Synthesis, characterization and reactivity study of aluminum compounds incorporating bi- and tri-dentate pyrrole-piperazine ligands

Substituted bi- and tri-dentate pyrrole-piperazine precursors, C₄H₃NH-[2-CH₂N(CH₂CH₂)₂NPh] (1) and C₄H₂NH-{2,5-[CH₂N(CH₂CH₂)₂NPh]₂} (2) are synthesized and the corresponding Al derivatives are afforded systematically in moderate yield. Reacting one equivalent of 1 with AlMe₃ in toluene generates an aluminum dimethyl compound, AlMe₂{C₄H₃N-[2-CH₂N(CH₂CH₂)₂NPh]} (3) in satisfactory yield. Similarly, the reaction of two equivalents of 1 and AlMe₃ affords [AlMe{C₄H₃N[2-CH₂N(CH₂CH₂)₂NPh]}₂] (4), which on repeating recrystallization in dichloromethane produces [AlMe{C₄H₃N[2-CH₂N(CH₂CH₂)₂NPh]}₂[NH(CH₂CH₂)₂NPh]] (4a) in small quantity. Furthermore, reactions of 3 with one equivalent of 1,3-diphenylpropane-1,3-dione and 2,6-diisopropylphenol in toluene results in the aluminum methyl compounds, [AlMe(PhCOCHCOPh){C₄H₃N[2-CH₂N(CH₂CH₂)₂NPh]}] (5) and [AlMe(O-2,6-ⁱPr₂C₆H₃){C₄H₃N[2-CH₂N(CH₂CH₂)₂NPh]}] (6), respectively. Interestingly on fractional recrystallization, compound 6 affords the μ²-oxo bridged dialuminum compound, [{Al(O-2,6-ⁱPr₂C₆H₃){C₄H₃N[2-CH₂N(CH₂CH₂)₂NPh]}}₂(μ-O)] (6a) while exposed to air. Additionally, the combination of the tridentate pyrrole ligand 2 and one equivalent of AlMe₃ in toluene produces [AlMe₂{C₄H₂N{2,5-[CH₂N(CH₂CH₂)₂NPh]₂}}] (7). Compounds 3, 4a, 5, and 6a are subjected to X-ray diffraction analysis to ensure the geometry around the Al atom. Overall, the influences of the new pyrrole-piperazine incorporated N-donor precursors on the structural factors, reactivity with small organic molecules and the associated mechanistic details are discussed.