Synthesis and Structural and Chemical Features of Formimidoyl-Functionalized Lithium Cyclopentadienides

Treatment of 6-dimethylaminofulvene (1) with lithium anilide (2a) yields lithium [(N-phenylformimidoyl)cyclopentadienide], 3a, by an anilide addition/dimethylamine elimination reaction sequence. Similar treatment of 1 with the reagents Li[HNAr] [Ar = p-tolyl (2b), −2,6-dimethylphenyl (2c), or −2,6-diisopropylphenyl (2d)] gave the corresponding Li[C5H4CHNAr] products 3b, 3c, and 3d in good yields (80−90% isolated) as THF adducts. Crystallization from THF gave 3d·3THF, which was characterized by X-ray diffraction. It contains monomeric units in the solid state. The lithium atom is coordinated to the imino nitrogen atom and to three THF molecules in a tetrahedral arrangement. The C5H4 unit shows some residual bond alternation. In crystalline 3d it exhibits no contact to the alkali metal. The structural features of 3d can be described by a resonance hybrid of mesomeric structures of iminium-cyclopentadienide (i.e., C5H4CHN(Li·3THF)Ar, 3A) and of fulvenoide character (i.e., C5H4CHN(Li·3THF)Ar, 3B). The structure of 3c·3THF in the solid state is analogous. Complex 3a crystallizes with 1 equiv of THF. It exhibits a cyclodimeric structure (3a·THF)2 in the crystal, where each Li atom is η5-Cp- and κN-coordinated to two different C5H4CHNPh- ligands. The electrostatic nature of the Li−C and Li−N interactions results in very similar structural features of the C5H4CHNAr- ligands in the complexes 3a, 3c, and 3d. The solution structures of these systems were characterized by temperature-dependent 1H and 7Li NMR spectra. The reagents 3a and 3d were employed in the synthesis of the respective bis[1,1‘-(N-aryl)formimidoyl]ferrocenes 4a and 4d (both isolated in ca. 80% yield). The complexes 4a and 4b were characterized by X-ray crystal structure analyses.