Mechanistic Studies of the Thermolysis of Tetraneopentyltitanium(IV). 1. Solution Evidence That Titanium Alkylidenes Activate Saturated Hydrocarbons

Abstract

Studies of the thermolysis of Ti(CH2CMe3)4 in solution have been carried out in parallel with studies of the chemical mechanism responsible for its conversion to titanium carbide under CVD conditions. In hydrocarbon solutions, the neopentyl complex thermolyzes to eliminate 2.1 equiv of neopentane as the principal organic product. A deuterium kinetic isotope effect (k α (H)/k α (D) = 5.2 ± 0.4) upon deuterating the alkyl groups at the α positions provides clear evidence that the initial step in the thermolysis is an α-hydrogen abstraction reaction to form neopentane. The activation parameters for this α-hydrogen abstraction process are ΔH ⧧ = 21.5 ± 1.4 kcal/mol and ΔS ⧧ = −16.6 ± 3.8 cal/(mol K). The titanium-containing product of this reaction is a titanium alkylidene, which in solution activates C−H bonds of both saturated and unsaturated hydrocarbon solvents such as benzene and cyclohexane. No activation of the C−F bonds of hexafluorobenzene is seen, however. Under special circumstances, a second thermolysis pathway for TiNp4 can be detected, γ-hydrogen activation, but this pathway is intrinsically about 25 times slower than the α-hydrogen abstraction process.

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

“…The nitride cluster works as a template to enforce a trivalent state of an endohedral Ti atom, which can be then oxidized or reduced to the Ti IV and Ti II states 13 . In addition to the Ti-NCFs, the only known Ti-EMFs are Ti@C 28 (seen only in the gas phase so far) 16 , the carbides Ti 2 C 2 @C 78 and Ti 2 C 2 @C 82 (refs 17,18), and a sulphide Ti 2 S@C 78 (ref. 19).…”

“…Due to such an electronic structure, L 2 Ti ¼ CR 2 molecules are very unstable in the pristine form and readily coordinate other groups to Ti or decompose 27 . As a result, (Cp) 2 Ti ¼ CR 2 species are frequently formed only in situ as transient intermediates 28,29 . 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 .…”

Metal–ligand multiple bonds as frustrated Lewis pairs for C–H functionalization

“…Intramolecular addition of a benzylic C–H bond across a Zr(IV) phosphinidene has been reported by Stephan [32]. Several C–H cleavage reactions have also been reported across alkylidenes and alkylidynes [44–46], and these may be viewed as the microscopic reverse of the α-hydrogen eliminations frequently utilized to generate such multiply bonded units.…”

Sequential Hydrocarbon C—H Bond Activations by 16-Electron Organometallic Complexes of Molybdenum and Tungsten

“…Thus, these processes involve triple C-H bond activations of the original hydrocarbon substrates. complexes A, Β and C belong to a small class of compounds that intermolecularly activate C-H bonds via their addition across the M=C bond of an alkylidene intermediate (6,7,8). However, A, B, and C are unique in their ability to activate aliphatic C-H bonds to form isolable, well-defined products.…”