5271-39-6Relevant articles and documents
[Cp2TiCH2CHMe(SiMe3)]+, an alkyl-titanium complex which (a) exists in equilibrium between a β-agostic and a lower energy γ-agostic isomer and (b) undergoes hydrogen atom
Dunlop-Briere, Alexandre F.,Baird, Michael C.,Budzelaar, Peter H. M.
, p. 17514 - 17527 (2013)
The compound [Cp2Ti(Me)(CD2Cl2)][B(C 6F5)4] reacts with trimethylvinylsilane (TMVS) to form the 1,2-insertion product [Cp2TiCH2CHMe(SiMe 3)]+ (III), which exists in solution as equilibrating β- and γ-agostic isomers. In addition, while free rotation of the β-methyl group results in a single, averaged γ-H atom resonance at higher temperatures, decoalescence occurs below ～200 K, and the resonance of the γ-agostic hydrogen atom at δ ～ -7.4 is observed. Reaction of [Cp2Ti(CD3)(CD2Cl2)]+ with TMVS results in the formation of [Cp2TiCH2CH(CD 3)(SiMe3)]+, which converts, via reversible β-elimination, to an equilibrium mixture of specifically [Cp 2TiCH2CH(CD3)(SiMe3)]+ and [Cp2TiCD2CD(CH3)(SiMe3)] +. Complementing this conventional process, exchange spectroscopy experiments show that the β-H atom of [Cp2TiCH 2CHMe(SiMe3)]+ undergoes exchange with the three hydrogen atoms of the β-methyl group (β-H/γ-H exchange) but not with the two α-H atoms. This exchange process is completely shut down when [Cp2TiCH2CH(CD3)(SiMe 3)]+ is used, suggesting an H/D kinetic isotope effect much larger (apparently >16 000) than the maximum possible for an over-the-barrier process. It is proposed that β-H/γ-H exchange is facilitated by quantum mechanical proton tunnelling in which a hydrogen atom of the 2-methyl group of the alkene-hydride deinsertion product [Cp 2TiH{CH2-CMe(SiMe3)}]+ undergoes reversible exchange with the hydride ligand via the allyl dihydrogen species [Cp2TiH2{(η3-CH2C(SiMe 3)CH2}]+. Complementing these findings, DFT calculations were carried out to obtain energies and NMR parameters for all relevant species and thence to obtain better insight into the agostic preference(s) of complex III and the observed exchange processes. In all cases where comparisons between experimental and calculated data were possible, agreement was excellent.
Russian-Doll-Like Molecular Cubes
Liu, Die,Li, Kaixiu,Chen, Mingzhao,Zhang, Tingting,Li, Zhengguang,Yin, Jia-Fu,He, Lipeng,Wang, Jun,Yin, Panchao,Chan, Yi-Tsu,Wang, Pingshan
, p. 2537 - 2544 (2021)
Nanosized cage-within-cage compounds represent a synergistic molecular self-assembling form of three-dimensional architecture that has received particular research focus. Building multilayered ultralarge cages to simulate complicated virus capsids is beli
Rare-Earth-Metal Pentadienyl Half-Sandwich and Sandwich Tetramethylaluminates–Synthesis, Structure, Reactivity, and Performance in Isoprene Polymerization
Barisic, Damir,Buschmann, Dennis A.,Schneider, David,Maichle-M?ssmer, C?cilia,Anwander, Reiner
supporting information, p. 4821 - 4832 (2019/03/13)
Targeting the synthesis of rare-earth-metal pentadienyl half-sandwich tetramethylaluminate complexes, homoleptic [Ln(AlMe4)3] (Ln=Y, La, Ce, Pr, Nd, Lu) were treated with equimolar amounts of the potassium salts K(2,4-dmp) (2,4-dmp=2,4-dimethylpentadienyl), K(2,4-dipp) (2,4-dipp=2,4-diisopropylpentadienyl), and K(2,4-dtbp) (2,4-dtbp=2,4-di-tert-butylpentadienyl). The reactions involving the larger rare-earth-metal centers lanthanum, cerium, praseodymium, and neodymium gave selectively the desired half-sandwich complexes [(2,4-dmp)La(AlMe4)2], [(2,4-dipp)La(AlMe4)2], and [(2,4-dtbp)Ln(AlMe4)2] (Ln=La, Ce, Pr, Nd) in high crystalline yields. Smaller rare-earth-metal centers yielded preferentially the sandwich complexes [(2,4-dmp)2Ln(AlMe4)] (Ln=Y, Lu) and [(2,4-dipp)2Y(AlMe4)]. Activation with fluorinated borate/borane co-catalysts gave highly active catalyst systems for the fabrication of polyisoprene, displaying molecular weight distributions as low as Mw/Mn=1.09 and a maximum cis-1,4 selectivity of 90.4 %. The equimolar reaction of half-sandwich complex [(2,4-dtbp)La(AlMe4)2] with B(C6F5)3 led to the isolation and full characterization of the single-component catalyst {{(2,4-dtbp)La[(μ-Me)2AlMe(C6F5)]}[Me2Al(C6F5)2]}2. The reaction of the latter complex with 10 equivalents of isoprene could be monitored by 1H NMR spectroscopy. Also, a donor-induced aluminato/gallato exchange was achieved with [(2,4-dtbp)La(AlMe4)2] and GaMe3(OEt2) leading to [(2,4-dtbp)La(GaMe4)2].
Nucleophilicity of Alkyl Zirconocene and Titanocene Precatalysts, and Kinetics of Activation by Carbenium Ions and by B(C6F5)3
Berionni, Guillaume,Kurouchi, Hiroaki,Eisenburger, Lucien,Mayr, Herbert
supporting information, p. 11196 - 11200 (2016/08/03)
Kinetics of activation of methyl and benzyl metallocene precatalysts by benzhydrylium ions, tritylium ions, and triarylborane B(C6F5)3were measured spectrophotometrically. The rate constants correlate linearly with the electrophilicity parameter E of the benzhydrylium and tritylium ions employed, allowing us to determine the σ-nucleophilicities of the metal–carbon bond of several zirconocenes and titanocenes. Bridging, substitution, metal, and ligand effects on the rates of metal–alkyl bond cleavage (M=Zr, Ti) were studied and structure–reactivity correlations were used to predict the kinetics of generation of metallocenium ions pairs, which are active catalysts in polymerization reactions and are highly electrophilic Lewis acids in frustrated Lewis pair catalysis.
