Bridging Halogen Research Articles

Synthesis, characterization, crystal structure, hirshfeld surface analysis and chromotropic properties of halo- bridged copper(II) complexes

In this study, two novel chromotropic binuclear copper(II) complexes were introduced. The complexes were denoted as [LCu(μ-Cl)Cl]2, 1 and [LCu(μ-Br)Br]2, 2 with L representing the chelating ligand N,N-dimethyl-N'-(9-methylanthracenyl)ethylenediamine. To investigate the structures of these complexes, various analytical techniques were employed, including FT-IR, EPR, UV–Vis spectroscopy, TG-DTA, molar conductivity measurements, and elemental analysis. The geometrical property of [LCu(μ-Br)Br]2 was determined using X-ray crystallography, while the intermolecular interactions were studied through Hirshfeld surface analysis. The characterization data provided strong evidence supporting the proposed binuclear structure, wherein two copper (II) centers are connected by two halogen bridges. Remarkably, these complexes exhibit solvatochromism and thermochromism properties, displaying a diverse range of colors in solutions with varying polarities and/or temperatures. Experimental findings suggest that the observed chromotropism can be attributed to the substitution of solvent molecules with the coordinated halogens in the complexes.

Key factors for connecting silver-based icosahedral superatoms by vertex sharing

Metal nanoclusters composed of noble elements such as gold (Au) or silver (Ag) are regarded as superatoms. In recent years, the understanding of the materials composed of superatoms, which are often called superatomic molecules, has gradually progressed for Au-based materials. However, there is still little information on Ag-based superatomic molecules. In the present study, we synthesise two di-superatomic molecules with Ag as the main constituent element and reveal the three essential conditions for the formation and isolation of a superatomic molecule comprising two Ag13−xMx structures (M = Ag or other metal; x = number of M) connected by vertex sharing. The effects of the central atom and the type of bridging halogen on the electronic structure of the resulting superatomic molecule are also clarified in detail. These findings are expected to provide clear design guidelines for the creation of superatomic molecules with various properties and functions.

A hybrid quantum dot:MXene bulk heterojunction for an efficient infrared self-powered photodetector

A Br-MXene:PbS QD bulk heterostructure (BHJ) with an intermediate halogen bridging is developed to boost charge transfer at the donor/acceptor interface. The photodetector based on this BHJ exhibits an enhanced photoresponse performance.

Switching from Oxygen Quenching Resistance to Linear Response by Smart Luminescent Iridium(III)-Based Metal-Organic Frameworks.

In this study, we utilize a photo-active Ir-metalloligand, Ir(C^N)2(L) (C^N = 2-(2,4-difluorophenyl) pyridine, L = [2,2'-bipyridine]-5,5'-dicarboxylic acid), to assemble with CdX2 under hydrothermal conditions, yielding highly emissive crystals of two-dimensional metal-organic frameworks (2D MOFs) (named Ir-Cd2X2, X = Cl, Br). The Ir-Cd2X2 MOFs exhibit μs-level phosphorescence lifetimes and more than 55% quantum yield (QY) at room temperature because of the rigid framework connected by Cd2X2 clusters. By immersing Ir-Cd2X2 in water solution for 5 min, a new MOF (Ir-Cd) was obtained, which is given a structure with hydrolyzed Cd-nodes by complete removal of halogen bridges as elucidated by single-crystal diffraction. Although the phosphorescence emission of pristine CdX2 MOFs exhibits oxygen quenching resistance, the converted Ir-Cd MOF possesses sensitively oxygen-responsive 3MLCT properties, showing a KSV value as high as 14.5 with strictly linear relation (R2 = 0.995). This work differs from the traditional method for improving oxygen-sensing metrics by enhancing QY and phosphorescence lifetime in Ir complexes, while also demonstrating that the transformation in the surrounding coordination environment on adjacent metal centers can also constitute key factors for improved photoluminescence stability or responsive properties in Ir-based heteronuclear MOFs, providing clues for the development of either oxygen quenching blockers or sensors suitable for different occasions.

Electrochemical CO2 reduction improved by tuning the Cu-Cu distance in halogen-bridged dinuclear cuprous coordination polymers

Cu dinuclear coordination polymers in which two Cu atoms are bridged by a halogen reduce CO2 to C2 products. The distance between the two Cu atoms is expected to affect the C–C coupling reaction efficiency, herein we propose that a shorter Cu-Cu distance and desorption of bridging halogen elements from Cu atoms facilitate C2 formation from experimental results and DFT calculations. In many previous studies, the influence of factors other than the organic ligands has been overlooked in the structural design of metal complexes. The C2 product selectivity is greatly influenced by surrounding elements as well as organic ligands.

A rare case of a 2:2:1 ternary cocrystal of pyridine sulfides and trithiocyanuric acid.

We report a rare case of a 2:2:1 ternary cocrystal consisting of two trithiocyanuric acid molecules, two bis(pyridin-4-yl) sulfide molecules and 1,4-bis(pyridin-4-yl)tetrasulfane, namely, 1,3,5-triazinane-2,4,6-trithione-4-(pyridin-4-ylsulfanyl)pyridine-1,4-bis(pyridin-4-yl)tetrasulfane (2/2/1), 2C3H3N3S3·2C10H8N2S·C10H8N2S4. This interesting crystal structure with five neutral molecules per asymmetric unit was synthesized and characterized by means of X-ray diffraction (XRD) experiments and quantum-chemical modelling. Among various specific interactions, hydrogen and halogen bridges have a significant role in stabilizing the crystal structure. In particular, the role played by stacking interactions has been revealed by structure analysis and theoretical calculations. Crystallization was spontaneous and reproducible. One of the components, 1,4-bis(pyridin-4-yl)tetrasulfane, has been characterized by XRD for the first time.

Novel binuclear antimony(III) halide complexes of 5-methoxy-2-mercaptobenzimidazole: synthesis, structural characterization, and biological studies

The new binuclear antimony(III) complexes corresponding to the formulas [SbCl3(µ2-S)(MtMBZIM)2]2 (1), {[SbBr2(µ2-Br)(MtMBZIM)2]2·H2O} (2) and {[SbI2(µ2-I)(MtMBZIM)2]2·H2O} (3) (MtMBZIM: 5-methoxy-2-mercaptobenzimidazole) have been synthesized and structurally characterized with regard to their melting point, elemental analysis, molar conductivity, FT-IR, and FT-Raman spectroscopic techniques, and TG-DTA analysis. The crystal structures of 1-3 have been determined by single-crystal X-ray diffraction. Compounds 1-3 are doubly bridged dimers and in each square pyramidal monomeric unit, the equatorial plane is formed by two sulfur and two halogen atoms in trans-S, trans-X arrangement. The monomeric units in 2 and 3 are linked to each other via halogen bridges, but in 1, they are linked to each other via sulfur bridges; to our knowledge, this binding type is the first example of trans-S, trans-X square pyramidal antimony(III) complexes. Newly synthesized complexes with their corresponding ligand have been tested for their in vitro cytotoxic activity against human adenocarcinoma cervix (HeLa) cells as well as for their antimicrobial activity. The influence of 1-3 on the catalytic peroxidation of the linoleic acid by the enzyme lipoxygenase (LOX) has been determined experimentally and theoretically.

Copper(I) chloride and copper(I) perchlorate π-complexes with 2-allylthio-5-methyl-1,3,4-thiadiazole: synthesis and crystal structure

By means of the alternating-current electrochemical technique two copper(I) π,σ-compounds [Cu 4 Cl 4 ( Almethia ) 2 ] ( 1 ) and [Cu 2 ( Almethia ) 3 ](ClO 4 ) 2 ( 2 ) with 2-allylthio-5-methyl-1,3,4-thiadiazole ( Almethia ) were obtained in a single crystal form and structurally characterized by X-ray diffraction. Diffraction data for 1 and 2 crystals were collected on an Kuma KM-4-CCD and Agilent Gemini A (with an Atlas CCD detector) diffractometers using Mo K a radiation (λ = 0.71073 Å). The collected diffraction data were processed using CrysAlis PRO program. The structures were solved by ShelXT and refined by least squares method on F 2 by ShelXL with the following graphical user interface of OLEX 2 . π-Complexes crystallize in the centrosymmetric space group P 2 1 /n : ( 1 ) a = 12.032(4), b = 8.289(3), c = 12.053(4) Å, b = 111.11(3)°, V = 1121.4(7) Å 3 , Z = 2; ( 2 ) a = 10.221(4), b = 22.777(6), c = 12.974(4) Å, b = 90.41(3)°, V = 3020.3(17) Å 3 , Z = 4. Оrganic ligand Almethia possesses a chelate-bridging function, being coordinated to the metal by means of unsaturated C=C bond and by the two nitrogen atoms of heterocyclic ring. Two crystallographically different copper(I) atoms in 1 possess different coordination environment: Cu(1) has distorted tetrahedral surrounding, arranged by allylic C=C bond, thiadiazole N atom of Almethia molecule and by two bridging halogen atoms; Cu(1) possesses the trigonal pyramidal environment arranged by N atom of the thiadiazole core and three chlorine atoms. Cu(1) deviates from a base of the trigonal pyramid (N4, Cl1 and Сl2) all-in-all by 0.13Å, while the apical Cl1 i (Symmetry code: ( і ) 1– x ; 1– y ; 1– z ) is removed from the metal center by 2.900(2)Å. Despite the last Cu2–Cl1 i distance is remarkably longer than the other analogue distances in the structure, this value still significantly shorter than the sum of van-der-Vaals radii of copper and chlorine. Thus, Almethia molecule causes a formation of stepped cubane tetramer Cu 4 Cl 4 . In contrast to 1 , in structure 2 two crystallographically independent copper(I) have distorted tetrahedral environment, arranged by allylic C=C bond and three N atoms of three different 1,3,4-thiadiazole molecules. Two of this molecules are π,σ-bonded to the metals and the third one is only σ-coordinated by two nitrogen atoms. Cationic [Cu 2 ( Almethia ) 3 ] 2+ fragments and ClO 4 – anions are only electrostatically bound. Keywords: copper(I), π-complex, 1,3,4-thiadiazole, crystal structure.

Extremely Strong Halogen Bond. The Case of a Double-Charge-Assisted Halogen Bridge.

The stable model of a double (±)charge-assisted halogen bridge has been built on the basis of searches of the Crystal Structure Database. The model, investigated by DFT theory, consists of quinuclidine-like cation derivatives and a set of simple anions. These charged fragments form halogen-bonded complexes of which the energy of complexation in some cases reaches 100 kcal/mol. Even for such strong interactions, the QTAIM characteristics are similar to those of the more classic, relatively weak halogen bonds. An important effect of complexation is the charge transfer measured by means of QTAIM and NBO. It can also be supposed, on the basis of detailed structural and QTAIM analysis, that the delocalization of the charge in a quinuclidine moiety occurs through space and not necessarily along formal bonds. The analysis of only partially charged and fully neutral counterparts of a double (±)charge-assisted halogen bridge shows significantly weaker bonding, being less than 10 kcal/mol.

Theoretical and Synthetic Study on the Existence, Structures, and Bonding of the Halide-Bridged [B2X7](-) (X = F, Cl, Br, I) Anions.

While hydrogen bridging is very common in boron chemistry, halogen bridging is rather rare. The simplest halogen-bridged boron compounds are the [B2X7](-) anions (X = F, Cl, Br, I), of which only [B2F7](-) has been reported to exist experimentally. In this paper a detailed theoretical and synthetic study on the [B2X7](-) anions is presented. The structures of [B2X7](-) anions have been calculated at the MP2/def2-TZVPP level of theory, and their local minima have been shown to be of C2 symmetry in all cases. The bonding situation varies significantly between the different anions. While in [B2F7](-) the bonding is mainly governed by electrostatics, the charge is almost equally distributed over all atoms in [B2I7](-) and additional weak iodine···iodine interactions are observed. This was shown by an atoms in molecules (AIM) analysis. The thermodynamic stability of the [B2X7](-) anions was estimated in all phases (gas, solution, and solid state) based on quantum-chemical calculations and estimations of the lattice enthalpies using a volume-based approach. In the gas phase the formation of [B2X7](-) anions from [BX4](-) and BX3 is favored in accord with the high Lewis acidity of the BX3 molecules. In solution and in the solid state only [B2F7](-) is stable against dissociation. The other three anions are borderline cases, which might be detectable under favorable conditions. However, experimental attempts to identify [B2X7](-) (X = Cl, Br, I) anions in solution by (11)B NMR spectroscopy and to prepare stable [PNP][B2X7] salts failed.

An Al/P-based frustrated Lewis pair as an efficient ambiphilic ligand: coordination of boron trihalides, rearrangement, and formation of HBX₂ complexes (X = Br, I).

The Al/P-based frustrated Lewis pair (FLP) Mes2P-C(═CH-Ph)-Al(CMe3)2 (1) reacted with boron halides BX3 (X = F, Cl, Br, I) as an ambiphilic ligand to form complexes (2-5) in which the boron atoms were coordinated to phosphorus and one of the halogen atoms to aluminum. Nonplanar five-membered heterocycles resulted that had five different ring atoms (AlCPBX). The distance of the bridging halogen atoms to the AlCPB plane increased steadily with the radius of the halogen atoms. Only the BF3 adduct showed a dynamic behavior in solution at room temperature with equivalent tert-butyl or mesityl groups in the NMR spectra, while in other cases, the rigid conformation led to the magnetic inequivalence of the substituents at Al and P with well-resolved signals for each group. The BBr3 and BI3 complexes underwent in solution at room temperature a spontaneous stereoselective rearrangement with the concomitant release of isobutene. The obtained products, Mes2P-(μ-C═CH-Ph)(μ-HBX2)-AlX(CMe3) (6 and 7) may be viewed as unique adducts of a modified new Al/P-based FLP, Mes2P-C(═CH-Ph)-AlX(CMe3) (X = Br, I), with dihalogenboranes, HBX2. The trapped boranes are either completely unknown (X = I) or unstable in the free form. Quantum-chemical calculations suggest an ionic rearrangement mechanism via the formation of a borenium cation, β-hydride elimination, and hydride transfer. The bromine migration from boron to aluminum corresponds to a formal suprafacial 1,3-sigmatropic rearrangement.

Solid-state NMR/NQR and first-principles study of two niobium halide cluster compounds

Two hexanuclear niobium halide cluster compounds with a [Nb6X12]2+ (X=Cl, Br) diamagnetic cluster core, have been studied by a combination of experimental solid-state NMR/NQR techniques and PAW/GIPAW calculations. For niobium sites the NMR parameters were determined by using variable Bo field static broadband NMR measurements and additional NQR measurements. It was found that they possess large positive chemical shifts, contrary to majority of niobium compounds studied so far by solid-state NMR, but in accordance with chemical shifts of 95Mo nuclei in structurally related compounds containing [Mo6Br8]4+ cluster cores. Experimentally determined δiso(93Nb) values are in the range from 2400 to 3000ppm. A detailed analysis of geometrical relations between computed electric field gradient (EFG) and chemical shift (CS) tensors with respect to structural features of cluster units was carried out. These tensors on niobium sites are almost axially symmetric with parallel orientation of the largest EFG and the smallest CS principal axes (Vzz and δ33) coinciding with the molecular four-fold axis of the [Nb6X12]2+ unit. Bridging halogen sites are characterized by large asymmetry of EFG and CS tensors, the largest EFG principal axis (Vzz) is perpendicular to the X-Nb bonds, while intermediate EFG principal axis (Vyy) and the largest CS principal axis (δ11) are oriented in the radial direction with respect to the center of the cluster unit. For more symmetrical bromide compound the PAW predictions for EFG parameters are in better correspondence with the NMR/NQR measurements than in the less symmetrical chlorine compound. Theoretically predicted NMR parameters of bridging halogen sites were checked by 79/81Br NQR and 35Cl solid-state NMR measurements.

Application of molecular docking and ONIOM methods for the description of interactions between anti-quorum sensing active (AHL) analogues and the Pseudomonas aeruginosa LasR binding site

Molecular docking methods were applied to simulate the coupling of a set of nineteen acyl homoserine lactone analogs into the binding site of the transcriptional receptor LasR. The best pose of each ligand was explored and a qualitative analysis of the possible interactions present in the complex was performed. From the results of the protein-ligand complex analysis, it was found that residues Tyr-64 and Tyr-47 are involved in important interactions, which mainly determine the antagonistic activity of the AHL analogues considered for this study. The effect of different substituents on the aromatic ring, the common structure to all ligands, was also evaluated focusing on how the interaction with the two previously mentioned tyrosine residues was affected. Electrostatic potential map calculations based on the electron density and the van der Waals radii were performed on all ligands to graphically aid in the explanation of the variation of charge density on their structures when the substituent on the aromatic ring is changed through the elements of the halogen group series. A quantitative approach was also considered and for that purpose the ONIOM method was performed to estimate the energy change in the different ligand-receptor complex regions. Those energy values were tested for their relationship with the corresponding IC50 in order to establish if there is any correlation between energy changes in the selected regions and the biological activity. The results obtained using the two approaches may contribute to the field of quorum sensing active molecules; the docking analysis revealed the role of some binding site residues involved in the formation of a halogen bridge with ligands. These interactions have been demonstrated to be responsible for the interruption of the signal propagation needed for the quorum sensing circuit. Using the other approach, the structure-activity relationship (SAR) analysis, it was possible to establish which structural characteristics and chemical requirements are necessary to classify a compound as a possible agonist or antagonist against the LasR binding site.

Halogen–halogen interaction in view of many-body approach

The many-body theory was applied in order to estimate the character of interaction in quadruple complex consisting of four bromomethane molecules. The tetrameric complex used as a model system was found in the crystal structure in which halogen bonds were stabilizing the solid state structure. Decomposition of interaction energy on two-, three- and four-body terms allowed to conclude that individual halogen bonds in tetramer rather does not cooperate forming the complex. The nonadditive contribution to interaction energy is negative, but of very small value in respect to total interaction energy (less than 0.01%), indicating very weak cooperativity of halogen bridges.

Group 14 structural variations: perhalo derivatives of the “dimetallenes”: dicarbenes, disilenes, digermenes, distannenes, and diplumbenes

Trends in the structural variations of all perhalo derivatives of dicarbenes and their Group 14 analogues have been studied. This included all M2X4 molecules, where M = C, Si, Ge, Sn, or Pb, and X = F, Cl, Br, or I. Mapping the potential energy surface of all molecules has uncovered several isomers. The stability of these isomers depends on both the Group 14 atoms and the halogen ligands. Several isomers were found stable; the ones that are global minima include (with their symmetries and an example in parenthesis): the typical ethene structure X2M=MX2 (D2h, F2C=CF2), an X3M–MX structure (CS, F3Si–SiF, a trifluorosilyl–silylene), another X3M–MX structure (C1, Cl3Si–SiCl), one more X3M–MX structure with a single halogen bridge (C1, I2Si–μI–SiI), a trans double halogen bridged structure (D2h, FSn–μF2–SnF), and another trans double-bridged structure with puckered ring (CS, IPb–μI2–PbI). Some of the other structures that are stable but are not the global minima include: a trans-bent structure X2M–MX2 (C2h, all X2Si–SiX2), cis double-bridged structure (C2v with planar ring, FPb–μF2–PbF, or with puckered ring, C2v, IGe–μI2–GeI), and even a square bipyramidal structure (D4h, Sn–μF4–Sn). The energy differences between some of the structures are small and the application of another computational method and using a different basis set might alter their relative stabilities. Reasons for the difference in the stabilities of isomers have been discussed.

Experimental and Theoretical Charge Density Study of the Chemical Bonding in Chlorokojic Acid Crystal Structure

The experimental charge density in the chlorokojic acid crystal structure was investigated on the basis of high-resolution X-ray diffraction data collected at 80(2) K. The nature of chemical bonding and halogen interactions has been studied by means of deformation densities and by topological analysis using the Bader's quantum theory of atoms in molecules (QTAIM). A comparison between the experimental and theoretical results, from calculations concerning gas phase and periodic DFT/B3LYP, was performed. For the intermolecular hydrogen bonds of O-H···O, C-H···O, and C-H···Cl types, full characteristics of bond critical points (BCP's) were provided indicating closed-shell interactions. The topological analysis of charge density distribution revealed the presence of intermolecular C-Cl···O halogen bridge and halogen-halogen interactions of C-Cl···Cl-C type forming supramolecular Cl(3) synthons, crucial for three-dimensional structure formation of chlorokojic acid. The pronounced anisotropy of electrostatic potential at the Cl atom clarifies both electrophilic and nucleophilic properties of the substituent.

Controlled Polymerization and Self-Assembly of Halogen-Bridged Diruthenium Complexes in Organic Media and Their Dielectrophoretic Alignment

Lipophilic paddlewheel biruthenium complexes [Ru(2)(μ-O(2)CR)(3)X](n) (O(2)CR = 3,4,5-tridodecyloxybenzoate, X = Cl, I) self-assemble in organic media to form halogen-bridged coordination polymers. The polymerization is accompanied by spectral changes in π(RuO,Ru(2)) → π*(Ru(2)) and π(axial ligand) → π*(Ru(2)) absorption bands. These polymeric complexes form lyotropic liquid crystals in n-decane at concentrations above ~100 unit mM. The bridging halogen axial ligands (X = Cl or I) exert significant influences on their electronic structures and self-assembling characteristics: the chloride-bridged polymers give hexagonally aligned ordered columnar structure (columnar hexagonal phase, Col(h)), whereas the iodide-bridged polymers form less ordered columnar nematic (Col(n)) phase, as revealed by small-angle X-ray diffraction measurements. Chloro-bridged coordination polymers dispersed in n-decane are thermally intact even at the elevated temperature of 70 °C. In contrast, iodo-bridged polymers show reversible dissociation and reassembly phenomena depending on temperature. These halogen-bridged coordination polymers show unidirectional alignment upon applying alternating current (ac) electric field as investigated by crossed polarizing optical microscopy and scanning electron microscopy. The unidirectionally oriented columns of chloro-bridged polymers are accumulated upon repetitive application of the ac voltage, whereas iodo-bridged coordination polymers show faster and reversible alignment changes in response to turning on-and-off the electric field. The controlled self-assembly of electronically conjugated linear complexes provide a potential platform to design electric field-responsive nanomaterials.

2‐{E(CH2CH2)2NCH2}C6H4]nBiX3–n (E = O, NMe; X = Cl, Br, I; n = 1–3) and [2‐(Me2NCH2)C6H4]BiBr2 – New Hypervalent Organobismuth(III) Compounds

AbstractThe homoleptic [2‐{E(CH2CH2)2NCH2}C6H4]3Bi [E = O (1), MeN (2)], the chlorides [2‐{E(CH2CH2)2NCH2}C6H4]2BiCl [E = O (3), MeN (4)] and the dichlorides [2‐{E(CH2CH2)2NCH2}C6H4]BiCl2 [E = O (5), MeN (6)] were synthesized by reaction of the corresponding ortho‐lithium derivative with BiCl3 in corresponding molar ratios. The dihalides [2‐{E(CH2CH2)2NCH2}C6H4]BiX2 [X = Br, E = O (7), MeN (9); X = I, E = O (8), MeN (10)] as well as [2‐(Me2NCH2)C6H4]BiBr2 (11) were obtained by halide exchange reactions between RBiCl2 and excess aqueous solution of KX. The compounds were characterized by multinuclear (1H, 13C) NMR spectroscopy and mass spectrometry. The crystal and molecular structures of 1–5 and 11 were determined by single‐crystal X‐ray diffraction. In all compounds the (C6H4CH2)N nitrogen atoms are coordinated to bismuth. The crystals of 1 and 2, as well as those of the chlorides 3 and 4, contain discrete molecules. For R3Bi intramolecular N→Bi interactions of medium strength [Bi(1)–N(1) 3.170(7) Å for 1 and 3.211(5) Å for 2] result in an overall distorted octahedral (C,N)3Bi core. In the monochlorides one nitrogen atom is strongly coordinated to the bismuth atom [2.660(11) Å for 3 and 2.744(14) Å for 4] trans to the halogen atom, whereas the second one is involved in a weaker intramolecular N→Bi interaction [3.095(11) Å for 3 and 3.061(14) Å for 4] trans to a carbon atom. The overall (C,N)2BiCl core is distorted square‐pyramidal. The crystals of the monohalides 5 and 11 contain discrete dimer units. The pendant arm nitrogen atom is strongly coordinated [2.548(9) Å for 5 and 2.485(13) Å for 11] trans to the bridging halogen atom, thus resulting in an overall distorted square‐pyramidal (C,N)BiX3 (X = Cl, Br) core. The six‐membered morpholinyl and piperazinyl rings in 1–5 adopt the chair conformation, which prevents intramolecular coordination of the oxygen or N(Me) nitrogen atoms to bismuth. Supramolecular architectures based on intermolecular Bi···Br interactions, Cl···H, and Br···H contacts are formed in the crystals of 3, 5, and 11.

One-Step and Two-Step Spin-Crossover Iron(II) Complexes of ((2-Methylimidazol-4-yl)methylidene)histamine

A tridentate ligand ((2-methylimidazol-4-yl)methylidene)histamine (abbreviated as H(2)L(2-Me)), that is, the 1:1 condensation product of 2-methyl-4-formylimidazole and histamine, was used for the syntheses of a new family of iron(II) spin-crossover (SCO) complexes with the general chemical formulas [Fe(H(2)L(2-Me))(2)]X(2) x solvent (X = Cl, ClO(4), and BPh(4); solvent = 2-PrOH and CH(3)CN) and [Fe(H(2)L(2-Me))(2)]X x Y x solvent (X = Cl and Br; Y = ClO(4), BF(4), PF(6), and AsF(6); solvent = EtOH and 2-PrOH). The complex cation [Fe(H(2)L(2-Me))(2)](2+) is a chiral species due to an octahedral coordination of two unsymmetrical tridentate ligands, has a ligand field strength around the spin-crossover point, and is hydrogen-bonded to anions to form a variety of network structures. The dichloride complexes [Fe(H(2)L(2-Me))(2)]Cl(2) x 2-PrOH x 0.5 H(2)O (1) and [Fe(H(2)L(2-Me))(2)]Cl(2) x 2-PrOH x H(2)O (1') have a one-dimensional (1D) structure, in which adjacent two chiral complex-cations are doubly bridged by two Cl(-) ions through NH(histamine)...Cl(-)...HN(2-methyl-4-formylimidazole) hydrogen bonds to give a chiral 1D rod. The chiral rods with the same chirality are stacked in the crystal lattices to give a conglomerate, 1, and those with the opposite chiralities are stacked to give a racemic compound, 1'. The enantiomeric circular dichromism spectra of 1 gave definitive evidence of the conglomerate. Compound 1 showed a two-step SCO, while the desolvated sample showed a steep one-step SCO at T(1/2) = 180 K. A series of complexes, [Fe(H(2)L(2-Me))(2)]Cl x X x EtOH (X = ClO(4) (2a), BF(4) (2b), PF(6) (2c), and AsF(6) (2d)), [Fe(H(2)L(2-Me))(2)]Cl x ClO(4) x 0.5(1-PrOH) x 1.5 H(2)O (2a'), and [Fe(H(2)L(2-Me))(2)]Br x ClO(4) x 0.5 EtOH (2a''), display an isomorphous two-dimensional (2D) network at room temperature (296 K), in which the structure is constructed by the NH...Cl(-) (or Br(-)) hydrogen bonds between the imidazole NH groups of [Fe(H(2)L(2-Me))(2)](2+) and the Cl(-) (or Br(-)) ion as a connector. The complexes showed a variety of SCO properties depending on the anion, solvent molecule, and the kind of bridging halogen ion. The complexes of ClO(4)(-) (2a, 2a', 2a'') and BF(4)(-) (2b) with smaller anions showed a two-step SCO with a wide temperature region of the intermediate state of (high-spin (HS) + low-spin (LS))/2 state, their values of (T(1/2,1), T(1/2,2)) being (75, 255 K), (100, 220 K), (110, 220 K), and (100, 260 K), respectively, where the crystal changes from monoclinic P2(1)/n in the HS state to triclinic P1 in the intermediate state. The complexes of PF(6)(-) (2c) and AsF(6)(-) (2d) with larger anions showed a one-step SCO at T(1/2) = 198 and 173 K, respectively, in which the crystal system and space group showed no change during the spin transition. The crystal solvent and halide ion also affected the completeness of the SCO in the lower-temperature region and the steepness of the SCO profile. The experimental results were correlated to the theoretical results based on an Ising-like model. [Fe(H(2)L(2-Me))(2)](BPh(4))(2) x CH(3)CN (3) has no network structure. [Fe(H(2)L(2-Me))(2)](ClO(4))(2) (4) assumes a chiral 3D network structure constructed by the hydrogen bonds between the imidazole groups of one enantiomorph [Fe(H(2)L(2-Me))(2)](2+) and the bridging ClO(4)(-) ion. Compounds 3 and 4 in the solid states are in the HS state, demonstrating that the formation of imidazole-Cl(-) or Br-hydrogen bonds can give SCO properties, but the hydrogen bond of imidazole-ClO(4)(-) cannot give SCO.

Multichanneled relaxation of halogen‐bridged diplatinum chain complexes photoexcited

AbstractEmploying a time‐dependent Hartree‐Fock method, we simulate photoexcitation and the following nonlinear lattice relaxation of quasi‐one‐dimensional binuclear platinum complexes with bridging halogens. High‐energy excitations above the electron‐hole continuum tend to relax into polarons, while excitons pumped within the optical gap can be stabilized into solitons.Both neutral and charged solitons appear as stable photoproducts, which is not the case with conventional platinum‐halide chains. When a pair of kink and antikink is born out of a self‐trapped exciton and recombine to disappear, excess spin and charge densities alternate and oscillate on them. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)