Synthesis and Crystal Structures of β-Diketonate-Associated Aluminum Complexes as Potential Catalysts for the Ring-Opening Polymerization of ε-Caprolactone

A couple of novel crystalline aluminum(III) derivatives containing tridentate Schiff base ligand (HL) and β-diketones (acetylacetone = acac, benzoylacetone = bnzac, and dibenzoylmethane = dbnz), namely, [Al(L)bnzac] [Al1], [Al(L)dnbz] [Al2], and [Al(L)acac] [Al3], are synthesized and characterized using different spectroscopic techniques and elemental analysis. Single crystal X-ray diffraction analysis of Al2 and Al3 exhibits hexacoordinated geometry around aluminum center atom which is also confirmed using density functional theory (DFT). The ring-opening polymerization (ROP) of caprolactone is evaluated to determine the catalytic potential of the complexes Al1-Al3 in the absence and presence of benzyl alcohol (BnOH). The effect of time and temperature is also examined and found that lesser steric effects of the ancillary ligand causes a higher polymerization rate. Gel permeation chromatography (GPC) is used to determine the molecular weight (M_n & M_w) and dispersity (Đ) values of polycaprolactone. Within first 2 h, Al3 exhibits excellent catalytic activity with 99% conversion at 110 °C (M_nGPC = 1948 gmol^-1, M_wGPC = 2865 gmol^-1, and Đ = 1.47). An end-group study is performed using matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) spectrometry, and ¹H NMR spectral analysis. Also, PCL is characterized using elemental, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) analyses. First-order kinetics are found in the monomer aligned with the activated monomer mechanism for the catalysts.