Synthesis and characterization of aluminum complexes incorporating Schiff base ligands derived from pyrrole-2-carboxaldehyde

Abstract

The reaction of 1,2-Ethylendiamine-N,N′-bis(1H-pyrrol-2-yl)methylene (L¹H), 1,2-benzenediamine-N,N′-bis(1H-pyrrol-2-yl)methylene (L²H) and 3,4-Dimethyl-1,2-benzenediamine-N,N′-bis(1 H-pyrrol-2-yl)methylene (L³H) with one equivalent of AlR₂R′ (R = alkyl, R′ = alkyl, Cl) in toluene afforded the monometallic complexes [LAlR′] (1–12). All the complexes were characterized by analytical and spectroscopic methods. Compounds 1, 3, 4, 6, 7, and 11 were characterized bya single-crystal X-ray structural analyses. Due to the rigidity imposed by the ligand, 1, 3 and 4, with an ethylene “backbone” connecting the two nitrogens, adopt a severely distorted tbp geometry (av. χ = 0.68). On the other hand 6, 7 and 11, with a phenylene backbone, adopt an intermediate tbp-sqp geometry (av. χ = 0.43). The study of the X-ray crystal structures suggests that, like group 13 Salen complexes, flexible bridges between the two imine groups favor a tbp geometry while more rigid backbones, like phenylene rings, lead to an intermediate sqp and tbp geometry. Based on the χ geometrical parameter for previously reported aluminum complexes with Schiff base ligands and those presented herein, it is possible to propose that upon the coordination geometries of five coordinated aluminum complexes are less tbp distorted (χ ≈ 1) the catalytic activity on ROP increases.