Abstract
A series of spiro-phenanthrene-monoketone/OH derivatives (L1–L6) were synthesized and fully characterized with 1H/13C NMR spectroscopy and elemental analyses. By treating ligands with AlMe3, oxygen-bridged binuclear aluminum complexes (Al1–Al6) were isolated and characterized by 1H/13C NMR spectroscopy. The molecular structures of ligands (L2, L4 and L5) and complex Al1 were determined by single crystal X-ray diffraction. In the presence of benzyl alcohol (BnOH), these aluminum complexes demonstrated high efficiency towards the ring-opening polymerization of ε-caprolactone (ε-CL), resulting in PCL in a linear manner with the BnO-end group. In addition, complexes Al1 and Al5 exhibited good catalytic activities even without BnOH. Moreover, complexes Al3 and Al6 with the bulkier substituent of iPr at the ortho-position of the arylamines demonstrated better catalytic activities than the analogs. Moreover, substituents on the backbone also affected catalytic behaviors.
Highlights
Complex D could catalyze the cycloaddition of CO2 with epoxies.12b The anilinotropone-based dimeric aluminum complexes (E) demonstrated high activities in the ring-opening polymerization (ROP) of rac-lactides with the assistance of benzyl alcohol (BnOH).13
It is noteworthy that the resonance of –NH for all aluminum complexes Al1–Al6 still remained, being similar to the reports of the –NH presence in aluminum complexes.8,12a the crystal structures of the organic compounds ligands. CCDC 2063065 (L2), L4 and L5 and aluminum complex Al1 were determined by single crystal X-ray diffraction (Fig. 1–4)
The results showed that there was no polymer observed at room temperature or 30 C when the ROP of 3-CL was conducted at a molar ratio [CL] : [Al] : [BnOH] of [250 : 1 : 1]
Summary
Biodegradable polyesters, such as polylactides (PLA) and polycaprolactone (PCL), have attracted much attention due to their biodegradability, biocompatibility and permeability, and have been extensive biomaterials in the pharmaceutical industries over the past few decades.1 These polyesters are commonly prepared by the ring-opening polymerization (ROP) of cyclic esters, such as 3-caprolactone (3-CL) or lactide, and catalyzed by metal complexes as prevalent catalysts.2 Aluminum complex catalysts3 have appealed with good activities and being nicely characterized by NMR spectra in order to illustrate the coordination mechanism of the ROP of cyclic esters.4 Beyond 3-CL, the stereoselective ROP of rac-lactides was rst achieved using the salen–Al complex catalyst.5 In general, the modi cations ofThe N^O bidentate aluminum complexes (A, Chart 1) with ve-membered ring coordination were veri ed to be in monomeric or dimeric forms that depended on the bulkiness of Al– R,10 but usually formed dimeric ones (B, Chart 1) derived from 2(1,3,5-dithiazinan-5-yl)ethanols,11 as well as other alcohols (C and D, Chart 1).12 Notably, complex D could catalyze the cycloaddition of CO2 with epoxies.12b The anilinotropone-based dimeric aluminum complexes (E) demonstrated high activities in the ring-opening polymerization (ROP) of rac-lactides with the assistance of BnOH.13 Subsequently, dimeric aluminum complexes (F) initiated the ROP of 3-CL14 with good controllability. Scheme 1 Synthesis of the spiro-phenanthrene-mono–OH derivatives L4–L6 and aluminum complexes Al1–Al6. Synthesis and characterization of the ligands (L1–L6) and aluminum complexes (Al1–Al6)
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