Cooperative Heterometallic Catalysts for Lactide Ring-Opening Polymerization: Combining Aluminum with Divalent Metals

Abstract

While homometallic (salen)Al catalysts display excellent performance in lactide ring-opening polymerization (ROP), heterometallic (salen)­Al complexes have yet to be reported. Herein, we describe four heterobimetallic (salen)Al catalysts and show that the choice of the heterometal is key. Cooperative Al/Mg and Al/Zn combinations improved the catalyst activity by a factor of up to 11 compared to the mono-Al analogue, whereas the mono-Mg and mono-Zn analogues were completely inactive. In contrast, Al/Li and Al/Ca heterocombinations stunted the polymerization rate. Kinetic and computational studies suggest that Al/Mg and Al/Zn cooperativity arises from the close intermetallic proximity facilitating chloride bridging (thus enhancing initiation), which promotes a rigid square pyramidal geometry around the Al center and further increases the available monomer coordination sites. This work also translates the use of ab initio molecular dynamics calculations to ROP, introducing a useful method of investigating catalyst flexibility and revealing that ligand strain and molecular rigidity can enhance heterometallic catalyst performance.

Advances in heterometallic ring-opening (co)polymerisation catalysis

“…Our group reported Al/Zn and Mg/Al complexes 12a-b 40 , which displayed good catalyst activities in rac -LA ROP, outperforming the mono-Al analogue by respective factors of two and 11 under identical conditions ( 12a , k obs = 1.8 × 10 −3 s −1 ; 12b , k obs = 8.8 × 10 −3 s −1 ; mono-Al k obs = 0.8 × 10 −3 s −1 ; toluene, 120 °C, 1:50:100 catalyst:propylene oxide (PO):rac-LA. The mono-Zn and mono-Mg analogues were completely inactive under the same conditions, while Ca/Al and Li/Al complexes 12c-d displayed lower activities than the mono-Al complex.…”

“…While detailed mechanistic studies have not yet been reported for heterometallic complexes, experimental observations indicate that the larger and more electropositive metal acts as the monomer coordination site, and the more Lewis acidic metal acts as the source of the M-alkoxide nucleophile 31 . Complexes where alkali metals (K/Li/Na) are combined with divalent (Mg/Zn) 31 – 35 , trivalent (Al/In/Y) 36 – 40 or tetravalent (Ge/Sn) elements have been most prevalent in ROP 41 . Combining non-toxic and earth abundant metals such as Al, Mg and Zn with alkali metals is particularly appealing from both economic and environmental perspectives 42 .…”

“…Combining Lewis acidic metals with more electronegative metals with weaker M-OR bonds may accelerate coordination and nucleophilic attack. Heterometallic complexes based on Al/Zn 46 , K/Zn 31 , La/Mg 59 , Li/In 38 , Li/Mg and Li/Zn 32 , Li/Sm 55 , Mg/Al 40 , Na/Sm 53 , Na/Zn 31 , Sm/Al 58 and Ti/Zn 47 have all displayed superior activities compared to the homometallic analogues. Most of these complexes feature a M-O-M′ framework, enabling intermetallic electronic communication and/or “ate”-type activation (vide supra), which can lead to enhanced nucleophilicity of the M- R bond (e.g.…”

Synthetic Strategy Towards Heterodimetallic Half-Sandwich Complexes Based on a Symmetric Ditopic Ligand

“…While the antiproliferative activity of the current complexes was only modest, the robust, facile and modular nature of the method to form these heterodimetallic compounds means that a wide range of systems could be synthesized and examined for biological activity. Furthermore, this method to heterodimetallic complexes may also find applications in the development of new catalysts ( Mata et al, 2014 ; Gaston et al, 2021 ; Patra and Maity, 2021 ).…”

Cation–π interactions enabling hard/soft Ti/Ag heterobimetallic cooperativity in lactide ring-opening polymerisation

“…Ag + ) near the Ti catalytic site allows the distribution of both mechanistic elements across both metals, hence providing cooperativity. 12 Thus, whereas the Ti atom offers restricted coordination sites, the Ag cation does not feature an initiating group but combining both allows Ag + to activate the monomer (i.e. as an electrophilic partner) and the Ti alkoxide moiety to act as the nucleophilic partner.…”