Cooperative Effect in Binuclear Zinc Catalysts in the ROP of Lactide

Abstract

Me 3 and Et 4) and dimeric ketodiiminate zinc alkyl complexes [L 1 (H)ZnR] 2 (R = Me 5 and Et 6; L 1 = (Me 2 NC 2 H 4 NC (Me)CH) 2 CO, L 2 = (Me 2 NC 3 H 6 NC(Me)-CH) 2 CO) were synthesized and spectroscopically characterized ( 1 H and 13 C NMR and IR). Diffusion-ordered NMR spectroscopy and single-crystal X-ray diffraction analysis (1, 2, and 4−6) proved their monomeric (1−4) and dimeric (5 and 6) structures in solution and solid states. Their catalytic activity in the ring-opening polymerization of lactide was studied under various conditions and compared to mononuclear β-ketoimine zinc complexes 7−10. Initiation reactions of the Et-substituted complexes 2, 4, and 6 are faster than for the corresponding Me-substituted complexes 1, 3, and 5, and kinetic studies with catalyst 2 proved the first-order dependency on both the monomer and the catalyst concentration. Quantum chemical calculations revealed that the activation barriers for the addition of CH 3 − to L-LA via a mononuclear mechanism for the mono-( 7) and binuclear (1) Me-substituted and the corresponding MeO-substituted complexes 1-OMe and 7-OMe, which are regarded as model compounds of the "active" catalyst, are similar. However, while the binuclear mechanism for complex 1 is slightly higher in energy than for the mononuclear mechanism, the binuclear pathway for the MeO-substituted complex 1-OMe is favored, clearly proving the beneficial cooperative effect between the two adjacent zinc atoms.

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

“…Recently, Schulz and co-workers reported the syntheses of ketodiiminate zinc-alkyl complexes (Figure 9) and studied their activity for the ROP of L-LA. 56 In general, bimetallic complexes 39−44 were highly active with the Zn-ethyl complexes giving enhanced TOF values compared to the Zn-methyl analogues, likely due to the higher nucleophilicity 9). These studies revealed that the transfer of the methoxy group to L-LA has a lower energy barrier for 50 compared to 49, as the second Zn center improves the activation of the carbonyl bond, resulting in faster polymerization.…”

Homoleptic and heteroleptic ketodiiminate zinc complexes for the ROP of cyclic l-lactide

“…MALDI-ToF mass spectra were recorded with a Bruker UltraeXtreme MALDI-ToF Scheme 2 Catalytically active mono-and bimetalic catalysts I, II and III. 36,37,42 mass spectrometer (Bruker Daltonik). The polymers were dissolved in THF (10 mg mL −1 ) using dihydroxy benzoic acid as the matrix in THF (20 mg mL −1 ).…”

“…26 In the last decades, main group metals such as alkaline or earth alkaline metals [27][28][29] as well as zinc have been intensively studied as catalysts for the ROP of lactides and lactones due to their high activity, low toxicity and biocompatibility. [30][31][32][33][34][35][36][37][38][39][40][41] In marked contrast, these type of metal complexes are rarely known for the preparation of cyclic polylactides. Recently, our group reported the synthesis of binuclear magnesium complexes and their capability for the synthesis of cPLA and cPCL with high TOF values up to 712 800 h −1 under mild reaction conditions.…”

Mechanism of ring‐opening polymerization of l‐lactide by lanthanide aryloxide: A theoretical study on the effect of the aryloxide ligands on the process

“…Metal alkoxides and derived phenolates, including those of Sn [7, 8], Al [9–11], Zn [12], Mg [13], Ti [14], and rare earth (Ln = Sc, Y, La‐Lu) compounds [15–17] have been reported as catalysts/initiators for the bulk ROP of different cyclic esters and lactides. In this work, we focus on the Ln complexes [18] which have been utilized to promote the ROP of cyclic esters with good catalytic performance [6, 19] for biomedical applications [20].…”