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 [C 5 H 4 CH(C⋮CCH 3 )N(CH 3 ) 2 ]Li ( 10 ). Likewise, the addition of p -tolyllithium, n -butyllithium, or phenyllithium resulted in the analogous substituted lithium cyclopentadienide systems [C 5 H 4 CH(R)N(CH 3 ) 2 ]Li, 11 (R = p -tolyl), 12 (R = C 4 H 9 ), 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 [C 5 H 4 CH(R)N(CH 3 ) 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 η 5 -coordinated to the Cp ligand and κN-bonded to the −N(CH 3 ) 2 substituent of the next ligand (Li−N 2.181(7) Å), thereby constructing an oligomeric chain of a (THF)Li(η 5 -Cp)(L-κN) type. The compound 11 crystallizes without incorporation of solvent. Here the oligomeric chain is constructed by η 5 -Cp coordination to lithium and again κN-bonding of the −N(CH 3 ) 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 η 5 -coordinated to one face of the C 5 H 4 ring system, whereas another lithium atom in the oligomeric chain structure is bonded in a η 2 -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 η 5 -coordinated in a metallocene fashion to two cyclopentadienides, whereas the adjacent Li2 atom is coordinated to two −N(CH 3 ) 2 groups, assisted by a weaker η 2 - to η 4 -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 [C 5 H 4 CH(R)NR 2 ‘] - ligand is thus suited to experimentally realize a whole family of novel oligomeric (CpLi) n structural types.