Dinucleating Ligand Platforms Supporting Indium and Zinc Catalysts for Cyclic Ester Polymerization

Metal Complexes in the Synthesis of Biodegradable Polymers: Achievements and Prospects

This review describes recent advances in the synthesis of homopolymers of lactide and related cyclic esters via ring-opening polymerization (ROP) in the presence of metal complexes based on group 1, 2, 4, 12, 13 and 14 metals. Particular attention is paid to the influence of the initiator structure on the properties of the obtaining homo- and copolymers. Also, a separate chapter is devoted to the study of metal complexes in the synthesis of copolymers of lactide and lactones. This review highlights the efforts made over the last ten years or so, and shows how main-group metals have received increasing attention in the field of the polymerization of lactide and related cyclic esters.

Bimetallic polymerization of lactide with binaphthol-derived bis-heteroscorpionate dizinc and dimagnesium complexes

Discrete bimetallic catalysts often provide enhanced reactivity and selectivity in lactone polymerization, making metal-metal cooperativity an important design principle for new catalyst development. However, the poor modularity of binucleating ligands limits structure-reactivity analysis and optimization. This report describes a modular, binucleating bis(pyrazolyl)alkane ligand series (1-R) bridged by a chiral binaphthol unit, prepared by nucleophile-catalyzed condensation between a dialdehyde and a bis(pyrazolyl)methanone. A bis(ethylzinc) complex was characterized by single-crystal X-ray diffraction, but in situ complexation with Zn(HMDS)₂ and Mg(HMDS)₂ provided more active catalysts for lactide polymerization (HMDS^- = hexamethyldisilazide). Structure-reactivity studies identified complexes of 1-Me₂ as the most active, and these catalysts show significant enhancements in rate compared to their monometallic analogues. Kinetic analysis resulted in first-order dependence on both mono- and bimetallic catalysts, suggesting metal-metal cooperativity as the basis for this rate enhancement. End-group analysis and low dispersity implicate a coordination-insertion mechanism through an alkoxide. Despite rapid transesterification observed by MALDI, we still demonstrated controlled polymerization in the block copolymerization of ε-caprolactone and L-lactide. Although we observed rate differences in the polymerization of L-lactide by opposite enantiomer catalysts, we did not observe catalyst-directed stereoselectivity in the polymerization of rac- or meso-lactide.

Cooperative Initiation in a Dinuclear Indium Complex for CO2 Epoxide Copolymerization

Dinuclear indium complexes have been synthesized and characterized. These include neutral and cationic indium complexes supported by a Schiff base ligand bearing a binaphthol linker. The new compounds were investigated for alternating copolymerization of CO₂ and cyclohexene oxide. In particular, the neutral indium chloride complex (±)-[(O_NapN_iN)InCl₂]₂ (4) showed high conversion of cyclohexene oxide and selectivity for poly(cyclohexene carbonate) formation without cocatalysts at 80 °C under various CO₂ pressures (2-30 bar). Importantly, the reactivity of the dinuclear indium chloride complex 4 is drastically different from that of the mononuclear indium chloride complex (±)-(NN_iO_tBu)InCl₂ (5), suggesting a cooperative initiation mechanism involving the two indium centers in 4.

A highly selective and sensitive salamo-salen-salamo hybrid fluorometic chemosensor for identification of Zn2+ and the continuous recognition of phosphate anions

A salamo-salen-salamo hybrid fluorescent chemical sensor (H₄L) was synthesized and characterized. It exhibits high selectivity and sensitivity to Zn²⁺ in physiological pH range. Meanwhile, its zinc(II) complex (L-Zn²⁺) continuously responses phosphate anions in DMF/H₂O (v/v, 9:1) solution. Moreover, the identification processes are explored using characterization methods such as UV-absorption spectra, IR spectra and ESI-MS spectrum. In addition, the coordination mechanism of H₂PO₄^- and Zn²⁺ were successfully exploited to make the chemical sensor reproducible. In short, the sensors H₄L and L-Zn²⁺ will be promising detection devices for Zn²⁺ and phosphate anions.

Efficient Bulky Organo-Zinc Scorpionates for the Stereoselective Production of Poly(rac-lactide)s

The direct reaction of the highly sterically demanding acetamidinate-based NNN'-scorpionate protioligand Hphbp^tamd [Hphbp^tamd = N,N'-di-p-tolylbis(3,5-di-tertbutylpyrazole-1-yl)acetamidine] with one equiv. of ZnMe₂ proceeds in high yield to the mononuclear alkyl zinc complex [ZnMe(κ³-phbp^tamd)] (1). Alternatively, the treatment of the corresponding lithium precursor [Li(phbp^tamd)(THF)] with ZnCl₂ yielded the halide complex [ZnCl(κ³-phbptamd)] (2). The X-ray crystal structure of 1 confirmed unambiguously a mononuclear entity in these complexes, with the zinc centre arranged with a pseudotetrahedral environment and the scorpionate ligand in a κ³-coordination mode. Interestingly, the inexpensive, low-toxic and easily prepared complexes 1 and 2 resulted in highly efficient catalysts for the ring-opening polymerisation of lactides, a sustainable bio-resourced process industrially demanded. Thus, complex 1 behaved as a single-component robust initiator for the living and immortal ROP of rac-lactide under very mild conditions after a few hours, reaching a TOF value up to 5520 h^-1 under bulk conditions. Preliminary kinetic studies revealed apparent zero-order dependence on monomer concentration in the absence of a cocatalyst. The PLA materials produced exhibited narrow dispersity values, good agreement between the experimental M_n values and monomer/benzyl alcohol ratios, as well as enhanced levels of heteroselectivity, reaching P_s values up to 0.74.

Phosphasalalen Rare-Earth Complexes for the Polymerization of rac-Lactide and rac-β-Butyrolactone

A series of new phosphasalalen pro-ligands, analogues of salalen but with an iminophosphorane replacing the imine functionality, and their corresponding rare-earth alkoxide and siloxide complexes were synthesized. The multinuclear NMR spectra and X-ray diffraction analyses revealed that, for the tert-butoxide and ethoxide complexes, the resulting phosphasalalen rare-earth product was composed of a mononuclear alkoxide and a binuclear complex containing bridged alkoxo and hydroxo groups, while an analogous binuclear complex was isolated as the sole product for the siloxide complex. All the complexes could catalyze the heteroselective ring-opening polymerization (ROP) of rac-lactide (P_r up to 0.77) with high catalytic activities and a controlled polydispersity. Remarkably, the yttrium and lutetium phosphasalalen complexes could also efficiently catalyze the ROP of rac-β-butyrolactone to produce syndiotactic polymers (P_r up to 0.73) while their salalen analogues were inert, revealing the special effects of the iminophosphorane moiety. Detailed end-group analyses and kinetic investigations suggested that the alkoxo-hydroxo-bridged complexes maintained their binuclear structures in the polymerization.

Recent advances in synthesis of organometallic complexes of indium

The indium complexes are being used in many applications like catalysis, optoelectronics, sensors, solar cells, biochemistry, medicine, infrared (IR) mirrors and thin-film transistors (TFTs). In organometallic complexes of indium, it forms different types of complexes with single, double, triple and tetra linkages by coordinating with numerous elements like C, N, O and S and also with some other elements like Se and Ru. So, the present study comprises all the possible ways to synthesize the indium complexes by reacting with different organic ligands; most of them are N-heterocyclic carbenes, amines, amides and phenols. The commonly used solvents for these syntheses are tetrahydrofuran, dichloromethane, toluene, benzene, dimethyl sulfoxide (DMSO) and water. According to the nature of the ligands, indium complexes were reported at different temperatures and stirring time. Because of their unique characteristics, the organometallic chemistry of group 13 metal indium complexes remains a subject of continuing interest in synthetic chemistry as well as material science.

From cyclic amines and acetonitrile to amidine zinc(ii) complexes

A seemingly simple combination of [Zn(quin)₂(H₂O)] (quin⁻ = quinaldinate) and a selected secondary cyclic amine, piperidine (pipe), pyrrolidine (pyro) or morpholine (morph), afforded in acetonitrile a number of products: anionic homoleptic quinaldinate, neutral heteroleptic quinaldinate/amine and quinaldinate/amidine complexes. The piperidine and pyrrolidine systems underwent reaction with acetonitrile to give amidines. The in situ formed piperidinoacetamidine (pipeam) or pyrrolidinoacetamidine (pyroam) coordinated to zinc(ii). Reactions with piperidine led to trans-[Zn(quin)₂(pipe)₂]·2CH₃CN (1), [Zn(quin)₂(pipe)]·cis-[Zn(quin)₂(pipe)₂] (2), pipeH[Zn(quin)₃]·CH₃CN (3), [Zn(quin)₂(pipeam)]·CH₃CN (4a), [Zn(quin)₂(pipeam)]·2CHCl₃ (4b), pipeamH[Zn(quin)₃] (5) and pipeamH[Zn(quin)₂(CH₃COO)]·acetamide (6) (pipeH⁺ and pipeamH⁺ denote protonated amine or amidine). By analogy, [Zn(quin)₂(pyro)₂] (7), pyroH[Zn(quin)₃]·CH₃CN (8), pyroH[Zn(quin)₂Cl] (9), [Zn(quin)₂(pyroam)]·CH₃CN·0.5pyroam·0.5H₂O (10a), [Zn(quin)₂(pyroam)]·2CHCl₃ (10b), [Zn(quin)₂(pyroam)]·CH₂Cl₂ (10c) and pyroamH[Zn(quin)₃] (11) were obtained in the pyrrolidine reactions. The morpholine system allowed isolation of only two novel products, trans-[Zn(quin)₂(morph)₂] (12) and morphH[Zn(quin)₃]·CH₃CN (13). Importantly, no amidine could be isolated. Instead, in autoclaves at 105 °C morpholine degraded to ammonia, as confirmed by mass spectrometry of the gas phase. pyroamH[Zn(quin)₃] exists in two polymorphs which differ in the binding modes of quinaldinate ligands. In 11triclinic, the metal ion of [Zn(quin)₃]⁻ features a five-coordinate environment, whereas that in 11monoclinic is surrounded by six donors. Stabilities of the [Zn(quin)₃]⁻ isomers were assessed with DFT calculations. The one with a six-coordinate zinc(ii) ion was found to be more stable than its five-coordinate counterpart. Favorable intermolecular interactions in the solid state stabilize both and reduce the energy difference between them. The calculations show the conversion of the five-coordinate [Zn(quin)₃]⁻ into its coordinatively saturated isomer to be an almost barrierless process.

Piperidine and pyrrolidine, both belonging to a group of secondary cyclic amines, reacted with acetonitrile in the presence of zinc(ii) to give the corresponding amidines that coordinated to the metal through their imine nitrogen.

Dinucleating Amino-Phenolate Platform for Zinc Catalysts: Impact on Lactide Polymerization

We report imine- and amine-based dinucleating ligands bearing a bisphenol backbone and explore their coordination chemistry with zinc to form zinc alkyl, alkoxide, acetate, and amide complexes. Full characterization of the complexes shows that this ligand framework can support dinuclear and trinuclear complexes. We explore the reactivity of the zinc alkyl and alkoxide complexes as catalysts for the ring opening polymerization of lactide and compared this reactivity to analogous mononuclear complexes. We show that 1) The amine-based complexes are more reactive than the imine-based analogues; 2) The trinuclear zinc alkyl species show unusual control and reproducibility for lactide polymerization; and 3) The extent of bimetallic cooperation is hampered by the ability of the ligand framework to form trinuclear clusters.

Dinuclear aluminum complexes bearing methylene-bridged phenoxy-imine ligands and their application in the ring-opening polymerization of rac-lactide

The first report on the use of dinuclear aluminum complexes supported by methylene-bridged phenoxy-imine ligands for the ring-opening polymerization of rac-lactide.

Indium phosphasalen catalysts showing high isoselectivity and activity in racemic lactide and lactone ring opening polymerizations

Study of a series of phosphasalen indium alkoxide complexes reveals that the substitution pattern at the phosphorus atoms can deliver outstanding isoselectivity with high rates.

Highly active rare-earth metal catalysts for heteroselective ring-opening polymerization of racemic lactide

Rare-earth metal complexes usually exhibit high activities in the ring-opening polymerization (ROP) of lactide, yet only a few scandium complexes have shown satisfactory activity. Herein, we report the synthesis of a series of chiral anilido-oxazoline-supported scandium and yttrium complexes that exhibit high activity in the ROP of racemic lactide (rac-LA). Complexes La-f-Ln(CH2SiMe3)2THF (La-f = 2-(2,6-R2PhN)-phenyl-4-(S)-R'-oxazoline; for 1a-f: L = La-f, Ln = Sc; for 2a-d: L = La-d, Ln = Y) were synthesized via the convenient one-pot reaction of Ln(CH2SiMe3)3(THF)2 (Ln = Sc, Y) with the corresponding proligands. The crystal structures of 1a, 1d, 1e, and 1f were isostructural, adopting a distorted trigonal bipyramidal configuration. Sc complexes 1 showed outstanding activity in the ROP of rac-LA with heteroselectivity. TOFs of up to 720 h-1 and 2910 h-1 were obtained in THF at room temperature and toluene at 60 °C, respectively. To our knowledge, these are the highest activities reported for Sc systems. Y complexes 2 showed higher activity and heteroselectivity than the Sc complexes, with TOFs of up to 1176 h-1 in THF at room temperature. Compared with the ortho-substituent on the anilido moiety, the bulky substituent at the chiral center of the oxazoline ring had a greater effect on controlling the heteroselectivity.

Modular bimetallic complexes with a sulfonamido-based ligand

A series of bimetallic complexes prepared with the ligands N,N,N',N'-tetramethylethane-1,2-diamine (TMEDA) and N,N',N''-[2,2',2''-nitrilotris(ethane-2,1-diyl)]tris(2,4,6-trimethylbenzenesulfonamido) ([MST]3-) is described. Four diiron compounds of the formulation (TMEDA)FeII(X)-(μ-OH)-FeIIIMST were prepared, in which the X- ligands are the anions OTf-, Br-, SCN-, or N3-. Additionally, two heterobimetallic compounds of the formulation (TMEDA)MII(OTf)-(μ-OH)-FeIIIMST (MII = CoII or NiII) were synthesized. All these compounds have similar spectroscopic and structural properties. The diiron compounds exhibit perpendicular-mode electron paramagnetic resonance spectra consistent with S = 1/2 spin ground states, which is expected for high-spin FeII and FeIII centres that are antiferromagnetically coupled. The heterobimetallic (TMEDA)NiII(OTf)-(μ-OH)-FeIIIMST complex had a spin state of S = 3/2 that also resulted from antiferromagnetic coupling between the high-spin NiII and FeIII centres. The modularity of this system is further demonstrated by the substitution of the TMEDA ligand with ethylenediamine (en); for this species two equivalents of en coordinate to the FeII centre to form [(en)2FeII-(μ-OH)-FeIIIMST]OTf. These results demonstrate that a modular bimetallic system has been developed in which the key components can be modified.

Indium Catalysts for Ring Opening Polymerization: Exploring the Importance of Catalyst Aggregation

Inexorably, the environmental persistence and damage caused by polyolefins have become major drawbacks to their continued long-term use. Global shifts in thinking from fossil-fuel to renewable biobased resources have urged researchers to focus their attention on substituting fossil-fuel based polymers with renewable and biodegradable alternatives on an industrial scale. The recent development of biodegradable polyesters from ring opening polymerization (ROP) of bioderived cyclic ester monomers has emerged as a promising new avenue toward this goal. Ever increasing numbers of metal-based initiators have been reported in the literature for the controlled ROP of cyclic esters, in particular for the polymerization of lactide to produce poly(lactic acid) (PLA). PLA has several material weaknesses, which hinder its use as a replacement for commodity plastics. Despite many advances in developing highly active and controlled catalysts for lactide polymerization, no single catalyst system has emerged to replace industrially used catalysts and provide access to PLA materials with improved properties. We reported the first example of indium(III) for the ring opening polymerization of lactide. Since then, indium(III) has emerged as a useful Lewis acid in initiators for the controlled polymerization of lactide and other cyclic esters. In particular, we have developed a large family of chiral dinuclear indium complexes bearing tridentate diaminophenolate ligands and tetradentate salen and salan ligands. Complexes within our tridentate ligand family are highly active initiators for the moderately isoselective living and immortal polymerization of rac-lactide, as well as other cyclic esters. We have shown that subtle steric effects influence aggregation in these systems, with polymerization typically proceeding through a dinuclear propagating species. In addition, profound effects on polymerization activities have been observed for central tertiary versus secondary amine donors in these and other related systems. In contrast, our well-controlled and highly active chiral indium salen systems are more isoselective than the tridentate analogues and polymerize lactide via a mononuclear propagating species. Again, we have noticed that subtle steric and electronic changes to the ligand can influence both polymerization activity and stereoselectivity via aggregation phenomena. Recently, we have reported a promising new chiral indium catalyst supported by a tetradentate salan ligand. This catalyst is remarkably water and air stable and can be activated by linear and branched alcohols to provide controlled access to multiblock copolymers in air. This catalyst represents an important step forward toward generating new, commercially relevant catalysts for ROP of cyclic esters to produce novel biodegradable polymers, and highlights the unique value of indium-based catalysts in the field.

Synthesis of Aluminum Complexes Bearing 8-Anilide-5,6,7-trihydroquinoline Ligands: Highly Active Catalyst Precursors for Ring-Opening Polymerization of Cyclic Esters

The stoichiometric reactions of 8-(2,6-R¹-4-R²-anilide)-5,6,7-trihydroquinoline (LH) with AlR₃ (R = Me or Et) afforded the aluminum complexes LAlR₂ (Al1⁻Al5,Al1: R¹ = ⁱPr, R² = H, R = Me; Al2: R¹ = Me, R² = H, R = Me; Al3: R¹ = H, R² = H, R = Me; Al4: R¹ = Me, R² = Me, R = Me; Al5: R¹ = Me, R² = Me, R = Et) in high yields. All aluminum complexes were characterized by NMR spectroscopy and elemental analysis. The molecular structures of complexes Al4 and Al5 were determined by single-crystal X-ray diffractions and revealed a distorted tetrahedral geometry at aluminum. In the presence of BnOH, complexes Al1⁻Al5 efficiently initiated the ring-opening homopolymerization of ε-caprolactone (ε-CL) and rac-lactide (rac-LA), respectively, in a living/controlled manner.

Aluminium complexes containing salicylbenzothiazole ligands and their application in the ring-opening polymerisation of rac-lactide and ε-caprolactone

This paper is the first report on the use of aluminium salicylbenzothiazole complexes for the ROP of rac-LA and ε-CL.

Zinc bimetallics supported by a xanthene-bridged dinucleating ligand: synthesis, characterization, and lactide polymerization studies

Di-zinc complexes of a new dinucleating xanthene-based bis(iminophenolate) ligand have been synthesized, and their coordination chemistry and lactide polymerization activity have been investigated.

Impact of aryloxy initiators on the living and immortal polymerization of lactide

This report describes two different methodologies for the synthesis of aryl end-functionalized poly(lactide)s (PLAs) catalyzed by indium complexes.

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.