Metallosalen Complexes Research Articles

Flexible metallosalen complex for pyrophosphate sensing in biological milieu: Use disassembly strategy for signal amplification

In the present work, we have designed and synthesized two dimeric fluorescein derivatives with flexible salen (1) or a rigid salophen (2) linker unit. The Fe(III) complexes of these compounds ([1.Fe]+ and [2.Fe]+) showed changes in solution color specifically upon addition of inorganic pyrophosphate (PPi) at pH 7.0 in PBS buffer. Along this, we also observe appearance of green fluorescence (turn-on) in the prescence of PPi. The mechanistic studies suggested PPi-induced demetallation process, which led to disassembly for molecular structure in the aqueous medium and release of highly fluorescent fluorescein aldehyde. The disassembly process was found to be dependent on electronic structure, rigidity of the linker unit, temperature, and local pH etc. The selectivity as well as sensitivity of [1.Fe]+ towards PPi was also found to be superior than that observed with [2.Fe]+. Since each disassembly process released two signalling unit (fluorescein), we could observe a huge increment in FL intensity even with small quantity of PPi (<10 µM). Finally, the sensory system was utilized for detection of PPi in diluted blood serum samples and inside living mammalian cells (HeLa, cervical cancer cells). Chemically-modified paper strips have been developed for the detection of PPi in water samples at outside laboratory conditions.

Transformation of carbon dioxide into valuable chemicals over bifunctional metallosalen catalysts bearing quaternary phosphonium salts

The chemical transformation of CO2 under mild conditions remains a great challenge because of its exceptional kinetic and thermodynamic stability. Two important reactions in the transformation of CO2 are the N-formylation reaction of amines using hydrosilanes and CO2, and the cycloaddition of CO2 to epoxides. Here, we report the high efficiency of bifunctional metallosalen complexes bearing quaternary phosphonium salts in catalyzing both of these reactions under solvent-free, mild conditions without the need for co-catalysts. The catalysts' bifunctionality is attributed to an intramolecular cooperative process between the metal center and the halogen anion. Depending on the reaction, this activates CO2 by permitting either the synergistic activation of Si–H bond via metal–hydrogen coordinative bond (M–H) or the dual activation of epoxide via metal–oxygen coordinative bond (M–O). The one-component catalysts are also shown to be easily recovered and reused five times without significant loss of activity or selectivity. The current results are combined with previous work in the area to propose the relevant reaction mechanisms.

Oxidation of cycloalkanes with molecular oxygen in the presence of salen metallocomplexes in thermomorphic conditions

The oxidation of cycloalkanes with molecular oxygen, catalyzed by two groups of metallosalen complexes, was studied. The first group consisted of salen complexes with different metals such as Mn, Fe, Co, Ni, Cu, Zn, while the second group was composed of manganese salen complexes with different substituents ( t -butyl electron-donating substituents and/or electron-withdrawing perfluoroalkyl substituents). Mn, Fe and Co salen complexes are the most active catalysts, while Ni, Cu and Zn salen complexes are far less efficient. The introduction of t -butyl electron-donating substituents into Mnsalen complex increases the catalytic activity and catalysts solubility in the reaction mixture. The introduction of perfluoroalkyl electron-withdrawing substituents enhances the catalytic activity and renders the solubility of the catalyst temperature dependent (thermomorphic behaviour), thus allowing one to recover them easily after the reaction by simply cooling the system to room temperature. The synthesis of two new manganese salen complexes with perfluoroalkyl substituents was elaborated. • New strategy for recovering homogeneous catalysts–thermomorphic effect • Fluorous catalysis without fluorous solvent • Synthesis and characterization of two new perfluoroalkyl manganese complexes • Influence of substituents on physico-chemical properties of salen complexes • Influence of substituents in salen complexes on their catalytic activity

Molecular Assembly Directed by Metal–Aromatic Interactions: Control of the Aggregation and Photophysical Properties of Zn–Salen Complexes by Aromatic Mercuration

There is widespread interest in non-covalent bonding and weak interactions, such as electrostatic interactions, hydrogen bonding, solvophobic/hydrophobic interactions, metal-metal interactions, and π-π stacking, to tune the molecular assembly of planar π-conjugated organic and inorganic molecules. Inspired by the roles of metal-aromatic interaction in biological systems, such as in ion channels and metalloproteins, herein, we report the first example of the use of Hg(2+) -aromatic interactions to selectively control the assembly and disassembly of zinc-salen complexes in aqueous media; moreover, this process exhibited significant "turn on" fluorescent properties. UV/Vis and fluorescence spectroscopic analysis of the titration of Hg(2+) ions versus complex ZnL(1) revealed that the higher binding affinity of Hg(2+) ions (compared to 13 other metal ions) was ascribed to specific interactions between the Hg(2+) ions and the phenyl rings of ZnL(1); this result was also confirmed by (1)H NMR spectroscopy and HRMS (ESI). Further evidence for this type of interaction was obtained from the reaction of small-molecule analogue L(1) with Hg(2+) ions, which demonstrates the proximity of the N-alkyl group to the aromatic protons during Hg(2+)-ion binding, which led to the consequential H/D exchange reaction with D(2) O. DFT modeling of such interactions between the Hg(2+) ions and the phenyl rings afforded calculated distances between the C and Hg atoms (2.29 Å) that were indicative of C-Hg bond-formation, under the direction of the N atom of the morpholine ring. The unusual coordination of Hg(2+) ions to the phenyl ring of the metallosalen complexes not only strengthened the binding ability but also increased the steric effect to promote the disassembly of ZnL(1) in aqueous media.

Synthesis of a Novel Rigid Artificial Superoxide Dismutase Based on Modified Hollow Mesoporous Silica Microspheres

Hollow silica microspheres (HSMs) that mimics superoxide dismutase (SOD) activity has been developed by using the HSMs modified with the novel metallosalen complex through the esterification between the carboxy and silanol. A novel metallosalen complex, named Fe-(bis(salicylaldehyde)-3,4-diaminobenzoic acid), with superoxide scavenging activity was prepared. We found that the novel complex has a higher superoxide scavenging activity than traditional complex, named Fe-(bis(salicylaldehyde)ethylenediamine), without the carboxy, the catalytic process was studied and explained. The property and SOD activity of the above metallosalen-modified HSMs were investigated. These novel rigid metallosalen-modified HSMs possess excellent SOD activity and it can be easily immobilized to fabricate biomimetic sensor or other catalytic unit/receptor, it solves many practical problems such as easy recovery and repeat utilization. In addition, the metallosalen can be replaced by other active centers to mimetic novel kind of enzymes.

Fe(III)-Salen and Salphen Complexes Induce Caspase Activation and Apoptosis in Human Cells

To explore the apoptotic and antitumor activities of metallo-salens, the authors have synthesized several Fe(III)-salen and salphen complexes and analyzed their effects on human cancer and noncancer cells. Their results demonstrated that Fe(III)-salen and salphen complexes affect cell viability and induce nuclear fragmentation and apoptosis in breast cancer (MCF7) cells. The IC(50) values for the active metallo-salen complexes ranged between 0.3 and 22 µM in MCF7 cells. Biochemically active Fe(III)-salen and salphen complexes induced caspase-3/7 activation and release of cytochrome c from the mitochondria to cytosol, suggesting the involvement of the mitochondrial pathway of apoptosis. Comparison of IC(50) values toward 3 different cell lines demonstrated that selected Fe(III)-salen complexes induce tumor cell-selective apoptosis in cultured cells. Overall, the studies demonstrated that Fe(III)-salen and salphen complexes induced efficient apoptosis in cultured human cells. The nature of the substituents and the bridging spacer between diamino groups play critical roles in determining the apoptotic activities of Fe(III)-salen and salphen complexes.

Access to multinuclear salen complexes using olefin metathesis

The use of olefin metathesis as a construction tool for multimetallic salen-based structures is described. The approach involves mono- and diallyl-functionalized metallosalen complexes that can be directly coupled by metathesis leading to dimetallic species or mixtures of linear and cyclic oligomers. The metathesis of bis-allyl Ni(salen) complexes has been studied in detail. At high concentration it is possible to selectively obtain di-Ni species rather than heavier oligomers while under dilute conditions cyclic rather than linear oligomers are preferentially obtained. A mono-allyl Zn(salphen) complex was efficiently coupled using metathesis to give the di-Zn(salphen) product, which was subsequently transmetalated with a variety of metals to yield dimetallic salens of potential catalytic interest. Finally, a tetranuclear Zn(4) macrocycle was also prepared using buildings blocks obtained by metathesis from commercially available precursors. The methods described herein allow for the facile construction of multi-centered Schiff base complexes of catalytic or supramolecular interest.

ChemInform Abstract: Cooperative Multimetallic Catalysis Using Metallosalens

AbstractReview: 120 refs.

Facile and scalable synthesis of a novel rigid artificial superoxide dismutase based on modified hollow mesoporous silica microspheres

A novel metallosalen complex, named Mn-(bis(salicylaldehyde)-3,4-Diaminobenzoic acid), with superoxide scavenging activity was prepared from manganese(III) acetate dehydrate and bis(salicylaldehyde)-3,4-Diaminobenzoic acid in ethanol. We found that the novel complex has a higher superoxide scavenging activity than traditional complex, named n-(bis(salicylaldehyde)ethylenediamine), without the carboxy, the catalytic process was studied and explained. Hollow silica microspheres (HSMs) that mimics superoxide dismutase activity has been developed by using the HSMs modified with the novel metallosalen complex through the esterification between the carboxy and silanol. The property and superoxide dismutase activity of the metallosalen-modified HSMs were investigated. These novel rigid metallosalen-modified HSMs possess excellent superoxide dismutase activity and it can be easily immobilized to fabricate biomimetic sensor or other catalytic unit/receptor, it solves many practical problems such as easy recovery and repeat utilization. An application test of this novel rigid artificial SOD was performed by fabricating a novel biomimetic sensor to determine superoxide based on mesoporous hollow silica microspheres receptor and rhodanine chemiluminescence detector. In addition, the metallosalen can be replaced by other active centers to mimic novel kind of enzymes and immobilized in/on other devices to establish novel biomimetic sensors.

Cooperative multimetallic catalysis using metallosalens

Metallosalen complexes are able to catalyze several transformations via a bimetallic, cooperative pathway. Various strategies to induce simultaneous activation by two metal centers with multinuclear catalysts have been reported to date, leading to higher reaction rates and selectivities in existing reactions. In some cases, new reactions could be realized that were previously not possible. The design principles, the most successful approaches and the catalytic applications of multinuclear metallosalen catalysts are discussed in this feature article.

Homochiral helical coordination polymers of metallosalen complexes with tunable pitches

Homochiral helical coordination polymers [ML] have been developed via the solvothermal reaction of the chiral di-3-pyridyl-functionalized salen complex (R,R)-(−)-N,N′-bis(3-tert-butyl-5-(3-pyridyl)salicylidene)-1,2-diaminocyclohexane (H2L) with various metal salts MX2 (MX2 = Zn(NO3)2, 1; CoCl2, 2; Ni(NO3)2, 3 and Mn(OAc)2, 4) and characterized by a variety of techniques including microanalysis, IR, thermogravimetric analysis (TGA), circular dichroism (CD) spectroscopy and powder and single-crystal X-ray diffraction. Salen L ligands link adjacent metal centers to form 1D helical chains using tetradentate N2O2 donors and pyridine groups, the pitch lengths of the helices can be tuned by metal cations.

CD spectroscopy as a fingerprint technique for the structural characterization and determination of absolute configurations of unsymmetric chiral Schiff base Ni(II) complexes

Unsymmetric Schiff base complexes have attracted more attention in recent years due to their diverse effects in catalytic reactions. Due to their high dissymmetry, unsymmetric metallosalen complexes are harder to prepare than symmetric ones. This means that X-ray crystallographic structural determination is sometimes unavailable, so their absolute configurations are determined by circular dichroism (CD) spectroscopy instead. Herein, some quadridentate unsymmetric metallosalen nickel(II) complexes were synthesized and their structures were characterized by CD spectra. An empirical rule for assignment of the absolute configurations of tetra-coordinated pseudo-planar Ni(II) complexes was put forward. Furthermore, a fingerprint was found to judge whether the metallosalen complexes are symmetric or unsymmetric.

A Modular Approach Towards Nonsymmetrical Bis(metallosalen) Building Blocks

AbstractNew bis(metallosalen) building blocks are described that can be accessed through di‐ or tetraimine precursors derived from the 3,3′‐diaminobenzidine backbone. A selective procedure for the introduction of two imine groups in 3,3′‐diaminobenzidine was developed to furnish starting materials for nonsymmetrical metallosalen complexes. The procedure proved to be widely applicable and several diimine derivatives were isolated and fully characterized. The diimine compounds were used to prepare the first nonsymmetrical bis(salen) complexes (M = Zn, Ni) with different peripheral groups upon treatment with various salicylaldehydes in a one‐pot procedure. The identity of the bis(metallosalen)s was verified by a range of analytical techniques, and for two of these derivatives the X‐ray molecular structures are described. The crystallographic data revealed that the biphenylene linker between the two N2O2 donor sets can adopt conformations with different dihedral angles. The supramolecular potential of the bis[Zn(salen)] derivatives was investigated by combining two of these complexes with a ditopic ligand to yield supramolecular assemblies with nanoscale dimensions.(© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2008)

Crossing the Borders Between Homogeneous and Heterogeneous Catalysis: Developing Recoverable and Reusable Catalytic Systems

Homogeneous and heterogeneous catalysis have developed independently as two separate disciplines. Homogeneous catalysis is characterized by the use of highly active, well-characterized compounds. In contrast, heterogeneous catalysis exhibits the advantage of easy separation of the catalyst from the products and can be easily adapted to continuous flow processes. In recent years, there is an emerging trend in catalysis that is bridging these two fields. On one hand, some of the complexes used in homogeneous catalysis are really precursors of nanoparticles that are species that have been traditionally subject of study in heterogeneous catalysis. On the other hand, the use of novel media allows the recovery and reuse of homogeneous catalysts, a hallmark of heterogeneous catalysis. Also, powerful experimental techniques can characterize the active sites in solids up to a much higher level of detail. In this review, we have selected two reaction types that are attracting much current interest, namely the enantioselective addition to aldehydes by chiral metallosalen complexes and the palladium catalyzed C–C cross coupling, and used them to illustrate a series of strategies based on new concepts that can serve to impart the advantages of homogeneous catalysis into heterogeneous catalysis and vice versa.

Self‐Assembly of a Homochiral Nanoscale Metallacycle from a Metallosalen Complex for Enantioselective Separation

Only half may enter: A homochiral porous nanoscale metallacycle has been efficiently self-assembled from semiflexible enantiopure metallosalen complexes with complementary coordination motifs. Single crystals of the macromolecule (see structure; C gray, Zn purple, O red, N blue) show a reversible and controllable dynamic behavior. Particularly, the metallacycle can be used to resolve small racemic alcohols with high enantioselectivity (over 99.0 % ee).

Self‐Assembly of a Homochiral Nanoscale Metallacycle from a Metallosalen Complex for Enantioselective Separation

Jeder zweite wird aufgenommen: Durch die Selbstorganisation flexibler enantiomerenreiner Metall-Salen-Komplexe mit komplementären Koordinationsstellen entsteht ein homochiraler Metallacyclus mit einer Nanopore (C grau, Zn violett, O rot, N blau), der im Einkristall eine reversible und steuerbare Dynamik zeigt. Mithilfe des Metallacyclus gelang die Racematspaltung kleiner Alkohole mit über 99 % ee. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2008/z704347_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Biomimetic Alkene Epoxidation and Alkane Hydroxylation with Sodium Periodate Catalyzed by Mn(III)-salen Supported on Amberlite IRA-200

The Mn(III)-salen, containing phosphonium groups at the 5,5′-positions of the salen ligand supported on Amberlite IRA-200 via electrostatic binding was used for the oxidation of alkenes and alkanes with sodium periodate at room temperature in the presence of imidazoles as axial ligands, and the effect of solvent, different axial ligands, and various oxygen donors was investigated. This heterogenized catalyst shows high catalytic activity in alkene epoxidation and alkane hydroxylation. It showed high selectivity in the epoxidation of stilbenes, α-pinene, and (R)-(+)-limonene, and exhibits a particular ability to epoxidize linear alkenes. The stability and reusability of this new heterogenized metallo-salen complex was also investigated. The catalyst was characterized by FTIR, UV-Vis, SEM, and thermal analysis.

α-Diazoacetates as Carbene Precursors: Metallosalen-Catalyzed Asymmetric Cyclopropanation

Readily available α-diazo compounds have been shown to be efficient carbene precursors for asymmetric cyclopropanation using ruthenium(NO)-salen and cobalt-salen complexes, as catalysts. The stereoselectivity of the cyclopropanation depends on the metal ion, its valency, the structural and electronic nature of the salen ligand, and the apical ligand of the salen complex used. Both trans- and cis-selective asymmetric intermolecular cyclopropanation can be realized under mild conditions by using a suitable metallosalen complex. Furthermore, a wide range of alkenyl α-diazoacetates and the related alkenyl diazomethyl ketones undergo intramolecular cyclopropanation in a highly enantioselective manner by using various metallosalen complexes, easily prepared in a modular fashion, as catalyst.

Selective alkene epoxidation and alkane hydroxylation with sodium periodate catalyzed by cationic Mn(III)-salen supported on Dowex MSC1

Dicationic Mn(III)-salen, containing phosphonium groups at 5,5′-positions of salen ligand, is supported on an ion-exchange resin, Dowex MSC1, via electrostatic interaction. The catalyst, Mn(salen)-Dowex, has been characterized by FTIR, UV–vis spectroscopic techniques, scanning electron micrograph (SEM), thermal and elemental analysis. This heterogenized homogeneous manganese(III)-salen complex can be used as catalyst in the alkene epoxidation with sodium periodate. This catalytic system shows a good activity in the epoxidation of cyclic, aromatic and especially linear alkenes and exhibits a high selectivity in epoxidation of α-pinene and R-(+)-limonene. Alkyl aromatic and cycloalkanes were oxidized efficiently to their corresponding alcohols and ketones in the presence of this catalyst. The stability and reusability of this new heterogenized metallosalen complex was also investigated.

Metallosalen complexes immobilized in zeolite NaX as catalysts of aerobic oxidation of cyclooctane

Physicochemical properties of metallosalen complexes entrapped within NaX zeolite and their catalytic activities in the oxidation of cyclooctane with dioxygen in the absence of a reducing agent were studied, and compared with free metallocomplexes. Broadening of the zeolite structure-sensitive vibrations observed after the encapsulation gives the evidence of the zeolite framework changes due to the presence of the “ship-in-the-bottle” species. EPR spectroscopy evidences the inclusion of metallosalen complexes since it distinguishes between encapsulated metallocomplexes and those adsorbed on the zeolite external surface. UV–vis spectra confirm the immobilization and stabilization of the metallocomplexes inside the zeolite structure. The most active catalysts are free metallosalen complexes. We have found that the activity of these complexes increases in the order Ni(salen), Fe(salen), Mn(salen), Cu(salen) and Co(salen) with triple difference in the catalytic activity between the most and the least active catalyst. Upon encapsulation their activity drops about two times. Only Ni(salen) after heterogenization does not change its catalytic activity. The reduction of catalytic activity of the encapsulated catalysts is probably governed by two factors: constraints in the formation of the intermediate complex and change of the redox potential of the metal in the salen complex.