Salen Manganese Research Articles

Synthesis of New Macrocyclic Chiral Manganese(III) Schiff Bases as Catalysts for Asymmetric Epoxidation

We describe a general synthetic strategy for the preparation of a series of macrocyclic chiral manganese(III) salen complexes. The developed reaction pathway allows the modulation of the different key groups, namely, the chiral diimine, the bulky substituents in positions 3 and 3', and the linker used in the macrocyclization of the Schiff base. The different complexes presented here illustrate these readily available structural variations. The catalytic properties of the catalysts (5 mol %) were improved for the asymmetric epoxidation of 2,2'-dimethylchromene with NaOCl or H2O2 as oxygen atom donor. A large range of enantiomeric excesses was obtained (ee values from 30% to 96%), depending on the features and the stability of the complexes. The most efficient catalyst, in terms of stereoinduction (ee value = 96%), contains a diiminocyclohexyl moiety, ethyl groups in positions 3 and 3', and a short polyether junction arm.

Encapsulation of chiral Mn(III) salen complexes into aluminium pillared clays: Application as heterogeneous catalysts in the epoxidation of styrene

Two chiral manganese(III) salen complexes bearing different chiral diimine bridges, complexes I and II, were encapsulated into three types of Al-pillared clays (Al-PILCs) by in situ generation of the complexes inside the PILCs; one of the PILCs used was derived from a Wyoming clay (Al-WYO) and the other two were from the same original clay Benavila: (i) Al-BEN, where no surfactant was used in the preparation and (ii) Al-TERG, where a polyalchol, i.e. tergitol was used as surfactant to act as an interlayer gallery template. The Al-pillared clay based materials were characterised by different techniques such as FTIR, UV–vis, AA, XPS, XRD and N 2 adsorption at 77 K. They were screened as heterogeneous catalysts in the epoxidation of styrene using iodosylbenzene or m-chloroperbenzoic acid as oxidants. The encapsulated complexes show, in general, moderate alkene conversion (16–40%), but very poor enantiomeric excesses (ee% = 0–14); nevertheless, for complex I encapsulated into Al-TERG using PhIO as oxidant 14 ee% was obtained, which is higher than the value found for the homogeneous phase reaction run under comparable conditions. Characterisation of the catalyst materials after two run suggests partial collapsing of the clay pillars.

Covalent attachment of chiral manganese(III) salen complexes onto functionalised hexagonal mesoporous silica and application to the asymmetric epoxidation of alkenes

Two modified Jacobsen-type catalysts were anchored onto an amine functionalised hexagonal mesoporous silica (HMS) using two distinct anchoring procedures: (i) one was anchored directly through the carboxylic acid functionalised diimine bridge fragment of the complex (CAT1) and (ii) the other through the hydroxyl group on the aldehyde fragment of the complex (CAT2), mediated by cyanuric chloride. The new heterogeneous catalyst, as well as the precedent materials, were characterised by elemental analyses, DRIFT, UV–vis, porosimetry and XPS which showed that the complexes were successfully anchored onto the hexagonal mesoporous silica. These materials acted as active heterogeneous catalysts in the epoxidation of styrene, using m-CPBA as oxidant, and α-methylstyrene, using NaOCl as oxidant. Under the latter conditions they acted also as enantioselective heterogeneous catalysts. Furthermore, when compared to the reaction run in homogeneous phase under similar experimental conditions, an increase in asymmetric induction was observed for the heterogenised CAT1, while the opposite effect was observed for the heterogenised CAT2, despite of CAT2 being more enantioselective than CAT1 in homogeneous phase. These results indicate that the covalent attachment of the Jacobsen catalyst through the diimine bridge leads to improved enantiomeric excess (%ee), whereas covalent attachment through one of the aldehyde fragments results in a negative effect in the %ee. Using α-methylstyrene and NaOCl as oxidant, heterogeneous catalyst reuse led to no significant loss of catalytic activity and enantioselectivity.

A metal–organic framework material that functions as an enantioselective catalyst for olefin epoxidation

A new microporous metal-organic framework compound featuring chiral (salen)Mn struts is highly effective as an asymmetric catalyst for olefin epoxidation, yielding enantiomeric excesses that rival those of the free molecular analogue. Framework confinement of the manganese salen entity enhances catalyst stability, imparts substrate size selectivity, and permits catalyst separation and reuse.

Enhanced catalase-like activity of manganese salen complexes in water: effect of a three-dimensionally fixed auxiliary

A new Mn(Salen) complex bearing an ureido group as an auxiliary that is three-dimensionally fixed by a cyclopentane ring fused to the Salen structure was developed. This compound exhibited considerably higher catalase-like activity than the original Mn(Salen), i.e., the cyclopentane-fused Mn(Salen) without the auxiliary, under near-physiological conditions.

Catalytic epoxidation of olefins and hydroxylation of alkanes with sodium periodate by water-soluble manganese(III)salen

The catalytic activity of a water-soluble Mn(salen)OAc complex in the epoxidation of alkenes and hydroxylation of alkanes was studied in acetonitrile, at room temperature, using sodium periodate as an oxygen source. The effect of various axial ligands as co-catalyst such as triethylamine, diethylamine, piperidine, 4-cyanopyridine, 2-methylpyridine, 4-methylpyridine, 4- tert-butylpyridine, 2-methylimidazole, pyrazine, quinalidine, morpholine, triphenylphosphine and dimethylformamide were investigated in the epoxidation of cyclooctene. Imidazole as a strong π-donor was the best axial ligand. The effect of different solvents was studied in the epoxidation of cyclooctene and CH 3CN/H 2O was chosen as solvent. The effect of the oxygen donors such as NaIO 4, Bu 4NIO 4, KHSO 5, H 2O 2, H 2O 2/urea, NaOCl and tert-BuOOH was also studied in the epoxidation of cyclooctene where NaIO 4 was selected as an oxygen donor.

Organo-Laponites as Novel Mesoporous Supports for Manganese(III) salen Catalysts

A Mn(III) salen complex was immobilized onto the Laponite surface using three different methodologies: method A, direct immobilization of the complex on the parent Laponite; method B, covalent anchoring through cyanuric chloride (CC); and method C, covalent anchoring through CC into a 3-aminopropyl)triethoxysilane (APTES) modified Laponite. All of the materials were characterized by FTIR, XPS, thermogravimetry, XRD, and nitrogen isotherms at 77 K, to gather information on the modifications introduced by the organo spacers within the Laponite surface, as well as on the anchored complex integrity; the Mn based materials were screened in the heterogeneous epoxidation of styrene. The results have shown that the immobilization of the manganese(III) salen complex by methods B and C have occurred at the edges of the clay particles through the spacers (APTES and CC) that have been anchored onto the Si-OH groups, whereas in method A, the complex is distributed throughout the clay surface, including the interlayer region. Therefore, the manganese loadings on the Laponites were as follows: materials prepared by method A >> method B > method C. All of the heterogeneous catalysts showed high styrene epoxide selectivity, with that prepared by method A showing comparable styrene epoxide selectivity as the homogeneous phase reaction. The styrene epoxide yields decrease in the following order: materials prepared by method A > method B > method C (1st cycles), which parallel the respective support catalytic activity and decreasing of manganese content. The heterogeneous catalysts prepared using methods B and C could be reused at least for four times, with the former exhibiting the most stable catalytic activity, but that prepared by method A showed a significant decrease after two catalytic cycles.

Mn(III) salen complex immobilised into pillared clays by in situ and simultaneous pillaring/encapsulation procedures : Application in the heterogeneous epoxidation of styrene

The complex [Mn(saldPh)Cl], chloride-[ N, N′-bis(salicylaldehyde)1,2-diphenylethylenediamine]manganese(III), was encapsulated into an aluminium pillared clay (denoted as Al-WYO) by three different methodologies. Method A—two step liquid phase methodology: (i) adsorption of manganese(II) chloride in methanolic solution within an aluminium pillared clay, followed by (ii) diffusion of the ligand N, N′-bis(salicylaldehyde)-1,2-diphenylethylenediamine in methanol. Method B—simultaneous/pillaring encapsulation of the manganese(III) salen complex, [Mn(saldPh)Cl], and pillaring of a montmorillonite clay with aluminium polyoxycations. In this case, the [Mn(saldPh)Cl] complex was dissolved in ethanol and added to the oligomer solution at a stage where the oligomeric species are already formed. Method C—the simultaneous encapsulation also occurred but the complex, also dissolved in ethanol, was added to the clay dispersion. All materials were characterised by XRD, XPS, atomic absorption spectroscopy, UV–Vis, and FTIR. In all cases the manganese(III) salen complex is mainly physically entrapped within the matrix, although some host–guest interactions with the matrix could be present. All new materials, [Mn(saldPh)Cl]@Al-WYO_A, [Mn(saldPh)Cl]@Al-WYO_B, and [Mn(saldPh)Cl]@Al-WYO_C show catalytic activity in the epoxidation of styrene at room temperature using PhIO as oxygen source in acetonitrile. The solids presented high styrene epoxide selectivity and were reused for four times, but a small decrease in the catalytic activity was observed. Nevertheless, the catalysts prepared by simultaneous pillaring/encapsulation, methods B and C, are more stable upon reuse than the catalyst prepared by method A. FTIR spectra of catalysts after the catalytic reactions suggest that during the reaction no structural changes of the PILC took place, but some active phase leaching and deactivation have occurred which are responsible for the decrease in the catalytic activity on reutilisation.

Styrene epoxidation catalysed by manganese(III) salen complex supported on activated carbons

The hydroxyl functionalised manganese(III) salen complex, bearing a large π delocalisation, [Mn(4-HOsaldPh)Cl], was immobilised onto a commercial activated carbon and its air and acid oxidised forms using three different metal loadings. For all the materials the manganese contents were determined by ICP-AES and they were screened as heterogeneous catalysts in the epoxidation of styrene, using iodosylbenzene as oxygen source and acetonitrile as reaction media. All the heterogeneous catalysts were as chemoselective towards the styrene epoxide as the homogeneous counterpart, with the exception of the complex supported onto the acid oxidised activated carbon, which exhibited the lowest styrene epoxide selectivities, a consequence of its acidic oxygen surface groups; the highest styrene conversions and styrene epoxide yields were obtained for that complex immobilised onto the air oxidised activated carbon. Generally, an increase in styrene conversion and styrene epoxide selectivity with the manganese(III) complex loading onto all supports was observed. Upon reuse all the heterogeneous catalysts do not lose their styrene epoxide selectivity, but generally a slight decrease in the styrene epoxide yield is observed; catalyst aging studies done for one of the material prepared, [Mn(4-OHsaldPh)Cl]@C2_1%, revealed high stability and the same catalytic efficiency for almost 2 months.

Nitrogen Atom Transfer between Manganese Complexes of Salen, Porphyrin, and Corrole and Characterization of a (Nitrido)manganese(VI) Corrole

The investigations of complete nitrogen atom transfer reactions from (nitrido)manganese(V) salen to manganese(III) complexes of porphyrins and corroles revealed that stabilization of the [Mn(N)]2+ moiety is in the order of corrole > porphyrin > salen. The first kinetic examination of this quite fundamental reaction exposed a large solvent effect on both the enthalpy and entropy activation energies. Oxidation of the (nitrido)manganese(V) corroles leads to the first (nitrido)manganese(VI) complexes that are coordinated by tetrapyrrolic ligands.

Parallel mechanistic studies on the counterion effect of manganese salen and porphyrin complexes on olefin epoxidation by iodosylarenes

Counterions of manganese(III) porphyrin complexes influence diastereoselectivity in cis-stilbene epoxidation and product distribution in cyclohexene epoxidation markedly. In the epoxidation of cis-stilbene by iodosylbenzene carried out in a solvent mixture of CH 3CN and CH 2Cl 2, trans-stilbene oxide is the major product in the reaction of manganese complexes bearing a ligating anion (i.e., Cl −), whereas cis-stilbene oxide is the dominant product in the reactions of manganese complexes bearing a poorly-ligating anion (i.e., CF 3 SO 4 - ). In cyclohexene epoxidation, the yields of allylic oxidation products such as cyclohexenol and cyclohexenone are higher when the counterion of the manganese catalysts is Cl − than when the counterion is CF 3 SO 4 - . The product selectivities are also dependent on the nature of iodosylarenes and the axial and porphyrin ligands of the manganese porphyrin catalysts. The observation that product selectivities are different depending on the iodosylarenes may indicate the involvement of multiple oxidants in oxygen atom transfer reactions. These results are compared with those observed in manganese salen-catalyzed epoxidation of olefins by iodosylarenes.

A Catalytic Langmuir Film as a Model for Heterogeneous and Homogeneous Catalytic Processes

An amphiphilic manganese salen complex forms Langmuir films that catalyze the epoxidation of cinnamyl alcohol with the urea/hydrogen peroxide complex (UHP) dissolved in the subphase (see picture). Compression of the monolayer changes the organization of the film and thus strongly modifies the rate of the reaction, which is enhanced for molecular areas of about 140–160 Å2.

The reactivity–selectivity principle in the oxidation of aryl methyl sulfides with sodium hypochlorite catalysed by (salen)MnIII complexes

The kinetics of oxygen atom transfer from four oxo(salen)manganese(V) complexes to various para-substituted phenyl methyl sulfides have been studied spectrophotometrically in 90% acetonitrile-10% water(v/v) at 20oC. Electron-releasing substituents in sulfides and electron-withdrawing substituents in oxo(salen)manganese(V) complexes enhance the rate of oxidation. Correlation analyses establish that there is an inverse relationship between reactivity and selectivity in both the sulfide and the complex series. Mathematical treatment of the results shows the operation of a valid reactivity-selectivity principle in this redox system.

Mesoporous MCM41-heterogenised (salen)Mn and Cu complexes as effective catalysts for oxidation of sulfides to sulfoxides

Chiral salen manganese and copper complexes immobilised on mesoporous silica supports have been investigated as catalysts in the sulfide to sulfoxide oxidation. The presence of stable manganyl [Mn(V) O] complexes into support channels have been established, being responsible for catalytic activity. The recovery and reusability of heterogenised Mn- and Cu-catalysts have been studied . Chiral salen manganese and copper complexes immobilised on mesoporous silica supports (silica, MCM-41) have been investigated as catalysts in the sulfide to sulfoxide oxidation; the supported catalysts are highly suited to reactions leading to high yields and high sulfoxide/sulfone selectivity while moderate asymmetric induction (up to 30%) was observed. The presence of stable manganyl [Mn(V) O] complexes into support channels have been established, being responsible for catalytic activity. The recovery and reusability of heterogenised Mn- and Cu-catalysts have been studied.

Mesoporous MCM41-heterogenised (salen)Mn and Cu complexes as effective catalysts for oxidation of sulfides to sulfoxidesIsolation of a stable supported Mn(V)O complex, responsible of the catalytic activity

Abstract Chiral salen manganese and copper complexes immobilised on mesoporous silica supports (silica, MCM-41) have been investigated as catalysts in the sulfide to sulfoxide oxidation; the supported catalysts are highly suited to reactions leading to high yields and high sulfoxide/sulfone selectivity while moderate asymmetric induction (up to 30%) was observed. The presence of stable manganyl [Mn(V) O] complexes into support channels have been established, being responsible for catalytic activity. The recovery and reusability of heterogenised Mn- and Cu-catalysts have been studied.

A site-selective dual anchoring strategy for artificial metalloprotein design.

Introducing nonnative metal ions or metal-containing prosthetic groups into a protein can dramatically expand the repertoire of its functionalities and thus its range of applications. Particularly challenging is the control of substrate-binding and thus reaction selectivity such as enantioselectivity. To meet this challenge, both non-covalent and single-point attachments of metal complexes have been demonstrated previously. Since the protein template did not evolve to bind artificial metal complexes tightly in a single conformation, efforts to restrict conformational freedom by modifying the metal complexes and/or the protein are required to achieve high enantioselectivity using the above two strategies. Here we report a novel site-selective dual anchoring (two-point covalent attachment) strategy to introduce an achiral manganese salen complex (Mn(salen)), into apo sperm whale myoglobin (Mb) with bioconjugation yield close to 100%. The enantioselective excess increases from 0.3% for non-covalent, to 12.3% for single point, and to 51.3% for dual anchoring attachments. The dual anchoring method has the advantage of restricting the conformational freedom of the metal complex in the protein and can be generally applied to protein incorporation of other metal complexes with minimal structural modification to either the metal complex or the protein.

Synthesis and Characterization of Novel Chiral Sulfonato—Salen—Manganese(III) Complex in a Zinc—Aluminum LDH Host.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Epoxidation by Layered Double Hydroxide-Hosted Catalysts. Catalyst Synthesis and Use in the Epoxidation of R-(+)-Limonene and (-)-α-Pinene Using Molecular Oxygen

A novel chiral sulphonato–salen–manganese(III) complex has been intercalated into a Zn(II)–Al(III) layered double hydroxide (LDH) host to produce a stable heterogeneous epoxidation catalyst. Powder X-ray diffraction, IR and UV–Visible (solid) spectroscopy and TGA confirmed the successful intercalation of the Mn complex within the LDH gallery height. The intercalated sulphonato–salen–manganese(III) complex was found to be an effective heterogeneous catalyst for the stereoselective epoxidation of R-(+)-limonene and (−)-α-pinene at room temperature and using molecular oxygen at atmospheric pressure as oxidant. At close to 100% conversion, R-(+)-limonene was converted into the corresponding epoxide with 93% selectivity and 43% de (diastereomeric excess), whereas (−)-α-pinene was converted with 93% selectivity and 98% de. The catalyst could be recycled without loss of efficiency.

Manganese(III) salen complexes anchored onto activated carbon as heterogeneous catalysts for the epoxidation of olefins

Manganese(III) salen complexes bearing hydroxyl groups on the aldehyde moiety were grafted onto the surface of an air oxidised activated carbon. The resulting carbon materials were characterised by elemental analysis, X-ray photoelectron spectroscopy and N 2 adsorption at 77 K. The catalytic activity of the novel manganese(III) salen based materials was tested in the epoxidation of styrene in acetonitrile, using iodosylbenzene as oxygen source, and compared with the corresponding homogeneous phase data. The heterogenised catalysts are chemoselective towards the styrene epoxide as their homogeneous counterparts, are resistant to leaching and can be re-used without loss of their catalytic activity.

Modulation of the catalytic activity of manganese(iii) salen complexes in the epoxidation of styrene: influence of the oxygen source

Several achiral Mn(III) salen complexes with different groups in the diimine bridge and in the aldehyde fragment were synthesised and their catalytic activity in the epoxidation of styrene was studied at room temperature, using two oxygen sources, NaOCl or PhIO, and in two solvents, CH3CN and CH2Cl2. These manganese(III) salen complexes present high chemoselectivities as homogeneous catalysts in the epoxidation of styrene, using either iodosylbenzene or sodium hypochlorite as oxygen sources. In general, when iodosylbenzene is used as oxidant higher styrene epoxide yields and lower yields of by-products, other than benzaldehyde, are obtained than with aqueous sodium hypochlorite solutions. It was possible to tune the catalytic activities of “[Mn(salen)X]” complexes by introduction of substituents in the diimine bridge and in the aldehyde fragment. The presence of bulky substituents in the diimine bridge always increases the catalytic activity of these complexes, regardless of the oxidant, an indication of steric tuning. However, the electronic tuning of the catalytic activity by introducing substituents in the 5 and 3 positions of the aldehyde fragment has different effects depending on the oxygen source. For the one-phase system resulting from the use of PhIO, electron withdrawing groups increase (electron donating groups decrease) the catalytic activity of the complexes, which probably results from destabilisation (stabilisation) of [OMn(V)(salen)X], the identified active species making them more (less) reactive. However, when NaOCl is used, the observed behaviour is the opposite: electron donating groups make the complexes better catalysts. The apparent similarity between the solubility of the complexes in the organic solvent and their catalytic activity seems to suggest that solubility must play a key role in their activity.