L. Yu. Ustynyuk

Activation of CH4 and H2 by zirconium(IV) and titanium(IV) cationic complexes. Theoretical DFT study

Abstract The reactions of methane and dihydrogen molecules with the cations [(η 5 -C 5 H 5 ) 2 MCH 3 ] + ( 1 ) and [(η 5 -C 5 H 5 ) 2 MH] + ( 3 ) (M=Zr, Ti) have been investigated by gradient-corrected density functional calculations. In the case of CH 4 , the active cationic center of 1 or 3 attracts the substrate molecule first to form an agostic complex in which its CH bond is already somewhat weakened. The σ-bonded ligand exchange reaction in the system 1 +CH 4 proceeds through a symmetric transition state with an activation barrier of 15.0 kcal mol −1 (11.6 kcal mol −1 ) for the Zr (Ti) complex. Hydrogen reacts with 1 exothermally, Δ H 0 =−7.1 kcal mol −1 (−8.6 kcal mol −1 ) for Zr (Ti), yielding 3 and CH 4 without an activation barrier. These theoretical results give insight into the mechanism of H/D exchange in methane in the presence of Ziegler–Natta-type catalysts observed experimentally by Grigoryan et al. It is shown that organometallic cationic complexes of Zr(IV) and Ti(IV) may prove to be promising systems for CH and HH bond activation under mild conditions.

DFT study of the mechanism of alkane hydrogenolysis by transition metal hydrides. 1. Interaction of silica-supported zirconium hydrides with methane

Model reactions of silica-supported zirconium hydrides (≡Si—O—)3ZrH and (≡Si—O—)2ZrH2 with methane, resulting in cleavage of a C—H bond in the methane molecule and the formation of (≡Si—O—)3ZrCH3 and (≡Si—O—)2Zr(H)CH3 as products were studied using the DFT approach with the PBE density functional. The processes proceed as bimolecular reactions without preliminary formation of agostic complexes. According to calculations, zirconium dihydrides (≡Si—O—)2ZrH2 are more reactive toward the methane C—H bonds than zirconium monohydrides (≡Si—O—)3ZrH. The calculated activation energies of the reactions with participation of zirconium dihydrides (≡Si—O—)2ZrH2 are in better agreement with the known experimental data for the Yermakov—Basset catalytic system.

Ion pairs formed in metallocene-based systems by the “double activation” mechanism and their catalytic activity

The DFT method is used to study the interaction between metallocene catalyst precursors used in ethylene and propylene polymerization and two molecules of the Al-containing cocatalyst Al(C6F5)3. The participation of two Al centers in metallocene activation accounts for the catalytic activity and other properties of the catalytic systems. Two energy parameters characterizing the number of active sites and the polymerization rate per site are calculated.

A study of the polymerization of ethylene on Ti(III) monocyclopentadienyl compounds by the density functional theory method

Density functional theory was used to study model ethylene reactions with CpTiIIIEt+A− (A− = CH3B(C6F5)3−, or B(C6F5)4−; A− can be absent) compounds. The polymerization of ethylene on an isolated CpTiEt+ cation is hindered because of equilibrium between the CpTi(C2H4)Et+ primary complex and the primary product of CpTiBu+ insertion. At the same time, the polymerization of ethylene on CpTiEt+A− ion pairs (A− = CH3B(C6F5)3− or B(C6F5)4−) is thermodynamically allowed (ΔE from −26.2 to −25.6 kcal/mol and ΔG298 from −10.9 to −10.4 kcal/mol) and is not related to overcoming substantial energy barriers (ΔE# = 8.2−12.3 kcal/mol and ΔG298≠) = 7.8−13.3 kcal/mol). The degree of polymerization can be low because of the effective occurrence of polymer chain termination by hydrogen transfer from the polymer chain to the monomer.

The activation of C-H bonds in C1-C3 alkanes by zirconium(III,IV) and titanium(III,IV) hydrides immobilized on the surface of SiO2: a density functional theory study

Model reactions of the (≡Si-O-)3MIVH (1), (≡Si-O-)2MIVH2 (2), and (≡Si-O-)2MIIIH (3) hydrides, where M = Ti and Zr, immobilized on the surface of silica with methane and propane were studied by the density functional theory with the PBE functional. The reactions involved the breaking of C-H alkane bonds and the formation of the (≡Si-O-)3MR, (≡Si-O-)2M(H)R, and (≡Si-O-)2MR products (R = Me, n-Pr, and i-Pr), respectively. Reactions with the participation of 1 and 2 were found to occur as bimolecular processes without the formation of agostic-type prereaction complexes. With 3, the reaction was accompanied by the formation of stable prereaction and postreaction complexes. The conclusion was drawn that dihydrides 2 and trivalent metal hydrides 3 were much more reactive with respect to alkane C-H bonds than monohydrides 1. All the systems studied were characterized by low reaction regioselectivities.

Hydrogenolysis and Hydroisomerization of Neopentane on Titanium and Zirconium Hydrides Stabilized on the Surface of SiO2: A Theoretical Study by Density Functional Theory

The model reactions of neopentane hydrogenolysis and hydroisomerization on transition metal hydrides (≡ Si−O)3MIVH (1), (=eqSi−O)2MIVH2 (2), and (≡Si−O)2MIIIH (3) (M = Zr, Ti) immobilized on the surface of silica were studied by density functional theory. It was shown that the hydrogenolysis of neopentane could occur on all the three types of metal hydrides, the catalytic activity of reaction centers increasing along the series M(IV) monohydrides 1 < M(IV) dihydrides 2 < M(III) hydrides 3. At the same time, the isomerization of the hydrocarbon skeleton of neopentane observed experimentally on titanium-based systems could only be explained by the participation of Ti(III) hydrides.

Activation of C--H bonds in C1-C3 alkanes by titanium(iv) and zirconium(iv) cationic complexes: a DFT study

A DFT study of a model reaction [(η5-C5H5)2MCH3]+ + RH ⇌ [(η5-C5H5)2MR]+ + CH4 (M = TiIV, ZrIV; R = Me, Et, Pr, Pri) was carried out with the PBE density functional. Exchange of σ-bonded ligand proceeds through the formation of agostic complexes [Cp2M(RH)CH3]+ followed by their isomerization into complexes [Cp2M(CH4)R]+via an inner-sphere migration of a hydrogen atom. The calculated rate constants for such migrations involving the primary and secondary C--H bonds of propane molecule differ by 930 times for TiIV complexes and by 47 times for ZrIV complexes, which is due to the effect of steric factors.

The thermodynamic parameters of formation of ions in the interaction of alkylaluminum with electron donors

The thermodynamic parameters of formation of ions in the system AlMe3 + electron donor (D) in the presence and absence of water microimpurities were determined. The structure of these ions was studied and their concentration in nonpolar media was estimated for the example of D = pyridine. The conclusion was drawn from the results of calculations that the participation of water resulted in a substantial increase in the concentration of ions.

A DFT study of ethylene polymerization by zirconocene catalysts. 1. Model system [Cp2ZrEt](+)+C2H4

A DFT study of ethylene polymerization by zirconocene catalysts was carried out. Stationary points corresponding to intermediates and transition states were located on the potential energy surface of the [Cp2ZrC2H5]++C2H4 model system. Three possible reaction mechanisms involving the formation of β-agostic complexes were considered. The energy and thermodynamic characteristics for different reaction pathways were calculated. Corresponding activation energies lie in the range 3.9–6.8 kcal mol−1.

A study of binuclear zirconium hydride catalysts of the hydrogenolysis of alkanes by the density functional theory method

Binuclear hydride centers containing two Zr(IV) atoms are suggested as promising catalysts for the hydrogenolysis of alkanes under mild conditions (T < 450 K, p ∼ 1 atm). Reactions of model compounds L2(H)Zr(X)2Zr(H)L2 (X = H, L = OSi≡ (4a), X = L = OMe (4d)), L(H)Zr(O)2Zr(H)L (L = OSi≡ (4b), Cp(4c)) and (≡SiO)2(H)Zr-O-Zr(H)(OSi≡)2 (4e and 4f) with the propane molecule were studied using the density functional theory method. The results show that centers of the 4a, 4e, and 4f types and especially 4b are promising catalysts of the hydrogenolysis of alkanes due to a high degree of unsaturation of two Zr atoms and their sequential participation in the splitting of the C-C bond and hydrogenation of ethylene formed as a result of splitting.