Substituent effects on spin-crossover Fe(II)N4O2 pyrenylhydrazone complexes

Multifunctional magnetic materials have broad application prospects in molecular switches and information storage. In this study, four mononuclear Fe(II) complexes are synthesized using a series of pyrenylhydrazone ligands HL1-4. Two deprotonated ligands are coordinated to the iron(II) ions in an enolic form, leading to neutral complexes FeII(Lx)2·xsol with a FeIIN4O2 octahedral coordination environment. Magnetic measurements suggest that complex Fe(L1)2·2ACE (1·2ACE, ACE = acetone) is mainly low spin below 300 K and complex Fe(L3)2·ACE (3·ACE) is high spin, whereas complexes Fe(L2)2 (2) and Fe(L4)2·6H2O (4·6H2O) exhibit gradual spin crossover behavior. The spin states of complexes 1-4 are confirmed by single-crystal X-ray diffraction analysis. The substituent effect on the magnetic properties of the complexes is significant in this system. Temperature-dependent fluorescence emission spectra show the coexistence but no coupling effect of spin crossover and fluorescence for complexes 2 and 4·6H2O.

Selective cyanide sensing using a Fe(III) complex of pyridoxal-beta alanine Schiff base

Fluorogenic chemosensors using pyridoxal derivatized ligands as fluorophore is a rapidly growing field of research. Here we report a new Fe(III) complex, [Fe(HBala-pydx)(Bala-pydx)] (H₂Bala-pydx is the Schiff base of pyridoxal with beta-alanine), which can serve as a sensitive and selective turn-on fluorescent sensor for the detection of cyanide(CN^-) in micromolar concentrations (L.O.D. is 1.134 µM), via the ligand displacement approach, in aqueous-acetonitrile medium. The Fe(III) complex is adequately characterized by analytical and spectroscopic methods along with X-ray crystal structure determination.

Mononuclear Iron(III) Complex with Unusual Temperature Change of Color and Magneto-Structural Properties: Synthesis, Structure, Magnetization, Multi-frequency ESR and DFT Study

From the reaction of 2-hydroxy-6-methylpyridine (L) with iron(II) tetrafluoroborate, a new mononuclear iron(III) octahedral complex [FeL6](BF4)3 has been isolated. The color of the complex is reversible changing from red at...

Thermodriven, Acidity-Driven, and Photodriven Spin-State Switching in Pyridylacylhydrazoneiron(II) Complexes at or above Room Temperature

The magnetic bistability of spin-crossover (SCO) materials is highly appealing for applications as molecular switches and information storage. However, switching of the spin state around room temperature remains challenging. In this work, we reported the successful manipulation of the spin states of two iron(II) complexes (1-Fe and 2-Fe) based on pyridylacylhydrazone ligands in manifold ways. Both complexes are stabilized in the low-spin (LS) state at room temperature because of the strong ligand-field strength imposed by the ligands. 2-Fe shows thermoinduced SCO above room temperature with a very large and reproducible hysteresis (>50 K), while 1-Fe remains in the LS state up to 400 K. Acidity-driven spin-state switching of the two complexes was achieved at room temperature as a result of the complex dissociation and release of iron(II) in its high-spin (HS) state. Recovery of the complex is feasible upon further alkalization treatment in the case of 1-Fe, allowing bidirectional modulation of the spin state of the metal center. Light-driven one-way switching from LS to HS is also achieved by virtue of E-to-Z isomerization at the C═N double bond, which results in dissociation of the complex because of the poor binding affinity in the Z configuration.

Potassic-Hastingsite from the Kedrovy District (East Siberia, Russia): Petrographic Description, Crystal Chemistry, Spectroscopy, and Thermal Behavior

In this work we report on a petrographic, crystal-chemical, and optical characterization, obtained from different analytical methods, of amphibole species. Potassic-hastingsite, ideally AKBCa2C(Fe2+4Fe3+)T(Si6Al2)O22W(OH)2, has been found in the Kedrovy district (East Siberia, Russia). The sample occurs as well-formed and large radially radiant aggregates of dark green, almost black crystals. The unit cell dimensions are a = 9.9724(3) Å, b = 18.2968(4) Å, c = 5.3573(1) Å, β = 104.945(3)°, V = 944.44(4) Å3, Z = 2. Site populations were determined by combining single-crystal structure refinement and electron probe microanalysis, and Fe3+/Fe2+ ratio was obtained from X-ray fluorescence analysis. Infrared, diffuse light UV/Vis/NIR absorption, and electron spin resonance spectra are presented and discussed. A thermoelastic behavior of a powder of potassic-hastingsite was studied by in situ high-temperature X-ray diffraction. A thermal expansion and subsequent significant contraction in the unit cell volume during a high-temperature X-ray powder diffraction experiment is observed as a consequence of the deprotonation process, which is locally balanced via oxidation of Fe2+. According to the data obtained for potassic-hastingsite, these processes occur within 400–600 °C. The thermal expansion of the mineral is anisotropic; the thermal expansivity coefficients αa:αb:αc (×10−6) = −18.06:9.59:−1.09 at 400 °C, −26.15:−1.52:2.22 at 600 °C and 23.77:−25.06:42.08 at 750 °C.

Iron Complexes of an Antiproliferative Aroyl Hydrazone: Characterization of Three Protonation States by Electron Paramagnetic Resonance Methods

Tridentate aroyl hydrazones are effective metal chelators in biological settings, and their activity has been investigated extensively for medicinal applications in metal overload, cancer, and neurodegenerative diseases. The aroyl hydrazone motif is found in the recently reported prochelator (AH1-S)2, which has shown antiproliferative proapoptotic activity in mammalian cancer cell lines. Intracellular reduction of this disulfide prochelator leads to the formation of mercaptobenzaldehyde benzoylhydrazone chelator AH1 and to iron sequestration, which in turn impacts cell growth. Herein, we investigate the iron coordination chemistry of AH1 to determine the structural and spectroscopic properties of the iron complexes in the solid state and in solution. A neutral iron(III) complex of 2:1 ligand-to-metal stoichiometry was isolated and characterized fully to reveal two different binding modes for the tridentate AH1 ligand. Specifically, one ligand binds in the monoanionic keto form, whereas the other ligand coordinates as a dianionic enolate. Continuous-wave electron paramagnetic resonance experiments in frozen solutions indicated that this neutral complex is one of three low-spin iron(III) complexes observed depending on the pH of the solution. Electron spin echo envelope modulation (ESEEM) experiments allowed assignment of the three species to different protonation states of the coordinated ligands. Our ESEEM analysis provides a method to distinguish the coordination of aroyl hydrazones in the keto and enolate forms, which influences both the ligand field and overall charge of the complex. As such, this type of analysis could provide valuable information in a variety of studies of iron complexes of aroyl hydrazones, ranging from the investigation of spin-crossover behavior to tracking of their distribution in biological samples.

Spin Crossover in Bipyridine Derivative Bridged One-Dimensional Iron(III) Coordination Polymer

Herein, the syntheses, solid-state molecular structures, and characterization of two types of one-dimensional FeIII coordination polymers showing thermally induced spin crossover are reported. The reaction of [Fe(acen)Cl] (acen2− = N,N′-ethylenebis(acetylacetonylideneaminate) with 3,3′-bpy or 4,4′-bpy (bpy = bipyridine) produced zigzag and linear one-dimensional chain complexes, [Fe(acen)(3,3′-bpy)][BPh4] (1) or [NEt3H][Fe(acen)(4,4′-bpy)][BPh4]2·0.5(4,4′-bpy) (2), respectively, as confirmed by single crystal X-ray diffraction analysis. Variable-temperature single crystal X-ray diffraction measurements, continuous-wave X-band electron paramagnetic resonance (EPR) spectra, 57Fe Mössßauer spectra, and DC magnetic susceptibility data revealed that complex 1 exhibited a gradual and complete spin crossover at a transition temperature of 212 K, while complex 2 undergoes an incomplete spin crossover even at 400 K.

Crystal structure of the mixed methanol and ethanol solvate of bis-{3,4,5-trimeth-oxy-N'-[1-(pyridin-2-yl)ethyl-idene]benzohydrazidato}zinc(II)

The unit cell of the title compound, [Zn(C17H18N3O4)2]·CH4O·C2H6O, contains two complex mol-ecules related by an inversion centre, plus one methanol and one ethanol solvent molecule per complex molecule. In each complex, two deprotonated pyridine aroylhydrazone ligands {3,4,5-trimeth-oxy-N'-[1-(pyridin-2-yl)ethyl-idene]benzohydrazide} coordinate to the ZnII ion through the N atoms of the pyridine group and the ketamine, and, additionally, through the O atom of the enolate group. In the crystal, dimers are formed by π-π inter-actions between the planar ligand moieties, which are further connected by C⋯O and C⋯C inter-actions. The inter-molecular inter-actions were investigated using Hirshfeld surface analysis and two-dimensional fingerprint plots, revealing that the most important contributions for the crystal packing are from H⋯H (44.8%), H⋯C/C⋯H (22.2%), H⋯O/O⋯H (18.7%) and C⋯C (3.9%) inter-actions.

Cu(ii)-induced twisting of the biphenyl core: exploring the effect of structure and coordination environment of biphenyl-based chiral copper(ii) complexes on interaction with calf-thymus DNA

Biphenyl core-based clip-like receptors get twisted after complexation with Cu²⁺. The extent of interaction of the optically active complexes with ct-DNA varies depending on the structure and coordination environment.

Observation of Proton Transfer Coupled Spin Transition and Trapping of Photoinduced Metastable Proton Transfer State in an Fe(II) Complex

An important technique to realize novel electron- and/or proton-based functionalities is to use a proton-electron coupling mechanism. When either a proton or electron is excited, the other one is modulated, producing synergistic functions. However, although compounds with proton-coupled electron transfer have been synthesized, crystalline molecular compounds that exhibit proton-transfer-coupled spin-transition (PCST) behavior have not been reported. Here, we report the first example of a PCST Fe(II) complex, wherein the proton lies on the N of hydrazone and pyridine moieties in the ligand at high-spin and low-spin Fe(II), respectively. When the Fe(II) complex is irradiated with light, intramolecular proton transfer occurs from pyridine to hydrazone in conjunction with the photoinduced spin transition via the PCST mechanism. Because the light-induced excited high-spin state is trapped at low temperatures in the Fe(II) complex-a phenomenon known as the light-induced excited-spin-state trapping effect-the light-induced proton-transfer state, wherein the proton lies on the N of hydrazone, is also trapped as a metastable state. The proton transfer was accomplished within 50 ps at 190 K. The bistable nature of the proton position, where the position can be switched by light irradiation, is useful for modulating proton-based functionalities in molecular devices.

Multiple adaptation of constitutional dynamic networks and information storage in constitutional distributions of acylhydrazones

We report a study of the behavior of four dynamic covalent libraries (DCLs) based on acylhydrazones aAbB and of the corresponding square constitutional dynamic networks (CDNs) NA-ND under the effect of three agents, namely, metal cations, base + metal cations and light irradiation; in particular, the successful switching of the CDN NB between two orthogonal distributions results, respectively, from metallo-selection and photo-selection. The four DCLs undergo triple adaptation when subjected to the three agents with the generation of specific CDN distributions characteristic of each of the four DCLs. The ternary outputs displayed by the DCLs present three states (-1, 0 and 1) related to three different constitutional distributions expressed in response to the triple inputs applied. This latter process amounts to the storage of molecular information in dynamic distributions rather than in selective interactions between complementary entities undergoing molecular recognition.

New Pyrazole-Hydrazone Derivatives: X-ray Analysis, Molecular Structure Investigation via Density Functional Theory (DFT) and Their High In-Situ Catecholase Activity

The development of low-cost catalytic systems that mimic the activity of tyrosinase enzymes (Catechol oxidase) is of great promise for future biochemistry technologic demands. Herein, we report the synthesis of new biomolecules systems based on hydrazone derivatives containing a pyrazole moiety (L1-L6) with superior catecholase activity. Crystal structures of L1 and L2 biomolecules were determined by X-ray single crystal diffraction (XRD). Optimized geometrical parameters were calculated by density functional theory (DFT) at B3LYP/6-31G (d, p) level and were found to be in good agreement with single crystal XRD data. Copper (II) complexes of the compounds (L1-L6), generated in-situ, were investigated for their catalytic activities towards the oxidation reaction of catechol to ortho-quinone with the atmospheric dioxygen, in an attempt to model the activity of the copper containing enzyme tyrosinase. The studies showed that the activities depend on four parameters: the nature of the ligand, the nature of counter anion, the nature of solvent and the concentration of ligand. The Cu(II)-ligands, given here, present the highest catalytic activity (72.920 μmol·L^-1·min^-1) among the catalysts recently reported in the existing literature.

Synthesis, characterization and antitumor activity of Ln(III) complexes with hydrazone Schiff base derived from 2-acetylpyridine and isonicotinohydrazone

In the present study, two isostructural lanthanide (Ln)(III) complexes, namely Ln(HL)₂(NO₃)(CH₃OH)₂)·CH₃OH, where Ln = La in complex 1 and Ce in complex 2, and hydrogen ligand (HL) = (E)-N'-[1-(2-pyridinyl)ethylidene]isonicotinohydrazone, have been isolated and characterized by elemental analysis, infrared spectra and single-crystal X-ray diffraction analysis. The results revealed that the acylhydrazone ligand HL in each complex was deprotonated as an anionic ligand and coordinated to the central La(III) ion via enolization of oxygen and nitrogen atoms. Furthermore, the antitumor effects and potential mechanisms of the two complexes were explored in the human lung cancer cell line A549 and in the human gastric cancer cell lines BGC823 and SGC7901. In the present study, the roles the two complexes on the proliferation and apoptosis of the above tumor cell lines were determined by MTT assay and Annexin V/propidium iodide flow cytometry, respectively. Furthermore, various apoptosis-associated key genes, including caspase 3, B cell lymphoma (Bcl)-2-associated X protein (Bax) and Bcl-2, were detected by western blotting to explore the possible antitumor mechanisms of the two complexes. The results revealed that the two complexes had comparable antitumor activities in terms of inhibiting proliferation and inducing apoptosis in tumor cell lines. The changes in the protein expression levels of caspase 3, Bax and Bcl-2 further verified the apoptosis-promoting mechanisms of the two complexes in tumor cell lines. These findings have a great potential in biomedical applications of novel Ln(III) complexes.

Iron(iii) complexes of 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol as spin-crossover molecular materials

Four new mononuclear Fe(iii) complexes are obtained by using 2-methyl-6-(pyrimidin-2-yl-hydrazonomethyl)-phenol. The perchlorate complex displays spin crossover with a hysteresis of 32 K, while the neutral complex exhibits a gradual incomplete spin transition.

Preparation of dihydroquinazoline carbohydrazone Fe(ii) complexes for spin crossover

Mononuclear spin-crossover complexes of dihydroquinazoline derivatives were synthesized to enable the spin crossover phenomenon via the variation of the solvent, anion and substituent.

Delicate modulated assembly of a new kind of trinuclear copper(ii) motif governed by N-containing agents

A new kind of trinuclear cupric motif was preparedin situby adopting a novel multidentate bihydrazide ligand, leading to five assembly styles that were governed by N-containing agents.

Absorption of CO2 and CS2 into the Hofmann-type porous coordination polymer: electrostatic versus dispersion interactions

Absorption of CO2 and CS2 molecules into the Hofmann-type three-dimensional porous coordination polymer (PCP) {Fe(Pz)[Pt(CN)4]}n (Pz = pyrazine) was theoretically explored with the ONIOM(MP2.5 or SCS-MP2:DFT) method, where the M06-2X functional was employed in the DFT calculations. The binding energies of CS2 and CO2 were evaluated to be -17.3 and -5.2 kcal mol(-1), respectively, at the ONIOM(MP2.5:M06-2X) level and -16.9 and -4.4 kcal mol(-1) at the ONIOM(SCS-MP2:M06-2X) level. It is concluded that CS2 is strongly absorbed in this PCP but CO2 is only weakly absorbed. The absorption positions of these two molecules are completely different: CO2 is located between two Pt atoms, whereas one S atom of CS2 is located between two Pz ligands and the other S atom is between two Pt atoms. The optimized position of CS2 agrees with the experimentally reported X-ray structure. To elucidate the reasons for these differences, we performed an energy decomposition analysis and found that (i) both the large binding energy and the absorption position of CS2 arise from a large dispersion interaction between CS2 and the PCP, (ii) the absorption position of CO2 is mainly determined by the electrostatic interaction between CO2 and the Pt moiety, and (iii) the small binding energy of CO2 comes from the weak dispersion interaction between CO2 and the PCP. Important molecular properties relating to the dispersion and electrostatic interactions, which are useful for understanding and predicting gas absorption into PCPs, are discussed in detail.

High-temperature spin crossover behavior in a nitrogen-rich Fe(III)-based system

A nitrogen-rich ligand bis(1H-tetrazol-5-yl)amine (H(3)bta) was employed to isolate a new Fe(III) complex, Na(2)NH(4)[Fe(III)(Hbta)(3)]·3DMF·2H(2)O (1). Single crystal X-ray diffraction revealed that complex 1 consists of Fe(III) ions in an octahedral environment where each metal ion is coordinated by three Hbta(2-) ligands forming the [Fe(III)(Hbta)(3)](3-) core. Each unit is linked to two one-dimensional (1-D) Na(+)/solvent chains creating a two-dimensional (2-D) network. In addition, the presence of multiple hydrogen bonds in all directions between ammonium cation and ligands of different [Fe(III)(Hbta)(3)](3-) units generates a three-dimensional (3-D) network. Magnetic measurements confirmed that the Fe(III) center undergoes a Spin Crossover (SCO) at high temperature (T(1/2) = 460(10) K).

Accessibility and selective stabilization of the principal spin states of iron by pyridyl versus phenolic ketimines: modulation of the 6A1 ↔ 2T2 ground-state transformation of the [FeN4O2]+ chromophore

Several potentially tridentate pyridyl and phenolic Schiff bases (apRen and HhapRen, respectively) were derived from the condensation reactions of 2-acetylpyridine (ap) and 2'-hydroxyacetophenone (Hhap), respectively, with N-R-ethylenediamine (RNHCH(2)CH(2)NH(2), Ren; R = H, Me or Et) and complexed in situ with iron(II) or iron(III), as dictated by the nature of the ligand donor set, to generate the six-coordinate iron compounds [Fe(II)(apRen)(2)]X(2) (R = H, Me; X(-) = ClO(4)(-), BPh(4)(-), PF(6)(-)) and [Fe(III)(hapRen)(2)]X (R = Me, Et; X(-) = ClO(4)(-), BPh(4)(-)). Single-crystal X-ray analyses of [Fe(II)(apRen)(2)](ClO(4))(2) (R = H, Me) revealed a pseudo-octahedral geometry about the ferrous ion with the Fe(II)-N bond distances (1.896-2.041 Å) pointing to the (1)A(1) (d(π)(6)) ground state; the existence of this spin state was corroborated by magnetic susceptibility measurements and Mössbauer spectroscopy. In contrast, the X-ray structure of the phenolate complex [Fe(III)(hapMen)(2)]ClO(4), determined at 100 K, demonstrated stabilization of the ferric state; the compression of the coordinate bonds at the metal center is in accord with the (2)T(2) (d(π)(5)) ground state. Magnetic susceptibility measurements along with EPR and Mössbauer spectroscopic techniques have shown that the iron(III) complexes are spin-crossover (SCO) materials. The spin transition within the [Fe(III)N(4)O(2)](+) chromophore was modulated with alkyl substituents to afford two-step and one-step (6)A(1) ↔ (2)T(2) transformations in [Fe(III)(hapMen)(2)]ClO(4) and [Fe(III)(hapEen)(2)]ClO(4), respectively. Previously, none of the X-salRen- and X-sal(2)trien-based ferric spin-crossover compounds exhibited a stepwise transition. The optical spectra of the LS iron(II) and SCO iron(III) complexes display intense d(π) → p(π)* and p(π) → d(π) CT visible absorptions, respectively, which account for the spectacular color differences. All the complexes are redox-active; as expected, the one-electron oxidative process in the divalent compounds occurs at higher redox potentials than does the reverse process in the trivalent compounds. The cyclic voltammograms of the latter compounds reveal irreversible electrochemical generation of the phenoxyl radical. Finally, the H(2)salen-type quadridentate ketimine H(2)hapen complexed with an equivalent amount of iron(III) to afford the μ-oxo-monobridged dinuclear complex [{Fe(III)(hapen)}(2)(μ-O)] exhibiting a distorted square-pyramidal geometry at the metal centers and considerable antiferromagnetic coupling of spins (J ≈ -99 cm(-1)).