Coordination of ε-Caprolactone to a Cationic Niobium(V) Alkoxide Complex: Fundamental Insight into Ring-Opening Polymerization via Coordination–Insertion

Abstract

We report three niobium-based initiators for the catalytic ring-opening polymerization (ROP) of ε-caprolactone, exhibiting good activity and molecular weight control. In particular, we have prepared on the gram-scale and fully characterized a monometallic cationic alkoxo-Nb(V) ε-caprolactone adduct representing, to the best of our knowledge, an unprecedented example of a metal complex with an intact lactone monomer and a functional ROP-initiating group simultaneously coordinated at the metal center. At 80 °C, all three systems initiate the immortal solution-state ROP of ε-caprolactone via a coordination−insertion mechanism, which has been confirmed through experimental studies, and is supported by computational data. Natural bond orbital calculations further indicate that polymerization may necessitate isomerization about the metal center between the alkoxide chain and the coordinated monomer. The observations made in this work are expected to inform mechanistic understanding both of amine tris(phenolate)-supported metal alkoxide ROP initiators, including various highly stereoselective systems for the polymerization of lactides and of coordination−insertion-type ROP protocols more broadly.

Unexpected Periodicity in Cationic Group 5 Initiators for the Ring-Opening Polymerization of Lactones

“…Whereas the activities of both 4a and 4b were negligible at 25 °C in chloroform- d , the hexafluoroantimonate salt, 3a , of a monocationic, monoalkoxo, bimetallic Nb complex readily polymerized β-BL both at 60 °C in toluene- d 8 and at 25 °C in chloroform- d , proceeding in both cases without a significant induction period (see Supporting Information). As we have reported previously for the ROP of ε-CL, the precatalyst 3a is activated via cleavage of a labile Nb−μ 2 F bond and coordination of the monomer to the cationic fragment, the active propagating species being identical with that for both 4a and 5a . Accordingly, the lack of an induction period associated with deployment of this species to the ROP of β-BL further confirms the role of the coordinated ε-CL molecule of 4a and 4b in the slow initiation observed for those systems, with monomer exchange at the coordinatively saturated metal center clearly being unfavorable.…”

“…Consistent with the polymerization of ε-caprolactone, the Ta species exhibited a significantly increased rate, relative to the Nb system, in the ROP of both rac-β-butyrolactone and δ-valerolactone. The expansion of the monomer scope in this work, in comparison to our previous studies, and differentiation of the metal-coordinated lactone from the wider monomer feed has permitted further illumination of the initiation mechanism and specifically of the intramolecular insertion event. This work represents the first examples of the Group 5-catalyzed ROP of rac -β-butyrolactone and of the tantalum-catalyzed ROP of δ-valerolactone.…”

“…In this case, coordination of the monomer to the metal center precedes intramolecular nucleophilic attack at the ester carbonyl group by the alkoxide moiety, which is followed by ring opening. With the exception of our recently reported stable alkoxo–niobium­(V) ε-CL adduct (see below), the intermediate formed on coordination of the monomer, prior to the insertion event, is not typically observable . After insertion and ring opening occur, the growing chain remains coordinated to the metal center, fulfilling the role of the alkoxide in subsequent propagation events.…”

“…This was the first reported example of a stable cyclic ester adduct of a metal alkoxide complex, and when heated to 80 °C in a solution of ε-CL in toluene, it initiated the controlled ROP of that monomer. Moreover, we demonstrated that the alkoxide moiety was the active initiating group and that ROP proceeded via a coordination–insertion mechanism . Encouraged by the results of that work, we report here the preparation of the analogous Ta­(V) species, 4b , and its utility in application to the catalytic ROP of ε-CL.…”

“…Analogous to the ε-CL adducts 4a and 4b , solid- and solution-state characterization showed complexes 5a and 5b to, similarly, be mutually isostructural (Figure ). Notably, in addition to being only the third and fourth known species in which both an alkoxide initiating group and a cyclic ester are concurrently coordinated at a metal center, following 4a and 4b , to our knowledge, 5a and 5b are the first metal complexes of δ-VL to be isolated and represent the first examples of solid-state structures containing δ-VL. The Nb–O and Ta–O bond lengths of the coordinated δ-VL and ethoxide moieties of 5a and 5b did not exhibit any statistically significant differences, either with one another or when compared to the relevant bonds of 4a and 4b .…”