Salen Moiety Research Articles

A Platinum-Aluminum Bimetallic Salen Complex for Pro-senescence Cancer Therapy.

Cell senescence is defined as irreversible cell cycle arrest, which can be triggered by telomere shortening or by various types of genotoxic stress. Induction of senescence is emerging as a new strategy for the treatment of cancer, especially when sequentially combined with a second senolytic drug capable of killing the resulting senescent cells, however severely suffering from the undesired off-target side effects from the senolytic drugs. Here, we prepare a bimetalic platinum-aluminum salen complex (Alumiplatin) for cancer therapy-a combination of pro-senesence chemotherapy with in situ senotherapy to avoid the side effects. The aluminum salen moiety, as a G-quadruplex stabilizer, enhances the salen's ability to induce cancer cell senescence and this phenotype is in turn sensitive to the cytotoxic activity of the monofunctional platinum moiety. It exhibits an excellent capability for inducing senescence, a potent cytotoxic activity against cancer cells both in vitro and in vivo, and an improved safety profile compared to cisplatin. Therefore, Alumiplatin may be a good candidate to be further developed into safe and effective anticancer agents. This novel combination of cell senescence inducers with genotoxic drugs revolutionizes the therapy options of designing multi-targeting anticancer agents to improve the efficacy of anticancer therapies.

Salen-Based Conjugated Microporous Polymers for Efficient Oxygen Evolution Reaction.

Exploring high-performance electrocatalysts, especially non-noble metal electrocatalysts, for the oxygen evolution reaction (OER) is critical to energy storage and conversion. Herein, we report for the first time that conjugated microporous polymers (CMPs) incorporating salen can be used as OER electrocatalysts with outstanding performances. The best OER electrocatalyst (salen-CMP-Fe-3) exhibits a low Tafel slope of 63 mV dec-1 and an overpotential of 238 mV at 10 mA cm-2 . DFT and Grand Canonical Monte Carlo calculations confirmed that the significantly improved electrocatalytic properties can be attributed to the intrinsic catalytic activity of the salen moiety and the enrichment effect of the pore structures. This work demonstrates that salen-based conjugated polymers are a type of metal-coordinated porous polymer that show excellent catalyst performance.

Enantiomeric Recognition of α-Aminoacids by a Uranyl Salen-Bis-Porphyrin Complex.

A novel uranyl salen-bis-porphyrin complex, in which two porphyrin subunits and salen moiety were directly linked, was synthesized for the recognition of tetrabutylammonium (TBA) amino acids. This uranyl salen complex, due to the presence of porphyrins with their fluorescence properties, represents the first example of a luminescence of uranyl salen complexes. UV/Vis measurements indicate the formation of 1:1 host–guest complexes, whereas UV-vis and fluorescence studies revealed that this complex acts as a receptor for the enantiomeric recognition of α-aminoacids derivatives, with high association constants and an excellent enantiomeric discrimination between the two enantiomers of phenylalanine–TBA.

Polynuclear Enantiopure Salen–Mesoionic Carbene Hybrid Complexes

Salen ligands and mesoionic carbene (MIC) ligands constitute important ligand classes in homogeneous catalysis. In this article we describe the first integration of a salen moiety and MIC donors in the same catalyst entity. Homo- and heterometallic polynuclear metal complexes could be selectively prepared by first installing a metal center (Ni(II), Pd(II)) in the N2O2 salen sphere followed by metalation of the mesoionic carbenes (Ag(I), Pd(II)). The silver complexes could be used for transmetalations to form Au(III) complexes. Several complexes showed catalytic activity in the 1,4-addition of an oxindole to a nitroolefin.

Multinuclear cobalt-salen complexes with phenylene linker for epoxide polymerizations

Di- and trinuclear cobalt (Co)–salen complexes with a benzene ring as a rigid linker were explored for epoxide polymerizations. The dinuclear Co–salen complex with a 1,2-phenylene linker showed higher catalytic activity than the dinuclear Co–salen complex with a 1,3-phenylene linker and the trinuclear Co–salen complex with a 1,3,5-benzenetriyl linker for the copolymerization of propylene oxide (PO) with carbon dioxide. A combination of the absolute configuration of the two Co–salen moieties was found to affect its catalytic activity. The optimized dinuclear Co–salen complex with a heterochiral combination demonstrated highest activity and maintained its catalytic activity under a low catalyst concentration. The heterochiral dinuclear Co–salen complex also showed high activity for the copolymerization of PO with cyclic anhydride. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2150–2159

Intriguing Indium-salen Complexes as Multicolor Luminophores.

The series of novel salen-based indium complexes (3-tBu-5-R-salen)In-Me (3-tBu-5-R-salen = N,N'-bis(2-oxy-3-tert-butyl-5-R-salicylidene)-1,2-diaminoethane, R = H (1), tBu (2), Br (3), Ph (4), OMe (5), NMe2 (6)) and [(3-tBu-5-NMe3-salen)In-Me](OTf)2 (7; OTf = CF3SO3-) have been synthesized and fully characterized by NMR spectroscopy and elemental analysis. All indium complexes 1-7 are highly stable in air and even aqueous solutions. The solid-state structures for 3-5, which were confirmed by single-crystal X-ray analysis, exhibit square-pyramidal geometries around the indium center. Both the UV/vis absorption and PL spectra of 1-7 exhibit significant intramolecular charge transfer (ICT) transitions based on the salen moieties with systematically bathochromic shifts, which depend on the introduction of various kinds of substituents. Consequently, the emission spectra of these complexes cover almost the entire visible region (λem = 455-622 nm).

Novel Oligopyrrole Carboxamide based Nickel(II) and Palladium(II) Salens, Their Targeting of Human G-Quadruplex DNA, and Selective Cancer Cell Toxicity.

DNA targeting by various metal complexes is a key strategy toward the restriction of cancer cell proliferation. Toward this end, we designed and synthesized novel salen-based Ni(II) and Pd(II) metal complexes with positively charged flanking side chains comprising N-methylpyrrole carboxamides of varying lengths. The compounds showed high specificity toward G-quadruplex DNA over duplex DNA. Sufficient inhibition of the telomerase activity was observed, which was ascertained by the prominent restriction of cancer cell proliferation in the long-term cell viability and telomerase inhibition assays. The compounds exhibited selective cancer cell death following an apoptotic pathway. Analysis of the binding mode showed partial stacking of the salen moiety over the G-tetrads and association of the pendant oligopyrrole carboxamide units with the grooves. The conjugation of the tetrad-binding metal salen core with groove-oriented flexible oligopyrrole moieties resulted in the high selectivity and stabilization of the human G-quadruplex DNA structures.

Ionic Polymer Microspheres Bearing a Co(III) -Salen Moiety as a Bifunctional Heterogeneous Catalyst for the Efficient Cycloaddition of CO2 and Epoxides.

We report a unique strategy to obtain the bifunctional heterogeneous catalyst TBB-Bpy@Salen-Co (TBB=1,2,4,5-tetrakis(bromomethyl)benzene, Bpy=4,4'-bipyridine, Salen-Co=N,N'-bis({4-dimethylamino}salicylidene)ethylenediamino cobalt(III) acetate) by combining a cross-linked ionic polymer with a Co(III) -salen Schiff base. The catalyst showed extra high activity for CO2 fixation under mild, solvent-free reaction conditions with no requirement for a co-catalyst. The synthesized catalyst possessed distinctive spherical structural features, abundant halogen Br(-) anions with good leaving group ability, and accessible Lewis acidic Co metal centers. These unique features, together with the synergistic role of the Co and Br(-) functional sites, allowed TBB-Bpy@Salen-Co to exhibit enhanced catalytic conversion of CO2 into cyclic carbonates relative to the corresponding monofunctional analogues. This catalyst can be easily recovered and recycled five times without significant leaching of Co or loss of activity. Moreover, based on our experimental results and previous work, a synergistic cycloaddition reaction mechanism was proposed.

Electronic Structure Description of a Doubly Oxidized Bimetallic Cobalt Complex with Proradical Ligands.

The geometric and electronic structure of a doubly oxidized bimetallic Co complex containing two redox-active salen moieties connected via a 1,2-phenylene linker was investigated and compared to an oxidized monomeric analogue. Both complexes, namely, CoL(1) and Co2L(2), are oxidized to the mono- and dications, respectively, with AgSbF6 and characterized by X-ray crystallography for the monomer and by vis-NIR (NIR = near-infrared) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) magnetometry, and density functional theory (DFT) calculations for both the monomer and dimer. Both complexes exhibit a water molecule coordinated in the apical position upon oxidation. [CoL(1)-H2O](+) displays a broad NIR band at 8500 cm(-1) (8400 M(-1) cm(-1)), which is consistent with recent reports on oxidized Co salen complexes (Kochem, A. et al., Inorg. Chem., 2012, 51, 10557-10571 and Kurahashi, T. et al., Inorg. Chem., 2013, 52, 3908-3919). DFT calculations predict a triplet ground state with significant ligand and metal contributions to the singularly occupied molecular orbitals. The majority (∼75%) of the total spin density is localized on the metal, highlighting both high-spin Co(III) and Co(II)L(•) character in the electronic ground state. Further oxidation of CoL(1) to the dication affords a low-spin Co(III) phenoxyl radical species. The NIR features for [Co2L(2)-2H2O](2+) at 8600 cm(-1) (17 800 M(-1) cm(-1)) are doubly intense in comparison to [CoL(1)-H2O](+) owing to the description of [Co2L(2)-2H2O](2+) as two non-interacting oxidized Co salen complexes bound via the central phenylene linker. Interestingly, TD-DFT calculations predict two electronic transitions that are 353 cm(-1) apart. The NIR spectrum of the analogous Ni complex, [Ni2L(2)](2+), exhibits two intense transitions (4890 cm(-1)/26 500 M(-1) cm(-1) and 4200 cm(-1)/21 200 M(-1) cm(-1)) due to exciton coupling in the excited state. Only one broad band is observed in the NIR spectrum for [Co2L(2)-2H2O](2+) as a result of the contracted donor and acceptor orbitals and overall CT character.

A DFT study on the recognition of α-amino acid derivatives by chiral uranyl–salen

Chirality is a fundamental property of matter that plays a main role in several branches of science, from optics to medicine. In particular, chiral molecules containing carboxylate anions (e.g., amino acids) have captured the interest of many researchers due to their large presence in enzymes as substrates, antibodies and cofactors. Here we report on the DFT study of two uranyl–salen receptors, which differ for the presence of two tert-butyl groups on the salen moiety, that have shown excellent enantioselective discrimination toward selected α-amino acid derivatives. Moreover, to clarify the high enantioselectivity values achieved by these uranyl–salen receptors, we combined titration and DOSY NMR data with DFT calculations on these complexes. Our results suggest that the counterion of the amino acid plays a crucial role in the enantioselective recognition: we found that a host–guest system with a high density of atoms, due to the close proximity of the sterically encumbered n-Bu4N+ cation to the uranyl-aminoacid complex, is essential for an efficient enantiodiscrimination.

Spectral, Computational, Electrochemical and Antibacterial Studies of Iron(III)–Salen Complexes

Herein, we report the solvothermal method for the synthesis of iron(III)–salen complexes (salen = N,N′-bis(salicylidene)ethylenediaminato) from salicylaldehyde, 2,2′-bis ethylenediamine and ferric chloride in a single step. The solvothermal method involves the synthesis of complexes with good quality having excellent control on the composition. An insight into the spectral, computational, electrochemical and antibacterial studies highlights the structural, redox and clinical aspects of the complexes. The calculated energy of spin multiplicity sextet (S = 6) obtained from DFT lies below doublet (S = 2) and quartet (S = 4) surfaces by 13.7 and 5.8 kcal mol−1, respectively, indicating that sextet is more stable compared to the other two multiplicities for eight iron(III)–salen complexes. The excitation energies and intensities of the electronic transitions calculated by the TDDFT method are in good agreement with the experimental values. Electron-donating substituents in the salen moiety produce a cathodic shift and electron-withdrawing groups the anodic shift in Fe(III)/Fe(II) redox potential values. The results of preliminary antibacterial study reveal that the iron(III)–salen complex was effective against clinically important Gram-negative bacterium (Escherichia coli) (7.2 μg ml−1) than the Gram-positive bacterium (Staphylococcus aureus) (158.3 μg ml−1).

Detection of copper(ii) and aluminium(iii) by a new bis-benzimidazole Schiff base in aqueous media via distinct routes

Synthesis and characterization of a bis-benzimidazole appended Schiff base ligand, 2-(3,5-bis(1H-benzimidazole-2-yl)-phenyliminomethyl)phenol (H3L) displaying excellent selectivity towards Cu2+and Al3+in mixed aqueous media.

A symmetric pseudo salen based turn-on fluorescent probe for sensitive detection and visual analysis of zinc ion

A turn-on fluorescence probe for zinc ion has been synthesized, which contains a symmetric pseudo salen moiety as zinc recognition site. The multidentate salen ligand chelates with zinc ions, and generates the rigidity of the zinc complex, which subsequently results in a dramatic fluorescence enhancement of the probe. Mass spectral and spectroscopic results reveal that the reaction proceeds rapidly in a 1 to 1 stoichiometric manner to form a clean zinc complex. We have demonstrated that the fluorescence probe not only has high selectivity for zinc ion over other biological relevant ions but also shows high sensitivity for zinc ion with a detection limit of 83nM. A fluorescence test paper has been developed by incorporating the probe on a polyamide fiber paper. The fluorescence color of the test paper became bright green–yellow under UV lamp when water sample containing zinc ions was dropped on it, suggesting its potential application in the measurement of zinc ions.

Thermal studies, structural characterization, and antimicrobial evaluation of coordinated metal complexes containing salen moiety

Using the bidentate ligand N,N′-bis(2-methoxybenzylidene)ethylenediamine salen Schiff base, Cu(II), Cr(III), and La(III) complexes were prepared and structurally characterized using several physicochemical techniques. The thermal behavior of the synthesized complexes was investigated using simultaneous thermogravimetry (TG) and differential thermal analysis (DTA) techniques. The geometrical configurations of the complexes are proposed to be octahedral. Antimicrobial and antifungal activities of the reported complexes were tested and evaluated in vitro using modified Kirby–Bauer disc diffusion and minimum inhibitory concentration methods. The prepared complexes show remarkable antimicrobial activity; in particular, the Cr(III) complex exhibits potential inhibition against Escherichia coli, Staphylococcus aureus, and Candida albicans. On the other hand, the lanthanum complex presents higher in vitro antimicrobial activity when compared with the copper complex. In vitro antimicrobial activity assays indicate that the novel complexes might serve as promising candidates for new therapeutic agents.

Homo- and Heterobimetallic Pd–, Ag–, and Ni–Hybrid Salen–Bis-NHC Complexes

salen ligands and N-heterocyclic carbenes (NHC) are among the most abundant structural ligand motifs for metal catalysis. In this article we present a modular approach to chiral ligands merging both ligand motifs for the preparation of bimetallic catalysts, in which one metal (M1) is coordinated by the salen moiety and the other metal (M2) binds to two NHCs. After selective complexation of M1 = PdII, NiII ion into the salen N2O2 coordination site, heterobimetallic M1/Ag(I) complexes were synthesized which could be further utilized for the preparation of homo- and heterobimetallic M1/PdII complexes by an oxidative transmetalation to Pd(0) as a key step of the catalyst synthesis. The structures of the bimetallic complexes and the intermetallic distances strongly depend on the counterions of M2 = PdII, as revealed by X-ray and UV–vis studies. In the absence of an anionic ligand with suitable Lewis basicity the salen oxygen atoms serve as bridging ligands for both metals. A preliminary investigation into catalysis showed that the complexes are capable of catalyzing the 1,4-addition of oxindoles to 2-nitrostyrene.

Pseudo-crown functionalized copper salen complexes forming electroactive polymers: Rationalization of Ba2+ interaction using XAS and DFT

A series of electroactive films based on the Cu(salen) motif was produced on Pt electrodes by potentiodynamic polymerization of [Cu(3-MeOsalen)] (1), [Cu(3-MeOsaltMe)] (2), [Cu(3-MeOsalpd)] (3) and [Cu(3-MeOsalophen)] (4). Coulometric assay showed coverages to be in the range Γ∼90–530nmolcm−2. During polymerization and subsequent redox cycling, the films had distinct i-E signatures, despite the fact that the imine bridge is notionally outside the site of electroactivity (the conjugated polymer spine). A combination of DFT and XAS was used to explore underlying structural reasons for the differing electrochemical signatures and the facility of the films to complex solution phase Ba2+ (a model for metal ion sensing). DFT was able to provide finer details, notably between the N- and O-donors in the Cu local environment, but its application was restricted to monomeric entities. The most significant structural effect of the nature of the imine bridge involvement was the extent of distortion from planarity of the salen moiety. This in turn alters the size of the pseudo-crown pocket formed by the two O-donors shared by the Cu and the two O-donors of the methoxy groups, such that the Ba2+ ion can only lie within the molecular plane in the cases of 1 and 2. XANES shows the Cu to be invariably present in an essentially square planar environment, irrespective of imine bridge, monomer or polymer environment, or Ba2+ complexation. The relevance of these outcomes to design criteria for materials of relevance to metal ion sensing is discussed.

Structural, photophysical, and mesomorphic properties of luminescent platinum(II)-salen Schiff base complexes

A series of square planar platinum(II) salen complexes containing 4-substituted alkoxy chains of aromatic rings, [Pt((4-CnH2n+1O)2salen)] (n=3 (1), 4 (2), 6 (3), 8 (4), 10 (5), 12 (6), 14 (7), 16 (8), and 18 (9)) has been prepared, and photophysical and mesomorphic properties have been investigated. Complexes 1–9 emitted intense phosphorescence from the mixed exited-state between the MLCT (MLCT=metal-to-ligand charge transfer) and LLCT (LLCT=ligand-to-ligand charge transfer) states both in dichloromethane solution and in solid states at room temperature. The red-shifted solid-state emission spectra from monomers in solution are caused by the supramolecular contact with one-dimensional stacking between neighboring chromophores (salen moieties) through the weak C–H⋯O type hydrogen bonding and van der Waals interactions in the solid state revealed by an X-ray crystallographic analysis for 2. Though 1–4 did not exhibit any mesophases (liquid–crystalline phases), 5–9 showed a lamello-columnar (ColL) mesophase with the high thermal stability, leading to the different phosphorescence spectra from both in solution and in solid states. This is affiliated with the appearance of the intermolecular dz2Pt(II)-dz2Pt(II) orbital interaction in a one-dimensional stacking distance with ca. 3.0Å due to self-assemblies in the liquid crystal which gives rise to the MMLCT (MMLCT=metal–metal-to-ligand charge transfer) transition. This is supported by absorption and emission measurements in the wide range from room temperature in the solid state to the clearing points (around 280°C). The relationship between molecular assemblies and photophysical properties is discussed.

Synthesis of new derivatives of cyclotriphosphazene substituted with the salen side groups

The reaction between hexachlorocyclotriphosphazenes (N3P3Cl6) and three equivalents of bis-salicylidene ethylene diamine (HOC6Hsu4-CH=N-CH2-)2 (salen) was investigated in two different experimental conditions. In the presence of anhydrous Na2CO3 in stirring tetrahydrofuran (THF) medium, the reaction led to a 50:50 mixture of tris[bis(salicylidene)ethylenediamine] cyclotriphosphazene, spiro-N3P3[(OC6H4-CH=N-CH2-)2]3 (1) and ansa (2) fully substituted products, whereas, in toluene solution, N3P3Cl6 with salen (3) and Al2O3/KOH afforded the spiro (1), as the only isolable compound. The spectroscopic data revealed that the oxygen atoms of salen moieties were selectively bonded to the three phosphorus atoms of the phosphazene ring. The new derivatives of cyclotriphosphazenes were characterized by IR spectrophotometry 1H NMR, 31P NMR spectral data and elemental analysis.

Chiral solvating agents for carboxylic acids based on the salen moiety

A series of chiral salen C 2- symmetric aminophenols 1–4, which were derived from (1 R,2 R)- or (1 S,2 S)-1,2-diphenylethane-1,2-diamine with the corresponding salicylic aldehyde through a [2+2] condensation reaction, have been prepared. The NMR studies demonstrated that they can function as chiral solvating agents for the determination of the enantiomeric purity of chiral carboxylic acids.

Fe(III) complex anchored SBA-15 is a new heterogeneous catalyst for the cleavage of aliphatic C [dbnd]C bond of styrene and its derivatives

Heterogenization of homogeneous Fe-complex on a mesoporous silica SBA-15 has been designed to gain the benefits of both homogeneous and heterogeneous catalytic processes emphasizing the influence of two different coordinating ligands in catalytic applications. With this aim, Fe(III) Schiff-base complex containing both (i) acetylacetonate and (ii) H 2salen ligands has been covalently anchored onto a mesoporous SBA-15 for the first time in three successive steps, (i) surface functionalization of SBA-15 with (3-aminopropyl)triethoxysilane, (ii) anchoring of Fe(III) acetylacetonate through Schiff-base condensation between the propylamine groups that functionalized with SBA-15 and carbonyl carbons of Fe(III) acetylacetonate, followed by (iii) tethering of H 2salen ligand to F(III) acetylacetonate, leading to desired heterogeneous catalyst, SBA-Fe(acac:salen). The N 2 adsorption, and low-angle XRD characterization techniques revealed the presence of structural and textural characteristics of parent SBA-15 in SBA-Fe(acac:salen) heterogeneous catalyst. The anchored acetylacetonate and salen moieties have been resolved by CHNS elemental analysis, TGA and FT-IR. The X-ray photoelectron spectroscopic analysis unveiled the existence of Fe 3+ species in SBA-Fe(acac:salen) heterogeneous catalyst. The SBA-Fe(acac:salen) catalyst exhibited a overwhelming performance in activating the aliphatic C C bond of styrene and its derivatives towards the transformation of corresponding aldehydes selectively through the oxidative cleavage in the presence of aqueous 30% H 2O 2 at room temperature. The high performance of this catalyst is due to isolated Fe 3+ species on the surfaces of hexagonally ordered mesoporous silica SBA-15 with optimal coordination environments with two different coordinating ligands.