Evidence for alpha-nitrogen participation in the internal C-H activation reaction at ((dimethylamino)methyl) cyclopentadienyl-derived methylzirconocene cations
J. Pflug et al., Evidence for alpha-nitrogen participation in the internal C-H activation reaction at ((dimethylamino)methyl) cyclopentadienyl-derived methylzirconocene cations, ORGANOMETAL, 18(19), 1999, pp. 3818-3826
Treatment of 6-(dimethylamino)-6-methylfulvene (1) with methyllithium follo
wed 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 s
itu [Zr](+)-CH3 cation system generated in this manner proved to be unstabl
e under the reaction conditions and instantaneously eliminated me thane wit
h 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 cryst
al structure analysis. It exhibits an eta(2)-coordination of the pendant fo
rmaldiminium 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 approxim
ate to -40 ppm relative to the (NMe2)-N-15 NMR resonance found for 3. Compl
ex 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)m
etallocene 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 featur
e 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, CH
3 is transferred from zirconium to boron, and 1 equiv of methane is liberat
ed 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 bo
th nitrogen atoms to zirconium: i.e., the presence of a (eta(2)-R2NCH2)Zrthree-membered-ring system, formed by C-H activation, and a kappa N-coordin
ated intact pendant -CMe2-NMe2 group. The latter is displaced upon the addi
tion of tert-butyl isocyanide to yield the (kappa C-isonitrile)(eta(2)-form
aldiminium)metallocene cation complex 9.