Oligomeric Chain Structures of Substituted (Dialkylaminomethyl)cyclopentadienyllithium Compounds: Structural Variations Characterized by Different Involvement of the Internal Amino Donor Ligands in the Solid State

Abstract

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.