The Lewis acid tris(pentafluorophenyl)borane adds to the (butadiene)group 4 metallocenes 1a−d (metallocene = Cp2Zr, Cp2Hf, (MeCp)2Zr, (Me3CCp)2Zr) to give the metallocene−(μ-C4H6)−borate−betaine complexes 2a−d. (Isoprene)zirconocene (1e) and (2-phenylbutadiene)zirconocene (1f) add the B(C6F5)3 reagent regioselectively at the carbon atom C-4 to give the complexes 2e and 2f, respectively. The complexes 2 all show a pronounced M···F−C interaction with one of the six ortho-B(C6F5)3 fluorine atoms. The resulting metallacyclic structures were characterized by X-ray diffraction of the complexes 2c and 2e (Zr···F ≈ 2.40 Å, angle Zr−F−C ≈ 140°). The bridging fluorine atom of the complexes in solution is characterized by an extreme upfield shift of its 19F NMR resonance (δ ≈ −210 to −220 ppm) relative to the signals of the remaining five o-F resonances of the B(C6F5)3 moiety (average δ ≈ −135 ppm). The 19F NMR spectra of the complexes 2 are dynamic even in the noncoordinating solvent toluene-d 8. All six o-fluorine signals equilibrate with coalescence temperatures around 240 K at 564 MHz to give a single resonance signal at high temperature. This fluorine equilibration process of the −B(C6F5)3 end of the metallocene−borate−betaine complexes 2 is very likely to proceed via a rate determining cleavage of the coordinative M···F−C interaction. From the activation barrier of this process, obtained from the dynamic fluorine NMR spectra, Zr···F bond dissociation energies of ca. 8.5 kcal/mol were estimated for the complexes 2. This magnitude of the M···F−C bond dissociation energy makes the internal fluorocarbon coordination a very suitable tool for protecting active electrophilic metal catalyst centers. The Zr···F−C bond of the complexes 2 is cleaved by the addition of the donor solvent THF with formation of acyclic 1,2-η2-allyl metallocene complexes.

Treatment of the ansa-metallocene complex [Me 2 Si(C 5 H 4 ) 2 ]Zr(butadiene) (s-cis-/s-trans-4) with B(C 6 F 5 ) 3 yields the corresponding ansa-zirconocene[C 4 H 6 B(C 6 F 5 ) 3 ] betaine 6. Complex 6 is a homogeneous single component Ziegler catalyst that actively polymerizes ethene and propene, respectively. With the olefins ethene, propene, 1-butene, 1-pentene, 1-hexene, or 1,5-hexadiene complex 6 undergoes a stoichiometric insertion reaction at -20 °C to generate the metallacyclic carbon-carbon coupling products 9a-9f, which feature an internal C4dC5 alkene coordination to the metal center and an intramolecular C6-Zr ion pair interaction. The rate of the overall 1-alkene insertion process 6 f 9 (k chem ) was measured, and the observed rate constant of the degenerate allyl inversion process of the starting material (6 f ent-6, k m(obs) ). This allows for a mathematical kinetic deconvolution of the two-step reaction sequence, namely the initial alkene addition process to 6 to generate the reactive (π-alkene)(σ-allyl)metallocene-type intermediate 8 (rate of formation (k 1 [alkene], rate of alkene dissociation: k -1 ) and the subsequent insertion reaction 8 f 9 (k 2 ) to give k 1 ()). This procedure quantitatively determines the two transition states involved. In each case of the 6 f 9b-f series the second transition state is higher than the first one: a general energy profile is observed in which the actual insertion step is rate-determining and is preceded by the alkene addition/alkene dissociation preequilibrium. For example, the rate of 1-pentene dissociation at the stage of the intermediate 8 to reform the starting material 6 is ∼70 times higher than the competing actually product forming alkene insertion reaction to yield 9d. The difference of transition-state energies ranges from ∆∆G q 2 ) 1.7 ( 0.4 kcal mol -1 for 1-butene insertion to ∆∆G q 2 ) 2.1 ( 0.4 kcal mol -1 for 1-pentene and 1-hexene insertion, respectively. The kinetic analysis of the alkene insertion reaction at the single component "constrained geometry" catalyst [Me 2 Si(C 5 H 4 )N t Bu]Zr[C 4 H 6 B(C 6 F 5 ) 3 ] 11 was carried out analogously. B(C 6 F 5 ) 3 addition to [Me 2 Si(C 5 H 4 )N t Bu]Zr(butadiene) 10 yields a 1.8:1 mixture of the stereoisomeric betaines 11A/ 11B ("supine/prone" orientation of the ligand). The 11A/11B complex mixture actively polymerizes ethene. At low temperature the 11A/11B mixture reacts stoichiometrically with the series of olefins listed above to give the mono-alkene insertion products 14a-f, each found in solution as a single diastereoisomer. The kinetic analysis shows an even more pronounced alkene addition/alkene dissociation preequilibrium step (k -1 /k 2 ≈ 130 for 1-pentene) followed by rate-determining insertion (∆∆G q 2 ranging from +2.2 to +3.0 kcal mol -1 ). Reaction profiles featuring the actual alkene insertion step as the kinetically controlling activation barrier could be characteristic for group 4 metallocene Ziegler catalysts and related systems.

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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