How a Thermally Unstable Metal Hydrido Complex Can Yield High Catalytic Activity Even at Elevated Temperatures

Abstract

Despite their instability in ethereal solvents, organotitanium hydride catalysts are successfully employed in catalysis at moderate to high temperatures (110 °C), even in the presence of alcohols. It is shown computationally (bond dissociation energy (BDE) analysis and energetic profile for regeneration) and experimentally (EPR studies and kinetic studies), with the specific example of hydrodefluorination (HDF), that despite the long standing belief, regeneration of Ti-H bonds from Ti-F bonds using silanes is endergonic. The resulting low concentration of Ti-H species is crucial for the catalytic stability of those systems. The resting state in the catalysis is a Ti-F species. The most promising silanes for regeneration are not the ones that have the strongest Si-F bond, but the ones that show the largest difference in Si-F and Si-H BDEs.

Self‐Assembly Reactions To Form Multinuclear Zirconium(III) and Titanium(III) Complexes with Imidazole Derivatives as Bridging Ligands

“…Multinuclear, especially trinuclear titanium or zirconium complexes are known, [22][23][24][25][26] but these complexes are not formed by self-assembly and the metal atoms do not occupy the vertices of the molecular architectures. Here, the increased steric demand hinders the formation of rectangles or squares.…”

Competition of Nucleophilic Aromatic Substitution, σ‐Bond Metathesis, and syn Hydrometalation in Titanium(III)‐Catalyzed Hydrodefluorination of Arenes

“…Subsequent F − elimination from HM9 ‐ 9 a , resulting in the intermediary formation of Cp 2 Ti + , a fluoride, and 9 a , is facile ( elim.TS9 ‐ 9 a , +9.8 kcal mol −1 ). Under catalytic conditions, that is, a large excess amount of silane, the strongly nucleophilic fluoride is likely trapped by silane and the fluoride is subsequently transferred to the Ti species (Scheme ); hydride transfer from the silane would not lead to the thermodynamic minimum, 4 ‐trimer . HM has been reported previously for Ru‐catalyzed HDF, and elimination can proceed with or without HF formation, depending on the substrate …”

“…Under catalytic conditions, that is, al arge excessa mount of silane, the strongly nucleophilic fluoridei s likely trapped by silane and the fluoridei ss ubsequentlyt ransferred to the Ti species( Scheme 3); [21] hydridet ransfer from the silane would not lead to the thermodynamic minimum, 4trimer. [8] HM hasb een reportedp reviously for Ru-catalyzed HDF,a nd elimination can proceed with or without HF formation, depending on the substrate. [5e, 6j] We were not able to locate az witterionic intermediate for substrate 9.H ydrometalation in pathway Bi si rreversible as F À eliminationh as al ower barrier.…”

“…Extended studies indicate a titanium(III) hydrido complex ( 1 ) as the active species, which forms a titanium(III) fluoride ( 4 ) via F/H exchange with fluorinated olefins ( 3 ; Scheme ). The resting state of this catalytic process is the trimeric metal fluoride species (Cp 2 TiF) 3 ( 4 ‐trimer; Cp=cyclopentadienyl) because catalyst regeneration using silanes is endergonic . Recently, the method was used as a synthetic tool to synthesize functionalized aromatic compounds inaccessible by conventional routes in high regioselectivity…”

“…The restings tate of this catalytic process is the trimeric metal fluorides pecies( Cp 2 TiF) 3 (4-trimer;C p= cyclopentadienyl) because catalyst regenerationu sing silanes is endergonic. [8] Recently,t he method wasu sed as as ynthetic tool to synthesize functionalized aromatic compoundsi naccessible by conventional routes in high regioselectivity. [9] Theoretical studies on group 4d 1 -metallocene-catalyzed HDF are unavailable but some theoretical studies on CÀF bond-activation reactions of archetypical fluoroarenes C 6 F 6 and C 6 F 5 Hw ith group 4d 0 -metal hydride systems have been published.…”