Spin Crossover Properties Research Articles

Structural studies of a manganese(iii) complex with spin-crossover and thermochromic properties

A new MnIII complex exhibits a gradual spin crossover phenomenon with single crystal thermochromism. A comparative structural study was conducted using octahedral deviation parameters.

Structure-function correlations in mononuclear manganese(iii) spin crossover systems with a big conjugated hexadentate Schiff-base ligand.

This paper reports the syntheses, crystal structures and magnetic properties of spin crossover (SCO) salts of formulae [Mn(naphth-sal-N-1,5,8,12)]SbF6 (1), [Mn(naphth-sal-N-1,5,8,12)]AsF6 (2), [Mn(naphth-sal-N-1,5,8,12)]PF6·1/2CH3OH (3) and [Mn(naphth-sal-N-1,5,8,12)]ClO4 (4), where (naphth-sal-N-1,5,8,12)2- (2,2'-((1E,14E)-2,6,10,14-tetraazapentadeca-1,14-diene-1,15-diyl)diphenolate) is a big conjugated hexadentate Schiff-base ligand. Magnetic susceptibility measurements demonstrated that complexes 1 and 2 showed a gradual one-step SCO between the high-spin (HS, S = 2) and low-spin (LS, S = 1) states without thermal hysteresis. The transition temperatures T1/2 of the SbF6 (1) and AsF6 (2) salts estimated from the magnetic susceptibility measurements are 164 and 171 K, respectively. The existence of the crystal solvent of complex 3 changes the supramolecular packing, leading to close ππ stacking interactions between the phenyl groups of the ligands. These close stacking interactions hinder the flexibility of the whole ligand, precluding the spin transformation of complex 3 and leading to its stabilization in the HS state in the temperature range of 2-300 K. For 4, crystal structure analysis indicates that the reduction in the anion size leads to close contacts between the naphthalene rings. These C-Hπ interactions provide a means for preventing the spin crossover occurring at low temperatures.

The substituent guest effect on four-step spin-crossover behavior

The fluoro substituent strategy on the guest in a three-dimensional Hofmann-type metal–organic framework is explored for four-step spin-crossover properties.

Switchable on-off spin-crossover properties of iron(ii) compounds by trimming intermolecular hydrogen bonds.

Reversible single crystal-to-single crystal transformations from [FeL2(NCS)2]·3MeOH (1·3MeOH, L = 2-(anthracen-10-yl)-1H-imidazo[4,5-f][1,10]phenanthroline) to 1 and 1·2H2O were reported, which were accompanied by on-off switching of the spin-crossover properties.

Controlled synthesis of [Fe(pyridine)2Ni(CN)4] nanostructures and their shape-dependent spin-crossover properties

Ultrathin nanosheets of the spin crossover (SCO) coordination polymer [Fe(py)2Ni(CN)4] (py = pyridine) have been achieved through a reverse micelle route. The nanosheets possess regular square shape with a thickness less than 10 nm and a main width of 600–800 nm. Through controlling the synthetic parameters, the nanostructures of [Fe(py)2Ni(CN)4] can be tuned to be square plate-, cube- and box-like shapes, respectively. The possible formation mechanism for these diversiform nanocrystals was proposed. Magnetic measurements and differential scanning calorimetry analyses revealed an interesting morphology-dependent SCO behavior of [Fe(py)2Ni(CN)4]. The hysteresis width decreases in the order of nanoplates (ca. 22 K), nanocubes (ca. 16 K), nanoboxes (ca. 4 K) and nanosheets (ca. 2 K). The enthalpy (ΔH) and entropy (ΔS) variations associated with the spin transitions of nanosheets and nanoboxes are significantly lower than those of nanocubes and nanoplates. Thus, the nanosheets and nanoboxes exhibit stronger nano-effect and their SCO properties are largely weakened owing to the large specific surface areas resulting from their unique morphologies.

Electronic Structure, Vibrational Spectra, and Spin-Crossover Properties of Vacuum-Evaporable Iron(II) Bis(dihydrobis(pyrazolyl)borate) Complexes with Diimine Coligands. Origin of Giant Raman Features.

The vibrational properties of spin-crossover complexes [Fe(H2B(pz)2)2(L)] (pz = pyrazole) containing L = 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen) ligands are investigated by temperature-dependent infrared and Raman spectroscopy. For comparison, the analogous cobalt(II) complexes [Co(H2B(pz)2)2(L)] (L = bipy and phen) and iron(II) compounds with L = 4,4'-dimethyl-2,2'-bipyridine and 4,7-dimethyl-1,10-phenanthroline coligands are studied. Highly intense, structured bands (giant Raman features, GRFs) are observed in the resonance Raman spectra of all Fe(II) complexes between 400 and 500 cm-1 at low temperatures in the HS state which, for the SCO complexes, is excited by the Raman laser. On the basis of magnetic field Mössbauer and saturation magnetization data electronic Raman effects are excluded to account for these features. Furthermore, detailed vibrational analysis also allows excluding a vibrational resonance Raman effect involving one of the modes of the individual complexes as a possible origin of the GRFs. Consequently, these features are attributed to coherent two-phonon excitation of metal-ligand stretching vibrations in molecular dimers coupled by π-π stacking interactions.

Amine-Functionalized Spin Crossover Building Blocks.

Bistable spin crossover complexes such as [Fe{HB(pz)3}2] (pzH = pyrazole) show promise for sensor applications and electrically‐controlled data storage units, but exploiting their potential hinges on their integration into a functional environment. We here present a system enabling such covalent post‐functionalization steps in both symmetric and asymmetric patterns, based on the amine‐functionalized complex [Fe{HB(4‐NH2pz)(pz)2}2], obtained by reduction of the nitro analogue. The building block aspects of [Fe{HB(4‐NH2pz)(pz)2}2] are showcased by its transformation into amide, imine and azo derivatives, which are structurally and magnetically characterized. All tris(pyrazolyl)borate complexes retain the spin crossover properties of their parent compound, with spin crossover temperatures ranging from 350 to 430 K. The transition parameters are correlated with the electronic properties of the functionalizing group, opening the possibility of fine‐tuning the spin crossover properties of the building block as it is integrated in the environment of choice.

Heteroleptic Iron(II) Spin-Crossover Complexes Based on a 2,6-Bis(pyrazol-1-yl)pyridine-type Ligand Functionalized with a Carboxylic Acid.

Two new heteroleptic complexes [Fe(1bppCOOH)(3bpp-bph)](ClO4)2·solv (1·solv, solv = various solvents; 1bppCOOH = 2,6-bis(1H-pyrazol-1-yl)isonicotinic acid; 3bpp-bph = 2,6-bis(5-([1,1'-biphenyl]-4-yl)-1H-pyrazol-3-yl)pyridine) and [Fe(1bppCOOH)(1bppCOOEt)](ClO4)2·0.5Me2CO (2·0.5Me2CO, 1bppCOOEt = ethyl 2,6-bis(1H-pyrazol-1-yl)isonicotinate) were designed and prepared. The heteroleptic compound 1·solv was obtained by the combination of stoichiometric amounts of Fe(ClO4)2, 1bppCOOH, and 3bpp-bph, and it was designed to fine-tune the spin crossover (SCO) properties with respect to the previously reported homoleptic compound [Fe(1bppCOOH)2](ClO4)2. Indeed, the introduction of a new substituted 3bpp ligand induces a weaker ligand field in addition to promoting the formation of π···π and C-H···π intermolecular interactions through the biphenyl groups. For the desolvated counterpart 1, this results in a shift of the SCO curve toward room temperature and the observation of a 13 K hysteresis width. Besides, compound 2·0.5Me2CO, which represents the first example of a heteroleptic complex containing two 1bpp tridentate ligands, stabilizes the LS state at room temperature confirming the same trend observed for the corresponding homoleptic compounds. Interestingly, both 1 and 2·0.5Me2CO heteroleptic complexes exhibit photoswitchable properties when irradiating with a 523 nm laser at 10 K. Preliminary characterization of the deposited complexes on native SiO2 by X-ray absorption measurements suggests oxidation and decomposition of the complexes.

Influence of Host-Guest and Host-Host Interactions on the Spin-Crossover 3D Hofmann-type Clathrates {FeII(pina)[MI(CN)2]2}·xMeOH (MI = Ag, Au).

The synthesis, structural characterization and magnetic properties of two new isostructural porous 3D compounds with the general formula {FeII(pina)[MI(CN)2]2}·xMeOH (x = 0-5; pina = N-(pyridin-4-yl)isonicotinamide; MI = AgI and x ∼ 5 (1·xMeOH); MI = AuI and x ∼ 5 (2·xMeOH)) are presented. The single-crystal X-ray diffraction analyses have revealed that the structure of 1·xMeOH (or 2·xMeOH) presents two equivalent doubly interpenetrated 3D frameworks stabilized by both argentophilic (or aurophilic) interactions and interligand C═O···HC H-bonds. Despite the interpenetration of the networks, these compounds display accessible void volume capable of hosting up to five molecules of methanol which interact with the host pina ligand and establish an infinite lattice of hydrogen bonds along the structural channels. Interestingly, the magnetic studies have shown that solvated complexes 1·xMeOH and 2·xMeOH display two- and four-step hysteretic thermally driven spin transitions, respectively. However, when these compounds lose the methanol molecules, the magnetic behavior changes drastically giving place to gradual spin conversions evidencing the relevant influence of the guest molecules on the spin-crossover properties. Importantly, since the solvent desorption takes place following a single-crystal-to-single-crystal transformation, empty structures 1 and 2 (x = 0) could be also determined allowing us to evaluate the correlation between the structural changes and the modification of the magnetic properties triggered by the loss of methanol molecules.

Iron(III) Azadiphenolate Compounds in a New Family of Spin Crossover Iron(II)–Iron(III) Mixed-Valent Complexes

A new family of mixed valent, double salt spin crossover compounds containing anionic FeIII and cationic FeII compounds i.e., [FeII{(pz)3CH}2][FeIII(azp)2]2·2H2O (4), [FeII(TPPZ)2][FeIII(azp)2]2]·H2O (5) and [FeII(TPPZ)2][FeIII(azp)2]2]·H2O·3MeCN (6) (where (pz)3CH = tris-pyrazolylmethane, TPPZ = 2,3,5,6, tetrapyridylpyrazine and azp2− = azadiphenolato) has been synthesized and characterised. This is the first time that the rare anionic spin crossover species, [FeIII(azp)2]−, has been used as an anionic component in double salts complexes. Single crystal structures and magnetic studies showed that compound 6 exhibits a spin transition relating to one of the FeIII centres of the constituent FeII and FeIII sites. Crystal structures of the anionic and cationic precursor complexes were also analysed and compared to the double salt products thus providing a clearer picture for future crystal design in double spin crossover materials. We discuss the effects that the solvent and counterion had on the crystal packing and spin crossover properties.

Photochromism and Photomagnetism Induced by Structural Disorder of a Crystalline Spin-Crossover FeII Complex

Photochromic and photoswitchable magnetic (photomagnetic) properties of spin-crossover (SCO) metal complexes in the solid state were observed almost at cryogenic temperature. Here we present a new crystalline complex [FeII(TMPT)6](ClO4)8 (TMPT = 1-(1,2,4-triazol-4-yl)-2,4,6-trimethyl-pyridinium) showing photochromism and reversible hysteresis shift, with one direction being triggered by thermal annealing and the other by irradiation at room temperature. IR/Raman and single-crystal X-ray diffraction data revealed that these behaviors are very probably induced by thermal disorder of ClO4– anions, which change intermolecular interactions and the ligand field strength of the FeII center. This discovery may inspire the design and synthesis of new photochromic and photomagnetic compounds through controlling structural disorder.

Fe(II) spin crossover complexes of a derivative of 2,6-bis(pyrazol-1-yl)pyridine (1-bpp) functionalized with a carboxylic acid in the 3-pyridyl position

The preparation of a new bis(pyrazol-1-yl)pyridine (1-bpp) derivative functionalized with a carboxylic acid in the 3-pyridyl position, bpp3-COOH ligand is reported together with the structure and spin-crossover (SCO) properties of [FeII(bpp3-COOH)2](ClO4)2·0.5EtOH·0.5H2O (1). Magnetic properties of 1 indicate that LS is favored. Desolvation leads to a gradual and incomplete SCO. Solvated and desolvated compounds show LIESST effect.

Effects of solvent vapor annealing on the crystallinity and spin crossover properties of thin films of [Fe(HB(tz)3)2

Abstract We investigated the solvent vapor annealing process of thermally evaporated thin films of the [FeII(HB(tz)3)2] (tz = 1,2,4-triazol-1-yl) spin crossover complex. Using grazing incidence X-ray diffraction, atomic force microscopy, and variable-temperature UV spectrophotometry, we show how the crystallinity, morphology, and spin crossover properties of the resulting films are altered by exposure to vapors of five different solvents: diethyl ether, acetone, dichloromethane, ethanol, and water. Remarkably, most solvents yielded highly oriented crystalline films displaying an abrupt and complete spin transition around 336 K. A notable exception is the case of dichloromethane—suggesting that the solvent ability to accept hydrogen bonding might be a key factor in controlling the film crystallinity. On the other hand, only water vapor treatment affords for smooth and continuous film morphology. The recrystallization process in controlled humidity was thus followed in situ by monitoring the UV absorbance of the films. This study revealed the critical importance of the relative humidity (RH): for values above a threshold of ca. 72% RH (at 298 K) a very fast (seconds) crystallization occurs leading to robust, crystalline films, whereas crystallization at lower RH values is slower (hours) and leads to less stable, semicrystalline films displaying incomplete spin transitions.

A Known Iron(II) Complex in Different Nanosized Particles: Variable-Temperature Raman Study of Its Spin-Crossover Behavior.

The spin-crossover (SCO) polymorph B (complex 1) of the known compound [FeII{N(CN)2}2(abpt)2], where abpt is 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole, has been prepared in three different particle sizes averaging ∼300 (sample 1a), ∼80 (sample 1b), and ∼20 nm (sample 1c). Two independent octahedral molecules possessing Fe1 and Fe2 were found to be present in the crystal of B. Magnetostructural relationships had established that at room temperature both FeII sites are in the high-spin state (HS-HS), whereas a decrease in the temperature to 90 K induces the complete high-spin to low-spin conversion of the Fe1 site, with Fe2 remaining in the high-spin state (LS-HS). The three samples have been characterized by elemental analyses, ATR spectra, solution UV/vis spectra (to exclude resonance Raman effects) and powder X-ray diffraction patterns, while their morphological characteristics have been examined by scanning electron microscopy (SEM). The SCO behavior of the originally prepared sample 1a has been monitored in detail by variable-temperature Raman studies in the 300-80 K range using mainly low-frequency ν(Fe-N) and δ(NFeN) modes and the ν(C≡N) mode of the axial dicyanamido groups as spin-sensitive vibrations. The new peaks that appear in the low-temperature Raman spectra of the LS-HS form of the complex are reproduced in the calculated spectrum of the LS state of [FeII{N(CN)2}2(abpt)2]. The influence of the average particle size on the SCO properties of 1 has also been studied by variable-temperature Raman spectra. The studies indicate that, during the HS-HS → LS-HS transition, the latter form of the complex appears at higher temperatures for the smaller particles; the T1/2 shift accomplished by manipulating the particle size within a range of roughly 1 order of magnitude (300-20 nm) may be as high as ∼30 K. The SCO features of 1, as deduced from the Raman study, are in excellent agreement with those derived from a traditional variable-temperature magnetic susceptibility study, indicating the utility of the former.

Electrical Voltage Control of the Pressure-Induced Spin Transition at Room Temperature in the Microporous 3D Polymer [Fe(pz)Pt(CN)4

Fine control and direct monitoring of the spin crossover properties driven by pressure at room temperature are reported for the porous three-dimensional coordination polymer {Fe(pz)[Pt(CN)4]} by using a homemade pressure cell that transforms a DC voltage into pressure. The pressure induced spin state switching is steadily driven through a piezoelectric ceramic element, which transforms the voltage 1−4 kV in pressures in the 0.001−0.035 GPa range. At the same time, the spin state is easily monitored through changes in the optical spectra of the title compound. The results demonstrate that {Fe(pz)[Pt(CN)4]} responds to as small pressure variations as 0.001 GPa (10 atm), thereby proving its efficacy to work as an effective pressure sensor.

Effect of nanostructuration on the spin crossover transition in crystalline ultrathin films.

Mastering the nanostructuration of molecular materials onto solid surfaces and understanding how this process affects their properties are of utmost importance for their integration into solid-state electronic devices. This is even more important for spin crossover (SCO) systems, in which the spin transition is extremely sensitive to size reduction effects. These bi-stable materials have great potential for the development of nanotechnological applications provided their intrinsic properties can be successfully implemented in nanometric films, amenable to the fabrication of functional nanodevices. Here we report the fabrication of crystalline ultrathin films (<1-43 nm) of two-dimensional Hofmann-type coordination polymers by using an improved layer-by-layer strategy and a close examination of their SCO properties at the nanoscale. X-ray absorption spectroscopy data in combination with extensive atomic force microscopy analysis reveal critical dependence of the SCO transition on the number of layers and the microstructure of the films. This originates from the formation of segregated nanocrystals in early stages of the growth process that coalesce into a continuous film with an increasing number of growth cycles for an overall behaviour reminiscent of the bulk. As a result, the completeness of the high spin/low spin transition is dramatically hindered for films of less than 15 layers revealing serious limitations to the ultimate thickness that might be representative of the performance of the bulk when processing SCO materials as ultrathin films. This unprecedented exploration of the particularities of the growth of SCO thin films at the nanoscale should encourage researchers to put a spotlight on these issues when contemplating their integration into devices.

Interplay between spin-crossover and luminescence in a multifunctional single crystal iron(ii) complex: towards a new generation of molecular sensors.

We present a new example of a mononuclear iron(ii) complex exhibiting a correlated spin-crossover (SCO) transition and strong fluorescence, whose coordination sphere is saturated, for the first time, by six phosphorescent ligands. The interplay between SCO and light emission properties in the thermal region of the spin transition was investigated by means of magnetic, fluorescence, optical absorption and optical microscopy measurements on a single crystal. Overall, the results show an excellent correlation between fluorescence and magnetic data of the present gradual transition, indicating an extreme sensitivity of the optical activity of the ligand to the spin state of the active iron(ii) ions. These results open the way for conceiving new prototypes of pressure and temperature sensors based on this synergy between SCO and luminescence properties. In particular, the fact that cooperative SCO material is not a prerequisite for obtaining such synergetic effects, is useful for the design of thin films or nanoparticles, in which the cooperativity is reduced, for appropriate implementation in nanosized devices to enhance the sensing properties at the nanoscale.

Spin Crossover and Magnetic Properties of Ba-Substituted Cobaltites

The dependence of the properties of the La0.45Ba0.55CoO3 – δ and La0.4Ba0.6CoO3 – δ cobaltites on the oxygen content has been studied. A decrease in the oxygen content leads to the stabilization of cobalt ions in the 3+ oxidation state and to a transition from a conducting state with positive exchange interaction to an antiferromagnetic insulating state with a long range magnetic order with decreasing temperature. Application of a magnetic field gradually transforms the antiferromagnetic state to the ferromagnetic one. The cubic lattice parameter is independent of temperature and an anomalous dependence of the magnetization, electrical conductivity, and magnetoresistance is treated within a spin crossover scenario. It is assumed that the high-temperature conducting phase and ferromagnetic clusters correspond to a collective intermediate spin state of cobalt ions, whereas the low-temperature antiferromagnetic phase corresponds to a mixed high-spin/low-spin state.

Spin switching in tris(8-aminoquinoline)iron(ii)(BPh4)2: quantitative guest-losing dependent spin crossover properties and single-crystal-to-single-crystal transformation.

As a derivative of 2-picolylamine, which contains rich protons favouring hydrogen bond formation to assemble a variety of valuable spin crossover (SCO) compounds, 8-aminoquinoline (aqin) will be a good candidate for constructing new mononuclear bistable state compounds. With the guidance of this view, two solvated compounds [Fe(aqin)3](BPh4)2·2(CH3CN) (1·2CH3CN) and [Fe(aqin)3](BPh4)2·1.5(CH3COCH3) (2·1.5CH3COCH3) were synthesized. The structural characterization and magnetic studies demonstrate that this strategy has been successful. Single-crystal diffraction reveals that both the mononuclear compounds have facial (fac-)-configuration cations, which form hydrogen bonds using -NH2 groups with solvent molecules (acetonitrile or acetone). Subsequent magnetic measurement shows the highly sensitive solvent-dependent occurrence of a spin transition above room temperature for both compounds. Interestingly, for compound 1·2CH3CN, in the successively repeated heating and cooling process, by monitoring the loss of solvent molecules by TGA, the shifting of the spin transition curve is found to be linearly dependent on the fraction of the residual solvent content. Additionally, the desolvated sample can re-solvate with CH3CN and recover the magnetic response reproducibly. Furthermore, after losing the acetonitrile molecules, the single-crystal-to-single-crystal transformation occurred to give 1.

Heteroatom substitution effects in spin crossover dinuclear complexes.

We probe the effect of heteroatom substitution on the spin crossover (SCO) properties of dinuclear materials of the type [Fe2(NCX)4(R-trz)5]·S (X = S, Se; S = solvent; R-trz = (E)-N-(furan-2-ylmethylene)-4H-1,2,4-triazol-4-amine (furtrz); (E)-N-(thiophen-2-ylmethylene)-4H-1,2,4-triazole-4-amine (thtrz)). For the furtrz family ([Fe2(NCX)4(furtrz)5]·furtrz·MeOH; X = S (furtrz-S) and X = Se (furtrz-Se)) gradual and incomplete one-step SCO transitions are observed (furtrz-S (T1/2 = 172 K) and furtrz-Se (T1/2 = 205 K)) and a structural evolution from [HS-HS] to [HS-LS] per dinuclear species. Contrasting this, within the thtrz family ([Fe2(NCX)4(thtrz)5]·4MeOH; X = S (thtrz-S) and X = Se (thtrz-Se)) more varied SCO transitions are observed, with thtrz-S being SCO-inactive (high spin) and thtrz-Se showing a rare complete two-step SCO transition (T1/2(1,2) = 170, 200 K) in which the FeII sites transition from [HS-HS] to [HS-LS] to [LS-LS] per dinuclear unit with no long range ordering of spin-states at the intermediate plateau. Detailed structure-function analyses have been conducted within this growing dinuclear family to rationalise these diverse spin-switching properties.