Salicylidene Ring Research Articles

The influence of axial ligands on the catalytic activity and enantioselectivity of salen‐Mn complexes in the asymmetric epoxidation

AbstractThe asymmetric epoxidation of indene catalyzed by Jacobsen catalysts with different axial ligands (Cl−, Br−, OAc−, CF3CO2−, N(CH3)2−, and NO3−) was theoretically investigated. The spin states and approach vectors were taken into consideration. The axial ligands can affect the steric conformation of the catalysts. Furthermore, the structure distortion during the formation of the transition state is responsible for the enantioselectivity, and this distortion is also highly related to the axial ligands. These ligands have nonnegligible influence on both the catalytic activity and enantioselectivity. Changing the axial ligand can even turn the enantioselectivity from (S, R)‐ to (R, S)‐enantiomer. The influence of axial ligand on the enantioselectivity is comparable with that induced by the salicylidene ring modification. Controlling the catalytic activity and enantioselectivity of salen‐Mn complexes by simply changing the axial ligand deserves to be paid more attention.

Design, synthesis, and biological activity of Schiff bases bearing salicyl and 7-hydroxycoumarinyl moieties

18 (11 novel) Schiff bases, derivatives of salicylaldehyde, 2-hydroxyacetophenone, and 6-acetyl-, 8-acetyl-, and 8-formyl-7-hydroxy-4-methylcoumarin were synthesized and characterized by their spectral studies. 6-Acetyl-7-hydroxy-4-methylcoumarin was prepared by novel method under microwave assistance. These Schiff bases were evaluated for antibacterial activities against 12 bacterial and six fungi strains in vitro. N-(3,5-Dichloro-2-hydroxybenzylidene)-4-aminobenzenesulfonic acid sodium salt proved to be the most active against Staphylococcus aureus and MRSA strains (MIC 0.0194 µmol/cm3). The substitution pattern, two chlorine atoms in the salicylidene ring and the SO3Na group, is probably the most beneficial for the activity against Gram-(+) bacteria strains. All Schiff bases were evaluated for cancer efficacy against CFPAC-1 and HeLa cell cultures originating from human pancreas cancer or human cervical cancer. Schiff bases derived from salicylaldehyde are highly effective in pancreas and cervical cancer cells; however, they demonstrate also substantial toxicity towards NIH3T3 cells. Derivatives of coumarin contain three highly selective compounds: 7-hydroxy-8-[(4-methoxyphenylimino)methyl]-4-methyl-2H-chromen-2-one, N-[(7-hydroxy-4-methyl-2-oxo-2H-chromen-8-yl)methylene]-4-aminobenzenesulfonic acid sodium salt, and 7-hydroxy-8-[1-(4-hydroxyphenylimino)ethyl]-4-methyl-2H-chromen-2-one suggesting more promising potential of the second group of substances.Graphical abstract

Spectroscopic characterization of N,N'-bis(salicylidene)pentane-1,3-diamine nickel(II) complex

The <TEX>$N_2O_2$</TEX> tetradentate Schiff base ligand, N,N'-bis(salicylidene)pentane-1,3-diamine (Salpn), coupled with 1:2 concentration ratio of 1,3-diaminopentane and salicylaldehyde was used to produce a series of macrocyclic Nikel(II) complexes. In the metal complexation, it was observed that Salpn macrocyclic ligand can adopt more than a metal ion giving an unique multinuclear metal complexes including Ni(II)Salpn and <TEX>$Ni(II)_3(Salpn)_2$</TEX>. Characteristic IR <TEX>${\upsilon}(M-O)$</TEX> peaks for Ni(II)Salpn and <TEX>$Ni(II)_3(Salpn)_2$</TEX> were observed to be <TEX>$1028cm^{-1}$</TEX> and <TEX>$1024cm^{-1}$</TEX>, respectively. Characteristic UV-Vis absorption <TEX>${\lambda}_{max}$</TEX> peaks for <TEX>$Ni(II)_3(Salpn)_2$</TEX> were observed to be 241nm and 401 nm. Structural characterization of <TEX>$Ni(II)_3(Salpn)_2$</TEX> by NMR exhibits that the salicylidene ring moiety has two different resonance signals originated from the magnetically asymmetric diligand and trinuclear bis complex. Complete NMR signal assignments and characterizations elucidating structural features of <TEX>$Ni(II)_3(Salpn)_2$</TEX> were described in detail.

Titanium–salan‐catalyzed asymmetric sulfoxidations with H2O2: design of more versatile catalysts

Titanium–salan complexes with 3,3’‐diphenyl substituents in the salicylidene rings of the salan ligand are efficient sulfoxidation catalysts, capable of catalyzing the asymmetric oxidation of bulky aryl benzyl sulfides with H2O2 with good to high enantioselectivities. In this paper, substituent effects on titanium‐salan‐catalyzed enantioselective oxidation of sulfides to sulfoxides have been systematically investigated. Titanium–salan catalysts with halogen substituents at the 5,5’‐positions (3,3’‐H2dihydrogen substituted) have been found to catalyze the oxidation of both bulky aryl benzyl sulfides and small alkyl phenyl sulfides with good to high enantioselectivities. Kinetic data witness a direct attack of the sulfide to the electrophilic active oxygen species; a consistent reaction mechanism is proposed. Copyright © 2013 John Wiley &amp; Sons, Ltd.

Gallium(III) complexes based on N,N′-bis(salicylidene)propane-1,3-diamine and its derivatives

The reactions of N,N′-bis(salicylidene)propane-1,3-diamine (salpropH2) and its substituted derivatives with Ga(acac)3 afforded the complexes [Ga(acac)(salprop)]·0.5H2O (1), [Ga(acac)(5Clsalprop)] (2), [Ga(acac)(5Brsalprop)] (3), [Ga(acac)(5NO2salprop)] (4) and [Ga(acac)(5Mesalprop)] (5), in high yields. The crystal structures of all complexes have been determined by single-crystal X-ray crystallography. All complexes are mononuclear with the Ga(III) atoms being in octahedral environments surrounded by one tetradentate chelate 5Rsalprop2− ligand and one bidentate chelate acac− ligand. The free ligands exhibit photoluminescence which was found to be increased upon complex formation. The substituted derivatives of the salpropH2 ligand bearing either electron-donating methyl groups or electron-withdrawing groups, such as Cl, Br and NO2 at the fifth position of both salicylidene rings were found to modify the emission maximum of the free ligands and the complexes.

Ligand dissociation/recoordination in fluorescent ionic zinc–salicylideneimine compounds: synthesis, characterization, photophysical properties, and 1H NMR studies

A series of ionic zinc-salicylideneimine compounds, [HNEt3][Zn(L)Cl2] (L = sal(H)-4-CN, 1; sal(Cl)-4-CN, 2; sal(Br)-4-CN, 3; sal(OMe)-4-CN, 4) and [NEt4][Zn(sal(H)-4-CN)Cl2] (5), have been synthesized and structurally characterized. Compounds 1-5 all display an intense fluorescence band in both solution (methanol, MeOH; acetonitrile, ACN; dimethylsulfoxide, DMSO; dichloromethane, DCM) and solid phases, with a maximum in the region of 515-560 nm and 529-573 nm, respectively. Detailed (1)H NMR and optical spectroscopic studies indicate the occurrence of ligand dissociation and recoordination in 1-4 in solution, leading to an equilibrium between the zinc-salicylideneimine complex species, [Zn(L)Cl2](-), and the salicylideneimine free ligand, HL. The tendency of ligand dissociation is related to the solvent, the concentration, and the substitution of the salicylidene ring. The greatest degree of ligand dissociation is observed for 1-4 when dissolved in noncoordinating, less polar DCM solvent, followed by coordinating, high polar DMSO solvent. The least degree of ligand dissociation occurs in solutions of coordinating, moderately polar MeOH and ACN solvents for 1-3 and 4, respectively. Dilute solutions are likely characterized by the high degree of ligand dissociation, whereas the equilibrium shifts in favor of the complex [Zn(L)Cl2](-) form at higher concentration. Furthermore, the electron-donating methoxy substitution in the salicylidene ring promotes a high tendency for ligand dissociation, while electron-withdrawing chloro and bromo groups cause the reverse tendency. The (1)H NMR spectrum of 5 shows only one set of proton resonances in the aromatic region corresponding to the complex [Zn(sal(H)-4-CN)Cl2](-) species, indicating that the lack of ammonium proton would protect the complex form from ligand dissociation.

Cis-Dioxo-molybdenum(VI) Schiff base complexes: Synthesis, crystal structure and catalytic performance for homogeneous oxidation of olefins

The synthesis of two Mo(VI) tetradentate Schiff base complexes derived from 2,2′-dimethylpropylenediamine and aromatic aldehydes, (MoO2{hnaphnptn} (1) and MoO2{salnptn(3-OMe)2} (2)) is reported. Full characterization of these complexes was accomplished with elemental analyses, spectroscopic studies (1H NMR, IR, and UV–Vis) and X-ray structure analysis. X-ray crystallography studies reveal that these complexes adopt a distorted octahedral six-coordinate configuration formed by tetradentate Schiff base ligand and two binding oxygen atoms. Catalytic performance of the prepared molybdenum complexes for oxidation of different olefins with tert-butyl hydroperoxide was evaluated. These complexes were found to be an efficient and selective catalyst for the homogeneous oxidation of various olefins. MoO2{salnptn(3-OMe)2} with a methoxy groups on the salicylidene ring of the ligand promote the effectiveness of the catalyst.

Tuning of the Emission Efficiency and HOMO–LUMO Band Gap for Ester-Functionalized {Al(salophen)(H2O)2}+ Blue Luminophors

A series of [AlL(H(2)O)(2)(NO(3))] complexes, with L standing for an ester substituted salophen-type ligand, has been synthesized, and the luminescence properties have been investigated. These derivatives differ by the nature of the ester-R group introduced at the C5 position of their salicylidene rings (i.e., phenyl, 7a,a'; naphthyl, 7b,b'; pentafluorophenyl, 7c,c'; and p-nitrophenyl, 7d) and by the bis-imino bridge (i.e., 1,2- phenylene, 7a-d; and 1,2-naphthalene, 7a'-c'). All the complexes are characterized by luminescence in the blue range, the chemical diversity having no effect on the emission wavelength (480-485 nm). However, the emission efficiency was found to be strongly dependent on the Schiff-base ligand with quantum yields ranging from ϕ = 22% to 44%, the highest values being for the salophen derivatives with the electron-withdrawing ester-R groups (7a, 34%; 7a', 23%; 7b, 31%; 7b', 22%; 7c, 40%; 7c', 29%, and 7d, 44%). Both the electrochemical data and DFT calculations show that the HOMO-LUMO band gap is modified as a function of the ester R group (from 2.92 to 3.16 eV, based on the redox potentials). The crystal structures for the N,N'-bis(5-(phenoxycarbonyl)salicylidene)-1,2-phenylenediamine and the N,N'-bis(5-(p-nitrophenoxycarbonyl)salicylidene)-1,2-phenylenediamine aluminum complexes (7a and 7d) are reported.

New molecular architectures by aggregation of tailored zinc(ii) Schiff-base complexes

This contribution reports the synthesis, characterization, 1H NMR, optical absorption and fluorescence properties of a bis(salicylaldiminato)Zinc(II) Schiff base complex, 1, having dipodal, alkyl ammonium bromide derivatized side chains in the salicylidene rings. The comparison of 1H NMR and optical spectroscopic properties of 1 with those of the closely related amphiphilic alkyl derivative allowed getting insights into the nature of species in solutions. Present data suggest the existence of Zn⋯Br interactions, thus avoiding intermolecular Zn⋯O interactions, as usually occur in the absence of other coordinating species. SEM images of samples obtained by drop-casting from dichloromethane solutions of 1 reveal the existence of branched nanostructures, consistent with intermolecular Zn⋯Br interactions. The present approach thus suggests a new method to unique supramolecular architectures for coordination metal complexes.

Structure and pulsed EPR characterization of N,N′-bis(5-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(iv) oxide and its adducts with propylene oxide

The role of steric hindrance in controlling the binding mode of propylene oxide to a novel vanadyl salen-type complex N,N'-bis(5-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide, [VO(3)], has been investigated using CW/pulse EPR, ENDOR and HYSCORE spectroscopy and compared to that of the parent complex N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamino-vanadium(IV) oxide, [VO(1)]. The single-crystal X-ray structure of [VO(3)]·HCCl(3) has been determined by X-ray analysis and is complemented by DFT calculations and circular dichroism measurements. The structure of the complex in frozen solution, as revealed by the EPR methods, is in good agreement with the X-ray and DFT analyses. Removal of the 'inner'tert-butyl groups from the salicylidene rings reduces the steric hindrance between the ligand and epoxide substrate. As a result the selectivity for binding single enantiomers of propylene oxide in these complexes is reversed in [VO(3)] relative to [VO(1)].

New copper(ii)-centered complexes with organometallic donor–acceptor substituted unsymmetrical Schiff base ligands

The synthesis and full characterization using various spectroscopic techniques and single-crystal X-ray diffraction analysis of a family of three robust copper(II)-centered complexes with organometallic donor–acceptor substituted unsymmetrical Schiff base ligands, namely, the binuclear derivatives Cu[Fc–C(O)CHC(CH3)N–X–NCH-(2-O,5-Br–C6H3)] (3: X = o-C6H4, 4: X = CH2CH2), and the corresponding ionic trinuclear counterpart of 4, [Cu{Fc–C(O)CHC(CH3)N–CH2CH2–NCH-(η6-2-O,5-Br–C6H3)RuCp*}][PF6] (5) (Fc = (η5-C5H5)Fe(η5-C5H4), Cp* = η5-C5Me5), have been explored. Single crystal X-ray diffraction analysis of complex 3 indicates a bowed structure of the unsymmetrical Schiff base scaffold. The Cu2+ ion is tetracoordinated in a square planar environment, with two nitrogen atoms and two oxygen atoms as donors. The EPR spectra of solid trinuclear complexes 5 and 7 (unsubstituted salicylidene ring and X = o-C6H4) at low temperature (68 K) consisted of fine-structure transitions (ΔMs = ±1) with zero-field splitting (ZFS) values of 0.0458 and 0.0415 cm−1, respectively, and a half-field signal (ΔMs = ±2) at ca. 1600 G, suggesting the formation of dimeric species (S = 1). In those dimers, Cu–Cu distances of 3.97 and 4.14 A, respectively, are computed from ZFS terms. The variable-temperature (2–300 K) magnetic susceptibilities of powdered samples of 5 and 7 have been measured, and an intermolecular antiferromagnetic interaction with exchange couplings of J = −1.05 and −0.65 cm−1, respectively, were found. The electrochemical behavior of both the neutral binuclear and ionic trinuclear compounds was investigated by cyclic voltammetry. Notably, the ability of the copper Schiff base core to transmit electronic effects between the two organometallic termini has been demonstrated by the observed anodic shift of the reversible voltammetric signal ascribed to the FeII/FeIII couple of the ferrocenyl donor fragment.

Keto–enol tautomerism in asymmetric Schiff bases derived from p-phenylenediamine

Reaction of dehydroacetic acid and p-phenylenediamine afforded a monosubstituted Schiff base, I, with the other amino group free. In further reactions with various salicylaldehyde derivatives, I served as a precursor for synthesis of asymmetric bis-Schiff bases. The synthesized compounds are thus comprised of two subunits, dehydroacetic ( dha) and salicylidene ( sal), which are bridged by the phenylene linker. All products were investigated by means of elemental analysis, FT-IR and NMR spectroscopy, thermal methods, powder X-ray diffraction and, when possible, by single crystal X-ray crystallography. Structural and spectroscopic studies revealed that in the bis-products, the dha subunit adopts the keto–amino tautomeric form, while the sal subunit adopts the enol–imino form. Tautomeric forms were not affected if a methoxo group was introduced on the salicylidene ring. Both tautomeric subunits are stabilized by strong resonance-assisted hydrogen bonds, RAHB. The two subunits of the prepared bis-Schiff bases predominantly retain in solution the same tautomeric forms as found in the solid state.

Aromaticity balance, π-electron cooperativity and H-bonding properties in tautomerism of salicylideneaniline: the quantum theory of atoms in molecules (QTAIM) approach

Topological analysis based on DFT calculations regarding proton transfer reaction in salicylideneaniline (SA) was performed to scrutinize possible changes in the intramolecular H-bond, π-electron delocalization and aromaticity levels of certain fragments. Quantum chemical calculations and natural bond orbital (NBO) analyses were carried out over a tautomeric ensemble whose members correspond to the molecules at different stages in tautomeric interconversion of SA. The elaboration of intramolecular hydrogen bonding in terms of the relevant topological parameters and the interpretation of certain dependencies regarding its strength were examined. The results show that delocalization index (DI) between donor and acceptor atom δ(O,N) is a useful topological parameter for describing H-bond strength, which is influenced by π-delocalization level within quasiaromatic chelate ring, indicating its resonance-assisted character. NBO analyses reveal that lone-pair (LP) population on N center also affects the strength of intramolecular H-bond in SA. Furthermore, π-electron transfer accompanying intramolecular proton migration in SA is brought into being through formally vacant non-Lewis type LP* orbital on the tautomeric proton. As a result of this, tautomeric protons in molecular entities near TS have hypovalent character due to the lack of electron population in the bonding orbital relative to that in LP* orbital. While H-bonds in the tautomeric ensemble of SA are predominantly partial covalent, molecular entities close to transition state have the strongest covalent H-bonds. The most important result is also that there are linear correlations between the orders of bonds (hydroxyl and amine) involving intramolecular H-bond and electron density values at the relevant BCPs due to partially covalent character of these bonds, contrary to exponential behavior as for purely covalent bonds. Quasiaromatic chelate ring formation is established not only to compel a reduced aromaticity of salicylidene ring but also to decrease in LP-population on N.

Synthesis, characterization and pronounced epoxidation activity of cis-dioxo-molybdenum(VI) tridentate Schiff base complexes using tert-butyl hydroperoxide

The synthesis and catalytic performance of novel cis-dioxo-Mo(VI) complexes containing simple ONO tridentate Schiff base ligands in the epoxidation of various olefins using tert-butyl hydroperoxide in desired times with excellent chemo- and stereoselectivity have been described. The study of turnover numbers and the UV–Vis spectra of the Mo complexes in the present epoxidation system indicate well the high efficiency and stability of the catalysts during the reaction. The electron-deficient and bulky groups on the salicylidene ring of the ligand promote the effectiveness of the catalyst.

Pronounced Catalytic Activity of Manganese(III)–Schiff Base Complexes in the Oxidation of Alcohols by Tetrabutylammonium Peroxomonosulfate

AbstractA novel and practical catalytic method for efficient and highly selective oxidation of a wide range of benzylic, allylic, aliphatic, primary, and secondary alcohols to the corresponding aldehydes and ketones using tetrabutylammonium peroxomonosulfate catalyzed by tetradentate Schiff base–MnIII complexes has been developed. Electron‐deficient and hindered alcohols required longer reaction times for oxidation in this catalytic system. The electron‐poor and hindered salicylidene ring of the ligand enhanced the catalytic activity and stability of Mn catalysts. The desired turnover numbers obtained in the oxidation reactions indicated the high efficiency and relative stability of these simple Schiff base complexes in this catalytic system.

Synthesis, Spectral, Structural, Second-Order Nonlinear Optical Properties and Theoretical Studies On New Organometallic Donor−Acceptor Substituted Nickel(II) and Copper(II) Unsymmetrical Schiff-Base Complexes

The synthesis, spectroscopic and structural characterization, linear and nonlinear optical properties, as well as the electrochemical behavior of a series of robust neutral binuclear M[Fc-C(O)CH=C(CH(3))N-X-N=CH-(2-O,5-R-C(6)H(3))] (M = Ni (4), Cu (5), X = o-C(6)H(4), R = H; M = Ni (9), X = CH(2)CH(2), R = OH), and their corresponding ionic trinuclear [M{Fc-C(O)CH=C(CH(3))N-X-N=CH-(eta(6)-2-O,5-R-C(6)H(3))RuCp*}][PF(6)] (6, 7, 10), M[ONNO]-type unsymmetrical Salophen and salen complexes featuring ferrocenyl (Fc) donor and the mixed sandwich acceptor [Cp*Ru(eta(6)- salicylidene)](+) as a push-pull moiety are reported in this paper (Fc = CpFe(eta(5)-C(5)H(4)); Cp = eta(5)-C(5)H(5); Cp* = eta(5)-C(5)Me(5)). The single-crystal X-ray structure of the bimetallic iron-nickel derivative 4 indicates a bowed structure of the unsymmetrical Schiff base skeleton. The Ni(II) ion is tetracoordinated in a square planar environment, with two nitrogen atoms and two oxygen atoms as donors. The new metalloligand [Fc-C(O)CH=C(CH(3))N(H)CH(2)CH(2)N=CH-(2,5-(OH)(2)C(6)H(3))] (8) obtained from the Schiff base condensation of 2,5-dihydroxobenzaldehyde with the half-unit precursor, Fc-C(O)CH=C(CH(3))N(H)CH(2)CH(2)NH(2) (2), is reported with its crystal structure showing partial delocalization of the heteroconjugated [O-C-C-C-N] frameworks with a dihedral angle between the respective planes of 60.76 degrees. Second order nonlinear optical (NLO) measurements were achieved using the Harmonic Light Scattering technique to probe the role of the M[ONNO] chromophores and of the pi-complexation of the salicylidene ring in the nonlinearity. All the complexes exhibit a second-order nonlinear response increasing with the nuclearity, the hyperpolarizability (beta) value of the trinuclear complex 10 being 1.5 time larger than that of the metalloligand 8 (beta = 155 x 10(-30) esu). A rationalization of the structural, electronic, and redox properties of the title compounds is provided, based on a theoretical investigation at the density functional theory (DFT) level. Their UV-visible spectra has been assigned with the help of time-dependent (TD) DFT calculations. They are dominated by LMCT, MLCT, and pi-pi* transitions.

Modulation of the luminescence quantum efficiency for blue luminophor {Al(salophen)}+ by ester-substituents

A series of Al(III) complexes of a salophen (N,N'-bis(salicylidene)-o-phenylenediamine) derivative with an ester substituent at the C5 position of the salicylidene rings have been prepared and their luminescence properties investigated. All exhibit a bright blue emission (lambda(em) = 478 nm) that is observed neither for other metal ions nor with the non-functionalized ligand. Moreover, the ester-R group (Me, (t)Bu, Ph) was found to significantly affect the quantum yields (varphi = 12 to 20%) of the luminophor. DFT calculations have been undertaken to reveal the influence of the ester and its R-group on the frontier molecular orbitals. Preparation and photoluminescence properties in solution and solid state are reported. The crystal structure of a pyridine oxide adduct [L(tBu)Al(PyO)(2)].NO(3), where L(tBu) stands for the tButyl-ester functionalized Schiff base, has been solved.

Critical Role of External Axial Ligands in Chirality Amplification of trans-Cyclohexane-1,2-diamine in Salen Complexes

A series of Mn(IV)(salen)(L)(2) complexes bearing different external axial ligands (L = Cl, NO(3), N(3), and OCH(2)CF(3)) from chiral salen ligands with trans-cyclohexane-1,2-diamine as a chiral scaffold are synthesized, to gain insight into conformational properties of metal salen complexes. X-ray crystal structures show that Mn(IV)(salen)(OCH(2)CF(3))(2) and Mn(IV)(salen)(N(3))(2) adopt a stepped conformation with one of two salicylidene rings pointing upward and the other pointing downward due to the bias from the trans-cyclohexane-1,2-diamine moiety, which is in clear contrast to a relatively planar solid-state conformation for Mn(IV)(salen)(Cl)(2). The CH(2)Cl(2) solution of Mn(IV)(salen)(L)(2) shows circular dichroism of increasing intensity in the order L = Cl < NO(3) << N(3) < OCH(2)CF(3), which indicates Mn(IV)(salen)(L)(2) adopts a solution conformation of an increasing chiral distortion in this order. Quantum-chemical calculations with a symmetry adapted cluster-configuration interaction method indicate that a stepped conformation exhibits more intense circular dichroism than a planar conformation. The present study clarifies an unexpected new finding that the external axial ligands (L) play a critical role in amplifying the chirality in trans-cyclohexane-1,2-diamine in Mn(IV)(salen)(L)(2) to facilitate the formation of a chirally distorted conformation, possibly a stepped conformation.

Ab Initio Investigation on the Second-Order Nonlinear Optical Responses in Keto−Enol Equilibria of Salicylideneanilines

The keto-enol (K-E) tautomerization equilibrium, more precisely, the keto-amine/enol-imine equilibrium, has been investigated for a series of substituted salicylideneanilines in view of designing compounds with a contrast of second-order nonlinear optical properties. Substituting the salicylidene ring by an acceptor group or the other ring by a donor prevents the K form from being stable, whereas in the other cases, the K form can easily be converted to the E form due to the small activation barrier, figuring out in most cases that the K form is metastable. For a representative set of donor/acceptor substituents, the E and K forms present a sufficiently large contrast of beta to allow its detection by using electric-field-induced second harmonic generation or hyper-Rayleigh scattering. The largest beta values are mainly associated with species bearing a donor in the para position of the salicylidene ring and an acceptor on the other ring whereas the largest beta values are generally found for the E form.

Syntheses, structure, redox and catalytic epoxidation properties of dioxomolybdenum(VI) complexes with Schiff base ligands derived from tris(hydroxymethyl)amino methane

Five kinds of dioxomolybdenum(VI) complexes with Schiff base ligands derived from tris(hydroxymethyl)amino methane are prepared and structurally characterized by X-ray crystallography, which reveals that these complexes adopt a distorted octahedral six-coordinate configuration formed by the tridentate Schiff base ligand, one coordinating water and two binding oxygen atoms. These complexes show good catalytic activities and selectivity in the epoxidation of cyclohexene with t-butylhydroperoxide, especially for complex 4, which could give a nearly 100% of epoxidation conversion and selectivity. Introduction of the electron-withdrawing group to the salicylidene ring of complex strongly increases the effectiveness of a catalyst, but decreases the redox stability of a complex.