Chromium Salen Complexes Research Articles

Functional and Degradable Polyester-co-polyethers from CO2, Butadiene, and Epoxides.

Carbon dioxide (CO2), as a renewable and nontoxic C1 feedstock, has been recognized as an ideal comonomer to prepare sustainable materials. In this regard, substantial focus has been dedicated to the ring-opening copolymerization of CO2 and epoxides, which results in the creation of aliphatic polycarbonates in most cases. Here, we report an unprecedented strategy to synthesize functional and degradable polyester-co-polyethers from CO2, butadiene, and epoxides via a CO2/butadiene-derived δ-valerolactone intermediate (EVP). Utilizing a chromium salen complex as the catalyst, the copolymerization of EVP and epoxides was successfully achieved to produce CO2/butadiene/epoxide terpolymers. The obtained polyester-co-polyethers with varied 39-93 mol % EVP content (equal to 18-28 wt % CO2 incorporation) show high thermal stability, tunable glass-transition temperatures, on-demand functionality, and good chemical degradability. This method extends the potential to access functional CO2-based polymers.

Post-Modification of Copolymers Obtained by ATRP for an Application in Heterogeneous Asymmetric Salen Catalysis.

Copolymers are valuable supports for obtaining heterogeneous catalysts that allow their recycling and therefore substantial savings, particularly in the field of asymmetric catalysis. This contribution reports the use of two comonomers: Azido-3-propylmethacrylate (AZMA) bearing a reactive azide function was associated with 2-methoxyethyl methacrylate (MEMA), used as a spacer, for the ATRP synthesis of copolymers, and then post-functionalized with a propargyl chromium salen complex. The controlled homopolymerization of MEMA by ATRP was firstly described and proved to be more controlled in molar mass than that of AZMA for conversions up to 63%. The ATRP copolymerization of both monomers made it possible to control the molar masses and the composition, with nevertheless a slight increase in the dispersity (from 1.05 to 1.3) when the incorporation ratio of AZMA increased from 10 to 50 mol%. These copolymers were post-functionalized with chromium salen units by click chemistry and their activity was evaluated in the asymmetric ring opening of cyclohexene oxide with trimethylsilyl azide. At an equal catalytic ratio, a significant increase in enantioselectivity was obtained by using the copolymer containing the largest part of salen units, probably allowing, in this case, the more favorable bimetallic activation of both the engaged nucleophile and electrophile. Moreover, the catalytic polymer was recovered by simple filtration and re-engaged in subsequent catalytic runs, up to seven times, without loss of activity or selectivity.

Chiral Chromium Salen@rGO as Multipurpose and Recyclable Heterogeneous Catalyst.

The first immobilization of a pyrene-tagged chromium salen complex through π-π noncovalent interactions on reduced graphene oxide (rGO) is described. A very robust supported catalytic system is obtained to promote asymmetric catalysis in repeated cycles, without loss of activity or enantioselectivity. This specific behavior was demonstrated in two different catalytic reactions (up to ten reuses) promoted by chromium salen complexes, the cyclohexene oxide ring-opening reaction and the hetero-Diels-Alder cycloaddition between various aldehydes and Danishefsky's diene. Furthermore, the chiral chromium salen@rGO has been found to be compatible with a multi-substrate type use, in which the structure of the substrate involved is modified each time the catalyst is reused.

Chromium-Salen Complex/Nitroxyl Radical Cooperative Catalysis: A Combination for Aerobic Intramolecular Dearomative Coupling of Phenols.

We describe an aerobic intramolecular dearomative coupling reaction of tethered phenols using a catalytic system consisting of a chromium-salen (Cr-salen) complex combined with a nitroxyl radical. This novel catalytic system enables formation of various spirocyclic dienone products including those unable to be accessed by previously reported methods efficiently under mild reaction conditions.

Mechanistic Study of Isotactic Poly(propylene oxide) Synthesis using a Tethered Bimetallic Chromium Salen Catalyst.

Initial catalyst dormancy has been mitigated for the enantioselective polymerization of propylene oxide using a tethered bimetallic chromium(III) salen complex. A detailed mechanistic study provided insight into the species responsible for this induction period and guided efforts to remove them. High-resolution electrospray ionization-mass spectrometry and density functional theory computations revealed that a μ-hydroxide and a bridged 1,2-hydroxypropanolate complex are present during the induction period. Kinetic studies and additional computation indicated that the μ-hydroxide complex is a short-lived catalyst arrest state, where hydroxide dissociation from one metal allows for epoxide enchainment to form the 1,2-hydroxypropanolate arrest state. While investigating anion dependence on the induction period, it became apparent that catalyst activation was the main contributor for dormancy. Using a 1,2-diol or water as chain transfer agents (CTAs) led to longer induction periods as a result of increased 1,2-hydroxyalkanolate complex formation. With a minor catalyst modification, rigorous drying conditions, and avoiding 1,2-diols as CTAs, the induction period was essentially removed.

Effect of Quaternary Phosphonium Salts as Cocatalysts on Epoxide/CO2 Copolymerization Catalyzed by salen-Type Cr(III) Complexes

Organic quaternary phosphonium salts are used in conjunction with chromium salen complexes in the alternating copolymerization of CO2 and cyclohexene oxide (CHO) or propylene oxide (PO). A highly e...

Visible light generation of chromium(V)-oxo salen complexes and mechanistic insights into catalytic sulfide oxidation

Visible light irradiation of the photo-labile salen-chromium(III) chlorate or bromate precursors produced salen-chromium(V)-oxo complexes that were spectroscopically and kinetically indistinguishable from those formed by chemical oxidation of chromium(III) salens with PhI(OAc)2. The photochemistry observed in this work is ascribed to the heterolytic cleavage of the O-X bond in the apical counterion that results in a two-electron oxidation of the metal to form the chromium(V)-oxo species. Second-order rate constants for oxidation reactions of 3 with organic substrates were determined under pseudo-first order condition, and particularly low level of reactivity for sulfide oxidations was observed. In this study, chromium(III) salen complexes effectively catalyzed the oxidation of aryl sulfides into sulfoxides with PhI(OAc)2 in the presence of a small amount of water. The competition product studies with the Hammett correlation plot indicated that the observed chromium(V)-oxo species is not likely to serve as the major oxidant for the sulfide oxidations catalyzed by chromium(III) salens with PhI(OAc)2.

Transduction of interaction between trace tryptophan and surface-confined chromium salen using impedance spectroscopy. A sensing device that works based on highly selective inhibition of mediator's Faradaic process

It is known that tryptophan (Trp) tends to be electropolymerized over a potential window that is mostly applied for voltammetric determination of Trp. Furthermore, over the applied potential range, most of Trp-sensors suffer from some possible interferences. Knowing these challenges motivated us to establish a novel sensing device able to monitor Trp before approaching the mentioned potential range, decreasing memory-effects and some sources of non-linearity during electrochemical measurements. However, considering its molecular structure, it is unrealistic to expect tryptophan oxidation in negative potentials. This work reports the application of a surface-confined chromium-salen complex that delivered a redox couple attributed to Cr(II)/Cr(III) at a low positive potentials. The recorded cyclic voltammograms (CVs) clarified that Trp has a high inhibitory activity toward the chromium oxidation peak in the alkaline medium at a low potential of −0.18 V vs. Ag/AgCl. The positive shift of the anodic peak potential of redox reaction disclosed that Trp drastically influenced the capability of the interfacial charge transfer of the modified electrode, the features that was exploited in establishing a sensitive Trp detection method using electrochemical impedance spectroscopy. The proposed sensor was successfully employed to determine Trp over a concentration range of 4.0−60.0 nM with the calculated detection limit of 0.78 nM.

The Kinetics of the Reaction of Carbon Dioxide with Propylene Oxide Catalyzed by a Chromium–Salen Complex

The kinetics of the reaction of CO2 with propylene oxide utilizing a salenCrCl/PPNCl active catalytic system is studied with varying reaction conditions (temperature, pressure, and cocatalyst/catalyst ratio). The reaction proceeds selectively to form cyclic propylene carbonate (PC) at [PPNCl]/[salenCrCl] ratios above two. The value of the effective activation energy of PC formation is found.

Alternating copolymerization of dihydrocoumarin and epoxides catalyzed by chromium salen complexes: a new route to functional polyesters.

We report the alternating ring-opening copolymerization of dihydrocoumarin with epoxides catalyzed by chromium(III) salen complexes. This process provides access to a range of perfectly alternating polyesters with high molecular weights and narrow molecular weight distributions. Atactic poly(cyclohexene dihydrocoumarate) and poly(cyclopentene dihydrocoumarate) were found to be semi-crystalline by differential scanning calorimetry.

Copolymerization of Carbon Dioxide and Propylene Oxide Catalyzed by Salen Complexes

Three salen complexes N,N'-bis(salicylidene)-1,2-phenylenediamino MⅢ Cl were prepared and employed for the copolymerization of carbon dioxide with propylene oxide. FT-IR and UV-Vis spectra confirmed the characteristic of metal salen complexes obtained. The central metal atoms in the salen complexes have great influence on the copolymerization of carbon dioxide with propylene oxide. The result shows that chromium metal is more effective to synthesize PPC copolymer. The structure of the resulting PPC was characterized by IR 1H NMR and GPC. 86.3% carbonate content of the PPC was achieved with chromium salen complexes.

Electropolymerization of chiral chromium–salen complexes: new materials for heterogeneous asymmetric catalysis

Electrochemical oxidation is described as a very efficient polymerization procedure for the heterogenization of metallic chiral catalysts. From chromium chiral complexes based on salen–thiophene ligands, this methodology provided an efficient access to various polymers. Recovered as insoluble powders, these materials were tested in different enantioselective heterogeneous catalytic reactions. Structural modifications were introduced on the salen core in order to evaluate their influence on redox polymer properties and on the enantioselectivity of the catalysis. Electrochemical experiments showed the particular stability of these deposited materials at the electrode surface and SEM analyses suggested the influence of the electropolymerization conditions on their morphology.

Ring-Opening Copolymerization of Maleic Anhydride with Epoxides: A Chain-Growth Approach to Unsaturated Polyesters

We report the ring-opening copolymerization of maleic anhydride with a variety of epoxides catalyzed by a chromium(III) salen complex. Quantitative isomerization of the cis-maleate form of all polymers affords the trans-fumarate analogues. Addition of chain transfer reagents yields low M(n), narrow PDI polymer samples. This method provides access to a range of new unsaturated polyesters with versatile functionality, as well as the first synthesis of high molecular weight poly(propylene fumarate).

Reusable Chiral Dicationic Chromium(III) Salen Catalysts for Aminolytic Kinetic Resolution of trans‐Epoxides

AbstractA series of new recyclable chiral dicationic chromium(III) salen complexes 1–10 bearing different substituents, viz., hydrogen, methyl, tert‐butyl, triphenylphosphinomethyl, triethylaminomethyl, methylimidazolium, methylpyridinium, methyl‐N,N‐dimethylpyridinium at the 3,3′‐ and 5,5′‐ positions of the salen unit with (1S,2S)‐(+)‐1,2‐diaminocyclohexane, (1S,2S)‐(−)‐1,2‐diphenyl‐1,2‐diaminoethane, and (S)‐(−)‐1,1′‐binaphthyl‐2,2′‐diamine collars have been synthesized and characterized by various physico‐chemical methods. These complexes were used as catalysts for the highly enantioselective aminolytic kinetic resolution of racemic trans‐epoxides with different anilines as nucleophiles at room temperature. With the use of catalyst 3, anti‐β‐amino alcohols were obtained in excellent yields (>99% with respect to the nucleophile) and enantioselectivities (ee>99%) with the concomitant recovery of corresponding epoxides in high optical purity (ee up to >99%) and quantitative yields in 12 h. The catalyst 3 is recyclable in the aminolytic kinetic resolution process and worked well up to six cycles with retention of enantioselectivity.

Binding of 4-(N,N-dimethylamino)pyridine to Salen- and Salan-Cr(III) Cations: A Mechanistic Understanding on the Difference in Their Catalytic Activity for CO2/Epoxide Copolymerization

The coordination chemistry of 4-(N,N-dimethylamino)pyridine (DMAP) with Salen- or Salan- (where the tetradentate N,N'-disubstituted bisaminophenoxide is designated as Salan, a saturated version of the Schiff-base Salen ligand) chromium complexes was studied by electrospray ionization mass spectrometry (ESI-MS). The relative stabilities of mono DMAP adducts of these chromium complexes were characterized by collision-induced dissociation (CID) and further discussed with regard to the activity in catalyzing CO(2)/epoxide copolymerization. [SalenCr](+) cations preferably bind two DMAP molecules to form six-coordinated complex ions, while [SalanCr](+) cations usually bind one molecule of DMAP to form five-coordinated complex ions, which were found to be relatively unstable. The remarkable difference in the coordination of DMAP to these two chromium complexes resulted in a significant difference in catalytic activity for the alternating copolymerization of CO(2) and propylene oxide. In the presence of 1 equiv of DMAP, the activity of the chromium-Salan complex 2a was up to 86 h(-1) of TOF at ambient temperature, which was about 30 times that of the corresponding chromium-Salen complex 1a. In sharp contrast to a long induction period up to 2 h with the use of 1a in conjunction with DMAP as catalyst, no initiation time or a very short one was observed in the binary 2a/DMAP catalyst systems. The initiator role of DMAP was confirmed by continuous determination of the propagating polymer species at various intervals using ESI-MS, which in combination with a kinetic study by means of infrared spectroscopy resulted in a mechanistic understanding on the difference in activity of the two catalyst systems for CO(2)/epoxide copolymerization.

Two-electron oxidation of N,N,N′,N′-tetramethylphenylenediamine with a chromium(v) salen complex

The oxidation of tetramethylphenylenediamine (TMPD) with (salen)CrVO+ generates initially the 2-electron product TMPD2+, followed by the reaction with excess TMPD to yield the radical cation, TMPD+. The kinetics of both TMPD/(salen)CrVO+ and TMPD/TMPD2+ reactions are acid-dependent, with TMPD being the most reactive form, and the doubly protonated TMPDH2(2+) exhibiting no discernible reactivity toward either (salen)CrVO+ or TMPD2+. The specific rate constants for the individual reactions are: TMPD/(salen)CrVO+, k=(6.28+/-0.50)x10(9) M(-1) s(-1), TMPDH+/(salen)CrVO+, (3.89+/-0.31)x10(5), TMPD/TMPD2+, (7.61+/-0.42)x10(8), and TMPDH+/TMPD2+, (3.46+/-0.22)x10(4).

Carbon dioxide/propylene oxide coupling reaction catalyzed by chromium salen complexes

A series of chromium/Schiff base complexes N,N′-bis(salicylidene)-1,2-phenylenediamino chromiumIII X were prepared and employed for the alternating copolymerization of carbon dioxide with racemic propylene oxide in the presence of (4-dimethylamino)pyridine. The effect of the complex structure and reaction conditions on the catalytic activity, the poly(propylene carbonate)/cyclic carbonate (PPC/PC) selectivity, and the polymer head-to-tail linkages was examined. The experiments indicated that N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-phenylenediamino chromiumIII (NO3) exhibited the highest PPC/PC selectivity as well as polymer head-to-tail linkages and N,N′-bis(3,5-dichlorosalicylidene)-1,2-phenylenediimino chromiumIII (NO3) possessed the highest catalytic activity among these chromium/Schiff base complexes. The structure of the produced copolymer was characterized by the IR, 1H NMR, and 13C NMR measurements. Almost 100% carbonate content of the resulting polycarbonate were obtained with the help of these effective catalyst systems under facile conditions.

Poly(propylene carbonate), old CO2 Copolymer with New Attractiveness

AbstractSummary: The catalytic synthesis of poly(propylene carbonate) (PPC) from propylene oxide and CO2 is mediated by zinc glutarate and chromium salen complexes. The determined solid state structure of the zinc glutarate was taken to model the polymerisation of ethylene oxide. It has a low activation energy for a reaction path involving two zinc atoms, where one binds the nucleophilic alkoxy chain end and one activates the epoxide. A similar pathway may operate in the alternating copolymerization PO/CO2 as is shown in the homogeneous chromium salen catalyst system. The material profile was determined of PPC with 93% carbonate linkages. A study of the transparent blend with poly(lactic acid) (PLA) shows that the polymers are not miscible, but rather compatible as they are finely dispersed and show linear behaviour in the mechanic properties. The permeability of oxygen, carbon dioxide and water of the composite PPC/PLA are also reported.

Stereochemistry Control of the Alternating Copolymerization of CO2 and Propylene Oxide Catalyzed by SalenCrX Complexes

Various poly(propylene carbonate)s (PPC)s with different head-to-tail linkages (from ∼60% to 93%) could be synthesized by the copolymerization of racemic propylene oxide (rac-PO) and CO2 with chromium Salen complexes as catalyst. The axial X group and the Salen ligand framework of the chromium complexes and the properties of cocatalyst significantly affect stereochemistry of the resulting polymer. Continuous determination of a polymer chain end group (initiating and chain growth species) at various times was achieved by in situ electrospray ionization mass spectrometry (ESI-MS), which in combination with the terpolymerization of CO2 with R-PO and cyclohexene oxide (CHO) gave us an insight into the mechanistic understanding of CO2/PO copolymerization and polycarbonates stereochemistry control. Apart from the effect of the ligand set of the chromium Salen complexes on polymer stereochemistry, a “polymer chain-end control” mechanism also significantly affects the stereochemistry of the resulting polycarbonates.

Role of the Cocatalyst in the Copolymerization of CO2 and Cyclohexene Oxide Utilizing Chromium Salen Complexes [J. Am. Chem. Soc. 2005, 127, 14026−14038

ADVERTISEMENT RETURN TO ISSUEPREVAddition/CorrectionNEXTORIGINAL ARTICLEThis notice is a correctionRole of the Cocatalyst in the Copolymerization of CO2 and Cyclohexene Oxide Utilizing Chromium Salen Complexes [J. Am. Chem. Soc.2005, 127, 14026−14038].Donald J. Darensbourg, Mackiewicz, and Jody L. RodgersCite this: J. Am. Chem. Soc. 2005, 127, 49, 17565Publication Date (Web):November 18, 2005Publication History Published online18 November 2005Published inissue 1 December 2005https://doi.org/10.1021/ja059920dCopyright © 2005 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views857Altmetric-Citations11LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (10 KB) Get e-Alerts Get e-Alerts