Evidence for α-nitrogen participation in the internal C-H activation reaction at ((dimethylamino)methyl) cyclopentadienyl-derived methylzirconocene cations

Treatment of 6-(dimethylamino)-6-methylfulvene (1) with methyllithium followed by the reaction of the resulting [C5H4-CMe2NMe2]Li reagent with CpZrCl3 gave the complex [(eta(5)-C5H4-CMe2NMe2)CpZrCl2] (2). Its treatment with 2 molar equiv of methyllithium furnished [(eta(5)-C5H4-CMe2NMe2)CpZr(CH3)(2)] (3). Complex 3 reacted with B(C6F5)(3) by methyl group transfer. The in situ [Zr](+)-CH3 cation system generated in this manner proved to be unstable under the reaction conditions and instantaneously eliminated me thane with formation of 4. In the course of this reaction a N-CH3 hydrogen atom was abstracted Complex 4 was stabilized by the addition of 1 equiv of the alkyl isocyanide RN=C (R = CMe3, n-C4H9, -CMe2-CH2CMe3) to yield the respective adducts 5. The complex [(eta(5):eta(2)(C,N)-C5H4-CMe2N(=CH2)-CH3)CpZr(kappa C-C=N-CMe3)](+)[CH3B(C6F5)(3)](-) (5a) was characterized by an X-ray crystal structure analysis. It exhibits an eta(2)-coordination of the pendant formaldiminium moiety to zirconium (d(Zr-N) = 2.318(7) Angstrom, d(Zr-C) = 2.272(7) Angstrom). The (eta(2)-R2NCH2)Zr moiety shows a characteristic N-15 NMR chemical shift (delta -376 ppm), deshielded by ca. Delta delta approximate to -40 ppm relative to the (NMe2)-N-15 NMR resonance found for 3. Complex 4 reacts with butadiene or isoprene by insertion into the Zr-CH2 bond of the (eta(2)-formaldiminium)Zr moiety to form the metallacyclic (pi-allyl)metallocene complexes 6a,b. The X-ray crystal structure analysis of 6b shows a close Zr-N contact at 2.491(4) Angstrom. A corresponding N-15 NMR feature was found at delta -357 ppm. The complex [(eta(5)-C5H4-CMe2NMe2)(2)Zr(CH3)(2)] reacts analogously: upon treatment with B(C6F5)(3) in a 1:1 ratio, CH3 is transferred from zirconium to boron, and 1 equiv of methane is liberated to give complex 8 (N-15 NMR signals at delta -357 and -372 ppm). Complex 8 was also characterized by X-ray diffraction. It shows coordination of both nitrogen atoms to zirconium: i.e., the presence of a (eta(2)-R2NCH2)Zr+ three-membered-ring system, formed by C-H activation, and a kappa N-coordinated intact pendant -CMe2-NMe2 group. The latter is displaced upon the addition of tert-butyl isocyanide to yield the (kappa C-isonitrile)(eta(2)-formaldiminium)metallocene cation complex 9.