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

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

Origin of Product Selectivity in Yttrium-Catalyzed Benzylic C–H Alkylations of Alkylpyridines with Olefins: A DFT Study

DFT studies have been conducted for the direct benzylic C­(sp³)–H alkylation of alkylpyridines with olefins catalyzed by a cationic half-sandwich yttrium alkyl complex. It has been found that, in the case of 2-<i>tert-</i>butyl-6-methylpyridine, the successive insertion of two molecules of ethylene, achieving butylation, was the outcome of kinetics. However, the continuous insertion of the third ethylene for hexylation was unfavorable both kinetically and thermodynamically in comparison with C–H activation to release the butylation product, which is in agreement with experimental results. The energy decomposition analyses disclosed that the steric repulsion between the two ^<i>t</i>Bu groups of pyridyl moieties made the C–H activation of the one-ethylene preinserted intermediate relatively unfavorable. In contrast, in the case of 2,6-lutidine, the resulting monoethylation intermediate via feasible ethylene insertion favorably promotes C–H activation of another molecule of 2,6-lutidine rather than undergoes successive ethylene insertion to give the monobutylation product because of the additional Y···N interaction between the metal and incoming 2,6-lutidine moiety to stabilize the C–H activation transition state. The subsequent ethylene insertion and C–H activation alternatively take place at the remaining α-methyl group and then at the resulting α-CH₂, finally yielding the multiethylation product. Interestingly, the Y-catalyzed C­(sp³)–H alkylation reactivity of alkylpyridines has been found to follow the order C_α–H (1°) > C_α′–H (2°) > C_α″–H (3°) > C_β–H (2°) > C_γ–H (1°). The calculations show a clear correlation between the energy barrier for C–H activation and the Y···N contacts of the corresponding transition state. The shorter the Y···N distance in the transition states, the lower the energy barrier for the C–H activation. Further analyses of charge population indicate that the NBO charge on the Y atom positively correlates well with the reactivity of the C–H bonds

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

Enantioselective α‑Hydroxylation by Modified Salen-Zirconium(IV)-Catalyzed Oxidation of β‑Keto Esters

The highly enantioselective α-hydroxylation of β-keto esters using cumene hydroperoxide (CHP) as the oxidant was realized by a chiral (1<i>S</i>,2<i>S</i>)-cyclo­hexane­diamine backbone salen-zirconium­(IV) complex as the catalyst. A variety of corresponding chiral α-hydroxy β-keto esters were obtained in excellent yields (up to 99%) and enantio­selectivities (up to 98% ee). The zirconium-catalyzed enantioselective α-hydroxylation of β-keto esters was scalable, and the zirconium catalyst was recyclable. The reaction can be performed in gram scale, and corresponding chiral products were acquired in 95% yield and 99% ee

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