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 [C 5 H 4 −CMe 2 NMe 2 ]Li reagent with CpZrCl 3 gave the complex [(η 5 -C 5 H 4 −CMe 2 NMe 2 )CpZrCl 2 ] ( 2 ). Its treatment with 2 molar equiv of methyllithium furnished [(η 5 -C 5 H 4 −CMe 2 NMe 2 )CpZr(CH 3 ) 2 ] ( 3 ). Complex 3 reacted with B(C 6 F 5 ) 3 by methyl group transfer. The in situ [Zr] + −CH 3 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−CH 3 hydrogen atom was abstracted. Complex 4 was stabilized by the addition of 1 equiv of the alkyl isocyanide RN⋮C (R = CMe 3, n -C 4 H 9, −CMe 2 CH 2 CMe 3 ) to yield the respective adducts 5 . The complex [(η 5:η 2 ( C, N )-C 5 H 4 −CMe 2 N( CH 2 )CH 3 )CpZr(κ C -C⋮N−CMe 3 )] + [CH 3 B(C 6 F 5 ) 3 ] - ( 5a ) was characterized by an X-ray crystal structure analysis. It exhibits an η 2 -coordination of the pendant formaldiminium moiety to zirconium ( d (Zr−N) = 2.318(7) Å, d (Zr−C) = 2.272(7) Å). The (η 2 -R 2 NCH 2 )Zr moiety shows a characteristic 15 N NMR chemical shift (δ −376 ppm), deshielded by ca. Δδ ≈ −40 ppm relative to the 15 NMe 2 NMR resonance found for 3 . Complex 4 reacts with butadiene or isoprene by insertion into the Zr−CH 2 bond of the (η 2 -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 15 N NMR feature was found at δ −357 ppm. The complex [(η 5 -C 5 H 4 −CMe 2 NMe 2 ) 2 Zr(CH 3 ) 2 ] reacts analogously: upon treatment with B(C 6 F 5 ) 3 in a 1:1 ratio, CH 3 is transferred from zirconium to boron, and 1 equiv of methane is liberated to give complex 8 ( 15 N 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 (η 2 -R 2 NCH 2 )Zr + three-membered-ring system, formed by C−H activation, and a κN-coordinated intact pendant −CMe 2 −NMe 2 group. The latter is displaced upon the addition of tert -butyl isocyanide to yield the (κ C -isonitrile)(η 2 -formaldiminium)metallocene cation complex 9 .