Hexadentate Schiff Base Ligand Research Articles

Cooperative effects of ligands and Counteranions in mononuclear Manganese(III) Schiff-base spin crossover complexes

The flexible hexadentate Schiff-base ligands (R-sal-N-1,5,8,12)2− can be easily obtained by modifying the different substituents at the 5 or 3,5 positions on the phenyl rings of the salicylaldehyde. We report in this paper the syntheses, crystal structures and magnetic properties of three new mononuclear manganese(III) complexes [Mn(R-sal-N-1,5,8,12)]Y (R = 5-H, Y = BF4 (1); R = 3,5-diCl, Y = SbF6 (2); R = 5-tBu, Y = ClO4 (3)). All the complexes have an analogous monocationic MnN4O2 surrounding offered by the flexible hexadentate ligand in a distorted octahedral geometry. Magnetic susceptibility measurements demonstrated that complex 1 maintains a high-spin (HS) state between 2 and 400 K due to the closing packing, while complex 2 shows a cooperative and complete one-step thermo-induced spin state switching with T1/2 of 194 K. As for complex 3, single-crystral X-ray structural studies show the existence of a large amount of HS manganese(III) ions at 100 K and it exhibits a surprisingly gradual SCO process with a hysteresis width of 6 K at lower temperature.

Synthesis, Characterization and Breast Anti-cancer Activity of Iron(II), Cobalt(II), Nickel(II) and Copper(II) Complexes with a Hexadentate Schiff Base Ligand Derived from 2,5-Dihydroxy-1,4-benzoquinone with 5-Amino-2-methylphenol

The complexes of Fe(II), Co(II), Ni(II), and Cu(II) Schiff base ligand derived from 2,5-dihydroxy-1,4-benzoquinone and 5-amino-2-methylphenol were synthesized. The ligand was synthesized by the reaction between the mentioned ketone and amine in 1:2 molar ratio, respectively. The four metal complexes were synthesized by refluxing the ligand with the related metal(II) chloride salts. The synthesized compounds were characterized using FTIR spectroscopy, UV-visible, 1H-NMR, conductivity, atomic absorption, magnetic susceptibility, and thermogravimetric analysis. According to the results, the chelation between metals and ligand occurs with the imine groups and the deprotonated hydroxyl groups of 2,5-dihydroxy-1,4-benzoquinone and 5-amino-2-methylphenol in the ligand. The conductivity test of the four complexes shows the non-electrolytic nature of them. The magnetic susceptibility values of Fe(II), Co(II), Ni(II), and Cu(II) complexes are 4.20, 4.11, 2.97, and 2.34 B.M, respectively. The thermogravimetric and atomic absorption analyses suggest the general chemical formula for the complexes is [M2(L)(H2O)6]. In addition, the ligand and one of its metal complexes (Co(II) complex) were examined against breast cancer cells, and they gave the IC50 of 101.24 and 129.2 µg/mL, respectively. This result suggests that Co(II) complex is a better anti-cancer agent in comparison with the ligand.

The cooperative effects of ligand substituent positions and solvents on spin crossover in Mn(III) complexes based on hexadentate Schiff-base ligands

Spin crossover (SCO), as a molecular bistable phenomenon, has attracted extensive interest. In this paper, the syntheses, crystal structures and magnetic properties of three Mn(III) hexadentate Schiff-base complexes [Mn(3-OMe-sal-N-1,5,8,12)]AsF6·H2O (1), [Mn(4-OMe-sal-N-1,5,8,12)]AsF6·H2O (2) and [Mn(5-OMe-sal-N-1,5,8,12)]AsF6(3) have been reported. The substituent positions of ligands and solvent molecules have a cooperative effect on SCO behavior. Complex 1 shows a complete SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis, and complex 2 shows a gradual incomplete SCO behavior. The transition temperatures of 2 (T1/2 = 241 K) is 98 K higher than that of 1 (T1/2 = 143 K), which indicates that the ligand field strength of (4-OMe-sal-N-1,5,8,12)2– is stronger than that of (3-OMe-sal-N-1,5,8,12)2–. Due to the absence of crystallizing solvent water molecule, complex 3 maintains a HS state in the test temperature range.

Compressed and Expanded Lattices - Barriers to Spin-State Switching in Mn3+ Complexes.

We report the structural and magnetic properties of two new Mn3+ complex cations in the spin crossover (SCO) [Mn(R-sal2323)]+ series, in lattices with seven different counterions in each case. We investigate the effect on the Mn3+ spin state of appending electron-withdrawing and electron-donating groups on the phenolate donors of the ligand. This was achieved by substitution of the ortho and para positions on the phenolate donors with nitro and methoxy substituents in both possible geometric isomeric forms. Using this design paradigm, the [MnL1]+ (a) and [MnL2]+ (b) complex cations were prepared by complexation of Mn3+ to the hexadentate Schiff base ligands with 3-nitro-5-methoxy-phenolate or 3-methoxy-5-nitro-phenolate substituents, respectively. A clear trend emerges with adoption of the spin triplet form in complexes 1a-7a, with the 3-nitro-5-methoxy-phenolate donors, and spin triplet, spin quintet and thermal SCO in complexes 1b-7b with the 3-methoxy-5-nitro-phenolate ligand isomer. The outcomes are discussed in terms of geometric and steric factors in the 14 new compounds and by a wider analysis of electronic choices of Mn3+ with related ligands by comparison of bond length and angular distortion data of previously reported analogues in the [Mn(R-sal2323)]+ family. The structural and magnetic data published to date suggest a barrier to switching may exist for high spin forms of Mn3+ in those complexes with the longest bond lengths and highest distortion parameters. A barrier to switching from low spin to high spin is less clear but may operate in the seven [Mn(3-NO2-5-OMe-sal2323)]+ complexes 1a-7a reported here which were all low spin in the solid state at room temperature.

The cooperative effects of substituents and anions in Mn(III) spin crossover complexes with hexadentate Schiff-base ligands

In this paper, the syntheses, crystal structures and magnetic properties of spin crossover (SCO) salts of formula [Mn(4-OMe-sal-N-1,5,8,12)]SbF6‧0·45H2O (1), [Mn(3-OMe-sal-N-1,5,8,12)]SbF6‧H2O (2), [Mn(4-OMe-sal-N-1,5,8,12)]ClO4‧H2O (3) and [Mn(3-OMe-sal-N-1,5,8,12)]Br (4) have been reported. Magnetic susceptibility measurements demonstrate that complex 1 shows a gradual one-step SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis. The transition temperatures T1/2 of the 4-OMe-SbF6 (1) salt estimated from the magnetic susceptibility measurement is 213 K. As for complex 2, π···π stacking interactions lead to more close stacking which hinder the flexibility of the whole ligand and it is stable in the HS state in the temperature range of 2–300 K. Both complexes 3 and 4 exhibit incomplete spin crossover behavior. These results are consistent with Hirshfeld analysis. Electrochemical analysis indicates that the change of methoxy‑substituent position from 4 to 3 decreases the electron density on the metal center.

Rotating magnetocaloric effect in highly anisotropic TbIII and DyIII single molecular magnets

The magnetocaloric effect (MCE) was investigated in highly anisotropic single crystals of two single molecule magnets (SMMs): [LnIII(ZnIIL)2]CF3SO3, where Ln = Tb, Dy and L = tripodal hexadentate Schiff base ligand. The structure of these paramagnetic compounds consists of identically oriented linear trinuclear clusters in a trigonal system with an easy direction c∥Zn–Ln–Zn array and a hard plane ab⊥Zn–Ln–Zn array. The magnitude of MCE measured for c∥H was significantly greater than MCE for ab∥H at a wide temperature range regardless of the studied SMM. Therefore, the rotating magnetocaloric effect (RMCE) was evaluated. The maxima of the magnetic entropy change for RMCE were obtained at 2.0 K and moderate fields: 3.9 J K−1 kg−1 at µ0H = 1.3 T for Ln = Tb and 3.3 J K−1 kg−1 at µ0H = 1.1 T for Ln = Dy. The relative efficiency of RMCE compared to the MCE measured in c∥H was as high as 99% at low magnetic fields.

Effect of halogen substituents on the intermolecular interactions and magnetic properties of Mn(III) complexes

Six mononuclear Mn(III) complexes with hexadentate Schiff-base ligand, the products of the reaction between a N,N′-bis(3-aminopropyl)ethylenediamine or triethylenetetramine and 5-X-salicylaldehyde have been synthesized: the I− salts of complexes 1–3 exhibit a gradual SCO behavior, [Mn(5-Cl-sal-N-1,5,8,12)]I (1) shows a nearly complete SCO with T1/2 = 125 K, whereas [Mn(5-Br-sal-N-1,5,8,12)]I (2) and [Mn(5-I-sal-N-1,5,8,12)]I (3) show a gradual and incomplete SCO above room temperature. [Mn(5-F-sal-N-1,4,7,10)]I (4) predominantly remains HS state at all temperatures. [Mn(5-I-sal-N-1,5,8,12)]NO3 (5) and [Mn(5-F-sal-N-1,5,8,12)]NO3 (6) show an almost full conversion to the HS state and a more gradual SCO having the T1/2 value of 282 K for 5 and 159 K for 6, respectively.

Solid-state lattice effects on high-spin Mn(III) complexes with hexadentate Schiff-base ligand

ABSTRACT This paper reports the syntheses, crystal structures, magnetics and spectroscopic properties of high-spin (HS) salts of formula [Mn(salk-N-1,5,8,12)]Y·S (Y = AsF6 for 1, Y = BF4, S = CH3OH for 2, Y = PF6, S = CH3OH for 3), where (salk-N-1,5,8,12)2− (2,2ʹ-((2E,14E)-3,7,10,14-tetraazahexadeca-2,14-diene-2,15-diyl)diphenolate) is a hexadentate Schiff-base ligand. Crystal structural analysis indicates that the extensive hydrogen bonds play a primary role in stabilisation of their HS states. As an aid to understanding this behaviour, the crystal structure of the known spin crossover compound [Mn(sal-N-1,5,8,12)]AsF6 (4) has been re-examined. To understand the impact of intermolecular forces to the different magnetic behaviours of salts 1 and 4, we evaluate of their supramolecular structures and confirm that intermolecular interactions can significantly affect the cooperativity. Furthermore, electrochemical studies on 1 and 4 shows that the choice of spin state is controlled by the solid-state lattice effects, i.e. cooperativity rather than molecular electronic effect.

Synthesis, identification, antibacterial, and dyeing applications of complexes of hexadentate (N4O2 donor) Schiff base ligands derived from curcumin with some transition and non–transition metal cations

Schiff base ligand (H2CANPT) was prepared by two steps: first, by the condensation of curcumin with 4-amino antipyrin produces4,4′-(((1E,3Z,5Z,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta-1,6-diene-3,5-diylidene)bis(azanylylidene))bis(1,5-dimethyl-2-phenyl-1,2-dihydro-3H-pyrazol-3-one) (CANP). Second, by the condensation of (CANP) with L-tyrosine produces2,2′-(((3Z,3′Z)-(((1E,3Z,5Z,6E)-1,7-bis(4-hydroxy-3-methoxyphenyl)hepta 1,6-diene-3,5-diylidene)bis(azanylylidene))bis(1,5-dimethyl-2-phenyl-1,2-dihydro-3-H-pyra-zole-4-yl-3-ylidene))bis(azanylylidene))bis(3-(4-hydroxyphenyl)propanoic acid) (H2CANPT). The resulted Schiff comported as hexadentate coordinated with (N4O2) atoms, then it was treated with some transition and non-transaction metal ions, such as (Mn(II), Fe(III), Co(II), Ni(II), Cu(II), Cd(II), Hg(II) chloride and Ag(I) as nitrate to synthesize complexes. All the prepared compounds were identified using FT-IR, UV-visible, elemental analysis (C.H.N), 1H and 13C-NMR, molar conductivity and magnetic susceptibility. Schiff base and its complexes were tested against two types of bacteria (Staphylococcus aureus and Escherichia Coli). Besides, they were tested as dyes on cotton fibres.

New Mononuclear Mn(III) Complexes with Hydroxyl-Substituted Hexadentate Schiff Base Ligands

This paper reports the syntheses, crystal structures and magnetic properties of Mn(III) hexadentate Schiff base complexes [Mn(4-OH-sal-N-1,5,8,12)]NO3(1) and [Mn(4-OH-sal-N-1,5,8,12)]ClO4(2), where (4-OH-sal-N-1,5,8,12)2− (4,4′-((1E,13E)-2,6,9,13-tetraazatetradeca-1,13-diene-1,14-diyl)bis(3-methoxyphenol) is a new hydroxyl-substituted hexadentate Schiff base ligand. The introduction of the (4-OH-sal-N-1,5,8,12)2− ligand induces more hydrogen bonding interactions, in addition to promoting the formation of intermolecular interactions among the cations. However, the close-packing structures of both complexes lead to their stabilization in the high-spin state in the temperature range of 2−300 K.

Construction of a 18-Metal Neodymium(III) Nanoring with NIR Luminescent Sensing to Antibiotics.

One 18-metal Nd(III) nanoring, [Nd18(L1)8(HL2)2(OAc)20(MeOH)8(EtOH)6(H2O)4]·2(MeOH)·6(H2O) (1), was constructed by the use of a hexadentate Schiff base ligand. For 1, the near-infrared (NIR) luminescence of Nd(III) was detected under the excitation of absorption band at 371 nm. The study of luminescent sensing properties exhibits that, even with the existence of other antibiotics, this Nd(III) nanoring displays high sensitivity and selectivity to nitrofuran antibiotics (NFAs). The luminescence quenching constants and limits of detection of 1 to NFAs are found to be 1.4 × 104 to 3.5 × 104 M-1 and 0.9-2.2 μM, respectively.

Front Cover: Stabilization of Pristine Low Resistive State in a Ruthenium(II)‐Polymer of a Hexadentate Schiff Base Ligand: Synthesis, Characterization, and Application as Non‐Volatile Memory Device (Eur. J. Inorg. Chem. 32/2020)

Front Cover: Stabilization of Pristine Low Resistive State in a Ruthenium(II)‐Polymer of a Hexadentate Schiff Base Ligand: Synthesis, Characterization, and Application as Non‐Volatile Memory Device (Eur. J. Inorg. Chem. 32/2020)

Stabilization of Pristine Low Resistive State in a Ruthenium(II)‐Polymer of a Hexadentate Schiff Base Ligand: Synthesis, Characterization, and Application as Non‐Volatile Memory Device

Metallo‐supramolecular polymers offer ample opportunity to develop new emergent materials with the potential for applications as future memories. In this context, we have synthesized a novel hybrid polymer of our developed Schiff base ligand with Ru(II)‐metal ion. The main objective of this study is to understand the effect of changing central metal ion on the ligand field stabilization energy, and the band gap of material as the biting angle of organic moiety changes accordingly with a change in cationic radius of the central metal ion. Furthermore, we have also investigated the application in non‐volatile memory devices. Complete characterization of the newly synthesized hybrid polymer has been performed to establish its structure as well as oxidation state of the central metal ion. A comparative study of the present case and the previously reported Fe(II)‐polymer provides a clear insight into the effect of incorporating a bigger size metal ion in the polymer chain. Non‐volatile memory properties of the polymer have shown a rare behaviour of Negative Differential Resistance (NDR) along with the Low Resistive State (LRS) as its pristine state. Remarkable charge retention and endurance test have also been demonstrated.

Structure-function correlations in mononuclear manganese(iii) spin crossover systems with a big conjugated hexadentate Schiff-base ligand.

This paper reports the syntheses, crystal structures and magnetic properties of spin crossover (SCO) salts of formulae [Mn(naphth-sal-N-1,5,8,12)]SbF6 (1), [Mn(naphth-sal-N-1,5,8,12)]AsF6 (2), [Mn(naphth-sal-N-1,5,8,12)]PF6·1/2CH3OH (3) and [Mn(naphth-sal-N-1,5,8,12)]ClO4 (4), where (naphth-sal-N-1,5,8,12)2- (2,2'-((1E,14E)-2,6,10,14-tetraazapentadeca-1,14-diene-1,15-diyl)diphenolate) is a big conjugated hexadentate Schiff-base ligand. Magnetic susceptibility measurements demonstrated that complexes 1 and 2 showed a gradual one-step SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis. The transition temperatures T1/2 of the SbF6 (1) and AsF6 (2) salts estimated from the magnetic susceptibility measurements are 164 and 171 K, respectively. The existence of the crystal solvent of complex 3 changes the supramolecular packing, leading to close ππ stacking interactions between the phenyl groups of the ligands. These close stacking interactions hinder the flexibility of the whole ligand, precluding the spin transformation of complex 3 and leading to its stabilization in the HS state in the temperature range of 2-300 K. For 4, crystal structure analysis indicates that the reduction in the anion size leads to close contacts between the naphthalene rings. These C-Hπ interactions provide a means for preventing the spin crossover occurring at low temperatures.

Synthesis, characterization, DFT calculations and catalase-like enzymatic activities of novel hexadentate Schiff base and its manganese complexes

We reported the synthesis of the manganese(II) and manganese(III) complexes of a novel hexadentate Schiff base ligand (HBME) (1) obtained from the condensation of triethylenetetramine and 2 equiv. of 1-(biphenyl)-2-hydroxyimino-2-(morpholino)-1-ethanone. Hexadentate Schiff base ligand reacted with manganese(II) perchlorate or manganese(III) acetate and sodium azide, to give rise to a series of mono and dinuclear complexes [Mn(BME)2tren)] (2), [Mn2(BME)2tren(N3)2] (3) and [Mn2(BME)2tren(N3)4] (4). Synthesized molecules were characterized by elemental analyses, ICP-MS, spectroscopic studies, thermal analyses, molar conductivity and magnetic susceptibility measurements. The quantum chemical calculations of the HBME and complexes (2–4) were performed in the ground state using the DFT of B3LYP level with 6-311G(d,p) and LanL2DZ basis sets, respectively. 1H and 13C NMR chemical shifts were calculated by using the gauge independent atomic orbital (GIAO) method and the electronic properties such as HOMO-LUMO energies were determined by the time-dependent DFT approach. The calculated spectroscopic values for the HBME were in very good agreement with experimental ones. Furthermore, catalytic activity of the complexes for the disproportionation of hydrogen peroxide was also investigated in the presence of 1-methylimidazole. All the complexes displayed catalytic activity for the disproportionation of H2O2 significantly, but the activities of the (3) and (4) were relatively higher than the other molecules.

Magnetic and structural correlations in [Fe(nsal2trien)] salts: the role of cation–anion interactions in the spin crossover phenomenon

Cation–anion and cation–solvent–anion interactions determine the SCO behaviour of six [FeIII(nsal2trien)] salts.

Polyether Hexadentate Schiff Base Ligand with Trivalent Chromium, Iron, Cobalt Ions

Abstract: Williamson etherification method used to prepared ether compound from salicylaldehyde with 1,3-dibromopropane and potassium carbonate as base, this compound used with 2-amino-4-chlorophenol to prepare polyether hexadentate Schiff base ligand (HSL) by the 1:2 molar condensations. Cr 3+, Fe 3+, Co 3+ ions were reacted with HSL and characterized by 1H NMR and 13CNMR technique, infrared, electronic spectra, elemental analysis, thermal analysis, conductivity and magnetic susceptibility measurements, from results the metal ion was bonded to the HSL through the phenol oxygen, imine nitrogen and ether oxygen atoms to suggest octahedral configurations. The biological activity of HSL with its complexes have been studied.

Synthesis and Characterisation of Transition Metal Complexes with new Schiff base Ligand

A new hexadentate Schiff base ligand derived from condensation of 4-aminoantipyrine with isophthaldehyde and thiosemicarbazide, and its metal complexes were characterized by elemental analyses, molar conductance, magnetic susceptibility measurement and spectral ( IR, Uv-Vis,1H NMR, Mass, EPR) studies. It has been originated that the Schiff base ligand with Co (II), Ni (II),Cu (II) and Zn (II) ions from mononuclear complexes on 1 : 1 (metal : ligand) stoichiometry. The conductivity data confirm the electrolytic nature of the complexes. The Co (II), Ni (II), and Zn (II) complexes show an octahedral configuration. Electronic absorption spectra of the Cu (II) complex suggest a square- planar geometry around the central metal ion. The IR spectral data suggest the involvement of sulphur and azomethine nitrogen in coordination to the central metal ion. The synthesized complexes have been subjected to antimicrobial study. The antimicrobial results indicated that the antimicrobialactivity of thecomplexesshow theenhanced activity in comparison to free ligand.These findings are giving suitable support for developing new antimicrobial agent and expand our scope for applications.

Syntheses and Structure Investigations of 3d Transition Metal Complexes with a Flexible N4O2-Donor Hexadentate Schiff-Base Ligand

The syntheses and structure investigations of four new 3d transition metal complexes (1–4) with a flexible N4O2-donor hexadentate Schiff-base ligand are described; three complexes (1, 2, and 4) of FeIII, CoIII, and CuII metal ions have been investigated by UV-vis, FT-IR, high-resolution mass spectrometry (HR-MS), and scanning electron microscopy–electron dispersive spectroscopy, as well as single crystal X-ray diffraction. The X-ray structure of NiII complex 3 is also reported. The molecular structures of the complexes (1–3) demonstrate distorted octahedral coordination geometry, each exhibiting 1 : 1 (M : L) ratios and the CuII complex 4 shows a trinuclear structure with a CuII : L ratio of 3 : 2 in the solid state, which has been proven by X-ray diffraction. On the other hand, a mononuclear species of the CuII complex is formed in solution, which has been identified by electrospray ionization HR-MS.

Triple-Stranded Cluster Helicates for the Selective Catalytic Oxidation of C-H Bonds.

Triple-stranded cluster helicates with heptametallic dicubane cores are synthesized by entrapping metals in the cavities of linear triple helicates based on a C2-symmetrical hexadentate Schiff-base ligand of ortho-substitued biphenol. The helicates are stable in both the solution and solid states, and the copper species could selectively catalyze the oxidation of C-H bonds of alkanes to ketones.