AlMe₃and ZnMe₂Adducts of a Titanium Imido Methyl Cation:  A Combined Crystallographic, Spectroscopic, and DFT Study

Abstract

A combined experimental and DFT study of the reactions of the titanium imido methyl cation [Ti(NtBu)(Me3[9]aneN3)Me]+ (4+) with AlMe3 and ZnMe2 is described. Reaction of 4+ with AlMe3 gave [Ti(NtBu)(Me3[9]aneN3)(mu-Me)2AlMe2]+ (7+), the first structurally characterized AlMe3 adduct of a transition metal alkyl cation and a model for the presumed resting state in MAO-activated olefin polymerizations. Reaction of 4+ with ZnMe2 also gave a methyl-bridged heterobinuclear species, namely [Ti(mu-NtBu)(Me3[9]aneN3)(mu-Me)2ZnMe]+ (8+), the first directly observed ZnMe2 adduct of a transition metal alkyl cation. At room temperature, all three metal-bound methyls of 8+ underwent rapid exchange with those of free ZnMe2, whereas at 233 K only the terminal Zn-Me group exchanged significantly. Addition of AlMe3 to 8+ quantitatively formed 7+ and ZnMe2. Reaction of 4+ with Cp2ZrMe2 gave [Ti(NtBu){Me2(mu-CH2)[9]aneN3}(mu-CH2)ZrCp2]+ (10+) via a highly selective double C-H bond activation reaction in which both alkyl groups of Cp2ZrMe2 were lost. DFT calculations on models of 7+ confirmed the approximately square-based pyramidal geometries for the bridging methyl groups. Calculations on 8+ found that the formation of the Ti(mu-Me)2Zn moiety is assisted by an Nimide-->Zn dative bond. DFT calculations for the sterically less encumbered methyl cation [Ti(NMe)(H3[9]aneN3)Me]+ found strong thermodynamic preferences for adducts featuring Nimide-->M (M = Al or Zn) interactions. This offers insight into recently observed structure-productivity trends in MAO-activated imido-based polymerization catalysts. Calculations on the metallocenium adducts [Cp2Ti(mu-Me)2AlMe2]+ and [Cp2Ti(mu-Me)2ZnMe]+ are described, each showing alpha-agostic interactions for the bridging methyl groups. For these systems and the imido ones, the coordination of AlMe3 to the corresponding monomethyl cation is ca. 30 kJ mol-1 more favorable than for ZnMe2.

Methyl Complexes of the Transition Metals

“…[34][35][36] In this context the first AlMe3 adduct of a TM-alkyl cation, a heterobimetallic species containing a titanium imido methyl cation, has been structurally characterized by Mountford et al This species is a model for the proposed resting state during MAO activated catalytic olefin polymerization (Scheme 5). 37 A unique structurally characterized AlMe3 adduct of an ansa-zirconoceneMe cation has been also reported. 38 Recent examples of nonmetallocene catalysts for ethylene polymerization are the neutral methyl complexes of titanium stabilized with bulky iminato ligands (L)TiMe2X (L = imidazolin-2-imide; X = Cl or Me).…”

Theoretical investigation on the reaction of N2O and CO catalyzed by Fe+(C6H6)

“…Finally, we will calculate the rate constant for these reactions using conventional transition state theory (TST). In recent years, DFT [13][14][15][16][31][32][33][34] has attracted considerable attention and has been widely applied to electronic structure calculations for systems containing transition metals. DFT methods have been shown to be particularly useful and computationally efficient for systems with a relatively large number of electrons.…”

Group 4 metal complexes for homogeneous olefin polymerisation: a short tutorial review

“…However, this was not structurally characterised [121]. In 2006, Mountford et al [122] reported the first cationic transition metal compound containing a M(l-Me 2 )AlMe 2 moiety characterised by X-ray crystallography (20, Fig. 8).…”

“…Indeed in some cases, it has been shown that a molecule of solvent would rather bind to the vacant site of the cation than [BF 20 ] -; this is a testament to the anion's non-coordinating nature [133][134][135] -can interact with cations sufficiently to form higher aggregates [137]. Marks et al [138] reported using similar PGSE NMR measurements in C 6 [121,122] quadruples or even hextuples in solution at higher concentrations. It is worth noting that the previously mentioned PGSE NMR experiments were carried out at concentrations above that which is usually used in single-site catalysis, and the existence of these aggregates at lower, more relevant, concentrations is more unlikely.…”