Syntheses, structures and catalytic activity of tetranuclear Mg complexes in the ROP of cyclic esters under industrially relevant conditions

Tetranuclear magnesium imino(phenolate) complexes Mg₄(L^1–4)₄ are excellent catalysts for the ROP of bulk rac-lactide and ε-caprolactone under industrially relevant conditions.

Magnesium and zinc complexes bearing NNO-tridentate ketiminate ligands: synthesis, structures and catalysis in the ring-opening polymerization of lactides

Magnesium and zinc complexes bearing pyrazoline-derived ketiminate ligands, [(L^1–3)₂M₂(μ-OBn)₂] and [M(L)₂] proceed through the “coordination–insertion” mechanism in the ROP of lactides.

Configurationally flexible zinc complexes as catalysts for rac-lactide polymerisation

Introduction of catalytic site flexibility (fast racemisation) into a previously reported zinc alkoxide catalyst afforded a slight preference for isotactic lactide polymerization via catalytic-site mediated chain-end control.

Mono-BHT heteroleptic magnesium complexes: synthesis, molecular structure and catalytic behavior in the ring-opening polymerization of cyclic esters

Numerous heteroleptic 2,6-di-tert-butyl-4-methylphenolate (BHT) magnesium complexes have been synthesized by treatment of (BHT)MgBu(THF)₂ with various alcohols. Molecular structures of the complexes have been determined by X-ray diffraction. The magnesium coordination number in [(BHT)Mg(μ-OBn)(THF)]₂ (3) and [(BHT)Mg(μ-O-tert-BuC₆H₄)(THF)]₂ (4) is equal to 4. Complexes formed from esters of glycolic and lactic acids, [(BHT)Mg(μ-OCH₂COOEt)(THF)]₂ (5) and [(BHT)Mg(μ-OCH(CH₃)COOCH₂COO^tBu)(THF)]₂ (6) contain chelate fragments with pentacoordinated magnesium. Compounds 3-6 contain THF molecules coordinated to magnesium atoms. Complex {(BHT)Mg[μ-O(CH₂)₃CON(CH₃)₂]}₂ (7) does not demonstrate any tendency to form an adduct with THF. It has been experimentally determined that complexes 3 and 5 are highly active catalysts of lactide polymerization. The activity of 4 is rather low, and complex 7 demonstrates moderate productivity. According to DOSY NMR experiments, compounds 3 and 5 retain their dimeric structures even in THF. The free energies of model dimeric [(DBP)Mg(μ-OMe)(Sub)]₂ and monomeric (DBP)Mg(OMe)(Sub)₂ products on treatment of [(DBP)Mg(μ-OMe)(THF)]₂ with a series of σ-electron donors (Sub) have been estimated by DFT calculations. These results demonstrate that the substitution of THF by Sub in a dimeric molecule is an energetically allowed process, whereas the dissociation of dimers is energetically unfavorable. DFT modeling of ε-CL and (dl)-lactide ROP catalyzed by dimeric and monomeric complexes showed that a cooperative effect of two magnesium atoms occurs within the ROP for binuclear catalytic species. A comparison of the reaction profiles for ROP catalyzed by binuclear and mononuclear species allowed us to conclude that the binuclear mechanism is favorable in early stages of ROP initiated by dimers 3 and 5.

Ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium iminophenoxides

Crown ether complexes of sodium and potassium iminophenoxides were demonstrated to catalyze the stereoselective polymerization of rac-lactide.

Crown ether complexes of potassium quinolin-8-olates: synthesis, characterization and catalysis toward the ring-opening polymerization of rac-lactide

Crown ether complexes of potassium quinolin-8-olates were synthesized and demonstrated to catalyze the stereoselective polymerization of rac-lactide.

Solvent dependence of the solid-state structures of salicylaldiminate magnesium amide complexes

There are challenges in using magnesium coordination complexes as reagents owing to their tendency to adopt varying aggregation states in solution and thus impacting the reactivity of the complexes. Many magnesium complexes are prone to ligand redistributionviaSchlenk equilibrium due to the ionic character within the metal–ligand interactions. The role of the supporting ligand is often crucial for providing stability to the heteroleptic complex. Strategies to minimize ligand redistribution in alkaline earth metal complexes could include using a supporting ligand with tunable sterics and electronics to influence the degree of association to the metal atom. Magnesium bis(hexamethyldisilazide) was reacted with salicylaldimines [1L=N-(2,6-diisopropylphenyl)salicylaldimine and2L= 3,5-di-tert-butyl-N-(2,6-diisopropylphenyl)salicylaldimine] in either nondonor (toluene) or donor solvents [tetrahydrofuran (THF) or pyridine]. The structures of the magnesium complexes were studied in the solid stateviaX-ray diffraction. In the nondonor solvent,i.e.toluene, the heteroleptic complex bis{μ-2-[(2,6-diisopropylphenyl)iminomethyl]phenolato}-κ3N,O:O;κ3O:N,O-bis[(hexamethyldisilazido-κN)magnesium(II)], [Mg2(C19H22NO)2(C6H18NSi2)2] or [1LMgN(SiMe3)2]2, (1), was favored, while in the donor solvent,i.e.pyridine (pyr), the formation of the homoleptic complex {2,4-di-tert-butyl-6-[(2,6-diisopropylphenyl)iminomethyl]phenolato-κ2N,O}bis(pyridine-κN)magnesium(II) toluene monosolvate, [Mg(C27H38NO)2(C5H5N)2]·C5H5N or [{2L2Mg2(pyr)2}·pyr], (2), predominated. Heteroleptic complex (1) was crystallized from toluene, while homoleptic complexes (2) and the previously reported [1L2Mg·THF] [Quinqueet al.(2011).Eur. J. Inorg. Chem.pp. 3321–3326] were crystallized from pyridine and THF, respectively. These studies support solvent-dependent ligand redistribution in solution.In-situ1H NMR experiments were carried out to further probe the solution behavior of these systems.

The Role of Nitrogen Donors in Zinc Catalysts for Lactide Ring-Opening Polymerization

The electronic effects of nitrogen donors in zinc catalysts for ring-opening polymerization of cyclic esters were investigated. Alkyl and benzyloxy zinc complexes supported by tridentate diamino- and aminoimino phenolate ligands were synthesized, and their solid-state and solution structures characterized. The solution-state structures showed that the alkyl complexes are mononuclear, while the alkoxy complexes are dimeric with the ligands coordinated with different denticities depending on the nature of the ligand donors. The catalytic activities of these compounds toward the ring-opening polymerization of racemic lactide were studied and showed that catalysts with secondary and imine nitrogen donors are more active than analogues with tertiary amines.

Group 4 metal complexes of Trost's semi-crown ligand: synthesis, structural characterization and studies on the ring-opening polymerization of lactides and ε-caprolactone

The synthesis of titanium(IV), zirconium(IV) and hafnium(IV) complexes of Trost's semi-crown ligand is described. All complexes were fully characterized by 1H and 13C NMR and mass spectroscopy. The molecular structures of the representative complexes 2, 3 and 4 were determined by single-crystal X-ray diffraction studies. The X-ray diffraction studies reveal that complexes 2, 3 and 4 crystallized in an orthorhombic crystal system. Complexes 2 and 3 have a monomeric structure in the solid state with distorted octahedral geometry around the metal center, whereas complex 4 was found to crystallize in a trimeric structure bridging with an oxygen atom, where the geometry around the titanium is distorted trigonal bipyramidal. The activities and stereoselectivities of these complexes toward the ring-opening polymerization (ROP) of lactides (L-LA and rac-LA) and ε-caprolactone (CL) have been investigated. Complexes 1–4 were found to be efficient single-component initiators for the ring-opening polymerization of cyclic esters and yielded high molecular weight polymers (Mn) with narrow molecular weight distributions (MWD).The microstructure of the resultant polylactides (PLAs) from rac-LA was determined. Complexes 2 and 3 afforded isotactic-enriched PLA (Pm = 0.78–0.71) with narrow MWD (1.07–1.04), on the other hand complexes 1 and 4 produced atactic PLA. Kinetic and post polymerization studies confirm that the polymerization proceeds through the coordination–insertion mechanism.

Metal complexes containing nitrogen-heterocycle based aryloxide or arylamido derivatives as discrete catalysts for ring-opening polymerization of cyclic esters

This perspective highlights some recent advances in the field of discrete N-heterocyclic aryloxide/arylamido metal complexes for ROP of lactones.

Different mechanisms at different temperatures for the ring-opening polymerization of lactide catalyzed by binuclear magnesium and zinc alkoxides

Two binuclear magnesium and zinc alkoxides supported by a bis-salalen type dinucleating heptadentate Schiff base ligand were synthesized and fully characterized. The two complexes are efficient initiators for the ring-opening polymerization (ROP) of L-lactide, affording polymers with narrow polydispersities and desirable molecular weights. Interestingly, the mechanisms for the ROP of lactide are different at different temperatures. At a high temperature of 130 °C, a coordination-insertion mechanism is reasonable for the bulk melt polymerization of lactide. At a low temperature, the alkoxide cannot initiate the ROP reaction; however, upon the addition of external benzyl alcohol into the system, the ROP of lactide can smoothly proceed via an "activated monomer" mechanism. In addition, these complexes display slight stereo-selectivity for the ring-opening polymerization of rac-lactide, affording partially isotactic polylactide in toluene with a Pm value of 0.59.

Mono-, di- and tetra-zinc complexes derived from an amino-benzotriazole phenolate ligand containing a bulkier N-alkyl pendant arm: synthesis, structure and catalysis for ring-opening polymerization of cyclic esters

Zinc complexes constructed from the amino-modified benzotriazole phenol pro-ligand, 2-(2H-benzotriazol-2-yl)-6-((diisopropylamino)methyl)-4-(2,4,4-trimethylpentan-2-yl)phenol ((C8DIA)BTP-H, 1), were synthesized stepwise and structurally characterized. The reaction of (C8DIA)BTP-H (1) with one equivalent of diethyl zinc (ZnEt2) generates a dimeric and four-coordinated zinc complex, [(μ-(C8DIA)BTP)ZnEt]2 (2), which is doubly bridged by two phenolate groups of C8DIABTP ligands. Further reaction of 2 with benzyl alcohol (BnOH) in stoichiometric proportions affords a tetranuclear zinc benzylalkoxide complex [(μ-OBn)((C8DIA)BTP)Zn]4 (3) that possesses a saddle-shaped core with four μ2-bridging benzylalkoxy groups upon four Zn centres. Interestingly, the di-nuclear Zn alkoxide [(μ-OBn)((C8DIA)BTP)Zn(DMAP)]2 (4) could be prepared by treatment of 3 with a stoichiometric amount of 4-(dimethylamino)pyridine (DMAP). ZnEt2 reacts with two equivalents of 1 in the presence of DMAP (1.0 mol equiv.) to yield a five-coordinated mononuclear zinc complex, [((C8DIA)BTP)2Zn(DMAP)] (5). All complexes adopt an N,O-bidentate coordination mode from the phenoxy oxygen atom and benzotriazole nitrogen atom, in which the nitrogen atom of the pendent arm substituent is not coordinated to the zinc centre. Ring-opening polymerization of ε-caprolactone and β-butyrolactone catalysed by 2 and 3 was investigated.

Iso-selective ring-opening polymerization of rac-lactide catalyzed by crown ether complexes of sodium and potassium naphthalenolates

Two crown ether complexes of sodium and potassium naphthalenolates were synthesized and entirely characterized. The two complexes can iso-selectively catalyze the ring-opening polymerization (ROP) of rac-lactide at room temperature and afford polylactides with desired molecular weights and narrow PDIs; the best isotacticity (Pm) achieved was 0.73.

Ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate catalyzed by magnesium and zinc complexes derived from binaphthyl-based iminophenolate ligands

Magnesium and zinc complexes derived from binaphthyl-based iminophenolate ligands efficiently initiated the ring-opening polymerization of rac-lactide and α-MeTMC, with the highest heteroselectivity (P_r = 0.84) or regioselectivity (X_reg = 0.98) obtained for the zinc complex.