208184-25-2Relevant academic research and scientific papers
Syntheses, structures, and properties of phenyltrihydroborate complexes of zirconocene and titanocene
Liu, Fu-Chen,Chen, Jung-Hua,Chen, Shou-Chon,Chen, Ko-Yu,Lee, Gene-Hsian,Peng, Shie-Ming
, p. 291 - 300 (2007/10/03)
The phenyltrihydroborate complexes, Cp2ZrCl{(μ-H) 2BHPh}, 1, and Cp2Zr{(μ-H)2BHPh}2 ? (1/2 toluene), 2, were prepared from the reactions of Cp 2ZrCl2 with one and two moles of LiBH3Ph. The Zr-H-B bonds in 2 are stable under vacuum at 100°C for hours without significant decomposition. An inductive effect has been proposed for this strong interaction. This hydrogen bridge bond can be broken upon reacting with the Lewis base N(C2H5)3 to produce (C 2H5)3N ? BH2Ph and the zirconium hydride compound Cp2ZrH{(μ-H)2BHPh}, 3. Compound 3 also can be prepared from the reaction of Cp2ZrHCl with LiBH3Ph. The reaction of 1 with the Lewis acid B(C6F 5)3 is solvent dependent, the metathesis product Cp 2ZrCl{(μ-H)2B(C6F5)2}, 4, was formed in the toluene solution, whereas the ionic complex [Cp 2ZrCl(OEt2)][HB(C6F5)3], 5, was isolated from the ether solution. The reaction of titanocene dichloride, Cp2TiCl2, with LiBH3Ph produced a 17-electron, paramagnetic complex, Cp2Ti{(μ-H)2BHPh}, 6. Single crystal X-ray structures of 1, 2, 3, 4, 5, and 6 were also determined. A coplanar structure of the four bridge hydrogens in 2 was observed.
Mechanistic aspects of the reactions of bis(pentafluorophenyl)borane with the dialkyl zirconocenes Cp2ZrR2 (R = CH3, CH2SiMe3, and CH2C6H5)
Spence, Rupert E.V.H.,Piers, Warren E.,Sun, Yimin,Parvez, Masood,MacGillivray, Leonard R.,Zaworotko, Michael J.
, p. 2459 - 2469 (2008/10/08)
The reactions of bis(pentafluorophenyl)borane with simple dialkyl zirconocenes Cp2ZrR2 (R = CH3, CH2SiMe3, CH2Ph) proceed via initial alkyl/hydride exchange to yield Cp2Zr-(H)R and RB(C6F5)2. Two reaction paths are then followed depending on whether further equivalents of HB(C6F5)2 are present or not. If present, HB(C6F5)2 reacts with the newly formed Zr-H moiety to form dihydridoborate compounds, ultimately yielding Cp2Zr[(μ-H)2B-(C6F5) 2]2, 1, and 2 equiv of RB(C6F5)2. Compound 1 was characterized by X-ray crystallography. In the absence of more HB(C6F5)2, the products of alkyl/hydride exchange react to eliminate RH and produce the borane-stabilized alkylidene compounds Cp2Zr(μ-CH2)[(μ-H)B(C6F5) 2], 2, and Cp2Zr{η3-CH(C6H 5)[μ-H)B(C6F5)2]}, 4. The latter compound is formed cleanly in 92percent yield and was characterized by X-ray crystallography. Mechanistic studies on these reactions involving partially deuterated compounds reveal that the alkyl/hyride exchange process is reversible and takes place via a stepwise alkide-abstraction-hydridereplacement sequence rather than a concerted, four-centered σ-bond metathesis type mechanism. This is most convincingly demonstrated by the observed inversion of stereochemistry observed when erythro-Cp2Zr[CH(D)CH(D)-t-C4H9](Cl) (3JHH = 12.82 ± 0.05 Hz) is treated with excess HB(C6F5)2, producing threo-(C6F5)2B-CH(D)CH(D)-t-C4H 9 (3JHH = 5.00 ± 0.05 Hz). Further experiments reveal a H/D scrambling process involving the borane proton and the Cα-H positions of the zirconium alkyl groups (R = CH3, CH2Ph). For example, treatment of Cp2Zr(CD2C6D5)2 with 1 equiv of HB(C6F5)2 leads to a mixture of isotopomers of 4 and toluene, including C6D5CH3 and C6D5CH2D, suggesting a scrambling process in which the borane engages in multiple contacts with the metallocene reagent prior to alkane elimination. The HTD scrambling event is proposed to involve hydridoborate attack of the remaining alkyl group on the forming metallocene cation as HB(C6F5)2 abstracts the other alkide ligand. The implications of these mechanistic studies within the realms of metallocene activation and metallocene-catalyzed hydroborations are discussed.
