Two [1,2,4-(Me₃C)₃C₅H₂]₂CeH Molecules are Involved in Hydrogenation of Pyridine to Piperidine as Shown by Experiments and Computations

Hydrogenation of pyridine to piperidine catalyzed by [1,2,4-(Me₃C)₃C₅H₂]₂CeH, abbreviated as Cp′₂CeH or [Ce]′-H, is reported. The reaction proceeds from Cp′₂Ce­(2-pyridyl), isolated from the reaction of pyridine with Cp′₂CeH, to Cp′₂Ce­(4,5,6-trihydropyridyl), and then to Cp′₂Ce­(piperidyl). The cycle is completed by the addition of pyridine, which generates Cp′₂Ce­(2-pyridyl) and piperidine. The net reaction depends on the partial pressure of H₂ and temperature. The dependence of the rate on the H₂ pressure is associated with the formation of Cp′₂CeH, which increases the rate of the first and/or second additions of H₂ but does not influence the rate of the third addition. Density functional theory calculations of several possible pathways are consistent with three steps, each of which are composed of two elementary reactions, (i) heterolytic activation of H₂ with a reasonably high energy, Δ<i>G</i>^⧧ = 20.5 kcal mol^–1, on Cp′₂Ce­(2-pyridyl), leading to Cp′₂CeH­(6-hydropyridyl), followed by an intramolecular hydride transfer with a lower activation energy, (ii) intermolecular addition of Cp′₂CeH to the C⁴C⁵ bond, followed by hydrogenolysis, giving Cp′₂Ce­(4,5,6-trihydropyridyl) and regenerating Cp′₂CeH, and (iii) a similar hydrogenation/hydrogenolysis sequence, yielding Cp′₂Ce­(piperidyl). The calculations reveal that step ii can only occur in the presence of Cp′₂CeH and that alternative intramolecular steps have considerably higher activation energies. The key point that emerges from these experimental and computational studies is that step ii involves two Cp′₂Ce fragments, one to bind the 6-hydropyridyl ligand and the other to add to the C⁴C⁵ double bond. In the presence of H₂, this second step is intermolecular and catalytic. The cycle is completed by reaction with pyridine to yield Cp′₂Ce­(2-pyridyl) and piperidine. The structures of Cp′₂CeX, where X = 2-pyridyl, 4,5,6-trihydropyridyl, and piperidyl, are fluxional, as shown by variable-temperature ¹H NMR spectroscopy

Qualitative Estimation of the Single-Electron Transfer Step Energetics Mediated by Samarium(II) Complexes: A “SOMO–LUMO Gap” Approach

Lanthanide II organometallic complexes usually initiate reactions via a single-electron transfer (SET) from the metal to a bonded substrate. Extensive mechanistic studies were carried out for lanthanide III complexes in which no change of oxidation state is involved. Some case-dependent strategies were reported by our group in order to account for a SET event in organometallic computed studies. In the present study, we show that analysis of DFT orbital spectra allows differentiating between exothermic and endothermic electron transfer. This methodology appears to be general; it allows differentiating between lanthanide centers and substituent effects on metallocenes. For that purpose, we considered mainly various samarocene adducts as well as a SmI₂ complex explicitly solvated by THF. Comparison between DFT methods and <i>ab initio</i> (CAS-SCF and HF) computational level revealed that the SOMO–LUMO gap computed at the DFT B3PW91 level, in combination with small-core RECPs and standard basis sets, offers a qualitative estimation of the energetics of the SET that is in line with both CAS-SCF calculations and experimental results when available. This orbital-based approach, based on DFT calculation, affords a fast and efficient methodology for pioneer exploration of the reactivity of lanthanide­(II) mediated by SET

Two [1,2,4-(Me₃C)₃C₅H₂]₂CeH Molecules are Involved in Hydrogenation of Pyridine to Piperidine as Shown by Experiments and Computations

Hydrogenation of pyridine to piperidine catalyzed by [1,2,4-(Me₃C)₃C₅H₂]₂CeH, abbreviated as Cp′₂CeH or [Ce]′-H, is reported. The reaction proceeds from Cp′₂Ce­(2-pyridyl), isolated from the reaction of pyridine with Cp′₂CeH, to Cp′₂Ce­(4,5,6-trihydropyridyl), and then to Cp′₂Ce­(piperidyl). The cycle is completed by the addition of pyridine, which generates Cp′₂Ce­(2-pyridyl) and piperidine. The net reaction depends on the partial pressure of H₂ and temperature. The dependence of the rate on the H₂ pressure is associated with the formation of Cp′₂CeH, which increases the rate of the first and/or second additions of H₂ but does not influence the rate of the third addition. Density functional theory calculations of several possible pathways are consistent with three steps, each of which are composed of two elementary reactions, (i) heterolytic activation of H₂ with a reasonably high energy, Δ<i>G</i>^⧧ = 20.5 kcal mol^–1, on Cp′₂Ce­(2-pyridyl), leading to Cp′₂CeH­(6-hydropyridyl), followed by an intramolecular hydride transfer with a lower activation energy, (ii) intermolecular addition of Cp′₂CeH to the C⁴C⁵ bond, followed by hydrogenolysis, giving Cp′₂Ce­(4,5,6-trihydropyridyl) and regenerating Cp′₂CeH, and (iii) a similar hydrogenation/hydrogenolysis sequence, yielding Cp′₂Ce­(piperidyl). The calculations reveal that step ii can only occur in the presence of Cp′₂CeH and that alternative intramolecular steps have considerably higher activation energies. The key point that emerges from these experimental and computational studies is that step ii involves two Cp′₂Ce fragments, one to bind the 6-hydropyridyl ligand and the other to add to the C⁴C⁵ double bond. In the presence of H₂, this second step is intermolecular and catalytic. The cycle is completed by reaction with pyridine to yield Cp′₂Ce­(2-pyridyl) and piperidine. The structures of Cp′₂CeX, where X = 2-pyridyl, 4,5,6-trihydropyridyl, and piperidyl, are fluxional, as shown by variable-temperature ¹H NMR spectroscopy

Experimental and DFT Computational Study of β‑Me and β‑H Elimination Coupled with Proton Transfer: From Amides to Enamides in Cp*₂MX (M = La, Ce)

The thermal rearrangement of the f-block metallocene amides Cp*₂MNR₁R₂, where R₁ is CHMe₂, R₂ is either CHMe₂ or CMe₃, and M is either La or Ce, to the corresponding enamides Cp*₂MNR₁[C­(Me)CH₂] and H₂ or CH₄, respectively, occurs when the solid amides are heated in sealed evacuated ampules at 160–180 °C for 1–2 weeks. The net reaction is a β-H or β-Me elimination followed by a γ-abstraction of a proton at the group from which the β-elimination occurs. When R₁ is either SiMe₃ or SiMe₂CMe₃ and R₂ is CMe₃, the enamide Cp*₂MNR₁[C­(Me)CH₂] is isolated, the result of β-Me elimination, but when R₂ is CHMe₂, the enamides Cp*₂MNR₁[C­(Me)CH₂] and Cp*₂NR₁[C­(H)CH₂] are isolated, the result of β-H and β-Me elimination. In the latter cases, both enamides are formed in similar amounts and the rates of the β-H and β-Me elimination steps must be similar. A two-step mechanism is developed from DFT calculations. The first step is migration of a hydride or a methyl anion to the Cp*₂M fragment, forming M–H or M–Me bonds as the NC bond in the intermediate imine forms. The enamide evolves from the metal-coordinated imine by abstraction of a proton from the γ-carbon of the intermediate imine. The two elementary steps involve significant geometrical changes within the N_αC_βC_γ set of atoms during the two-step elimination process that are in large part responsible for the relatively high activation barriers for the net reaction, which may be classified as a proton-coupled hydride or methyl anion transfer reaction

Experimental and DFT Computational Study of β‑Me and β‑H Elimination Coupled with Proton Transfer: From Amides to Enamides in Cp*₂MX (M = La, Ce)

The thermal rearrangement of the f-block metallocene amides Cp*₂MNR₁R₂, where R₁ is CHMe₂, R₂ is either CHMe₂ or CMe₃, and M is either La or Ce, to the corresponding enamides Cp*₂MNR₁[C­(Me)CH₂] and H₂ or CH₄, respectively, occurs when the solid amides are heated in sealed evacuated ampules at 160–180 °C for 1–2 weeks. The net reaction is a β-H or β-Me elimination followed by a γ-abstraction of a proton at the group from which the β-elimination occurs. When R₁ is either SiMe₃ or SiMe₂CMe₃ and R₂ is CMe₃, the enamide Cp*₂MNR₁[C­(Me)CH₂] is isolated, the result of β-Me elimination, but when R₂ is CHMe₂, the enamides Cp*₂MNR₁[C­(Me)CH₂] and Cp*₂NR₁[C­(H)CH₂] are isolated, the result of β-H and β-Me elimination. In the latter cases, both enamides are formed in similar amounts and the rates of the β-H and β-Me elimination steps must be similar. A two-step mechanism is developed from DFT calculations. The first step is migration of a hydride or a methyl anion to the Cp*₂M fragment, forming M–H or M–Me bonds as the NC bond in the intermediate imine forms. The enamide evolves from the metal-coordinated imine by abstraction of a proton from the γ-carbon of the intermediate imine. The two elementary steps involve significant geometrical changes within the N_αC_βC_γ set of atoms during the two-step elimination process that are in large part responsible for the relatively high activation barriers for the net reaction, which may be classified as a proton-coupled hydride or methyl anion transfer reaction

Two [1,2,4-(Me₃C)₃C₅H₂]₂CeH Molecules are Involved in Hydrogenation of Pyridine to Piperidine as Shown by Experiments and Computations

Hydrogenation of pyridine to piperidine catalyzed by [1,2,4-(Me₃C)₃C₅H₂]₂CeH, abbreviated as Cp′₂CeH or [Ce]′-H, is reported. The reaction proceeds from Cp′₂Ce­(2-pyridyl), isolated from the reaction of pyridine with Cp′₂CeH, to Cp′₂Ce­(4,5,6-trihydropyridyl), and then to Cp′₂Ce­(piperidyl). The cycle is completed by the addition of pyridine, which generates Cp′₂Ce­(2-pyridyl) and piperidine. The net reaction depends on the partial pressure of H₂ and temperature. The dependence of the rate on the H₂ pressure is associated with the formation of Cp′₂CeH, which increases the rate of the first and/or second additions of H₂ but does not influence the rate of the third addition. Density functional theory calculations of several possible pathways are consistent with three steps, each of which are composed of two elementary reactions, (i) heterolytic activation of H₂ with a reasonably high energy, Δ<i>G</i>^⧧ = 20.5 kcal mol^–1, on Cp′₂Ce­(2-pyridyl), leading to Cp′₂CeH­(6-hydropyridyl), followed by an intramolecular hydride transfer with a lower activation energy, (ii) intermolecular addition of Cp′₂CeH to the C⁴C⁵ bond, followed by hydrogenolysis, giving Cp′₂Ce­(4,5,6-trihydropyridyl) and regenerating Cp′₂CeH, and (iii) a similar hydrogenation/hydrogenolysis sequence, yielding Cp′₂Ce­(piperidyl). The calculations reveal that step ii can only occur in the presence of Cp′₂CeH and that alternative intramolecular steps have considerably higher activation energies. The key point that emerges from these experimental and computational studies is that step ii involves two Cp′₂Ce fragments, one to bind the 6-hydropyridyl ligand and the other to add to the C⁴C⁵ double bond. In the presence of H₂, this second step is intermolecular and catalytic. The cycle is completed by reaction with pyridine to yield Cp′₂Ce­(2-pyridyl) and piperidine. The structures of Cp′₂CeX, where X = 2-pyridyl, 4,5,6-trihydropyridyl, and piperidyl, are fluxional, as shown by variable-temperature ¹H NMR spectroscopy

Ethylene–Butadiene Copolymerization by Neodymocene Complexes: A Ligand Structure/Activity/Polymer Microstructure Relationship Based on DFT Calculations

Ethylene/butadiene copolymerization can be performed by neodymocene catalysts in the presence of an alkylating/chain transfer agent. A variety of polymerization activities and copolymer microstructures can be obtained depending on the neodymocene ligands. For a set of four catalysts, namely (C₅Me₅)₂NdR, [Me₂Si­(3-Me₃SiC₅H₃)₂]­NdR, [Me₂Si­(C₅H₄)­(C₁₃H₈)]­NdR and [Me₂Si­(C₁₃H₈)₂]­NdR, we report a DFT mechanistic study of this copolymerization reaction performed in the presence of dialkylmagnesium. Based on the modeling strategy developed for the ethylene homopolymerization catalyzed by (C₅Me₅)₂NdR in the presence of MgR₂, our model is able to account for the following: (i) the formation of Nd/Mg heterobimetallic complexes as intermediates, (ii) the overall differential activity of the catalysts, (iii) the copolymerization reactivity indexes, and (iv) the specific microstructure of the resulting copolymers, including branching and cyclization. The analysis of the reaction mechanisms and the energy profiles thus relates ligand structure, catalyst activity, and polymer microstructure and sets the basis for further catalyst developments

Deciphering the Mechanism of Coordinative Chain Transfer Polymerization of Ethylene Using Neodymocene Catalysts and Dialkylmagnesium

Ethylene polymerizations were performed in toluene using the neodymocene complex (C₅Me₅)₂NdCl₂Li­(OEt₂)₂ or {(Me₂Si­(C₁₃H₈)₂)­Nd­(μ-BH₄)­[(μ-BH₄)­Li­(THF)]}₂ in combination with <i>n</i>-butyl-<i>n</i>-octylmagnesium used as both alkylating and chain transfer agent. The kinetics were followed for various [Mg]/[Nd] ratios, at different polymerization temperatures, with or without ether as a cosolvent. These systems allowed us to (i) efficiently obtain narrowly distributed and targeted molar masses, (ii) characterize three phases during the course of polymerization, (iii) estimate the propagation activation energy (17 kcal mol^–1), (iv) identify the parameters that control chain transfer, and (v) demonstrate enhanced polymerization rates and molar mass distribution control in the presence of ether as cosolvent. This experimental set of data is supported by a computational investigation at the DFT level that rationalizes the chain transfer mechanism and the specific microsolvation effects in the presence of cosolvents at the molecular scale. This joint experimental/computational investigation offers the basis for further catalyst developments in the field of coordinative chain transfer polymerization (CCTP)

Dialkenylmagnesium Compounds in Coordinative Chain Transfer Polymerization of Ethylene. Reversible Chain Transfer Agents and Tools To Probe Catalyst Selectivities toward Ring Formation

A range of dialkenylmagnesium compounds ([CH₂CH­(CH₂)_<i>n</i>]₂Mg; <i>n</i> = 1–6) were synthesized and used as chain transfer agents (CTA) with either (C₅Me₅)₂NdCl₂Li­(OEt₂)₂ (<b>1</b>) or [Me₂Si­(C₁₃H₈)₂Nd­(BH₄)₂Li­(thf)]₂ (<b>2</b>) neodymium precursors for the polymerization of ethylene. In all cases, the systems followed a controlled coordinative chain transfer polymerization mechanism. The intramolecular insertion of the vinyl group on the CTA in growing chains is possible and led to the formation of cyclopentyl, cyclohexyl, and possibly cycloheptyl chain ends. While the production of cyclopentyl- or cyclohexyl-capped polyethylene chains can be quantitative (<i>n</i> = 2–5), the integrity of this double bond can also be kept if <i>n</i> is higher than 6. In comparison to <b>1</b>/CTA catalytic systems, <b>2</b>/CTA catalytic systems showed a higher propensity to produce cycloalkyl chain ends. This was ascribed to the lower steric demand around the active site, as shown by DFT calculations. In addition, the formation of bis­(cyclopentylmethyl)­magnesium from dipentenylmagnesium using a catalytic amount of <b>2</b> was shown

Dialkenylmagnesium Compounds in Coordinative Chain Transfer Polymerization of Ethylene. Reversible Chain Transfer Agents and Tools To Probe Catalyst Selectivities toward Ring Formation

A range of dialkenylmagnesium compounds ([CH₂CH­(CH₂)_<i>n</i>]₂Mg; <i>n</i> = 1–6) were synthesized and used as chain transfer agents (CTA) with either (C₅Me₅)₂NdCl₂Li­(OEt₂)₂ (<b>1</b>) or [Me₂Si­(C₁₃H₈)₂Nd­(BH₄)₂Li­(thf)]₂ (<b>2</b>) neodymium precursors for the polymerization of ethylene. In all cases, the systems followed a controlled coordinative chain transfer polymerization mechanism. The intramolecular insertion of the vinyl group on the CTA in growing chains is possible and led to the formation of cyclopentyl, cyclohexyl, and possibly cycloheptyl chain ends. While the production of cyclopentyl- or cyclohexyl-capped polyethylene chains can be quantitative (<i>n</i> = 2–5), the integrity of this double bond can also be kept if <i>n</i> is higher than 6. In comparison to <b>1</b>/CTA catalytic systems, <b>2</b>/CTA catalytic systems showed a higher propensity to produce cycloalkyl chain ends. This was ascribed to the lower steric demand around the active site, as shown by DFT calculations. In addition, the formation of bis­(cyclopentylmethyl)­magnesium from dipentenylmagnesium using a catalytic amount of <b>2</b> was shown