PNP-Ligated Heterometallic Rare-Earth/Ruthenium Hydride Complexes Bearing Phosphinophenyl and Phosphinomethyl Bridging Ligands

The reaction of rare-earth bis­(alkyl) complexes containing a bis­(phosphinophenyl)­amido pincer (PNP), LnPNP_<i>i</i>Pr(CH₂SiMe₃)₂ (<b>1-Ln</b>, Ln = Y, Ho, Dy), with ruthenium trihydride phosphine complexes, Ru­(C₅Me₅)­H₃PPh₃ and Ru­(C₅Me₅)­H₃PPh₂Me, gave the corresponding bimetallic Ln/Ru complexes bearing two hydride ligands and a bridging phosphinophenyl (μ-C₆H₄PPh₂-κ<i>P</i>:κ<i>C</i>¹, <b>2a-Ln</b>) or a bridging phosphinomethyl ligand (μ-CH₂PPh₂-κ<i>P</i>:κ<i>C</i>, <b>2b-Ln</b>), respectively. Reaction of <b>2a-Y</b> with CO gas at 1 atm and at 20 °C in toluene-<i>d</i>₈ afforded the complex <b>3a-Y</b>, which bears a bridging pseudooxymethylene ligand (μ-OCH­(<i>o</i>-C₆H₄)­PPh₂-κ<i>P</i>:κ<i>O</i>) and a bridging hydride ligand on the Y/Ru centers. Computational studies by the DFT method suggested that <b>3a-Y</b> was formed in two steps: first the coordination of CO (Δ<i>G</i>(B3PW91) = 22.9; Δ<i>G</i>(M06) = 14.9 kcal/mol) and migratory insertion of the Y–C₆H₄ group (Δ<i>G</i>^⧧(B3PW91) = 13.3; Δ<i>G</i>^⧧(M06) = 16.7 kcal/mol), followed by a rapid intramolecular hydride migration to the resulting acyl group. Complex <b>2b-Y</b> reacted with organic nitriles (<i>t</i>BuCN, CH₃CN, PhCN), an aldimine (PhNCHPh), an isonitrile (<i>t</i>BuNC), and group IX transition-metal carbonyls (M­(C₅Me₅)­(CO)₂, M = Rh, Ir) via insertion of the reactive Y–CH₂ group into the unsaturated bond. These reactions afforded complexes with new ligand scaffolds, including a bridging alkylideneamidophosphine (<b>4b-Y</b>), an amidophosphine (<b>7b-Y</b>), an η²-iminoacylphosphine (<b>8b-Y</b>), and oxycarbenephosphine (<b>9b-Y</b> and <b>10b-Y</b>) ligands at the binuclear Y/Ru core. All of these reaction products were structurally characterized by X-ray crystallography, NMR spectroscopy, and elemental analyses

PNP-Ligated Heterometallic Rare-Earth/Ruthenium Hydride Complexes Bearing Phosphinophenyl and Phosphinomethyl Bridging Ligands

The reaction of rare-earth bis­(alkyl) complexes containing a bis­(phosphinophenyl)­amido pincer (PNP), LnPNP_<i>i</i>Pr(CH₂SiMe₃)₂ (<b>1-Ln</b>, Ln = Y, Ho, Dy), with ruthenium trihydride phosphine complexes, Ru­(C₅Me₅)­H₃PPh₃ and Ru­(C₅Me₅)­H₃PPh₂Me, gave the corresponding bimetallic Ln/Ru complexes bearing two hydride ligands and a bridging phosphinophenyl (μ-C₆H₄PPh₂-κ<i>P</i>:κ<i>C</i>¹, <b>2a-Ln</b>) or a bridging phosphinomethyl ligand (μ-CH₂PPh₂-κ<i>P</i>:κ<i>C</i>, <b>2b-Ln</b>), respectively. Reaction of <b>2a-Y</b> with CO gas at 1 atm and at 20 °C in toluene-<i>d</i>₈ afforded the complex <b>3a-Y</b>, which bears a bridging pseudooxymethylene ligand (μ-OCH­(<i>o</i>-C₆H₄)­PPh₂-κ<i>P</i>:κ<i>O</i>) and a bridging hydride ligand on the Y/Ru centers. Computational studies by the DFT method suggested that <b>3a-Y</b> was formed in two steps: first the coordination of CO (Δ<i>G</i>(B3PW91) = 22.9; Δ<i>G</i>(M06) = 14.9 kcal/mol) and migratory insertion of the Y–C₆H₄ group (Δ<i>G</i>^⧧(B3PW91) = 13.3; Δ<i>G</i>^⧧(M06) = 16.7 kcal/mol), followed by a rapid intramolecular hydride migration to the resulting acyl group. Complex <b>2b-Y</b> reacted with organic nitriles (<i>t</i>BuCN, CH₃CN, PhCN), an aldimine (PhNCHPh), an isonitrile (<i>t</i>BuNC), and group IX transition-metal carbonyls (M­(C₅Me₅)­(CO)₂, M = Rh, Ir) via insertion of the reactive Y–CH₂ group into the unsaturated bond. These reactions afforded complexes with new ligand scaffolds, including a bridging alkylideneamidophosphine (<b>4b-Y</b>), an amidophosphine (<b>7b-Y</b>), an η²-iminoacylphosphine (<b>8b-Y</b>), and oxycarbenephosphine (<b>9b-Y</b> and <b>10b-Y</b>) ligands at the binuclear Y/Ru core. All of these reaction products were structurally characterized by X-ray crystallography, NMR spectroscopy, and elemental analyses

Computational Analyses of the Effect of Lewis Bases on Styrene Polymerization Catalyzed by Cationic Scandium Half-Sandwich Complexes

The styrene polymerizations catalyzed by cationic half-sandwich rare-earth metal complexes [(η⁵-C₅Me₅)­Sc­(CH₂SiMe₃)­(THF)_<i>n</i>]⁺ (<i>n</i> = 0 (<b>A</b>), 1 (^<b>thf</b><b>A</b>)), [(η⁵-C₅Me₅)­Sc­(CH₂C₆H₄NMe₂-<i>o</i>)]⁺ (<b>B</b>), and [(η⁵-C₅Me₅)­Sc­(C₆H₄OMe-<i>o</i>)]⁺ (<b>C</b>) have been computationally studied. It has been found that THF as an external Lewis base has no effect on the regioselectivity in the chain initiation step. However, it can make activity lower toward styrene insertion. THF is computationally proposed to move away from the Sc center during chain propagation and thus has no effects on stereoselectivity. Aminobenzyl as an internal Lewis base in <b>B</b> results in no regioselectivity at the chain initiation stage and has no effect on syndioselectivity during chain propagation. The internal Lewis base anisyl induces high-isotactic chain-end microstructure. The discrepancy in chain-end microstructures induced by aminobenzyl and anisyl groups could be ascribed to the different coordination capability of oxygen and nitrogen atoms to Sc metal. The size of the metal-involved ring in the bare cationic species plays an important role in the control of chain-end microstructure of the resulting polystyrene

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

Computational Studies on Isospecific Polymerization of 1-Hexene Catalyzed by Cationic Rare Earth Metal Alkyl Complex Bearing a <i>C</i>₃ <i>i</i>Pr-trisox Ligand

1-Hexene polymerization catalyzed by dicationic rare earth metal alkyl species [Ln­(<i>i</i>Pr-trisox)­(CH₂SiMe₃)]²⁺ (Ln = Sc and Y; trisox = trisoxazoline) has been computationally studied by using QM/MM approach. It has been found that the initiation of 1-hexene polymerization kinetically prefers 1,2-insertion (free energy barrier of 17.23 kcal/mol) to 2,1-insertion (free energy barrier of 20.05 kcal/mol). Such a preference of 1,2-insertion has been also found for chain propagation stage. The isotactic polymerization was computed to be more kinetically preferable in comparison with syndiotactic manner, and the dicationic system resulted in lower insertion free energy barrier and more stable insertion product in comparison with the monocationic system. The stereoselectivity was found to follow chain-end mechanism, and the isospecific insertion of 1-hexene is mainly controlled by kinetics. In addition, the current computational results, for the first time, indicate that the higher activity of Sc species toward 1-hexene polymerization in comparison with the Y analogue could be ascribed to lower insertion barrier, easier generation of the active species, and its larger chemical hardness

Mechanistic Insights into Ring Cleavage and Contraction of Benzene over a Titanium Hydride Cluster

Carbon–carbon bond cleavage of benzene by transition metals is of great fundamental interest and practical importance, as this transformation is involved in the production of fuels and other important chemicals in the industrial hydrocracking of naphtha on solid catalysts. Although this transformation is thought to rely on cooperation of multiple metal sites, molecular-level information on the reaction mechanism has remained scarce to date. Here, we report the DFT studies of the ring cleavage and contraction of benzene by a molecular trinuclear titanium hydride cluster. Our studies suggest that the reaction is initiated by benzene coordination, followed by H₂ release, C₆H₆ hydrometalation, repeated C–C and C–H bond cleavage and formation to give a MeC₅H₄ unit, and insertion of a Ti atom into the MeC₅H₄ unit with release of H₂ to give a metallacycle product. The C–C bond cleavage and ring contraction of toluene can also occur in a similar fashion, though some details are different due to the presence of the methyl substituent. Obviously, the facile release of H₂ from the metal hydride cluster to provide electrons and to alter the charge population at the metal centers, in combination with the flexible metal–hydride connections and dynamic redox behavior of the trimetallic framework, has enabled this unusual transformation to occur. This work has not only provided unprecedented insights into the activation and transformation of benzene over a multimetallic framework but it may also offer help in the design of new molecular catalysts for the activation and transformation of inactive aromatics

H–H and N–H Bond Cleavages of Dihydrogen and Ammonia by a Bifunctional Imido (NH)-Bridged Diiridium Complex: A DFT Study

The mechanisms of H–H and N–H bond cleavages of dihydrogen and ammonia mediated by the diiridium μ₂-imido complex [(Cp*Ir)₂(μ₂-H)­(μ₂-NH)]⁺ (<b>A</b>; Cp* = η⁵-C₅Me₅) were theoretically investigated with the density functional theory (DFT) method. Both oxidative addition and metal–ligand cooperation modes have been studied for H–H and N–H bond cleavages, respectively. The H–H bond cleavage most likely occurs through competitive oxidative addition and metal–ligand cooperation, and both cleavage modes have similar overall free energy barriers (24.3 and 25.7 kcal/mol, respectively). The ligand-assisted N–H bond heterolytic cleavage mechanism is proposed for the NH₃ reaction, and the general oxidative addition pathway can be reasonably ruled out, as it is kinetically unfavorable

H–H and N–H Bond Cleavages of Dihydrogen and Ammonia by a Bifunctional Imido (NH)-Bridged Diiridium Complex: A DFT Study

The mechanisms of H–H and N–H bond cleavages of dihydrogen and ammonia mediated by the diiridium μ₂-imido complex [(Cp*Ir)₂(μ₂-H)­(μ₂-NH)]⁺ (<b>A</b>; Cp* = η⁵-C₅Me₅) were theoretically investigated with the density functional theory (DFT) method. Both oxidative addition and metal–ligand cooperation modes have been studied for H–H and N–H bond cleavages, respectively. The H–H bond cleavage most likely occurs through competitive oxidative addition and metal–ligand cooperation, and both cleavage modes have similar overall free energy barriers (24.3 and 25.7 kcal/mol, respectively). The ligand-assisted N–H bond heterolytic cleavage mechanism is proposed for the NH₃ reaction, and the general oxidative addition pathway can be reasonably ruled out, as it is kinetically unfavorable