those of the rare earth metals, the LUMO energy of the active species was lowered and thus the catalytic activity was high. Therefore, the LUMO energy of the active species could be adopted as a potential criterion to estimate the activity of a catalytic system for styrene polymerization. This work reveals for the first time the power of the pyridyl-methylene fluorenyl ligand and the nature of the factors influencing the catalytic performance

Nature of the Entire Range of Rare Earth Metal-Based Cationic Catalysts for Highly Active and Syndioselective Styrene Polymerization

Because of the steric bulkiness and the η⁵/κ¹-constrained-geometry-configuration (CGC) geometry, the entire range of pyridyl-methylene-fluorenyl-stabilized rare earth metal bisalkyl complexes, (Flu-CH₂-Py)­Ln­(CH₂SiMe₃)₂(THF)<i>_x</i> (Flu = fluorenyl; Py = pyridyl; for <b>1</b>, Ln = Sc and <i>x</i> = 0; for <b>2–11</b>, Ln = Lu, Tm, Er, Ho, Y, Dy, Tb, Gd, Nd, or Pr and <i>x</i> = 1), and monoalkyl complex, (Flu-CH₂-Py)₂La­(CH₂SiMe₃) (THF) (<b>12</b>), has been successfully achieved for the first time via the sequential salt metathesis reactions. Activated by [Ph₃C]­[B­(C₆F₅)₄] and Al^<i>i</i>Bu₃, complexes <b>1–9</b> showed high activity and perfect syndioselectivity for styrene polymerization, while the large Nd- and Pr-attached precursors <b>10</b> and <b>11</b> exhibited slightly decreased syndioselectivity but rather low activity; the monoalkyl La precursor <b>12</b> was completely inert. The activity increased with the decrease in the rare earth metal size, in striking contrast to the literature that has shown that a large metal facilitates a high activity, which was also not a result of an enthalpic effect (Δ<i>H</i>^⧧) or an entropic effect (Δ<i>S</i>^⧧) according to Eyring plots. The types of organoborates and the aluminum alkyls, the electron donors, and the polarity of the reaction medium, which affected the coordination of styrene to the active species, aroused significantly different catalytic activity, indicating that styrene coordination played the key role in the polymerization process. On the basis of this, the density functional theory calculation of the active species in the model of [(Flu-CH₂-Py)­Ln-<i>n</i>C₁₇H₁₉]⁺ revealed whenever the orbitals of the pyridyl-methylene fluorenyl ligand overlapped with those of the rare earth metals, the LUMO energy of the active species was lowered and thus the catalytic activity was high. Therefore, the LUMO energy of the active species could be adopted as a potential criterion to estimate the activity of a catalytic system for styrene polymerization. This work reveals for the first time the power of the pyridyl-methylene fluorenyl ligand and the nature of the factors influencing the catalytic performance

Theoretical Mechanistic Studies on the <i>trans</i>-1,4-Specific Polymerization of Isoprene Catalyzed by a Cationic La–Al Binuclear Complex

This paper reports a DFT study on <i>trans</i>-1,4-specific polymerization of isoprene catalyzed by the cationic heterobimetallic half-sandwich complex [(C₅Me₅)­La­(AlMe₄)]⁺. The possible structures of the active species, viz., [(C₅Me₅)­La­(μ₂-Me)₃AlMe]⁺ (<b>A</b>), [(C₅Me₅)­La­(μ₂-Me)₂AlMe₂]⁺ (<b>B</b>), and [(C₅Me₅)­La­(Me)­(μ₂-Me)­AlMe₂]⁺ (<b>C</b>), have been investigated. On the basis of the chain initiation and the structure transformations among these three species, <b>C</b> has been proposed to be the true active species smoothly producing <i>trans</i>-1,4-polyisoprene observed experimentally. Both La/Al bimetal-cooperating monomer insertion and La-center-based insertion pathways have been calculated, and the latter is found to be more favorable, where the AlMe₃ moiety serves as a ligand coordinating to the La center via a methyl group. In contrast to this, in the Y analogous system, the AlMe₃ ligand is proposed to leave away from the Y center during the chain propagation and the <i>cis</i>-1,4-selectivity is preferred, showing a consistence with experimental results. Such a situation could be ascribed to the smaller ionic radius of Y and thermodynamically favorable dissociation of AlMe₃ from Y center in comparison with the La system. These results suggest that such an alkylaluminum compound plays a crucial role in the regulation of selectivity in the polymerization system investigated