Spin crossover bistability in three mutually perpendicular interpenetrated (4,4) nets

Exploring the equilibrium and non-equilibrium properties of a cooperative trinuclear spin-crossover chain: The role of elastic frustration

Among the large family of spin-crossover (SCO) solids, recent investigations focused on polynuclear SCO materials, whose specific molecular configurations allow the presence of multi-step transitions and elastic frustration. In this contribution, we develop the first elastic modeling of thermal and dynamical properties of trinuclear SCO solids. For that, we study a finite SCO open chain constituted of successive elastically coupled trinuclear (A=B=C) blocks, in which each site (A, B, and C) may occupy two electronic configurations, namely, low-spin (LS) and high-spin (HS) states, accompanied with structural changes. Intra- and inter-molecular springs couple the sites inside and between trimers. The model also includes the change of length inside and between the trinuclear units subsequent to the spin states changes. First, we studied the mechanical relaxation of a LS chain initially prepared with HS distances, from which we dissected the dynamics of the atomic displacements for various strengths of intra- and inter-molecular elastic constants. Second, we investigated the thermal properties of the chain at equilibrium, which revealed the existence of a rich variety of behaviors, going from: gradual LS to HS transition to multiple spin transitions with the presence of self-organized spin state structures in the plateaus. The latter were identified as emerging from antagonist short- and long-range elastic interactions between intra- and inter-block size changes. The present model opens several possible extensions, among which are the cases of coupled non-linear trimer molecules as well as that of inter-chain interactions with block-block interactions, leading to unexpected hysteretic spin transitions.

Ising-Like Model Hasn't Yet Said Its Last Word: Exact and Mean-Field Investigations of 2, 3 and 4-Body Interactions in 1D Ising Chain of Binuclear Spin-Crossover Solids

We investigate the static properties of a new class of 1D Ising-like Hamiltonian for binuclear spin-crossover materials accounting for two-, three-, and four-body short-range interactions between binuclear units of spins( s 1 A , s 1 B ) ${(s_1^A, s_1^B )}$ and( s 2 A , s 2 B ) ${(s_2^A, s_2^B )}$ . The following 2-, 3-, and 4-bodyJ 1 ( s 1 A + s 1 B ) ( s 2 A + s 2 B ) ${J_1 (s_1^A + s_1^B )(s_2^A + s_2^B )}$ ,K 1 s 1 A s 1 B ( s 2 A + s 2 B ) ${K_1 s_1^A s_1^B (s_2^A + s_2^B )}$ , andK 2 ( s 1 A s 1 B ) ( s 2 A s 2 B ) ${K_2 (s_1^A s_1^B )(s_2^A s_2^B )}$ terms are considered, in addition to intra-binuclear interactions, such as effective ligand-field energy and exchange-like coupling. An exact treatment is carried out within the frame of the transfer matrix method, leading to a 4×4 matrix from which, we obtained the thermal evolution of the thermodynamic quantities. Several situations of model parameter values were tested, among which that of competing intra- and inter-molecular interactions, leading to the occurrence of (i) one-step spin transition, (ii) two-, three-, and four-step transitions, obtained with a reasonable number of parameters. To reproduce first-order phase transitions, we accounted for inter-chains interactions, treated in the mean-field approach. Hysteretic multi-step transitions, recalling experimental observations, are then achieved. Overall, the present model not only suggests new landscapes of interaction configurations between SCO molecules but also opens new avenues to tackle the complex behaviors often observed in the properties of SCO materials.

Thermally and Photoinduced Spin-Crossover Behavior in Iron(II)-Silver(I) Cyanido-Bridged Coordination Polymers Bearing Acetylpyridine Ligands

We report two new cyanido-bridged Fe(II)-Ag(I) coordination polymers using different acetylpyridine isomers, {Fe(4acpy)2[Ag(CN)2]2} 1 and {Fe(3acpy)[Ag(CN)2]2} 2 (4acpy = 4-acetylpyridine; 3acpy = 3-acetylpyridine) displaying thermally and photoinduced spin crossover (SCO). In both cases, the acetylpyridine ligand directs the coordination polymer structure and the SCO of the materials. Using 4-acetylpyridine, a two-dimensional (2D) structure is observed in 1 made of layers stacked on each other by silver-ketone interactions leading to a complete SCO and reversible thermally and photoswitching of the magnetic and optical properties. Changing the acetyl group to a 3-position, a completely different structure is obtained for 2. The unexpected coordination of the carbonyl group to the Fe(II) centers induces a three-dimensional (3D) structure, leading to statistical disorder around the Fe(II) with three different coordination spheres, [N6], [N4O2], and [N5O]. This disorder gives rise to an incomplete thermally induced SCO with a poor photoswitchability. These results demonstrate that the choice of the acetyl position on the pyridine dictates the structural characteristics of the compounds with a direct impact on the SCO behavior. Remarkably, this work opens interesting perspectives for the future design of Fe-Ag cyanido coordination polymers with judiciously substituted pyridine ligands to tune the thermally and photoinduced SCO properties.

Pressure-induced multi-step and self-organized spin states in an electro-elastic model for spin-crossover solids

Spin transition materials are known to exhibit a rich variety of behaviors under several stimuli, among which pressure leads to major changes in their electronic and elastic properties. From an experimental point of view, thermal spin transitions under isotropic pressure showed transformations from (i) hysteretic to continuous transformations where the hysteresis width vanishes beyond some threshold pressure value; this is the conventional case. In several other cases very pathological and unexpected behaviours emerged, like (ii) persistent hysteresis under pressure; (iii) non-uniform behavior of the thermal hysteresis width which first increases with pressure and then decreases and vanishes at higher pressures; (iv) furthermore, double step transitions induced by pressure are also often obtained, where the pressure triggers the appearance of a plateau during the thermal transition, leading to two-step transitions, and finally (v) other non-conventional re-entrant transitions, where the thermal hysteresis vanishes at some pressure and then reappears at higher pressure values are also observed. In the present theoretical study, we investigate this problem with an electro-elastic description of the spin-crossover phenomenon by solving the Hamiltonian using a Monte Carlo technique. The pressure effect is here introduced directly in the lattice parameters, the elastic constants and ligand field energy. By considering spin state-dependent compressibility, we demonstrate that a large panel of experimental observations can be qualitatively described with this model. Among them, we quote (i) the conventional pressure effect decreasing the hysteresis width, (ii) the unconventional cases with pressure causing a non-monotonous behavior of the hysteresis width, (iii) re-entrant, as well as (iv) double step transitions accompanied with various types of spin state self-organization in the plateau regions.

Thermo- and photoinduced spin state switching in an iron(II) 2D coordination network associated with large light-induced thermal hysteresis and tuning of dimensionality via ligand modulation

Three iron(ii) complexes, [Fe(L1)2(NCS)2(MeOH)2] (1), [Fe(L1)2(NCSe)2(MeOH)2] (2), and [Fe(L2)2(NCS)2]n (3) (L1 = 2,5-dipyridyl-3,4-ethylenedioxythiophene and L2 = 2,5-diethynylpyridinyl-3,4-ethylenedioxythiophene), have been synthesized using redox-active luminescent ethylenedioxythiophene (EDOT)-based ligands, and characterized by variable temperature single-crystal X-ray diffraction, (photo)magnetic, optical reflectivity, and spectroscopy studies. Magneto-structural investigations revealed that 1 and 2 are mononuclear with a FeN4O2 octahedral coordination geometry and remain in a high-spin (HS) (S = 2) state in a temperature range of 2-280 K. Interestingly, a 2D coordination network structure with FeN6 surrounding each iron center was observed for 3, which exhibits reversible thermo-induced spin-state switching between the paramagnetic high-spin (HS) (S = 2) and diamagnetic low-spin (LS) (S = 0) states at around 105 K (T1/2). Furthermore, optical reflectivity and photomagnetic measurements at low temperature confirmed that 3 shows reversible ON/OFF switching between the photoinduced excited paramagnetic HS metastable state and diamagnetic LS state under light irradiation (ON mode using red light and OFF mode using green light). Finally, the photoinduced excited HS state can be reversibly relaxed back to the diamagnetic ground LS state by heating the system at ca. 88 K (TLIESST = 88 K) (light-induced excited spin state trapping (LIESST) effect). Furthermore, 3 also showed an exciting and unique 18 K wide light-induced thermal hysteresis (LITH) effect above liquid nitrogen temperature (100 K). DFT and CASSCF level theoretical calculations were utilized to better understand the magneto-structural correlations of these complexes.

Spin transition in a ferrous chloride complex supported by a pentapyridine ligand

Ferrous chloride complexes [Fe^IIL_xCl] commonly attain a high-spin state independently of the supporting ligand(s) and temperature. Herein, we present the first report of a complete spin crossover with T_1/2 = 80 K in [Fe^II(Py5OH)Cl]⁺ (Py5OH = pyridine-2,6-diylbis[di(pyridin-2-yl)methanol]). Both spin forms of the complex are analyzed by X-ray spectroscopy and DFT calculations.

Spin crossover in the Prussian blue analogue FePt(CN)6 induced by pressure or X-ray irradiation

Pressure and X-ray irradiation induced spin crossover is found in Prussian blue analogue FePt(CN)₆.

A thermal- and light-induced switchable one-dimensional rare loop-like spin crossover coordination polymer

Rare loop-like isostructural one-dimensional coordination polymer (1D-CP) systems formulated as {Fe(DPIP)₂(NCSe)₂}_n·4DMF (1) and {Fe(DPIP)₂(NCSe)₂}_n·4DMF (2) were obtained by self-assembling Fe^II and pseudohalide NCX^-(X = S, Se) ions in presence of the V-shaped bidentate bridging ligand, namely, N,N'-dipyridin-4-ylisophthalamide (DPIP), and were characterized by elemental analysis, IR spectroscopy, TGA, single crystal X-ray diffraction and powder X-ray diffraction. The magnetic studies show that complex 2 undergoes a complete thermally induced spin crossover (SCO) behavior centered at T_1/2 = 120 K with ca. 5 K thermal hysteresis loop and light-induced excited spin state trapping effect (LIESST) with T_LIESST = 65 K. However, either the homologous X = S (1) or the desolvated form of complex 2 is high spin at all temperatures, proving further the concerted synergy for the SCO of 2 between the intrinsic ligand field and that indirectly induced via hydrogen bond interaction. The current results provide valuable information for the design of new 1D SCO systems via the rational control of the cooperated effects derived from the intramolecular coordination bond and the intermolecular supramolecular interactions.

A luminescent Pt2Fe spin crossover complex

A heterotrinuclear luminescent [Pt₂Fe] spin crossover (SCO) complex was developed, synthesized, and investigated.

Guest-dependent single-ion magnet behaviour in a cobalt(ii) metal-organic framework

Single-ion magnets (SIMs) are the smallest possible magnetic devices for potential applications in quantum computing and high-density information storage. Both, their addressing in surfaces and their organization in metal-organic frameworks (MOFs) are thus current challenges in molecular chemistry. Here we report a two-dimensional 2D MOF with a square grid topology built from cobalt(ii) SIMs as nodes and long rod-like aromatic bipyridine ligands as linkers, and exhibiting large square channels capable to host a large number of different guest molecules. The organization of the cobalt(ii) nodes in the square layers improves the magnetic properties by minimizing the intermolecular interactions between the cobalt(ii) centres. Moreover, the SIM behaviour was found to be dependent on the nature of the aromatic guest molecules. The whole process could be followed by single-crystal X-ray diffraction, providing comprehensive evidence of the putative role of the solvent guest molecules that leave a "fingerprint" on the 2D structures and thus, on the cobalt environment.

Anion directed structural diversity in zinc complexes with conformationally flexible quinazoline ligand: structural, spectral and theoretical studies

In this paper, we report the synthesis, and structure of four new complexes of conformationally flexible 6-chloro-4-phenyl-2-(pyridin-2-yl)quinazoline ligand (L) with Zn(ii). Significance of ring twisting on supramolecular assembly and photophysical properties have also been highlighted.

Highly unusual interpenetration isomers of electroactive nickel bis(dithiolene) coordination frameworks

The metalloligand [Ni(pedt)2](-) (pedt = 1-(pyridine-4-yl)ethylene-1,2-dithiolate) has been incorporated into two multi-dimensional structures for the first time. These coordination frameworks represent highly unusual interpenetration isomers and exhibit solid state redox and optical properties that reflect the electronically delocalised nature of the metalloligand.

Two-Dimensional Coordination Polymers with Spin Crossover Functionality

In the solid state, the propagation of spin crossover (SCO) information is governed by a complex interplay between inner and outer coordination sphere effects. In this way, lattice cooperativity can be enhanced through solid state packing interactions (i.e. hydrogen-bonding and π-stacking) and via coordinatively linking spin switching sites (i.e. coordination polymers). SCO framework materials have successfully provided an avenue for enhanced cooperativity and additional function as host–guest sensors via their potential porosity. In this review, we explore two-dimensional SCO coordination polymers: (1) spin crossover frameworks (SCOFs) consisting of (4,4) grids and (2) Hofmann-type materials where layers are separated by organic ligands. These families have each allowed the elucidation of important structure–function properties and provided a novel platform for molecular sensing applications. Towards advancing the field of infinite polymeric SCO materials, two-dimensional materials can offer flexible porosity, potentially leading to novel spin state-switching functionality.

Two- and one-step cooperative spin transitions in Hofmann-like clathrates with enhanced loading capacity

Naphthalene and nitrobenzene clathrates of a new Hofmann-like porous MOF with improved loading capacity display highly cooperative spin crossover behaviour.

Spin state switching in iron coordination compounds

The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligo- and polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

Role of Fe-N-C geometry flip-flop in bistability in Fe(tetrazol-2-yl)4(C2H5CN)2-type core based coordination network

[Fe(ebtz)(2)(C(2)H(5)CN)(2)](ClO(4))(2) was prepared in the reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(ClO(4))(2)·6H(2)O in propionitrile. The compound crystallizes as a one-dimensional (1D) network, where bridging of neighboring iron(II) ions by two ebtz ligand molecules results in formation of a [Fe(ebtz)(2)](∞) polymeric skeleton. The 1D chains are assembled into supramolecular layers with axially coordinated nitrile molecules directed outward. The complex in the high spin (HS) form reveals a very rare feature, namely, a bent geometry of the Fe-N-C(propionitrile) fragment (149.1(3)° at 250 K). The HS to low spin (LS) HS→LS transition triggers reorientation of the propionitrile molecule resulting in accommodation of a typical linear geometry of the Fe-N-C(nitrile) fragment. The switching of the propionitrile molecule orientation in relation to the coordination octahedron is associated with increase of the distance between the supramolecular layers. When the crystal is in the LS phase, raising the temperature does not cause reduction of the distance between supramolecular layers, which contributes to further stabilization of the more linear geometry of Fe-N-C(C(2)H(5)) and the LS form of the complex. Thus, a combination of Fe-N-C(C(2)H(5)) geometry lability and lattice effects contributes to the appearance of hysteretic behavior (T(1/2)(↓) ≈ 112 K, T(1/2)(↑) ≈ 141 K).

HS⇄LS transition in iron(II) two-dimensional coordination networks containing tris(tetrazol-1-ylmethyl)methane as triconnected building block

Novel tripodal ligand 1,1',1''-tris(tetrazol-1-ylmethyl)methane (111tz) and products of its reactions with perchlorate as well as with tetrafluoroborate salts of iron(II) are presented. The isostructural complexes, [Fe(111tz)₂](ClO₄)₂ and [Fe(111tz)₂](BF₄)₂, were isolated as two-dimensional (2D) coordination networks revealing a honeycomb-like pattern with cages occupied by disordered anions. 111tz molecules act as a tridentate ligand bridging three adjacent Fe(II) ions, and the nitrogen N4 atom of six tetrazole rings (tz) is placed in octahedron vertices of FeN₆ chromophores. The complexes, crystallizing in the P3 space group, were characterized by variable-temperature single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Variable-temperature magnetic susceptibility measurements show that both systems undergo abrupt and complete spin transition with T(1/2)(↑) = T(1/2)(↓) = 176 K for perchlorate and T(1/2)(↑) = 193.8 and T(1/2)(↓) = 192.8 K for the tetrafluoroborate analogue. Change of spin state in both complexes is accompanied by a thermochromic effect. The HS→LS transition in [Fe(111tz)₂](ClO₄)₂ involves shortening of the Fe-N4 bond lengths from 2.171(2) Å (293 K) to 2.002(1) Å (100 K). In [Fe(111tz)₂](BF₄)₂, lowering of temperature from 293 to 100 K is accompanied by shortening of the Fe-N4 distances from 2.179(2) to 1.987(2) Å, respectively. Perchlorate in [Fe(111tz)₂](ClO₄)₂ or tetrafluoroborate anions in [Fe(111tz)₂](BF₄)₂ are engaged in the formation of intermolecular contacts within as well as with the neighboring 2D layer.

1D and 2D assembly structures by imidazole···chloride hydrogen bonds of iron(II) complexes [Fe(II)(HL(n-Pr))3]Cl·Y (HL(n-Pr) = 2-methylimidazol-4-yl-methylideneamino-n-propyl; Y = AsF6, BF4) and their spin states

Two Fe(II) complexes fac-[Fe(II)(HL(n-Pr))(3)]Cl·Y (Y = AsF(6) (1) and BF(4) (2)) were synthesized, where HL(n-Pr) is 2-methylimidazole-4-yl-methylideneamino-n-propyl. Each complex-cation has the same octahedral N(6) geometry coordinated by three bidentate ligands and assumes facial-isomerism, fac-[Fe(II)(HL(n-Pr))(3)](2+) with Δ- and Λ-enantiomorphs. Three imidazole groups per Δ- or Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) are hydrogen-bonded to three Cl(-) ions or, from the viewpoint of the Cl(-) ion, one Cl(-) ion is hydrogen-bonded to three neighbouring fac-[Fe(II)(HL(n-Pr))(3)](2+) cations. The 3 : 3 NH···Cl(-) hydrogen bonds between Δ- or Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) and Cl(-) generate two kinds of assembly structures. The directions of the 3 : 3 NH···Cl(-) hydrogen bonds and hence the resulting assembly structures are determined by the size of the anion Y, though Y is not involved into the network structure and just accommodated in the cavity. Compound 1 has a 1D ladder structure giving a larger cavity, in which the Δ- and Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) enantiomorphs are bridged by two NH···Cl(-) hydrogen bonds. Compound 2 has a 2D network structure with a net unit of a cyclic trimer of {fac-[Fe(II)(HL(n-Pr))(3)](2+)···Cl(-)}(3) giving a smaller cavity, in which Δ- or Λ-fac-[Fe(II)(HL(n-Pr))(3)](2+) species with the same chirality are linked by NH···Cl(-) hydrogen bonds to give a homochiral 2D network structure. Magnetic susceptibility and Mössbauer spectral measurements demonstrated that compound 1 showed an abrupt one-step spin crossover with 4.0 K thermal hysteresis of T(c↓) = 125.5 K and T(c↑) = 129.5 K and compound 2 showed no spin transition and stayed in the high-spin state over the 5-300 K temperature range.

A supramolecular spin crossover Fe(III) complex and its Cr(III) isomer: stabilization of water-chloride cluster within supramolecular host

The metal complexes, [M(Hdammthiol)(2)]Cl·3H(2)O [M = Cr(III) (1), Fe(III) (2)] [where H(2)dammthiol is the thiol form of the ligand, diacetylmonoxime morpholine N-thiohydrazone] were synthesized by metal template reactions of diacetylemonoxime with morpholine N-thiohydrazide in the presence of CrCl(3)·6H(2)O and FeCl(3)·6H(2)O. Both the complexes (1 and 2) were characterized by single crystal X-ray crystallography, spectroscopic (IR and UV-vis), Mössbauer and TGA analyses. The single crystal X-ray studies of both complexes show that the supramolecular hosts, constructed by the discrete mononuclear complexes, form supramolecular channels along the c-axis which are filled up by water-chloride clusters. In both complexes, the 1D water-chloride chain with chair-like architecture within the supramolecular hosts presents novelty. The magnetic measurement study of Fe(III) complex shows a spin crossover from S = 1/2 at 2.5 K to S = 5/2 at 300 K. At very low temperature, the presence of strong cooperative hydrogen bonding interactions stabilizes the S = 1/2 state.

Temperature-dependent in situ ligand cyclization via C═C coupling and formation of a spin-crossover iron(II) coordination polymer

The reaction of N(1),N(2)-bis(pyridin-4-ylmethylene)ethane-1,2-diamine (L) with Fe(NCS)(2) under various temperatures gave rise to three iron(II) coordination polymers, namely, one-dimensional [Fe(L')(NCS)(2)] (1), two-dimensional [Fe(L)(2)(NCS)(2)]·H(2)O (2), and one-dimensional [Fe(L)(2)(NCS)(2)]·2CH(2)Cl(2)·4MeOH (3). The formation of 1 involved an in situ C═C coupling reaction, L to L' [L' = 5,6-di(pyridin-4-yl)-1,2,3,4-tetrahydropyrazine], which was catalyzed by cyanide ions decomposed from thiocyanates; the manganese(II) (1a) and zinc(II) (1b) analogues of 1 were also synthesized for comparison. Magnetic studies showed that complex 1 underwent a pressure-dependent one-step incomplete spin transition whereas complexes 2 and 3 were paramagnetic in the whole temperature range.

Tetrazol-2-yl as a Donor Group for Incorporation of a Spin-Crossover Function Based on Fe(II) Ions into a Coordination Network

The coordination polymer {[Fe(pbtz)3](ClO4)2 . 2EtOH}infinity (1) has been prepared in a reaction between Fe(ClO4)2 . 6H2O and 1,3-di(tetrazol-2-yl)propane (pbtz). The formation of the second product {[Fe(pbtz)3](ClO4)2}infinity (2) was also noticed. Both complexes crystallize in the R3 space group. The single-crystal X-ray diffraction study of 1 (295, 90 and 230 K) revealed that the 2-substituted tetrazole rings (2tz) coordinate monodentately to the metal ions, forming Fe(2tz)6 cores. There are two crystallographically independent iron(II) ions in 1. At 295 K the Fe-N4 bond lengths are equal to 2.173(5) and 2.196(5) A for Fe1 and 2.176(5) and 2.190(4) A for Fe2. The pbtz ligand molecules act as N4,N4' connectors, bridging central atoms in the three directions, which leads to the formation of the 3D network. The crystal lattice of 1 is solvated by ethanol molecules. At 295 K the solvent and ligand molecules are disordered. The results of temperature-dependent magnetic susceptibility measurements (5-300 K), and the single-crystal X-ray diffraction studies (90 K) have exhibited that 1 undergoes the thermally induced spin transition HS<-->LS (SCO). The chiMT(T) dependence shows in the range 200-75 K gradual SCO. Below 75 K the transition is finished and approximately 20% of the HS fraction is present in the sample. The HS-->LS transition is accompanied by a shortening of the Fe-N bonds of 0.15 A. At 90 K the ligand molecules are ordered. The presence of 2 in the reaction product was disclosed accidentally, and only the X-ray diffraction studies (250, 90 K) were performed. Also in 2 iron(II) ions serve as topological nodes of the 3D network. Despite the same network topology, 2 crystallizes without ethanol molecules solvating the crystal lattice. The pbtz molecules bridge the neighboring iron(II) ions, coordinating through N4,N4' atoms of the 2-substituted tetrazole rings forming the Fe(2tz)6 cores. At 250 K the Fe-N bond lengths are equal to 2.208(5) and 2.218(5) A. In contrast to 1, the cooling of the crystal of 2 from 250 to 90 K does not involve the shortening of the Fe-N bond lengths. At this temperature, the Fe-N distances remain characteristic for the HS form of the complex and are equal to 2.203(3) and 2.208(3) A.