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 RNtC (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(dCH 2 )-CH 3 )CpZr(κC-CtN-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. ∆δ ≈ -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 (κC-isonitrile)(η 2formaldiminium)metallocene cation complex 9.

Alkyl-and aryllithium reagents add cleanly to the electrophilic carbon center C6 of 6-(N,Ndimethylamino)fulvenes to yield the corresponding substituted cyclopentadienyllithium systems Li[C 5 H 4 -CR 1 R 2 NMe 2 ]. Subsequent treatment with ZrCl 4 ‚2THF gives the corresponding Cp-functionalized zirconocene dichlorides. These were reacted with methyllithium to give the (C 5 H 4 CR 1 R 2 NMe 2 ) 2 Zr(CH 3 ) 2 complexes 11a (R 1 ) R 1 ) CH 3 ) and 11b (R 1 ) CH 3 , R 2 ) Ph), respectively. Treatment of 11 with tris(pentafluorophenyl)borane was carried out to generate the corresponding alkylmetallocene cations (12), which turned out to be unstable under the reaction conditions applied (-20°C) with regard to liberation of 1 equiv of methane by CH activation at a methyl group adjacent to nitrogen and formation of the spiro-metallocene complex systems 13. CH activation may be a major reaction pathway open to alkylzirconocene cation systems under suitable reaction conditions.

Treatment of 6-dimethylaminofulvene (8) with propynyllithium (9a) results in a clean nucleophilic addition at the electrophilic fulvene carbon atom C6 to yield the substituted lithium cyclopentadienide system [C5H4CH(C⋮CCH3)N(CH3)2]Li (10). Likewise, the addition of p-tolyllithium, n-butyllithium, or phenyllithium resulted in the analogous substituted lithium cyclopentadienide systems [C5H4CH(R)N(CH3)2]Li, 11 (R = p-tolyl), 12 (R = C4H9), or 13 (R = Ph), respectively. These four Li−cyclopentadienide systems were characterized by single crystal X-ray structural analyses. This has revealed a series of gradually different oligomeric [C5H4CH(R)N(CH3)2]Li structural aggregate types in the solid state. Compound 10 crystallizes with 1 equiv of THF that is coordinated to Li. The metal is η5-coordinated to the Cp ligand and κN-bonded to the −N(CH3)2 substituent of the next ligand (Li−N 2.181(7) Å), thereby constructing an oligomeric chain of a (THF)Li(η5-Cp)(L-κN) type. The compound 11 crystallizes without incorporation of solvent. Here the oligomeric chain is constructed by η5-Cp coordination to lithium and again κN-bonding of the −N(CH3)2 donor of the adjacent ligand to the metal center. In the solvent-free system 11 the resulting Li−N linkage (2.032(5) Å) is much shorter than in 10. In 12 a beginning coordination of the lithium atom to both its neighboring Cp rings is observed. A Li cation is η5-coordinated to one face of the C5H4 ring system, whereas another lithium atom in the oligomeric chain structure is bonded in a η2-fashion from the other face, assisted by κN-coordination of the attached dimethylamino substituent. Eventually, two very different coordination geometries around lithium are characteristic for the oligomeric chain structure of 13:  atom Li1 is rather symmetrically η5-coordinated in a metallocene fashion to two cyclopentadienides, whereas the adjacent Li2 atom is coordinated to two −N(CH3)2 groups, assisted by a weaker η2- to η4-Cp coordination to the attached Cp ring systems. Supporting quantum chemical calculations have revealed rather shallow minima and easy conversions between these structural types. The [C5H4CH(R)NR2‘]- ligand is thus suited to experimentally realize a whole family of novel oligomeric (CpLi) n structural types.

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