Isotacticrac-Lactide Polymerization with Copper Complexes: The Influence of Complex Nuclearity

Understanding catalytic synergy in dinuclear polymerization catalysts for sustainable polymers

Understanding the chemistry underpinning intermetallic synergy and the discovery of generally applicable structure-performances relationships are major challenges in catalysis. Additionally, high-performance catalysts using earth-abundant, non-toxic and inexpensive elements must be prioritised. Here, a series of heterodinuclear catalysts of the form Co(III)M(I/II), where M(I/II) = Na(I), K(I), Ca(II), Sr(II), Ba(II) are evaluated for three different polymerizations, by assessment of rate constants, turn over frequencies, polymer selectivity and control. This allows for comparisons of performances both within and between catalysts containing Group I and II metals for CO2/propene oxide ring-opening copolymerization (ROCOP), propene oxide/phthalic anhydride ROCOP and lactide ring-opening polymerization (ROP). The data reveal new structure-performance correlations that apply across all the different polymerizations: catalysts featuring s-block metals of lower Lewis acidity show higher rates and selectivity. The epoxide/heterocumulene ROCOPs both show exponential activity increases (vs. Lewis acidity, measured by the pKa of [M(OH2)m]n+), whilst the lactide ROP activity and CO2/epoxide selectivity show linear increases. Such clear structure-activity/selectivity correlations are very unusual, yet are fully rationalised by the polymerization mechanisms and the chemistry of the catalytic intermediates. The general applicability across three different polymerizations is significant for future exploitation of catalytic synergy and provides a framework to improve other catalysts.

Isoselective Polymerization of rac-Lactide by Magnesium Initiators Bearing Achiral Di(2-pyridyl)methyl Substituted Aminophenolate Ligands

Reactions of achiral di(2-pyridyl)methyl substituted aminophenols L^1-6H (2-{N-R³-N-[di(2-pyridyl)methyl]aminomethyl}-4-R¹-6-R²-C₆H₂OH: R¹ = R² = ^tBu, R³ = ⁿBu (L¹H), R³ = ⁿhexyl (L²H), R³ = cyclohexyl (L³H); R¹ = R² = cumyl, R³ = ⁿBu (L⁴H), R³ = ⁿhexyl (L⁵H), R³ = cyclohexyl (L⁶H)) with {Mg[N(SiMe₃)₂]₂}₂ ([L^1-6H]:[Mg] = 1:1) afforded a series of magnesium silylamido complexes 1-6. In the solid state, the magnesium center of 3, 4, and 6 is penta-coordinated by the tetradentate aminophenloate ligand and one silylamido ligand to form a seriously distorted square-pyramidal geometry as confirmed by X-ray crystallography diffraction analysis. VT ¹H NMR and ROESY experiments further indicate that these magnesium complexes are also five-coordinated in solutions where the coordination of either of the two pyridyl pendants to the magnesium center is maintained. Complexes 1-6 are highly active toward the ring-opening polymerization of rac-lactide (rac-LA) at r.t. both in toluene and in tetrahydrofuran, capable of polymerizing 500 equiv of monomer to high conversions just within minutes. Among them, complex 3 exhibited the highest iso-stereoselectivity, affording moderately isotactic polylactide in toluene (P_m = 0.75). It is found that the isoselectivities and activities of these magnesium complexes toward the polymerization of rac-LA are closely associated with the substituents at the ortho-position of the phenoxide unit and on the skeleton nitrogen atom of the ligand. On the basis of NMR spectroscopic studies, the formation of isotactic PLAs with dominant stereoblock sequences was witnessed by using these magnesium complexes as initiators, and the inequivalent coordination of two pyridyl pendant arms in these magnesium complexes might be the source of exerting isoselective control.

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.

Mimicking the Cu Active Site of Lytic Polysaccharide Monooxygenase Using Monoanionic Tridentate N-Donor Ligands

In an effort to prepare small molecule mimics of the active site of lytic polysaccharide monooxygenase (LPMO), three monoanionic tridentate N donor ligands comprising a central deprotonated amide group flanked by two neutral donors were prepared, and their coordination chemistry with Cu(I) and Cu(II) was evaluated. With Cu(I), a dimer formed, which was characterized by X-ray crystallography and NMR spectroscopy. A variety of mononuclear and dinuclear Cu(II) species with a range of auxiliary ligands (MeCN, Cl^-, OH^-, OAc^-, OBz^-, CO₃ ^2-) were prepared and characterized by X-ray diffraction and various spectroscopies (UV-vis, EPR). The complexes exhibit structural similarities to the LPMO active site.

Controlling polymer stereochemistry in ring-opening polymerization: a decade of advances shaping the future of biodegradable polyesters

This review highlights recent developments in the field of biodegradable polymeric materials intended to replace non-degradable conventional plastics, focusing on studies from the last ten years involving the stereoselective ring-opening polymerization of cyclic esters. This encompasses exciting advances in both catalyst design and monomer scope. Notably, the last decade has seen the emergence of metal-free stereocontrolled ROP for instance, as well as the synthesis and stereocontrolled polymerization of new types of chiral monomers. This study will emphasize recent stereoselective polymerization catalysts and chiral monomers and will focus on stereocontrol quantification, the mechanisms of stereocontrol and their differentiation if reported and studied for a specific catalyst system.

Lactide polymerization using a sterically encumbered, flexible zinc complex

4-(tert-Butyl)-2-trityl-6-(di-(2-picolyl)amine)phenol, LH, was prepared from paraformaldehyde, 4-(tert-butyl)-2-tritylphenol and di-(2-picolyl)amine. Reaction with Zn(N(SiMe3)2)2 gave LZnN(SiMe3)2. The complex was shown by X-ray diffraction study, variable temperature NMR and DFT calculations to coordinate only one pyridine ligand, which allows for fast and facile complex isomerisation. LZnN(SiMe3)2 was active in rac-lactide polymerization, but in contrast to previous complexes of this type did not show any evidence for isotactic monomer enchainment via a catalytic-site mediated chain-end control mechanism. Addition of alcohol led to increased activity, but the complex was unstable in the presence of free alcohol.

Synthesis, structures, and catalytic efficiency in ring opening polymerization of rac-lactide with tridentate vs. bidentate cobalt(ii), zinc(ii), and cadmium(ii) complexes containing N-substituted N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)amine ligands

A series of Co(ii), Zn(ii), and Cd(ii) complexes supported by 1-(3,5-dimethyl-1H-pyrazol-1-yl)-N-((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-N-(furan-2-ylmethyl)methanamine (L_A) and N,N-bis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)-4-isopropylaniline (L_B) were synthesized. The direct chelation of CoCl₂·6H₂O, ZnCl₂, and CdBr₂·4H₂O by the ligands produced [L_nMX₂] (L_n = L_A or L_B; M = Zn or Co, with X = Cl; M = Cd, with X = Br) complexes in high yields. Structural studies revealed that [L_BCoCl₂] and [L_BZnCl₂] adopted distorted tetrahedral geometries, as L_B coordinated the metal centers in a bidentate fashion, while L_A coordinated the metal centers in a tridentate fashion through the nitrogen atoms of the pyrazole and amine moieties, so that [L_ACoCl₂] and [L_AZnCl₂] exhibited trigonal bipyramidal geometries and [L_ACdBr₂] a square pyramidal geometry. [L_BCdBr₂] has two Cd-containing structures per unit cell, whereby one Cd center adopted a distorted tetrahedral geometry and the other exhibited square bipyramidal geometry. The in situ-generated alkyl derivatives of the synthesized complexes were assessed in the ring-opening polymerization of rac-lactide. Heterotactic polylactides (PLAs) were furnished with all complexes. The [L_BZnCl₂]/MeLi system produced PLA with a superior heterotactic bias (P_r up to 0.94) at −25 °C. PLAs with wide-ranging polydispersity indices (1.16–2.23) and low molecular weights were produced in all cases, irrespective of the specific M(II) center and ancillary ligand utilized.

Co(ii), Zn(ii), and Cd(ii) complexes supported by bis-pyrazolyl ligands were applied to the ring-opening polymerization of rac-lactide to produce heterotactic polylactides (PLAs) with superior heterotactic bias i.e. P_r up to 0.94 at −25 °C.

Highly Isoselective Ring-Opening Polymerization of rac-Lactide Using Chiral Binuclear Aluminum Catalyst

The microstructure can significantly affect the physical and mechanical properties of polymers. Five chiral binuclear aluminum methyl complexes (rac1 to rac5) are synthesized by using chiral binaphthalene diamine ligand and characterized by both ¹ H and ¹³ C NMR spectroscopy. The crystal structures of rac3 and rac5 are also identified by single crystal X-ray diffraction. In the presence of ⁱ PrOH initiator, these aluminum complexes catalyze the ring-opening polymerization (ROP) of rac-lactide (rac-LA) in a controlled manner and produce polymers with high to excellent isoselectivity (P_m up to 0.93). Even with a catalyst loading as low as 0.1%, polylactide with a P_m of 0.83 can still be obtained. Kinetic studies reveal the first-order dependence on monomer concentration whereas kinetic resolution polymerization provides the evidence to support that these binuclear aluminum catalysts catalyzed ROP of rac-LA adopts enantiomorphic site control mechanism. Furthermore, this strategy can also be applied to the ROP of both ε-caprolactone and δ-valeroactone.

Reactions of a pincer proligand with copper iodide: bridging instead of C–H metalation

Various precursors of divalent copper have been treated with the meta-disubstituted phenylene-based proligand POC(H)OP (1,3-(i-Pr2PO)2C6H4) with the objective of preparing classical pincer complexes (POCOP)CuX. However, in no case was such species obtained, presumably owing to the difficult C–H metallation step. Analogous reactions of monovalent precursors were also unsuccessful, whereas reaction of POC(H)OP with CuI under different conditions gave the non-metallated adducts {(μ, κP, [Formula: see text]-POC(H)OP)Cu(μ-Ι)}2, 1, {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ-Ι)2(DMAP)2}, 2 (DMAP = 4-dimethylaminopyridine), and {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ3-Ι)2}2, 3. Treating 1 with DMAP gave the adduct 2, whereas 3 could be obtained by treating 1 with BuLi or by sublimation of 1. The solid state structures of these complexes revealed the tetrahedral geometry that might be anticipated for the d10 Cu(I) centers, in addition to fairly close I–H distances; on the other hand, no C–H interaction (agostic or otherwise) was observed with the Cu centers in any of these structures. The unsuccessful metallation of the C(2)–H moiety is thought to be a result of the strong preference of monovalent copper center to form bridging interactions with iodide and the POC(H)OP ligand; this appears to prevent the approach of the central carbon of the ligand to the Cu centers.

Dinuclear iminophenoxide copper complexes in rac-lactide polymerisation

Dinuclear bis(R'-(R''-iminomethyl)phenoxide) copper complexes L2Cu2(μ-OR)2 were prepared from the reaction of copper methoxide with ROH and LH (ROH = dimethylaminoethanol or pyridylmethanol, R' = H, 4,6-tBu, 1,3-Cl, R'' = benzyl, cyclohexyl, diphenylmethyl and 2,6-dimethylphenyl). Preparation was complicated by formation of homoleptic L2Cu and only 9 of the 24 possible combinations could be prepared. All complexes were characterized by single crystal X-ray diffraction studies and crystallized as dinuclear penta-coordinated complexes. Homoleptic complexes L2Cu were inactive in lactide polymerization at room temperature. Most heteroleptic complexes showed modest to good activities with full conversion in less than 6 h at room temperature. Complexes with R' = H showed poor molecular weight control, complexes with R' = Cl were inactive in polymerization. In pyridylmethoxide-containing complexes, only one alkoxide initiated chain growth. All complexes produced atactic polymer.

Ligand-Based Storage of Protons and Electrons in Dihydrazonopyrrole Complexes of Nickel

A newly developed dihydrazonopyrrole ligand and corresponding Ni complexes have been synthesized and thoroughly characterized. Electrochemical studies and chemical reactivity tests show that these complexes can reversibly store both electrons and protons, or equivalently H-atoms, via ligand-based events. The stored H-atom equivalent can be transferred to small molecules such as acetonitrile or oxygen. Furthermore, this series of complexes can adopt a variety of different coordination modes. In addition to one e^- reactivity, the two e^- electrophilic oxidation of phosphines is also demonstrated. Taken together, these results show that dihydrazonopyrrole complexes represent a geometrically and electronically flexible scaffold for controlling the flow of both electrons and protons.

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.

pH-Dependent structural diversity of a 2-pyridinemethanol Cu complex and its relatively strong magnetic exchange coupling via hydrogen bonding

The diversity of the self-assembled structures of a Cu complex via hydrogen bonding, including a square planar unit, is reported. The Cu complex forms two self-assembled structures by hydrogen bonding, depending on the acidity of the recrystallization conditions. A linear chain structure can be produced under acidic conditions, and a three-dimensional network structure is observed under basic conditions. The linear chain structure is constructed from intermolecular sharing of a hydrogen atom between two 2-pyridinemethanolate units, with an OO distance of 2.412(1) Å and an O-H-O bond angle of 170(3)°. The linear chain structure exhibits relatively strong magnetic coupling (J = -9.91(2) cm^-1) via hydrogen bonding between Cu atoms; this coupling was also confirmed by electron spin resonance experiments. Thermochromic behavior was also observed for the complex.

Phosphasalen Indium Complexes Showing High Rates and Isoselectivities in rac-Lactide Polymerizations

Polylactide (PLA) is the leading bioderived polymer produced commercially by the metal-catalyzed ring-opening polymerization of lactide. Control over tacticity to produce stereoblock PLA, from rac-lactide improves thermal properties but is an outstanding challenge. Here, phosphasalen indium catalysts feature high rates (30±3 m^-1  min^-1 , THF, 298 K), high control, low loadings (0.2 mol %), and isoselectivity (P_i =0.92, THF, 258 K). Furthermore, the phosphasalen indium catalysts do not require any chiral additives.

Synthesis of aluminum complexes supported by 2-(1,10-phenanthrolin-2-yl)phenolate ligands and their catalysis in the ring-opening polymerization of cyclic esters

Phenanthroline-phenolate based N,N,O-chelate aluminum complexes were demonstrated to catalyze the ROP of ε-caprolactone, rac-lactide, and rac-β-butyrolactone, as well as their block copolymerization.

Copper(II) complexes with imino phenoxide ligands: synthesis, characterization, and their application as catalysts for the ring-opening polymerization of rac-lactide

ABSTRACT

Four new copper complexes based on bidentate imino phenoxide ligands were synthesized and characterized by IR, UV-Vis spectroscopy, ESI mass spectrometry, single crystal X-ray diffraction, and electrochemistry. The crystal structures revealed that the copper(II) atoms are surrounded by phenolate oxygen and imine nitrogen atoms of two ligands in a distorted square-planar geometry. The existence of ligand-centered, as well as Cu(II)-centered quasi-reversible and reversible redox reactions are observed in the cyclic voltammetry experiments of all the complexes. All complexes are able to catalyze the ring-opening polymerization of rac-lactide yielding polymers with moderate molecular weights and moderately broad molecular weight distributions.

GRAPHICAL ABSTRACT

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.

Synthesis and characterization of dinuclear rare-earth complexes supported by amine-bridged bis(phenolate) ligands and their catalytic activity for the ring-opening polymerization of l-lactide

Reactions of amine-bridged bis(phenolate) protio-ligands N,N-bis(3,5-di-tert-butyl-2-hydroxybenzyl)aminoacetic acid (L(1)-H3) and N,N-bis[3,5-bis(α,α'-dimethylbenzyl)-2-hydroxybenzyl]aminoacetic acid (L(2)-H3), with 1 equiv. M[N(SiMe3)2]3 (M = La, Nd, Sm, Gd, Y) in THF at room temperature yielded the neutral rare-earth complexes [M2(L)2(THF)4] (L = L(1), M = La (), Nd (), Sm (), Gd (), Y (); L = L(2), M = La (), Nd (), Sm (), Gd (), Y ()). All of these complexes were characterized by single-crystal X-ray diffraction, elemental analysis and in the case of yttrium and lanthanum complexes, (1)H NMR spectroscopy. The molecular structure of revealed dinuclear species in which the eight-coordinate lanthanum centers were bonded to two oxygen atoms of two THF molecules, to three oxygen atoms and one nitrogen atom of one L(1) ligand, and two oxygen atoms of the carboxyl group of another. Complexes were also dinuclear species containing seven-coordinate metal centers similar to , albeit with bonding to one rather than two carboxyl group oxygens of another ligand. Further treatment of with excess benzyl alcohol provided dinuclear complex [La2(L(1))2(BnOH)6] (), in which each lanthanum ion is eight-coordinate, bonded to three oxygen atoms and one nitrogen atom of one ligand, three oxygen atoms of three BnOH molecules, as well as one oxygen atom of bridging carboxyl group of the other ligand. In the presence of BnOH, complexes efficiently catalyzed the ring-opening polymerization of l-lactide in a controlled manner and gave polymers with relatively narrow molecular weight distributions. The kinetic and mechanistic studies associated with the ROP of l-lactide using /BnOH initiating system have been performed.

Structures of potassium calix[4]arene crown ether inclusion complexes and application in polymerization of rac-lactide

Reaction of 1,3-dipropoxy-p-tert-butyl-calix[4]arene (L(1)H2) with KN(SiMe3)2 afforded a one-dimensional (1D) chain complex [K2L(1)]n (1). Upon reaction with 1 equivalent 18-crown-6, complex 1 can convert to complex [K2(18-crown-6)L(1)] (2) which possesses a sandwich structure. Treatment of two calix[4]arene-crown ligands of 1,3-dihydroxy-p-tert-butyl-calix[4]arene-crown-5 (L(2)H2) and 1,2-dihydroxy-p-tert-butyl-calix[4]arene-crown-5 (L(3)H2) with KN(SiMe3)2 gave the dinuclear complex [K2L(2)] (3) and the mononuclear complex [K(THF)L(3)H] (4), respectively. Complexes 1-4 were all characterized by single-crystal X-ray diffraction techniques. The variable temperature (1)H NMR spectrum indicates there is a quick rotation equilibrium of the two phenoxy groups in complex 3. In addition, complexes 1-4 have been tested for the ring-opening polymerization (ROP) of rac-lactide and the results showed that complexes 2 and 3 are highly active for the ROP of rac-lactide. The obtained polymers displayed low dispersity values (Đ) and the molecular weights are close to the calculated ones. Furthermore, complexes 2 and 3 show moderate isoselectivities of Pm = 0.67 and Pm = 0.73, respectively.