Show Spin Crossover Research Articles

Co-crystallisation as a modular approach to the discovery of spin-crossover materials.

Herein we present co-crystallisation as a strategy for materials discovery in the field of switchable spin crossover (SCO) systems. Using [Fe(3-bpp)2]·2A (where 3-bpp = 2,6-bis(pyrazol-3-yl)pyridine, A = BF4−/PF6−) as a starting point, a total of 11 new cocrystals have been synthesised with five different dipyridyl coformers. Eight of these systems show spin crossover behaviour, and all show dramatically different switching properties from the parent complex. The cocrystals have been studied by variable temperature single-crystal X-ray diffraction and SQUID magnetometry to develop structure–property relationships. The supramolecular architecture of the cocrystals depends on the properties of the coformer. With linear, rigid coformer molecules leading to 1D supramolecular hydrogen-bonded chains, while flexible coformers form 2D sheets and bent coformers yield 3D network structures. The SCO behaviour of the cocrystals can be modified through changing the coformer and thus co-crystallisation presents a rapid, facile and highly modular tool for the discovery of new switchable materials. The wider applicability of this strategy to the design of hybrid multifunctional materials is also discussed.

Temperature and Counterion Dependent Spin Crossover in a Hexaamineiron(II) Complex

AbstractBased on previous results with [Fe(tame)2]Cl2.MeOH (tame=1,1,1‐tris(aminomethyl)ethane), which exhibits temperature dependent spin crossover, we report a series of isostructural rhombohedral salts [Fe(tame)2]X2.MeOH (X=Br−, ClO4−, BF4−) and examine their temperature dependent structures. In the case of [Fe(tame)2]Br2.MeOH, temperature dependent single crystal Vis‐NIR spectroscopy is reported as a complement to single crystal X‐ray diffraction results. The [Fe(tame)2]Br2.MeOH compound does show spin crossover behaviour but at very low temperatures (<100 K) and the spin active complex cation could not be converted exclusively to its low spin form even at 12 K. This is significantly different to its relative [Fe(tame)2]Cl2.MeOH which is entirely low spin at 60 K. The isostructural [Fe(tame)2]X2.nMeOH (X=ClO4− (n=0.5) and BF4− (n=1)) compounds show no spin crossover at the temperatures examined and remain exclusively in their high spin form. Removal of the MeOH solvent leads to another isostructural compound [Fe(tame)2](ClO4)2, which shows a remarkable reversible loss of crystallinity below 200 K that could be restored by warming to temperatures above 200 K. The fluoride and trifluoromethanesulfonate salts of [Fe(tame)2]2+ crystallise in monoclinic lattices and show no spin crossover behaviour.

Iron(II) spin crossover complexes with a sulfur rich ligand backbone

One goal that many scientists pursue is the unification of several interesting chemical or physical properties in one system, as only multifunctional materials will meet the challenges of today's technologies. With this background, three novel iron(II) coordination compounds with a Schiff base-like N2O2 coordinating ligand L bearing a sulfur-rich backbone are investigated in this work. Two of the complexes, the mononuclear [FeL(py)2] (py = pyridine) and the coordination polymer {[FeL(bpee)]}n [bpee = trans-1,2-bis(4-pyridyl)ethene], show spin crossover behavior followed using magnetic susceptibility measurements and Mössbauer spectroscopy. To get a closer insight into different states of the complex spin crossover behavior of {[FeL(bpee)]}n, XPS measurements were conducted at different temperatures. Furthermore, impedance spectroscopic measurements at variable temperatures were performed to get insight into the electrical conductivity of this system. All iron(II) complexes were electrochemically characterized using cyclovoltammetric measurements, supplemented by DFT computation. Apparently, the extension by a sulfur backbone leads to a stabilization of the HOMO. Due to this, the complexes are more difficult to oxidize than comparable systems. With {[FeL(azpy)]}n, another coordination polymer, this time axially decorated by the redox-active ligand azpy (azpy = 4,4′-azopyridine), is investigated, which is, however, a pure high spin complex.

Study of Spin Crossover Property of a Series of X‐OMe‐SalEen (X=6, 5 and 4) Based Fe(III) Complexes

AbstractThree complexes bearing the molecular formula [Fe(X‐OMe‐SalEen)2]NCY⋅nH2O; X=6, Y=S, n=0 (1); X=5, Y=S, n=1.5 H2O (2) and X=4, Y=Se, n=0 (3), where 6‐OMe‐SalEen=2‐((E)‐(2‐(ethylamino)ethylimino)methyl)‐6‐methoxyphenol, 5‐OMe‐SalEen=2‐((E)‐(2‐(ethylamino)ethylimino)methyl)‐5‐methoxyphenol, and 4‐OMe‐SalEen=2‐((E)‐(2‐(ethylamino)ethylimino)methyl)‐4‐methoxyphenol, have been synthesized, characterized and their spin crossover behavior has been studied. The ligands differ in the position of the methoxy group (ortho, meta, and para) of the phenoxide ring directly bonded to the Fe center. Complex 1 and 3 comprise FeIII centers, whereas rare crystallographically independent high‐spin and low‐spin FeIII centers are present in complex 2. Complex 1 shows a sharp and hysteric spin crossover behavior with a hysteresis loop having 4 K width. Complex 2 shows spin crossover for the FeIIILS in solvated form which after desolvation becomes stable at the high spin. Complex 3 shows abrupt and incomplete spin crossover above room temperature. Magneto‐structural correlation and UV‐Vis spectroscopy have been done to study spin‐crossover behavior and the spin transition temperature of the complexes.

Thermally induced spin transition in Fe(pyrazine)[Fe(CN)5NO

Transition metal nitroprussides form a family of coordination polymers with interesting physical properties. From their three-dimensional (3D) phases, by inducing rupture of the axial T–NC bond through organic molecules (L) with a high ability to form a T–L coordination bond, pillared transition metal nitroprussides can be obtained. For monodentate molecules, for example, pyridine and its derivatives, the pillars are formed in the interlayer region by pairs of molecules coupled through their dipole moments, and the resulting solid has a two-dimensional (2D) structure with T(L)2[Fe(CN)5NO] as the formula unit. In this contribution, we report the use of a bidentate molecule, pyrazine, to obtain pillared ferrous nitroprusside. According to the refined crystal structure, this solid has a 3D framework, which results from the pyrazine molecule (L) coordinating to two iron atoms to afford Fe(pyrazine)[Fe(CN)5NO]. This material shows spin-crossover (SCO) behavior with thermal hysteresis of about 40 K, which is associated with relatively large structural changes, for instance, a reversible variation of 12% in the unit cell volume. The thermally induced spin transition is accompanied by a notable color change of the material. The main features of this thermally activated spin transition in the material under study are discussed herein on the basis of its structural characterization at 100 and 300 K, magnetic and differential scanning calorimetry (DSC) measurements, and Raman and Mössbauer spectra for both the low- and high-spin phases. The reversibility of this thermally activated spin transition has been verified by magnetic, Mössbauer, X-ray powder diffraction (XRD), and infrared (IR) data. To the best of our knowledge, this is the first report on the preparation and characterization of Fe(pyrazine)[Fe(CN)5NO] and its thermally induced SCO behavior.

Influence of CF3 Substituents on the Spin Crossover Behavior of Iron(II) Coordination Polymers with Schiff Base‐like Ligands

A Schiff base‐like ligand bearing CF3 substituents was synthesized and converted to iron(II) coordination polymers [{FeL(Lax)}n] using five different bridging ligands Lax. The structure of the coordination polymers was investigated using powder X‐ray diffraction and single‐crystal X‐ray diffraction in the case of [{FeL(bipy)}n]. The later revealed an untypical ABAB pattern of alternating equatorial ligands rotated by 180° with regard to each other along the chain. The temperature‐dependent magnetic behavior was investigated with a SQUID magnetometer and the spin states at room temperature were confirmed by 57Fe‐Mössbauer spectroscopy. Three out of five coordination polymers show spin crossover behavior in the temperature range between 50 and 400 K with different kind of curve progressions (abrupt, gradual, step‐wise). The other two coordination polymers are either fully highspin or fully low spin.

Directing self-assembly in solution towards improved cooperativity in Fe(iii) complexes with amphiphilic tridentate ligands.

An amphiphilic iron(iii) complex with a tridentate Schiff-base ligand was prepared by condensation of a hexadecyloxy functionalised salycylaldehyde with a diamine followed by complexation with FeCl2 and anion methathesis with NaClO4. The complex shows spin crossover both in the solid state and solution. However in solution self-assembly and consequently aggregation of individual molecules form concentration dependent particles with sizes of 300 nm for higher concentrations, or 5 nm for lower concentrations. Aggregate formation was confirmed by NANO-flex 180° DLS Size, scan-rate dependent cyclic voltammetry and scanning electron microscopy. Molecular simulations were used to investigate the self-assembly of the complex in solution, including the role of residual water molecules. The simulations showed the self-assembly of reverse micelle-like structures when a small water cluster is inserted in solution, whereas no large aggregates formed in dehydrated environments. The perchlorate anions were found near the metal centres, stabilizing the aggregates around the water pool. Simulations of pre-assembled structures further showed the lack of stability of large aggregates in the absence of water. The larger aggregates promoted efficient communication between the iron(iii) centres and the compound displayed spin crossover in solution at around 220 K with a 10 K hysteresis window, as measured by NMR and SQUID magnetometry.

Synthesis of iron(II) complexes with asymmetric N2O2 coordinating Schiff base-like ligands and their spin crossover properties

The synthesis of new Schiff base-like ligands with asymmetric substituents pattern and their iron complexes with pyridine as axial ligand is described. Two of the ligands and one of the iron(II) complexes were characterized by single crystal X-ray structure analysis. One of the the iron(II) complexes shows spin crossover behavior while the others remain in the high spin state. The influence of the reduced symmetry of the ligand on the properties of the complexes is discussed.

On the Magnetic Coupling and Spin Crossover Behavior in Complexes Containing the Head-to-Tail [FeII2(μ-SCN)2] Bridging Unit: A Magnetostructural Experimental and Theoretical Study.

A new dinuclear complex [{Fe(tpc-OBn)(NCS)(μ-NCS)}2] (1) based on the tripodal tpc-OBn ligand (tpc-OBn = tris(2-pyridyl)benzyloxymethane), containing bridging μ-κN:κS-SCN and terminal κN-SCN thiocyanate ligands, has been prepared and characterized by single crystal X-ray diffraction, magnetic studies, and DFT theoretical calculations. This complex represents the first example of dinuclear FeII complex with double μ-κN:κS-SCN bridges in a head-to-tail configuration that exhibits ferromagnetic coupling between metal ions (JFeFe = +1.08 cm-1). Experimental and theoretical magnetostructural studies on this kind of infrequent FeII dinuclear complex containing a centrosymmetrically [Fe2(μ-SCN)2] bridging fragment show that the magnitude and sign of the magnetic coupling parameter, JFeFe, depend to a large extent on the Fe-N-C (α) angle, so that JFeFe decreases linearly when α decreases. The calculated crossover point below which the magnetic interactions change from ferromagnetic to antiferromagnetic is found at 162.3°. In addition, experimental results obtained in this work and those reported in the literature suggest that large Ntripodal-FeII distances and bent N-bound terminal κN-SCN ligands favor the high spin state of the FeII ions, while short Ntripodal-FeII distances and almost linear Fe-N-C angles favor a stronger ligand field, which enables the FeII ions to show spin crossover (SCO) behavior.

Synthesis of Anion-Mixed Crystals of Assembled Complexes Bridged by 1,2-Bis(4-pyridyl)ethane and the Ligand Field of Fe(NCS)(NCBH3) Unit

Abstract Anion-mixed iron(II) assembled complexes using NCS and NCBH3 as anionic ligand and bridged by 1,2-bis(4-pyridyl)ethane (bpe) are synthesized under several conditions for various ratios of NCS to NCBH3. Elemental analysis showed that the experimentally observed fractions of NCS tend to be more than those in the preparation. Some structures are determined by using single crystal X-ray structural analysis. Powder X-ray diffraction patterns are compared among the anion-mixed crystals. 57Fe Mössbauer spectroscopy reveals that a new doublet appears and it is different from the doublets of Fe(NCS)2 and Fe(NCBH3)2 units and the relative area of this new doublet reaches maximum when the fraction of NCS is around 1.0 per each iron, suggesting that the new doublet is ascribed to an Fe(NCS)(NCBH3) unit. Corresponding anion-mixed complexes enclathrating p-dichlorobenzene (p-DCB) are synthesized. Superconducting quantum interference device (SQUID) measurement and Mössbauer spectroscopy reveal that the complexes show spin-crossover (SCO) phenomenon and the relative area of the low-spin state is more than the expected value. The existence of Fe(NCS)(NCBH3) is supported by the results of SQUID measurement and density functional theory (DFT) calculation. Ligand field splitting strength of Fe(NCS)(NCBH3) unit is estimated to be intermediate between Fe(NCS)2 and Fe(NCBH3)2 units. Complexes enclathrating p-DCB display propeller conformation of bpe around the iron atom, while complexes without a guest molecule have distorted propeller conformation of bpe around the iron atom. It is suggested that such difference determines whether the SCO is on or off.

Triggering Two-Step Spin Bistability and Large Hysteresis in Spin Crossover Nanoparticles via Molecular Nanoengineering

The local entrapment of the spin crossover complex Fe(II)-tris[2-(2′-pyridyl)benzimidazole] into the pluronic polymeric matrix (P123, PEG20–PPG70–PEG20, MW ∼ 5800) yielded the formation of magnetic nanoparticles of ∼26 nm (SCO-Np). Formation of SCO-Np was driven by the emergence of noncovalent interactions between the aromatic −NH group of the benzimidazole moieties present in Fe(II)-tris[2-(2′-pyridyl)benzimidazole] with the aliphatic ether (−O−) groups of the pluronic polymeric matrix. The nanoparticles show spin crossover behavior, two-step spin bistability, and wide magnetic hysteresis, expressed in the temperature range of 170–280 K (ΔTmax = 38 K). The neat SCO molecules, Fe(II)-tris[2-(2′-pyridyl)benzimidazole], on the contrary show only first-order spin transition and negligible hysteresis. The developed matrix-confinement approach of SCO molecules shown in this work yielded an unprecedented and significant improvement of the magnetic cooperativity compared to the neat spin crossover system, despit...

Synthesis, Crystal Structures and Magnetic Properties of Composites Incorporating an Fe(II) Spin Crossover Complex and Polyoxometalates

[Fe(dppOH)2]2+ (dppOH = 2,6-di(pyrazol-1-yl)-4-(hydroxymethyl)pyridine) is known to show spin crossover (SCO) behavior and light-induced excited spin state transitions (LIESST). Here, we show that the SCO properties of the [Fe(dppOH)2]2+ complex can be altered by a crystal engineering approach employing counter anion exchange with polyoxometalate (POM) anions. Using this strategy, two new composite materials (TBA)[Fe(dppOH)2][PMo12O40] (1) and [Fe(dppOH)2]3[PMo12O40]2 (2) (TBA = tetra-n-butylammonium) have been isolated and studied by single crystal X-ray diffraction and magnetic susceptibility measurements. 1 was found to be in a high spin state at 300 K and showed no spin crossover behavior due to a dense packing structure induced by hydrogen bonding between the hydroxyl group of the dppOH ligands and the POM anions. Conversely, 2 contains two crystallographically unique Fe centers, where one is in the low spin state whilst the other is locked in a high spin state in a manner analogous to 1. As a result, 2 was found to show partial spin crossover behavior around 230 K with a decrease in the χmT value of 1.9 emu·mol−1·K. This simple approach could therefore provide a useful method to aid in the design of next generation spin crossover materials.

Effect of Chelate Ring Size in Iron(II) Isothiocyanato Complexes with Tetradentate Tripyridyl‐alkylamine Ligands on Spin Crossover Properties

Abstract Iron(II) complexes of the type [Fe(L)(NCS)2] with tetradentate ligands L are well known to show spin crossover properties. However, this behavior is quite sensitive in regard to small changes of the ligand system. Starting from the thoroughly investigated complex [Fe(tmpa)(NCS)2] [tmpa = tris(2‐pyridylmethyl)amine, also abbreviated as tpa in the literature] we modified the ligand by increasing systematically the chelate ring sizes from 5 to 6 thus obtaining complexes [Fe(pmea)(NCS)2], [Fe(pmap)(NCS)2], and [Fe(tepa)(NCS)2] [pmea = N,N‐bis[(2‐pyridyl)methyl]‐2‐(2‐pyridyl)ethylamine, pmap = N,N‐bis[2‐(2‐pyridyl)ethyl]‐(2‐pyridyl)methylamine, and tepa = tris[2‐(2‐pyridyl)ethyl]amine]. All complexes were structurally characterized and spin crossover properties were investigated using Mößbauer spectroscopy, magnetic measurements, and IR/Raman analyses. The results demonstrated that only the iron complexes with tmpa and pmea showed spin crossover properties, whereas the complexes with the ligands pmap and tepa only formed high spin complexes. Furthermore, DFT calculations supported these findings demonstrating again the strong influence of ligand environment. Herein the effect of increasing the chelate ring sizes in iron(II) isothiocyanato complexes with tetradentate tripyridyl‐alkylamine ligands is clearly demonstrated.

Drastic effect of lattice propionitrile molecules on the spin-transition temperature of a 2,2'-dipyridylamino/s-triazine-based iron(II) complex.

Reaction of iron(II) selenocyanate (obtained from Fe(ClO4)2 and KNCSe) with 2-(N,N-bis(2-pyridyl)amino)-4,6-bis(pentafluorophenoxy)-(1,3,5)triazine (L1(F)) in propionitrile produces the compound [Fe(L1(F))2(NCSe)2]·2CH3CH2CN (1(NCSe)·2PrCN), which shows spin-crossover (SCO) properties characterized by a T(1/2) of 283 K and a ΔT80 (i.e., temperature range within which 80% of the transition considered occurs) of about 65 K. Upon air exposure, 1(NCSe)·2PrCN gradually converts to a new SCO species that exhibits different properties, as reflected by T(1/2) = 220 K and ΔT80 = 70 K. Various characterization techniques, namely, IR spectroscopy, thermogravimetric analysis, and thermodiffractometric studies, reveal that the new phase is obtained through the loss of the lattice propionitrile molecules within several days upon air exposure or several hours upon heating above 390 K.

Dimeric Ni(II)2 and polymeric Ni(II)4Fe(II) type complexes bridged with Cl− and CN− ligands: X-ray structures and magnetic properties of a dimeric complex of [(tren)Ni(μ-Cl)2Ni(tren)](ClO4)2 and a polymeric complex of {[Fe(CN)6][Ni(tren)]2[Ni(tren)(H2O)]2}Cl2(ClO4)2·4H2O

Abstract A dimeric complex of [(tren)Ni(μ-Cl)2Ni(tren)](ClO4)2 (1) and a polymeric complex of {[Fe(CN)6][Ni(tren)]2[Ni(tren)(H2O)]2}Cl2(ClO4)2·4H2O (2), are isolated from the reaction of potassium ferricyanide, nickel chloride and tren and their X-ray structures and magnetic properties were characterized. Two [Ni(tren)]2 + units are linked through bis μ-Cl ligands forming dimeric structure of 1. In complex 2, there are two kinds of Ni(II) unit, terminal ([Ni(tren)(H2O)]2 +) and bridged ([Ni(tren)]2 +), and this bridged [Ni(tren)]2 + units are linked with two different Fe(II) ions through μ-NC ligands forming one-dimensional polymeric rectangular chain. 1 shows a weak ferromagnetic coupling between two Ni(II) ions and 2 shows spin crossover in Fe(II) ion.

Explanation of the site-specific spin crossover in Fe(mtz)6(BF4)2

The spin crossover behavior of the two [Fe(mtz)6](2+) complexes occupying different lattice sites in Fe(mtz)6(BF4)2 is addressed by combining quantum chemical calculations with a careful analysis of the crystal structure. It is first established from the calculations that the energy difference between high spin and low spin states depends on the orientation of the tetrazole ligands; small rotation angles favor the low spin state, while for angles larger than ∼20° the high spin state is more stable. The crystal structure shows that the two complexes have different average rotation angles of the ligands. It is larger for the site that remains HS down to low temperatures and smaller for the site that shows spin crossover to LS. The origin of the different rotation angles is found to be determined by a subtle interplay amongst steric repulsion between the ligands, HF interactions between the complex and the counterions, and intersite interactions involving NH contacts and π-π interactions between the N[double bond, length as m-dash]N double bonds of the tetrazole rings.

Anion‐Dependent Spin Crossover and Coordination Assembly Based on [Fe(tpa)]2+ [tpa = tris(2‐pyridylmethyl)amine] and [N(CN)2]–: Square, Zigzag, Dimeric, and [4+1]‐Cocrystallized Complexes

AbstractA series of FeII complexes based on [Fe(tpa)]2+ building units and [N(CN)2]– bridging ligands with different anions for charge balance has been prepared and characterized. They have a general formula [Fe(tpa){N(CN)2}]x·Y·Z, where x = 4 (14+), Y = 4ClO4–, and Z = 2H2O for 1·(ClO4)4(H2O)2; x = n (2n+), Z = 0, and Y = nClO4–, nPF6–, nAsF6–, and nBPh4– for 2·(ClO4)n, 2·(PF6)n, 2·(AsF6)n, and 2·(BPh4)n, respectively; x = 2 (32+), Z = 0, and Y = 2BPh4– for 3·(BPh4)2; x = 4 (44+), Y = 3SbF6– + [N(CN)2]–, and Z = [Fe(tpa){N(CN)2}2] (5) + 4H2O for (4+5). Single‐crystal X‐ray diffraction studies reveal that these complexes show various structures that are affected by different counteranions, i.e., square 1·(ClO4)4(H2O)2, zigzag polymers [2·(ClO4)n, 2·(PF6)n, 2·(AsF6)n, and 2·(BPh4)n], dimeric 3·(BPh4)2, and [4+1]‐cocrystallized (4+5). Magnetic studies show that the square complex 1·(ClO4)4(H2O)2 undergoes a two‐step complete spin crossover, whereas the dimer 3·(BPh4)2 is high‐spin in the whole temperature range, and all zigzag polymer 2·anions show spin‐crossover (SCO) behavior, the transition temperature (T1/2) of which increases with increasing anion size. Interestingly, complex (4+5) displays a gradual two‐step spin crossover, which is different from that of 1·(ClO4)4(H2O)2 and originates from the cocrystallized spin‐crossover tetranuclear and monomeric species. Detailed studies on the crystal structures and magnetic properties have unveiled a remarkable anion‐dependent formation of different structures and SCO behavior of these complexes.

Cobalt Complexes with “Click”-Derived Functional Tripodal Ligands: Spin Crossover and Coordination Ambivalence

We demonstrate the use of a Cu(I) catalyzed "Click" reaction in the synthesis of novel ligands for spin crossover complexes. The reaction between azides and alkynes was used to synthesize the reported tripodal ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBTA, and the new ligands tris[(1-cyclohexyl-1H-1,2,3-triazol-4-yl)methyl]amine, TCTA, and tris[(1-n-butyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBuTA. Reactions of TBTA with Co(ClO(4))(2) lead to complexes of the form [Co(TBTA)(CH(3)CN)(3)](ClO(4))(2), 1, and [Co(TBTA)(2)](ClO(4))(2), 2, where complex formation can be controlled by the metal/ligand ratio and the complexes 1 and 2 can be chemically and reversibly switched from one form to another in solution resulting in coordination ambivalence. The benzyl substituents of TBTA in 2 show intramolecular C-H-π T-stacking that generates a chemical pressure to stabilize the low spin (LS) state at lower temperatures. The structural parameters of 2 are consistent with a Jahn-Teller active LS Co(II) (elongation) ion showing four short and two long bonds. 2 shows spin-crossover (SCO) behavior in the solid state and in solution with a high T(0) close to room temperature which is driven by the T-stacking. 1 remains high spin (HS) between 2 and 400 K. Reversible chemical switching is observed between 1 and 2 at room temperature, with an accompanying change in the spin state from HS to LS. The importance of the intramolecular T-stacking in driving the SCO behavior is proven by comparison with two analogous compounds that lack an aromatic substituent and remain HS down to very low temperatures.

Unconventional Spin Crossover in Dinuclear and Trinuclear Iron(III) Complexes with Cyanido and Metallacyanido Bridges

AbstractA nonsymmetrical triamine, 1,6‐diamino‐4‐azahexane, was Schiff‐condensed with (X‐substituted) o‐salicylaldehyde to yield pentadentate ligands X‐L5: salpet and MeBu‐salpet. These ligands form mononuclear, dinuclear, and trinuclear FeIII complexes, whose structures were determined by single‐crystal X‐ray analysis. Of the mononuclear complexes, [FeIII(salpet)Cl] and [FeIII(MeBu‐salpet)Cl] are high spin (S = 5/2), whereas [FeIII(salpet)CN]·MeOH is low spin (S = 1/2). The dinuclear and trinuclear complexes show a kind ofthermally induced spin crossover. The dinuclear complex [L5FeIII(CN)FeIIIL5](ClO4)·2H2O (L5 = salpet) is a mixed‐spin assembly: the C‐coordinated FeIII center is low spin (L) and the N‐coordinated FeIII center is high spin (H) at low temperature; an antiferromagnetic interaction occurs between them. This LH reference state is mixed with the LL one. Upon heating, the system shows an increasing content of the HH state. Also, the dinuclear complex [L5FeIII(CN)FeIIIL5](BPh4)·2MeCN (L5 = MeBu‐salpet) exhibits a spin transition between LH and HH spin pairs. The mixed‐valence trinuclear complex [L5FeIII{FeII(CN)5(NO)}FeIIIL5]·0.5MeOH·3.75H2O (L5 = salpet) shows spin crossover with a residual high‐spin fraction at liquid He temperature owing to the LL + LH ground state. The metallacyanido‐bridged complex [L5FeIII{Ni(CN)4}FeIIIL5]·2MeOH (L5 = MeBu‐salpet) contains a high‐spin pair, HH, over the whole temperature interval with a ferromagnetic exchange interaction. A theoretical model was outlined that allows simultaneous fitting of all available experimental data (magnetic susceptibility, magnetization, high‐spin mole fraction obtained from the Mössbauer spectra) on a common set of parameters. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Parallel Crystallization of a “Static” and a Spin‐Crossover Polymorph of an Iron(II) Complex from the Same Solution

AbstractThe ligand 2,6‐bis(1H‐pyrazol‐1‐yl)‐4‐(thiocyanatomethyl)pyridine (L) has been prepared from the hydroxymethyl precursor by OH/Br exchange and subsequent thiocyanate substitution. Two polymorphs of the unsolvated iron(II) complex [Fe(L)2](BF4)2 crystallize at room temperature from the same solution (MeOH), and side by side. One (yellow) is high spin between 4 and 350 K, whereas the other (red‐brown) shows spin crossover, with a thermal spin‐transition centred at 272 K (width ca. 50 K). Both forms have been fully characterized by variable‐temperature structure determination and magnetic susceptibility measurements. At a given temperature, the solid‐state structures differ strongly with respect to the coordination geometry at iron(II), the orientation of the thiocyanatomethyl substituents, and cation‐anion contacts. The implications of these structural differences for the observed spin behaviour are discussed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)