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 Cp 2 ZrR 2 (R = CH 3, CH 2 SiMe 3, CH 2 Ph) proceed via initial alkyl/hydride exchange to yield “Cp 2 Zr(H)R” and RB(C 6 F 5 ) 2 . Two reaction paths are then followed depending on whether further equivalents of HB(C 6 F 5 ) 2 are present or not. If present, HB(C 6 F 5 ) 2 reacts with the newly formed Zr−H moiety to form dihydridoborate compounds, ultimately yielding Cp 2 Zr[(μ-H) 2 B(C 6 F 5 ) 2 ] 2, 1, and 2 equiv of RB(C 6 F 5 ) 2 . Compound 1 was characterized by X-ray crystallography. In the absence of more HB(C 6 F 5 ) 2, the products of alkyl/hydride exchange react to eliminate RH and produce the borane-stabilized alkylidene compounds Cp 2 Zr(μ-CH 2 )[(μ-H)B(C 6 F 5 ) 2 ], 2, and Cp 2 Zr{η 3 -CH(C 6 H 5 )[(μ-H)B(C 6 F 5 ) 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 σ-bond metathesis type mechanism. This is most convincingly demonstrated by the observed inversion of stereochemistry observed when erythro- Cp 2 Zr[CH(D)CH(D)- t -C 4 H 9 ](Cl) ( 3 J HH = 12.82 ± 0.05 Hz) is treated with excess HB(C 6 F 5 ) 2, producing threo -(C 6 F 5 ) 2 B−CH(D)CH(D)- t -C 4 H 9 ( 3 J HH = 5.00 ± 0.05 Hz). Further experiments reveal a H/D scrambling process involving the borane proton and the C α −H positions of the zirconium alkyl groups (R = CH 3, CH 2 Ph). For example, treatment of Cp 2 Zr(CD 2 C 6 D 5 ) 2 with 1 equiv of HB(C 6 F 5 ) 2 leads to a mixture of isotopomers of 4 and toluene, including C 6 D 5 CH 3 and C 6 D 5 CH 2 D, 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(C 6 F 5 ) 2 abstracts the other alkide ligand. The implications of these mechanistic studies within the realms of metallocene activation and metallocene-catalyzed hydroborations are discussed.