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We clicked a salen ligand onto a thiol-ethane bridged periodic mesoporous organosilica (Salen-PMO) using a photo-initiated thiol-ene click reaction. This process resulted in a covalently bonded salen ligand on the PMO material. The final BET surface area amounts 511 m 2 /g and the pore size diameter is approximately 5.5 nm. The functionalized PMO material showed an excellent carbon dioxide uptake capacity of 1.29 mmol/g at 273 K and 1 bar. More importantly, by coordinating a MoO 2 2+ complex onto the Salen-PMO material, we obtained a heterogeneous catalyst with a good catalytic performance for the epoxidation of cyclohexene. The catalyst was highly reusable, as no decrease in its activity was observed for at least four runs (99% conversion). Finally, the metal-free Salen-PMO showed an exceptional catalytic performance in the cycloaddition of CO 2 to epoxides. The obtained results clearly demonstrate the versatility of the Salen-PMO material not only as metal-free catalyst but also as a support material to anchor metal complexes for specific catalytic applications. With the same catalytic platform, we were able to firstly create epoxides out of alkenes, and subsequently turn these epoxides into cyclic carbonates, consuming CO 2 .

A cationic covalent organic polymer with bifunctional active site was synthesized, which was treated by N, N'-bis(5-bromomethylsalicylaldehyde)ethylenediamine (salen ligand) and tris(1H-imidazol-1-yl) triazine (TIT) in the presence of aluminum ethoxide. The bifunctional cationic covalent organic polymer was investigated by various characterization technologies including PXRD, FT-IR, XPS, TG, SEM, EDS, N2-adsorption and CO2-adsorption. In this polymer, aluminum acts as lewis acid site and bromine ion acts as nucleophile, cooperatively catalyzing the cycloaddition reaction of CO2 and epoxides. Due to its cooperative effect, a higher catalytic activity was found to exhibit 98.1% conversion of epichlorohydrin under optimized conditions (Initial pressure 1.0 MPa, 0.57 mol% catalyst of COP-Al, 90 °C, reaction time 18 h, in the absence of a co-catalyst). Notably, the heterogeneous catalyst still showed good activity and stability after five cycles. A salen-based cationic covalent organic polymers (COP-Al) was used as a bifunctional catalyst for the cycloaddition reaction of CO2 and epoxides with high activity under solvent-free and co-catalyst-free conditions.

Oxidation of nickel salen complexes results in highly reactive species, undergoing polymerisation within microseconds. As a result, oxidation chemistry of NiSalens is available only for sterically hindered complexes. In the present work, an unexpectedly high stability of a nickel complex with a thiophene-based salen ligand was reported and studied. Based on the electrochemical and spectroscopic studies supported by quantum chemical calculations, a rationale for the stability of oxidized nickel complexes with thiophene-based salen ligands was proposed. The introduction of the thiophene core leads to a depletion of spin density on peripheral carbon atoms of the oxidized complex, which switches off its intermolecular reactivity. Instead of this, the initial ligand-centered radical cation undergoes an intramolecular process, namely intervalence tautomerisation, which leads to a highly stable metal-centered oxidized state.

A hetero-tetranuclear CeNi3 complex with a macrocyclic ligand catalysed the aerobic oxygenation of a methylene group adjacent to a carbonyl group under visible-light radiation to produce the corresponding α-diketones. The visible-light induced homolysis of the Ce–O bond of a bis(enolate) intermediate is proposed prior to aerobic oxygenation.

This work describes the application of a library of iron(III)-salen catalysts in the production of biodiesel from vegetable oils. The conversion of neutral soybean oil is complete within two hours at 160–180 °C with low catalyst loading (0.10 mol%). A comparative screening reveals that the catalysts containing acetate as a fifth ligand are the most performing, and these have been conveniently used to convert acidic and waste cooking oils (WCO). WCOs were used as received without further purification to produce biodiesel in high yield (85–90%) under optimized conditions (2 h at 180 °C, catalyst loading 0.1 mol%, oil to alcohol molar ratio 1:20). The iron content in the lipophilic and hydrophilic phases of the crude mixture was investigated and the residual concentration in biodiesel was found to be in the order of 10–14 ppm, comparable to that contained in biodiesels from other sources. Graphical Abstract

The Co(III)-catalyzed reaction of 1,3-dicarbonyl compounds with 2-(diazoacetyl)-2H-azirines, prepared by a simplified procedure from 2H-azirin-2-carbonyl chlorides, led in high yields to the formation of 2-(diazoacetyl)pyrroles, while leaving the diazoacetyl function intact. The intramolecular aromatic substitution reaction of 2-(diazoacetyl)pyrroles, catalyzed by Cu(OTf)2, provided selectively previously unknown benzo[e]- and hetero[e]-fused indol-7-oles in good yields. Formylation of benzo[e]indol-4-ol led selectively to the 5-formyl derivative, which is a good precursor for an unusual salen ligand and its Ni-complex. Triflates prepared from benzo[e]indol-4-oles gave various 4-substituted benzo[e]indoles carrying aryl, 2-thienyl, 2-pyridyl, and alkynyl groups, in excellent yields using cross-coupling reactions. 4-(2-Pyridyl)benzo[e]indoles, upon treatment with BF3·Et2O/Et3N, afforded a new type of fluorescent boron complexes with large Stokes shifts.

This work deals with the synthesis, spectroscopic studies and catalytic evaluation of the novel chiral salen (L1H2) and (L2H2) ligands and their chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. Initially, the reaction of 5-azidomethyl salicylaldehyde and (R)-(−)-2-amino-1-butanol in absolute ethanol afforded a new chiral salen ligand (L1H2). Then, a novel chiral salen ligand (L2H2) have been prepared from chiral salen ligand (L1H2) for the synthesis of boronate [L2(B1-4)] complexes through click reaction approach under ambient conditions. The reaction of chiral salen (L1H2) and (L2H2) ligands with various boronic acids afforded a new tetra-coordinated mononuclear chiral boronate [L1(B1-4)] and [L2(B1-4)] complexes. All the compounds are remarkably stable crystalline solids and were obtained in good yields. For the full characterization of newly synthesized chiral salen ligands and their boronate complexes, the FT-IR, UV–Vis, NMR (1H, 13C, and 11B), LC-MS, and elemental analysis techniques have been used. The well-shaped chiral boronate compounds were investigated as catalyst for the asymmetric transfer hydrogenation (ATH) of aromatic ketones under appropriate settings. Particularly, it was proved that the ferrocene-based boronate compounds can afford an efficient catalytic conversion compared to the other boronate complexes in the asymmetric transfer hydrogenation catalytic studies.

Two novel Zn2Dy2 Schiff base cluster complexes composed of [DyZn2(salen)2Cl2]+ and [Dy(bpdc)2]− complex-fragment building blocks (bpdc2− = 2,2′-bipyridine-6,6′-dicarboxylate) have been prepared, which exhibit interesting T-shaped metal cores and ferromagnetic interactions. Two-step magnetic relaxation was observed under a 1500 Oe dc field for both cluster complexes, and their effective energy barrier can be regulated by the bis-imine chain in the Schiff base. Complex 1, [Dy2Zn2(La)2(bpdc)2Cl2)]·3MeOH [H2La = N,N′- bis(3-methoxysalicylidene)-1,3-diaminopropane], has the Ueff/k value of 20.2(0.3) K and the τ0 value of 6.7(0.2) × 10−8 s for the fast relaxation (FR) phase as well as the Ueff/k value of 243.2(4.9) K and the τ0 value of 6.0(0.3) × 10−10 s for the slow relaxation (SR) phase; while complex 2, [Dy2Zn2(Lb)2(bpdc)2Cl2)]·MeOH·H2O [H2Lb = N,N′-bis(3-methoxysalicylidene)-1,2-diaminoethane], possesses the Ueff/k value of 22.0(0.4) K and the τ0 value of 4.6(0.1) × 10−8 s for the FR phase as well as the Ueff/k value of 205.7(4.4) K and the τ0 value of 1.7(0.2) × 10−9 s for the SR phase. Furthermore, the two complexes exhibit dual emission centered on not only the lanthanide(III) ion but also the ligand in the visible region, of which the metal-centered emission is closely related to the length of the bis-imine chain on the salen ligand. The average fluorescence lifetime (τ) of 1 (10.6 ns) is an order of magnitude longer than that of 2 (0.86 ns).

Manganese(iii) salen has been developed as a new catalytic motif for alcohol dehydrogenation and the mechanism has been elucidated., The first example of a manganese(iii) catalyst for the acceptorless dehydrogenation of alcohols is presented. N,N′-Bis(salicylidene)-1,2-cyclohexanediaminomanganese(iii) chloride (2) has been shown to catalyze the direct synthesis of imines from a variety of alcohols and amines with the liberation of hydrogen gas. The mechanism has been investigated experimentally with labelled substrates and theoretically with DFT calculations. The results indicate a metal–ligand bifunctional pathway in which both imine groups in the salen ligand are first reduced to form a manganese(iii) amido complex as the catalytically active species. Dehydrogenation of the alcohol then takes place by a stepwise outer-sphere hydrogen transfer generating a manganese(iii) salan hydride from which hydrogen gas is released.

The design of nanostructured catalysts based on earth‐abundant metals that mediate important reactions efficiently, selectively and with a broad scope is highly desirable. Unfortunately, the synthesis of such catalysts is poorly understood. We report here on highly active Ni catalysts for the reductive amination of ketones by ammonia employing hydrogen as a reducing agent. The key functions of the Ni‐salen precursor complex during catalyst synthesis have been identified: (1) Ni‐salen complexes sublime during catalyst synthesis, which allows molecular dispersion of the metal precursor on the support material. (2) The salen ligand forms a nitrogen‐doped carbon shell by decomposition, which embeds and stabilizes the Ni nanoparticles on the γ‐Al2O3 support. (3) Parameters, such as flow rate of the pyrolysis gas, determine the carbon supply for the embedding process of Ni nanoparticles.

A new ligand H2L with pyridine and salen moieties and its coordination polymers (CPs) [Mn(L)Cl]·DMF (1) and [Fe(L)Cl]·DMF (2) were synthesized and their photocatalytic activity for the conversion of CO2 into CO under visible-light irradiation was investigated. This is the first instance of pyridyl-salen-ligand based CPs for photocatalyzing CO2 reduction.

A bis-hypochlorite adduct of a manganese(IV) salen complex having a chiral (R,R)-cyclohexane-1,2-diamine linkage (2-tBu) is successfully prepared and characterized by various spectroscopic methods. The reactions of 2-tBu with various organic substrates show that 2-tBu is capable of sulfoxidation, epoxidation, chlorination, and hydrogen abstraction reactions. However, the enantioselectivity of the epoxidation reactions by 2-tBu is much lower than that reported for the catalytic reactions by Jacobsen’s catalyst. The low enantioselectivity is consistent with a planar conformation of the salen ligand, which is suggested by circular dichroism spectroscopy. This study suggests that 2-tBu is not a reactive intermediate of Jacobsen’s enantioselective epoxidation catalysis.

Several aluminium alkyl and alkyloxide complexes supported by a thioether-bridged salen ligand were synthesized by the protonolysis reaction. Reactions of N,N’-bis(3,5-di-tert-butylsalicylidene)-2,2’-diaminophenylsulfide (H2L) with one or two equivalents of AlMe3 in toluene at 100 °C afforded the corresponding mono- and binuclear aluminium methyl complexes LAlMe (1) and L(AlMe2)2 (2). Treatment of the in situ generated complex 1 with one equivalent of ROH (R = PhCH2, iPr, CH3OCH2CH2) produced the desired aluminium alkyloxide complexes LAlOCH2Ph (3), LAl(OiPr) (4) and LAlO(CH2)2OCH3 (5). All the molecular structures were established by single crystal X-ray diffraction studies. It was found that aluminium alkyloxide complexes 3-5 showed higher catalytic activity for the homopolymerization of e-caprolactone (e-CL) than corresponding aluminium methyl complexes 1 and 2. Furthermore, these alkyloxide complexes were also efficient initiators for the copolymerization of L-lactide and e-CL to give practically random copolymers.

A series of bifunctional metal-salen complexes bearing ionic liquid substituents were investigated as catalysts for the synthesis of cyclic carbonates from carbon dioxide (CO2) and epoxides using density functional theory (DFT) and energy span model. Detailed studies reveal that the three-step mechanism involving the ring-opening of propylene oxide (PO), CO2 insertion, and ring-closure to the cyclic carbonate is kinetically more favorable than the two-step mechanism. The effect of ionic liquids, epoxide substituents and metal center of the metal-salen complexes were explored. Amino functionalized ionic liquids substituents tethered to the salen ligand of the metal-salen complexes performed well. Epoxide substituents affect the cycloaddition mainly through the electronic effect and the substituent that lowers the negative charges of the ring carbon atom to be attacked favors the cycloaddition. When the salen ligand is fixed, the catalytic activity metal-salen complexes can be measured by the interaction between the oxygen atom (O) of epoxides and the central metal (M) and there is a linear relationship between the energy span and the M-O bond length, which can be used as a descriptor to screen CO2 conversion catalysts.

An inexpensive and effective cobalt coordinated conjugated microporous polymer (Co-CMP-2) made up from cross-linked an ethanediamine-based salen ligand with 1,3,5-triethynylbenzene. Co-CMP-2 exhibited extremely high catalytic efficiency in the synthesis of cyclic carbonates from CO2 and epoxides, which is superior to that of previously reported Co-CMP. Co-CMP-2 achieved a TOF of 23300 h−1 for ethylene carbonate. Moreover, the catalyst can be reused more than 10 times without significant loss in activity.

A variety of unique Al(salen) complexes functionalized by imidazolium-based ionic liquid (IL) moieties with the salen ligand at the two sides of 3,3′-position have been successfully prepared, rather than familiar 5,5′-position reported previously. The catalytic activity obtained by these bifunctional catalysts could be superior to those of the binary type catalysts in the formation of five-membered heterocyclic compounds from the cycloaddition reaction of CO2 and three-membered heterocyclic compounds (including terminal epoxides and N -substituted aziridines), presumably due to the distinguished intramolecularly synergistic catalysis, which might lead to perform the cycloaddition reaction at ambient conditions and retain excellent yield and unprecedented chemo- or regioselectivity. Moreover, the polyether-based trifunctional Al(salen) catalysts with the best catalytic performance could be regenerated and reused at least eight times without any obvious decreases in catalytic activity. Finally, the kinetic investigation suggested the structure of catalysts had important influences on the catalytic activity, thereby proposing the possible reaction mechanism.

A simple cooperative catalytic system was successfully developed for the solvent-free N-formylation of amines with CO2 and hydrosilanes under ambient conditions, which was composed of a Zn(salen) catalyst and quaternary ammonium salt. These commercially available binary components activated the Si-H bonds effectively, owing to the intermolecular synergistic effect between Lewis base and transition metal center (LB-TM), and subsequently facilitated the insertion of CO2 to form the active silyl formats, thereby leading to excellent catalytic performance at a low catalyst loading. Furthermore, the bifunctional Zn(salen) complexes, with two imidazolium-based ionic-liquid (IL) units at the 3,3'-position of salen ligand, acted as intramolecularly cooperative catalysts, and the solvent-regulated separation resulted in facile catalyst recycling and reuse.

Two Lewis acid–base bifunctional monometallic aluminum–salen complexes were prepared based on a new type of salen ligand with two N-methylhomopiperazine moieties at the 3,3′-position. The Al(salen) complexes proved to be efficient and recyclable homogeneous catalysts towards the organic solvent-free synthesis of cyclic carbonates from epoxides and CO2 in the absence of a co-catalyst, in which >90% yield of cyclic carbonate could be obtained under relatively mild conditions. The catalysts can be easily recovered and reused five times without significant loss of activity and selectivity. Furthermore, the Lewis acid–base cooperative activation mechanism by the bifunctional Al(salen) complexes was proposed according to experimental data.

The catalytic activity of cobalt(III)–salen ion catalyzed selective H2O2 oxidation of organic sulfides to sulfoxides is examined using spectrophotometric technique. The catalytic reaction proceeds through Michaelis–Menten kinetics and the rate of the reaction is highly sensitive to the nature of the substituent present in the substrate as well as in the salen ligand. The product analyses show that the aryl methyl sulfides are selectively oxidized to the corresponding sulfoxides. Based on the spectral and kinetic studies two possible mechanisms have been proposed.

A salen ligand and its derivatives salan and salalen, bearing the same substituents on the phenolate rings, have been synthesized and then used for the preparation of the corresponding Fe(iii) complexes. All complexes were characterized by MALDI ToF mass spectroscopy, Evans method, and UV-Vis and IR spectroscopy. Moreover, for the salalen-based iron complex, the X-ray structure was studied. A comparison of their behaviour in the catalysis of CO2 with benchmark epoxide substrates allowed us to individuate an order of reactivity of the different classes of complexes. A salen Fe(iii) complex was prepared with a longer bridge between the two nitrogen atoms of the ligand skeleton in order to evaluate the influence of the flexibility on the activity of the catalyst. Finally, the same complexes were tested as catalysts in the ring-opening polymerization of l-lactide and e-caprolactone.

Various metal-salen catalysts were prepared for use in the direct synthesis of diphenyl carbonate (DPC) from phenol and carbon dioxide. We found that metal-salen complexes containing titanium as central metal species retained suitable Lewis acid property for the reaction. It was revealed that the catalytic activity of Ti-salen complexes could be controlled by introducing appropriate substituents into salen ligand. Insertion of phosphonium salts into para-position of aromatic aldehyde of salen ligand enhanced solubility of the catalyst in the methanol-phenol solution, and tert-butyl substituent in the salen ligand induced selective formation of DPC due to steric effect. In addition, introduction of various bridging groups into salen ligand caused change in electronic property of central metal atom. Among the catalysts tested, Ti-(t-butyl)salphen(PPh3)Cl showed the best catalytic performance at 100 °C and 60 bar. The catalytic system utilizing Ti-(t-butyl)salphen(PPh3)Cl catalyst was then optimized by conducting the reaction at various reaction temperatures and pressures.

Cobalt (II) Salen complex catalysts were synthesized and immobilized through covalent bonds on synthetic silica (SBA-15) and commercial supports, including silica (SiO2), alumina (Al2O3) and granular activated carbon (C). Fourier transform infrared spectroscopy (FTIR), ultraviolet-visible (UV–vis), diffuse reflectance ultraviolet-visible spectroscopy (DR–UV–vis) and X-ray diffraction (XRD) were used to characterize and follow the synthesis of the cobalt (II) Salen complex The UV–vis absorption spectra confirmed the formation of the Salen ligand and its coordination with the Co (II). FTIR spectra showed the incorporation of the Co Salen complex and its anchoring to the support was confirmed by DR–UV–vis. The catalytic activity was evaluated measuring the oxidation of phenol by air in a buffer solution using micellar electrokinetic chromatography (MEKC). The effects of support, initial substrate concentration, pH, oxidant (O2 vs H2O2) and temperature were investigated. The best catalytic performance was obtained when the Co Salen complex was immobilized on SBA-15 (Co Salen/SBA-15). Conversely, the results suggest that the activated carbon promoted the irreversible adsorption of the substrate. The Co Salen support was also characterized by scanning electron microscopy (SEM) to elucidate the morphology. Elemental analysis using energy dispersive X-ray spectroscopy (EDX) exhibited better Co Salen dispersion on the SiO2 and SBA-15 supports. The cobalt loading calculated from atomic absorption spectroscopy (AAS) was 0.22, 0.20, 0.14 and 0.09 mmol/g for the Co Salen supported on SiO2, SBA-15, Al2O3 and C, respectively. A specific surface area of 736.13 m2/g and a nanoporosity of 3.62 nm were obtained for SBA-15 using N2 physisorption analysis; this high specific surface area is characteristic of SBA-15 and plays a critical role in the reported activity. According to these results and the corresponding data analysis, the catalyst promoted the oxidation of phenol with a reversible first-order kinetic reaction in air, as observed from the data obtained at 3 different temperatures (25, 38 and 50 °C). These results enabled the calculation of activation energy values of 63 and 57 kJ/mol. In addition, the Co Salen/SBA-15 showed very good selectivity toward the production of catechol, which is one of the most important precursors in the chemical industry.

The oxovanadium(IV)-salen ion catalyzed H2O2 oxidation of N,N-dimethylaniline forms N-oxide as the product of the reaction. The reaction follows Michaelis–Menten kinetics and the rate of the reaction is accelerated by electron donating groups present in the substrate as well as in the salen ligand. This peculiar substituent effect is accounted for in terms of rate determining bond formation between peroxo bond of the oxidant and the N-atom of the substrate in the transition state. Trichloroacetic acid (TCA) shifts the λmax value of the oxidant to the red region and catalyzes reaction enormously. The cleavage of NO bond by vanadium complex leads to moderate yield of the product. But the percentage yield of the product becomes excellent in the presence of TCA.

The oxovanadium(IV)–salen-catalyzed H2O2 oxidation of phenols gives o-catechol and hydroquinone as the reaction products. The reaction follows Michaelis–Menten-type kinetics, and the rate is accelerated by electron-donating substituents on both the substrate and the salen ligand. This peculiar substituent effect is accounted for in terms of bond formation between peroxo group of the oxidant and the benzene ring of the substrate in the transition state. A suitable mechanism for the reaction is proposed.

A chiral dimeric macrocyclic V(V) salen complex based on a chiral macrocyclic salen ligand derived from 1R, 2R-(−) diaminocyclohexane with trigol bisaldehyde was synthesized and evaluated as an efficient catalyst (5 mol%) for asymmetric addition of potassium cyanide and trimethylsilyl cyanide to various N-benzylimines at −20 °C. The dimeric chiral macrocyclic salen complex was found to be an efficient recyclable catalyst to give a high yield (91%) of α-aminonitrile and high chiral induction (ee up to 99%) in the case of 2-methoxy substituted N-benzylimines in 16 h. The catalytic system worked well with both organic and inorganic cyanide sources and the catalyst was recycled up to five cycles with retention of enantioselectivity. Based on NMR investigation and kinetics a probable mechanism was proposed for the catalytic enantioselective Strecker reaction.

A series of neutral rare-earth metal aryloxides and amides supported by a new pentadentate (N2O3) salen ligand were synthesized, and their catalytic behaviors for the ring-opening polymerization of rac-lactide (rac-LA) were explored. The protolysis reactions of N,N′-bis(3,5-di-tert-butylsalicylidene)-2,2′-diaminodiphenyl ether (LH2) with (ArO)3Ln(THF) (ArO = 2,6-But2-4-MeC6H2O) and Ln[N(SiMe3)2]3 in a 1 : 1 molar ratio in THF gave the neutral rare-earth metal aryloxides LLn(OAr)(THF)n [n = 0, Ln = Sc (1), Yb (2); n = 1, Ln = Y (3), Sm (4) and Nd (5)] and rare-earth metal amides LLnN(SiMe3)2 [Ln = Yb (6), Y (7)], respectively. X-ray structural determination showed that complexes 1, 2, 6 and 7 have monomeric structures, in which the coordination geometry around the rare-earth metal atom can be best described as a distorted trigonal prism. Complexes 3 and 5 are THF-solvated monomers and each of the rare-earth metal atoms is seven-coordinated to form a distorted capped trigonal prism. It was found that all of these complexes can efficiently initiate the ring-opening polymerization (ROP) of rac-LA to give heterotactic-rich polylactides (PLAs). The highly heterotactic PLA (Pr up to 0.93) was obtained using complex 2 as the initiator at a polymerization temperature of 0 °C. The observed increasing order of activity of 1 < 2 < 3 < 4 ≈ 5 is in agreement with the order of their ionic radii, whereas the order of stereoselectivity is in the reverse order. The rare-earth metal salen amides can initiate rac-LA polymerization in a controlled manner, while polymerization using the rare-earth metal salen aryloxides is less controlled at room temperature.

An iron(III) complex of tetradentate N,N′-disubstituted bis(aminophenoxide) (designated as salan, a saturated version of the corresponding salen ligand) with a sterically hindered organic base anchored on the ligand framework, can selectively mediate the conversion of carbonyl sulfide to sulfur-containing polymers by the copolymerization with epoxides. This single-site catalyst exhibits broad substrate scope, and the resultant copolymers have completely alternating structures. In addition, this catalyst is efficient in producing diblock copolymers, suggesting a living polymerization nature.

We report here a unique example of an in situ generated aluminum initiator stabilized by a C2-symmetric salen ligand which shows a hitherto unknown high activity for the ROP of rac-lactide at room temperature. Using a simple and robust catalyst system, which is prepared from a salen complex and an onium salt, this convenient route employs readily available reagents that afford polylactide in good yields with narrow polydispersity indices, without the need for time-consuming and expensive processes that are typically required for catalyst preparation and purification. In line with the experimental evidence, DFT studies reveal that initiation and propagation proceed via an external alkoxide attack on the coordinated monomer.

Salen ligand synthesized from 2-hydroxybenzaldehyde and 2-hydroxy-1-naphthaldehyde was used as a palladium chelating ligand for the immobilization of the catalytic site. Mesoporous silica supported palladium catalysts were prepared by immobilizing Pd(OAc)2 onto a mesoporous silica gel through the coordination of the imine-functionalized mesoporous silica gel. The prepared catalysts were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray (EDX), inductivity couple plasma (ICP), nitrogen adsorption-desorption, and Fourier transform infrared (FT-IR) spectroscopy. The solid catalysts showed higher activity for the hydroamination of C-(tetra-O-acetyl-β-D-galactopyranosyl)allene with aromatic amines compared with the corresponding homogenous catalyst. The heterogeneous catalytic system can be easily recovered by simple filtration and reused for up to five cycles with no significant loss of catalytic activity.

Two dinuclear aluminum alkyl complexes bearing salen ligand with alkyl backbone of different length between the nitrogen atoms (1,3-propylene (1) and 1,5-pentylene (2)) and the related hemi-salen aluminum complex (3) were tested as catalysts in the ring-opening copolymerization of cyclohexene oxide (CHO) and limonene oxide (LO) with succinic anhydride (SA) and phthalic anhydride (PA). The effects of different reaction conditions (cocatalyst and reaction solvent) on the productivity and selectivity of the reactions were evaluated. The comparison of the catalytic behavior of the dinuclear complexes bearing the aluminum reactive centers at different distances as well as of their mononuclear counterpart suggested that the copolymerization follows a monometallic pathway differently from what observed in the homopolymerization of CHO.

A series of monometallic salen aluminum complexes were prepared by covalent linkage of the imidazolium-based ionic liquid moieties containing various polyether chains with the salen ligand at the two sides of the 5,5′-position. The salen aluminum complexes proved to be efficient and recyclable homogeneous catalysts towards the organic solvent-free synthesis of cyclic carbonates from epoxides and CO2 in the absence of a co-catalyst. The catalysts presented excellent “CO2 capture” capability due to the molecules containing polyether chains and the metal aluminum center, in which >90% yield of cyclic carbonate could be obtained under mild conditions. The catalysts can be easily recovered and six times reused without significant loss of activity and selectivity. Moreover, based on experimental and previous work, the “CO2 capture and activation” cycloaddition reaction mechanisms by monometallic or bimetallic salen aluminum complexes were both proposed.

International audience; A new chiral uranyl(VI) complex incorporating a tetradenate salen ligand is reported. Thesynthesized uranyl complex is studied by microanalyses, spectroscopic and X–ray diffractionstudies. The structural studies reveal a slightly distorted pentagonal bipyramid coordinationenvironment around uranyl ion. Interestingly, the uranyl complex was found to be potentialvisible light active catalyst for C–H bond functionalization of dialkylanilines, and affordedmoderate to excellent yield of corresponding –aminonitriles when exposed to visible light for 8hours in the presence of NaCN and acetic acid as cyanide source, and H2O2 as oxidant.

Porous organic polymers (POPs) represent interesting candidate materials for carbon dioxide (CO2) adsorption applications owing to the permanently porous nature of the structures and the ability to vary metalloligand centres that can be incorporated as a potential means of property tuning. This work reports the synthesis and characterisation of four transition metal complexes (using M=Mn, Ni, Fe, and Pd) of the bis-bromo salen ligand, and the incorporation of these complexes into POPs with tris-(p-ethynyl)-triphenylamine to yield metallated polymers (POPMn, POPNi, POPFe, and POPPd). The POPs were shown to possess Brunauer–Emmett–Teller (BET) surface areas of up to 650m2g−1. Overall, this work provides further insight into the potential of permanently porous polymeric materials in post-combustion capture applications.

The structure of (BeSalen)2 is reported. The incompatibility of the geometry of the beryllium with the inflexibility of the Salen ligand gives rise to a rare dimeric bis-didentate motif.

A series of novel chiral salen Mn(III) complexes possessing a thermoregulated phase-transfer function was prepared for the first time. The preparation involved the introduction of the “smart” poly(N-isopropylacrylamide) (PNIPAAm) group to the framework of the chiral salen Mn(III) complex by covalently connecting to one side of the 5-position in the salen ligand (complex 1 ) or by axially grafting onto the metal center of the complex (complex 2 ). Characterization results suggested the presence of the PNIPAAm moiety and active sites in the complexes. The thermoresponsive PNIPAAm groups imparted inverse temperature-dependent water solubility to the PNIPAAm-based complexes, which allowed them to undergo thermoregulated phase-separable catalysis during oxidative kinetic resolution (OKR) in water. Excellent enantioselectivity (up to 96%) with high kinetic resolution efficacy (13.1) was achieved over complex 1 in the aqueous OKR of α -methylbenzyl alcohols, and this efficacy was significantly higher than that obtained over neat complex. The PNIPAAm-based complexes can also be easily recovered by simply regulating the temperature and be reused for four times without obvious loss of enantioselectivity and activity.

The catalysis effect of [Fe(III)-salen]Cl complex on the decolorization and degradation of Malachite Green in the Fenton system was investigated in this study. A significant decolorization enhancement was observed in the presence of only trace amount of the complex without additional equipment or external energy. The effects of reaction parameters were studied and the results showed that the increase of [Fe(III)-saler]Cl complex, molar ratio of salen ligand to Fe(III) and initial concentration of H2O2 favor the increase of decolorization rate. The increase of initial concentration of the dye leads to the decrease of decolorization rate and the optimum pH was the original pH of Malachite Green solution. Under the optimal condition, the decolorization and TOC removal rates after 24 min treatment were 97.94% and 54.35% respectively. Hydroxyl radical was identified as the main active oxidant species in the [Fe(III)-salen]Cl complex catalyzed Fenton system. The degradation intermediates of Malachite Green were analyzed and a possible reaction mechanism was proposed. The decolorization of Malachite Green obeyed the pseudo-first order equation and the activation energy was calculated by the related kinetic constants, which was only 49.13% of Fe(III) Fenton system. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.

Chiral vanadium(IV)–salen complexes with variable spacer length were immobilized on polysiloxanes, which were used as catalytically active stationary phase in on-column reaction gas chromatography, on-column reaction electrophoresis, in flow-through reactors and as coating of laboratory glassware. These devices modified with the catalytically active stationary phase were employed in asymmetric sulfoxidations using hydrogen peroxide as the oxidant. The here presented approach combines catalysis and chromatographic analysis in a single step and allows to study the kinetics of reactions and to optimize reaction conditions. The initial reaction kinetics and enantioselectivity of the sulfoxidation of benzylphenylsulfide was studied in dependency of the spacer length of the immobilized salen ligand. A drastic influence of the spacer length on the enantioselectivity in the here investigated sulfoxidation was measured.

Two Lewis acid–base bifunctional monometallic aluminum–salen complexes were prepared based on a new type of salen ligand with two N-methylhomopiperazine moieties at the 3,3′-position. The Al(salen) complexes proved to be efficient and recyclable homogeneous catalysts towards the organic solvent-free synthesis of cyclic carbonates from epoxides and CO2 in the absence of a co-catalyst, in which >90% yield of cyclic carbonate could be obtained under relatively mild conditions. The catalysts can be easily recovered and reused five times without significant loss of activity and selectivity. Furthermore, the Lewis acid–base cooperative activation mechanism by the bifunctional Al(salen) complexes was proposed according to experimental data.

The synthesis and characterization of aluminum alkyl and alkoxide complexes bearing racemic 6,6′-dimethylbiphenyl-bridged salen-type ligands, and their catalysis in the ring-opening polymerization (ROP) of rac-lactide are reported. Reactions of AlMe3 with various amounts of the proligands L1−4H2 (6,6′-[(6,6′-dimethyl-[1,1′-biphenyl]-2,2′-diyl)bis(nitrylomethilidyne)]-bis(2-R1-4-R2-phenol): L1H2, R1 = R2 = Me; L2H2, R1 = tBu, R2 = Me; L3H2, R1 = R2 = cumyl; L4H2, R1 = Br, R2 = tBu) afforded the corresponding mono- and dinuclear aluminum methyl complexes [L1−3AlMe (1–3), L1−4Al2Me4 (4–7)]. Aluminum alkoxide complexes L2AlOiPr (8), L2AlOBn (9), and α-alkoxy ester complexes L2Al(OCMe2CO2Me) (10), L2Al[(S)-OCHMeCO2Me] (11) were prepared via in situ alcoholysis of the parent aluminum methyl complex 2 with the corresponding alcohols. The molecular structures of mononuclear complexes 1–3, dinuclear complex 6, alkoxide complexes 8–9 and α-alkoxy ester complexes 10–11 were established by single-crystal X-ray diffraction studies. Two broad resonances at about 69–70 ppm and 25–41 ppm were observed in the 27Al NMR spectra of complexes 10 and 11, indicating the existence of both four- and five-coordinate aluminum centers in solution, which results from the dissociation of one N donor of the salen ligand, accompanied by an association and dissociation equilibrium of the carbonyl group of the α-alkoxy ester ligand to the aluminum center. Complex 11 is also a rare example of an O-lactate model complex that mimics the first insertion of L-LA. All complexes were investigated for the ROP of rac-LA at 110 °C in toluene. The polymerization initiated by complexes 1–3 in the presence of iPrOH showed living features, affording PLAs with narrow molecular weight distributions (PDIs = 1.03–1.05) and 65–73% isotacticities. Particularly, complex 8 showed an “immortal” behavior for the polymerization of rac-LA in the presence of excess alcohol. Compared with the mononuclear counterparts, the tetra-coordinate dinuclear aluminum complexes enabled a few fold boosts in activity, but gave atactic PLAs with broadened PDIs.

A chiral dimeric macrocyclic V(V) salen complex based on a chiral macrocyclic salen ligand derived from 1R, 2R-(−) diaminocyclohexane with trigol bisaldehyde was synthesized and evaluated as an efficient catalyst (5 mol%) for asymmetric addition of potassium cyanide and trimethylsilyl cyanide to various N-benzylimines at −20 °C. The dimeric chiral macrocyclic salen complex was found to be an efficient recyclable catalyst to give a high yield (91%) of α-aminonitrile and high chiral induction (ee up to 99%) in the case of 2-methoxy substituted N-benzylimines in 16 h. The catalytic system worked well with both organic and inorganic cyanide sources and the catalyst was recycled up to five cycles with retention of enantioselectivity. Based on NMR investigation and kinetics a probable mechanism was proposed for the catalytic enantioselective Strecker reaction.

A series of conjugated multi-structured fluorescent probe molecules based on a salen ligand were designed and investigated in dimethyl sulfoxide solvent using a quantum-chemical method. The results indicate that the one-photon absorption and fluorescence emission spectra (λ (O) and λ (EM)) of these molecules generally show redshifts (of 23.1-74.5 and 22.7-116.6 nm, respectively) upon the coordination of the molecules to Zn(2+). Large Stokes shifts (1511.2-11744.1 cm(-1)) were found for the molecules, meaning that interference between λ (O) and λ (EM) can be avoided for these molecules. The two-photon absorption spectra of the molecules usually present blueshifts, but the two-photon absorption cross-section (δ) greatly increases (by 221.5-868.0 GM) upon the coordination of the molecules with Zn(2+). Most of the molecules show strong two-photon absorption peaks in the range 678.2-824.4 nm, i.e., in the near-infrared region. In a word, the expanded π-conjugated frameworks of these molecules lead to redshifted λ (O) and λ (EM) and enhanced δ values. Moreover, (L-phenyl)​2 and (L-phenyl-ethynyl)2 are the most suitable of the multi-structured molecules examined in this work for use as two-photon fluorescent probes for zinc ion detection in vivo. Graphical Abstract Scheme of the calculated transition energies (E0k and E0n) and the transition dipole moments (M0k and Mkn). NTO 109, NTO 197 and NTO 228 of Zn(L-phenyl-ethynyl), Zn2(L-phenyl-ethynyl)2 and Zn3(L-phenyl)3 for one-photon absorption, respectively.

Three dinuclear aluminum alkyl complexes of the general formula LAl2Me4, where L are salen ligands with an alkyl backbone of different lengths between the nitrogen atoms (1,3-propylene (1), 1,5-pentylene (2) and 1,12-dodecaylene (3)), have been prepared through alkane elimination reactions between each ligand and two equivalents of AlMe3. The related hemi-salen aluminum complex 4 was prepared by an analogous reaction between a phenoxy–imine ligand and a single equivalent of AlMe3. The activities of these aluminum complexes in the ring-opening polymerization (ROP) of rac-lactide and of several epoxides have been investigated and compared. The dinuclear complex 1, bearing the salen ligand with the shortest alkyl bridge, was the most active in the ROP of LA producing isotactic enriched PLA. Otherwise, the other complexes (2 and 3), in which the metal centers are remote, produced atactic PLA with inferior activity. Analogous differences in terms of activity emerged in the ROP of epoxides. The comparison of the catalytic behavior of the dinuclear complexes as well as their mononuclear counterparts suggests the cooperation between the two aluminum metal centers of the dinuclear species in which these are close enough.

Cyclic voltammetry, controlled-potential (bulk) electrolysis, gas chromatography (GC), gas chromatography–mass spectrometry (GC–MS), and high-performance liquid chromatography–electrospray ionization–mass spectrometry (HPLC–ESI–MS) have been employed to investigate the direct reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (CFC-113) at a carbon cathode in dimethylformamide (DMF) containing 0.10 M tetra-n-butylammonium tetrafluoroborate (TBABF4) as well as the nickel(I) salen-mediated reduction of CFC-113 in the same medium. Cyclic voltammograms for direct reduction of CFC-113 show two irreversible cathodic peaks attributable to formation of, first, 1-chloro-1,2,2-trifluoroethene (CFC-1113) and, second, trifluoroethene (HFC-1123); bulk electrolyses of CFC-113 at potentials corresponding to each of the cathodic peaks afford CFC-1113 and HFC-1123, respectively, in essentially quantitative yield. Cyclic voltammograms for reduction of nickel(II) salen in the presence of CFC-113 provide evidence for the catalytic formation of CFC-1113 and HFC-1123, and bulk electrolyses involving electrogenerated nickel(I) salen lead to mixtures of CFC-1113 and HFC-1123, although 1,1,1,2-tetrafluoroethane (HFC-134a) has been detected as a minor product. It has been determined with the aid of HPLC–ESI–MS that the salen ligand of the nickel(I) catalyst is modified during the catalytic reduction of CFC-113. A mechanistic scheme is proposed for the direct reduction of CFC-113.

A series of novel chiral salen Mn(III) complexes with inherent phase-transfer capability were prepared by covalent linkage of the limidazolium IL moieties containing various PEG chains with chiral salen ligand at two sides of 5,5′-position. Technologies of characterization well suggested the presence of polyether chain, the IL linker, and the intact active sites in the complexes. The amphipathic nature of the PEG chain allowed the PEG-based catalysts to undergo built-in phase transfer, which in turn increased the reaction rates in water/organic biphasic systems. Enantioselective epoxidation of styrene, α-methylstyrene, indene, 1,2-dihydronaphthalene, 6-cyano-2,2-dimethylchromene, and 6-nitro-2,2-dimethylchromene catalyzed by the complexes with NaOCl gave >99% conversions within 60 min. The enantiomeric excess ( ee ) for the epoxides was in the range of 68–93%, except for styrene ( ee , 35%) and α-methylstyrene ( ee , 42%). Furthermore, the PEG-based complexes were stable and could be separated from the reaction mixture by control of the solvent.

Ionic ligands derived from a salen ligand containing two proximal 1,3-disubstituted imidazolium ionic liquid cores form cobalt(III) complexes capable of selectively oxidizing veratryl alcohol, a lignin model compound, to veratraldehyde using air as the source of oxygen. These complexes are easy to prepare, inexpensive, water stable, and soluble in ionic liquids, making them viable candidates for use as oxidation catalysts.

Two new one-component aluminium-based catalysts for the reaction between epoxides and carbon dioxide have been prepared. The catalysts are composed of aluminium–salen chloride complexes with trialkylammonium groups directly attached to the aromatic rings of the salen ligand. With terminal epoxides, the catalysts induced the formation of cyclic carbonates under mild reaction conditions (25–35 °C; 1–10 bar carbon dioxide pressure). However, with cyclohexene oxide under the same reaction conditions, the same catalysts induced the formation of polycarbonate. The catalysts could be recovered from the reaction mixture and reused.

A novel enantiopure salen ligand bearing a diphenylphosphine oxide on the 3-position of one aromatic ring was synthesized and combined with Ti(O i -Pr) 4 as a monometallic bifunctional catalyst for asymmetric cyanosilylation reaction of aldehydes with trimethylsilyl cyanide (TMSCN). The catalyst system exhibited excellent activity and moderate enantioselectivity. The addition of TMSCN to 4-nitrobenzaldehyde in the presence of 1 mol% catalyst loading could complete within 10 min at ambient temperature. An intramolecularly cooperative catalysis was observed in this system wherein the central metal Ti(IV) is suggested to play a role of Lewis acid to activate aldehydes while the appended diphenylphosphine oxide worked as Lewis base to activate TMSCN.

Metal complexes based on the salen ligand are highly versatile compounds that have many interesting properties, including magnetism, catalysis, and reversible oxygen binding. The marriage of complexes of salen with porous materials is a promising way to combine the attractive properties of the salen complex with those of the porous structure. In this Microreview, we discuss several recent examples of published research that illustrate the power of combining these two disparate areas of research. Approaches based on supramolecular chemistry are emphasized.