[OSSO]-Type Chromium(III) Complexes for the Reaction of CO2 with Epoxides.

In this work, four new mononuclear Cr(III) complexes (2-5) bearing bis-thioether-diphenolate, [OSSO]-type ligands, were synthesized and characterized. These complexes in combination with bis(triphenylphosphine)iminium chloride (PPNCl) promoted the coupling of CO2 with epoxides. Depending on the type of substrate and the conditions, the reaction results in the selective formation of either polycarbonate or cyclic carbonate. For example, the reactions in the presence of complex 2 led to the exclusive formation of poly(cyclohexene carbonate, PCHC) from cyclohexene oxide (CHO) (TOF up to 39 h-1 , at T=45-100 °C, time=24 h, pCO2 =20 bar, epoxide/2 (mol/mol)=1000, and PPNCl/2=0.5-2.0 mol %). Under the same conditions and PPNCl/2=0.5-5.0 mol %, the reactions of CO2 with styrene oxide (SO), epichlorohydrin (ECH), 1,2 epoxydodecane (EDD), and allyl glycidyl ether (AGE) have shown selective conversion to the corresponding cyclic carbonates (TOF up to 41 h-1 ).

Stereoregular Polymerization of Acyclic Terpenes.

The growing environmental pollution and the expected depleting of fossil resources have sparked interest in recent years for polymers obtained from monomers originating from renewable sources. Furthermore, nature can provide a variety of building blocks with special structural features (e. g. side groups or stereo-elements) that cannot be obtained so easily via fossil-based pathways. In this context, terpenes are widespread natural compounds coming from non-food crops, present in a large variety of structures, and ready to use as monomers with or without further modifications. The present review aims to provide an overview of how chemists can stereospecifically polymerize terpenes, particularly the acyclic ones like myrcene, ocimene, and farnesene, using different metal catalyst systems in coordination-insertion polymerization. Attention is also paid to their copolymers, which have recently been disclosed, and to the possible applications of these bio-based materials in various industrial sectors such as in the field of elastomers. © 2021 The Authors. ChemPlusChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution Non-Commercial License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited and is not used for commercial purposes.

Versatile Preparation of Branched Polylactides by Low-Temperature, Organocatalytic Ring-Opening Polymerization in N-Methylpyrrolidone and Their Surface Degradation Behavior.

We describe how the organocatalytic, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-based lactide ring-opening polymerization can be effectively performed in a very polar solvent, N-methylpyrrolidone (NMP). Due to a low ceiling temperature, this "living" mechanism has been unreported to date, but we here demonstrate that through a combination of low temperature and repeated monomer additions (starve-fed process), this mechanism enables the generation of a plethora of multifunctional homo- and (stereo)block-poly(lactide)s (PLAs) with exquisite control of the molecular weight dispersity (typically D < 1.1) and topology (from linear through 4-, 6-, or 8-armed stars and up to ∼140 armed combs). They are scarcely obtainable or inaccessible through more classical synthetic methods due to the poor solubility of multifunctional initiators (polyols) in most organic solvents and monomer melts. In these precisely designed structures, branching significantly altered the nature of the materials' hydrolytic degradation, allowing them to acquire a pronounced surface character (as opposed to the bulk degradation of linear polymers). Finally, we have assessed the amenability of this method to in situ block copolymerization by using the tacticity of PLLA blocks in PLLA-b-PDLLA versus PDLLA-b-PLLA (L-LA polymerized before or after DL-LA) as a sensitive method to detect (stereochemical) defects.

NSSN-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of l-Lactide.

A new class of zirconium and hafnium complexes coordinated by linear dianonic tetradentate NSSN ligands is reported. The ligands feature two amide functions coupled with two thioether groups linked by a central flexible ethane bridge and two lateral rigid phenylene bridges and differ for the substituents on the aniline nitrogen atoms, i.e., isopropyl, cyclohexyl, or mesityl substituents: NSSN-iPr, NSSN-Cy, or NSSN-Mes. They were prepared by reacting 2-aminothiophenol with dibromoethane to afford the NSSN ligands without substituents on the aniline nitrogen atoms, which were subsequently alkylated through a reductive amination of acetone or cyclohexanone or palladium-catalyzed cross-coupling reaction with mesityl bromide. The corresponding zirconium and hafnium complexes 1-5 were obtained through a transamination reaction between the neutral ligands and Zr(NMe2)4 or Hf(NMe2)4 [(NSSN-iPr)Zr(NMe2)2 (1), (NSSN-Cy)Zr(NMe2)2 (2), (NSSN-Mes)Zr(NMe2)2 (3), (NSSN-iPr)Hf(NMe2)2 (4), and (NSSN-Cy)Hf(NMe2)2 (5)]. They were characterized in solution by NMR spectroscopy and in solid state by X-ray diffraction analysis (except for 3). All complexes present an octahedral coordination geometry with a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands. The catalytic performances of 1-5 in the ring-opening polymerization of cyclic esters were investigated. Complex 1 was the most active: its polymerization activity was superior to those generally displayed by zirconium complexes featuring OSSO ligands and compared well with those of the most active group 4 complexes operating in a toluene solution.

Sustainable Myrcene-Based Elastomers via a Convenient Anionic Polymerization.

Soluble heterocomplexes consisting of sodium hydride in combination with trialkylaluminum derivatives have been used as anionic initiating systems at 100 °C in toluene for convenient homo-, co- and ter-polymerization of myrcene with styrene and isoprene. In this way it has been possible to obtain elastomeric materials in a wide range of compositions with interesting thermal profiles and different polymeric architectures by simply modulating the alimentation feed and the (monomers)/(initiator systems) ratio. Especially, a complete study of the myrcene-styrene copolymers (PMS) was carried out, highlighting their tapered microstructures with high molecular weights (up to 159.8 KDa) and a single glass transition temperature. For PMS copolymer reactivity ratios, rmyr = 0.12 ± 0.003 and rsty = 3.18 ± 0.65 and rmyr = 0.10 ± 0.004 and rsty = 3.32 ± 0.68 were determined according to the Kelen-Tudos (KT) and extended Kelen-Tudos (exKT) methods, respectively. Finally, this study showed an easy accessible approach for the production of various elastomers by anionic copolymerization of renewable terpenes, such as myrcene, with commodities.

NIR multiphoton ablation of cancer cells, fluorescence quenching and cellular uptake of dansyl-glutathione-coated gold nanoparticles.

Theranostics based on two-photon excitation of therapeutics in the NIR region is an emerging and powerful tool in cancer therapy since this radiation deeply penetrates healthy biological tissues and produces selective cell death. Aggregates of gold nanoparticles coated with glutathione corona functionalized with the dansyl chromophore (a-DG-AuNPs) were synthesized and found efficient nanodevice for applications in photothermal therapy (PTT). Actually the nanoparticle aggregation enhances the quenching of radiative excitation and the consequent conversion into heat. The a-DG-AuNPs are readily internalized in Hep G2 where the chromophore acts as both antenna and transducer of the NIR radiation under two-photons excitation, determining efficient cell ablation via photothermal effect.

A Novel [OSSO]-Type Chromium(III) Complex as a Versatile Catalyst for Copolymerization of Carbon Dioxide with Epoxides.

A new chromium(III) complex, bearing a bis-thioether-diphenolate [OSSO]-type ligand, was found to be an efficient catalyst in the copolymerization of CO2 and epoxides to achieve poly(propylene carbonate), poly(cyclohexene carbonate), poly(hexene carbonate) and poly(styrene carbonate), as well as poly(propylene carbonate)(cyclohexene carbonate) and poly(propylene carbonate)(hexene carbonate) terpolymers.

Cyclic Polyester Formation with an [OSSO]-Type Iron(III) Catalyst.

The efficient formation of cyclic polyesters from the ring-opening polymerization of lactide, ϵ-caprolactone, and ß-butyrolactone catalyzed by a 1,4-dithiabutanedyl-2,2'-bis(4,6-dicumylphenol) [OSSO]-FeCl complex activated with cyclohexene oxide was achieved. The catalyst was very active (initial turnover frequency up to 2718 h-1 ), robust, and worked with a monomer/Fe ratio up to 10 000. The formation of cyclic polymers was supported by using high-resolution matrix-assisted laser desorption ionization (MALDI) MS, and the average ring size (≈5 kDa for cyclic polylactide) independent of the reaction conditions. A monometallic ring-opening polymerization/cyclization mechanism was proposed from the results of a kinetic investigation.

Regioselective Ring-Opening of Glycidol to Monoalkyl Glyceryl Ethers Promoted by an [OSSO]-FeIII Triflate Complex.

A FeIII -triflate complex, bearing a bis-thioether-di-phenolate [OSSO]-type ligand, was discovered to promote the ring-opening of glycidol with alcohols under mild reaction conditions (0.05 mol % catalyst and 80 °C). The reaction proceeded with high activity (initial turnover frequency of 1680 h-1 for EtOH) and selectivity (>95 %) toward the formation of twelve monoalkyl glyceryl ethers (MAGEs) in a regioselective fashion (84-96 % yield of the non-symmetric regioisomer). This synthetic approach allows the conversion of a glycerol-derived platform molecule (i.e., glycidol) to high-value-added products by using an Earth-crust abundant metal-based catalyst.

Aerobic Oxidation and Oxidative Esterification of 5-Hydroxymethylfurfural by Gold Nanoparticles Supported on Nanoporous Polymer Host Matrix.

The aerobic oxidation and oxidative esterification of 5-hydroxymethylfurfural (HMF) catalyzed by gold nanoparticles (AuNPs) supported on a semicrystalline nanoporous multiblock copolymer matrix consisting of syndiotactic poly(styrene)-cis-1,4-poly(butadiene) (sPSB) have been investigated. Depending on the reaction parameters (support nanoporosity, presence of water, solvent, temperature, cocatalyst, oxygen pressure), the conversion of HMF can be finely addressed to the formation of the desired oxidation product, such as 2,5-diformylfuran (DFF), 5-formylfuran-2-carboxylic acid (FFCA), methyl 5-(hydroxymethyl)furan-2-carboxylate (MHMFC), dimethyl furan-2,5-dicarboxylate (DMFC), and furan-2,5-dicarboxylic acid (FDCA), under optimized reaction conditions. The AuNP-sPSB catalyst is highly effective and selective because the polymer support acts as a conveyor and concentrator of the reactants toward the catalytic sites.

First Attempt of Glycidol-to-Monoalkyl Glyceryl Ethers Conversion by Acid Heterogeneous Catalysis: Synthesis and Simplified Sustainability Assessment.

The selective preparation of monoalkylglyceryl ethers (MAGEs) is a task for researchers owing to their broad range of applications. In this work, green feedstocks such as glycidol and alcohols were used to prepare MAGEs under mild reaction conditions (80 °C, 3 h, 0.5 mol % catalyst) in the presence of acid heterogeneous catalysts. Nafion shows the best performances in terms of conversion and selectivity to MAGES and also high stability. A comparison of the environmental performances with the most consolidated pathway from glycerol has shown that the usage of glycidol (recovered as a value-added product from Epicerol process) and Nafion leads to a lower impact on ecosystems. In addition, results achieved from a simplified socio-economic analysis show that the innovative route here proposed has potential (at the laboratory scale) of enhancing potential gains and of reducing the social implications resulting from externalities associated with environmental impacts (e.g., CO2 equivalents).

Phenylene-Bridged OSSO-Type Titanium Complexes in the Polymerization of Ethylene and Propylene.

The dichloro titanium complexes (OSSO tBu)TiCl2 (1) and (OSSOCum)TiCl2 (2) bearing o-phenylene-bridged OSSO-type ligands [OSSO tBu-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-di-tert-butyphenol) and OSSOCum-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenol)] were prepared and characterized. The X-ray structure of 1 revealed that Ti atom has an octahedral coordination geometry with an fac-fac wrapping of the [OSSO] ligand. In solution at 25 °C, 1 mainly retains the C 2 symmetric structure, whereas 2 shows an equilibrium between C 2- and C 1-symmetric stereoisomers. Activation of 2 with (Ph3C)[B(C6F5)4] led to a highly active catalytic system with an activity of 238 kgPE·molcat -1·bar-1·h-1; linear polyethylene with a T m of 122 °C and M w of 107 kDa were obtained under these conditions. Catalyst 1 displayed the moderate activity of 59 kgPE·molcat -1·bar-1·h-1. Gel permeation chromatography analysis revealed the formation of high-molecular-weight polyethylenes with very large distributions of the molecular weights, indicating a low control of the polymerization process, probably becaue of the presence of different active species in solution. Density functional theory investigation provides a rational for the relative high-molecular-weight polymers obtained with these complexes. The precatalyst 2 was also active in propylene polymerization producing atactic oligomers terminated with unsaturated end groups.

Glycidol, a Valuable Substrate for the Synthesis of Monoalkyl Glyceryl Ethers: A Simplified Life Cycle Approach.

The disposal of any waste by recovering it within the production plant represents the ultimate goal of every biorefinery. In this scenario, the selective preparation of monoalkyl glyceryl ethers (MAGEs) starting from glycidol, obtained as byproduct in the epichlorohydrin production plant, represents a very promising strategy. Here, we report the synthesis of MAGEs through the reaction of glycidol with alcohols catalyzed by a green homogeneous Lewis acids catalyst, such as BiIII triflate, under very mild reaction conditions. To evaluate the green potential of the proposed alternative, a simplified life cycle assessment (LCA) approach was followed by comparing the environmental performance of the proposed innovative route to prepare MAGEs with that of the most investigated pathway from glycerol. A considerable reduction of all impact categories considered was observed in our experimental conditions, suggesting that the glycidol-to-MAGEs route can be a valuable integration to the glycerol-to-MAGEs chain. Thanks to the use of primary data within the LCA model, the results achieved are a very good approximation of the real case.

Stereorigid OSSO-Type Group 4 Metal Complexes in the Ring-Opening Polymerization of rac-Lactide.

The synthesis and characterization of a series of group 4 metal complexes of general formula {OSSOX}M(OR)2 (X = R = tBu, M = Zr (1); X = cumyl, M = Zr, R = tBu (2); X = cumyl, M = Ti, R = iPr (4); X = cumyl, M = Hf, R = tBu (5)) and {OSSOX}2Zr (X = Cl (3)) supported by o-phenylene-bridged bis(phenolato) ligands (OSSOtBu-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-di-tert-butyphenol); OSSOCum-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-bis(2-phenylpropan-2-yl)phenol); OSSOCl-H = 6,6'-((1,2-phenylenebis(sulfanediyl))bis(methylene))bis(2,4-dichlorophenol)) are described herein. Complexes 1-5 were readily obtained by σ-bond metathesis reactions between the proligand and the appropriate homoleptic metal precursor. The reaction with OSSOCl yielded the bis-ligand complex{OSSOCl}2Zr (3) regardless of the OSSOCl-H/Zr(OtBu)4 molar ratio or experimental conditions. All complexes were characterized in solution using NMR spectroscopy and, in the case of 2, by single-crystal X-ray diffraction experiments. These complexes show a fac-fac ligand wrapping and a cis relationship between the other two monodentate ligands; zirconium and hafnium complexes 1-3 and 5 are configurationally stable, whereas titanium complex 4 is fluxional in solution at room temperature. The complexes tested in the ring-opening polymerization (ROP) of racemic-lactide showed, except in the case of 3, moderate rates and good levels of polymerization control. Upon addition of an exogenous alcohol (isopropyl alcohol or tert-butyl alcohol) efficient binary catalytic systems were achieved. Polymerizations were well-controlled, as testified by the linear growth of the molecular weight as polymerization proceeded, narrow polydispersity indices, and molecular weights close to those expected on the basis of added alcohol amounts. Experimental and theoretical evidence is provided that ROP reactions operate according to an activated monomer mechanism.

Glycidol: an Hydroxyl-Containing Epoxide Playing the Double Role of Substrate and Catalyst for CO2 Cycloaddition Reactions.

Glycidol is converted into glycerol carbonate (GC) by coupling with CO2 in the presence of tetrabutylammonium bromide (TBAB) under mild reaction conditions (T=60 °C, PCO2 =1 MPa) in excellent yields (99 %) and short reaction time (t=3 h). The unusual reactivity of this substrate compared to other epoxides, such as propylene oxide, under the same reaction conditions is clearly related to the presence of a hydroxyl functionality on the oxirane ring. Density functional theory calculations (DFT) supported by 1 H NMR experiments reveal that the unique behavior of this substrate is a result of the formation of intermolecular hydrogen bonds into a dimeric structure, activating this molecule to nucleophilic attack, and allowing the formation of GC. Furthermore, the glycidol/TBAB catalytic system acts as an efficient organocatalyst for the cycloaddition of CO2 to various oxiranes.

Synthesis of Monoalkyl Glyceryl Ethers by Ring Opening of Glycidol with Alcohols in the Presence of Lewis Acids.

The present work deals with the production of monoalkyl glyceryl ethers (MAGEs) through a new reaction pathway based on the reaction of glycidol and alcohols catalyzed by Lewis acid-based catalysts. Glycidol is quantitatively converted with high selectivity (99 %) into MAGEs under very mild reaction conditions (80 °C and 0.01 mol % catalyst loading) in only 1 h using Al(OTf)3 or Bi(OTf)3 as catalyst. The proposed method enhances the choice of possible green synthetic approaches for the production of value-added products such as MAGEs.

Thioether-triphenolate bimetallic iron(III) complexes as robust and highly efficient catalysts for cycloaddition of carbon dioxide to epoxides.

The selective and effective synthesis of organic carbonates under mild conditions, starting from carbon dioxide and oxiranes, catalyzed by metal complexes is currently a focus of interest for both industrial and academic researchers. We recently developed a novel thioether-triphenolate iron(III) catalyst (Ct-BU) that has proven to be highly active for the coupling of CO2 with epoxides, resulting in cyclic organic carbonates under solvent-free conditions. In the current work, the properties of this novel class of catalysts were extensively investigated. In particular, the steric properties of the ligand were modulated by changing the substituents of the aromatic rings in order to obtain a deeper knowledge of the relationship between the complex structure and catalytic performance/selectivity for these iron complexes. Notably, the less steric demanding iron(III) CH complex synthesized shows, when activated by n-tetrabutylammonium bromide, an impressive turnover frequency (TOF) of 3800 h(-1) for the formation of propylene carbonate and glycerol carbonate which are, by far, the highest reported for an iron based catalyst and compares well with the most active catalyst based on other metals.

{2-[2,2-Bis(4,4-dimethyl-4,5-di-hydro-1,3-oxazol-2-yl-κN)prop-yl]pyridine}-dichlorido-iron(II).

The title compound,[FeCl2(C18H25N3O2)], has a distorted tetra-hedral Cl2N2 coordination of the Fe(II) atom as a result of the constraints imposed by the 2-[2,2-bis-(4,4-dimethyl-4,5-di-hydro-1,3-oxazol-2-yl)prop-yl]pyridine ligand. The pyridine ring is almost perpendicular to the six-membered chelated ring containing the metal atom [dihedral angle between their mean planes = 88.5 (1)°].