Abstract
(Alkenyl)zirconocene chlorides Cp2Zr(Cl)(CHCHR) (4, R = H, phenyl, n-butyl, or cyclohexyl) react with “Cp2Zr” generated from various precursors to yield the neutral dinuclear alkenyl-bridged bis(metallocene) complexes 6 [Cp2Zr(μ-Cl)(μ-η1:η2-CHCHR)ZrCp2]. The complexes 6 are regioselectively protonated by treatment with (HNMe2Ph+)(BPh4-) (8a) to yield the unusually structured products 9 [Cp2Zr(μ-Cl)(μ-η1:η2-C1H2C2HR)ZrCp2+], which exhibit an unsymmetrically bridged hydrocarbyl ligand containing a novel type of a hypercoordinated carbon center (C1) inside the rigid organometallic framework. From the NMR analysis and an X-ray crystal structure determination of the example 10c [(MeCp)2Zr(μ-Cl)(μ-C1H2C2H-n-butyl)Zr(MeCp)2+], it is evident that the hypercarbon atom C1 is coordinated to four close neighboring atoms (Zr1, Zr2, C2, and H1a) in a distorted square-planar arrangement with the remaining C1−H1b bond being oriented perpendicular to it. H1b thus marks the apex of a distorted square pyramid; the hypercarbon atom C1 is located in the center of the basal plane. This unusual structural coordination geometry around C1 is determined by the stereoelectronic features of the two adjacent group 4 bent metallocene units. From the dynamic features of complex 9a (R = H), a stabilization energy of ca. 10 kcal mol-1 is estimated for the uncommon coordination mode of C1, which is favored here, relative to a “normal” sp3-hybridized structure in a C2v-symmetric metallacyclic framework. Part of the pronounced thermodynamic stabilization of this unusual pentacoordinate carbon geometry originates from the strong α-agostic Zr···H1a−C1 interaction in the basal plane. From the monodeuterated derivative 9a-C1HD, an energy difference of 220 cal mol-1 between D and H favoring the bridging position was determined.
Published Version
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