Mechanistic aspects of the reactions of bis(pentafluorophenyl)borane with the dialkyl zirconocenes Cp2ZrR2 (R = CH3, CH2SiMe3, and CH2C6H5)

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[(mu-H)(2)B(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(mu-CH2)[(mu-H)B(C6F5)(2)], 2, and Cp2Zr{eta(3)-CH(C6H5)[(mu-H)B(C6F5)(2)]}, 4. The latter compound is formed cleanly in 92% 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 - hydride-replacement sequence rather than a concerted, four-centered sigma-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) ((3)J(HH) = 12.82 +/- 0.05 Hz) is treated with excess HB(C6F5)(2), producing threo-(C6F5)(2)B-CH(D)CH(D)-t-C4H9 ((3)J(HH) = 5.00 +/- 0.05 Hz). Further experiments reveal a H/D scrambling process involving the borane proton and the C-alpha-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 H/D 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.