Synthesis, structure and catalytic capability of two yttrium diphenylglycinate complexes

Reaction of [Y(OiPr)3] with 2,2′-diphenylglycine (LH3) in toluene afforded, following work-up (MeCN), the complex [Y3(OH)(LH2)9Na(NCMe)3]·12MeCN (1·12MeCN). A molecular structure determination reveals a unique cluster with a central Y3(OH) unit bearing 9 amino/carboxylate ligands derived from LH3, and in addition incorporation of the drying agent sodium. Conducting the reaction in Et2O with repeated removal of solvent led, following work-up (MeCN), to the complex [Y6O2(OH)2(LH2)12]·12MeCN (2·12MeCN). Complex 2·12MeCN contains three unique Y atoms coordinated by LH2, hydroxide and an oxide forming a cluster containing six Y3+ ions. Preliminary catalytic screening of 1·12MeCN, 2·12MeCN and [Y(OiPr)3] reveals activity for the co-polymerization of propylene oxide and CO2, with turnover numbers (TONs) <1, however in the presence of bis(triphenylphosphine)iminium chloride (PPNCl), TOFs are significantly improved, particularly for 1 (TOF 11.56) and 2 (TOF 10.67). In all cases, oligomeric products were formed with relatively small molecular weights (Mn ≤ 1670 Da) and with low dispersity (Đ ≤ 1.28). In the case of the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) or δ-valerolactone (δ-VL), the systems perform best as melts and afford low to moderate molecular weight products (≤ 6840 Da for PCL and ≤8910 Da for PVL) with good control (Đ ≤ 1.35). Depending on the conditions, cyclic or linear products (with H/OH end groups) were observed. Kinetic studies for ε-CL (over 3 h) revealed the rate order 2 > 1 ≈ [Y(OiPr)3], whilst for δ-VL, [Y(OiPr)3] exhibited the fastest rate over the initial 100 min, but at 60 min its rate significantly slowed and at ca. 120 min was surpassed by complex 1. The catalytic depolymerization of polyethylene terephthalate (PET) using 1 and 2 has also been investigated, with the system employing 1 performing better in terms of conversion and bis(2-hydroxyethyl) terephthalate (BHET) yield. However, these systems exhibited no activity for ethylene polymerization under the conditions employed herein.