Four-Coordinate Titanium Alkylidene Complexes:  Synthesis, Reactivity, and Kinetic Studies Involving the Terminal Neopentylidene Functionality

Abstract

A series of titanium complexes containing a terminal neopentylidene functionality have been prepared by a one electron oxidatively induced α-hydrogen abstraction from the corresponding bis-neopentyl precursor (Nacnac)Ti(CH2 tBu)2 (Nacnac- = [Ar]NC(CH3)CHC(CH3)N[Ar], Ar = 2,6-(CHMe2)2C6H3), among them (Nacnac)TiCHtBu(OTf) and (Nacnac)TiCHtBu(I). It was determined that bulky alkyl groups bound to titanium as well as a bulky coordinating anion from the oxidant are needed to promote α-hydrogen abstraction. Complex (Nacnac)TiCHtBu(OTf) serves as a template for other four-coordinate titanium neopentylidene complexes such as (Nacnac)TiCHtBu(X) (X- = Cl, Br, and BH4). Complexes (Nacnac)TiCHtBu(X) undergo cross-metathesis reactivity with the imine functionality of the Nacnac- ligand forming the imido complexes (HtBuCC(Me)CHC(Me)N[Ar])TiNAr(X) (X- = OTf, Cl, Br, I, BH4). In addition, C−H activation of two tertiary carbons also takes place to afford the titanacycles Ti[2,6-(CMe2)(CHMe2)C6H3]NC(Me)CHC(Me)N[2,6-(CMe2)(CHMe2)C6H3](X) (X- = OTf, Cl, Br and η2-BH4). Kinetic studies in C6D6 reveal the formation of (HtBuCC(Me)CHC(Me)N[Ar])TiNAr(I) from (Nacnac)TiCHtBu(I) to be independent of solvent (C6D6, Et2O−d10, THF-d8) and the reaction to be first order in titanium (k = 8.06 × 10-4 s-1 at 57 °C, with activation parameters ΔH ⧧ = 21.3(2) kcal/mol, ΔS ⧧ = −8(3) cal/mol K). Compound (Nacnac)TiCHtBu(OTf) reacts with various substrates to afford products in which the alkylidene functionality has been significantly transformed. When the alkylidene derivatives (NacnactBu)TiCHtBu(X) (X- = OTf, I; NacnactBu - = [Ar]NC(tBu)CHC(tBu)N[Ar]) were prepared, the intramolecular cross-metathesis transformation observed with (Nacnac)TiCHtBu(X) was inhibited completely.

Organometallic consequences of a redox reaction: Terminal trimethylsilylmethylidene titanium complexes prepared by a one-electron oxidation step

“…1, the molecule structure adopts a pseudo tetrahedral geometry whereby the b-diketiminate ligand occupies the remaining two coordination sites. The Ti-C distances (2.104(3) and 2.139(3) Å ) are within range to similar bisalkyl titanium complexes reported by us [36][37][38] and Budzelaar and co-workers [39]. Although all hydrogens in the molecule were located and refined isotropically, their location does not suggest an a-hydrogen agostic interaction taking place with the Ti(III) center.…”

“…In addition, the 13 C NMR spectrum clearly exposes a highly deshielded alkylidene carbon resonance at 319.4 ppm with a J C-H component of 106 Hz. This latter feature suggests moderate a-hydrogen agostic interaction taking place with the metal center and presents 2-THF to be less electrophilic at Ti(IV), when compared to (nacnac)Ti@CH t Bu(OTf) (J C-H = 96 Hz) [38,36]. Formation of a OTf À complex in 2-THF is also evident from the 19 F NMR spectrum (singlet resonance at À77.8 ppm).…”

“…1 H, 13 C, and 19 F NMR spectra are essentially analogous to its solvated counterpart, 2-THF. Despite this similarity, the 1 H NMR spectrum shows a slight deviation in the chemical shift for the alkylidene hydrogen (12.39 ppm), while the 13 C NMR spectrum displays a further downfield chemical shift at 320.1 ppm, congruently with a stronger TiÁ Á ÁH a-hydrogen agostic interaction (when compared to 2-THF) taking place with the four-coordinate metal center (J C-H = 98 Hz) [38,36]. This feature implies that coordination of THF is not a necessity for the stability of 2, and that binding of the Lewis base to the Ti(IV) center reduces the a-hydrogen agostic interaction.…”

“…Based on 1 H NMR spectra, the latter product does not appear to be the intramolecular double C-H metallated product: an analogous byproduct formed from the decomposition of (nacnac)Ti@CH t Bu(OTf), namely the complex Ti(OTf){[2,6-(CMe 2 )(CHMe 2 )C 6 H 3 ]NC(Me)} 2 CH [38,36]. We do however, observe SiMe 4 in the mixture, but have not been able to separate the second metal-based product.…”

Endohedral fullerene with μ3-carbido ligand and titanium-carbon double bond stabilized inside a carbon cage

“…For example, the archetypical Schrock carbene (Cp) 2 33 , whereas X 2 Ti ¼ CH 2 (X ¼ H, F, Cl) species were isolated in low-temperature inert gas matrices 34,35 . At the same time, Ti alkylidenes with a higher coordination number, when Ti-based LUMO is further coordinated, can form stable molecules 22,23,[36][37][38][39] .…”