Salen Complexes Research Articles

The Complex Reactivity of [(salen)Fe]2(μ-O) with HBpin and Its Implications in Catalysis.

We report a detailed study into the method of precatalyst activation during alkyne cyclotrimerization. During these studies we have prepared a homologous series of Fe(III)-μ-oxo(salen) complexes and use a range of techniques including UV-vis, reaction monitoring studies, single crystal X-ray diffraction, NMR spectroscopy, and LIFDI mass spectrometry to provide experimental evidence for the nature of the on-cycle iron catalyst. These data infer the likelihood of ligand reduction, generating an iron(salan)-boryl complex as a key on-cycle intermediate. We use DFT studies to interrogate spin states, connecting this to experimentally identified diamagnetic and paramagnetic species. The extreme conformational flexibility of the salan system appears connected to challenges associated with crystallization of likely on-cycle species.

Powder crystal structure analysis of salen metal complexes with phenylalkyl groups

Powder crystal structure analysis of salen metal complexes with phenylalkyl groups

Cobalt−Salen complexes bearing crown ether as metal-templated catalyst for cycloaddition of carbon dioxide and epoxide

Metallosalen presents very important applications in both homogenous and heterogeneous catalysis. Correspondingly, porous polymeric metallosalen combined with the performance of organometallics and heterogeneous polymers possess potential in CO2 fixation reactions. Herein, a family collection of K@SCs were conceived and obtained by combining cobalt-salen complexes with different groups and diaminobenzene-18-crown-6 complexed with KI, which allows multiphase catalytic immobilization of CO2 to epoxides. Catalyst K@SC-3 can fix CO2 into a variety of substituted epoxides in >90% yields to the corresponding cyclic carbonates in co-catalyst-free and solvent-free conditions. Furthermore, K@SC-3 can be recovered and reused at least six times by simple centrifugal separation, demonstrating superior stability. The kinetics experiments were also carried out at varied temperatures catalyzed by K@SC-3. It is concluded that a first-order equation of epoxide was proved and the activation energy was calculated to be 27.13 KJ/mol.

Chirality and thermal stability of four salen complexes: Crystallographic, theoretical and TGA studies

Four complexes of the general formula [M(L)] where (M = Cu, Ni, Pd or Co) and H2L is 2,2′-[cis-1,2-diaminocyclohexanediylbis(nitrilo-methylidyne)]bis(5-diethyl-amino]phenol were synthesized. The four complexes were characterized using FTIR and high-resolution mass spectroscopy. The crystal structures of [Cu(L)], [Ni(L)] and [Pd(L)] were determined. The asymmetric unit of [Cu(L)] or[Ni(L)] complexe consists of two enantiomer independent molecules which are proved to be two different enantiomers. The two enantiomers arise from the chirality of cis-1,2-disubstitutedcyclohexane. The thermal stability of the four complexes was investigated using thermal gravimetric analysis (TGA). The thermal stability of the ligand in the four complexes and that of free ligand follow the order [Ni(L)] > H2L ∼ [Cu(L)] > [Co(L)] > [Pd(L)]. The thermal stability trend was rationalized using three factors; the ionic characters of the M−N and M−O bonds, the strength M−O and M−N bonds (taking bond order as criterion) and the back donation from the metal to the antibonding orbitals localized on the ligand. NBO and QTAIM analysis indicate that ionic characters of the bonds of Cu-O and Cu-N are higher than the ionic characters in the corresponding bonds in the other three complexes. Also, the back donation from Cu center to the ligand is minimal, this rationalizes the fact the free H2L and the ligand in the [Cu(L)] complex decompose at similar temperatures. For the other three complexes, the M−O and M−N bonds have lower ionic characters and similar bond orders. Thermal stability is rationalized using stabilization energy due to ligand to metal back donation. Plotting of the decomposition temperature against the stabilization energy due to back donation gave a straight line with correlation coefficient R2 = 0.9906.

Chiral Metal Salen Complexes as Chiral Electrocatalysts for Asymmetric Electrochemical Carboxylation of Acetophenone

AbstractAsymmetric electrochemical carboxylation is the direct and effective method to fix and utilize CO2, and the selection of chiral catalysts is crucial. In this paper, a series of chiral metal salen complexes are prepared as chiral catalysts to achieve enantioselective conversion of acetophenone with CO2 to synthesis chiral 2‐hydroxy‐2‐phenylpropionic acid under electrochemical conditions. Under the catalysis of NiIIL1, the enantiomeric excess (ee) value of 90 % and yield of 59 % are obtained. By cyclic voltammetry and potentiostatic electrolysis, the effects of metal centers and tert‐butyl groups in ligand structures on the catalytic results are studied and the possible reaction mechanism is proposed.

Computational Evaluation of the Potential Pharmacological Activity of Salen-Type Ligands in Alzheimer's Disease.

Alzheimer's disease (AD) is the most common form of dementia representing from 60% to 70% of the cases globally. It is a multifactorial disease that, among its many pathological characteristics, has been found to provoke the metal ion dysregulation in the brain, along with an increase in the oxidative stress. There is proof that metallic complexes formed by the amyloid-β peptide (Aβ) and extraneuronal copper can catalyze the production of reactive oxygen species, leading to an increase in oxidative stress, promoting neuronal death. Due to this interaction, bioavailable copper has become an important redox active target to consider within the search protocols of multifunctional agents for AD's treatment. In this study, we examined by using bioinformatics and electronic structure calculations the potential application of 44 salen-type copper chelating ligands and 12 further proposed molecules as possible multifunctional agents in the context of AD. The candidates were evaluated by combining bioinformatic tools and electronic structure calculations, which allowed us to classify the molecules as potential antioxidants, redistributor-like compounds, and the newly proposed suppressor mechanism. This evaluation demonstrate that salen-type ligands exhibit properties suitable for interfering in the chain of copper-induced oxidative stress reactions present in AD and potential redistributor and suppressor activity for copper ions. Finally, a novel set of plausible candidates is proposed and evaluated. According to the evaluated criteria, a subset of 13 salen-type candidates was found to exhibit promissory pharmacological properties in the AD framework and were classified according to three plausible action mechanisms.

Heteronuclear Fe(III)/Mn(III) salen complex as electron transfer mediator for laccase-based biocathodes: Structural investigations, molecular docking, and electrochemical studies

A novel hetero-trinuclear Fe(III)/Mn(III) complex derived from a salen ligand has been investigated using elemental analysis, infrared spectroscopy, and single-crystal X-ray crystallography. The studied complex crystallized as orthorhombic with the Mn(III) ion coordinated in the equatorial plane by the phenolate O and imine N atoms of the ligand. Two axial positions are taken up by an O atom from water and a N atom from the cyanide group. The [Fe(CN)6] core in the crystal structure links the two [Mn(salen)(H2O)] moieties together. In addition, the isolated complexes have been utilized as mediators for the electron transfer between the laccase and the electrode. Moreover, using structural data for the laccase, computational docking simulations were also performed towards the hydrophobic pocket on the surface of the laccase to determine how the complex behaved in the vicinity of a so-called hydrophobic pocket on the surface outside of the Type-1 site. Additionally, the parameters of mediator concentration, temperature, and electrode conditions (CNT, MXene) were used to investigate the electron transfer mechanism.

A Highly Active Nickel Catalyst for the Selective Hydrogenation of Functionalized Nitroarenes

AbstractThe catalytic hydrogenation of nitroarenes using hydrogen gas as reducing agent to form substituted aniline derivates is an important reaction in industry and academic research. Here, we report on the nitroarene hydrogenation capability of a nanostructured nickel catalyst synthesized from a nickel salen complex and a commercially available porous silica support. Our catalyst allows the selective hydrogenation of nitroarenes under mild conditions while tolerating various functional groups and hydrogenation‐sensitive examples such as iodine, nitrile or olefinic groups. The use of the nickel salen complex as metal precursor is crucial for the high activity and selectivity observed.

Nickel-Cobalt Salen Organometallic Complexes Encapsulated in Mesoporous NaA Nanozeolite for Electrocatalytic Quantification of Ascorbic Acid and Paracetamol.

Goal of current study was fabrication of novel voltammetric nanosensor for the synchronize quantification of ascorbic acid (AA) and paracetamol (PAR) by nickel-cobalt salen complexes encapsulated in the supercages of NaA nanozeolite modified carbon paste electrode (NiCoSalenA/CPE). For this purpose, NiCoSalenA nanocomposite was firstly prepared and characterized by various methods. Also, cyclic voltammetry (CV), choronoamperometry (CHA) and differential pulse voltammetry (DPV) were utilized to evaluate performance of the modified electrodes. The effects of pH and modifier amount were considered on the electrochemical oxidation of AA and PAR on the surface of NiCoSalenA/CPE. Results from this method indicated that pH of 3.0 in phosphate buffer solution (0.1M) and 15 wt% of NiCoSalenA nanocomposite in the modified CPE results in the maximum current density. The oxidation signals of AA and PAR was amplified affectively at NiCoSalenA/CPE versus unmodified CPE. The limit of detection (LOD) and linear dynamic range (LDR) for the simultaneous measurement of them were founds to be 0.82 and 2.73-80.70 for AA and 0.51 µM, 1.71-32.50 and 32.50-137.60 µM for PAR, respectively. The catalytic rate constants (kcat) were attained to be 3.73 × 107 and 1.27 × 107 cm3 mol-1s-1 for AA and PAR via CHA method, respectively. Also, the amounts of diffusion coefficient (D) were found to be 1.12 × 10-7 and 1.92 × 10-7 cm2 s-1 for AA and PAR, respectively. The average value of electron transfer rate constant between NiCoSalenA/CPE and PAR was obtained to be 0.016s-1. The NiCoSalen-A/CPE displayed worthy stability, repeatability and extraordinary recovery for simultaneous measurements of AA and PAR. Application of offered sensor was confirmed by quantifying concentrations of AA and PAR in human serum solution as a real sample.

Design and development of salen-type Schiff bases as potential antivirus agents: Experimental and theoretical approach

Five new salen-type Schiff base compounds (LH2-L4H2) have been designed and synthesized and their interaction with SARS-CoV-2, HIV virus and DNA polymerase IV have been studied by in silico approaches. The newly synthesized salen-type Schiff base ligands have been derived from the condensation of substituted salicylaldehydes and ethylenediamine in MeOH. 1H and 13C NMR, FT-IR and UV-Vis spectral techniques have been applied in order to confirm the structural elucidation of the desired products. The crystal structure of the compound LH2 is determined by the single crystal X-ray diffraction. Density functional theory (DFT) calculations have been employed to evaluate the optimized electronic structure, HOMO-LUMO, energy gap, and global parameters. Extensive classical molecular dynamics simulations have been performed to investigate the consequences of docking of the synthesized ligand, LH2 on a protein moiety (PDB id: 7O46). Molecular docking studies have been performed on compounds (LH2-L4H2) to predict the binding mode and interactions between the ligands and the main protease of the SARS-CoV-2 (PDB ID: 7O46) for COVID-19 and HIV virus (PDB ID: 1UUI). The molecular docking results show that compounds (LH2-L4H2) with SARS-CoV-2 and HIV virus exhibit good binding affinity at binding site of receptor protein. As potential drug candidates, Swiss-ADME and target predictions (pharmacokinetics and drug-likeness prediction) analyses have also been studied and the results are compared with Chloroquine (CQ) and Hydroxychloroquine (HCQ) as anti-SARS-CoV-2 drugs. Salen-type ligands have also been docked into the DNA polymerase IV (PDB ID: 5YUX) for surface binding and intercalation. Overall, the present study offers the therapeutic potential for a series of compounds (LH2-L4H2) for biomedical applications with reference to coronavirus (SARS-CoV-2) and HIV virus.

Synthesis, spectroscopic investigation, molecular docking, ADME/T toxicity predictions, and DFT study of two trendy ortho vanillin-based scaffolds

In this article, we have synthesized two contemporary ortho-vanillin-based Salen-type ligands (H2L1/H2L2) characterized by modern spectroscopic tools. EDX analysis supports the elemental composition (C, N, O, and Br). SEM examined the morphology of the synthesized compounds. The molecular geometry was optimized in the gas phase using B3LYP-D3/6-311G (d, p) level. The global reactivity parameters, HOMO-LUMO energy gap (Δ), atomic properties, MESP, and ADME/T, vividly explore the chemical reactivity and toxicity of two Salen-type ligands. The DFT simulated IR/NMR characterized essential structural assignments, and UV-Visible spectra were employed to predict the optical properties. The article demonstrated in silico molecular docking against the Gm + ve Bacillus subtilis (6UF6), and Gm -ve Proteus Vulgaris establishes the ligand binding ability with essential amino acids through conventional H-bonding or other significant interactions. The docking simulation is compared for two compounds better than the control drugs and confirms the antimicrobial activity. The theoretical drug-like properties have been explored in-depth by ADME/T using the SWISSADME database. The analysis estimated the molecule's lipophilicity, the consensus P0/W, and water solubility. Thus, using various pharmaco-logical parameters, toxicity explains where the electron-withdrawing Br group plays a more toxic effect in H2L2 than in H2L1.

Chromium Complexes Supported by Salen-Type Ligands for the Synthesis of Polyesters, Polycarbonates, and Their Copolymers through Chemoselective Catalysis.

Salen, Salan, and Salalen chromium (III) chloride complexes have been investigated as catalysts for the ring-opening copolymerization reactions of cyclohexene oxide (CHO) with CO2 and of phthalic anhydride (PA) with limonene oxide (LO) or cyclohexene oxide (CHO). In the production of polycarbonates, the more flexible skeleton of salalen and salan ancillary ligands favors high activity. Differently, in the copolymerization of phthalic anhydride with the epoxides, the salen complex showed the best performance. Diblock polycarbonate-polyester copolymers were selectively obtained by one-pot procedures from mixtures of CO2, cyclohexene oxide, and phthalic anhydride with all complexes. In addition, all chromium complexes were revealed to be very active in the chemical depolymerization of polycyclohexene carbonate producing cyclohexene oxide with high selectivity, thus offering the opportunity to close the loop on the life of these materials.

A screening process for carbonation of vegetable oils using an aluminum(salen) complex with a further application as weldable polymers

AbstractCarbon dioxide (CO2) occurs naturally, though its emissions have been increasing due to anthropogenic activities, and its increasing atmospheric concentration levels are causing a greenhouse effect. In efforts to develop new carbon dioxide utilization (CDU) methodologies, the catalyzed reaction of CO2 with epoxidized vegetable oil, obtained from Brazilian macaw oil and Baru oil, to form carbonated oils for novel and sustainable monomers was explored. A screening process is carried out to develop the best reaction conditions, by varying catalyst/cocatalyst loading, reaction time, CO2 pressure, and the reaction temperature, resulting in conversions of 100%. The aluminum(salen) complex shows a selective and efficient catalyst activity. Both carbonated oils are reacted with amines (1,6‐diaminohexane, lysine, and 4,4′‐methylenebis [cyclohexylamine]) to provide weldable polyhydroxyurethanes. Polymers synthesized from lysine provide a more selective reaction and higher cross‐linked structures, with fewer side reactions involving the glyceride groups. All the synthesized polymers are thermally stable above 200°C and differential scanning calorimetry (DSC) analysis shows two main thermal events, related to the glass transition (Tg) and the topology‐freezing transition temperature (Tv). The Tv result indicates that the polymer has weldable properties due to chemical bond exchange. Thus, these polymers can be healed into different shapes upon exposure to red light (660 nm).

Manganese Salan Complexes as Catalysts for Hydrosilylation of Aldehydes and Ketones

Manganese has attracted significant recent attention due to its abundance, low toxicity, and versatility in catalysis. In the present study, a series of manganese (III) complexes supported by salan ligands have been synthesized and characterized, and their activity as catalysts in the hydrosilylation of carbonyl compounds was examined. While manganese (III) chloride complexes exhibited minimal catalytic efficacy without activation of silver perchlorate, manganese (III) azide complexes showed good activity in the hydrosilylation of carbonyl compounds. Under optimized reaction conditions, several types of aldehydes and ketones could be reduced with good yields and tolerance to a variety of functional groups. The possible mechanisms of silane activation and hydrosilylation were discussed in light of relevant experimental observations.

Uncovering the mechanism of water-promoted electrochemical degradation of NiSalen polymers

Polymeric nickel complexes with salen-type ligands (NiSalen) can serve as useful materials for a range of applications, including sensors, catalytic systems and energy storage devices. Although they surpass convenient conductive polymers in some aspects, they are still in the shadow of them. What pushes researchers away from NiSalen polymers is the low reproducibility of their electrochemical response, which sometimes can vary from day to day. Despite the convenient conductive polymers, which electrochemistry is often tolerant to moisture, most of the NiSalen polymers are highly water sensitive, which causes the uncertainty of their behavior, including the degradation of electrochemical activity and electrical conductance. In the present study, we demonstrate how NiSalen polymers degrade in the presence of H2O, and how the 3-substituent alternates the ultimate site of the H2O attack on the polymer. By a combination of the computational, electrochemical and spectroelectrochemical techniques, we demonstrate that the water-promoted degradation of the NiSalen polymer occurs only while it is oxidized. We have found that H2O attacks the Ni atom of the polymer, switching off charge transport along the polymer chain, but an introduction of CH3O-groups in the ligand structure changes the binding mode of H2O, protecting the polymer from degradation.

Electrochemical Epoxidation Catalyzed by Manganese Salen Complex and Carbonate with Boron-Doped Diamond Electrode.

Epoxides are essential precursors for epoxy resins and other chemical products. In this study, we investigated whether electrochemically oxidizing carbonate ions could produce percarbonate to promote an epoxidation reaction in the presence of appropriate metal catalysts, although Tanaka and co-workers had already completed a separate study in which the electrochemical oxidation of chloride ions was used to produce hypochlorite ions for electrochemical epoxidation. We found that epoxides could be obtained from styrene derivatives in the presence of metal complexes, including manganese(III) and oxidovanadium(IV) porphyrin complexes and manganese salen complexes, using a boron-doped diamond as the anode. After considering various complexes as potential catalysts, we found that manganese salen complexes showed better performance in terms of epoxide yield. Furthermore, the substituent effect of the manganese salen complex was also investigated, and it was found that the highest epoxide yields were obtained when Jacobsen's catalyst was used. Although there is still room for improving the yields, this study has shown that the in situ electrochemical generation of percarbonate ions is a promising method for the electrochemical epoxidation of alkenes.

Aluminum Salen Complexes Modified with Unsaturated Alcohol: Synthesis, Characterization, and Their Activity towards Ring-Opening Polymerization of ε-Caprolactone and D,L-Lactide.

A highly efficient one-step approach to the macromonomer synthesis using modified aluminum complexes as catalysts of ring-opening polymerization (ROP) of ε-caprolactone and D,L-lactide was developed. The syntheses, structures, and catalytic activities of a wide range of aluminum salen complexes, 3a-c, functionalized with unsaturated alcohol (HO(CH2)4OCH=CH2) are reported. X-Ray diffraction studies revealed a tetragonal pyramidal structure for 3c. Among the complexes 3a-c, the highest activity in bulk ROP of ε-caprolactone and D,L-lactide was displayed by 3b, affording polyesters with controlled molecular weights at low monomer to initiator ratios (Mn up to 15,000 g mol-1), relatively high polydispersities (Ð~1.8) and high number-average functionalities (Fn up to 85%).

Synthesis, Characterization, and Study of Catalytic Activity of Chiral Cu(II) and Ni(II) Salen Complexes in the α-Amino Acid C-α Alkylation Reaction.

A new family of Cu(II) and Ni(II) salen complexes was synthesized and fully characterized through various physicochemical methods. Their catalytic activity was evaluated in the phase transfer Cα-alkylation reaction of the Schiff bases of D,L-alanine ester and benzaldehyde derivatives. It was found that the introduction of a chlorine atom into the ortho- and para-positions of the phenyl ring of the substrate resulted in an increase in both the chemical yield and the asymmetric induction (ee 66-98%). The highest enantiomeric excess was achieved in the case of a Cu(II) salen complex based on (S,S)-cyclohexanediamine and salicylaldehyde at -20 °C. The occurrence of a bulky substituent in the ligand present in the complexes led to a drastic decrease in ee and chemical yield. For instance, the introduction of bulky substituents at positions 3 and 5 of the phenyl ring of the catalyst resulted in a complete loss of the stereoselectivity control in the alkylation reaction.

Impact of added ligand oxides in the sulfoxidation of phenylmercaptoacetic acids with oxovanadium(IV)‐salen complexes

AbstractThe mechanistic aspects covering the effect of added ligand oxides, picoline N‐oxide (PicNO), pyridine N‐oxide (PyNO), and triphenyl phosphine oxide (TPPO) on the selective sulfoxidation of phenylmercaptoacetic acids to PSAA by H2O2 catalyzed by oxovanadium(IV)‐salen complexes have been studied spectrophotometrically in 100% acetonitrile medium. Hydroperoxovanadium(V)‐salen is the reactive species in this sulfoxidation. The rate of sulfoxidation is found to be enhanced significantly by increasing the concentrations of PMAA, salen, and H2O2. The ascertained order of reactivity among the oxovanadium(IV)‐salen complexes is 5,5′‐dimethyl > unsubstituted salen > 5,5′‐dichloro. The reaction is found to be impeded by the addition of ligand oxide and introduction of substituents in PMAA. The observed order of rate retardation among ligand oxides is TPPO < PyNO < PicNO. Downward Hammett plot obtained has been rationalized by change in rate‐determining step from coordination of PMAA to active species for electron‐withdrawing substituents to transfer of oxygen to PMAA for electron‐donating substituents within the same mechanism.

An Al-salen complex for the fluorometric detection of pyrophosphate in water via a disassembly approach

This report describes the selective fluorometric detection of pyrophosphate in water with a simple Al[Formula: see text]-salen complex. The Al-based probe is synthesized in two steps in 30% yield and disassembled in the presence of pyrophosphate into its molecular building blocks. The released 3-chlorosalicylaldehyde signaling unit leads to a detectable signal with a 29-fold increase in fluorescence ([Formula: see text] = 513 nm; [Formula: see text] = 388 nm). At this emission wavelength, we did not observe a response from the Al[Formula: see text]-salen probe despite its intrinsic blue fluorescence ([Formula: see text] = 460 nm;[Formula: see text][Formula: see text] = 347 nm). The Al[Formula: see text]-complex shows excellent discrimination of pyrophosphate over other ions including phosphate containing adenosine triphosphate, phytic acid, or glyphosate and only fluoride inhibits the pyrophosphate-triggered disassembly process. Related salen-based probes with the same hydrophobic ligand framework but either Zn[Formula: see text] or Fe[Formula: see text] ions instead of Al[Formula: see text] were not sufficiently robust and therefore not suitable for analytical applications in pure water at pH 7.4.