Salen-type Schiff-base Ligand Research Articles

Hydroxylation of Phenol Catalyzed by Oxovanadium(IV) of Salen-Type Schiff Base Complexes with Hydrogen Peroxide

Active and highly selective catalytic systems of oxovanadium(IV) salen-type have been prepared and characterized by various physico-chemical techniques. Substituted salen-type Schiff base ligands were prepared from 3-ethoxy salicylaldehyde with 1,2-diaminobenzene and 1,8-diaminonaphthalene abbreviated (EtOsalphen) and (EtOsalnaph), respectively. The catalytic activity of the complexes for hydroxylation of phenol to catechol and hydroquinone using H2O2 as an oxidant has been studied. The best suited reaction conditions were obtained by considering the effect of solvent, concentration of substrate, reaction time, concentration of catalyst and temperature. Under the optimized reaction conditions, VO-(EtOsalphen) catalyst shows high conversion (71%) at a short reaction time (2 h) with selectivity of 92.5% towards catechol, while VO-(EtOsalnaph) complex also shows higher conversion (76.6%) after longer reaction time (6 h) with almost similar selectivity to catechol (94.2%). Synthesis and characterization of active systems of oxovanadium (IV) salen-type Schiff base has. The catalytic activities of these complexes for hydroxylation of phenol using H2O2 as an oxidant has been studied and were found to be a highly active and selective towards catechol formation.

A Chiral Salen-type Schiff Base Cu2+ Complex for the Vinylic-type Polymerization of Norbornene

The chiral Salen-type Schiff base ligand H 2 L (H 2 L = (s)-(+)2,2′-bis(2-hydroxy-3-methoxybenzylideneamino)-1,1′-binaphthyl), was selected to obtain the new chiral Cu2+ complex CuL (1), which was found to catalyze the polymerization of norbornene (NBE). The complex shows the distinctively distorted geometry from a square-planar reactive center, this feature being suggested to account for the efficient catalysis on the vinylic-type polymerization of NBE, with moderate molecular weights and narrow molecular weight distributions. From the involvement of a chiral Salen-type Schiff base ligand H 2 L (H 2 L = (s)-(+)2,2′-bis(2-hydroxy-3-methoxybenzylideneamino)-1,1′-binaphthyl), the new chiral Cu2+ complex CuL (1) was obtained, and found to catalyze the polymerization of Norbornene (NBE). The polymerization results showed that the Cu2+ complex 1, endowing the distinctive distortion from a square-planar reactive center, could efficiently catalyze the vinylic-type polymerization of NBE, with moderate molecular weights and narrow molecular weight distributions.

Salen-type nickel(II), palladium(II) and copper(II) complexes having chiral and racemic camphoric diamine components

Chiral and racemic Salen-type Schiff-base ligands (H 2L 1, H 2L 2 and H 2L 3), condensed between D-(+)- and D,L-camphoric diamine (also known as (1 R,3 S)-1,2,2-trimethylcyclopentane-1,3-diamine) and 2-hydroxybenzaldehyde or 3,5-dibromo-2-hydroxybenzaldehyde with a 1:2 molar ratio, have been synthesized and characterized. A series of new nickel(II), palladium(II) and copper(II) complexes of these chiral and racemic ligands exhibiting different coordination number (4, 5 and 6) have been characterized with the formulae [NiL 1]·CH 3OH ( 3), [NiL 1]·H 2O ( 4), [NiL 2] ( 5), [PdL 2] ( 6), [Cu 2(L 2) 2(H 2O)] ( 7) and [NiL 3(DMF)(H 2O)] ( 8). Different solvent molecules in 3 and 4 (methanol and water molecules) as well as different apical ligands in 7 and 8 (water and DMF molecules) are involved in different O–H···O hydrogen bonding interactions to further stabilize the structures. UV–Vis (UV–Vis), circular dichroism (CD) spectra and thermogravimetric (TG) analyses for the metal complexes have also been carried out.

A novel di-iron(III) structure based on an ageless ligand

Abstract Salen-type Schiff base ligands possess a N2O2 coordination site able to chelate transition metal ions. Reaction of such a ligand with FeCl3·6H2O leads to the formation of a genuine dinuclear L3Fe2 complex in which the ligand plays a double role. The structural determination confirms that each iron ion is wrapped up by a chelating tetradentate ligand while the third one ensures a bridge in between the two Fe ions and completes to six the metal coordination sphere. Such a bridge implies a non planar coordination of the chelating ligands and induces helicity around each metal ion. Crystals are true racemates made of homochiral Λ–Λ and Δ–Δ pairs. The magnetic study confirms the absence of a spin exchange interaction through the saturated diamino chain and the presence of an axial single ion zero field splitting (ZFS) D equal to −0.7 cm−1. The Mossbauer spectra show a unique asymmetric quadrupole-split doublet with isomer shift values, δ, of 0.342(2), 0.438(2) and 0.450(2) mm s−1, and quadrupole splittings, ΔEQ, of 0.826(4), 0.836(4) and 0.827(4) mm s−1 at 293, 80 and 4.5 K, respectively. These values are in agreement with a high-spin iron(III) site in a N3O3 ligand environment.

Unsymmetrical exo-dentate IN− ligand for further self-assembly with the Zn–Nd Salen-type Schiff-base ligands

With the hetero-binuclear Zn–Nd complex from the ethylene- or phenylene-linkered Salen-type Schiff-base ligand as the building block, further self-assembly of the unsymmetrical exo-dentate IN− (IN−=isonicotinic) anion gave a 2D layer polymeric [ZnL1Nd(IN)(NO3)2]n (1) (H2L1=N,N′-bis(3-methoxy-salicylidene)ethylene-1,2-diamine) or a discrete binuclear [ZnL2Nd(IN)(NO3)2(DMF] (2) (H2L2=N,N′-bis(3-methoxy-salicylidene)phenylene-1,2-diamine) complex, respectively. The change of linker flexibility of two Salen-type Schiff-base ligands (H2L1 and H2L2) resulted in the difference of structures and NIR luminescent properties of their mixed-ligands complexes.

Hetero-trinuclear near-infrared (NIR) luminescent Zn2Ln complexes from Salen-type Schiff-base ligands

With the Zn–Schiff-base [ZnL11(Py)] or [ZnL22(Py)] from the simple Salen-type Schiff-base ligand H22L11 or H22L22 (H22L11: N,N′-bis(salicylidene)ethylene-1,2-diamine; H22L22: N,N′-bis(salicylidene)phenylene-1,2-diamine) without the outer O2O2 portion as the precursor, two series of eight trinuclear Zn2Ln arrayed complexes (Ln = Nd (1 or 5), Yb (2 or 6), Er (3 or 7) or Gd (4 or 8)) have been obtained by the further reaction with Ln(NO3)3·6H2O, respectively. The results of photophysical studies show that the strong and characteristic NIR luminescence with emissive lifetimes in microsecond ranges, has been sensitized from the excited state (1LC and 3LC or only 1LC) of the ligand, whereas the Zn(II)-based visible luminescence is mostly quenched because of quite effective intramolecular energy transfer from the ligand-centered excited state of the Zn(II)–Schiff base complex to Ln(III) ions.

Construction and NIR luminescent property of hetero-bimetallic Zn–Nd complexes from two chiral salen-type Schiff-base ligands

Two new near-infrared (NIR) luminescent Zn–Nd complexes [ZnL 1Nd(OAc)(NO 3) 2] ( 3) and [ZnL 2Nd(DMF) 2(NO 3) 3] ( 4) have been obtained with two salen-type Schiff-base ligands H 2L 1 and H 2L 2 , ( H 2L 1 = N, N′-bis(3-methoxysalicylidene)-(1s, 2s)-(−)1,2-dipheneylethylenediamine and H 2L 2 = N, N′-bis(3-methoxysalicylidene)-(s)-2,2-diamine-1,1′-binaphthyl) from the reaction of different chiral diamines with o-vanillin. The X-ray crystal structure analysis reveals that both of them crystallize in the chiral space groups with P2(1), a = 10.1669(6), b = 19.3775(11), c = 17.4639(10) Å, β = 94.8710(10)°, V = 3428.1(3) Å 3, Z = 4 for 3, and C2, a = 22.1914(13), b = 9.7886(6), c = 22.0138(13) Å, β = 118.9590(10)°, V = 4372.5(4) Å 3, Z = 4 for 4. Complexes 3– 4 are both dinuclear Zn–Nd structures, while suitable choice of chiral Schiff-base ligands could induce the different complexions of ligands and metal ions, and the functional control of ligand character shows a potentially effective way to the fine-tuning properties of NIR luminescence from Nd ions.

Synthesis and spectroscopic characterization of asymmetric Schiff bases derived from 4′-formylbenzo-15-crown-5 containing recognition sites for alkali and transition metal guest cations

Asymmetric salen-type Schiff base ligands have been synthesized via a stepwise approach. In the first step, mono-Schiff base compounds were prepared by condensation of salicylaldehyde, 2-hydroxy-3-methoxybenzaldehyde, 2-hydroxy-5-methoxybenzaldehyde and 2-hydroxy-1-naphthaldehyde with hydrazine hydrate. These compounds were then reacted with 4′-formylbenzo-15-crown-5 to prepare asymmetric ligands. 1H-NMR spectra indicate that the mono- and asymmetric bis-Schiff base compounds exist in both (E) and (Z) isomeric forms in CDCl3 solution. The asymmetric crown compounds form crystalline 1:1 (Na+:ligand) complexes with sodium perchlorate. Homo-metallic Ni(II) and Zn(II) complexes with 1:2 (metal:ligand) stoichiometries have also been synthesized. The results indicate that the Schiff base ligands coordinate through the azomethine nitrogen and phenolic oxygen.

A new dinuclear CuII–SmIIIcomplex with a salen-type Schiff base ligand

The title complex (systematic name: diaqua­{6,6′-dimeth­oxy-2,2′-[1,2-ethanediylbis(nitrilo­methyl­idyne)]­diphenolato}­di­nitrato­samarium(III)copper(II) nitrate), [CuSm(C18H18N2O4)(NO3)2(H2O)2]NO3, is composed of a discrete dinuclear cation with a salen-type Schiff base ligand and one uncoordinated nitrate anion. The copper and samarium are doubly bridged by two phenolate O atoms provided by the Schiff base ligand. There are some classical inter­molecular hydrogen bonds (O—H⋯O), and some weak O⋯Cu and N⋯Cu inter­actions are also observed. These inter­actions generate a two-dimensional infinite layer.

Single-Chain Magnet (NEt4)[Mn2(5-MeOsalen)2Fe(CN)6] Made of MnIII−FeIII−MnIII Trinuclear Single-Molecule Magnet with an ST = 9/2 Spin Ground State

The cyano-bridged trinuclear compound, (NEt(4))[Mn(2)(salmen)(2)(MeOH)(2)Fe(CN)(6)] (1) (salmen(2)(-) = rac-N,N'-(1-methylethylene)bis(salicylideneiminate)), reported previously by Miyasaka et al. (ref 19d) has been reinvestigated using combined ac and dc susceptibility measurements. The strong frequency dependence of the ac susceptibility and the slow relaxation of the magnetization show that 1 behaves as a single-molecule magnet with an S(T) = (9)/(2) spin ground state. Its relaxation time (tau) follows an Arrhenius law with tau(0) = 2.5 x 10(-)(7) s and Delta(eff)/k(B) = 14 K. Moreover, below 0.3 K, tau saturates around 470 s, indicating that quantum tunneling of the magnetization becomes the dominant process of relaxation. (NEt(4))[Mn(2) (5-MeOsalen)(2)Fe(CN)(6)] (2) (5-MeOsalen(2)(-) = N,N'-ethylenebis(5-methoxysalicylideneiminate)) is a heterometallic one-dimensional assembly made of the trinuclear [Mn(III)(SB)-NC-Fe(III)-CN-Mn(III)(SB)] (SB is a salen-type Schiff-base ligand) motif similar to 1. Compound 2 has two types of bridges, a cyano bridge (-NC-) and a biphenolate bridge (-(O)(2)-), connecting Mn(III) and Fe(III) ions and the two Mn(III) ions, respectively. Both bridges mediate ferromagnetic interactions, as shown by modeling the magnetic susceptibility above 10 K with g(av) = 2.03, J(Mn)(-)(Fe)/k(B) = +6.5 K, and J'/k(B) = +0.07 K, where J' is the exchange coupling between the trimer units. The dc magnetic measurements of a single crystal using micro-SQUID and Hall-probe magnetometers revealed a uniaxial anisotropy (D(T)/k(B) = -0.94 K) with an easy axis lying along the chain direction. Frequency dependence of the ac susceptibility and time dependence of the dc magnetization have been performed to study the slow relaxation of the magnetization. A mean relaxation time has been found, and its temperature dependence has been studied. Above 1.4 K, both magnetic susceptibility and relaxation time are in agreement with the dynamics described in the 1960s by R. J. Glauber for one-dimensional systems with ferromagnetically coupled Ising spins (tau(0) = 3.7 x 10(-)(10) s and Delta(1)/k(B) = 31 K). As expected, at lower temperatures below 1.4 K, the relaxation process is dominated by the finite-size chain effects (tau'(0) = 3 x 10(-)(8) s and Delta(2)/k(B) = 25 K). The detailed analysis of this single-chain magnet behavior and its two regimes is consistent with magnetic parameters independently estimated (J'and D(T)) and allows the determination of the average chain length of 60 nm (or 44 trimer units). This work illustrates nicely a new strategy to design single-chain magnets by coupling ferromagnetically single-molecule magnets in one dimension.

Substituent effect on redox potential of nitrido technetium complexes with Schiff base ligand: Theoretical calculations

Theoretical calculations based on the density functional theory (DFT) were performed to understand the effect of substituents on the molecular and electronic structures of technetium nitrido complexes with salen type Schiff base ligands. Optimized structures of these complexes are square pyramidal. The electron density on a Tc atom of the complex with electron withdrawing substituents is lower than that of the complex with electron donating substituents. The HOMO energy is lower in the complex with electron withdrawing substituents than that in the complex with electron donating substituents. The charge on Tc atoms is a good measure that reflects the redox potential of [TcN(L)] complex.

Unsymmetrical salen-type ligands: high yield synthesis of salen-type Schiff bases containing two different benzaldehyde moieties

Salen-type Schiff base ligands incorporating two different benzylidene moieties and a diamine backbone were synthesized in high yield (80–90%) under mild conditions via a stepwise approach. In the first step, anhydrous hydrochloric acid was used to selectively protect one amino group of the vicinal diamine backbone. The resulting ammonium salt was added to a substituted benzaldehyde providing access to a mono-imine product. This compound reacted with an equivalent of a different benzaldehyde in the presence of triethylamine to afford an unsymmetrical salen-type ligand. This synthetic method allows facile access to a plethora of salen-type Schiff base ligands with easily tunable steric and electronic properties.

Synthesis and characterization of bulky salen-type complexes of Co, Cu, Fe, Mn and Ni with amphiphilic solubility properties

Several new bulky salen-type Schiff base ligands and their complexes with first-row transition metals Co, Cu, Fe, Mn and Ni have been synthesized and characterized. The ligands contain tert-butyl and methyl(triphenylphosphonium chloride) substituents in aromatic rings providing flexible solubility properties. Crystal structures of some of the complexes were determined.

Copper(I) complexes with N,O-donor Schiff base ligands related to intermediate forms of the TPQ cofactor in amine oxidases

Neutral complexes (PPh3)2Cu(L) were obtained from (PPh3)2Cu(NO3) and the deprotonated forms of the three Schiff base ligands 2-(benzylideneimino)phenol (BimOH), 4-(benzylideneimino)resorcinol (Bim(OH)2) and N-(2,4-dihydroxy-5-isopropylphenyl)acetamide (DipaH3). The ligands were designed to model aminated intermediate forms of the topaquinone (TPQ) cofactor in the enzymatic cycle of copper-dependent amine oxidases. The ligands L are coordinated to the metal centers through imine-N and phenolate-O donor atoms to form five-membered chelate rings, in contrast to the six-membered chelate rings of salen-type Schiff base ligands. Compound (Bim(OH)O)Cu(PPh3)2·0.5CH3OH was structurally analyzed, it exhibits intramolecular aryl–aryl interactions between the ligands and intermolecular hydrogen bonds involving the free phenolic hydroxyl substituent, the coordinated phenolate oxygen atom and the methanol solvate molecule.

Polymorphs of [(2S,3S)-α-Me-N,N'-bis(salicylidene)butane-2,3-diaminato]bis(pyridine)cobalt(III) tetrafluoroborate hydrate

The title compound, {2,2'-[butane-2,3-diylbis(nitrilo-1-ethylidyne) diphenolato - O,N, N,'O'} bis (pyridine-N)-cobalt(III) tetrafluoroborate hydrate, [Co(C 20 H 22 N 2 O 2 )(C 5 H 5 N) 2 ]BF 4 .xH 2 O or [Co{α-Me-sal 2 -(S,S)-bn}(py) 2 ]BF 4 .xH 2 O (py is pyridine), showed polymorphs of crystals grown at the same time; one crystallized in the hexagonal system (x = 1.5) with a very long c axis and the other was in the monoclinic system (x = 1.7). The cell axes of the two structures were related simply to each other by a m ≃ ah, b m ≃ a h + 2b h and c m ≃ c h /2, where the axes of the hexagonal and monoclinic cells are identified by subscripts h and m, respectively. The complex cations, as well as the molecular arrangements in both structures, are quite similar to each other. The cation adopts octahedral coordination formed by the tetradentate salen-type Schiff base ligand in the basal plane and two trans pyridine ligands. The cation has an approximate twofold axis of rotation through the Co atom. The two methyl groups of the C-C bridge take an antiperiplanar conformation. The counter-ion is located nearly in the basal plane of the cation. The cations form dimers via π-π stacking between pyridine rings. Layers of cation dimers and anions stack alternately along c. The hexagonal structure includes 1.5 water molecules per Co atom and the monoclinic structure has 1.7 water molecules.

Spectroscopic and magnetic properties and structure of a five-coordinate, O 2-binding cobalt(II) Schiff base complex and of the copper(II) analogue

The cobalt(II) complex CoL and the copper(II) complex CuL of the pentadentate O 2N 2N′ Salen-type Schiff base ligand H 2L = N, N′-bis(3-t-butyl-2-hydroxy-5-methylbenzyliden)-1, 7-diamino-4-methyl-4-azaheptane were prepared, characterized and subjected to single crystal X-ray analysis. The {MO 2N 2N′} coordination core forms a trigonal bipyramid differing in the degree of distortion, as expressed by the O-M-O angles 138.62(8)° [O-Co-O] and 155.21(7)° [O-Cu-O]. The distances between the metal and the aliphatic tertiary nitrogen atoms N′ are markedly different, 2.123(2) [Co-N′] and 2.300(2)Å[Cu-N′]. In contrast to normal Salen complexes the imine nitrogen atoms are located trans to each other. Solution room temperature electron spectra show a broad absorption in the near-IR range at 6350 (CoL) and 8200 (CuL) cm −1. The temperature dependence of the effective magnetic moment for high-spin CoL ( S =3/2) shows a maximum at 50 K, which is reproduced by the following set of magnetic parameters, reflecting the zero-field splitting; g 1 = 1.68, g 1 = 1.89, g = 3.21, D/hc = −38.9cm −1, E/hc = 1.7cm −1, α 110 = 0, zJ/hc = −1.00cm −1. Variable temperature electron paramagnetic resonance experiments prove that CoL, dissolved in toluene, N, N-dimethylformamide or pyridine, binds dioxygen reversibly.