Discrete Divalent Rare-Earth Cationic ROP Catalysts: Ligand-Dependent Redox Behavior and Discrepancies with Alkaline-Earth Analogues in a Ligand-Assisted Activated Monomer Mechanism

A Highly Active and Selective Zirconium-Based Catalyst System for the Industrial Production of Poly(lactic acid)

The biodegradable, aliphatic polyester poly(lactic acid), PLA, is a leading bio-based alternative to petrochemical-derived plastic materials across a range of applications. Widely reported in the available literature as a benchmark for PLA production via the bulk ring-opening polymerization of lactides is the use of divalent tin catalysts, and particularly tin(II) bis(2-ethylhexanoate). We present an alternative zirconium-based system that combines an inexpensive Group IV metal with the robustness, high activity, control, and designed compatibility with existing facilities and processes, that are required for industrial use. We have carried out a comprehensive kinetic study and applied a combined experimental and theoretical approach to understanding the mechanism by which the polymerization of lactide proceeds in the presence of this system. In the laboratory-scale (20 g) polymerization of recrystallized racemic d,l-lactide (rac-lactide), we have measured catalyst turnover frequencies up to at least 56,000 h-1, and confirmed the reported protocols' resistance toward undesirable epimerization, transesterification, and chain scission processes, deleterious to the properties of the polymer product. Further optimization and scale-up under industrial conditions have confirmed the relevance of the catalytic protocol to the commercial production of melt-polymerized PLA. We were able to undertake the efficient preparation of high-molecular-weight PLA on the 500-2000 g scale, via the selective and well-controlled polymerization of commercial polymer-grade l-lactide under challenging, industrially relevant conditions, and at metal concentrations as low as 8-12 ppm Zr by weight ([Zr] = 1.3 × 10-3 to 1.9 × 10-3 mol %). Under those conditions, a catalyst turnover number of at least 60,000 was attained, and the activity of the catalyst was comparable to that of tin(II) bis(2-ethylhexanoate).

Bonding analysis in ytterbium(II) distannyl and related tetryls

The syntheses of the ytterbium(II) distannyl [Yb{Sn(SiMe₃)₃}₂·(thf)₄] (Yb-Sn) and of its digermyl analogue [Yb{Ge(SiMe₃)₃}₂·(thf)₃] (Yb-Ge) are presented. The compounds were characterised by multinuclear high-resolution solution NMR spectroscopy, including ¹⁷¹Yb NMR, and by X-ray diffraction crystallography. The bonding and electronic properties of the two complexes, along with those of the known ytterbium(II) disilyl derivative [Yb{Si(SiMe₃)₃}₂·(thf)₃] (Yb-Si) and those of the congeneric calcium distannyl [Ca{Sn(SiMe₃)₃}₂·(thf)₄] (Ca-Sn), were investigated in detail by DFT calculations. This analysis points at a primarily ionic Yb-tetrel bonding, with a small covalent contribution, attributed principally to the 5d(Yb) participation. This weak covalent character is found to be larger for the distannyl Yb-Sn than for its lighter Si- and Ge-derivatives. The covalent component is also found to be greater in Yb-Sn than in Ca-Sn, due to the availability of the 5d(Yb) orbitals for bonding.

Application of the Redox-Transmetalation Procedure to Access Divalent Lanthanide and Alkaline-Earth NHC Complexes*

Divalent lanthanide and alkaline-earth complexes supported by N-heterocyclic carbene (NHC) ligands have been accessed by redox-transmetalation between air-stable NHC-AgI complexes and the corresponding metals. By using the small ligand 1,3-dimethylimidazol-2-ylidene (IMe), two series of isostructural complexes were obtained: the tetra-NHC complexes [LnI2 (IMe)4 ] (Ln=Eu and Sm) and the bis-NHC complexes [MI2 (IMe)2 (THF)2 ] (M=Yb, Ca and Sr). In the former, distortions in the NHC coordination were found to originate from intermolecular repulsions in the solid state. Application of the redox-transmetalation strategy with the bulkier 1,3-dimesitylimidazol-2-ylidene (IMes) ligand yielded [SrI2 (IMes)(THF)3 ], while using a similar procedure with Ca metal led to [CaI2 (THF)4 ] and uncoordinated IMes. DFT calculations were performed to rationalise the selective formation of the bis-NHC adduct in [SrI2 (IMe)2 (THF)2 ] and the tetra-NHC adduct in [SmI2 (IMe)4 ]. Since the results in the gas phase point towards preferential formation of the tetra-NHC complexes for both metal centres, the differences between both arrangements are a result of solid-state effects such as slightly different packing forces.

Alkali and Alkaline Earth Metal Complexes as Versatile Catalysts for Ring-Opening Polymerization of Cyclic Esters

Biodegradable polyesters such as poly(ϵ-caprolactone) (PCL) and poly(lactic acid) (PLA) have been considered for use in several areas, such as drug delivery devices, sutures, tissue engineering, and GBR membranes, due to its bio-renewability, biodegradability, and biocompatibility. Several synthetic techniques for the preparation of polyesters have been reported in the literature, amongst which the ring-opening polymerization (ROP) of cyclic esters is the most efficient. A convenient approach to access iso-selective PLAs is polymerization of racemic lactide (rac-LA), which shows excellent stereoregularity without the need for costly chiral auxiliaries or ligands. In this personal account, we review a series of methods that have been practiced to the synthesis of biodegradable polyesters from various cyclic monomers using alkali and alkaline earth metal complexes as efficient catalysts.

Application of the Molecular Invariom Model for the Study of Interactions Involving Fluorine Atoms in the {$${\text{Yb}}_{{\text{2}}}^{{{\text{II}}}}$$(μ2-OCH(CF3)2)3(μ3-OCH(CF3)2)2YbIII(OCH(CF3)2)2(THF)(Et2O)} Complex

Abstract

The electron density distributions obtained by the quantum-chemical (density functional theory) calculations and molecular invariom model in the trimeric ytterbium complex with the hexafluoroisopropoxide ligands {$${\text{Yb}}_{{\text{2}}}^{{{\text{II}}}}$$(μ2-OR)3(μ3-OR)2YbIII(OR)2(THF)(Et2O)} (I) (where R is CH(CF3)2, and THF is tetrahydrofuran) are compared. The main topological characteristics of the electron density at the critical points (3, –1) corresponding to the interactions of the ytterbium atoms in the coordination sphere obtained using two studied approaches demonstrate excellent agreement. The maximum divergence between the density functional calculations and molecular invariom model is observed for the intramolecular interactions involving the fluorine atoms (F···F, F···H, and F···O) in the structure of complex I. Geometry optimization leads to a higher number of these interactions in the complex. The energy corresponding to these interactions also increases. However, the main topological characteristics for the F···X interactions (X = F, H, O), which can be localized in the framework of both methods, remain within the transferability index range. An analysis of the deformation electron density shows that the Fδ–···Fδ– interactions are determined by the correspondence of the region of electron density concentration on one of the fluorine atoms to the region of electron density depletion on the second fluorine atom regardless of the method of measuring the electron density distribution.

Synthesis of amino-phenolate manganese complexes and their catalytic activity in carbon dioxide activation and oxidation reactions

Two manganese(III) compounds were studied as catalysts for the reaction of carbon dioxide with propylene oxide, styrene oxide, and cyclohexene oxide, and formed cyclic carbonate products selectively under solvent free conditions in the presence of an ionic co-catalyst such as TBAB or PPNCl. Variable temperature kinetic studies allowed the activation energy for propylene carbonate formation to be determined (64 kJ mol−1). The catalysts showed good stability in these reactions and overall turnover numbers (TON) of up to 6000 were observed. The complexes showed low activity for the aerobic oxidation of 4-methyoxybenzylalcohol to the corresponding aldehyde, achieving up to 40% conversion in 72 h. However, near quantitative conversion of 1,2-diphenyl-2-methoxyethanol to provide up to 90% yield of benzaldehyde could be achieved over the course of 5 days. Both complexes showed similar reactivity in the two catalytic processes, and this is likely due to the weakly coordinating nature of the pendant donor within the tetradentate ligand.

Alkaline Earth Metal-Mediated Highly Iso-selective Ring-Opening Polymerization of rac-Lactide

Alkaline earth (Ae) metal complexes of the aminophosphine borane ligand are highly active and iso-selective catalysts for the ring-opening polymerization (ROP) of rac-lactide (LA). The polymerization reactions are well controlled and produce polylactides with molecular weights that are precise and narrowly distributed. Kinetic studies reveal that the ROP of rac-LA catalyzed by all Ae metal complexes had a first-order dependency on LA concentration as well as catalyst concentration. A plausible reaction mechanism for Ae metal complex-mediated ROP of rac-LA is discussed, based on controlled kinetic experiments and molecular chain mobility.

Zinc Complexes for PLA Formation and Chemical Recycling: Towards a Circular Economy

A series of Zn^II complexes, based on propylenediamine Schiff bases, have been prepared and fully characterized. X-ray crystallography and NMR spectroscopy identified significant differences in the solid and solution state for the Zn^II species. All complexes have been applied to the ring-opening polymerization of l-lactide with emphasis on industrial conditions. High conversion and good molecular weight control were generally achievable for Zn(A-D)₂ , and high-molecular-weight poly(lactic acid) (PLA) was prepared in 1 min at a 10 000:1:33 [lactide]/[Zn]/[BnOH] loading. The more active Zn^II catalysts were also applied to PLA degradation to alkyl lactate under mild conditions. Zn(A-B)₂ demonstrated high activity and selectivity in this process with PLA being consumed within 1 h at 50 °C. Zn(C-D)₂ were shown to be less active, and these observations can be related to the catalysts' structure and the degradation mechanism. Initial results for the degradation of poly(ethylene terephthalate) and mixed feeds are also presented, highlighting the broader applicability of the systems presented.

Coordination Ring-Opening Polymerization of Cyclic Esters: A Critical Overview of DFT Modeling and Visualization of the Reaction Mechanisms

Ring-opening polymerization (ROP) of cyclic esters (lactones, lactides, cyclic carbonates and phosphates) is an effective tool to synthesize biocompatible and biodegradable polymers. Metal complexes effectively catalyze ROP, a remarkable diversity of the ROP mechanisms prompted the use of density functional theory (DFT) methods for simulation and visualization of the ROP pathways. Optimization of the molecular structures of the key reaction intermediates and transition states has allowed to explain the values of catalytic activities and stereocontrol events. DFT computation data sets might be viewed as a sound basis for the design of novel ROP catalysts and cyclic substrates, for the creation of new types of homo- and copolymers with promising properties. In this review, we summarized the results of DFT modeling of coordination ROP of cyclic esters. The importance to understand the difference between initiation and propagation stages, to consider the possibility of polymer-catalyst coordination, to figure out the key transition states, and other aspects of DFT simulation and visualization of ROP have been also discussed in our review.

DFT Visualization and Experimental Evidence of BHT-Mg-Catalyzed Copolymerization of Lactides, Lactones and Ethylene Phosphates

Catalytic ring-opening polymerization (ROP) of cyclic esters (lactides, lactones) and cyclic ethylene phosphates is an effective way to process materials with regulated hydrophilicity and controlled biodegradability. Random copolymers of cyclic monomers of different chemical nature are highly attractive due to their high variability of characteristics. Aryloxy-alkoxy complexes of non-toxic metals such as derivatives of 2,6-di-tert-butyl-4-methylphenoxy magnesium (BHT-Mg) complexes are effective coordination catalysts for homopolymerization of all types of traditional ROP monomers. In the present paper, we report the results of density functional theory (DFT) modeling of BHT-Mg-catalyzed copolymerization for lactone/lactide, lactone/ethylene phosphate and lactide/ethylene phosphate mixtures. ε-Caprolactone (ε-CL), l-lactide (l-LA) and methyl ethylene phosphate (MeOEP) were used as examples of monomers in DFT simulations by the Gaussian-09 program package with the B3PW91/DGTZVP basis set. Both binuclear and mononuclear reaction mechanistic concepts have been applied for the calculations of the reaction profiles. The results of calculations predict the possibility of the formation of random copolymers based on l-LA/MeOEP, and substantial hindrance of copolymerization for ε-CL/l-LA and ε-CL/MeOEP pairs. From the mechanistic point of view, the formation of highly stable five-membered chelate by the products of l-LA ring-opening and high donor properties of phosphates are the key factors that rule the reactions. The results of DFT modeling have been confirmed by copolymerization experiments.

Preparation of star-shaped functionalized polylactides by metal porphyrin complexes as both catalysts and cocatalysts

Several aluminum porphyrin complexes as catalysts and a copper porphyrin complex as a cocatalyst were prepared. These complexes were characterized by 1H NMR and elemental analysis. These complexes are used for L-lactide polymerization. The kinetic data of the polymerization using complex 2 as catalyst revealed that the polymeric rates were first-ordered in both the monomer and catalyst. There is a linear relationship between lactide conversion and the number-averaged molecular weight of PLA.

Tetradentate iminophenolate copper complexes in rac-lactide polymerization

Copper(II) nitrate complexes of 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)phenol, 2-(((2-((2-aminoethyl)amino)ethyl)imino)methyl)-4,6-dichlorophenol, 2-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol and (2,4-di-tert-butyl-6-(((2-(piperazin-1-yl)ethyl)imino)methyl)phenol, as well as a copper(II) acetate complex of 2-(((2-(piperidin-1-yl)ethyl)imino)methyl)phenol, have been prepared and characterized by X-ray diffraction studies. In combination with benzyl alcohol, all complexes are active in rac-lactide polymerization at 140 °C in molten monomer to provide moderately heterotactic polylactic acid. Most complexes showed complicated reaction kinetics, indicative of two interconverting active species. Molecular weight control was poor and a strong tendency toward intramolecular transesterification led to oligomeric products. There was no indication that the basic site of the ligand is participating in the polymerization reaction by deprotonation of the alcohol nucleophile.

Synthesis of biodegradable and biorenewable polylactides initiated by aluminum complexes bearing m-xylylenediamine derivatives via the ring-opening polymerization of lactides

Aluminum complexes derived from m-xylylenediamine were synthesized and investigated as initiators for l-lactide and rac-lactide polymerization.

Alkali metal complex-mediated ring-opening polymerization of rac-LA, ε-caprolactone, and δ-valerolactone

Catalytic ring opening polymerization (ROP) of rac-lactide, ε-caprolactone, and δ-valerolactone using alkali metal (Li, Na, K) complexes as competent catalysts are reported.

Cyclooctatetraenyl calcium and strontium amido complexes

The first examples of Ca and Sr amido complexes built on a cyclooctatetraenyl platform are reported.

Mixed Allyl Rare-Earth Borohydride Complexes: Synthesis, Structure, and Application in (Co-)Polymerization Catalysis of Cyclic Esters

A series of new trivalent rare-earth allyl-borohydride complexes with the formula [RE(BH₄ )₂ (C₃ H₅ )(thf)_x ] (RE=Sc (1), x=2; RE=Y (2) and La (3), x=3) were synthesized by reaction of the corresponding rare-earth trisborohydrides [RE(BH₄ )₃ (thf)_x ] with half an equivalent of bis(allyl)magnesium. The complexes were fully characterized by determining their X-ray structure. Similar to their previously described Nd (4) and Sm (5) analogues, these complexes display a monomeric structure with two terminal trihapto BH₄ groups, one π-η³ allyl ligand, three THF molecules for complexes 2 and 3, and two THF molecules for complex 1. The catalytic behavior of complexes 1-5 toward the ring-opening polymerization (ROP) of l-lactide (l-LA) and ϵ-caprolactone (ϵ-CL) was assessed. The Nd complex featured the best activity toward l-LA (turnover frequency (TOF)=1300 h^-1 ) and the order was Nd>La>Sm>Y>Sc. Complexes 1-3 were found very active for the ROP of ϵ-CL (TOF=166 000 h^-1 ), which is in line with the already established exceptionnally high performance of complexes 4 and 5. With both monomers, it was shown that the borohydride moiety was the preferentially initiating group, rather than the allyl one. The random copolymerization of l-LA and ϵ-CL was performed with complexes 1-5, in the absence or in the presence of benzyl alcohol as a chain-transfer agent, affording copolymers with ϵ-caprolactone up to 62 % inserted. The copolymers synthesized display a variety of microstructures, that is, blocky, random, or quasi-alternating.

Tethered cationic alkaline earth – olefin complexes

The utilisation of a specifically tailored aminofluoroalcohol possessing both a methoxy and an olefin dangling side arms enables the preparation of the first examples of discrete calcium- and strontium-olefin cationic complexes.

Calcium, Strontium and Barium Homogeneous Catalysts for Fine Chemicals Synthesis

The large alkaline earths (Ae), calcium, strontium and barium, have in the past 15 years yielded a brand new generation of heteroleptic molecular catalysts for the production of fine chemicals. However, the integrity of these complexes is often plagued by ligand redistribution equilibria in solution. This personal account retraces the paths followed in our research group towards the design of stable heteroleptic alkalino-earth complexes, including the use of intramolecular noncovalent Ae···H-Si and Ae···F-C interactions. Their implementation as homogenous precatalysts for reactions such as the intramolecular and intermolecular hydroamination and hydrophosphination of activated alkenes, the hydrophosphonylation of ketones, and the dehydrogenative coupling of amines and hydrosilanes that enable the efficient and controlled formations of CP, CN, or SiN σ-bonds, is presented in a synthetic perspective that highlights their overall outstanding catalytic performance.

P,O-Phosphinophenolate zinc(II) species: synthesis, structure and use in the ring-opening polymerization (ROP) of lactide, ε-caprolactone and trimethylene carbonate

The P,O-type phosphinophenol proligands (1·H, 2-PPh2-4-Me-6-Me-C6H2OH; 2·H, 2-PPh2-4-Me-6-(t)Bu-C6H2OH) readily react with one equiv. of ZnEt2 to afford in high yields the corresponding Zn(II)-ethyl dimers of the type [(κ(2)-P,O)Zn-Et]2 (3 and 4) with two μ-OPh bridging oxygens connecting the two Zn(II) centers, as determined by X-ray diffraction (XRD) studies in the case of 3. Based on diffusion-ordered NMR spectroscopy (DOSY), both species 3 and 4 retain their dimeric structures in solution. The alcoholysis reaction of Zn(II) alkyls 3 and 4 with BnOH led to the high yield formation of the corresponding Zn(II) benzyloxide species [(κ(2)-P,O)Zn-OBn]2 (5 and 6), isolated in a pure form as colorless solids. The centrosymmetric and dimeric nature of Zn(II) alkoxides 5 and 6 in solution was deduced from DOSY NMR experiments and multinuclear NMR data. Though the heteroleptic species 5 is stable in solution, its analogue 6 is instable in CH2Cl2 solution at room temperature to slowly decompose to the corresponding homoleptic species 8via the transient formation of (κ(2)-P,O)2Zn2(μ-OBn)(μ–κ(1):κ(1)-P,O) (6′). Crystallization of compound 6 led to crystals of 6′, as established by XRD analysis. The reaction of ZnEt2 with two equiv. of 1·H and 2·H allowed access to the corresponding homoleptic species of the type [Zn(P,O)2] (7 and 8). All gathered data are consistent with compound 7 being a dinuclear species in the solid state and in solution. Data for species 8, which bears a sterically demanding P,O-ligand, are consistent with a mononuclear species in solution. The Zn(II) alkoxide species 5 and the [Zn(P,O)2]-type compounds 7 and 8 were evaluated as initiators of the ring-opening polymerization (ROP) of lactide (LA), ε-caprolactone (ε-CL) and trimethylene carbonate (TMC). Species 5 is a well-behaved ROP initiator for the homo-, co- and ter-polymerization of all three monomers with the production of narrow disperse materials under living and immortal conditions. Though species 7 and 8 are ROP inactive on their own, they readily polymerize LA in the presence of a nucleophile such as BnOH to produce narrow disperse PLA, presumably via an activated-monomer ROP mechanism.

C–F→Ln/An interactions in synthetic f-element chemistry

C–F→Ln/An interactions have been increasingly recognized as a key aspect of f-element chemistry over the last two decades. This Perspective summarizes the literature on the nature of C–F→Ln/An contacts, their role in the structural and coordination chemistry of f-block elements and their applications for C–F bond activation.

Synthesis, structure and luminescent properties of lanthanide fluoroalkoxides

A set of new lanthanide fluoroalkoxides have been synthesized and characterized, some of them revealed ultraviolet photoluminescence.

Stereoselective Alkali-Metal Catalysts for Highly Isotactic Poly(rac-lactide) Synthesis

A high degree of chain end control in the isoselective ring-opening polymerization (ROP) of rac-lactide is a challenging research goal. In this work, eight highly active sodium and potassium phenolates as highly isoselective catalysts for the ROP of rac-lactide are reported. The best isoselectivity value of Pm = 0.94 is achieved. The isoselective mechanism is chain-end control through the analysis of the stereoerrors in the microstructure of a final polymer; thus, isotactic multiblock structure polymers are obtained, and the highest melt point can reach 192.5 °C. The donating group in phenolate can clearly accelerate the ROP reaction, potassium complexes are more active than the analogous sodium complexes, and the big spacial hindrance of the ligand can decrease the activity. The high isoselectivities of these complexes mostly result from their sandwich structure constructed by the plane of the crown and the plane of the anthryl group.

Preparation of biocompatible, biodegradable and sustainable polylactides catalyzed by aluminum complexes bearing unsymmetrical dinaphthalene-imine derivatives via ring-opening polymerization of lactides

Aluminum complexes bearing unsymmetrical dinaphthalene-imine ligands were employed for lactide polymerization.

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.

Ring-opening polymerization of lactide using chiral salen aluminum complexes as initiators: high productivity and stereoselectivity

Chiral salen aluminum complexes have been synthesized and investigated as initiators for l-lactide and rac-lactide polymerization.

Stereoselective ring-opening polymerization of rac-lactides catalyzed by titanium complexes containing N,N-bidentate phenanthrene derivatives

Titanium complexes bearing N,N-bidentate diamido derived from Schiff bases were investigated as catalysts for lactide polymerization.

Hemi-salen aluminum catalysts bearing N, N, O-tridentate type binaphthyl-Schiff-base ligands for the living ring-opening polymerisation of lactide

Hemi-salen aluminum complexes bearing tridentate N, N, O-type binaphthyl-Schiff-base ligands were synthesized and studied as catalysts for living polymerisation of lactide.

Titanium complexes with octahedral geometry chelated by salen ligands adopting β-cis configuration for the ring-opening polymerisation of lactide

Titanium complexes (Ti atom located in an octahedral geometry) based on quadridentate salen ligands as initiators for lactide polymerisation.

Magnesium complexes bearing N,N-bidentate phenanthrene derivatives for the stereoselective ring-opening polymerization of rac-lactides

Magnesium complexes based on N,N-bidentate phenanthrene derivatives derived from Schiff bases were synthesized and investigated as catalysts for rac-lactide polymerization.

Comparing a series of 8-quinolinolato complexes of aluminium, titanium and zinc as initiators for the ring-opening polymerization of rac-lactide

The preparation, characterization and properties of a series of 8-hydroxy quinolinolate complexes of Ti(iv), Al(iii) and Zn(ii) are presented. The complexes are active initiators for rac-lactide polymerizations: their performances are compared.