Bimetallic fluorine-substituted anilido-aldimine zinc complexes for CO2/(cyclohexene oxide) copolymerization

Regioselective nucleophilic aromatic substitution of an o-fluorine occurs to afford fluorine-substituted o-phenylene-bridged bis(anilido-aldimine) compounds o-C6H4[(C6H2R2)N=CH-C6F4-(H)N(C6H3R'2)]2 when Li(H)N-C6H3R'2 (R' = iPr, Et, Me) is reacted with o-C6H4[(C6H2R2)N=CH-C6F5]2 (R = iPr, Et, Me) in a nonpolar solvent such as diethyl ether or toluene. Successive additions of Me2Zn and SO2 gas to the bis(anilido-aldimine) compounds afford quantitatively dinuclear mu-methylsulfinato zinc complexes o-C6H4[[(C6H2R2)N=CH-C6F4-N(C6H3R'2)-kappa2N,N]Zn(mu-OS(O)Me)]2 (R = iPr, R' = iPr, 3a; R = iPr, R' = Me, 3c; R = Et, R' = (i)Pr, 3d; R = Et, R' = Et, 3e; R = Et, R' = Me, 3f; R = Me, R' = iPr, 3g; R = Me, R' = Et, 3h; R = Me, R' = Me, 3i). The molecular structure of 3c was confirmed by X-ray crystallography. Fluorine-substituted complexes 3a-i show significantly higher TOF (turnover frequencies) than the unfluorinated analogues for CO2/(cyclohexene oxide) copolymerization. The TOF is highly sensitive to the substituents R and R', and the highest TOF (2480 h(-1)) is obtained with 3g (R = Me, R' = iPr). Complex 3g is less sensitive to the residual protic impurities present in the monomers and shows activity at such a low catalyst concentration as [Zn]:[cyclohexene oxide] = 1:50,000, at which the unfluorinated analogue is completely inactive. By realizing the activity at such an extremely low [Zn]:[cyclohexene oxide] ratio, we achieve a high TON (turnover number) up to 10,100. High-molecular-weight polymers (M(n), 100,000-200,000) are obtained with a rather broad molecular-weight distribution (M(w)/M(n), 1.3-2.5). The obtained polymers are not perfectly alternating, and variable carbonate linkages (65-85%) are observed depending on the N-aryl ortho substituents R and R' and the polymerization conditions.     

Asymmetric alternating copolymerization of cyclohexene oxide and CO2 with dimeric zinc complexes

Dimeric zinc complex 2a [ = Et(2)Zn(2)(1a)(2)] has been synthesized by the reaction of Et(2)Zn and (S)-diphenyl(pyrrolidin-2-yl)methanol (1a-H). X-ray crystallography revealed that the alkoxide ligand replaced one of the two ethyl groups of Et(2)Zn and formed a five-membered chelate ring through a Zn-N dative bond. Two zinc centers were bridged by oxygen atoms to form a Zn(2)O(2) four-membered ring with a syn relationship between the two ethyl groups on the zinc centers. Dimeric zinc complex 2a was an active catalyst for asymmetric alternating copolymerization of cyclohexene oxide and CO(2). An MALDI-TOF mass spectrum of the obtained copolymer showed that the copolymerization was initiated by the insertion of CO(2) into Zn-alkoxide to give [(S)-diphenyl(pyrroridin-2-ly)methoxy]-[C(=O)O-(1,2-cyclohexylene)-O](n)-H (copolymer I), including chiral ligand 1a as an initiating group. Complex 3a-OEt ( = EtZn(1a)(2)ZnOEt), in which an ethoxy group replaced one of the two ethyl groups in 2a, also polymerized cyclohexene oxide and CO(2) with higher catalytic activity and enantioselectivity than 2a and afforded EtO-[C(=O)O-(1,2-cyclohexylene)-O](n)-H ( = copolymer III), including an ethoxy group as an initiating group. Throughout the studies, dimeric zinc species are indicated to be the active species for the copolymerization. It is also depicted that the substituent on the aryl moiety in diaryl(pyrrolidin-2-yl)methanol 2b-e influenced the polymerization activity.     

Stereoregular polycarbonate synthesis: Alternating copolymerization of CO2 with aliphatic terminal epoxides catalyzed by multichiral cobalt(III) complexes

Completely stereoregular polycarbonate synthesis was achieved with the use of unsymmetric multichiral cobalt‐based complexes bearing a derived chiral BINOL and an appended 1,5,7‐triabicyclo[4.4.0] dec‐5‐ene as catalyst for the copolymerization of CO₂ and aliphatic terminal epoxides at mild conditions. The (S,S,S)‐Co(III) complex 1c with sterically hindered substituent group is more stereoregular catalyst for the copolymerization of CO₂ and racemic propylene oxide to afford a perfectly regioregular poly(propylene carbonate) (PPC), with >99% head‐to‐tail linkages, >99% carbonate linkages, and a K_rel of 24.4 for the enchainment of (R)‐epoxide over (S)‐epoxide. The isotactic PPC exhibits an enhanced glass transition temperature of 47 °C, which is 10–12 °C higher than that of the corresponding irregular polycarbonate. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011.     

Role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide utilizing chromium salen complexes

The mechanism of the copolymerization of cyclohexene oxide and carbon dioxide to afford poly(cyclohexylene)carbonate catalyzed by (salen)CrN3 (H2salen = N,N,'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylene-diimine) in the presence of a broad range of cocatalysts has been studied. We have previously established the rate of copolymer formation to be very sensitive to both the electron-donating ability of the salen ligand and the [cocatalyst], where N-heterocyclic amines, phosphines, and ionic salts were effective cocatalysts. Significant increases in the rate of copolymerization have been achieved with turnover frequencies of approximately 1200 h(-1), thereby making these catalyst systems some of the most active and robust thus far uncovered. Herein we offer a detailed explanation of the role of the cocatalyst in the copolymerization of CO2 and cyclohexene oxide catalyzed by chromium salen derivatives. A salient feature of the N-heterocyclic amine- or phosphine-cocatalyzed processes is the presence of an initiation period prior to reaching the maximum rate of copolymerization. Importantly, this is not observed for comparable processes involving ionic salts as cocatalysts, e.g., PPN+ X-. In these latter cases the copolymerization reaction exhibits ideal kinetic behavior and is proposed to proceed via a reaction pathway involving anionic six-coordinate (salen)Cr(N3)X- derivatives. By way of infrared and 31P NMR spectroscopic studies, coupled with in situ kinetic monitoring of the reactions, a mechanism of copolymerization is proposed where the neutral cocatalysts react with CO2 and/or epoxide to produce inner salts or zwitterions which behave in a manner similar to that of ionic salts.     

Copolymerization of cyclohexene oxide and carbon dioxide using (salen)Co(III) complexes: synthesis and characterization of syndiotactic poly(cyclohexene carbonate)

Synthetic routes to a series of new (salen-1)CoX (salen-1 = N,N'-bis(salicylidene)-1,2-diaminoalkane; X = halide or carboxylate) species are described and the X-ray crystal structures of two (salen-1)CoX (salen- = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminocyclohexane; X = Cl, I) complexes are presented. (R,R)-(salen-)CoX (X = Cl, Br, I, OAc, pentafluorobenzoate (OBzF(5))) catalysts are active for the copolymerization of cyclohexene oxide (CHO) and CO(2), yielding syndiotactic poly(cyclohexene carbonate) (PCHC), a previously unreported PCHC microstructure. Variation of the salen ligand and reaction conditions, as well as the inclusion of [PPN]Cl ([PPN]Cl = bis(triphenylphosphine)iminium chloride) cocatalysts, has dramatic effects on the polymerization rate and the resultant PCHC tacticity. Catalysts rac-(salen-)CoX (salen- = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminopropane; X = Br, OBzF(5)) have high activities for CHO/CO(2) copolymerization, yielding syndiotactic PCHCs with up to 81% r-centered tetrads. Using Bernoullian statistical methods, PCHC tetrad and triad sequences were assigned in the (13)C NMR spectra of these polymers in the carbonyl and methylene regions, respectively.     

Effect of supercritical CO2 on the copolymerization behavior of cyclohexene oxide/CO2 and copolymer properties with DMC/Salen‐Co(III) catalyst system

The copolymerization of cyclohexene oxide (CHO) and carbon dioxide (CO₂) was carried out under supercritical CO₂ (scCO₂) conditions to afford poly (cyclohexene carbonate) (PCHC) in high yield. The scCO₂ provided not only the C1 feedstock but also proved to be a very efficient solvent and processing aid for this copolymerization system. Double metal cyanide (DMC) and salen‐Co(III) catalysts were employed, demonstrating excellent CO₂/CHO copolymerization with high yield and high selectivity. Surprisingly, our use of scCO₂ was found to significantly enhance the copolymerization efficiency and the quality of the final polymer product. Thermally stable and high molecular weight (MW) copolymers were successfully obtained. Optimization led to excellent catalyst yield (656 wt/wt, polymer/catalyst) and selectivity (over 96% toward polycarbonate) that were significantly beyond what could be achieved in conventional solvents. Moreover, detailed thermal analyses demonstrated that the PCHC copolymer produced in scCO₂ exhibited higher glass transition temperatures (T_g ～ 114 °C) compared to polymer formed in dense phase CO₂ (T_g ～ 77 °C), and hence good thermal stability. Additionally, residual catalyst could be removed from the final polymer using scCO₂, pointing toward a green method that avoids the use of conventional volatile organic‐based solvents for both synthesis and work‐up. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 2785–2793     

Perfectly alternating copolymerization of CO2 and epichlorohydrin using cobalt(III)-based catalyst systems

Selective transformations of carbon dioxide and epoxides into biodegradable polycarbonates by the alternating copolymerization of the two monomers represent some of the most well-studied and innovative technologies for potential large-scale utilization of carbon dioxide in chemical synthesis. For the most part, previous studies of these processes have focused on the use of aliphatic terminal epoxides or cyclohexene oxide derivatives, with only rare reports concerning the synthesis of CO(2) copolymers from epoxides containing electron-withdrawing groups such as styrene oxide. Herein we report the production of the CO(2) copolymer with more than 99% carbonate linkages from the coupling of CO(2) with epichlorohydrin, employing binary and bifunctional (salen)cobalt(III)-based catalyst systems. Comparative kinetic studies were performed via in situ infrared measurements as a function of temperature to assess the activation barriers for the production of cyclic carbonate versus copolymer involving two electronically different epoxides: epichlorohydrin and propylene oxide. The relative small activation energy difference between copolymer versus cyclic carbonate formation for the epichlorohydrin/CO(2) process (45.4 kJ/mol) accounts in part for the selective synthesis of copolymer to be more difficult in comparison with the propylene oxide/CO(2) case (53.5 kJ/mol). Direct observation of the propagating polymer-chain species from the binary (salen)CoX/MTBD (X = 2,4-dinitrophenoxide and MTBD = 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene) catalyst system by means of electrospray ionization mass spectrometry confirmed the perfectly alternating nature of the copolymerization process. This observation in combination with control experiments suggests possible intermediates involving MTBD in the CO(2)/epichlorohydrin copolymerization process.     

Alternating copolymerization of carbon dioxide and cyclohexene oxide catalyzed by salen CoIII(acetate) complexes

A series of Co^III carboxylate based upon N,N,O,O-tetradentate Schiff base ligand framework have been prepared. X-ray diffraction analysis confirms that these Schiff base Co^III carboxylate are all monomeric species with a six-coordinated central Co in their solid structures. The activities and polycarbonate selectivity of these complexes toward the copolymerization of epoxide (cyclohexene oxide and propylene oxide) and carbon dioxide have been investigated in the presence of bis(triphenylphosphine)iminium chloride. Copolymerization experiments indicate that [bis(α-methyl-3,5-di-tertbutyl-salicylaldehyde) ethylenediiminato] Co^IIIOOCH₃ exhibits the highest activity and polycarbonate selectivity among these Co^III carboxylate. The resultant copolymer contained almost 100 % carbonate linkages with the molecular weight up to 71.8 kg mol^?1 as well as narrow polymer dispersity index (polymer dispersity index?=?1.5). The substituents and the mode of the bridging part between the two nitrogen atoms both exert significant influences upon the progress of the copolymerizations, influencing both the polycarbonate selectivity and the rate of copolymerization.     

Highly regio- and stereo-selective copolymerization of CO2 with racemic propylene oxide catalyzed by unsymmetrical (S,S,S)-salenCo(III) complexes

A novel unsymmetrical (S,S,S)-salen ligand bearing a derived chiral-BINOL was synthesized by the reaction of the condensation product of (1S,2S)-1,2-diaminocyclohexane mono(hydrogen chloride) with 3-adamanyl-5-tert-butyl-2-hydroxybenzaldehyde and 3-formoyl-2-hydroxy-2′-alkyloxy-1,1′-binaphthyl, which originated from (S)-1,1′-bi-2-naphthol via a four-step reaction. The cobalt complexes of this ligand, in conjunction with a nucleophilic cocatalyst, exhibited excellent activity in catalyzing asymmetric, regio- and stereo-selective copolymerization of CO₂ and racemic propylene oxide. The highest record of kinetic resolution coefficient (K _rel) was obtained with the use of binary catalyst system consisting of (S,S,S)-salenCo(III) complex 1c and bulky bis(triphenylphosphine)-iminium chloride (PPNCl). The resulting poly(propylene carbonate)s have more than 99% carbonate linkages and more than 98% head-to-tail content.     

Pentamethylcyclopentadienyl-iridium(III) complexes with pyridylamino ligands: synthesis and applications as asymmetric catalysts for Diels-Alder reactions

Reaction of the dimer [(Cp*IrCl)2(P-Cl)2] with chiral pyridylamino ligands (pyam, L1-L5) in the presence of NaSbF6 gave complexes [Cp*IrCl(pyam)][SbF6] 1-5 as diastereomeric mixtures, which have been fully characterised, including the X-ray molecular structure determination of the complexes (S(Ir),R(N),R(C))-[Cp*IrClL1][SbF6] 1a and (R(Ir),S(N),S(C))-[Cp*IrClL5][SbF6] 5a. Treatment of these cations with AgSbF6 affords the corresponding aqua species [Cp*Ir(pyam)(H2O)][SbF6]2 6-10 which have been also fully characterised. The molecular structure of the complex (S(Ir),R(N),R(C))-[Cp*IrL,(H2O)][SbF6]2 6 has been determined by X-ray diffractometric methods. The aqua complexes [Cp*Ir(pyam)(H2O)][SbF6]2 (6, pyam = L2 (7), L3 (8)) evolve to the cyclometallated species [Cp*Ir{kappa3(N,N',C)-(R)-(C6H4)CH(CH3)NHCH2C5NH4}][SbF6] (11), [Cp*Ir{kappa3(N,N',C)-(R)-(C10H6)CH(CH3)-NHCH2C5NH4)}][SbF6] (12), and [Cp*Ir{kappa3(N,N',C)-(R)-(C10H6)CH(CH3)NHCH2C9NH6)}][SbF6] (13) respectively, via intramolecular activation of an ortho C-H aryl bond. Complexes 6-10 are enantioselective catalysts for the Diels-Alder reaction between methacrolein and cyclopentadiene. Reaction occurs rapidly at room temperature with good exo : endo selectivity (from 81 : 19 to 98 : 2) and moderate enantioselectivity (up to 72%). The involved intermediate Lewis acid-dienophile compounds [Cp*Ir(pyam)(methacrolein)][SbF]2 (pyam = L4 (14), L5 (15)) have been isolated and characterised.     

Cyclohexene oxide/CO2 copolymerization catalyzed by chromium(III) salen complexes and N-methylimidazole: effects of varying salen ligand substituents and relative cocatalyst loading

A detailed mechanistic study into the copolymerization of CO2 and cyclohexene oxide utilizing CrIII(salen)X complexes and N-methylimidazole, where H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine and other salen derivatives and X = Cl or N3, has been conducted. By studying salen ligands with various groups on the diimine backbone, we have observed that bulky groups oriented perpendicular to the salen plane reduce the activity of the catalyst significantly, while such groups oriented parallel to the salen plane do not retard copolymer formation. This is not surprising in that the mechanism for asymmetric ring opening of epoxides was found to occur in a bimetallic fashion, whereas these perpendicularly oriented groups along with the tert-butyl groups on the phenolate rings produce considerable steric requirements for the two metal centers to communicate and thus initiate the copolymerization process. It was also observed that altering the substituents on the phenolate rings of the salen ligand had a 2-fold effect, controlling both catalyst solubility as well as electron density around the metal center, producing significant effects on the rate of copolymer formation. This and other data discussed herein have led us to propose a more detailed mechanistic delineation, wherein the rate of copolymerization is dictated by two separate equilibria. The first equilibrium involves the initial second-order epoxide ring opening and is inhibited by excess amounts of cocatalyst. The second equilibrium involves the propagation step and is enhanced by excess cocatalyst. This gives the [cocatalyst] both a positive and negative effect on the overall rate of copolymerization.     

Alternating copolymerization of cyclohexene oxide and carbon dioxide catalyzed by noncyclopentadienyl rare‐earth metal bis(alkyl) complexes

The syntheses of several dialkyl complexes based on rare‐earth metal were described. Three β‐diimine compounds with varying N‐aryl substituents (HL¹=(2‐CH₃O(C₆H₄))N?C(CH₃)CH?C(CH₃)NH(2‐CH₃O(C₆H₄)), HL² = (2,4,6‐(CH₃)₃ (C₆H₂))N?C(CH₃)CH?C(CH₃)NH(2,4,6‐(CH₃)₃(C₆H₂)), HL³ = PhN?C(CH₃)CH(CH₃) NHPh) were treated with Ln(CH₂SiMe₃)₃(THF)₂ to give dialkyl complexes L¹Ln (CH₂SiMe₃)₂ (Ln = Y ( 1a ), Lu ( 1b ), Sc ( 1c )), L²Ln(CH₂SiMe₃)₂(THF) (Ln = Y ( 2a ), Lu ( 2b )), and L³Lu(CH₂SiMe₃)₂(THF) (3). All these complexes were applied to the copolymerization of cyclohexene oxide (CHO) and carbon dioxide as single‐component catalysts. Systematic investigation revealed that the central metal with larger radii and less steric bulkiness were beneficial for the copolymerization of CHO and CO₂. Thus, methoxy‐modified β‐diiminato yttrium bis(alkyl) complex 1a , L¹Y(CH₂SiMe₃)₂, was identified as the optimal catalyst, which converted CHO and CO₂ to polycarbonate with a TOF of 47.4 h^?1 in 1,4‐dioxane under a 15 bar of CO₂ atmosphere (T_p=130 °C), representing the highest catalytic activity achieved by rare‐earth metal catalyst. The resultant copolymer contained high carbonate linkages (>99%) with molar mass up to 1.9 × 10⁴ as well as narrow molar mass distribution (M_w/M_n = 1.7). © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6810–6818, 2008     

Aluminum salen complexes and tetrabutylammonium salts: a binary catalytic system for production of polycarbonates from CO2 and cyclohexene oxide

A series of complexes of the form (salen)AlZ, where H2salen = N,N'-bis(salicylidene)-1,2-phenylenediimine and various other salen derivatives and Z = Et or Cl, have been synthesized. Several of these complexes have been characterized by X-ray crystallography. An investigation of the utilization of these aluminum derivatives along with both ionic and neutral bases as cocatalysts for the copolymerization of carbon dioxide and cyclohexene oxide has been conducted. By studying the reactivity of these complexes for this process as substituents on the diimine backbone and phenolate rings are altered, we have observed that aluminum prefers electron-withdrawing groups on the salen ligands, thereby producing an electrophilic metal center to be most active toward production of polycarbonates from CO2 and cyclohexene oxide. For example, the complex derived from H2salen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-ethylenediimine is essentially inactive when compared to the analogous derivative containing nitro substituents in the 3-positions of the phenolate groups. This is to be contrasted with the catalytic activity observed for the (salen)CrX systems, where electron-donating salen ligands greatly enhanced the reactivity of these complexes for the coupling of CO2 and epoxides. While (salen)AlZ complexes are capable of producing poly(cyclohexene oxide) carbonate with low amounts of polyether linkage along with small quantities of cyclic carbonate byproducts, their reactivities, covering a turnover frequency range of 5.2-35.4 mol of epoxide consumed/(mol of Al x h), are greatly reduced when compared to their (salen)CrX analogues under identical reaction conditions.     

Anionic chiral cobalt(III) complexes as catalysts of asymmetric synthesis of cyanohydrins

Chiral coordinatively saturated cobalt(III) complexes with Schiff bases of enantio-pure amino acids are formed as Λ and Δ-isomers, which are not transformed into each other under normal conditions. These complexes catalyze the formation of enantiomerically enriched cyanohydrins from aldehydes and Me₃SiCN under homo-and heterogeneous catalysis. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 793–799, May, 2006.     

Iron(III) Schiff base complexes with asymmetric tetradentate ligands: synthesis,spectroscopy, and antimicrobial properties

The synthesis and characterization is reported of four iron(III) complexes of general formula [Fe(pythsalX)(H₂O)₂]Cl₂, derived from the NSNO-donor tetradentate Schiff base ligands pythsalHX ([5-X-N-(2-pyridylethylsulfanylethyl)salicylideneimine] (X = OMe, N₂Ph, I, NO₂). The complexes were characterized by physico-chemical and spectroscopic methods. The thermal stabilities of both the free Schiff bases and their complexes were studied by differential scanning calorimetry and thermogravimetric analyses. The spectroscopic data suggest that the Schiff base ligands coordinate through deprotonated phenolic oxygen, imine, and pyridine-type nitrogens and the thioether sulfur atoms to give an octahedral geometry around the iron(III) atom in all these complexes. The free Schiff bases and their complexes have been screened for antimicrobial activities and the results show that the free Schiff bases are more potent antibacterials than the complexes.     

Ir(III) complexes of diamine ligands for asymmetric ketone hydrogenation

The use of a combination of IrCl₃ with a series of ligands derived from the C2-symmetric diamine diphenylethanediamine (DPEN) forms a catalyst capable of the asymmetric hydrogenation of ketones in up to 85% ee.     

Chiral Ti(IV) complexes of hexadentate Schiff bases as precatalysts for the asymmetric addition of TMSCN to aldehydes and the ring opening of cyclohexene oxide

Chiral dinuclear titanium(IV) complexes (generated in situ from hexadentate Schiff bases and titanium tetra-isopropoxide) have been found to be more effective catalysts for the asymmetric addition of trimethylsilyl cyanide to aldehydes and the ring opening of cyclohexene oxide than their mononuclear analogues. The best results were obtained for benzaldehyde (86% enantiomeric excess) and cyclohexene oxide (89% enantiomeric excess).     

Conditions for the synthesis of Sb(III) fluoride complexes

Complexation of Sb(III) fluorides with alkali metal, ammonium, and thallium cations in the MF-SbF₃-H₂O systems at the molar ratios MF : SbF₃=(0.01–2) : 1 is studied by preparative chemistry, pH-metry, ₁₉F NMR, IR and X-ray powder diffraction methods. The crystals of Sb₃O₂F₅, MSb₄F₁₃, MSb₃F₁₀, MSb₂F₇, M₂Sb₃F₁₁, M₃Sb₄F₁₅, MSbF₄, M₂SbF₅ are synthesized and the factors influencing their formation are discussed. Some types of Sb(III) fluoride complexes are shown to be formed in the MX-SbF₃-H₂O systems (X = Cl^–, NO₃^–, BF₄^–, SCN^–, ClO₄^–).Translated from Koordinatsionnaya Khimiya, Vol. 30, No. 12, 2004, pp. 833–837.Original Russian Text Copyright © 2004 by Zemnukhova, Kovaleva, Fedorishcheva, Konshin, Davidovich.