Synthesis, structure and DFT calculation of bi- and tridentate ketiminate ligand-assisted aluminum complexes: potential catalyst for styrene oxide–CO2 cycloaddition reaction

Abstract

A series of ketiminate ligands, L¹H–L⁴H, are first synthesized by coupling benzoylacetone with phenylhydrazine, 2-methoxy-5-methylaniline, (+/−)-tetrahydrofurfurylamine, and N-(2-aminoethyl)-piperidine, respectively, in adequate yields. Deprotonation of L¹H–L⁴H with one equivalent of AlMe₃ in toluene generates the aluminum derivatives, L¹AlMe₂ (1), L²AlMe₂ (2), L³AlMe₂ (3) and L⁴AlMe₂ (4). Later, reacting 2 with one equivalent of 2,6-diisopropylphenol in methylene chloride affords the aluminum mono-phenoxide, [L²AlMe(O-2,6-ⁱPrC₆H₃)] (5), which after a recrystallization process produces the dinuclear aluminum derivative, [L²Al(O-2,6-ⁱPrC₆H₃)(μ-OH)]₂·2CH₂Cl₂ (5a). All the precursors and metal derivatives are in good agreement with ¹H and ¹³C NMR spectroscopic results. Furthermore, single crystal X-ray diffraction analysis reveals that in 2 and 4, the central Al atom exists in a distorted tetrahedral and trigonal bipyramidal environment, respectively. Similarly, in 5 the geometry around the Al atom is four-coordinated, whereas in 5a, a doubly hydroxo bridged dinuclear aluminum derivative is formed and the geometry can be elucidated as an edge-sharing bis-trigonal bipyramid. DFT computation was accomplished, which is in accordance with experimental evidence regarding geometrical configuration. Integrating the aluminum derivatives, 1–4, their catalytic activity for the styrene oxide–CO₂ cycloaddition reaction was investigated and the best conversion rate was observed for catalyst 3 at a temperature of 120 °C and low catalyst loading, (0.05 mol%) along with the co-catalyst TBAI (tetra-n-butylammonium iodide).