Structural and Spectroscopic Evidence for Intramolecular Agostic M···H−C and Dative Zr←F−C(ortho) Interactions in the Zwitterionic Metal Complexes [(C5H4)SiMe2(N-t-Bu)]M(+)(μ-C4H6)B(−)(C6F5)3, M = Ti, Zr

The reactions of equimolar amounts of [(C5H4)SiMe2(N-t-Bu)]M(C4H6) (M = Ti, Zr) and B(C6F5)3 proceed cleanly with the formation of the zwitterionic complexes [(C5H4)SiMe2(N-t-Bu)]M(+)(μ-C4H6)B(−)(C6F5)3. X-ray structural analyses of these novel compounds indicate that the π-allyl unit within the asymmetrical bridging cis-butadiene moiety adopts the Z-configuration. The structure of the zwitterionic Ti-betaine complex is stabilized by a pair of Ti(+)···H−CB(−) agostic interactions, whereas the structure of the Zr analogue features a dative Zr←F−C(ortho) interaction with a Zr−F distance of 2.324(1) Å and a single Zr(+)···H−CB(−) agostic interaction. Variable-temperature 19F NMR data show that the broad ortho-F resonance for the three freely rotating perfluorophenyl groups at room temperature separates upon cooling (193 K) to produce six distinct signals at δ −124.8, −129.6, −131.8, −133.4, −135.7, and −189.2, consistent with the coordination of an ortho-F atom to the electrophilic Zr center. From a line shape analysis the free energy barrier for disruption of this Zr←F−C(ortho) interaction is estimated to be ca. 9.8 kcal/mol (T c = 223 K).