Evidence for α-Nitrogen Participation in the Internal C−H Activation Reaction at ((Dimethylamino)methyl)cyclopentadienyl-Derived Methylzirconocene Cations

Abstract

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 [(η5-C5H4−CMe2NMe2)CpZrCl2] (2). Its treatment with 2 molar equiv of methyllithium furnished [(η5-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 methane 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, −CMe2CH2CMe3) to yield the respective adducts 5. The complex [(η5:η2(C,N)-C5H4−CMe2N(CH2)CH3)CpZr(κC-C⋮N−CMe3)]+[CH3B(C6F5)3]- (5a) was characterized by an X-ray crystal structure analysis. It exhibits an η2-coordination of the pendant formaldiminium moiety to zirconium (d(Zr−N) = 2.318(7) Å, d(Zr−C) = 2.272(7) Å). The (η2-R2NCH2)Zr moiety shows a characteristic 15N NMR chemical shift (δ −376 ppm), deshielded by ca. Δδ ≈ −40 ppm relative to the 15NMe2 NMR resonance found for 3. Complex 4 reacts with butadiene or isoprene by insertion into the Zr−CH2 bond of the (η2-formaldiminium)Zr moiety to form the metallacyclic (π-allyl)metallocene complexes 6a,b. The X-ray crystal structure analysis of 6b shows a close Zr−N contact at 2.491(4) Å. A corresponding 15N NMR feature was found at δ −357 ppm. The complex [(η5-C5H4−CMe2NMe2)2Zr(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 (15N NMR signals at δ −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 (η2-R2NCH2)Zr+ three-membered-ring system, formed by C−H activation, and a κN-coordinated intact pendant −CMe2−NMe2 group. The latter is displaced upon the addition of tert-butyl isocyanide to yield the (κC-isonitrile)(η2-formaldiminium)metallocene cation complex 9.