Zinc Complexes of Piperazinyl‐Derived Aminephenolate Ligands: Synthesis, Characterization and Ring–Opening Polymerization Activity

Abstract

A series of zinc complexes was prepared from 4,6‐disubstituted (R,R′) o‐[(4‐methyl‐1‐piperazinyl)methyl]phenols ([ONNR′,R]H) and characterized by elemental analysis, 1H and 13C{1H} NMR spectroscopy, and X‐ray crystallography. Reaction of these products with diethylzinc gave Zn[ONNtBu,tBu]2 (1) as a monometallic complex and {[μ‐ONNR′,R]ZnEt}2 [R′ = Me, R = tBu, (2); R′ = R = tBu (3); R′ = R = tAm (4)] bimetallic species that have distorted tetrahedral environments about the Zn centres. Reaction of 3 and 4 with alcohols gave {[ONNtAm,tAm]Zn(μ‐OR″)}2 [R″ = Bn (5); R″ = Et (6)] bimetallic species in which the Zn centres are bridged by benzyl alkoxide and ethoxide groups, respectively. A morpholinyl derived ligand was also synthesized and characterized (L4H), and its 1:1 stoichiometric reaction with ZnEt2 resulted in complex 7, {[μ‐ONOtBu,tBu]ZnEt}2. The reactivity of complexes 2–7 in the ring‐opening polymerization of rac‐lactide (LA) and ϵ‐caprolactone (ϵ‐CL) was studied. Reactions of carbon dioxide with cyclohexene oxide in the presence of 6 or 7/ROH (R = Bn, Et) afforded cyclohexene carbonate.

Ring-opening polymerization of rac-lactide mediated by tetrametallic lithium and sodium diamino-bis(phenolate) complexes

“…The control of macromolecular features is also much improved when BnOH is used, both in terms of PDIs and agreement between M ncal vs. M n . All the generated polymers have molecular weights higher than the theoretical values (M ncal ), which might be attributed to intermolecular transesterification reactions, 75,76 but are less significant in reactions when BnOH is present. Notably, the obtained dispersities (with or without BnOH) were more narrow than previously reported values 27,73,74 for polymers prepared in the melt-phase suggesting a more controlled polymerization.…”

Magnesium amino-bis(phenolato) complexes for the ring-opening polymerization of rac-lactide

“…Complexes of amino-bis( phenolates) exhibit interesting structural and electronic properties, as well as catalytic behaviour; the variety of metal complexes with this ligand class has also been reviewed. 9 Derivatives of this ligand class have been used in main group and d-block metal chemistry, including lithium, [10][11][12][13][14][15][16][17][18] magnesium, [19][20][21][22][23][24][25] calcium, 26 rareearths, 27,28 zinc, 20,[29][30][31][32][33][34][35][36][37] aluminum, 7,38,39 zirconium 40,41 and titanium. 42 Many of these complexes have been reported to be excellent initiators for the ROP of cyclic esters such as lactide and ε-caprolactone.…”

Ring-opening polymerization of cyclic esters with lithium amine-bis(phenolate) complexes

“…However, aminophenolato alkali and alkaline earth metal complexes have been shown to display both interesting structural and catalytic behaviour in their own right. Numerous derivatives of this ligand class have been used in main group and transition metal chemistry, including lithium, [3][4][5][6][7][8][9][10][11] magnesium, [12][13][14][15][16][17][18] calcium, 19 rareearths, 20,21 zinc, 13,[22][23][24][25][26][27][28][29][30] aluminium, [31][32][33] zirconium 34,35 and titanium. 36 Many of these complexes have been reported to be excellent initiators for the ring-opening polymerization (ROP) of cyclic esters such as lactide and ε-caprolactone.…”