Incorporating Sodium to Boost the Activity of Aluminium TrenSal Complexes towards rac‐Lactide Polymerisation

Abstract

Four novel homo-and heterometallic sodium and/or aluminium complexes based on the TrenSal ligand, [LH 3 ], have been synthesised and fully characterised, including by single-crystal X-ray diffraction experiments. While [LAl] was completely inactive towards rac-lactide ring-opening polymerisation, incorporating sodium to form heterometallic [LNaAlMe] changes the aluminium geometry from octahedral to tetrahedral, leading to good catalytic activity in the presence of co-initiator BnOH (k obs = 3.19 × 10 À 2 min À 1 ; room temperature, toluene solvent) and good polymerisation control. Under identical conditions, homometallic sodium complexes showed higher activities in rac-lactide polymerisation than [LNaAlMe], with [LNa 3 ] being extremely active (k obs = 1.21 min À 1 ) but displaying unusual second-order monomer dependency and poor polymerisation control. 1 H NMR spectroscopic studies suggest that polymerisation with [LNaAlMe] or [LH 2 Na]/BnOH follows an activated monomer mechanism, whereas [LNa 3 ] operates via simultaneous coordination-insertion and activated monomer mechanisms. Overall, heterometallic [LNaAlMe] provides a balance of good activity and control compared to the homometallic analogues.

Main group metal polymerisation catalysts

“…A similar phenomenon has also been observed with heterometallic Na/Al catalysts for LA ROP based on the TrenSal ligand, which outperforms the monometallic Al complex yet is slower than the homometallic sodium analogues. 66 However, these heterometallic alkali metal/ aluminium catalysts can deliver improved polymerisation control compared to alkali metal catalysts.…”

Exploiting Multimetallic Cooperativity in the Ring-Opening Polymerization of Cyclic Esters and Ethers

“…For example, 121 gave 90% conversion of rac-LA in 0.5 h at 25 °C, whereas A heterometallic Na/Al complex based on the TrenSal ligand was also reported to follow an AMM for rac-LA ROP (123, Figure 19). 96 While the monometallic Al complex 124 was completely inactive under identical conditions, the mono-and trimetallic Na analogues 125 and 126 displayed higher activities than that of heterometallic 123. Trimetallic 126 was the most active (k obs = 1.21 min −1 ) but with poor polymerization control (RT, toluene, [rac-LA]:[Cat]:[BnOH] 100:1:1), a combination that is often observed with alkali metal catalysts in LA ROP.…”

“…, 132 or 133) with a metal benzoxide salt (Mg(OBn) 2 , Ca(OBn) 2 or Zn(OBn) 2 ) gave high conversions of rac-LA (50−89%) in just 5 s, albeit with moderate control over the dispersities (Đ = 1.2− Further examples of heterometallic complexes for cyclic ester ROP centered on group 1 and 2 metal combinations. Bimetallic 121 outperformed monometallic 122 and heterometallic Mg/Li 120 for rac-LA and L-LA ROP; mono-and trimetallic sodium complexes 125 and 126 outperformed heterometallic 123 and homometallic aluminum 124 in rac-LA ROP (124 was inactive); heterometallic 127 displayed good activity in rac-LA ROP 80,95,96. …”

Aluminum-Containing Heterobimetallic Complexes as Versatile Platforms for Homogeneous Catalysis

“…Later in 2022, Garden and co-workers reported TrenSal-based complexes as catalyst for rac -LA ROP . The mononuclear [LAl] was found to be inactive, but incorporating Na + changed aluminum geometry from octahedral to tetrahedral in [LNaAlMe] ( 4 , Figure ), leading to good catalytic activity.…”