Dinuclear zirconium complex bearing a 1,5-bridged-calix[8]arene ligand as an effective catalyst for the synthesis of macrolactones

Abstract

“Macrocyclization of Cycles with a Macrocycle.” The synthesis of a dinuclear zirconium complex bearing a 1,5-m-xylene-diyl-bridged calix[8]arene ligand and tert-butoxide groups, via protonolysis of the calix proligand with two equivalents of zirconium(IV) tert-butoxide, has been accomplished. ¹H, ¹³C and two-dimensional NMR spectroscopy revealed for the complex a conformationally locked solution structure with C_2v symmetry, where the two zirconium atoms lie in close proximity. The elucidated structure has been validated through DFT calculations. The complex has been successfully used as a catalyst in the ring opening polymerization (ROP) of lactones such as rac-lactide (LA), ε-caprolactone (ε-CL) and β-butyrolactone (β-BL). The activities for the ROP of LA compares well with those for the so far reported most active group 4 complexes. Interestingly, the MALDI-TOF-MS investigations on the resulting polymer architecture revealed the highly selective formation of cyclic polymers (70–80 mol%). The cooperation of the intramolecular zirconium atoms has been proved comparing the polymerization results with those obtained using a mononuclear zirconium complex featuring a calix[4]arene ligand.