Niobium and Tantalum complexes derived from the acids Ph2C(X)CO2H (X = OH, NH2): synthesis, structure and ROP capability

Abstract

Reaction of benzilic acid, Ph₂C(OH)(CO₂H), L¹H₂, with equimolar amounts of M(OR)₅ (M = Nb, Ta) led, following work-up, to the tetranuclear complexes [Nb₄(OEt)₈(L¹)₄(μ-O)₂] (1) or [Ta₄(OEt)₈(L¹)₄(μ-O)₂]·0.5MeCN (2·0.5MeCN), respectively. Similar use of 2,2′-diphenylglycine, Ph₂C(NH₂)(CO₂H), L²H₃, led to the isolation of the dinuclear complexes [Nb₂(OEt)₄(L²H₂)₄(μ-O)]·2MeCN (3·2MeCN) or [Ta₂(OEt)₄(L²H₂)₄(μ-O)]·2.25MeCN (4·2.25MeCN). The molecular structures of 1–4 are reported. Complexes 1–4 have been screened for their potential to act as catalysts for the ring opening polymerization (ROP) of ε-caprolactone (ε-CL) and rac-lactide (r-LA), with or without benzyl alcohol (BnOH) present. In the case of ε-CL, complex 1 displayed best activity with >99% conversion at 100 °C, whilst 3 and 4 were virtually inactive under the same conditions. All complexes show moderate activities towards the ROP of r-LA at 160 °C, with 1–3 producing heterotactic enriched PLA while 4 afforded isotactic enriched PLA. Copolymerization studies revealed the most efficient system involved the initial addition of r-LA (for 6 h) followed by ε-CL (for 42 h), which led to 99% conversion for each of the monomers. Block copolymers of PLA-b-CL and PCL-b-LA and random copolymers PLA-co-CL were successfully synthesized by adjusting the feed sequence.