Metal Salen Research Articles

Immobilization of Ru(II)(salen)(PPh3)2 on Mesoporous MCM-41/SBA-15: Characterization and Catalytic Applications

Ru(II)(salen)(PPh3)2 immobilized on MCM-41 and SBA-15 modified with aminopropyl groups as linkers has been synthesized and characterized by elemental analysis, TEM, FTIR, BET, and TGA. Elemental analysis shows that the grafted samples contain 0.7–0.8 wt.% Ru. The retaining of long range ordering of the mesoporous MCM-41 and SBA-15 supporting materials after grafting is evident from TEM and N2 adsorption/desorption measurements. FTIR and TGA spectra show the formation of metal salen complexes with the amino groups acting as connectors to the SiO2 surface. Both grafted materials were successfully applied as catalysts for the olefination of various aldehydes with very good yields and high E-selectivity. The catalyst materials are recyclable for several catalytic runs.

Crystal structures and mesomorphism of metal–salen complexes with 4-substituted long alkoxy chains

Crystal structures and mesomorphism of metal–salen complexes with 4-substituted long alkoxy chains

Correlation between Active Center Structure and Enhanced Dioxygen Binding in Co(salen) Nanoparticles: Characterization by In Situ Infrared, Raman, and X-ray Absorption Spectroscopies

The structure and ligand environment of Co(salen) nanoparticles and unprocessed Co(salen) have been determined by the combined application of infrared, Raman, X-ray absorption near edge structure (XANES), and extended X-ray absorption fine structure (EXAFS) spectroscopies, and X-ray diffraction (XRD) experiments before and during interaction with O2. The Co(salen) nanoparticles were prepared by the precipitation with compressed antisolvent (PCA) technique using commercially obtained Co(salen) [denoted as unprocessed Co(salen)] as the parent compound. The unprocessed Co(salen) particles exist as dimer species with a square-pyramidal coordination geometry that display no measurable O2 binding at room temperature. In sharp contrast, the Co(salen) nanoparticles show near-stoichiometric O2 adsorption, as demonstrated by microbalance gas binding experiments. The spectroscopy results indicate the presence of CoII centers with distorted tetrahedral geometry in the Co(salen) nanoparticles with no evidence of metallic Co clusters, confirmed by the lack of Co−Co contributions at bonding distances in the EXAFS spectra and the presence of characteristic features of CoII in the XANES spectra. The EXAFS data also indicate that there are on average two Co−N and two Co−O bonds with a distance of 1.81 ± 0.02 and 1.90 ± 0.02 Å, respectively, consistent with typical metal salen structures. Upon O2 binding on the Co(salen) nanoparticles, the XANES results indicate oxidation of the CoII to CoIII, consistent with the vibrational data showing new bands associated with oxygen species bonded to Co centers and the increase in the oxygen coordination number from 1.8 to 2.9 in the EXAFS data. The results indicate that the enhanced O2 binding properties of Co(salen) nanoparticles are related to the unique distorted tetrahedral geometry, which is not observed in the unprocessed samples that contain mainly dimers with square planar geometry. The results presented here provide a fundamental relationship between active center structure and properties of novel molecule-based nanomaterials.

Metal salen complexes as ethylene polymerization catalysts - effect of catalytic system composition on its activity and properties of polymerization products

The series of salen-type complexes differing in the metal center (M = Ti, Zr, V) and in the coordination spheres (different substituent at salen ligands) after activation with methylalumoxane (MAO) or Et 2 AlCl were studied as catalytic systems in ethylene polymerization. The vanadium and titanium complexes were found to be more active in combination with alkylaluminum compound (Et 2 AlCl) then with MAO, while zirconium complex was active only in combination with MAO. The activity of complexes activated by Et2AlCl decreases in the order V > Ti » Zr ≡ 0, and when MAO is used as the cocatalyst the activity changes the order: Ti > Zr > V. The substituent type in the salen ligand also affects the catalyst activity; the best results were obtained for the catalyst having Cl or Br as substituents. The type of substituent has no significant effect on polyethylene properties, while the type of cocatalyst and the type of metallic centre play here the essential role. The salen titanium complexes in combination with Et 2 AlCl usually produce low molecular weight polyethylene with narrow polydispersity, also accompanied by oligomers. Other catalytic systems give linear high molecular weight polyethylene (M w = 200 000-700 000).

Complexities in the Ring-Opening Polymerization of Lactide by Chiral Salen Aluminum Initiators

The preparation of (R, R)- and (S, S)-salen Al(OR) complexes, where R = Et, CH2(i)Pr, CH2(t)Bu, and CH2CH(S)MeCl, are reported, along with their reactions with rac-lactide (salen = N, N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino). Rapid, reversible coordination of LA to the salen metal complex is observed, and it is shown that the relative rates of alcohol/alkoxide exchange are comparable to the NMR time scale while the rate of chain transfer involving (R, R)-salenAl(O-R-R) and (S, S)-salenAl(O-S-R) is much faster than the initial rate of ring opening of the LA monomer. For a primary [Al-OR] moiety, the ring opening of rac-LA is much faster than the ring-opening polymerization/enchainment of LA, and in the initial ring-opening event, the diastereoselectivity is dependent on the solvent, the chirality of the salen ligand, and the OR group. Irrespective of the initiator group OR or the solvent, the system moves to a pseudostatic equilibrium concentration of L- and D-LA which is dependent on the nature of the chirality of the salen ligand. Further studies show that the relative rate of trans-esterification is slower than the rate of LA enchainment and that the rate of epimerization is the slowest reaction in the system. Adventitious water leads to loss of catalytic activity and formation of the inert oxo-bridged compound [(salen)Al]2(mu-O) which has been structurally characterized.

Synthesis of benzhydrol derivatives by metal imidozalen catalysed electrophilic addition of aromatic aldehyde to hydrocarbons under solvothermal condition

Salen metal complexes act as good catalysts for reactions in homogenised, heterogenised and heterogenised-homogeneous conditions. A simple method for the synthesis of benzhydrol derivatives is proposed using anisole and substituted benzaldehydes in the presence of a salen metal catalyst at high temperature and pressure using Morey autoclave. The compounds were isolated in good yield and were characterized by spectral and analytical methods.

Energetics and Dynamics of Electron Transfer and Proton Transfer in Dissociation of MetalIII(salen)−Peptide Complexes in the Gas Phase

Time- and collision energy-resolved surface-induced dissociation (SID) of ternary complexes of Co(III)(salen)+, Fe(III)(salen)+, and Mn(III)(salen)+ with several angiotensin peptide analogues was studied using a Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially equipped to perform SID experiments. Time-resolved fragmentation efficiency curves (TFECs) were modeled using an RRKM-based approach developed in our laboratory. The approach utilizes a very flexible analytical expression for the internal energy deposition function that is capable of reproducing both single-collision and multiple-collision activation in the gas phase and excitation by collisions with a surface. The energetics and dynamics of competing dissociation pathways obtained from the modeling provides important insight on the competition between proton transfer, electron transfer, loss of neutral peptide ligand, and other processes that determine gas-phase fragmentation of these model systems. Similar fragmentation behavior was obtained for various Co(III)(salen)-peptide systems of different angiotensin analogues. In contrast, dissociation pathways and relative stabilities of the complexes changed dramatically when cobalt was replaced with trivalent iron or manganese. We demonstrate that the electron-transfer efficiency is correlated with redox properties of the metal(III)(salen) complexes (Co > Fe > Mn), while differences in the types of fragments formed from the complexes reflect differences in the modes of binding between the metal-salen complex and the peptide ligand. RRKM modeling of time- and collision-energy-resolved SID data suggests that the competition between proton transfer and electron transfer during dissociation of Co(III)(salen)-peptide complexes is mainly determined by differences in entropy effects while the energetics of these two pathways are very similar.

Oxidation of cyclohexane over iron and copper salen complexes simultaneously encapsulated in zeolite Y

Fe(salen) and Cu(salen) complexes simultaneously encapsulated in zeolite Y by the flexible ligand method showed much higher activity than the neat Cu(salen) and Fe(salen) or the Cu(salen)/Y and Fe(salen)/Y or their physical mixtures in the oxidation of cyclohexane with aqueous H 2 O 2 solution as a result of the existence of a synergetic effect. Iron and copper salen complexes have been simultaneously encapsulated in zeolite Y by using the flexible ligand method, as substantiated by infrared spectroscopy, diffuse reflectance UV–vis spectroscopy, thermo-gravimetric and differential-thermal analyses and N 2 adsorption/desorption experiments at −196 °C. The prepared material showed much higher activity than the neat Cu(salen) and Fe(salen) or the Cu(salen)/Y and Fe(salen)/Y or their physical mixtures in the oxidation of cyclohexane with H 2 O 2 . The existence of a synergetic effect is the cause of this high activity. This synergetic effect may originate from the formation of dinuclear salen complexes and/or the interaction of adjacent partially coordinated Cu(salen) and Fe(salen) complex molecules through the lattice oxygen of the zeolitic host. This assumption is supported by extensive investigations of the catalytic performance of different types of catalysts as well as cyclic voltammetry and diffuse reflectance UV–vis spectroscopy. The optimum molar ratio of Cu 2+ to Fe 3+ in the prepared material is 1.93:1. The reaction time and temperature strongly influenced the catalytic performance. Although this catalyst was not highly stable because of degradation of metal salen complexes in the presence of H 2 O 2 aqueous solution, this work provides a new route for designing and preparing highly active “ship-in-a-bottle” materials.

Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

The intercalation of transition metal salen complexes into layered MoS2

Exfoliation-restack method has been employed to synthesize the intercalation compounds based on the cationic complexes [M(Salen)]+ (M = Mn3+, Fe3+, Co3+; Salen = N, N′-ethylene-bis(salicylaldimine)) into the layered MoS2. Their conductivity is in the range of 0.04–0.1 S/cm, which is much higher than the pristine MoS2. Magnetic measurement indicated that the intercalation compounds [Mn(Salen)]0.18MoS2 · 0.25H2O and [Fe(Salen)]0.12MoS2 · 0.3H2O exhibit the temperature-dependent paramagnetism, which obviates from the Curie–Weiss law due to the temperature-independent paramagnetism of the exfoliated MoS2 slabs, while [Co(Salen)]0.14MoS2 · 0.5H2O exhibits the almost temperature-independent paramagnetism. All three intercalation compounds do not show magnetic spin crossover behavior.

Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO2

Making Plastics from Carbon Dioxide: Salen Metal Complexes as Catalysts for the Production of Polycarbonates from Epoxides and CO₂

Immobilization of ruthenium(II) salen complexes on poly(4-vinylpyridine) and their application in catalytic aldehyde olefination

Two Ru(II)(salen)(PPh 3) 2 complexes grafted on poly(4-vinylpyridine) have been synthesized and characterized. An elemental analysis shows that both grafted samples contain ca. 0.6 wt % Ru. FTIR spectra confirm the formation of metal–salen complexes attached to the carrier polymer by an interaction between the ruthenium(II) compounds with the pyridine nitrogen atoms of the poly(4-vinylpyridine). Immobilization of both Ru(II) salen complexes on the polymer increases their thermal stability as demonstrated by TG–MS analysis. The grafted materials were applied as catalysts for the olefination of various aldehydes at 60 °C under an inert gas atmosphere, showing comparable yields as their homogeneous congeners and high trans-selectivities. The ruthenium(II) compound with a larger salen ligand shows a better recyclability and selectivity than the derivative with the smaller ligand.

Synthesis, characterization and magnetic properties of transition metal salen complexes intercalated into layered MnPS 3

Three new intercalation compounds based on the cationic complexes [M(salen)] + (M = Mn 3+, Fe 3+, Co 3+; salen = N, N′-ethylene-bis(salicylaldimine)) and the layered MnPS 3 have been synthesized and characterized by XRD, IR, TGA and elemental analysis. Compared with pure MnPS 3, the lattice expansion (Δ d) of 9.6–9.7 Å for [Fe(salen)] 0.56Mn 0.72PS 3·0.5H 2O and [Mn(salen)] 0.26Mn 0.87PS 3·0.5H 2O indicates that the guest molecular is arranged with its C 2 axis in a titled way to the host layer. While [Co(Salen)] 0.70Mn 0.65PS 3·H 2O was expanded by about 14 Å suggesting that the guest is arranged in another way with its C 2 axis parallel to host layer. Their magnetic properties were studied with SQUID. The intercalate [Fe(salen)] 0.56Mn 0.72PS 3·0.5H 2O and [Mn(salen)] 0.26Mn 0.87PS 3·0.5H 2O exhibit magnetic transition from paramagnetism to ferrimagnetism at about 35 K. However, [Co(salen)] 0.70Mn 0.65PS 3·H 2O only exhibits paramagnetism in the whole range of the measured temperature. All three intercalates do not show spin crossover behavior.

Metal Salen Derivatives as Catalysts for the Alternating Copolymerization of Oxetanes and Carbon Dioxide To Afford Polycarbonates

Metal salen derivatives of chromium and aluminum, along with n-Bu4NX (X = Cl or N3) salts, have been shown to be effective catalysts for the selective coupling of CO2 and oxetane (trimethylene oxide) to provide the corresponding polycarbonate with only trace quantities of ether linkages. The formation of copolymer is suggested, based on circumstantial evidence, not to proceed via the intermediacy of trimethylene carbonate, which was observed as a minor product of the coupling reaction. For a reaction catalyzed by (salen)CrCl in the presence of n-Bu4NN3 as the cocatalyst, both matrix-assisted laser desorption ionization time-of-flight mass spectrometry and infrared spectroscopy revealed an azide end group in the copolymer.

Asymmetric cyanohydrin synthesis from aldehydes and ketones using chiral metal (salen) complex as catalyst

Abstract Chiral Al(salen) is used as catalyst for enantioselective cyanosilylation of aldehydes and ketones.

Metal salen complexes incorporating triphenoxymethanes: efficient, size selective anion binding by phenolic donors with a visual report

A metal salen complex has been designed to orientate four phenol groups into a tetrahedral array that tightly binds fluoride ion though four OH...F hydrogen bonding interactions.

Asymmetric Catalysis of Carbon—Carbon Bond‐Forming Reactions Using Metal(salen) Complexes

AbstractFor Abstract see ChemInform Abstract in Full Text.

On the Formation of Aliphatic Polycarbonates from Epoxides with Chromium(III) and Aluminum(III) Metal–Salen Complexes

A DFT-based description is given of the CO2/epoxide copolymerization with a catalyst system consisting of metal (chromium, iron, titanium, aluminum)-salen complexes (salen = N,N'-bis(3,5-di-tert-butylsalicyliden-1,6-diaminophenyl) in combination with either chloride, acetate, or dimethylamino pyridine (DMAP) as external nucleophile. Calculations indicate that initiation proceeds through nucleophilic attack at a metal-coordinated epoxide, and the most likely propagation reaction is a bimolecular process in which a metal-bound nucleophile attacks a metal-bound epoxide. Carbon dioxide insertion occurs at a single metal center and is most likely the rate-determining step at low pressure. The prevalent chain terminating/degradation-the so-called backbiting, a reaction leading to formation of cyclic carbonate from the polymer chain-would involve attack of a carbonate nucleophile rather than an alkoxide at the last unit of the growing chain. The backbiting of a free carbonato chain end is particularly efficient. Anion dissociation from six-coordinate aluminum is appreciably easier than from chromium-salen complexes, indicating the reason why in the former case cyclic carbonate is the sole product. Experimental data were gathered for a series of chromium-, aluminum-, iron-, and zinc-salen complexes, which were used in combination with external nucleophiles like DMAP and mainly (tetraalkyl ammonium) chloride/acetate. Aluminum complexes transform PO (propylene oxide) and CO2 to give exclusively propylene carbonate. This is explained by rapid carbonate anion dissociation from a six-coordinate complex and cyclic formation. CO2 insertion or nucleophilic attack of an external nucleophile at a coordinated epoxide (at higher CO2 pressure) are the rate-determining steps. Catalysis with [Cr(salen)(acetate/chloride)] complexes leads to the formation of both cyclic carbonate and polypropylene carbonate with various quantities of ether linkages. The dependence of the activity and selectivity on the CO2 pressure, added nucleophile, reaction temperature, and catalyst concentration is complex. A mechanistic description for the chromium-salen catalysis is proposed comprising a multistep and multicenter reaction cycle. PO and CO2 were also treated with mixtures of aluminum- and chromium-salen complexes to yield unexpected ratios of polypropylene carbonate and cyclic propylene carbonate.

Cover Picture: On the Formation of Aliphatic Polycarbonates from Epoxides with Chromium(III) and Aluminum(III) Metal–Salen Complexes (Chem. Eur. J. 21/2005)

A better understanding of the formation of aliphatic polycarbonates from propylene oxide and CO2 is presented by G. A. Luinstra, F. Molnar, B. Rieger et al. in their Full Paper on page 6298 ff. They describe experimental and DFT investigations for the homogeneous reaction using chromium(III)–and aluminum(III)–salen catalysts as well as for their mixtures.

Asymmetric Catalysis of Carbon-Carbon Bond-Forming Reactions Using Metal(salen) Complexes

The use of metal(salen) complexes in the asymmetric syntheses of cyanohydrins and amino acids have been developed as important and reliable methods of obtaining these highly versatile chiral intermediates. Thus, we have developed a number of titanium and vanadium(salen) complexes, with ligands derived from (R,R)-cyclohexane-1,2-diamine, which catalyze the synthesis of enantiomerically enriched 0-protected cyanohydrins from various carbonyl compounds and cyanide sources. Nickel, copper and cobalt(salen) complexes have also been developed, as catalysts for the preparation of non-racemic amino acids under phase-transfer conditions.