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

"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 C2v 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.