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Inorganics
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Spin-Crossover Complexes
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Spin-Crossover Complexes

A special issue of Inorganics (ISSN 2304-6740). This special issue belongs to the section "Coordination Chemistry".

Deadline for manuscript submissions: closed (31 July 2017) | Viewed by 78742

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editor

Prof. Dr. Kazuyuki Takahashi

E-Mail Website
Guest Editor
Department of Chemistry, Graduate School of Science, Kobe University, 1-1 Rokkodai, Nada, Kobe, Hyogo 657-8501, Japan
Interests: functional molecular materials; molecular conductors; molecular magnets; molecular dielectrics; molecular optical materials; spin-crossover; valence tautomerism; thermo- and photochromism; photoluminescence; chrage-transfer; proton-transfer; phase transition; crystal engineering
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Spin-crossover (SCO) phenomenon is originated from the intrinsic bistability of the d-electron configuration, created by the competition between ligand-field splitting and spin-pairing energies in a first coordination sphere of transition metal ions. Since Cambi’s visionary finding of SCO in 1931, considerable knowledge concerning syntheses, crystal structures, magnetic and thermodynamic properties, spectroscopies, molecular orbital calculations, and theories of SCO complexes has been accumulated in a very large number of inorganic molecular coordination compounds, and, in addition, inorganic cobaltates and bioinorganic molecular systems. Recent studies which have focused on other electronic properties exhibited by SCO complexes themselves, control of molecular assemblies, and moreover, multifunctionalization of SCO complexes with either different electronic properties or porous frameworks will open the possibility toward future practical applications of SCO complexes. Thus, the fundamentals and applications of SCO complexes afford a very exciting research field in inorganic coordination chemistry and will be attracting growing attention in a wide range of relevant research fields. This Special Issue aims at collecting research and review contributions of recent advances in all aspects of SCO and related phenomena and sharing given extensive knowledge with a broader audience by means of an open access way. I invite you to contribute papers in the following research areas and allow your research to impact the next generation trend in this promising field.

Kazuyuki Takahashi
Guest Editor

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Keywords

  • Spin-Crossover (SCO)
  • Valence Tautomerism
  • Charge-transfer-induced Spin Transition
  • Stimulus-induced Spin Transition
  • Ligand-driven Spin-state Conversion
  • Multifunctionality of SCO and related complexes
  • Crystal Engineering of SCO and related complexes

Published Papers (15 papers)

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Editorial

Jump to: Research, Review

4 pages, 322 KiB  
Editorial
Spin-Crossover Complexes
by Kazuyuki Takahashi
Inorganics 2018, 6(1), 32; https://doi.org/10.3390/inorganics6010032 - 01 Mar 2018
Cited by 20 | Viewed by 5591
Abstract
Spin-crossover (SCO) is a spin-state switching phenomenon between a high-spin (HS) and low-spin (LS) electronic configurations in a transition metal center. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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Research

Jump to: Editorial, Review

1707 KiB  
Article
Metal Substitution Effect on a Three-Dimensional Cyanido-Bridged Fe Spin-Crossover Network
by Kenta Imoto, Shinjiro Takano and Shin-ichi Ohkoshi
Inorganics 2017, 5(4), 63; https://doi.org/10.3390/inorganics5040063 - 24 Sep 2017
Cited by 6 | Viewed by 3759
Abstract
We report the CoII-substitution effect on a cyanido-bridged three-dimensional FeII spin-crossover network, Fe2[Nb(CN)8](4-pyridinealdoxime)8·2H2O. A series of iron–cobalt octacyanidoniobate, (FexCo1−x)2[Nb(CN)8](4-pyridinealdoxime)8·zH [...] Read more.
We report the CoII-substitution effect on a cyanido-bridged three-dimensional FeII spin-crossover network, Fe2[Nb(CN)8](4-pyridinealdoxime)8·2H2O. A series of iron–cobalt octacyanidoniobate, (FexCo1−x)2[Nb(CN)8](4-pyridinealdoxime)8·zH2O, was prepared. In this series, the behavior of FeII spin-crossover changes with the CoII concentration. As the CoII concentration increases, the transition of the spin-crossover becomes gradual and the transition temperature of the spin-crossover shifts towards a lower temperature. Additionally, this series shows magnetic phase transition at a low temperature. In particular, (Fe0.21Co0.79)2[Nb(CN)8](4-pyridinealdoxime)8·zH2O exhibits a Curie temperature of 12 K and a large coercive field of 3100 Oe. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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3814 KiB  
Article
Investigation of the Spin Crossover Properties of Three Dinulear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type
by Alexander R. Craze, Natasha F. Sciortino, Mohan M. Badbhade, Cameron J. Kepert, Christopher E. Marjo and Feng Li
Inorganics 2017, 5(4), 62; https://doi.org/10.3390/inorganics5040062 - 21 Sep 2017
Cited by 21 | Viewed by 5177
Abstract
The investigation of new spin-crossover (SCO) compounds plays an important role in understanding the key design factors involved, informing the synthesis of materials for future applications in electronic and sensing devices. In this report, three bis-bidentate ligands were synthesized by Schiff base condensation [...] Read more.
The investigation of new spin-crossover (SCO) compounds plays an important role in understanding the key design factors involved, informing the synthesis of materials for future applications in electronic and sensing devices. In this report, three bis-bidentate ligands were synthesized by Schiff base condensation of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively. Their dinuclear Fe(II) triple helicates were obtained by complexation with Fe(BF4)2·6H2O in acetonitrile. The aim of this study was to examine the influence of the steric nature of the ligand central atom (–X–, where X = CH2, S or O) on the spin-crossover profile of the compound. The magnetic behaviours of these compounds were investigated and subsequently correlated to the structural information from single-crystal X-ray crystallographic experiments. All compounds [Fe2(L1)3](BF4)2 (1), [Fe2(L2)3](BF4)2 (2) and [Fe2(L3)3](BF4)2 (3), demonstrated approximately half-spin transitions, with T1/2 values of 155, 115 and 150 K respectively, corresponding to one high-spin (HS) and one low-spin (LS) Fe(II) centre in a [LS–HS] state at 50 K. This was also confirmed by crystallographic studies, for example, bond lengths and the octahedral distortion parameter (∑) at 100 K. The three-dimensional arrangement of the HS and LS Fe(II) centres throughout the crystal lattice was different for the three compounds, and differing extents of intermolecular interactions between BF4− counter ions and imidazole N–H were present. The three compounds displayed similar spin-transition profiles, with 2 (–S–) possessing the steepest nature. The shape of the spin transition can be altered in this manner, and this is likely due to the subtle effects that the steric nature of the central atom has on the crystal packing (and thus inter-helical Fe–Fe separation), intermolecular interactions and Fe–Fe intra-helical separations. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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2479 KiB  
Article
Structure and Spin State of Iron(II) Assembled Complexes Using 9,10-Bis(4-pyridyl)anthracene as Bridging Ligand
by Saki Iwai, Keisuke Yoshinami and Satoru Nakashima
Inorganics 2017, 5(3), 61; https://doi.org/10.3390/inorganics5030061 - 12 Sep 2017
Cited by 4 | Viewed by 3982
Abstract
Assembled complexes, [Fe(NCX)2(bpanth)2]n (X = S, Se, BH3; bpanth = 9,10-bis(4-pyridyl)anthracene), were synthesized. The iron for the three complexes was in temperature-independent high spin state by 57Fe Mössbauer spectroscopy and magnetic susceptibility measurement. X-ray structural [...] Read more.
Assembled complexes, [Fe(NCX)2(bpanth)2]n (X = S, Se, BH3; bpanth = 9,10-bis(4-pyridyl)anthracene), were synthesized. The iron for the three complexes was in temperature-independent high spin state by 57Fe Mössbauer spectroscopy and magnetic susceptibility measurement. X-ray structural analysis revealed the interpenetrated structure of [Fe(NCS)2(bpanth)2]n. In the local structure around the iron atom, the coordinated pyridine planes were shown to be a parallel type, which is in accordance with the results investigated by density functional theory (DFT) calculation. This complex (X = S) has CH–π interactions between the H atom of coordinated pyridine and the neighboring anthracene of the other 2D grid. It was suggested that the interpenetrated structure was supported by the stabilization of CH–π interaction, and this intermolecular interaction forced the relatively unstable parallel structure. That is, the unstable local structure is compensated by the stabilization due to intermolecular interaction, which controlled the spin state as high spin state. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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3443 KiB  
Article
Structural Dynamics of Spin Crossover in Iron(II) Complexes with Extended-Tripod Ligands
by Philipp Stock, Dennis Wiedemann, Holm Petzold and Gerald Hörner
Inorganics 2017, 5(3), 60; https://doi.org/10.3390/inorganics5030060 - 05 Sep 2017
Cited by 15 | Viewed by 8072
Abstract
Selective manipulation of spin states in iron(II) complexes by thermal or photonic energy is a desirable goal in the context of developing molecular functional materials. As dynamic spin-state equilibration in isolated iron(II) complexes typically limits the lifetime of a given spin state to [...] Read more.
Selective manipulation of spin states in iron(II) complexes by thermal or photonic energy is a desirable goal in the context of developing molecular functional materials. As dynamic spin-state equilibration in isolated iron(II) complexes typically limits the lifetime of a given spin state to nanoseconds, synthetic strategies need to be developed that aim at inhibited relaxation. Herein we show that modulation of the reaction coordinate through careful selection of the ligand can indeed massively slow down dynamic exchange. Detailed structural analysis of [FeL]2+ and [ZnL]2+ (L: tris(1-methyl-2-{[pyridin-2-yl]-methylene}hydrazinyl)phosphane sulfide) with crystallographic and computational methods clearly reveals a unique trigonal-directing effect of the extended-tripod ligand L during spin crossover, which superimposes the ubiquitous [FeN6] breathing with trigonal torsion, akin to the archetypal Bailar twist. As a consequence of the diverging reaction coordinates in [FeL]2+ and in the tren-derived complex [Fe(tren)py3]2+, their thermal barriers differ massively, although the spin crossover energies are close to identical. As is shown by time-resolved transient spectroscopy and dynamic 1H-NMR line broadening, reference systems deriving from tren (tris-(2-aminoethyl)amine), which greatly lack such trigonal torsion, harbor very rapid spin-state exchange. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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3608 KiB  
Article
Modification of Cooperativity and Critical Temperatures on a Hofmann-Like Template Structure by Modular Substituent
by Takashi Kosone, Takeshi Kawasaki, Itaru Tomori, Jun Okabayashi and Takafumi Kitazawa
Inorganics 2017, 5(3), 55; https://doi.org/10.3390/inorganics5030055 - 16 Aug 2017
Cited by 12 | Viewed by 3445
Abstract
In a series of Hofmann-like spin crossover complexes, two new compounds, {Fe(3-F-4-Methyl-py)2[Au(CN)2]2} (1) and {Fe(3-Methyl-py)2[Au(CN)2]2} (2) (py = pyridine) are described. The series maintains a uniform 2-dimentional [...] Read more.
In a series of Hofmann-like spin crossover complexes, two new compounds, {Fe(3-F-4-Methyl-py)2[Au(CN)2]2} (1) and {Fe(3-Methyl-py)2[Au(CN)2]2} (2) (py = pyridine) are described. The series maintains a uniform 2-dimentional (2-D) layer structure of {Fe[Au(CN)2]2}. The layers are combined with another layer by strong aurophilic interactions, which results in a bilayer structure. Both coordination compounds 1 and 2 at 293 K crystallize in the centrosymmetric space groups P21/c. The asymmetric unit contains two pyridine derivative ligands, one type of Fe2+, and two types of crystallographically distinct [Au(CN)2]− units. Compound 1 undergoes a complete two-step spin transition. On the other hand, 2 maintains the characteristic of the high-spin state. The present compounds and other closely related bilayer compounds are compared and discussed in terms of the cooperativity and critical temperature. The bilayer structure is able to be further linked by substituent-substituent contact resulting in 3-dimentional (3-D) network cooperativity. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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3011 KiB  
Article
Spin-Singlet Transition in the Magnetic Hybrid Compound from a Spin-Crossover Fe(III) Cation and π-Radical Anion
by Kazuyuki Takahashi, Takahiro Sakurai, Wei-Min Zhang, Susumu Okubo, Hitoshi Ohta, Takashi Yamamoto, Yasuaki Einaga and Hatsumi Mori
Inorganics 2017, 5(3), 54; https://doi.org/10.3390/inorganics5030054 - 16 Aug 2017
Cited by 6 | Viewed by 5398
Abstract
To develop a new spin-crossover functional material, a magnetic hybrid compound [Fe(qsal)2][Ni(mnt)2] was designed and synthesized (Hqsal = N-(8-quinolyl)salicylaldimine, mnt = maleonitriledithiolate). The temperature dependence of magnetic susceptibility suggested the coexistence of the high-spin (HS) Fe(III) cation and [...] Read more.
To develop a new spin-crossover functional material, a magnetic hybrid compound [Fe(qsal)2][Ni(mnt)2] was designed and synthesized (Hqsal = N-(8-quinolyl)salicylaldimine, mnt = maleonitriledithiolate). The temperature dependence of magnetic susceptibility suggested the coexistence of the high-spin (HS) Fe(III) cation and π-radical anion at room temperature and a magnetic transition below 100 K. The thermal variation of crystal structures revealed that strong π-stacking interaction between the π-ligand in the [Fe(qsal)2] cation and [Ni(mnt)2] anion induced the distortion of an Fe(III) coordination structure and the suppression of a dimerization of the [Ni(mnt)2] anion. Transfer integral calculations indicated that the magnetic transition below 100 K originated from a spin-singlet formation transformation in the [Ni(mnt)2] dimer. The magnetic relaxation of Mössbauer spectra and large thermal variation of a g-value in electron paramagnetic resonance spectra below the magnetic transition temperature implied the existence of a magnetic correlation between d-spin and π-spin. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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2382 KiB  
Communication
Halogen Substituent Effect on the Spin-Transition Temperature in Spin-Crossover Fe(III) Compounds Bearing Salicylaldehyde 2-Pyridyl Hydrazone-Type Ligands and Dicarboxylic Acids
by Takumi Nakanishi, Atsushi Okazawa and Osamu Sato
Inorganics 2017, 5(3), 53; https://doi.org/10.3390/inorganics5030053 - 12 Aug 2017
Cited by 11 | Viewed by 4342
Abstract
Four Fe(III) spin-crossover (SCO) compounds, [Fe(HL1)2](HCl4TPA) (1-Cl), [Fe(HL1)2](HBr4TPA) (1-Br), [Fe(HL2)2](HCl4TPA) (2-Cl), and [Fe(HL2)2](HBr4TPA) (2-Br) (HL1 = 4-chloro-2-nitro-6-(1-(2-(pyridine-2-yl)hydrazono)ethyl)phenolate; HL2 = [...] Read more.
Four Fe(III) spin-crossover (SCO) compounds, [Fe(HL1)2](HCl4TPA) (1-Cl), [Fe(HL1)2](HBr4TPA) (1-Br), [Fe(HL2)2](HCl4TPA) (2-Cl), and [Fe(HL2)2](HBr4TPA) (2-Br) (HL1 = 4-chloro-2-nitro-6-(1-(2-(pyridine-2-yl)hydrazono)ethyl)phenolate; HL2 = 4-bromo-2-nitro-6-(1-(2-(pyridine-2-yl)hydrazono)ethyl)phenolate; HCl4TPA = 2,3,5,6-tetrachloro-4-carboxybenzoate; and HBr4TPA = 2,3,5,6-tetrabromo-4-carboxybenzoate), were synthesized to investigate the halogen substituent change effect in salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids in SCO complexes to the spin-transition temperature. Crystal structure analyses showed that these compounds were isostructural. In addition, a one-dimensional hydrogen–bonded column was formed by the dicarboxylic acid anion and weak hydrogen bonds between the Fe(III) complexes. From Mössbauer spectroscopy and magnetic property measurements, these compounds were confirmed to exhibit gradual SCO. The spin-transition temperature can be shifted by changing the halogen substituent in the salicylaldehyde 2-pyridyl hydrazone-type ligands and dicarboxylic acids without changing the molecular arrangement in the crystal packing. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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1600 KiB  
Article
Pybox-Iron(II) Spin-Crossover Complexes with Substituent Effects from the 4-Position of the Pyridine Ring (Pybox = 2,6-Bis(oxazolin-2-yl)pyridine)
by Akifumi Kimura and Takayuki Ishida
Inorganics 2017, 5(3), 52; https://doi.org/10.3390/inorganics5030052 - 08 Aug 2017
Cited by 29 | Viewed by 5056
Abstract
Spin-crossover (SCO) behavior of a series of [Fe(X-pybox)2](ClO4)2 was investigated, where X-pybox stands for 4-X-substituted 2,6-bis(oxazolin-2-yl)pyridine with X = H, Cl, Ph, CH3O, and CH3S. We confirmed that the mother compound [Fe(H-pybox)2](ClO [...] Read more.
Spin-crossover (SCO) behavior of a series of [Fe(X-pybox)2](ClO4)2 was investigated, where X-pybox stands for 4-X-substituted 2,6-bis(oxazolin-2-yl)pyridine with X = H, Cl, Ph, CH3O, and CH3S. We confirmed that the mother compound [Fe(H-pybox)2](ClO4)2 underwent SCO above room temperature. After X was introduced, the SCO temperatures (T1/2) were modulated as 310, 230, and 330 K for X = Cl, Ph, and CH3S, respectively. The CH3O derivative possessed the high-spin state down to 2 K. Crystallographic analysis for X = H, Cl, CH3O, and CH3S was successful, being consistent with the results of the magnetic study. Distorted coordination structures stabilize the HS (high-spin) state, and the highest degree of the coordination structure distortion is found in the CH3O derivative. A plot of T1/2 against the Hammett substituent constant σp showed a positive relation. Solution susceptometry was also performed to remove intermolecular interaction and rigid crystal lattice effects, and the T1/2’s were determined as 260, 270, 240, 170, and 210 K for X = H, Cl, Ph, CH3O, and CH3S, respectively, in acetone. The substituent effect on T1/2 became very distinct, and it is clarified that electron-donating groups stabilize the HS state. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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3725 KiB  
Article
Heteroleptic and Homoleptic Iron(III) Spin-Crossover Complexes; Effects of Ligand Substituents and Intermolecular Interactions between Co-Cation/Anion and the Complex
by Wasinee Phonsri, Luke C. Darveniza, Stuart R. Batten and Keith S. Murray
Inorganics 2017, 5(3), 51; https://doi.org/10.3390/inorganics5030051 - 01 Aug 2017
Cited by 15 | Viewed by 4933
Abstract
The structural and magnetic properties of a range of new iron(III) bis-tridentate Schiff base complexes are described with emphasis on how intermolecular structural interactions influence spin states and spin crossover (SCO) in these d5 materials. Three pairs of complexes were investigated. The [...] Read more.
The structural and magnetic properties of a range of new iron(III) bis-tridentate Schiff base complexes are described with emphasis on how intermolecular structural interactions influence spin states and spin crossover (SCO) in these d5 materials. Three pairs of complexes were investigated. The first pair are the neutral, heteroleptic complexes [Fe(3-OMe-SalEen)(thsa)] 1 and [Fe(3-MeOSalEen)(3-EtOthsa)] 2, where 3-R-HSalEen = (E)-2-(((2-(ethylamino)ethyl)imino)methyl)-6-R-phenol and 3-R-H2thsa = thiosemicarbazone-3-R-salicylaldimine. They display spin transitions above room temperature. However, 2 shows incomplete and gradual change, while SCO in 1 is complete and more abrupt. Lower cooperativity in 2 is ascribed to the lack of π–π interactions, compared to 1. The second pair, cationic species [Fe(3-EtOSalEen)2]NO3 3 and [Fe(3-EtOSalEen)2]Cl 4 differ only in the counter-anion. They show partial SCO above room temperature with 3 displaying a sharp transition at 343 K. Weak hydrogen bonds from cation to Cl− probably lead to weaker cooperativity in 4. The last pair, CsH2O[Fe(3-MeO-thsa)2] 5 and Cs(H2O)2[Fe(5-NO2-thsa)2] 6, are anionic homoleptic chelates that have different substituents on the salicylaldiminate rings of thsa2−. The Cs cations bond to O atoms of water and the ligands, in unusual ways thus forming attractive 1D and 3D networks in 5 and 6, respectively, and 5 remains HS (high spin) at all temperatures while 6 remains LS (low spin). Comparisons are made to other literature examples of Cs salts of [Fe(5-R-thsa)2]− (R = H and Br). Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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4245 KiB  
Article
High-Temperature Wide Thermal Hysteresis of an Iron(II) Dinuclear Double Helicate
by Shiori Hora and Hiroaki Hagiwara
Inorganics 2017, 5(3), 49; https://doi.org/10.3390/inorganics5030049 - 28 Jul 2017
Cited by 15 | Viewed by 5264
Abstract
Two new dinuclear iron(II) complexes (1·PF6 and 1·AsF6) of the general formula [FeII2(L2C3)2](X)4·nH2O·mMeCN (X = PF6, n = m = 1.5 for 1·PF6 and X [...] Read more.
Two new dinuclear iron(II) complexes (1·PF6 and 1·AsF6) of the general formula [FeII2(L2C3)2](X)4·nH2O·mMeCN (X = PF6, n = m = 1.5 for 1·PF6 and X = AsF6, n = 3, m = 1 for 1·AsF6) have been prepared and structurally characterized, where L2C3 is a bis-1,2,3-triazolimine type Schiff-base ligand, 1,1′-[propane-1,3-diylbis(1H-1,2,3-triazole-1,4-diyl)]bis{N-[2-(pyridin-2-yl)ethyl]methanimine}. Single crystal X-ray structure analyses revealed that 1·PF6 and 1·AsF6 are isostructural. The complex-cation [FeII2(L2C3)2]4+ of both has the same dinuclear double helicate architecture, in which each iron(II) center has an N6 octahedral coordination environment. Neighboring helicates are connected by intermolecular π–π interactions to give a chiral one-dimensional (1D) structure, and cationic 1D chains with the opposite chirality exist in the crystal lattice to give a heterochiral crystal. Magnetic and differential scanning calorimetry (DSC) studies were performed only for 1·AsF6, since the thermal stability in a high-temperature spin crossover (SCO) region of 1·PF6 is poorer than that of 1·AsF6. 1·AsF6 shows an unsymmetrical hysteretic SCO between the low-spin–low-spin (LS–LS) and high-spin–high-spin (HS–HS) states at above room temperature. The critical temperatures of warming (Tc↑) and cooling (Tc↓) modes in the abrupt spin transition area are 485 and 401 K, respectively, indicating the occurrence of 84 K-wide thermal hysteresis in the first thermal cycle. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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1767 KiB  
Article
Synthesis, Crystal Structures and Magnetic Properties of Composites Incorporating an Fe(II) Spin Crossover Complex and Polyoxometalates
by Satoshi Kuramochi, Takuya Shiga, Jamie M. Cameron, Graham N. Newton and Hiroki Oshio
Inorganics 2017, 5(3), 48; https://doi.org/10.3390/inorganics5030048 - 22 Jul 2017
Cited by 8 | Viewed by 5013
Abstract
[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 [...] Read more.
[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. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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Review

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4920 KiB  
Review
Control of Spin-Crossover Phenomena in One-Dimensional Triazole-Coordinated Iron(II) Complexes by Means of Functional Counter Ions
by Akira Sugahara, Hajime Kamebuchi, Atsushi Okazawa, Masaya Enomoto and Norimichi Kojima
Inorganics 2017, 5(3), 50; https://doi.org/10.3390/inorganics5030050 - 19 Aug 2017
Cited by 21 | Viewed by 5317
Abstract
The spin-crossover (SCO) phenomenon between a high-spin and a low-spin state has attracted much attention in the field of materials science. Among the various kinds of SCO complexes, the triazole-bridged iron(II) polymeric chain system, [Fe(II)(R-trz)3]X2·xH2O [...] Read more.
The spin-crossover (SCO) phenomenon between a high-spin and a low-spin state has attracted much attention in the field of materials science. Among the various kinds of SCO complexes, the triazole-bridged iron(II) polymeric chain system, [Fe(II)(R-trz)3]X2·xH2O (where trz is triazole and X is the anion), exhibiting the SCO phenomenon with thermal hysteresis around room temperature, has been extensively studied from the viewpoint of molecular memory and molecular devices. In connection with this system, we have controlled the SCO phenomenon according to the characteristic properties of counter ions. In the case of X being CnH2n+1SO3−, the spin transition temperature (T1/2) increases with increasing the length (n) of the alkyl chain of the counter ion and saturates above n = 5, which is attributed to the increase in the intermolecular interaction of the alkyl chains of CnH2n+1SO3−, called the fastener effect. The hysteresis width of T1/2 decreases with increasing n, showing the even-odd, also known as parity, effect. In the cases where X is toluenesulfonate (tos: CH3C6H4SO3−) and aminobenzenesulfonate (abs: NH2C6H4SO3−), T1/2 and its hysteresis width vary drastically with the structural isomerism (ortho-, metha-, and para-substitution) of counter ions, which implies the possibility of photoinduced spin transition by means of the photoisomerization of counter ions. From this strategy, we have synthesized [Fe(II)(NH2-trz)3](SP150)2·2H2O (SP150 = N-alkylsulfonated spiropyran) and investigated the SCO phenomenon. Moreover, we have developed [Fe(II)(R-trz)3]@Nafion films exhibiting spin transition around room temperature, where the Nafion membrane behaves as a counter anion as well as a transparent substrate, and investigated the photogenerated high-spin state below 35 K. The lifetime of the photogenerated high-spin state strongly depends on the intensity of irradiated light. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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2907 KiB  
Review
Design and Control of Cooperativity in Spin-Crossover in Metal–Organic Complexes: A Theoretical Overview
by Hrishit Banerjee, Sudip Chakraborty and Tanusri Saha-Dasgupta
Inorganics 2017, 5(3), 47; https://doi.org/10.3390/inorganics5030047 - 20 Jul 2017
Cited by 32 | Viewed by 6347
Abstract
Metal organic complexes consisting of transition metal centers linked by organic ligands, may show bistability which enables the system to be observed in two different electronic states depending on external condition. One of the spectacular examples of molecular bistability is the spin-crossover phenomena. [...] Read more.
Metal organic complexes consisting of transition metal centers linked by organic ligands, may show bistability which enables the system to be observed in two different electronic states depending on external condition. One of the spectacular examples of molecular bistability is the spin-crossover phenomena. Spin-Crossover (SCO) describes the phenomena in which the transition metal ion in the complex under the influence of external stimuli may show a crossover between a low-spin and high-spin state. For applications in memory devices, it is desirable to make the SCO phenomena cooperative, which may happen with associated hysteresis effect. In this respect, compounds with extended solid state structures containing metal ions connected by organic spacer linkers like linear polymers, coordination network solids are preferred candidates over isolated molecules or molecular assemblies. The microscopic understanding, design and control of mechanism driving cooperativity, however, are challenging. In this review we discuss the recent theoretical progress in this direction. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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7330 KiB  
Review
Supramolecular Control of Spin Crossover Phenomena Using Various Amphiphiles
by Keita Kuroiwa
Inorganics 2017, 5(3), 45; https://doi.org/10.3390/inorganics5030045 - 14 Jul 2017
Cited by 8 | Viewed by 5400
Abstract
An aspect of nanochemistry that has attracted significant attention is the formation of nanoarchitectures from the self-assembly of metal complexes, based on the design of compounds having cooperative functionalities. This technique is currently seen as important within the field of nanomaterials. In the [...] Read more.
An aspect of nanochemistry that has attracted significant attention is the formation of nanoarchitectures from the self-assembly of metal complexes, based on the design of compounds having cooperative functionalities. This technique is currently seen as important within the field of nanomaterials. In the present review, we describe the methods that allow tuning of the intermolecular interactions between spin crossover (SCO) complexes in various media. These approaches include the use of lipophilic derivatives, lipids, and diblock copolypeptide amphiphiles. The resulting supramolecular assemblies can enhance the solubility of various SCO complexes in both organic and aqueous media. In addition, amphiphilic modifications of coordination systems can result in metastable structures and dynamic structural transformations leading to unique solution properties, including spin state switching. The supramolecular chemistry of metal complexes is unprecedented in its scope and potential applications, and it is hoped that the studies presented herein will promote further investigation of dynamic supramolecular devices. Full article
(This article belongs to the Special Issue Spin-Crossover Complexes)
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