Cyclopentadienylaluminum donor–acceptor complexes – Molecular and supramolecular structure

Lewis acid–base complexes of cyclopentadienylaluminum derivatives MexCp3−x Al (x = 0–2) and trimethylaluminum with selected aromatic amines (L): dmap = 4-dimethylaminopyridine, py–Me = 4-methylpyridyne, were synthesized and characterized by 1H, 13C, 27Al NMR: Cp3Al · dmap (1), Cp3Al · py–Me (2), MeCp2Al · dmap (3), MeCp2Al · py–Me (4), Me2CpAl · dmap (5), Me2CpAl · py–Me (6), Me3Al · py–Me (7). 1H NMR studies of 3–6 revealed small amounts of the ligand redistribution products. The crystal structures of 1, 2 and 3 were determined by single X-ray diffraction studies. The compounds 1, 2 and 3 are monomeric with Cp ligands bonded to the aluminum center in η1(σ), η1(π) manner. The change of Cp–Al bond character from η1(π) to η1(σ) was found to reasonable correlate with the aromaticity of Cp− ligand described by HOMA index. Analysis of close intra- and intermolecular contacts showed presence of CH⋯π interactions leading to the formation of 2-D supramolecular networks. It was found that these interactions impact on the coordination sphere of aluminum and the conformation of Cp ring.