Search

Heterometallic cyano‐bridged spin‐crossover complexes form a large family of switchable compounds with different structural motives and diverse transition characteristics. Here we report on the hysteretic water‐dependent spin transitions found in the family of [Fe(pyridazine)2M(CN)4] frameworks (M = Ni, Pd, Pt). The structure of three new spin‐crossover compounds is built of cyanometallic layers supported by pyridazine ligands. The frameworks contain water guest molecules that can be removed upon heating. Spin transition was found in both hydrated and dehydrated compounds, while the removal of water stimulated a complete spin state switch. Mössbauer spectroscopy revealed two different high‐spin centers in hydrated frameworks, and only one of them is switchable. The compounds display a pronounceable thermochromism, changing their color from red in low‐spin to yellow in high‐spin. The work shows a perspective of 1,2‐diazine application for the design of switchable complexes. [ABSTRACT FROM AUTHOR]

The spin state of catalytically active 3d metal centers plays a significant role in their activity in enzymatic processes and organometallic catalysis. Here the catalytic activity of an FeII coordination compound that can undergo a cooperative switch between low-spin (LS) and high-spin (HS) states is reported. Catalytic measurements in the temperature range 291-318 K revealed a drastic drop of the catalytic activity on conversion of the metal centers from the LS to the HS form. For thermoswitchable complex [Fe(NH2trz)3]Br2 (NH2trz = 4-amino-1,2,4-tiazole; Tup = 305 K), the activation energy is found to be considerably lower for the LS state (158 kJ mol-1) compared to the HS state (305 kJ mol-1). Mössbauer analysis revealed that this is related to higher conversion of the LS complex on oxidation. Comparisons with another polymorph of [Fe(NH2trz)3]Br2 (Tup = 301 K) and [Fe(NH2trz)3](ClO4)2 (Tup = 240 K) are made. These results show the potential of spin-crossover compounds to compare the catalytic activity of different spin states of the same material when other means of differentiation are limited. [ABSTRACT FROM AUTHOR]

1,2,4-triazoles attract attention as actively used medications and ligands for constructing coordination architectures. In this paper we describe four optically active 4-substituted 1,2,4-triazoles that have been prepared by Bayer's synthesis from the corresponding aliphatic chiral amines. This approach tends to be universal towards different triazoles and permits to conserve a homochirality of substrates. Novel asymmetric molecules have been characterized by spectroscopic techniques and their structures have been retrieved from the single crystal X-ray analysis. Chiro-optical studies of these heterocycles have been made by means of circular dichroism spectroscopy. [ABSTRACT FROM AUTHOR]

A high temperature spin crossover (Tup = 367 K) was detected in a metal–organic framework [Fe(pz){Ag(CN)2}2]·MeCN (pz = pyrazine). Upon heating, this solvate released acetonitrile guest molecules, which slightly shifted the transition temperature of the complex (Tup = 370 K and Tdown = 356 K). [ABSTRACT FROM AUTHOR]

Spin transitions are a spectacular example of molecular switching that can provoke extreme electronic and structural reorganizations in coordination compounds. A new 3D cyanoheterometallic framework, [Fe(pz){Au(CN)2}2], has been synthesized in which a highly cooperative spin crossover has been observed at 367 and 349 K in heating and cooling modes, respectively. Mössbauer spectroscopy revealed a complete transition between the diamagnetic and paramagnetic states of the iron centres. The low-spin-to-high-spin transition induced a drastic structural distortion involving a large one-directional expansion (ca. 10.6 %) and contraction (ca. 9.6 %) of the lattice. Negative thermal expansion along the c axis was detected below and above the transition temperature. [ABSTRACT FROM AUTHOR]

The diversity of spin crossover (SCO) complexes that, on the one hand, display variable temperature, abruptness and hysteresis of the spin transition, and on the other hand, are spin-sensitive to the various guest molecules, makes these materials unique for the detection of different organic and inorganic compounds. We have developed a homochiral SCO coordination polymer with a spin transition sensitive to the inclusion of the guest 2-butanol, and these solvates with ( R)- and ( S)-alcohols demonstrate different SCO behaviours depending on the chirality of the organic analyte. A stereoselective response to the guest inclusion is detected as a shift in the temperature of the transition both from dia- to para- and from para- to diamagnetic states in heating and cooling modes respectively. Furthermore, the Mössbauer spectroscopy directly visualizes how the metallic centres in a chiral coordination framework differently sense the interaction with guests of different chiralities. [ABSTRACT FROM AUTHOR]

New methods are proposed for the synthesis of spin-crossover nano- and micro-objects. Several nano-objects that are based upon the spin-crossover complex [Fe(hptrz)3](OTs)2 (hptrz=4-heptyl-1,2,4-triazole, Ts= para-toluenesulfonyl) were prepared in homogeneous media. The use of various reagents (Triton X-100, PVP, TOPO, and PEGs of different molecular weights) as stabilizing agents yielded materials of different size (6 nm-2 μm) and morphology (nanorods, nanoplates, small spherical particles, and nano- and micro-crystals). In particular, when Triton X-100 was used, a variation in the morphology from nanorods to nanoplates was observed by changing the nature of the solvent. Interestingly, the preparation of the nanorods and nanoplates was always accompanied by the formation of small spherical particles. Alternatively, when PEG was used, 200-400 nm crystals of the complex were obtained. In addition, a very promising polymer-free synthetic method is discussed that was based on the preparation of relatively stable FeII-triazole oligomers in CHCl3. Their specific treatment led to micro-crystals, small nanoparticles, or gels. The size and morphology of all of these objects were characterized by TEM and by dynamic light scattering (DLS) where possible. Their spin-crossover behavior was studied by optical and magnetic measurements. The spin-transition features for large particles (>100 nm) were very similar to that of the bulk material, that is, close to room temperature with a hysteresis width of up to 8 K. The effects of the matrix and/or size-reduction led to modification of the transition temperature and an abruptness of the spin transition for oligomeric solutions and small nanoparticles of 6 nm in size. [ABSTRACT FROM AUTHOR]

Abstract: Synthesis of the [Fe(hptrz)3](OTs)2 spin crossover nanoparticle samples was performed. Transmission electronic microscopy was used to determine the size and size distribution of the nanoparticle samples for which a size range of 35–70nm was found – depending on the composition and the stoichiometry of the reverse micelle hexane-tergitol-water system. The factors susceptible to influence the aggregation degree of the nanoparticles, i.e., the starting experimental conditions, the experiment time and the effect of sonication were also studied. In comparison with the bulk powder sample, these nano-materials still exhibit spin crossover near room temperature with a hysteresis loop. The size-properties relationship concerning the present compound and analogue complexes within this family is also discussed. [Copyright &y& Elsevier]

A series of framework coordination polymers reveals the use of 1,2,4,5-tetrazines as efficient bridging ligands towards silver(I) and copper(I) ions. All four nitrogen atoms were functional as lone pair donors leading to an unprecedented m4-coordination of the ligands (1,2,4,5-tetrazine, ttz; 3,6-dimethyl-1,2,4,5-tetrazine, Me2ttz) in [Ag(ttz)(X)] (X = NO3, 1; ClO4, 2), [Ag2(Me2ttz)(NO3)2] (3), [Ag2(Me2ttz)(H2O)2(ClO4)2] (4), [Ag3(Me2ttz)(H2O)2(CF3SO3)3] (5) and [Cu4Cl4(Me2ttz)] (6). In 1 and 2, μ4-tetrazines and silver ions (AgN4, Ag-N 2.42-2.53Å ) compose a 3D framework of {42;84} topology. Structures 3 and 4 were based on disilver-tetrazine ribbons, while in 5 μ4-tetrazines interconnect silver-triflate chains. In 6, μ4-ligands connect inorganic layers sustained by copper-chloride squares, hexa- and octagons (1.974(2) and 1.981(2)Å ). Multiple N-coordination to metal ions enhances the ability of the electron deficient tetrazine system for anion … π binding. Compounds 1 and 2 exhibit very short interactions of this type with corresponding O… C(N) separations down to a record value of 2.78Å and O… π 2.61Å . For 6, π-acidity of tetrazine was reflected by contacts Cl … π of 3.30Å . Results of high level ab initio calculations (RI-MP2/aug-cc-pVTZ) were in good agreement with experimental results, and were suggestive of the progressive enhancement of the π-acidity by increasing the number of Ag(I) ions N-coordinated to tetrazine. [ABSTRACT FROM AUTHOR]

Spin crossover compounds are considered to be a viable alternative for creating display, memory and switching devices due to the bistability of their magnetic, optical, mechanical and electrical properties. This Letter presents the study of the dielectric and transport properties of the [Fe(Htrz)2(trz)](BF4) (Htrz = 1H-1,2,4-triazole) complex in a wide temperature and frequency range. Our results reveal a singular behavior of the dielectric modulus upon the spin transition in conjunction with the switching of the conductivity between the high spin and low spin states. [ABSTRACT FROM AUTHOR]

As one of the most likely alternatives to traditional vapor compression refrigeration technology, solid refrigeration technology based on the barocaloric effect (BCE) has attracted extensive attention in recent years. Spin-crossover (SCO) compounds are considered suitable for working at low driving pressures due to high-pressure sensitivity and small hysteresis width. However, the entropy change (ΔSSCO) of the SCO compound is smaller than that of other excellent barocaloric materials (plastic crystals and two-dimensional perovskites). Here, we report the BCE of the SCO compound {Fe(pz)2(BH3CN)2} (pz = pyrazine) with a smaller molar mass and a third source of entropy change besides electron and vibrational entropy changes. Compound {Fe(pz)2(BH3CN)2} exhibits high pressure sensitivity ( d T 1 / 2 d P = 20.2 K kbar−1) as well as entropy change (Δ S SCO = 202 J kg−1 K−1). The maximum values of reversible isothermal entropy change (ΔSit,rev,max) and adiabatic temperature change (ΔTad,rev,max) at 1 kbar are only 103 J kg−1 K−1 and ∼0 K, respectively, due to the hysteresis behavior. However, at sufficiently high driving pressures, ΔSit,rev,max exceeds 200 J kg−1 K−1, and ΔTad,rev,max can reach ∼47 K, which exceeds all SCO compounds reported in BCE studies and is comparable to some plastic-crystalline and two-dimensional perovskite barocaloric materials. The excellent BCE of the SCO compound {Fe(pz)2(BH3CN)2} is mainly due to its small molar mass, which makes the unit mass compound exhibit higher ΔSSCO, while the introduction of the third source of entropy change—the reorientation entropy change (ΔSreo), only plays a small role. This is expected to promote the practical application of SCO compounds as barocaloric refrigerants. [ABSTRACT FROM AUTHOR]

Chemical composition is leading among the numerous factors that determine the spin transition properties of coordination compounds. Classic dicyanometallic bridges {M(CN)2}− are commonly used to build Hofmann-like spin-crossover frameworks, but some extended bridges are also synthetically available. In this paper, we describe a successful synthesis of two very similar spin-crossover frameworks that differ in the cyanometallic bridges involved, namely [Fe(etpz)2{Ag(CN)2}2] (1) and {Fe(etpz)2[Ag2(CN)3][Ag(CN)2]} (2) (where etpz = 2-ethylpyrazine). Magnetic and Mössbauer studies demonstrated the occurrence of abrupt one-step high-spin (HS) ↔ low-spin (LS) transitions for both complexes. The spin transition temperatures are T1/2 ↓ = 233 K and T1/2 ↑ = 243 K for 1 and T1/2 ↓ = 188 K and T1/2 ↑ = 191 K for 2 with thermal hysteresis loops of 10 K for 1 and 3 K for 2. The bridging mononuclear [Ag(CN)2]− units and FeII cations assemble to form infinite 2D layers in the structure of 1. Interestingly, compound 2 forms 2D layers of FeII cations bridged by both binuclear [Ag2(CN)3]− and mononuclear [Ag(CN)2]− units. The structures of 1 and 2 comprise different types of intermolecular interactions including Ag⋯Ag and Ag⋯Netpz, which induce the creation of supramolecular 3D frameworks. The synergy between metallophilic interactions and the spin transition is also confirmed by the variation of Ag⋯Ag distances during spin crossover. The characterization of such analogues allowed us to analyze in detail the effect of the cyanometallic bridge on the structure of new frameworks and on the bistability in Hofmann-like complexes. [ABSTRACT FROM AUTHOR]

In the crystal structure of the title compound, [Cu(NO3)2(C3H5N3)4], the CuII atom is situated on an inversion center (Wyckoff position 2c of space group P21/n) and shows an octahedral [N4O2] coordination environment. The axial positions are occupied by O atoms of nitrate anions, while the equatorial positions are taken up by the N atoms of four 3-aminopyrazole ligands. As a result of the tautomerism of the latter, two coordinate with the N¹-atom of 3-aminopyrazole while the other two with the N²-atom. The presence of pyrrole-like N--H groups and amine substituents as donor groups leads to numerous intra- and intermolecular hydrogen-bonding interactions, which were quantified by Hirshfeld surface analysis. [ABSTRACT FROM AUTHOR]

The crystal structure of the title salt, bis(3-carboxy-1-methylpyridinium) octabromide, 2C7H8NO2+⋅Br82-, consists of 3-carboxy-1-methylpyridinium (N-methylnicotinic acid) cations, which are stacked between relatively rare [Br8]2- anions. The polybromide [Br8]2- anion has point group symmetry 1 and can be described as being composed of two [Br3]- anions connected with a Br2 molecule in a Z-shaped manner. Contacts between neighboring octabromide anions ensure the creation of pseudo-polymeric chains propagating along [111]. The organic cations are located between anionic chains and are connected to each other through O--H⋯O hydrogen bonds and to the [Br8]2- anions through ⋯Br interactions that induce the creation of a supramolecular triperiodic network. In addition, the presence of weak C--H⋯Br contacts leads to the creation of layers, which align parallel to (112). [ABSTRACT FROM AUTHOR]

Lead halide hybrid perovskites incorporating chiral organic cations attract considerable attention due to their promising application in multifarious optoelectronic devices. However, the examples of chiral hybrid perovskites are still limited, which greatly impedes their further studies in various optoelectronic fields. Herein, we report on new low-dimensional lead-halide hybrid perovskites incorporating the enantiopure chiral α-amino acid L -proline. Two hybrid perovskites (L -proH)PbBr3·H2O (Pro-PbBr3) and (L -proH)4Pb3Br10·4H2O (Pro-Pb3Br10) have been synthesized by employing different ratios of organic and inorganic precursors. According to structural analysis, the inorganic sublattice of compound Pro-PbBr3 is built of one-dimensional (1D) [PbX3]∞n− lead halide chains, whereas the inorganic sublattice of compound Pro-Pb3Br10 is built upon a rare two-dimensional (2D) [Pb3Br10]∞4n− honeycomb-type inorganic framework. Hirshfeld surface analysis revealed an important role of various hydrogen bonding interactions in providing the binding between organic and inorganic parts of these hybrid perovskites. The optical band gap values of new hybrid perovskites as estimated using the Tauc plot approach are 4.19 eV (Pro-PbBr3) and 4.13 eV (Pro-Pb3Br10). Also, new compounds display low-temperature broadband photoluminescence which can be attributed to the self-trapped excitons. These results show the potential of α-proline for constructing novel and highly demanded chiral hybrid perovskites, which will hold great promise for further optoelectronic applications. [ABSTRACT FROM AUTHOR]

We studied the effect of non-hydrostatic pressure on the hysteretic spin crossover in coordination complexes. By introducing into an Ising-like model a double distribution of the interactions and gap energy, respectively, we were able to generate the major hysteresis loop and the first-order reversal curve (FORC) diagram for spin-crossover systems of 106 hysterons (like-spin domains). We show that, for high pressure gradients around the spin-crossover system, the thermal hysteresis loop takes an asymmetric shape, in good agreement with the experimental data on pressure effect recorded at low temperatures, below the solidification of the pressure transmitting medium. Interestingly, the FORC diagram method seems to be much more sensitive to local changes than the "bulk" parameters, which characterize the major hysteresis loop. [ABSTRACT FROM AUTHOR]

3D hybrid perovskites based upon small organic cations gave start to a new intensively growing class of semiconducting materials. Here we report on the elaboration of quantum dots of a recently emerged new perovskite (AzrH)PbBr3 (AzrH = aziridinium cation). By employing the antisolvent precipitation technique and stabilization with a cationic surfactant we succeeded in obtaining quantum dots that display tunable luminescence. This piece of work shows the perspective of aziridinium-based materials for the elaboration of advanced photonic nanostructures. [ABSTRACT FROM AUTHOR]

Novel chiral hybrid perovskites are highly demanded for various advanced applications such as spintronics, optoelectronics, photovoltaics etc. However, the scope of these new materials is still limited. Herein, we present new 2D hybrid perovskites based upon chiral α-amino acid L -histidine. The generalized formula of these new compounds can be denoted as (L -HisH)2PbBrxI4−x (where L -His = L -histidine; x = 4, 3, 2, 1, 0.4 and 0). All perovskites are characterized by a very similar structural motif that consists of corner-sharing lead halide octahedra arranged in one-layer thin inorganic slabs interleaved by organic layers established by L -histidinium(1+) cations. L -Histidine provides a breaking of spatial parity of these perovskites that results in their non-centrosymmetric crystal structures. These compounds show a multiband absorption up to 590 nm for iodide perovskite. In addition, new compounds display pronounced single-peak photoluminescence, which finely blue shifts upon the gradual substitution of iodine by bromine. New perovskites exhibit excellent thermal stability up to 490 K and 445 K for bromide and iodide compounds, respectively. These results show the ability of L -histidine to produce novel and highly demanded chiral hybrid perovskites. [ABSTRACT FROM AUTHOR]

Single crystals of two spin-crossover (SCO) cyanometallic coordination polymers based on the pyridazino[4,5-d]pyridazine ligand (pp) of the composition [Fe(pp)M(CN)4]∙G (where M = Pd, Pt; G = guest molecules) were obtained by a slow diffusion technique. A single-crystal X-ray analysis showed that both compounds adopted the structure of porous 3D frameworks, consisting of heterometallic cyano-bridged layers and interlayer pillar pp ligands, with a total solvent accessible volume of ca. 160 Å3 per iron(II) ion (about 37% of the unit cell volume). These frameworks displayed hysteretic SCO behaviour with T1/2 of 150/190 K (heating/cooling) for Pd complex and 135/170 K (heating/cooling) for Pt complex, which was confirmed by variable-temperature SCXRD experiments. This research shows the perspective of using pp ligand for building porous MOFs with spin transitions. [ABSTRACT FROM AUTHOR]

Poly[[μ4-4,4′-bipyridazine-μ5-sulfato-disilver(I)] monohydrate], {[Ag2(SO4)(C8H6N4)]·H2O} n, (I), and poly[[aqua-μ4-pyridazino[4,5- d]pyridazine-μ3-sulfato-disilver(I)] monohydrate], {[Ag2(SO4)(C6H4N4)(H2O)]·H2O} n, (II), possess three- and two-dimensional polymeric structures, respectively, supported by N-tetradentate coordination of the organic ligands [Ag—N = 2.208 (3)–2.384 (3) Å] and O-pentadentate coordination of the sulfate anions [Ag—O = 2.284 (3)–2.700 (2) Å]. Compound (I) is the first structurally examined complex of the new ligand 4,4′-bipyridazine; it is based upon unprecedented centrosymmetric silver–pyridazine tetramers with tetrahedral AgN2O2 and trigonal–bipyramidal AgN2O3 coordination of two independent AgI ions. Compound (II) adopts a typical dimeric silver–pyridazine motif incorporating two kinds of square-pyramidal AgN2O3 AgI ions. The structure exhibits short anion–π interactions involving noncoordinated sulfate O atoms [O...π = 3.041 (3) Å]. [ABSTRACT FROM AUTHOR]

Spin‐crossover complexes with multistep transitions attract much attention due to their potential applications as multi‐switches and for data storage. A four‐step spin crossover is observed in the new iron(II)‐based cyanometallic guest‐free framework compound Fe(2‐ethoxypyrazine)2{Ag(CN)2}2 during the transition from the low‐spin to the high‐spin state. A reverse process occurs in three steps. Crystallographic studies reveal an associated stepwise evolution of the crystal structures. Multiple transitions in the reported complex originate from distinct FeII sites which exist due to the packing of the ligand with a bulky substituent. [ABSTRACT FROM AUTHOR]

In the title compound [Fe2(C2O4)(C8H8N4)4](CH3C6H4SO3)2·2.75H2O, the two FeII ions have a highly distorted octa­hedral FeN4O2 environment formed by two bidentate triazole-based chelating ligands and a bis-bidentate oxalate bridging anion that connects the metal ions. Stabilization within the crystal structure is provided via a system of O--H⋯O and N--H⋯O hydrogen bonding, which determines the formation of a two-dimensional architecture along the a-axis direction. [ABSTRACT FROM AUTHOR]

Solvatomorphism and polymorphism play a crucial role in defining the characteristics of spin crossover (SCO) in coordination compounds. In this paper we describe a detailed characterization of the SCO bis[hydrotris(1,2,3-triazol-1-yl)borate]iron(II) complex ([Fe(HB(1,2,3-tz)3)2]) 1. Five solvatomorphs of 1·2Solv (where Solv = DMSO, MeOH, and CHCl3), 1·7DMSO and 1·2DMF·H2O were obtained via recrystallization from the respective organic solvents. Solvatomorphs 1·2MeOH and 1·2CHCl3 undergo single-crystal-to-single-crystal transformations leading to 1 upon heating above 313 K. Single-crystal X-ray diffraction analysis revealed π–π stacking and C–H⋯N contacts between molecules of the complex both in the solvated and desolvated crystals. The spin crossover and thermal properties of 1 and its solvates were investigated by means of calorimetry, thermogravimetry, Mössbauer spectroscopy, variable temperature magnetic susceptibility, Raman spectroscopy and powder X-ray diffraction measurements. The anhydrous [Fe(HB(1,2,3-tz)3)2] complex, obtained via dehydration of dodecahydrate, undergoes structural phase transitions upon the first heating in the 303–493 K range followed by reversible, gradual spin crossover upon further thermal cycling, with a transition temperature of T1/2 = 373 K. [ABSTRACT FROM AUTHOR]

Reconfigurable radio‐frequency components are in high demand for modern communication systems as they can be involved in multiband and multistandard electronic devices. The key part of such components is an active switching element. This work offers a way to obtain an efficient microwave switch using vanadium dioxide‐poly (methyl methacrylate) composite. Differential scanning calorimetry, SQUID magnetometery, and impedance spectroscopy measurements were used to characterize the phase transition in the proposed composite. Temperature induced metal‐insulator transition occurs at technologically attractive 341 K. The transition leads to a change of microwave transmission trough VO2‐PMMA composite from −4.9 dB for low‐temperature monoclinic form to −5.8 dB for high‐temperature rutile form. This provides an ability to tune the material's transparency in the microwave range, while the shaping polymer matrix provides the proper mechanical processability of the switching element. [ABSTRACT FROM AUTHOR]

This study describes the synthesis of the first aziridinium-based compounds, namely hybrid perovskites (AzrH)PbHal3 (where AzrH = aziridinium, Hal = Cl, Br or I). This highly reactive species was stabilized in 3D lead halide frameworks and was found to be a small enough organic cation to promote the formation of semiconducting organo-inorganic materials. [ABSTRACT FROM AUTHOR]

We report on a molecular cationic iron(II) complex with a 4-amino-1,2,4-triazole ligand and a tetraiodomercurate anion exhibiting an incomplete spin crossover (SCO). The complex exhibits an unusual disordered structure with a linear arrangement of ligand and water molecules that can potentially accommodate up to four iron atoms, but both terminal metal positions have half chemical occupancies, while occupancies of all ligands are full. This corresponds to the crystallisation of disordered trinuclear complexes arranged into 1D supramolecular chains. Iron cations have different N6 or N3O3 coordination environments, leading to the thermally induced SCO in two thirds of the metal centres. This SCO behaviour was characterised by magnetic susceptibility measurements and Mössbauer spectroscopy. [ABSTRACT FROM AUTHOR]

In the title polymeric coordination compound, {[FePt(CN)4(H2O)2]·1.33CH3OH}n, the FeII cation (site symmetry 4/mm.m) is coordinated by the N atoms of four cyanide anions (CN-) and the O atoms of two water molecules, forming a nearly regular [FeN4O2] octahedron. According the Fe--N and Fe--O bond lengths, the FeII atom is in the high-spin state. The cyanide anions act in a bridging manner to connect the FeII and PtII atoms. The [Pt(CN)4]2- moieties (Pt with site symmetry 4/mm.m) have a perfect square-planar shape. The latter anion is located perpendicular to the FeN4 plane, thus ensuring the creation of a three-dimensional framework. The crystal structure features methanol solvent molecules of which 4/3 were located per FeII cation. These solvent molecules are located in hexagonal pores; they interact with coordinating water molecules through weak hydrogen bonds. Other guest molecules could not be modelled in a satisfactory way and their contribution to the scattering was removed by a mask procedure. [ABSTRACT FROM AUTHOR]

9-Amino­acridinium chloride N,N-di­methyl­formamide monosolvate, C13H11N2+Cl−·C3H7NO, crystallizes in the monoclinic space group P21/c. The salt was crystallized from N,N-di­methyl­formamide. The asymmetric unit consists of two C13H11N2+Cl− formula units. The 9-amino­acridinium (9-AA) mol­ecules are protonated with the proton on the N atom of the central ring. This N atom is connected to an N,N-di­methyl­formamide mol­ecule by a hydrogen bond. The H atoms of the amino groups create short contacts with two chloride ions. The 9-AA cations in adjacent layers are oriented in an anti­parallel manner. The mol­ecules are linked via a network of multidirectional π–π inter­actions between the 9-AA rings, and the whole lattice is additionally stabilized by electrostatic inter­actions between ions. [ABSTRACT FROM AUTHOR]

[Display omitted] • FeII Hofmann-type complexes form a big class of spin-crossover (SCO) compounds. • Different ligand types are suitable for the formation of these complexes. • Inclusion of guests significantly influences their spin-crossover characteristics. • FeII cyanometallic complexes can be processed to different types of SCO nanoobjects. • These complexes can display specific features demanded for practical application. Spin-crossover (SCO) compounds represent a huge class of switchable coordination complexes that can change their electronic configurations under the influence of different external stimuli. SCO provokes a spectacular switch of physical and chemical properties of complexes making them perspective as active elements of sensors, actuators, electronic and optical components, and other devices. In this article we offer a brief overview of cyanometallic frameworks exhibiting SCO properties. Building blocks that tend to provide new materials with desired spin-crossover characteristics are discussed. Special attention is paid on how the host–guest interactions in SCO frameworks affect their transition properties making them suitable for sensing applications. The effect of size reduction to nanometre scale on switchable properties of cyanometallates is described. Additionally, some outstanding features of SCO cyanometallates such as multistep transitions, mechanical actuation and switchable luminescence are highlighted. [ABSTRACT FROM AUTHOR]

[Display omitted] • Nucleation and growth phenomena are associated with the thermally-induced SCO. • Significant kinetic effects on the hysteresis behaviour are observed. • Ultra-slow switching rates and phase-boundary propagation velocities are observed. • Unique SCO properties are associated with numerous microstructural defects. Nucleation and growth phenomena associated with the thermally-induced spin transition was studied using optical microscopy in single crystals of a cyanoheterometallic coordination polymer [Fe(1,6-naphthyridine) 2 (Ag(CN) 2) 2 ]. Despite the fact that the spin-crossover phenomenon occurs near room temperature (ca. 290–300 K), the experimental results show the existence of significant kinetic effects on the hysteretic behaviour of the spin transition in this compound. Ultra-slow switching rates and phase-boundary propagation velocities were observed (down to 3 nm s−1), several orders of magnitude lower than customary. Our observations indicate that these peculiar spin-crossover properties (slow switching kinetics, scan-rate-dependent transition temperatures) are presumably related to the presence of numerous microstructural defects in the crystals, which is an intrinsic characteristic of this compound. [ABSTRACT FROM AUTHOR]

Chiral hybrid organic–inorganic perovskites (HOIPs) attract attention due to their potential for application in optoelectronic, spintronic and photovoltaic devices. However, to this day the range of available chiral HOIPs is still scarce. In this context, we synthesized and thoroughly characterized two pairs of enantiomeric HOIPs incorporating enantiopure α-alanine, the simplest chiral amino acid: (AlaH)PbX3 (Ala = R-alanine or S-alanine, X = Br or I). Single-crystal XRD measurements revealed an unusual 2D anionic motif, which consists of haloplumbate octahedra bonded together in an edge-sharing manner along the c axis and in a corner-sharing manner along the b axis. Four compounds undergo melting/crystallization at unusually low temperatures. New compounds also display photoluminescence in the visible range: 380–640 nm (X = Br) and 480–640 nm (X = I). Electronic spectroscopy reveals an optical response typical for semiconductive materials with optical band gaps of 3.12 eV (X = Br) and 2.48 eV (X = I). These results show that the incorporation of simple and easily accessible α-alanine (and other chiral amino acids) as a cationic part is an efficient way for obtaining highly demanding chiral HOIPs. [ABSTRACT FROM AUTHOR]

Hofmann-like cyanometallic complexes represent one of the biggest and well-known classes of FeII spin-crossover compounds. In this paper, we report on the first FeII Hofmann clathrate analogues with unsubstituted 1,2,3-triazole, which exhibit temperature induced spin transition. Two new coordination polymers with the general formula [FeII(1,2,3-triazole)2MII(CN)4] (M = Pt, Pd) undergo abrupt hysteretic spin crossover in the range of 190–225 K as revealed by magnetic susceptibility measurements. Two compounds are isostructural and are built of infinite cyanometallic layers which are supported by 1,2,3-triazole ligands. The thermal hysteresis loop is very stable at different scan rates from 0.5 to 10 K min−1. The compounds display strong thermochromic effect, changing their colour from pink in the low-spin state to white in the high-spin state. Our findings show that 1,2,3-triazole is suitable for elaboration of spin-crossover Hofmann clathrate analogues, and its use instead of more classical azines can advantageously expand this family of complexes. [ABSTRACT FROM AUTHOR]

Spin‐crossover complexes form one of the biggest families of switchable compounds. Here we show a new way to obtain spin‐crossover materials by direct synthesis from metallic iron. Four complexes of FeII with 4‐R‐1,2,4‐triazoles have been synthesized by a direct metal oxidation: [Fe(NH2trz)3]SO4, [Fe(NH2trz)3](BF4)2 and two polymorphs of [Fe(Htrz)2(trz)]BF4. Surprisingly high quality of PXRD pattern of [Fe(NH2trz)3]SO4 allowed to perform Rietveld refinement and obtain reliable crystal structure of the latter. All compounds have been studied by magnetic susceptibility measurements, optical reflectivity, differential scanning calorimetry and Raman spectroscopy. Importantly, spin‐crossover characteristics of the complexes obtained by this unusual method are preserved, thus revealing a new effective approach of "direct synthesis" towards switchable coordination compounds. [ABSTRACT FROM AUTHOR]

In the title compound [Fe(bpe)(Hbpe)Au(CN)2][Au(CN)2]2·bpe·2H2O}n [where bpe = 1,2-bis(pyridin-4-yl)ethene, C12H10N2], the FeII ion is coordinated in a distorted octahedral [FeN4O2] environment by two dicyanoaurate anions, two water molecules and two partially protonated 1,2-di(4-pyridyl)ethylene molecules. Dicyanoaurate anions bridge the FeII cations, forming infinite chains, which propagate along the a-axis direction. The chains are connected via aurophilic interactions with two non-coordinated dicyanoaurate anions for each FeII ion. The polymeric chains interact with each other via π–π stacking between the guest bpe molecules and multiple hydrogen bonds. [ABSTRACT FROM AUTHOR]

Two new 2D spin-crossover (SCO) analogues of Hofmann clathrates of composition [Fe(phth)2MII(CN)4] (where phth = phthalazine; MII = Pd, Pt) have been synthesized and their structures and switchable behaviour have been characterized. Single-crystal X-ray analysis reveals that the Pt and Pd derivatives contain FeII centres equatorially surrounded by four equivalent μ4-[MII(CN)4]2− groups. Two crystallographically equivalent phthalazine (phth) ligands occupy the axial positions of each FeII site, completing its octahedral coordination environment. The stabilization of these structures is realized via supramolecular C–H⋯M interactions and π–π stacking. Temperature-dependent magnetic susceptibility measurements showed that Pt (T1/2↓ = 211 K and T1/2↑ = 218 K) and Pd (T1/2↓ = 202 K and T1/2↑ = 207 K) derivatives display cooperative spin crossover with narrow thermal hysteresis loops. In addition, spin crossover in these complexes was characterized by optical measurements, differential scanning calorimetry, and IR and Raman spectroscopy. This research shows that the use of phthalazine leads to the production of new SCO systems with attractive transition characteristics and opens up new perspectives for the design of switchable complexes based on fused bicyclic azine ligands. [ABSTRACT FROM AUTHOR]

In the asymmetric unit of the title coordination compound, {[Cu(CN)(C4H3OC2H5N2)][Cu(CN)]}n, there are two Cu atoms with different coordination environments. One CuI ion is coordinated in a triangular coordination geometry by the N atom of the 2-eth­oxy­pyrazine mol­ecule and by two bridging cyanide ligands, equally disordered over two sites exchanging C and N atoms, thus forming polymeric chains parallel to the c axis. The other Cu atom is connected to two bridging cyanide groups disordered over two sites with an occupancy of 0.5 for each C and N atom, and forming an almost linear polymeric chain parallel to the b axis. In the crystal, the two types of chain, which are orthogonal to each other, are connected by cuprophilic Cu∙∙∙Cu inter­actions [2.7958 (13) Å], forming two-dimensional metal–organic coordination layers parallel to the bc plane. The coordination framework is further stabilized by weak long-range (electrostatic type) C—H∙∙∙π inter­actions between cyano groups and 2-eth­oxy­pyrazine rings. [ABSTRACT FROM AUTHOR]

In the title polymeric complex, {[FePt(CN)4(H2O)2]·2C3H6O}n, the FeII cation has an octa­hedral [FeN4O2] geometry being coordinated by two water mol­ecules and four cyanide anions. The Pt cation is located on an inversion centre and has a square-planar coordination environment formed by four cyanide groups. The tetra­cyano­platinate anions bridge the FeII cations to form infinite two-dimensional layers that propagate in the bc plane. Two guest mol­ecules of acetone per FeII are located between the layers. These guest acetone mol­ecules inter­act with the coordinated water mol­ecules by O—H···O hydrogen bonds. [ABSTRACT FROM AUTHOR]

In the title polymeric complex, {[Fe(CN)2(C10H8N2)2(H2O)2][Au(CN)2]}n, the FeII ion, which is located on a twofold rotation axis, has a slightly distorted FeN4O2 octahedral geometry. It is coordinated by two phenylpyrazine molecules, two water molecules and two dicyanoaurate anions, the Au atom also being located on a second twofold rotation axis. In the crystal, the coordinated dicyanoaurate anions bridge the FeII ions to form polymeric chains propagating along the b-axis direction. In the crystal, the chains are linked by Owater--H···Ndicyanoaurate anions hydrogen bonds and aurophillic interactions [Au···Au = 3.5661 (3) Å], forming layers parallel to the bc plane. The layers are linked by offset π-π stacking interactions [intercentroid distance = 3.643 (3) Å], forming a supramolecular metal-organic framework. [ABSTRACT FROM AUTHOR]

In the title metal-organic framework, [Fe(C6H8N2)2{Cu(CN)2}2]n, the low-spin FeII ion lies at an inversion centre and displays an elongated octahedral [FeN6] coordination environment. The axial positions are occupied by two symmetryrelated bridging 2-ethylpyrazine ligands, while the equatorial positions are occupied by four N atoms of two pairs of symmetry-related cyanide groups. The CuI centre is coordinated by three cyanide carbon atoms and one N atom of a bridging 2-ethylpyrazine molecule, which form a tetrahedral coordination environment. Two neighbouring Cu atoms have a short Cu···Cu contact [2.4662 (7) Å] and their coordination tetrahedra are connected through a common edge between two C atoms of cyanide groups. Each Cu2(CN)2 unit, formed by two neighbouring Cu atoms bridged by two carbons from a pair of μ-CN groups, is connected to six FeII centres via two bridging 2-ethylpyrazine molecules and four cyanide groups, resulting in the formation of a polymeric three-dimensional metal-organic coordination framework. [ABSTRACT FROM AUTHOR]

A new iron(ii)-based spin-crossover compound with thermal hysteresis operating under ambient conditions is reported. This complex exhibits a high reproducibility of the spin transition in many successive thermal cycles, stability of both spin states at room temperature and an attractive operational temperature range. [ABSTRACT FROM AUTHOR]

In the title metal-organic framework, [Fe(C4H3BrN2)2{Cu(CN)2}2]n, the FeII cation is located on an inversion center and has a slightly elongated octa­hedral coordination environment [FeN6], ligated by two pyrazine N atoms of symmetry-related bridging 2-bromo­pyrazine mol­ecules in the axial positions and by four N atoms of pairs of symmetry-related cyanido groups in the equatorial positions. The CuI center has a fourfold coordination environment [CuC3N], with an almost perfect trigonal–pyramidal geometry, formed by three cyanido C atoms and an N atom of a bridging 2-bromo­pyrazine mol­ecule. Copper(I) centers related by a twofold rotation axis are bridged by two carbon atoms from a pair of μ-CN groups, resulting in Cu2(CN)2 units. Each Cu2(CN)2 unit is linked to six FeII cations via a pair of linear CN units, the pair of μ-CN groups and two bridging 2-bromo­pyrazine ligands, resulting in the formation of a metal–organic framework, which is additionally stabilized by the short CuCu contacts of 2.4450 (7) Å. [ABSTRACT FROM AUTHOR]

A pentanuclear CoII–CoIII cluster complex has been developed by a solvothermal synthesis. Its highly stable metal-mixed iron–cobalt derivatives display robust spin crossover (T1/2 = 268 K) controlled by the degree of substitution. [ABSTRACT FROM AUTHOR]

In the title polymeric complex, {[Fe(C12H10N2)2(H2O)4](CH3C6H4SO3)2·2CH3OH}n, the FeII cation, located on an inversion centre, is coordinated by four water mol­ecules in the equatorial positions and two 1,2-bis­(pyridin-4-yl)ethene mol­ecules in the axial positions. This results in a distorted octa­hedral geometry for the [N2O4] coordination polyhedron. The 1,2-bis­(pyridin-4-yl)ethene mol­ecules bridge the FeII cations, forming polymeric chains running along the a-axis direction. Stabilization of the crystal structure is provided by O—H···O hydrogen bonds; these are formed by coordinated water mol­ecules as donors towards the O atoms of the methanol mol­ecules and tosyl­ate anions as acceptors of protons, leading to the formation of a three-dimensional supra­molecular network. Weak C—H···O hydrogen bonds are also observed in the crystal. [ABSTRACT FROM AUTHOR]

In the title polymeric complex, [Au2Fe(CN)4(C4H3IN2)2(C2H6O)]n, the FeII cation is coordinated by two iodo­pyrazine mol­ecules, one ethanol mol­ecule and three di­cyano­aurate anions in a distorted N5O octa­hedral geometry. In the crystal, the di­cyano­aurate anions bridge the FeII cations to form polymeric chains propagating along the b-axis direction. Stabilization of the crystal structure is provided by O—H···N hydrogen bonds and π–π stacking between parallel iodo­pyrazine rings of neighbouring chains, the centroid–centroid distances being 3.654 (10) and 3.658 (9) Å. [ABSTRACT FROM AUTHOR]

Discovery of spin-crossover (SCO) behavior in the family of FeII-based Hofmann clathrates has led to a "new rush" in the field of bistable molecular materials. To date this class of SCO complexes is represented by several dozens of individual compounds, and areas of their potential application steadily increase. Starting from Fe2+, square planar tetracyanometalates MII(CN)42- (MII = Ni, Pd, Pt) and 2-substituted pyrazines Xpz (X = Cl, Me, I) as coligands we obtained a series of nine new Hofmann clathrate-like coordination frameworks. X-ray diffraction reveals that in these complexes FeII ion has a pseudo-octahedral coordination environment supported by four µ4-tetracyanometallates forming its equatorial coordination environment. Depending on the nature of X and M, axial positions are occupied by two 2X-pyrazines (X = Cl and MII = Ni (1), Pd (2), Pt (3); X = Me and MII = Ni (4), Pd (5)) or one 2X-pyrazine and one water molecule (X = I and MII = Ni (7), Pd (8), Pt (9)), or, alternatively, two distinct FeII positions with either two pyrazines or two water molecules (X = Me and MII = Pt (6)) are observed. Temperature behavior of magnetic susceptibility indicates that all compounds bearing FeN6 units (1-6) display cooperative spin transition, while FeII ions in N5O or N4O2 surrounding are high spin (HS). Structural changes in the nearest FeII environment upon low-spin (LS) to HS transition, which include ca. 10% Fe-N distance increase, lead to the cell expansion. Mössbauer spectroscopy is used to characterize the spin state of all HS, LS, and intermediate phases of1-9 (see abstract figure). Effects of a pyrazine substituent and MII nature on the hyperfine parameters in both spin states are established. [ABSTRACT FROM AUTHOR]

Linter cellulose fiber based nanocomposites were prepared by functionalizing the fibers with [Fe(hptrz)3](OTs)2 spin-crossover nanoparticles and acridine orange dye molecules. The composites were characterized by electronic (UV-vis and fluorescence) and vibrational (Raman, FTIR) spectroscopies as well as by electronic and optical microscopies. The cellulose sheets prepared from the nanocomposite show significant thermochromic and thermofluorescent effects associated with the spin transition phenomenon, providing scope for paper thermometry and anti-counterfeit applications. [ABSTRACT FROM AUTHOR]

Here we describe the elaboration and investigation of composites prepared from the spin-crossover (SCO) complex [Fe(pyrazine){Au(CN) 2 } 2 ] and poly(vinylidene fluoride) matrix, with different contents of the active phase (10–35 wt%). Optical measurements demonstrated that all composites preserve temperature induced hysteretic spin transitions. Tensile mechanical analysis showed a non-linear change of the Young's modulus upon increase of the complex content. Thermomechanical analysis upon a constant strain demonstrated pronouncable alterations of the applied stress in the SCO region. Composites with smaller loads require less stress to attain a given strain in the high-spin state, however for those with high loads the behaviour is more sophisticated. The modelling of stress vs. temperature behaviour revealed that this may be interpreted through a miscellaneous interplay of expansion of both the polymer and the complex, the contribution of both components to the elastic properties of composites, and the effects of SCO on these properties. [Display omitted] • Insertion of a spin-crossover complex into PVDF leads to switchable composites. • Reinforcement of the films with low loads of the complex is observed. • Thermomechanical behaviour of composites reveals mechanical effects of SCO. • Basic tensile properties of SCO composites are emerged. [ABSTRACT FROM AUTHOR]

Building a better SQUID: A prototype for a SQUID‐like magnetometry device for the indirect detection of room‐temperature switching in spin‐crossover nanoparticles has been developed and used in the study of [Fe(hptrz)3](OTs)2 (hptrz=4‐heptyl‐1,2,4‐triazole, OTs=p‐toluenesulfonyl) nanoparticles, as a proof of concept for this novel micromagnetometry approach. The method provides significant benefits over conventional SQUID and nano‐SQUID techniques. [ABSTRACT FROM AUTHOR]

Ein Prototyp eines SQUID ‐ artigen magnetometrischen Messgeräts, das Schaltvorgänge in Spin ‐ Crossover ‐ Nanopartikeln bei Raumtemperatur indirekt detektiert, wurde entwickelt und an Nanopartikeln aus [Fe(hptrz)3](OTs)2 getestet (hptrz=4 ‐ Heptyl ‐ 1,2,4 ‐ triazol, OTs=p ‐ Toluolsulfonyl). Der mikromagnetometrische Ansatz hat klare Vorteile gegenüber herkömmlichen SQUID ‐ und Nano ‐ SQUID ‐ Techniken. [ABSTRACT FROM AUTHOR]

Nano-objects and thin films displaying molecular spin-crossover phenomena have recently attracted much attention. However, the investigation of spin crossover at reduced sizes is still a big challenge. Here we demonstrate that surface plasmon polariton waves propagating along the interface between a metal and a dielectric layer can be used to detect the spin-state changes in the latter with high sensitivity, even at the nanometer scale. [ABSTRACT FROM AUTHOR]