Room-temperature spin-transition iron compounds

Abstract  Since the discovery of the spin transition phenomenon in tris(N,N-dialkyldithiocarbamato) iron(III) complexes [1], numerous investigations have been devoted to this field of molecular magnetism. The spin transition phenomenon is probably the most spectacular example of bistability in molecular chemistry. However, it is a challenge to obtain spin transition materials when working under ambient conditions (e.g. room temperature and pressure), which would be highly advantageous for potential applications. So far, only some Fe(II) and Fe(III) molecular systems have shown temperature-induced spin transitions around and even above room temperature. Within this review we discuss the characteristics of this class of bistable compounds in detail and we try to draw more general conclusions regarding the integration, implementation and application of spin transition compounds as switching elements in hybrid molecular devices. Graphical abstract        

Fe(II)自旋交叉分子材料

自旋交叉配合物具有理想的分子双稳态,可用作新型的热开关、光开关和信息存储器件。本文对近三年来Fe(II)自旋交叉分子材料的重要研究进展进行了综述,主要讨论了转变温度在室温附近的Fe(II)自旋交叉配合物以及具有光致激发自旋态捕获(LIESST)效应和多功能的Fe(II)自旋交叉分子材料,并对Fe(II)自旋交叉分子材料的应用前景作了探讨。     

Mechanism of reversible spin transition with a thermal hysteresis loop in [FeIII(qsal)2][Ni(dmise)2] · 2CH3CN: Selenium analogue of the precursor of an Fe(III) spin-crossover molecular conducting system

A novel Fe(III) spin-crossover complex, [Fe(qsal)₂][Ni(dmise)₂] · 2CH₃CN 1 [qsalH = N-(8-quinolyl)-salicylaldimine, dmise = 4,5-dithiolato-1,3-dithiole-2-selone] was prepared. The magnetic susceptibility measurements revealed 1 exhibited a cooperative spin transition with a thermal hysteresis loop of 15 K. The high and the low temperature structures of 1 indicated three-dimensional intermolecular π?π interactions play a key role in the cooperative spin transition, accompanying a reversible molecular slipping of π-dimer of Ni(dmise)₂ along the molecular long axis. The transfer integral calculation for 1 suggested the π-dimer of Ni(dmise)₂ is in the spin singlet state.     

Heptanuclear high-spin complex compounds based on S-donors

The precursors [Fe(III)(^SYL)Cl] (^SYLH₂) = N,N′-bis(1-hydroxy-Y-2-benzyliden)-1,6-diamino-3-thiohexane, (Y = H, 3EtO, 5Me) are high-spin (S = 5/2) complexes. The precursors are combined with [Fe(II)(CN)₆]⁴⁻ and [Co(III)(CN)₆]³⁻ to yield star-shaped heptanuclear clusters, [Fe(II)(CN–Fe(III)^SYL)₆]Cl₂ and [Co(III)(CN–Fe(III)^SYL)₆]Cl₃. The star-shaped compounds are high-spin (HS) systems at room temperature. On cooling to 20 K some of the iron(III) centers perform some HS–HS transition.     

Magnetic moment and Mössbauer spectral studies of spin-crossover in tris(N,N′-dialkyldithiocarbamato)iron(III) complexes and their thermal decomposition

Room temperature Mössbauer spectra of tris(N,N-dialkyldithiocarbamato)iron(III) complexes [(R₂NCS₂)₃Fe] (R = Me, Et, n-Pr, i-Pr, n-Bu and i-Bu) exhibit an asymmetric doublet which can be resolved into two doublets, each corresponding to high and low spin states in equilibrium. The quadrupole splitting (E _Q ), in general, increases with the molecular weight of the alkyl group in both the cases. Plots of magnetic moment (_eff) versus temperature show that dimethyl-, diethyl-, di-n-propyl- and di-n-butyl-substituted dithiocarbamato complexes are equilibrium mixtures of high and low spin states at room temperature, but increasingly adopt low spin at the liquid nitrogen temperature. However, the di-i-propyl- and di-i-butyl-substituted dithiocarbamato complexes exhibit primarily low spin state in the 77–350 K range, with a small contribution (<15%) of high spin state. Fe—S stretching vibrations in far i.r. region also show spin equilibrium states. Thermogravimetric studies show fast decomposition in the 200–300 °C range, yielding Fe(SCN)₃ as an intermediate product followed by slow decomposition, leading finally to constant weight corresponding to Fe₂O₃ at ca. 650 °C. Mössbauer spectra of the final products of all the complexes exhibit a six line spectrum with H _eff = 517 ± 3 kOe corresponding to that of -Fe₂O₃ without any possibility of Fe₂S₃ as proposed in literature.     

Room temperature conductance switching in a molecular iron(iii) spin crossover junction

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction. The junction is constructed from [Fe^III(qsal-I)₂]NTf₂ (qsal-I = 4-iodo-2-[(8-quinolylimino)methyl]phenolate) molecules self-assembled on graphene surfaces with conductance switching of one order of magnitude associated with the high and low spin states of the SCO complex. Normalized conductance analysis of the current–voltage characteristics as a function of temperature reveals that charge transport across the SCO molecule is dominated by coherent tunnelling. Temperature-dependent X-ray absorption spectroscopy and density functional theory confirm the SCO complex retains its SCO functionality on the surface implying that van der Waals molecule—electrode interfaces provide a good trade-off between junction stability while retaining SCO switching capability. These results provide new insights and may aid in the design of other types of molecular devices based on SCO compounds.

Herein, we report the first room temperature switchable Fe(iii) molecular spin crossover (SCO) tunnel junction.     

Decoupled Spin Crossover and Structural Phase Transition in a Molecular Iron(II) Complex

Crystalline [Fe(bppSMe)₂][BF₄]₂ ( 1 ; bppSMe=4‐(methylsulfanyl)‐2,6‐di(pyrazol‐1‐yl)pyridine) undergoes an abrupt spin‐crossover (SCO) event at 265±5 K. The crystals also undergo a separate phase transition near 205 K, involving a contraction of the unit‐cell a axis to one‐third of its original value (high‐temperature phase 1; Pbcn, Z=12; low‐temperature phase 2; Pbcn, Z=4). The SCO‐active phase 1 contains two unique molecular environments, one of which appears to undergo SCO more gradually than the other. In contrast, powder samples of 1 retain phase 1 between 140–300 K, although their SCO behaviour is essentially identical to the single crystals. The compounds [Fe(bppBr)₂][BF₄]₂ ( 2 ; bppBr=4‐bromo‐2,6‐di(pyrazol‐1‐yl)pyridine) and [Fe(bppI)₂][BF₄]₂ ( 3 ; bppI=4‐iodo‐2,6‐di(pyrazol‐1‐yl)‐pyridine) exhibit more gradual SCO near room temperature, and adopt phase 2 in both spin states. Comparison of 1 – 3 reveals that the more cooperative spin transition in 1 , and its separate crystallographic phase transition, can both be attributed to an intermolecular steric interaction involving the methylsulfanyl substituents. All three compounds exhibit the light‐induced excited‐spin‐state trapping (LIESST) effect with T(LIESST=70–80 K), but show complicated LIESST relaxation kinetics involving both weakly cooperative (exponential) and strongly cooperative (sigmoidal) components.     

Retracted: Light-Induced Bistability in Iron(III) Spin-Transition Compounds of 5 X-Salicylaldehyde Thiosemicarbazone (X=H,Cl, Br)

The iron(III) spin-crossover compounds [Fe(Hthsa)(thsa)] ⋅ H₂O ( 1 ), [Fe(Hth5Clsa)(th5Clsa)₂] ⋅ H₂O ( 2 ), and [Fe(Hth5Brsa)(th5Brsa)₂] ⋅ H₂O ( 3 ) (H₂thsa=salicylaldehyde thiosemicarbazone, H₂th5Clsa=5-chlorosalicylaldehyde thiosemicarbazone, and H₂th5Brsa=5-bromosalicylaldehyde thiosemicarbazone) have been synthesized and their spin-transition properties investigated by magnetic susceptibility, Mössbauer spectroscopy, and differential scanning calorimetry measurements. The three compounds exhibit an abrupt spin transition with a thermal hysteresis effect. The more polarizable the substituent on the salicylaldehyde moiety, the more complete is the transition at room temperature with an increased degree of cooperativity. The molecular structures of 1 and 2 in the high-spin state are revealed. The occurrence of the light-induced excited-spin-state trapping phenomenon appears to be dependent on the substituent incorporated into the 5-position of the salicylaldehyde subunit. Whereas the compounds with an electron-withdrawing group (-Br or -Cl) exhibit light-induced trapped excited high-spin states with great longevity of metastability, the halogen-free compound does not, even though strong intermolecular interactions (such as hydrogen-bonding networks and π stacking) operate in the system. For compound 2 , the surface level of photoconversion is less than 35 %. In contrast, compound 3 displays full photoexcitation.     

Bis-chelate Fe(III) complex of an azomethine at the focal point of a branched ester functionalized with cyclohexylbenzoic acid

A Fe(III) complex with Cl counter ion based on a branched Schiff base has been synthesized and studied. The compound was produced by the reaction of the Schiff base with FeCl₃ at room temperature in benzene–ethanol. The complex is symmetric, i.e., bis-chelate, with an octahedral coordination of Fe. The compound revealed phase transitions of the “solid–solid” type. The complex displayed a temperature-induced spin transition (S?=?1/2???5/2) which was detected by EPR.     

Tuning the transition temperature and cooperativity of bapbpy-based mononuclear spin-crossover compounds: interplay between molecular and crystal engineering

In this study, we show that 1) different isomers of the same mononuclear iron(II) complex give materials with different spin‐crossover (hereafter SCO) properties, and 2) minor modifications of the bapbpy (bapbpy=N6,N6′‐di(pyridin‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine) ligand allows SCO to be obtained near room temperature. We also provide a qualitative model to understand the link between the structure of bapbpy‐based ligands and the SCO properties of their iron(II) compounds. Thus, seven new trans‐[Fe{R₂(bapbpy)}(NCS)₂] compounds were prepared, in which the R₂bapbpy ligand bears picoline ( 9 – 12 ), quin‐2‐oline ( 13 ), isoquin‐3‐oline ( 14 ), or isoquin‐1‐oline ( 15 ) substituents. From this series, three compounds ( 12 , 14 , and 15 ) have SCO properties, one of which ( 15 ) occurs at 288 K. The crystal structures of compounds 11 , 12 , and 15 show that the intermolecular interactions in these materials are similar to those found in the parent compound [Fe(bapbpy)(NCS)₂] ( 1 ), in which each iron complex interacts with its neighbors through weak N? H ??? S hydrogen bonding and π–π stacking. For compounds 12 and 15 , hindering groups located near the N? H bridges weaken the N? S intermolecular interactions, which is correlated to non‐cooperative SCO. For compound 14 , the substitution is further away from the N? H bridges, and the SCO remains cooperative as in 1 with a hysteresis cycle. Optical microscopy photographs show the strikingly different spatio‐temporal evolution of the phase transition in the noncooperative SCO compound 12 relative to that found in 1 . Heat‐capacity measurements were made for compounds 1 , 12 , 14 , and 15 and fitted to the Sorai domain model. The number n of like‐spin SCO centers per interacting domain, which is related to the cooperativity of the spin transition, was found high for compounds 1 and 14 and low for compounds 12 and 15 . Finally, we found that although both pairs of compounds 11 / 12 and 14 / 15 are pairs of isomers their SCO properties are surprisingly different.     

Synthesis, structural characterization, and magnetic properties of the metallamacrocycle [Fe6(C11H11N2O3)6(C4H9NO)6]

Abstract  

The 18-metallacrown-6 metallamacrocycle [Fe₆(pmshz)₆(C₄H₉NO)₆] has been synthesized by the self-assembly reaction of iron ions with N-substituted salicylhydrazide ligands. Six Fe(III) ions and six deprotonated N-propanoyl-4-methylsalicylhydrazide (H₃ pmshz) ligands construct a planar 18-membered ring based on Fe–N–N–Fe linkage. Because of the coordination, the ligand enforces the stereochemistry of the Fe(III) ions as a propeller shape with alternating …ΔΛΔΛ… configurations. There is a strong antiferromagnetic exchange interaction between the paramagnetic iron centers.     

One-pot, three-component reaction of isocyanides, dialkyl acetylenedicarboxylates, and non-cyclic anhydrides: synthesis of 2,5-diaminofuran derivatives and dialkyl (E)-2-[(N-acyl-N-alkylamino)carbonyl]-2-butenedioates

Abstract  

Isocyanides, dialkyl acetylenedicarboxylates, and non-cyclic anhydrides, for example acetic anhydride or benzoic anhydride, react in one-pot to afford 2,5-diaminofuran derivatives and dialkyl (E)-2-[(N-acyl-N-alkylamino)carbonyl]-2-butenedioates in fairly good yields at room temperature.     

Stabilization of the Low‐Spin State in a Mononuclear Iron(II) Complex and High‐Temperature Cooperative Spin Crossover Mediated by Hydrogen Bonding

The tetrapyridyl ligand bbpya (bbpya=N,N‐bis(2,2′‐bipyrid‐6‐yl)amine) and its mononuclear coordination compound [Fe(bbpya)(NCS)₂] ( 1 ) were prepared. According to magnetic susceptibility, differential scanning calorimetry fitted to Sorai’s domain model, and powder X‐ray diffraction measurements, 1 is low‐spin at room temperature, and it exhibits spin crossover (SCO) at an exceptionally high transition temperature of T_1/2=418 K. Although the SCO of compound 1 spans a temperature range of more than 150 K, it is characterized by a wide (21 K) and dissymmetric hysteresis cycle, which suggests cooperativity. The crystal structure of the LS phase of compound 1 shows strong N?H???S intermolecular H‐bonding interactions that explain, at least in part, the cooperative SCO behavior observed for complex 1 . DFT and CASPT2 calculations under vacuum demonstrate that the bbpya ligand generates a stronger ligand field around the iron(II) core than its analogue bapbpy (N,N′‐di(pyrid‐2‐yl)‐2,2′‐bipyridine‐6,6′‐diamine); this stabilizes the LS state and destabilizes the HS state in 1 compared with [Fe(bapbpy)(NCS)₂] ( 2 ). Periodic DFT calculations suggest that crystal‐packing effects are significant for compound 2 , in which they destabilize the HS state by about 1500 cm^?1. The much lower transition temperature found for the SCO of 2 compared to 1 appears to be due to the combined effects of the different ligand field strengths and crystal packing.     

The possible production of natural flavours by amino acid degradation

Abstract  

This work describes the degradation of phenylalanine and tryptophane catalysed by their complexes with Fe(II), Co(II), and Cu(II). The influences of the central atom and of the reaction conditions on the degradation of the amino acids were observed. The necessary condition of the degradation is the possibility of a redox reaction on the central atom between M(II) and M(III). Moreover, the coordination sphere of the central cation of the transition metal must not be sterically shielded. The necessary conditions are fulfilled only in the Fe(II) complexes. The degradation is strictly anaerobic because due to the influence of oxygen, an irreversible oxidation of Fe(II) to Fe(III) proceeds. This reaction results in 5-hydroxy-1H-indol instead of the mixture of the degradation products, such as benzaldehyde, phenylacetaldehyde, and phenylacetic acid. The influence of the temperature on the catabolism is very important because the reaction accelerates with temperature increase. The phenylalanine anion acts as a reducing agent, and Fe(II) is spontaneously reduced to Fe(0).     

Spin Transitions S= 5/2 ⇄ S= 1/2 in Iron(III) Chelates with Azomethin Ligands

Results of S= 5/2 S= 1/2 spin transition studies in Fe(III) chelates with azomethin ligands showing different dentate numbers are generalized. The current state of the problem is considered and the criteria for optimization of the synthesis of Fe(III) complexes with different spin states are discussed.     

Ionic liquid 1-(4-nitritobutyl)-3-methylimidazolium chloride as a new reagent for the efficient N-nitrosation of secondary amines under mild conditions

Abstract  

1-(4-Nitritobutyl)-3-methylimidazolium chloride has been developed as a new reagent for efficient nitrosation of secondary amines at 0 °C to room temperature. A variety of N-nitrosamines were prepared in excellent yields by use of this task-specific ionic liquid under mild and heterogeneous conditions.     

Spin transition in heptanuclear star-shaped iron(III)–antimony(V) NCS- and CN-bridged compounds

The precursor [Fe^III(L)Cl] (LH₂ = N,N′-bis(2′-hydroxy-benzyliden)-1,6-diamino-3-azahexane) has been prepared and Mössbauer spectroscopy assigned a high-spin (S = 5/2) state at room temperature. The precursor is combined with the bridging units [Sb^V(X)₆]^? (X = CN^?, NCS^?) to yield star-shaped heptanuclear clusters [(LFe^III–X)₆Sb^V]Cl₅. The star-shaped compounds are in general high-spin systems at room temperature. On cooling to 20 K some of the iron(III) centers switch to the low-spin state as indicated by Mössbauer spectroscopy, i.e. multiple electronic transitions. While the cyano-bridged complex performs a multiple spin transition the thiocyanate-compound shows no significant population at both temperatures.     

Rapid cooling experiments and use of an anionic nuclear probe to sense the spin transition of the 1D coordination polymers [Fe(NH2trz)3]SnF6n x H2O (NH2trz=4-amino-1,2,4-triazole)

[Fe(NH₂trz)₃]SnF₆ ? n H₂O (NH₂trz=4‐amino‐1,2,4‐triazole; n=1 ( 1 ), n=0.5 ( 2 )) are new 1D spin‐crossover coordination polymers. Compound 2 exhibits an incomplete spin transition centred at around 210 K with a thermal hysteresis loop approximately 16 K wide. The spin transition of 2 was detected by the Mössbauer resonance of the ¹¹⁹Sn atom in the SnF₆^2? anion primarily on the basis of the evolution of its local distortion. Rapid‐cooling ⁵⁷Fe Mössbauer and superconducting quantum interference device experiments allow dramatic widening of the hysteresis width of 2 from 16 K up to 82 K and also shift the spin‐transition curve into the room temperature region. This unusual behaviour of quenched samples on warming is attributed to activation of the molecular motion of the anions from a frozen distorted form towards a regular form at temperatures well above approximately 210 K. Potential applications of this new family of materials are discussed.     

Study on the spin-crossover transition in [Fe(cis-/trans-stpy)4(X)2] (stpy: styrylpyridine, X: NCS, NCBH3) under high pressure toward ligand-driven light-induced spin change

Toward the realization of a ligand-driven light-induced spin change (LD-LISC) around room temperature, we have investigated the spin-crossover phenomenon in [Fe(stpy)₄(X)₂] (stpy = styrylpyridine, X = NCS, NCBH₃) under high pressure. The spin transition temperature increases from 110 to 220 K with increasing applied pressure up to 0.75 GPa for [Fe(trans-stpy)₄(NCS)₂], while [Fe(cis-stpy)₄(NCS)₂] shows the high-spin state in the temperature region between 2 and 300 K even at 0.75 GPa. In the case of X = NCBH₃, due to the stronger ligand field of NCBH₃, the spin transition temperature increases from 240 to 360 K with increasing applied pressure up to 0.50 GPa for [Fe(trans-stpy)₄(NCBH₃)₂]. In the case of [Fe(cis-stpy)₄(NCBH₃)₂], the spin state is the high-spin state in the temperature region between 2 and 300 K. However, the spin transition appears at 125 K under 0.5 GPa and the transition temperature increases with increasing applied pressure. In this way, we have decided the applied pressure region of 0.65-1.09 GPa where [Fe(stpy)₄(NCBH₃)₂] undergoes LD-LISC at room temperature.     

Control of charge-transfer-induced spin transition temperature on cobalt-iron Prussian blue analogues

The electronic and spin states of a series of Co-Fe Prussian blue analogues containing Na(+) ion in the lattice, Na(x)()Co(y)()Fe(CN)(6) x zH(2)O, strongly depended on the atomic composition ratio of Co to Fe (Co/Fe) and temperature. Compounds of Co/Fe = 1.5 and 1.15 consisted mostly of the Fe(III)(t(2g)(5)e(g)(0), LS, S = 1/2)-CN-Co(II)(t(2g)(5)e(g)(2), HS, S = 3/2) site and the Fe(II)(t(2g)(6)e(g)(0), LS, S = 0)-CN-Co(III)(t(2g)(6)e(g)(0), LS, S = 0) site, respectively, over the entire temperature region from 5 to 350 K. Conversely, compounds of Co/Fe = 1.37, 1.32, and 1.26 showed a change in their electronic and spin states depending on the temperature. These compounds consisted mainly of the Fe(III)-CN-Co(II) site (HT phase) around room temperature but turned to the state consisting mainly of the Fe(II)-CN-Co(III) site (LT phase) at low temperatures. This charge-transfer-induced spin transition (CTIST) phenomenon occurred reversibly with a large thermal hysteresis of about 40 K. The CTIST temperature (T(1/2) = (T(1/2) descending + T(1/2) ascending)/2) increased from 200 to 280 K with decreasing Co/Fe from 1.37 to 1.26. Furthermore, by light illumination at 5 K, the LT phase of compounds of Co/Fe = 1.37, 1.32, and 1.26 was converted to the HT phase, and the relaxation temperature from this photoproduced HT phase also strongly depended on the Co/Fe ratio; 145 K for Co/Fe = 1.37, 125 K for Co/Fe = 1.32, and 110 K for Co/Fe = 1.26. All these phenomena are explained by a simple model using potential energy curves of the LT and HT phases. The energy difference of two phases is determined by the ligand field strength around Co(II) ions, which can be controlled by Co/Fe.