Zinc 8-aminotrihydroquinolines appended with pendant N-diphenylphosphinoethyl arms as exceptionally active catalysts for the ROP of ε-CL

Through activation with LiCH2SiMe3 or LiN(SiMe3)2, zinc(ii) chloride complexes bearing 5,6,7-trihydroquinolin-8-amines appended with pendant diphenyl phosphine units displayed remarkable catalytic activity for ROP of ε-caprolactone.

Lignin-Derived Non-Heme Iron and Manganese Complexes: Catalysts for the On-Demand Production of Chlorine Dioxide in Water under Mild Conditions

A lignin-derived ligand, bis(2-hydroxy-3-methoxy-5-propylbenzyl)glycine (DHEG), was synthesized from 2-methoxy-4-propylphenol (dihydroeugenol (DHE)) and the amino acid glycine. Two mononuclear iron and manganese complexes of DHEG were prepared, characterized, and employed for the oxidation of chlorite to chlorine dioxide in aqueous solution. Peroxyacetic acid (PAA) was used as a "green" oxidant in the redox reactions to initiate the formation of high-valent Fe and Mn (IV)-OH intermediates. EPR studies verified the formation of a high-valent Mn^IV species. Both Fe and Mn complexes catalyzed chlorite oxidation with bimolecular rate constants of 32 and 144 M^-1 s^-1, respectively, at pH 4.0 and 25 °C. The Mn complex was found to be more efficient for chlorite oxidation with a turnover frequency of 17 h^-1 and remained active during subsequent additions of PAA. The rate of ClO₂ decomposition with PAA/Mn-DHEG was first order in PAA and increased significantly as pH increased. A mechanism that accounts for all observations is presented.

Stereoselective polymerization of rac-lactide catalyzed by zwitterionic calcium complexes

Two zwitterionic calcium complexes L¹CaN(SiMe₃)₂(THF) (1) and L²CaN(SiMe₃)₂ (2) via protolysis reaction were synthesized. At −75 °C, 1 gave a heterotactic sequence enriched polylactide, whilst 2 produced an isotactic sequence enriched polymer.

Preparation and Characterization of Self-Assembled Poly(l-Lactide) on the Surface of β-Tricalcium Diphosphate(V) for Bone Tissue Theranostics

This work was aimed to obtain and characterize the well-defined biocomposites based on β-tricalcium diphosphate(V) (β-TCP) co-doped with Ce³⁺ and Pr³⁺ ions modified by poly(l-lactide) (PLLA) with precise tailored chain length and different phosphate to polymer ratio. The composites as well as β-tricalcium diphosphate(V) were spectroscopically characterized using emission spectroscopy and luminescence kinetics. Morphological and structural properties were studied using X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). The self-assembled poly(l-lactide) in a shape of rose flower has been successfully polymerized on the surface of the β-tricalcium diphosphate(V) nanocrystals. The studied materials were evaluated in vitro including cytotoxicity (MTT assay) and hemolysis tests. The obtained results suggested that the studied materials may find potential application in tissue engineering.

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.

Palladium(II) complexes of a bridging amine bis-(phenolate) ligand featuring κ2 and κ3 coordination modes

Bidentate and tridentate coordination of a 2,4-di-tert-butyl-substituted bridging amine bis-(phenolate) ligand to a palladium(II) center are observed within the same crystal structure, namely di-chlorido-({6,6'-[(ethane-1,2-diylbis(methyl-aza-nedi-yl)]bis-(methyl-ene)}bis-(2,4-di-tert-butyl-phenol))palladium(II) chlorido-(2,4-di-tert-butyl-6-{[(2-{[(3,5-di-tert-butyl-2-hy-droxy-phen-yl)meth-yl](meth-yl)amino}-eth-yl)(meth-yl)amino]-meth-yl}phenolato)palladium(II) methanol 1.685-solvate 0.315-hydrate, [PdCl2(C34H56N2O2)][PdCl(C34H55N2O2)]·1.685CH3OH·0.315H2O. Both complexes exhibit a square-planar geometry, with unbound phenol moieties participating in inter-molecular hydrogen bonding with co-crystallized water and methanol. The presence of both κ2 and κ3 coordination modes arising from the same solution suggest a dynamic process in which phenol donors may coordinate or dissociate from the metal center, and offers insight into catalyst speciation throughout Pd-mediated processes. The unit cell contains di-chlorido-({6,6'-[(ethane-1,2-diylbis(methyl-aza-nedi-yl)]bis-(methyl-ene)}bis-(2,4-di-tert-butyl-phenol))palladium(II), {(L 2)PdCl2}, and chlorido-(2,4-di-tert-butyl-6-{[(2-{[(3,5-di-tert-butyl-2-hy-droxy-phen-yl)meth-yl](methyl)amino}eth-yl)(meth-yl)amino]-meth-yl}phenolato)palladium(II), {(L 2 X)PdCl}, mol-ecules as well as fractional water and methanol solvent mol-ecules.

Bimetallic and trimetallic zinc amino-bis(phenolate) complexes for ring-opening polymerization of rac-lactide

Di- and trinuclear zinc complexes of an amino-bis(phenolate) ligand show good lactide polymerization activity in both solution and melt.

Bio- and chemocatalysis cascades as a bridge between biology and chemistry for green polymer synthesis

The development and integration of bio- and chemocatalytic processes to convert renewable or biomass feedstocks into polymers is a vibrant field of research with enormous potential for environmental protection and the mitigation of global warming. Here, we review the biotechnological and chemical synthetic strategies for producing platform monomers from bio-based sources and transforming them into eco-polymers. We also discuss their advanced bio-application using the example of polylactide (PLA), the most valuable green polymer on the market.

DFT calculations as a ligand toolbox for the synthesis of active initiators for ROP of cyclic esters

New heteroleptic zinc dimeric complexes bearing an aminophenolate ligand of a single-site initiator framework were synthesized and characterized by spectroscopic methods, X-ray analysis, and DFT calculations. The theoretical study, verified by the experimental data, explains the catalytic behaviour in the ROP of lactide in the examined zinc complexes. The presented simple DFT protocol constitutes a valuable method for the qualification of the ancillary ligand to rationally design new complexes to improve their catalytic activity.

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.

Aluminum methyl, alkoxide and α-alkoxy ester complexes supported by 6,6'-dimethylbiphenyl-bridged salen ligands: synthesis, characterization and catalysis for rac-lactide polymerization

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6'-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe3 with various amounts of the proligands (6,6'-[(6,6'-dimethyl-[1,1'-biphenyl]-2,2'-diyl)bis(nitrylomethilidyne)]-bis(2-R(1)-4-R(2)-phenol): , R(1) = R(2) = Me; , R(1) = (t)Bu, R(2) = Me; , R(1) = R(2) = cumyl; , R(1) = Br, R(2) = (t)Bu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [AlMe (), Al2Me4 ()]. Aluminum alkoxide complexes AlO(i)Pr (), AlOBn (), and α-alkoxy ester complexes Al(OCMe2CO2Me) (), Al[(S)-OCHMeCO2Me] () were prepared via in situ alcoholysis of the parent aluminum methyl complex with the corresponding alcohols. The molecular structures of mononuclear complexes , dinuclear complex , alkoxide complexes and α-alkoxy ester complexes were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69-70 ppm and 25-41 ppm were observed in the (27)Al NMR spectra of complexes and , indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex is also a rare example of an O-lactate model complex that mimics the first insertion of l-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes in the presence of (i)PrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03-1.05) and 65-73% isotacticities. Particularly, complex showed an "immortal" behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.

Diastereoselective synthesis of chiral aminophenolate magnesium complexes and their application in the stereoselective polymerization of rac-lactide and rac-β-butyrolactone

A series of magnesium silylamido complexes supported by chiral aminophenolate ligands were synthesized, and their catalytic behaviors toward the ring-opening polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored.

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.

Stereoselective Polymerization of rac-Lactide Catalyzed by Zinc Complexes with Tetradentate Aminophenolate Ligands in Different Coordination Patterns: Kinetics and Mechanism

A series of monomeric zinc silylamido complexes bearing [NNNO]-type tetradentate aminophenolate ligands, LZnN(SiMe3)2 [L = {(2-R(1))ArCH2N[(CH2)2R(2)]CH2(4-R(4)-6-R(3))C6H2O-}, R(1) = NMe2, R(2) = N(i)Pr2, R(3) = R(4) = Cl (1), R(3) = R(4) = cumyl (3); R(1) = NMe2, R(2) = NEt2, R(3) = R(4) = cumyl (2), R(3) = CPh3, R(4) = Me (4); R(1) = NEt2, R(2) = NEt2, R(3) = CPh3, R(4) = Me (5); R(1) = NMe2, R(2) = (S)-1-butylpyrrolidin-2-yl, R(3) = R(4) = cumyl (6), R(3) = CPh3, R(4) = Me (7)], have been synthesized via reactions of Zn[N(SiMe3)2]2 and 1 equiv of the corresponding aminophenols. The monomeric nature and versatile coordination patterns of these complexes in the solid state were further confirmed by X-ray diffraction studies on complexes 2, 3, 5, and 7. In complex 3, the N,N-diisopropylamino group on the pendant side arm does not coordinate to the metal center; only the remaining three donors of the aminophenolate ligand and the silylamido group interact with the zinc center. By contrast, in complexes 2, 5, and 7, the amino group of the aryl moiety does not coordinate to the metal center, while the amino group on the pendant side arm coordinates. At room temperature, the above-mentioned structural features of these complexes are retained in solution, as confirmed by (1)H NMR spectroscopy. Complexes 1-7 proved to be efficient initiators for the ring-opening polymerization of rac-lactide (rac-LA) at ambient temperature, and the polymerizations were better controlled in the presence of 2-propanol. The coordination pattern of the aminophenolate ligand exerted a significant influence on the stereoselectivity of the corresponding complex toward the polymerization of rac-LA, leading to the production of heterotactic biased polylactides (PLAs) by complexes 1 and 3 (Pm = 0.40-0.46) and moderately to highly isotactic PLAs by complexes 2 and 4-7 (Pm = 0.70-0.81). Detailed kinetic investigations revealed a first-order dependence on the monomer concentration for all complexes and different orders in the initiator concentration ranging from 1.78 to 1.81. The nature of the solvent as well as the molar ratio of the zinc complex and 2-propanol also displayed certain influence on the order of rac-LA polymerization in the initiator concentration. Factional orders of 1.80, 1.38, and 1.11 were obtained by using complex 5/(i)PrOH (1:1) in toluene and tetrahydrofuran and complex 5/(i)PrOH (1:2) in toluene, respectively. On the basis of DOSY and (1)H and (13)C NMR studies of zinc alkoxide model complexes "LZn(OCMe2COOMe)" as well as the fractional orders of 1.78-1.81 in the initiator concentration, activation/insertion processes likely involving more than one monomeric active species were then hypothesized.

Alkali-Metal Monophenolates with a Sandwich-Type Catalytic Center as Catalysts for Highly Isoselective Polymerization of rac-Lactide

Highly isoselective ring-opening polymerization (ROP) of rac-lactide is a challenge for sodium and potassium complexes under mild conditions. In this work, three sodium and potassium complexes with a sandwich-type catalytic center are highly active catalysts for the polymerization of rac-lactide and show high isoselectivities with P_m values of 0.72-0.82. The best isoselectivity of P_m = 0.82 is the highest value for alkali-metal complexes under mild conditions. The molecular weights of the obtained PLA are close to the theoretical values, and the molecular weight distributions are narrow.

Beyond stereoselectivity, switchable catalysis: some of the last frontier challenges in ring-opening polymerization of cyclic esters

Metal-based catalysts and initiators have played a pivotal role in the ring-opening polymerization (ROP) of cyclic esters, thanks to their high activity and remarkable ability to control precisely the architectures of the resulting polyesters in terms of molar mass, dispersity, microstructure, or tacticity. Today, after two decades of extensive research, the field is slowly reaching maturity. However, several challenges remain, while original concepts have emerged around new types or new applications of catalysis. This Review is not intended to comprehensively cover all of these aspects. Rather, it provides a personal overview of the very recent progress achieved in some selected, important aspects of ROP catalysis--stereocontrol and switchable catalysis. Hence, the first part addresses the development of new metal-based catalysts for the isoselective ROP of racemic lactide towards stereoblock copolymers, and the use of syndioselective ROP metal catalysts to control the monomer sequence in copolymers. A second part covers the development of ROP catalysts--primarily metal-based catalysts, but also organocatalysts--that can be externally regulated by the use of chemical or photo stimuli to switch them between two states with different catalytic abilities. Current challenges and opportunities are highlighted.

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.

Designing ancillary ligands for heteroleptic/homoleptic zinc complex formation: synthesis, structures and application in ROP of lactides

Synthesis and characterization of a series of new amino-phenol/naphthol ligands (L1,2-H) have been developed and their respective zinc complexes (1 and 2-Zn) have been synthesized.

Synthesis and structures of bis-ligated zinc complexes supported by tridentate ketoimines that initiate L-lactide polymerization

Eight bis-ligated, homoleptic, zinc complexes were synthesized through the reaction of NNO Schiff base ketoimines bearing varying substituents with diethyl zinc in an inert atmosphere glovebox at room temperature and isolated in 62-95% yield. The complexes were characterized with (1)H, (13)C, and (19)F nuclear magnetic resonance spectroscopy, absorbance spectroscopy, high resolution mass spectrometry, elemental analysis, and single crystal X-ray crystallography. The complexes were shown to adopt distorted octahedral coordination geometry around zinc. The (1)H and (19)F NMR spectra of complexes 1-7 showed stable zinc coordination at 300 K while the effect of steric encumbrance and two trifluoromethyl groups in complex 8 was investigated with variable temperature NMR. The bis-ligated zinc complexes were effective initiators for the ring opening polymerization of L-lactide into poly-L-lactic acid (PLLA). With [L-lac]/[Zn complex] = 50, the bis-ligated zinc complexes yielded percentage conversion of 14-98% with polymerization times varying from 15-1440 min, where the longest reaction times were required when two trifluoromethyl groups were present. The addition of 4-fluorophenol co-catalyst resulted in up to a 5-fold increase in the percentage conversion in toluene solution and up to a 14-fold increase in bulk melt polymerization with reductions in the poly-dispersity index values for the isolated PLLA. Addition of 4-fluorophenol to complex 1 was studied with (1)H and (19)F NMR and appeared to yield an in situ generated zinc alkoxide complex.

Magnesium complexes containing biphenyl-based tridentate imino-phenolate ligands for ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate

A series of racemic 2-[(2'-(dimethylamino)biphenyl-2-ylimino)methyl]-4-R(2)-6-R(1)-phenols (L¹H-L⁴H) were reacted with {Mg[N(SiMe3)2]2}2 to provide four heteroleptic magnesium complexes (L¹⁻⁴)MgN(SiMe3)2·(THF)n (1, R(1) = (t)Bu, R(2) = Me, n = 1; 2, R(1) = R(2) = CMe2Ph, n = 0; 3, R(1) = CPh3, R(2) = (t)Bu, n = 1; 4, R(1) = Br, R(2) = (t)Bu, n = 0), which have been fully characterized. X-ray structural determination shows that complex 1 possesses a monomeric structure, but complex 4 is dimeric with C2-symmetry where the two metal centers are bridged by two phenolate oxygen atoms of the ligands. The coordination geometry around the magnesium center in these complexes can be best described as a distorted tetrahedral geometry. The heteroleptic complexes 1-4 efficiently initiate the ring-opening polymerization of rac-lactide and α-methyltrimethylene carbonate (α-MeTMC) and the polymerizations are better controlled in the presence of 2-propanol. In general, the introduction of a bulky ortho-substituent on the phenoxy unit results in increases of both the catalytic activity and the stereo- or regioselectivity of the corresponding magnesium complex. Microstructure analyses of the resulting PLAs revealed that P(r) values range from 0.46 to 0.81, depending on the catalyst and the polymerization conditions. For racemic α-MeTMC, detailed analyses using (1)H and (13)C NMR spectroscopy indicated the preferential ring-opening of α-MeTMC at the most hindered oxygen-acyl bond (X(reg) = 0.65-0.86).