Molecular Structure of a Hydridoniobocene Complex [Nb(η⁵‐C₅H₄SiMe₃)₂(H)₃] and Its Use as Catalyst for the Ring‐Opening Polymerization of Cyclic Esters

Abstract

The second polyhydridoniobocene complex that was characterized by X‐ray diffraction is reported. On the basis of H–H distances and H–Nb–H angles, [Nb(η5‐C5H4SiMe3)2(H)3] (1) is classified as a “compressed hydride”. Compound 1 acts as an efficient single‐component initiator for the ring‐opening polymerization of ϵ‐caprolactone and δ‐valerolactone. ϵ‐Caprolactone and δ‐valerolactone are both polymerized within a few hours to yield high‐to‐medium‐molecular‐weight polymers with medium to broad polydispersities. Polymer end‐group analysis showed that the polymerization proceeds through a coordination–insertion mechanism based on the cleavage of the ring between the oxygen atom and the acyl carbon atom.

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

“…consideration in developing any process for which they might be employed. 63−66 In addition to a recent report from Plaman and Durr regarding the catalytic application of phenoxyimino Nb and Ta ethoxides, 56 and Al-Khafaji and co-worker's work encompassing the use of calixarene-supported NbCl 5 as a ROP initiator, 57 reports from the groups of Chakraborty and Redshaw concerning, respectively, the use of Nb and Ta imino phenoxides and tetraphenolate-supported Nb and Ta species, 51,55 and a poorly controlled trihydridoniobocene system reported by Otero and co-workers, 54 remain the only examples of these metals being successfully applied to the polymerization of ε-caprolactone. Several vanadium-catalyzed protocols have also been reported for the ROP of cyclic esters, including for εcaprolactone.…”

“…Complexes of the heavier group five metals, niobium and tantalum, have received little attention as initiators for the ROP of cyclic esters, with only a small handful of reports in the literature. ,– Indeed, the authors are aware of no established industrial catalytic processes mediated by Nb-based species, although this metal does have various specialist metallurgical uses, , as well as having more recently garnered interest in relation to potential energy material applications. – Importantly, ethical sourcing of Nb and Ta is a key consideration in developing any process for which they might be employed. – …”

Investigating the Ring-Opening Polymerization Activity of Niobium and Tantalum Ethoxides Supported by Phenoxyimine Ligands

“… 20 Regardless, there have been some reports of successful polymerization of LA and ε-caprolactone (CL) either through the addition of benzyl alcohol to Nb or Ta halide complexes 21 or via a coordination-insertion mechanism with metal alkoxides or hydrides. 22 − 24 Of these limited examples, a family of molecules synthesized by Saha and co-workers suggests that phenoxyimine derivatives of heavy group V alkoxides are capable of remarkable polymerization control. 23 Inspired by these results and previous work on group IV phenoxyimines, 25 − 27 we sought to explore a new family of Nb and Ta ethoxide catalysts for the ROP of LA and CL.…”

Vanadium, niobium and tantalum

“…67 Ring-opening polymerisation of cyclic esters was catalysed by [Nb(Z 5 -C 5 H 4 SiMe 3 ) 2 (H) 3 ]. 68 Niobium pentachloride continues to be a useful catalyst. For instance, [NbCl 5 ] was found to catalyse: (i) the intermolecular cycloaddition of alkynes with aryl nitriles to give substituted pyrimidine derivatives; 69 (ii) the multicomponent Povarov reaction to convert aniline derivatives, benzaldehyde and enol ethers to furano-and pyrano-quinoline derivatives; 70 (iii) the multicomponent reaction between 3,5-dimethoxyphenol, dimethyl malonate and aryl aldehydes to give substituted 4-aryl-3,4-dihydrocoumarins; 71 (iv) the dehydrogenation of fructose to 5-hydroxymethylfurfural.…”