Special Issue "Synthesis and Applications of New Spin Crossover Compounds"

A special issue of Crystals (ISSN 2073-4352).

Deadline for manuscript submissions: closed (31 May 2019).

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Special Issue Editor

Prof. Takafumi Kitazawa
E-Mail Website
Guest Editor
Toho University, Research Center for Materials with Integrated Properties, Tokyo, Japan
Tel. +81-47-472-5077
Interests: Coordination Chemistry, Spin crossover, Crystal chemistry
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Special Issue Information

Dear Colleagues,

The magnetochemistry research area of spin crossover (SCO) behavior in coordination compounds might be potentially associated with smart materials, promising materials for applications as components of memory devices, displays and sensors. 3-d block transition metal coordination compounds with d4-d7 configuration in an octahedral crystal field have a possibility of SCO phenomena between the low spin (LS) and the high spin (HS) states, being accompanied by color changes. Octahedral iron(II) SCO systems with 3d6, which can be transited between the diamagnetic (t2g)6 and the paramagnetic (t2g)4(eg)2 configuration might be widely and deeply investigated as smart materials.

Coordination materials with bistable systems between the LS and the HS states, usually triggered by external stimuli, such as temperature, light, pressure and guest molecule inclusion, are a family of potential candidates for smart materials because the change of the crystal cell volume between the HS and LS states is very large. Various SCO iron(II) coordination compounds, especially those with polymeric 1D, 2D and 3D frameworks, have been intensively investigated because their polymeric linked framework structures enhance cooperative effects which work among the SCO species resulting in SCO behavior with large hysteresis and transition temperature ranges near RT.

The interdisciplinary regions chemistry related to spin crossover phenomena are also welcome.

We are hoping to offer a portal for some of this exciting new research associated with molecular magnetism, spin crossover phenomena, coordination polymers, metallosupramolecular chemistry, multiproperty materials and functional materials with an attractive and valuable Special Issue of the open access journal, Crystals.

Prof. Dr. Takafumi Kitazawa
Guest Editor

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Keywords

  • Spin crossover
  • High spin
  • Low spin
  • Switching magnetic materials
  • Sensor
  • Smart materials
  • Supuramolecular coordination polymer
  • Inclusion compounds with magnetism

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Editorial

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Open AccessEditorial
Synthesis and Applications of New Spin Crossover Compounds
Crystals 2019, 9(8), 382; https://doi.org/10.3390/cryst9080382 - 25 Jul 2019
Cited by 1
Abstract
The spin crossover (SCO) between multi-stable states in transition metal material is one of the attractive molecular switching phenomena which is responsive to various external stimuli such as temperature, pressure, light, electromagnetic field, radiation, nuclear decay, soft-X-ray, guest molecule inclusion, chemical environments and [...] Read more.
The spin crossover (SCO) between multi-stable states in transition metal material is one of the attractive molecular switching phenomena which is responsive to various external stimuli such as temperature, pressure, light, electromagnetic field, radiation, nuclear decay, soft-X-ray, guest molecule inclusion, chemical environments and so forth [...] Full article

Research

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Open AccessArticle
Structural Insights into the Two-Step Spin-Crossover Compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2
Crystals 2019, 9(6), 316; https://doi.org/10.3390/cryst9060316 - 19 Jun 2019
Cited by 1
Abstract
The crystal structure of the polymeric spin crossover compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2 has been solved and its temperature dependence followed by means of single-crystal and powder X-ray diffraction. This compound presents a two-step spin transition with relatively abrupt steps centred [...] Read more.
The crystal structure of the polymeric spin crossover compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2 has been solved and its temperature dependence followed by means of single-crystal and powder X-ray diffraction. This compound presents a two-step spin transition with relatively abrupt steps centred at ca. 170 K and 145 K and a plateau at around 155 K. The origin of the two-step transition is discussed in light of these structural studies. The observations are compatible with a mostly disordered state between the two steps, consisting of mixing of high-spin and low-spin species, while weak substructure reflections in the mixed phase could indicate some degree of long-range order of the high-spin and low-spin sites. Full article
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Open AccessArticle
Spin Transition in the Cu(hfac)2 Complex with (4-Ethylpyridin-3-yl)-Substituted Nitronyl Nitroxide Caused by the “Asymmetric” Structural Rearrangement of Exchange Clusters in the Heterospin Molecule
Crystals 2019, 9(6), 285; https://doi.org/10.3390/cryst9060285 - 01 Jun 2019
Cited by 1
Abstract
Methods for the synthesis of binuclear [Cu(hfac)2LEt]2 and tetranuclear [[Cu(hfac)2]4(LEt)2] heterospin compounds based on copper hexafluoroacetylacetonate [Cu(hfac)2] and 2-(4-ethylpyridin-3-yl)-4,5-bis(spirocyclopentyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt), were developed. The crystals [...] Read more.
Methods for the synthesis of binuclear [Cu(hfac)2LEt]2 and tetranuclear [[Cu(hfac)2]4(LEt)2] heterospin compounds based on copper hexafluoroacetylacetonate [Cu(hfac)2] and 2-(4-ethylpyridin-3-yl)-4,5-bis(spirocyclopentyl)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt), were developed. The crystals of the complexes are elastic and do not crash during repeated cooling–heating cycles. It was found that a singlet–triplet conversion occurred in all of the {Cu(II)–O•–N<} exchange clusters in the molecules of the binuclear [Cu(hfac)2LEt]2 which led to spin coupling with cooling. The transition occurred in a wide temperature range with a maximum gradient ΔχT at ≈180 K. The structural transformation of the crystals takes place at T < 200 K and is accompanied by the lowering of symmetry from monoclinic to triclinic, twinning, and a considerable shortening of the Cu–ONO distance (2.19 and 1.97 Å at 295 and 50 K, respectively). For the tetranuclear [[Cu(hfac)2]4(LEt)2], two structural transitions were recorded (at ≈154 K and ≈118 K), which led to a considerable change in the spatial position of the Et substituent in the nitronyl nitroxyl fragment. The low-temperature process was accompanied by a spin transition recorded as a hysteresis loop on the χT(T) curve during the repeated cooling–heating cycles (T½↑ = 122 K, T½↓ = 115 K). This transition is unusual because it causes spin coupling in half of all of the {>N–•O–Cu2+} terminal exchange clusters, leading to spin compensation for only two paramagnetic centers of the six centers in the molecule. Full article
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Open AccessArticle
Iron(II) Spin Crossover Complex with the 1,2,3-Triazole-Containing Linear Pentadentate Schiff-Base Ligand and the MeCN Monodentate Ligand
Crystals 2019, 9(6), 276; https://doi.org/10.3390/cryst9060276 - 28 May 2019
Cited by 2
Abstract
A mononuclear iron(II) complex bearing the linear pentadentate N5 Schiff-base ligand containing two 1,2,3-triazole moieties and the MeCN monodentate ligand, [FeIIMeCN(L3-Me-3Ph)](BPh4)2·MeCN·H2O (1), have been prepared (L3-Me-3Ph = [...] Read more.
A mononuclear iron(II) complex bearing the linear pentadentate N5 Schiff-base ligand containing two 1,2,3-triazole moieties and the MeCN monodentate ligand, [FeIIMeCN(L3-Me-3Ph)](BPh4)2·MeCN·H2O (1), have been prepared (L3-Me-3Ph = bis(N,N′-1-Phenyl-1H-1,2,3-triazol-4-yl-methylideneaminopropyl)methylamine). Variable-temperature magnetic susceptibility measurements revealed an incomplete one-step spin crossover (SCO) from the room-temperature low-spin (LS, S = 0) state to a mixture of the LS and high-spin (HS, S = 2) species at the higher temperature of around 400 K upon first heating, which is irreversible on the consecutive cooling mode. The magnetic modulation at around 400 K was induced by the crystal-to-amorphous transformation accompanied by the loss of lattice MeCN solvent, which was evident from powder X-ray diffraction (PXRD) studies and themogravimetry. The single-crystal X-ray diffraction studies showed that the complex is in the LS state (S = 0) between 296 and 387 K. In the crystal lattice, the complex-cations and B(1)Ph4 ions are alternately connected by intermolecular CH···π interactions between the methyl group of the MeCN ligand and phenyl groups of B(1)Ph4 ions, forming a 1D chain structure. The 1D chains are further connected by P4AE (parallel fourfold aryl embrace) interactions between two neighboring complex-cations, constructing a 2D extended structure. B(2)Ph4 ions and MeCN lattice solvents exist in the spaces of the 2D layer. DFT calculations verified that the 1,2,3-triazole-containing ligand L3-Me-3Ph gives a stronger ligand field around the octahedral coordination environment of the iron(II) ion than the analogous imidazole-containing ligand H2L2Me (= bis(N,N′-2-methylimidazol-4-yl-methylideneaminopropyl)methylamine) of the known compound [FeIIMeCN(H2L2Me)](BPh4)1.5·Cl0.5·0.5MeCN (2) reported by Matsumoto et al. (Nishi, K.; Fujinami, T.; Kitabayashi, A.; Matsumoto, N. Tetrameric spin crossover iron(II) complex constructed by imidazole⋯chloride hydrogen bonds. Inorg. Chem. Commun. 2011, 14, 1073–1076), resulting in the much higher spin transition temperature of 1 than that of 2. Full article
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Open AccessArticle
Solvent Effects on the Spin Crossover Properties of Iron(II) Imidazolylimine Complexes
Crystals 2019, 9(2), 116; https://doi.org/10.3390/cryst9020116 - 22 Feb 2019
Cited by 3
Abstract
A series of Fe(II) complexes, fac-[Fe(4-ima-Bp)3](Y)2⋅sol (Y = ClO4; sol = 3EtOH 1, 3MeOH 2; Y= BF4; sol = EtOH⋅4H2O 3, 4H2O 4 and 3.5MeCN 5) [...] Read more.
A series of Fe(II) complexes, fac-[Fe(4-ima-Bp)3](Y)2⋅sol (Y = ClO4; sol = 3EtOH 1, 3MeOH 2; Y= BF4; sol = EtOH⋅4H2O 3, 4H2O 4 and 3.5MeCN 5) have been prepared and structurally and magnetically characterized. The low temperature structures of 1, 2 and 5 have been determined by X-ray crystallography with LS Fe(II) centres found in all cases. Extensive C–H···π interactions between the cations form 2D layers, which are linked to one another through N-H···O and O-H···O hydrogen bonds, resulting in high cooperativity. Despite 5 containing MeCN, N-H···O/F hydrogen bonds, and C–H···π and π-π interactions combine to give similar 2D layers. Magnetic measurements reveal moderately abrupt spin crossover for 1-4; becoming more gradual and only 50% complete in 1 due to solvent loss. The MeCN solvate shows more gradual SCO and reinforces how subtle changes in packing can significantly influence SCO behaviour. Full article
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Open AccessArticle
Threefold Spiral Structure Constructed by 1D Chains of [[M(NCS)2(bpa)2]·biphenyl]n (M = Fe, Co; bpa = 1,2-bis(4-pyridyl)ethane)
Crystals 2019, 9(2), 97; https://doi.org/10.3390/cryst9020097 - 14 Feb 2019
Cited by 1
Abstract
Assembled complexes [[M(NCS)2(bpa)2]·biphenyl]n (M = Fe, Co; bpa = 1,2-bis(4-pyridyl)ethane) have been synthesized because [Fe(NCBH3)2(bpa)2·biphenyl]n has a novel threefold spiral structure and shows stepwise spin-crossover phenomenon. We attempted to obtain spiral [...] Read more.
Assembled complexes [[M(NCS)2(bpa)2]·biphenyl]n (M = Fe, Co; bpa = 1,2-bis(4-pyridyl)ethane) have been synthesized because [Fe(NCBH3)2(bpa)2·biphenyl]n has a novel threefold spiral structure and shows stepwise spin-crossover phenomenon. We attempted to obtain spiral structures for [[Fe(NCS)2(bpa)2]·biphenyl]n and [[Co(NCS)2(bpa)2]·biphenyl]n using a one-step diffusion method, while the reported spiral structure of [[Fe(NCBH3)2(bpa)2]·biphenyl]n was obtained by diffusion method after synthesizing Fe(II)-pyridine complex. X-ray structural analysis revealed that [[Fe(NCS)2(bpa)2]·biphenyl]n and [[Co(NCS)2(bpa)2]·biphenyl]n had a chiral propeller structure of pyridines around the central metal, and they had a novel spiral structure and chiral space group P3121 without the presence of chiral auxiliaries. It was shown that the host 1D chain, having a chiral propeller structure of pyridines around the central metal along with its concerted interaction with an atropisomer of biphenyl, made a threefold spiral structure. Full article
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Open AccessArticle
High-Temperature Cooperative Spin Crossover Transitions and Single-Crystal Reflection Spectra of [FeIII(qsal)2](CH3OSO3) and Related Compounds
Crystals 2019, 9(2), 81; https://doi.org/10.3390/cryst9020081 - 02 Feb 2019
Cited by 2
Abstract
New Fe(III) compounds from qsal ligand, [Fe(qsal)2](CH3OSO3) (1) and [Fe(qsal)2](CH3SO3)·CH3OH (3), along with known compound, [Fe(qsal)2](CF3SO3) (2), [...] Read more.
New Fe(III) compounds from qsal ligand, [Fe(qsal)2](CH3OSO3) (1) and [Fe(qsal)2](CH3SO3)·CH3OH (3), along with known compound, [Fe(qsal)2](CF3SO3) (2), were obtained as large well-shaped crystals (Hqsal = N-(8-quinolyl)salicylaldimine). The compounds 1 and 2 were in the low-spin (LS) state at 300 K and exhibited a cooperative spin crossover (SCO) transition with a thermal hysteresis loop at higher temperatures, whereas 3 was in the high-spin (HS) state below 300 K. The optical conductivity spectra for 1 and 3 were calculated from the single-crystal reflection spectra, which were, to the best of our knowledge, the first optical conductivity spectra of SCO compounds. The absorption bands for the LS and HS [Fe(qsal)2] cations were assigned by time-dependent density functional theory calculations. The crystal structures of 1 and 2 consisted of a common one-dimensional (1D) array of the [Fe(qsal)2] cation, whereas that of 3 had an unusual 1D arrangement by π-stacking interactions which has never been reported. The crystal structures in the high-temperature phases for 1 and 2 indicate that large structural changes were triggered by the motion of counter anions. The comparison of the crystal structures of the known [Fe(qsal)2] compounds suggests the significant role of a large non-spherical counter-anion or solvate molecule for the total lattice energy gain in the crystal of a charged complex. Full article
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Open AccessArticle
Spatio-temporal Investigations of the Incomplete Spin Transition in a Single Crystal of [Fe(2-pytrz)2{Pt(CN)4}]·3H2O: Experiment and Theory
Crystals 2019, 9(1), 46; https://doi.org/10.3390/cryst9010046 - 16 Jan 2019
Cited by 3
Abstract
Optical microscopy technique is used to investigate the thermal and the spatio-temporal properties of the spin-crossover single crystal [Fe(2-pytrz)2{Pt(CN)4}]·3H2O, which exhibits a first-order spin transition from a full high-spin (HS) state at high temperature to an intermediate, [...] Read more.
Optical microscopy technique is used to investigate the thermal and the spatio-temporal properties of the spin-crossover single crystal [Fe(2-pytrz) 2 {Pt(CN) 4 }]·3H 2 O, which exhibits a first-order spin transition from a full high-spin (HS) state at high temperature to an intermediate, high-spin low-spin (HS-LS) state, below 153 K, where only one of the two crystallographic Fe(II) centers switches from the HS to HS-LS state. In comparison with crystals undergoing a complete spin transition, the present transformation involves smaller volume changes at the transition, which helps to preserving the crystal’s integrity. By analyzing the spatio-temporal properties of this spin transition, we evidenced a direct correlation between the orientation and shape of HS/HS-LS domain wall with the crystal’s shape. Thanks to the small volume change accompanying this spin transition, the analysis of the experimental data by an anisotropic reaction-diffusion model becomes very relevant and leads to an excellent agreement with the experimental observations. Full article
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Open AccessArticle
Anion Influence on Spin State in Two Novel Fe(III) Compounds: [Fe(5F-sal2333)]X
Crystals 2019, 9(1), 19; https://doi.org/10.3390/cryst9010019 - 29 Dec 2018
Cited by 4
Abstract
Structural and magnetic data on two iron (III) complexes with a hexadentate Schiff base chelating ligand and Cl or BPh4 counterions are reported. In the solid state, the Cl complex [Fe(5F-sal2333)]Cl, 1, is high spin between [...] Read more.
Structural and magnetic data on two iron (III) complexes with a hexadentate Schiff base chelating ligand and Cl or BPh4 counterions are reported. In the solid state, the Cl complex [Fe(5F-sal2333)]Cl, 1, is high spin between 5–300 K while the BPh4 analogue [Fe(5F-sal2333)]BPh4, 2, is low spin between 5–250 K, with onset of a gradual and incomplete spin crossover on warming to room temperature. Structural investigation reveals different orientations of the hydrogen atoms on the secondary amine donors in the two salts of the [Fe(5F-sal2333)]+ cation: high spin complex [Fe(5F-sal2333)]Cl, 1, crystallizes with non-meso orientations while the spin crossover complex [Fe(5F-sal2333)]BPh4, 2, crystallizes with a combination of meso and non-meso orientations disordered over one crystallographic site. Variable temperature electronic absorption spectroscopy of methanolic solutions of 1 and 2 suggests that both are capable of spin state switching in the solution. Full article
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Open AccessArticle
Effect of Transition Metal Substitution on the Charge-Transfer Phase Transition and Ferromagnetism of Dithiooxalato-Bridged Hetero Metal Complexes, (n-C3H7)4N[FeII1−xMnIIxFeIII(dto)3]
Crystals 2018, 8(12), 446; https://doi.org/10.3390/cryst8120446 - 28 Nov 2018
Cited by 1
Abstract
The dithiooxalato-bridged iron mixed-valence complex (n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato) undergoes a novel charge-transfer phase transition (CTPT) accompanied by electron transfer between adjacent FeII and FeIII sites. The [...] Read more.
The dithiooxalato-bridged iron mixed-valence complex (n-C3H7)4N[FeIIFeIII(dto)3] (dto = dithiooxalato) undergoes a novel charge-transfer phase transition (CTPT) accompanied by electron transfer between adjacent FeII and FeIII sites. The CTPT influences the ferromagnetic transition temperature according to the change of spin configuration on the iron sites. To reveal the mechanism of the CTPT, we have synthesized the series of metal-substituted complexes (n-C3H7)4N[FeII1-xMnIIxFeIII(dto)3] (x = 0–1) and investigated their physical properties by means of magnetic susceptibility and dielectric constant measurements. With increasing MnII concentration, x, MnII-substituted complexes show the disappearance of CTPT above x = 0.04, while the ferromagnetic phase remains in the whole range of x. These results are quite different from the physical properties of the ZnII-substituted complex, (n-C3H7)4N[FeII1-xZnIIxFeIII(dto)3], which is attributed to the difference of ion radius as well as the spin states of MnII and ZnII. Full article
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Open AccessArticle
Synthesis, Structure, and Photomagnetic Properties of a Hydrogen-Bonded Lattice of [Fe(bpp)2]2+ Spin-Crossover Complexes and Nicotinate Anions
Crystals 2018, 8(11), 439; https://doi.org/10.3390/cryst8110439 - 21 Nov 2018
Cited by 3
Abstract
In this paper, we report on the synthesis, crystal structure, and photomagnetic properties of the spin-crossover salt of formula [Fe(bpp)2](C6H4NO2)2·4H2O (1·4H2O) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; C6H [...] Read more.
In this paper, we report on the synthesis, crystal structure, and photomagnetic properties of the spin-crossover salt of formula [Fe(bpp)2](C6H4NO2)2·4H2O (1·4H2O) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; C6H4NO2 = nicotinate anion). This compound exhibits a 3D supramolecular architecture built from hydrogen bonds between iron(II) complexes, nicotinate anions, and water molecules. As synthesized, the hydrated material is low-spin and desolvation triggers a low-spin (LS) to high-spin (HS) transformation. Anhydrous phase 1 undergoes a partial spin crossover (T1/2= 281 K) and a LS to HS photomagnetic conversion with a T(LIESST) value of 56 K. Full article
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Open AccessArticle
Soft X-ray Absorption Spectroscopy Study of Spin Crossover Fe-Compounds: Persistent High Spin Configurations under Soft X-ray Irradiation
Crystals 2018, 8(11), 433; https://doi.org/10.3390/cryst8110433 - 19 Nov 2018
Cited by 1
Abstract
Metal-organic complex exhibiting spin crossover (SCO) behavior has drawn attention for its functionality as a nanoscale spin switch. The spin states in the metal ions can be tuned by external stimuli such as temperature or light. This article demonstrates a soft X-ray–induced excited [...] Read more.
Metal-organic complex exhibiting spin crossover (SCO) behavior has drawn attention for its functionality as a nanoscale spin switch. The spin states in the metal ions can be tuned by external stimuli such as temperature or light. This article demonstrates a soft X-ray–induced excited spin state trapping (SOXEISST) effect in Hofmann-like SCO coordination polymers of FeII(4-methylpyrimidine)2[Au(CN)2]2 and FeII(pyridine)2[Ni(CN)4]. A soft X-ray absorption spectroscopy (XAS) study on these polymers showed that the high spin configuration (HS; S = 2) was prevalent in Fe2+ ions during the measurement even at temperatures much lower than the critical temperatures (>170 K), manifesting HS trapping due to the X-ray irradiation. This is in strong contrast to the normal SCO behavior observed in FeII(1,10-phenanthroline)2(NCS)2, implying that the structure of the ligand chains in the polymers with relatively loose Fe-N coordination might allow a structural adaptation to stabilize the metastable HS state under the soft X-ray irradiation. Full article
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Open AccessArticle
New Iron(II) Spin Crossover Complexes with Unique Supramolecular Networks Assembled by Hydrogen Bonding and Intermetallic Bonding
Crystals 2018, 8(11), 415; https://doi.org/10.3390/cryst8110415 - 05 Nov 2018
Cited by 1
Abstract
Two spin crossover (SCO) coordination polymers assembled by combining FeII octahedral ion, 4-cyanopyridine (4-CNpy) and [Au(CN)2] liner unit are described. These compounds, Fe(4-CNpy)2[Au(CN)2]2·1/2(4-CNpy) (1a) and {Fe(4-CNpy)2[Au(CN)2]2 [...] Read more.
Two spin crossover (SCO) coordination polymers assembled by combining FeII octahedral ion, 4-cyanopyridine (4-CNpy) and [Au(CN)2] liner unit are described. These compounds, Fe(4-CNpy)2[Au(CN)2]2·1/2(4-CNpy) (1a) and {Fe(4-CNpy)2[Au(CN)2]2}-{Fe(H2O)2[Au(CN)2]2} (1b), present quite different supramolecular networks that show different magnetic behaviors. Compound 1a crystallizes in the centrosymmetric space group Pbcn. The asymmetric unit contains two 4-CNpy, one type of Fe2+, and two types of crystallographically distinct [Au(CN)2] units which form Hofmann-like two dimensional layer structures with guest spaces. The layers are combined with another layer by strong gold-gold intermetalic interactions. Compound 1b crystallizes in the centrosymmetric space group Pnma. The bent bismonodentate [AuI(CN)2] units and FeII ions form a complicated interpenetrated three dimensional structure. In addition, 1b exhibits ferromagnetic interaction. Full article
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Open AccessArticle
Impurity-Induced Spin-State Crossover in La0.8Sr0.2Co1−xAlxO3
Crystals 2018, 8(11), 411; https://doi.org/10.3390/cryst8110411 - 31 Oct 2018
Cited by 1
Abstract
We have prepared a set of polycrystalline samples of La0.8Sr0.2Co1xAlxO3 (0x0.2), and have measured the magnetization as functions of temperature and magnetic field. We find that [...] Read more.
We have prepared a set of polycrystalline samples of La 0.8 Sr 0.2 Co 1 x Al x O 3 ( 0 x 0.2 ), and have measured the magnetization as functions of temperature and magnetic field. We find that the average spin number per Co ion ( S Co ) evaluated from the room-temperature susceptibility is around 1.2–1.3 and independent of x. However, we further find that S Co evaluated from the saturation magnetization at 2 K is around 0.3–0.7, and decreases dramatically with x. This naturally indicates that a significant fraction of the Co 3 + ions experience a spin-state crossover from the intermediate- to low-spin state with decreasing temperature in the Al-substituted samples. This spin-state crossover also explains the resistivity and the thermopower consistently. In particular, we find that the thermopower is anomalously enhanced by the Al substitution, which can be consistently explained in terms of an extended Heikes formula. Full article
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Open AccessArticle
Iron(II) Spin Crossover (SCO) Materials Based on Dipyridyl-N-Alkylamine
Crystals 2018, 8(11), 401; https://doi.org/10.3390/cryst8110401 - 24 Oct 2018
Cited by 1
Abstract
We present here a new series of spin crossover (SCO) Fe(II) complexes based on dipyridyl-N-alkylamine and thiocyanate ligands, with the chemical formulae [Fe(dpea)2(NCS)2] (1) (dpea = 2,2’-dipyridyl-N-ethylamine), I-[Fe(dppa)2(NCS)2], ( [...] Read more.
We present here a new series of spin crossover (SCO) Fe(II) complexes based on dipyridyl-N-alkylamine and thiocyanate ligands, with the chemical formulae [Fe(dpea)2(NCS)2] (1) (dpea = 2,2’-dipyridyl-N-ethylamine), I-[Fe(dppa)2(NCS)2], (2) II-[Fe(dppa)2(NCS)2], and (2’) (dppa = 2,2’-dipyridyl-N-propylamine). The three complexes displayed nearly identical discrete molecular structures, where two chelating ligands (dpea (1) and dppa (2 and 2’)) stand in the cis-positions, and two thiocyanato-κN ligands complete the coordination sphere in the two remaining cis-positions. Magnetic studies as a function of temperature revealed the presence of a complete high-spin (HS) to low-spin (LS) transition at T1/2 = 229 K for 1, while the two polymorphs I-[Fe(dppa)2(NCS)2] (2) and II-[Fe(dppa)2(NCS)2] (2’) displayed similar magnetic behaviors with lower transition temperatures (T1/2 = 211 K for 2; 212 K for 2’). Intermolecular contacts in the three complexes indicated the absence of any significant interaction, in agreement with the gradual SCO behaviors revealed by the magnetic data. The higher transition temperature observed for complex 1 agrees well with the more pronounced linearity of the Fe–N–C angles recently evidenced by experimental and theoretical magnetostructural studies. Full article
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Open AccessArticle
Evolution of Spin-Crossover Transition in Hybrid Crystals Involving Cationic Iron Complexes [Fe(III)(3-OMesal2-trien)]+ and Anionic Gold Bis(dithiolene) Complexes Au(dmit)2 and Au(dddt)2
Crystals 2018, 8(10), 382; https://doi.org/10.3390/cryst8100382 - 03 Oct 2018
Cited by 1
Abstract
Hybrid ion-pair crystals involving hexadentate [Fe(III)(3-OMesal2-trien)]+ spin-crossover (SCO) cationic complexes and anionic gold complexes [Au(dmit)2] (1) (dmit = 4,5-dithiolato-1,3-dithiole-2-thione) and [Au(dddt)2] (2) (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were synthesized and studied by [...] Read more.
Hybrid ion-pair crystals involving hexadentate [Fe(III)(3-OMesal2-trien)]+ spin-crossover (SCO) cationic complexes and anionic gold complexes [Au(dmit)2] (1) (dmit = 4,5-dithiolato-1,3-dithiole-2-thione) and [Au(dddt)2] (2) (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) were synthesized and studied by single-crystal X-ray diffraction, P-XRD, and SQUID magnetometry. Our study shows that both complexes have similar 1:1 stoichiometry but different symmetry and crystal packing. Complex 1 has a rigid structure in which the SCO cations are engaged in strong π-interplay with molecular surrounding and does not show SCO transition while 2 demonstrates a reversible transition at Tsco = 118 K in a much “softer”, hydrogen-bonded structure. A new structural indicator of spin state in [Fe(sal2-trien)]+ complexes based on conformational analysis has been proposed. Aging and thermocycling ruined the SCO transition increasing the residual HS fraction from 14 to 41%. Magnetic response of 1 is explained by the AFM coupled dimers S = 5/2 with J1 = −0.18 cm−1. Residual high-spin fraction of 2, apart from a contribution of the weak dimers with J12 = J34 = −0.29 cm−1, is characterized by a stronger interdimer coupling of J23 = −1.69 cm−1, which is discussed in terms of possible involvement of neutral radicals [Au(dddt)2]. Full article
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Open AccessArticle
Solvent Effects on the Spin-Transition in a Series of Fe(II) Dinuclear Triple Helicate Compounds
Crystals 2018, 8(10), 376; https://doi.org/10.3390/cryst8100376 - 23 Sep 2018
Cited by 2
Abstract
This work explores the effect of lattice solvent on the observed solid-state spin-transition of a previously reported dinuclear Fe(II) triple helicate series 13 of the general form [FeII2L3](BF4)4(CH3CN)n, [...] Read more.
This work explores the effect of lattice solvent on the observed solid-state spin-transition of a previously reported dinuclear Fe(II) triple helicate series 13 of the general form [FeII2L3](BF4)4(CH3CN)n, where L is the Schiff base condensation product of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively, and 1 is the complex when L = L1, 2 when L = L2 and 3 when L = L3 (Craze, A.R.; Sciortino, N.F.; Bhadbhade, M.M.; Kepert, C.J.; Marjo, C.E.; Li, F. Investigation of the Spin Crossover Properties of Three Dinuclear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type. Inorganics. 2017, 5 (4), 62). Desolvation of 1 and 2 during measurement resulted not only in a decrease in T1/2 and completeness of spin-crossover (SCO) but also a change in the number of steps in the spin-profile. Compounds 1 and 2 were observed to change from a two-step 70% complete transition when fully solvated, to a single-step half complete transition upon desolvation. The average T1/2 value of the two-steps in the solvated materials was equivalent to the single T1/2 of the desolvated sample. Upon solvent loss, the magnetic profile of 3 experienced a transformation from a gradual SCO or weak antiferromagnetic interaction to a single half-complete spin-transition. Variable temperature single-crystal structures are presented and the effects of solvent molecules are also explored crystallographically and via a Hirshfeld surface analysis. The spin-transition profiles of 13 may provide further insight into previous discrepancies in dinuclear triple helicate SCO research reported by the laboratories of Hannon and Gütlich on analogous systems (Tuna, F.; Lees, M. R.; Clarkson, G. J.; Hannon, M. J. Readily Prepared Metallo-Supramolecular Triple Helicates Designed to Exhibit Spin-Crossover Behaviour. Chem. Eur. J. 2004, 10, 5737–5750 and Garcia, Y.; Grunert, C. M.; Reiman, S.; van Campenhoudt, O.; Gütlich, P. The Two-Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy. Eur. J. Inorg. Chem. 2006, 3333–3339). Full article
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Open AccessArticle
Mosaicity of Spin-Crossover Crystals
Crystals 2018, 8(9), 363; https://doi.org/10.3390/cryst8090363 - 13 Sep 2018
Cited by 1
Abstract
Real crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state [...] Read more.
Real crystals are composed of a mosaic of domains whose misalignment is evaluated by their level of “mosaicity” using X-ray diffraction. In thermo-induced spin-crossover compounds, the crystal may be seen as a mixture of metal centres, some being in the high-spin (HS) state and others in the low spin (LS) state. Since the volume of HS and LS crystal packings are known to be very different, the assembly of domains within the crystal, i.e., its mosaicity, may be modified at the spin crossover. With little data available in the literature we propose an investigation into the temperature dependence of mosaicity in certain spin-crossover crystals. The study was preceded by the examination of instrumental factors, in order to establish a protocol for the measurement of mosaicity. The results show that crystal mosaicity appears to be strongly modified by thermal spin-crossover; however, the nature of the changes are probably sample dependent and driven, or masked, in most cases by the characteristics of the crystal (disorder, morphology …). No general relationship could be established between mosaicity and crystal properties. If, however, mosaicity studies in spin-crossover crystals are conducted and interpreted with great care, they could help to elucidate crucial crystal characteristics such as mechanical fatigability, and more generally to investigate systems where phase transition is associated with large volume changes. Full article
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Open AccessArticle
Cobalt(II) Terpyridin-4′-yl Nitroxide Complex as an Exchange-Coupled Spin-Crossover Material
Crystals 2018, 8(4), 155; https://doi.org/10.3390/cryst8040155 - 02 Apr 2018
Cited by 6
Abstract
Spin-crossover (SCO) was studied in [Co(L)2](CF3SO3)2, where L stands for diamagnetic 2,2′:6′,2′′-terpyridine (tpy) and its paramagnetic derivative, 4′-{4-tert-butyl(N-oxy)aminophenyl}-substituted tpy (tpyphNO). The X-ray crystallographic analysis clarified the Co-N bond length change (Δ [...] Read more.
Spin-crossover (SCO) was studied in [Co(L)2](CF3SO3)2, where L stands for diamagnetic 2,2′:6′,2′′-terpyridine (tpy) and its paramagnetic derivative, 4′-{4-tert-butyl(N-oxy)aminophenyl}-substituted tpy (tpyphNO). The X-ray crystallographic analysis clarified the Co-N bond length change (Δd) in high- and low-temperature structures; Δdcentral = 0.12 and Δddistal = 0.05 Å between 90 and 400 K for L = tpy and Δdcentral = 0.11 and Δddistal = 0.06 Å between 90 and 300 K for L = tpyphNO. The low- and high-temperature structures can be assigned to approximate low- and high-spin states, respectively. The magnetic susceptibility measurements revealed that the χmT value of [Co(tpyphNO)2](CF3SO3)2 had a bias from that of [Co(tpy)2](CF3SO3)2 by the contribution of the two radical spins. The tpy compound showed a gradual SCO around 260 K and on cooling the χmT value displayed a plateau down to 2 K. On the other hand, the tpyphNO compound showed a relatively abrupt SCO at ca. 140 K together with a second decrease of the χmT value on further cooling below ca. 20 K. From the second decrease, Co-nitroxide exchange coupling was characterized as antiferromagnetic with 2JCo-rad/kB = −3.00(6) K in the spin-Hamiltonian H = −2JCo-rad(SCo·Srad1 + SCo·Srad2). The magnetic moment apparently switches double-stepwise as 1 μB 3 μB 5 μB by temperature stimulus. Full article
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