FeIII Complexes Research Articles

Vanadium(V) Complexes with Siderophore Vitamin E-Hydroxylamino-Triazine Ligands

Novel vitamin E chelate siderophore derivatives and their VV and FeIII complexes have been synthesised and the chemical and biological properties have been evaluated. In particular, the α- and δ-tocopherol derivatives with bis-methyldroxylamino triazine (α-tocTHMA) and (δ-tocDPA) as well their VV complexes, [V2VO3(α-tocTHMA)2] and [V2IVO3(δ-tocTHMA)2], have been synthesised and characterised by infrared (IR), nuclear magnetic resonance (NMR), electron paramagnetic resonance (EPR) and ultra violet-visible (UV-Vis) spectroscopies. The dimeric vanadium complexes in solution are in equilibrium with their respefrctive monomers, H2O + [V2VO2(μ-O)]4+ = 2 [VVO(OH)]2+. The two amphiphilic vanadium complexes exhibit enhanced hydrolytic stability. EPR shows that the complexes in lipophilic matrix are mild radical initiators. Evaluation of their biological activity shows that the compounds do not exhibit any significant cytotoxicity to cells.

Iron-Catalyzed Highly Enantioselective Addition of Silyl Enol Ethers to α,β-Unsaturated 2-Acyl Imidazoles.

A chiral FeII(N4) complex (N4 = (R,R)-N,N'-bis(2-isopropylquinolin-8-yl)-1,2-diphenylethane-1,2-diamine) was developed for the asymmetric conjugate addition of silyl enol ethers, including both acyclic ones and cyclohexenone-derived ones, to α,β-unsaturated 2-acyl imidazoles. This FeII complex is an effective chiral Lewis acid and was applied in the synthesis of an array of chiral 1,5-dicarbonyl synthons and cyclohexenone derivatives with high yields and enantioselectivities (up to 99% ee).

Synthesis and characterization of trigonal bipyramidal FeIII complexes and their solution behavior

A series of air-stable trigonal bipyramidal FeIII complexes supported by a redox non-innocent NNN pincer ligand, CztBu(PyrR)2− (R = iPr, Me, or H), were synthesized, fully characterized, and utilized for the investigation of the interaction between acetone and the FeIII center. The magnetic moments determined from the paramagnetic 1H NMR spectra in conjunction with EPR and Mössbauer spectroscopy indicate the presence of a high-spin ferric center. Cyclic voltammetry studies feature two quasi-reversible events corresponding to a metal-centered FeIII/II reduction around −0.40 V (vs. Fc) and a ligand-centered CztBu(PyrR)2/CztBu(PyrR)2+ oxidation at potentials near +0.70 V (vs. Fc). UV–Visible spectroscopy in CH2Cl2 showcases ligand–metal charge transfer (LMCT) bands, as well as coordination of acetone to CztBu(PyrH)2FeCl2. In situ IR spectroscopy and solution conductivity (̺κ) measurements of CztBu(PyrR)2FeCl2 with varied equivalents of acetone reveal that acetone is weakly associated with the iron center.

Electronic Structure of Tetrahedral, S = 2, [Fe{(EPiPr2)2N}2], E = S, Se, Complexes: Investigation by High-Frequency and -Field Electron Paramagnetic Resonance, 57Fe Mössbauer Spectroscopy, and Quantum Chemical Studies.

In this work, we assessed the electronic structures of two pseudotetrahedral complexes of FeII, [Fe{(SPiPr2)2N}2] (1) and [Fe{(SePiPr2)2N}2] (2), using high-frequency and -field EPR (HFEPR) and field-dependent 57Fe Mössbauer spectroscopies. This investigation revealed S = 2 ground states characterized by moderate, negative zero-field splitting (zfs) parameters D. The crystal-field (CF) theory analysis of the spin Hamiltonian (sH) and hyperfine structure parameters revealed that the orbital ground states of 1 and 2 have a predominant dx2-y2 character, which is admixed with dz2 (∼10%). Although replacing the S-containing ligands of 1 by their Se-containing analogues in 2 leads to a smaller |D| value, our theoretical analysis, which relied on extensive ab initio CASSCF calculations, suggests that the ligand spin-orbit coupling (SOC) plays a marginal role in determining the magnetic anisotropy of these compounds. Instead, the dx2-y2β → dxyβ excitations yield a large negative contribution, which dominates the zfs of both 1 and 2, while the different energies of the dx2-y2β → dxzβ transitions are the predominant factor responsible for the difference in zfs between 1 and 2. The electronic structures of these compounds are contrasted with those of other [FeS4] sites, including reduced rubredoxin by considering a D2-type distortion of the [Fe(E-X)4] cores, where E = S, Se; X = C, P. Our combined CASSCF/DFT calculations indicate that while the character of the orbital ground state and the quintet excited states' contribution to the zfs of 1 and 2 are modulated by the magnitude of the D2 distortion, this structural change does not impact the contribution of the excited triplet states.

Investigation of spectroscopic, thermal, and biological properties of FeII, CoII, ZnII, and RuII complexes derived from azo dye ligand

In this study, the novel azo dye FeII, CoII, ZnII, and RuII complexes with an azo dye ligand (L: (E)-4-((4-methoxyphenyl)diazenyl)-3-methyl-1H-pyrazol-5-amine) were synthesized. The structures of the complexes were characterized by FT-IR, elemental analysis, UV-Vis, mass spectra (FeII and CoII), NMR spectra (only diamagnetic ZnII and RuII), thermogravimetric and differential thermal analysis (TGA-DTA). All of the newly synthesized compounds were tested for their biochemical properties, including enzyme inhibitory and antioxidant activities. According to the in vitro ABTS, DPPH, CUPRAC, and FRAP antioxidant methods, the ligand and its metal complexes showed close antioxidant activities to the standards (BHA, BHT, ascorbic acid, and α-tocopherol). Enzyme inhibitions of the metal complexes were investigated against glutathione S-transferase (GST), acetylcholinesterase (AChE), and butyrylcholinesterase (BChE) enzymes. The best inhibition values (Ki) were observed for Ru(II) complex against GST (14.36 ± 2.16 µM), AChE (16.86 ± 2.74 µM), and BChE (14.12 ± 2.04 µM).

Gelatin-Derived 1D Carbon Nanofiber Architecture with Simultaneous Decoration of Single Fe-Nx Sites and Fe/Fe3 C Nanoparticles for Efficient Oxygen Reduction.

Exploring high-performance non-precious-metal electrocatalysts for the oxygen reduction reaction (ORR) is critical. Herein, a scalable and cost-effective strategy is reported for the construction of one-dimensional carbon nanofiber architectures with simultaneous decoration of single Fe-Nx sites and highly dispersed Fe/Fe3 C nanoparticles for efficient ORR, through the FeIII -complex-assisted electrospinning of gelatin nanofibers with subsequent pre-oxidation and carbonization. Results show that the presence of a FeIII complex enables the 1D gelatin nanofibers to be well retained during the pre-oxidation process. Owing to the distinct 1D nanofiber structure and the synergistic effect of Fe/Fe3 C and Fe-Nx sites, the resulting electrocatalyst is highly active for ORR with a half-wave potential of 0.885 V (outperforming commercial Pt/C) and a superior electrochemical stability in alkaline electrolytes. Similarly, it also shows a high power density (144.7 mW cm-2 ) and a superior stability in Zn-air batteries. This work opens a path for the design and synthesis of 1D carbon electrocatalyst for efficient ORR catalysis.

Unveiling the High Catalytic Activity of a Dinuclear Iron Complex for the Oxygen Evolution Reaction.

The dinuclear iron complex [(H2O)-FeIII-(ppq)-O-(ppq)-FeIII-Cl]3+ (FeIII(ppq), ppq = 2-(pyrid-2'-yl)-8-(1″,10″-phenanthrolin-2″-yl)-quinoline) demonstrates a catalytic activity about one order of magnitude higher than the mononuclear iron complex [Cl-FeIII(dpa)-Cl]+ (FeIII(dpa), dpa = N,N-di(1,10-phenanthrolin-2-yl)-N-isopentylamine) for the oxygen evolution reaction (OER). However, the mechanism behind such an unusually high activity has remained largely unclear. To solve this puzzle, a decomposition-and-reaction mechanism is proposed for the OER with the dinuclear FeIII(ppq) complex as the initial state of the catalytic agent. In this mechanism, the high-valent dinuclear iron complex first dissociates into two mononuclear moieties, and the oxidized mononuclear iron complexes directly catalyze the formation of an O-O bond through a nitrate attack pathway with nitrate functioning as a cocatalyst. Density functional theory calculations reveal that it is the electron-deficient microenvironment around the iron center that gives rise to the remarkable catalytic activity observed experimentally. Therefore, the outstanding performance of the FeIII(ppq) catalyst can be ascribed to the high reactivity of its mononuclear moieties in a high oxidation state, which is concomitant with the structural stability of the low-valent dinuclear complex. The theoretical insights provided by this study could be useful for the optimization and design of novel iron-based water oxidation catalysts.

Proton-Coupled Electron-Transfer Reactivity Controls Iron versus Sulfur Oxidation in Nonheme Iron-Thiolate Complexes.

Reaction of the five-coordinate FeII(N4S) complexes, [FeII(iPr3TACN)(abtX)](OTf) (abt = aminobenzenethiolate, X = H, CF3), with a one-electron oxidant and an appropriate base leads to net H atom loss, generating new FeIII(iminobenzenethiolate) complexes that were characterized by single-crystal X-ray diffraction (XRD), as well as UV-vis, EPR, and Mössbauer spectroscopies. The spectroscopic data indicate that the iminobenzenethiolate complexes have S = 3/2 ground states. In the absence of a base, oxidation of the FeII(abt) complexes leads to disulfide formation instead of oxidation at the metal center. Bracketing studies with separated proton-coupled electron-transfer (PCET) reagents show that the FeII(aminobenzenethiolate) and FeIII(iminobenzenethiolate) forms are readily interconvertible by H+/e- transfer and provide a measure of the bond dissociation free energy (BDFE) for the coordinated N-H bond between 64 and 69 kcal mol-1. This work shows that coordination to the iron center causes a dramatic weakening of the N-H bond and that Fe- versus S-oxidation in a nonheme iron complex can be controlled by the protonation state of an ancillary amino donor.

FeIII, CuII and ZnII Complexes of the Rigid 9-Oxido-phenalenone Ligand-Spectroscopy, Electrochemistry, and Cytotoxic Properties.

The three complexes [Fe(opo)3], [Cu(opo)2], and [Zn(opo)2] containing the non-innocent anionic ligand opo− (opo− = 9-oxido-phenalenone, Hopo = 9-hydroxyphenalonone) were synthesised from the corresponding acetylacetonates. [Zn(opo)2] was characterised using 1H nuclear magnetic resonance (NMR) spectroscopy, the paramagnetic [Fe(opo)3] and [Cu(opo)2] by electron paramagnetic resonance (EPR) spectroscopy. While the EPR spectra of [Cu(opo)2] and [Cu(acac)2] in dimethylformamide (DMF) solution are very similar, a rather narrow spectrum was observed for [Fe(opo)3] in tetrahydrofuran (THF) solution in contrast to the very broad spectrum of [Fe(acac)3] in THF (Hacac = acetylacetone, 2,4-pentanedione; acac− = acetylacetonate). The narrow, completely isotropic signal of [Fe(opo)3] disagrees with a metal-centred S = 5/2 spin system that is observed in the solid state. We assume spin-delocalisation to the opo ligand in the sense of an opo− to FeIII electron transfer. All compounds show several electrochemical opo-centred reduction waves in the range of −1 to −3 V vs. the ferrocene/ferrocenium couple. However, for CuII and FeIII the very first one-electron reductions are metal-centred. Electronic absorption in the UV to vis range are due to π–π* transitions in the opo core, giving Hopo and [Zn(opo)2] a yellow to orange colour. The structured bands ranging from 400 to 500 for all compounds are assigned to the lowest energy π−π* transitions. They show markedly higher intensities and slight shifts for the CuII (brown) and FeIII (red) complexes and we assume admixing metal contributions (MLCT for CuII, LMCT for FeIII). For both complexes long-wavelength absorptions assignable to d–d transitions were detected. Detailed spectroelectrochemical experiments confirm both the electrochemical and the optical assignments. Hopo and the complexes [Cu(opo)2], [Zn(opo)2], and [Fe(opo)3] show antiproliferative activities against HT-29 (colon cancer) and MCF-7 (breast cancer) cell lines in the range of a few µM, comparable to cisplatin under the same conditions.

SYNTHESIS, CHARACTERIZATION AND ORR ACTIVITY OF METAL-N4-NANOCOMPOSITES

Inspired by nature, Metal-N4-complexes have been utilized as potential models to catalyze the sluggish oxygen reduction reaction (ORR) at cathode in fuel cells. Herein, we designed and synthesized dibenzo-N4-[14]annulene metal complexes of FeII and CoII metal ions and their nanocomposites with carbon black and characterized by various techniques. Computational and spectroscopic results suggested that the prepared complexes have saddle octahedral geometry. Further, the electrocatalytic activity of prepared nanocomposites towards ORR was investigated by using cyclic and linear sweep voltammetric techniques. The results exhibited that these nanocomposites are significantly ORR active; however, Fe-nanocomposite showed high ORR activity as compared to Co-composite. Theoretical studies indicated that the HOMO of Fe-complex was very near to the antibonding LUMO orbitals of dioxygen as compared to Co-complex, endowing high ORR activity.

Study of interaction of metal ions with methylthymol blue by chemometrics and quantum chemical calculations

In this study, we determine the acidity constants of methylthymol blue (MTB) and association constants of its complexes with the ZnII, CuII, and FeII metal ions (MIs), through theoretical and experimental means. The complexes were characterized using UV–Visible absorption spectroscopy combined with soft/hard chemometrics methods and quantum chemical calculations. Quantum chemical calculations revealed that electronic transitions in the UV–Visible spectra of MTB have mixed n → π* and π → π* characters. The results of molar ratio and multivariate curve resolution alternating least squares (MCR-ALS) revealed the formation of successive 1:2 and 1:1 complexes (MI:MTB) for the ZnII and CuII systems. However, the formation of successive 1:1 and 2:1 complexes are suggested for FeII by the molar ratio and MCR-ALS. The majority of transitions observed in the UV–Visible spectra of the Zn(MTB) and Cu(MTB) complexes have ligand-to-ligand charge transfer (LLCT) characters. However, the transitions in the UV–Visible spectrum of the Fe(MTB) complex have LLCT and metal-to-ligand charge transfer (MLCT) characters. For the Fe2(MTB) complex, the lowest energy transition of has an LLCT character. However, its higher energy transitions are a mixture of LLCT, MLCT, and metal-to-metal charge transfer (MMCT) characters. The correlation between experimental and computed wavelengths revealed that the 1:1 complexes of ZnII and CuII prefer square pyramidal geometries. However, the FeII complexes always show octahedral geometry.

Iron(II/III) Halide Complexes Promote the Interconversion of Nitric Oxide and S-Nitrosothiols through Reversible Fe-S Interaction.

Heme and non-heme iron in biology mediate the storage/release of NO• from S-nitrosothiols as a means to control the biological concentration of NO•. Despite their importance in many physiological processes, the mechanisms of N-S bond formation/cleavage at Fe centers have been controversial. Herein, we report the interconversion of NO• and S-nitrosothiols mediated by FeII/FeIII chloride complexes. The reaction of 2 equiv of S-nitrosothiol (Ph3CSNO) with [Cl6FeII2]2- results in facile release of NO• and formation of iron(III) halothiolate. Detailed spectroscopic studies, including in situ UV-vis, IR, and Mössbauer spectroscopy, support the interaction of the S atom with the FeII center. This is in contrast to the proposed mechanism of NO• release from the well-studied "red product" κ1-N bound S-nitrosothiol FeII complex, [(CN)5Fe(κ1-N-RSNO)]3-. Additionally, FeIII chloride can mediate NO• storage through the formation of S-nitrosothiols. Treatment of iron(III) halothiolate with 2 equiv of NO• regenerates Ph3CSNO with the FeII source trapped as the S = 3/2 {FeNO}7 species [Cl3FeNO]-, which is inert toward further coordination and activation of S-nitrosothiols. Our work demonstrates how labile iron can mediate the interconversion of NO•/thiolate and S-nitrosothiol, which has important implications toward how Nature manages the biological concentration of free NO•.

Investigation of iron-ammine and amido complexes within a C3-symmetrical phosphinic amido tripodal ligand.

The primary and secondary coordination spheres can have large regulatory effects on the properties of metal complexes. To examine their influences on the properties of monomeric Fe complexes, the tripodal ligand containing phosphinic amido groups, N,N',N''-[nitrilotris(ethane-2,1-diyl)]tris(P,P-diphenylphosphinic amido) ([poat]3-), was used to prepare [FeII/IIIpoat]-/0 complexes. The FeII complex was four-coordinate with 4 N-atom donors comprising the primary coordination sphere. The FeIII complex was six-coordinate with two additional ligands coming from coordination of O-atom donors on two of the phosphinic amido groups in [poat]3-. In the crystalline phase, each complex was part of a cluster containing potassium ions in which KO[double bond, length as m-dash]P interactions served to connect two metal complexes. The [FeII/IIIpoat]-/0 complexes bound an NH3 molecule to form trigonal bipyramidal structures that also formed three intramolecular hydrogen bonds between the ammine ligand and the O[double bond, length as m-dash]P units of [poat]3-. The relatively negative one-electron redox potential of -1.21 V vs. [FeIII/IICp2]+/0 is attributed to the phosphinic amido group of the [poat]3- ligand. Attempts to form the FeIII-amido complex via deprotonation were not conclusive but isolation of [FeIIIpoat(NHtol)]- using the p-toluidine anion was successful, allowing for the full characterization of this complex.

Solvent-dependent fac/mer-isomerization and self-assembly of triply helical complexes bearing a pivot part.

Tris–chelate metal complexes of unsymmetrical bidentate ligands can form two geometric stereoisomers, facial (fac) and meridional (mer) isomers. Due to the small difference in their properties, the highly-selective synthesis of one of the isomers is challenging. We now designed a series of tripodal ligands with a tris(3-(2-(methyleneoxy)ethoxy)phenyl)methane pivot. Surprisingly, the ratio of the fac/mer isomers of the triply helical FeII complexes significantly changed depending on the solvents. To the best of our knowledge, this is the first example of fac/mer isomerism of a labile tris(2,2′-bipyridine) FeII complex governed by the solvent. Furthermore, well-defined self-assemblies were quantitatively produced by imine bond formation with a suitable diamine. The supramolecular assemblies contained only the fac isomer even though a mixture of the two isomers existed in solution before the condensation reaction. Namely, the self-assembly formation effectively adjusted the geometries of the building unit that results in the suitable supramolecular structure.

Activation of C-H bonds by a nonheme iron(IV)-oxo complex: mechanistic evidence through a coupled EDXAS/UV-Vis multivariate analysis.

The understanding of reactive processes involving organic substrates is crucial to chemical knowledge and requires multidisciplinary efforts for its advancement. Herein, we apply a combined multivariate, statistical and theoretical analysis of coupled time-resolved X-ray absorption (XAS)/UV-Vis data to obtain detailed mechanistic information for on the C-H bond activation of 9,10-dihydroanthracene (DHA) and diphenylmethane (Ph2CH2) by the nonheme FeIV-oxo complex [N4Py·FeIV(O)]2+ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine) in CH3CN at room temperature. Within this approach, we determine the number of key chemical species present in the reaction mixtures and derive spectral and concentration profiles for the reaction intermediates. From the quantitative analysis of the XAS spectra the transient intermediate species are structurally determined. As a result, it is suggested that, while DHA is oxidized by [N4Py·FeIV(O)]2+ with a hydrogen atom transfer-electron transfer (HAT-ET) mechanism, Ph2CH2 is oxidized by the nonheme iron-oxo complex through a HAT-radical dissociation pathway. In the latter process, we prove that the intermediate FeIII complex [N4Py·FeIII(OH)]2+ is not able to oxidize the diphenylmethyl radical and we provide its structural characterization in solution. The employed combined experimental and theoretical strategy is promising for the spectroscopic characterization of transient intermediates as well as for the mechanistic investigation of redox chemical transformations on the second to millisecond time scales.

Second-sphere effects on H2O2 activation by non-heme FeII complexes: role of a phenol group in the [H2O2]-dependent accumulation of FeIVO vs. FeIIIOOH.

Redox metalloenzymes achieve very selective oxidation reactions under mild conditions using O2 or H2O2 as oxidants and release harmless side-products like water. Their oxidation selectivity is intrinsically linked to the control of the oxidizing species generated during the catalytic cycle. To do so, a second coordination sphere is used in order to create a pull effect during the activation of O2 or H2O2, thus ensuring a heterolytic O–O bond cleavage. Herein, we report the synthesis and study of a new non-heme FeII complex bearing a pentaazadentate first coordination sphere and a pendant phenol group. Its reaction with H2O2 generates the classical FeIIIOOH species at high H2O2 loading. But at low H2O2 concentrations, an FeIVO species is generated instead. The formation of the latter is directly related to the presence of the 2nd sphere phenol group. Kinetic, variable temperature and labelling studies support the involvement of the attached phenol as a second coordination sphere moiety (weak acid) during H2O2 activation. Our results suggest a direct FeII → FeIVO conversion directed by the 2nd sphere phenol via the protonation of the distal O atom of the FeII/H2O2 adduct leading to a heterolytic O–O bond cleavage.

Direct Synthesis of Spin‐Crossover Complexes: An Unexpectedly Revealed New Iron‐Triazolic Structure

Spin‐crossover complexes form one of the biggest families of switchable compounds. Here we show a new way to obtain spin‐crossover materials by direct synthesis from metallic iron. Four complexes of FeII with 4‐R‐1,2,4‐triazoles have been synthesized by a direct metal oxidation: [Fe(NH2trz)3]SO4, [Fe(NH2trz)3](BF4)2 and two polymorphs of [Fe(Htrz)2(trz)]BF4. Surprisingly high quality of PXRD pattern of [Fe(NH2trz)3]SO4 allowed to perform Rietveld refinement and obtain reliable crystal structure of the latter. All compounds have been studied by magnetic susceptibility measurements, optical reflectivity, differential scanning calorimetry and Raman spectroscopy. Importantly, spin‐crossover characteristics of the complexes obtained by this unusual method are preserved, thus revealing a new effective approach of “direct synthesis” towards switchable coordination compounds.

Role of Coordination Number and Geometry in Controlling the Magnetic Anisotropy in FeII, CoII, and NiII Single‐Ion Magnets

Since the last decade, the focus in the area of single-molecule magnets (SMMs) has been shifting constructively towards the development of single-ion magnets (SIMs) based on transition metals and lanthanides. Although ground-breaking results have been witnessed for DyIII -based SIMs, significant results have also been obtained for some mononuclear transition metal SIMs. Among others, studies based on CoII ion are very prominent as they often exhibit high magnetic anisotropy or zero-field splitting parameters and offer a large barrier height for magnetisation reversal. Although CoII possibly holds the record for having the largest number of zero-field SIMs known for any transition metal ion, controlling the magnetic anisotropy in these systems are is still a challenge. In addition to the modern spectroscopic techniques, theoretical studies, especially ab initio CASSCF/NEVPT2 approaches, have been used to uncover the electronic structure of various CoII SIMs. In this article, with some selected examples, the aim is to showcase how varying the coordination number from two to eight, and the geometry around the CoII centre alters the magnetic anisotropy. This offers some design principles for the experimentalists to target new generation SIMs based on the CoII ion. Additionally, some important FeII /FeIII and NiII complexes exhibiting large magnetic anisotropy and SIM properties are also discussed.

FeIII and FeII Phosphasalen Complexes: Synthesis, Characterization, and Catalytic Application for 2-Naphthol Oxidative Coupling.

We report on the synthesis and characterization of three iron(III) phosphasalen complexes, [FeIII (Psalen)(X)] differing in the nature of the counter-anion/exogenous ligand (X- =Cl- , NO3 - , OTf- ), as well as the neutral iron(II) analogue, [FeII (Psalen)]. Phosphasalen (Psalen) differs from salen by the presence of iminophosphorane (P=N) functions in place of the imines. All the complexes were characterized by single-crystal X-ray diffraction, UV/Vis, EPR, and cyclic voltammetry. The [FeII (Psalen)] complex was shown to remain tetracoordinated even in coordinating solvent but surprisingly exhibits a magnetic moment in line with a FeII high-spin ground state. For the FeIII complexes, the higher lability of triflate anion compared to nitrate was demonstrated. As they exhibit lower reduction potentials compared to their salen analogues, these complexes were tested for the coupling of 2-naphthol using O2 from air as oxidant. In order to shed light on this reaction, the interaction between 2-naphthol and the FeIII (Psalen) complexes was studied by cyclic voltammetry as well as UV/Vis spectroscopy.

Thermochromic Meltable Materials with Reverse Spin Transition Controlled by Chemical Design

AbstractWe report a series of meltable FeII complexes, which, depending on the length of aliphatic chains, display abrupt forward low‐spin to high‐spin transition or unprecedented melting‐triggered reverse high‐spin to low‐spin transition on temperature rise. The reverse spin transition is perfectly reproducible on thermal cycling and the obtained materials are easily processable in the form of thin film owing to their soft‐matter nature. We found that the discovered approach represents a potentially generalizable new avenue to control both the location in temperature and the direction of the spin transition in meltable compounds.