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Compared with aggregate spin behavior, single‐molecule spin behavior can be accurately understood, controlled, and applied at the level of basic building blocks. The potential of single‐molecule electronic and nuclear spins for monitoring and control represents a beacon of promise for the advancement of molecular spin devices, which are fabricated by connecting a single molecule between two electrodes. Metal complexes, celebrated for their superior magnetic attributes, are widely used in the devices to explore spin effects. Moreover, single‐molecule electrical techniques with high signal‐to‐noise ratio, temporal resolution, and reliability help to understand the spin characteristics. In this review, the focus is on the devices with metal complexes, especially single‐molecule magnets, and systematically present experimental and theoretical state of the art of this field at the single‐molecule level, including the fundamental concepts of the electronic and nuclear spin and their basic spin effects. Then, several experimental methods developed to regulate the spin characteristics of metal complexes at single‐molecule level are introduced, as well as the corresponding intrinsic mechanisms. A brief discussion is provided on the comprehensive applications and the considerable challenges of single‐molecule spin devices in detail, along with a prospect on the potential future directions of this field. [ABSTRACT FROM AUTHOR]

The influence of the spin state of the metal centre in spin crossover compounds on the aromaticity of the ligands has been investigated for iron(II)tris-bipyridine (Fe(bpy)32+), and Fe(II)(formazanate)2 (as a truncated model and the full phenyl substituted compound). It was found that the aromaticity of the bipyridine ligands is unaffected by changing the spin state of the central iron atom, but that of the formazanate ligands is reduced upon transition to the high-spin state. This change in aromaticity is rationalized using the symmetry selection rules for aromaticity in terms of virtual excitations from occupied to empty orbitals. A further consequence of this loss in aromaticity is a shift to higher energy in the ring vibrations of the formazanate compounds that can be observed in either its IR or Raman spectrum; this prediction has been confirmed here. This change in aromaticity as a consequence of change in spin state can be regarded as an indication for non-innocent ligands. [ABSTRACT FROM AUTHOR]

Spin‐crossover (SCO) metal‐organic cages capable of switching between high‐spin and low‐spin states have the potential to be used as magnetic sensors and switches. Variation of the donor strength of heterocyclic aldehyde subcomponents in imine‐based ligands can tune the ligand field for a FeII center, which results in both homoleptic and heteroleptic cages with diverse SCO behaviors. The tetrahedral SCO cage built from 1‐methyl‐1H‐imidazole‐2‐carbaldehyde is capable of encapsulating various guests, which stabilize different cage spin states depending on guest size. Conversely, the SCO tetrahedron exhibits different affinities for guests in different spin states, which is inferred to result from subtle structural differences of the cavity caused by the change in metal center spin state. Examination of SCO thermodynamics across a series of host–guest complexes enabled sensitive probing of guest fit to the host cavity, providing information complementary to binding‐constant determination. [ABSTRACT FROM AUTHOR]

Two new Fe(II)-based dinuclear triple helicates having the formula {[Fe2(L)3]·(CF3SO3)4·6.5H2O·CH3OH} (complex 1) and {[Fe2(L)3]·(ClO4)4·7H2O·1.35CH3OH} (complex 2), displaying near room temperature spin transition have been synthesized and the effect of intermolecular interactions and co-operativity between metal centers on the spin crossover (SCO) has been studied. Picolinimidamide-based ligand system is chosen to provide maximum intermolecular interactions. Variable-temperature single crystal X-ray diffraction (SCXRD), magnetic study, and Hirshfeld analysis reveal that complex 1 shows a multistep spin transition, whereas, complex 2 shows an abrupt spin transition from [LS–LS] ↔ [HS–HS]. In complex 2 the presence of perchlorate anion induces high intermolecular O–H interaction that enhances the cooperativity resulting in high T1/2 of 330 K. This study accentuates the interplay between anion effect, crystal packing, and supramolecular interactions in tuning the magnetic properties of SCO compounds. [ABSTRACT FROM AUTHOR]

Control of the spin state of metal complexes is important because it leads to a precise control over the physical properties and the chemical reactivity of the metal complexes. Currently, controlling the spin state in metal complexes is challenging because a precise control of the properties of the secondary coordination sphere is often difficult. It has been shown that non-covalent interactions in the secondary coordination sphere of transition metal complexes can enable spin state control. Here we exploit this strategy for fluorinated triazole ligands and present mononuclear CoII and FeII complexes with "click"-derived tripodal ligands that contain mono-fluorinated benzyl substituents on the backbone. Structural characterization of 1 and 2 at 100 K revealed Co–N bond lengths that are typical of high spin (HS) CoII complexes. In contrast, the Fe–N bond lengths for 3 are characteristic of a low spin (LS) FeII state. All complexes show an intramolecular face-to-face non-covalent interaction between two arms of the ligand. The influence of the substituents and of their geometric structure on the spin state of the metal center was investigated through SQUID magnetometry, which revealed spin crossover occurring in compounds 1 and 3. EPR spectroscopy sheds further light on the electronic structures of 1 and 2 in their low- and high-spin states. Quantum-chemical calculations of the fluorobenzene molecule were performed to obtain insight into the influence of fluorine-specific interactions. Interestingly, this work shows that the same fluorinated tripodal ligands induce SCO behavior in both FeII and CoII complexes. [ABSTRACT FROM AUTHOR]

Complexation of Fe[ClO4]2·6H2O by 1 equiv. 2,6-bis((4S)-4-phenyl-4,5-dihydrooxazol-2-yl)pyridine ((S)-L1Ph) and 2,6-bis((4R)-4-phenyl-4,5-dihydrothiazol-2-yl)pyridine ((R)-L2Ph) cleanly affords [Fe((S)-L1Ph)((R)-L2Ph)][ClO4]2; [Fe((R)-L1iPr)((S)-L2iPr)][ClO4]2 (L1iPr = 2,6-bis(4-isopropyl-4,5-dihydrooxazol-2-yl)pyridine; L2iPr = 2,6-bis(4-isopropyl-4,5-dihydrothiazol-2-yl)pyridine) was prepared by a similar route. The compounds exhibit thermal spin-crossover in solution, at temperatures midway between the corresponding [Fe((R)-L1R)((S)-L1R)][ClO4]2 and [Fe((R)-L2R)((S)-L2R)][ClO4]2 (R = Ph or iPr) species. The spin states of [Fe(LR)(bimpy)][ClO4]2 and [Fe(LR)(bpp)][ClO4]2 (LR = L1R or L2R; bimpy = 2,6-bis(1H-benzimidazol-2-yl)pyridine; bpp = 2,6-di(pyrazol-1-yl)pyridine) are also reported, with most examples exhibiting gradual spin-crossover in solution and the solid state. Although some products undergo partial ligand exchange in solution by 1H NMR, their solution T½ values appear unaffected by this and correlate well with their spin state energies from gas phase DFT calculations. The high-spin state of [Fe(L2R)(bpp)]2+ is more stabilised than expected, compared to the other [Fe(LR)L]2+ complexes studied (L = bimpy, bpp or terpy). That is explained by an interplay between the relative σ-basicities and π-acidities of the two ligands in each molecule. The steric influence of their phenyl or isopropyl 'R' substituents stabilises the heteroleptic complexes by up to 5 kcal mol−1, compared to analogues lacking these groups. [ABSTRACT FROM AUTHOR]

The fine-tuning of intermolecular or intramolecular non-covalent interactions (NCIs) and thus the precise synthesis of metal complexes in which the spin states can be controlled by NCIs remains challenging, even though several such complexes have been intensively studied. In this regard, we present mononuclear cobalt(II) and iron(II) complexes with "click"-derived tripodal ligands that contain fluorinated benzyl substituents in the secondary coordination sphere. The complexes were co-crystallized with different solvent molecules to decipher the effect of the crystallized solvents on NCIs, and on the spin state of the metal ion. Additionally, the fluorine-specific interactions in the secondary coordination sphere were examined. We present a first structure-property correlation between the nature of interaction of the (per)fluorinated aromatic substituents on the ligand periphery, and the spin state of the metal complexes. In particular, the TF5TA containing ligand show interesting stacking motifs depending on the used solvent, and these interactions have an influence on the spin state of the cobalt(II) complexes. Furthermore, the iron(II) complex thereof, Fe(TF5TA)2(BF4)2·2EtOH displays spin crossover (SCO). [ABSTRACT FROM AUTHOR]

Understanding the relationship between the electronic state of active sites and N2 reduction reaction (NRR) performance is essential to explore efficient electrocatalysts. Herein, atomically dispersed Fe and Mo sites are designed and achieved in the form of well‐defined FeN4 and MoN4 coordination in polyphthalocyanine (PPc) organic framework to investigate the influence of the spin state of FeN4 on NRR behavior. The neighboring MoN4 can regulate the spin state of Fe center in FeN4 from high‐spin (dxy2dyz1dxz1dz21dx2−y21) to medium‐spin (dxy2dyz2dxz1dz21), where the empty d orbitals and separate d electron favor the overlap of Fe 3d with the N 2p orbitals, more effectively activating N≡N triple bond. Theoretical modeling suggests that the NRR preferably takes place on FeN4 instead of MoN4, and the transition of Fe spin state significantly lowers the energy barrier of the potential determining step, which is conducive to the first hydrogenation of N2. As a result, FeMoPPc with medium‐spin FeN4 exhibits 2.0 and 9.0 times higher Faradaic efficiency and 2.0 and 17.2 times higher NH3 yields for NRR than FePPc with high‐spin FeN4 and MoPPc with MoN4, respectively. These new insights may open up opportunities for exploiting efficient NRR electrocatalysts by atomically regulating the spin state of metal centers. [ABSTRACT FROM AUTHOR]

To study the effect of the chelate ring size on the magnetic properties of thioether‐based iron(II) metal complexes, two ligands have been envisaged, synthesised and characterised. The two ligands correspond to the bidentate benzylpicolylthioether (PySBn) and tetradentate 2,3‐bis(((2‐pyridylmethyl)thio)methyl)quinoxaline (QuinoxS). Five iron(II) complexes have been synthesised, containing either two bidentate ligands or one tetradentate ligand, and two N‐bond NCE co‐ligands (E=S, Se or BH3): trans‐[FeII(PySBn)2(NCE)2] (1 a–b) and cis‐[FeII(QuinoxS)(NCE)2] (2 a–c), a for E=S, b for E=Se and c for E=BH3. The iron(II) complexes have been characterised by standard techniques, X‐ray crystallography (except for complex 1 a) and VT‐magnetic measurements in the solid state. X‐ray crystallography showed that all the complexes are isolated in the high spin (HS) state, based on the relatively long Fe−L bond lengths, Fe−N>2.0 Å and Fe−S≈2.5–2.6 Å. VT‐magnetic measurements demonstrated that complexes 1 a and 2 a‐c are stabilised in the HS‐state, showing orbital contribution to g and zero field splitting. However, complexes 1 b shows a relatively abrupt, hysteretic, and incomplete at the low‐end spin conversion, with T1/2↓=92, T1/2↑=98 K and ΔT1/2=6 K at 5 K min−1, moreover, the hysteresis loop is scan rate dependent increasing up to 11 K at 10 K min−1. An analysis of structural and electronic parameters has been performed to rationalise the differing magnetic properties of the metal complexes, such as metallacycle size, bond lengths and angles, and cis‐ vs. trans‐coordination mode. A comparison with the literature‐reported spin crossover iron(II) complexes in N4S2 coordination sphere containing NCE co‐ligands has been conducted as well, finding that, as previously reported, the Fe−N−C(E) bond angle is diagnostic for determining the spin lability of the metal complexes, and in addition we have found that the N−C(E) bond length is too useful. [ABSTRACT FROM AUTHOR]

Recently, "host–guest chemistry" aspects of material science have received much attention, particularly in relation to moderating the functions of materials. In particular, magnetic properties involving the "magnetic ordering" and "spin crossover" of host–guest metal complex systems have been actively investigated in terms of their host–guest chemistry. That is, systems in which the magnetic properties are sensitive to perturbation by the uptake of guests (which thus act as chemical stimuli). Such guests (solvents, gases and organic molecules) very often influence the structures of both discrete metallosupramolecular assemblies as well as those of coordination polymers/metal‐organic frameworks (CPs/MOFs), providing a means for fine‐tuning their magnetic behavior, including the switching of their spin states. In this minireview, we report recent progress in the development and investigation of magnetic materials of both the CP and MOF categories as well as of discrete complexes whose properties are modulated by guests. [ABSTRACT FROM AUTHOR]

It is promising and challenging to manipulate the electronic structures and functions of materials utilizing both metal‐to‐metal charge transfer (MMCT) and spin‐crossover (SCO) to tune the valence and spin states of metal ions. Herein, a metallocyanate building block is used to link with a FeII‐triazole moiety and generates a mixed‐valence complex {[(Tp4‐Me)FeIII(CN)3]9[FeII4(trz‐ph)6]}⋅[Ph3PMe]2⋅[(Tp4‐Me)FeIII(CN)3] (1; trz‐ph=4‐phenyl‐4H‐1,2,4‐triazole). Moreover, MMCT occurs between FeIII and one of the FeII sites after heat treatment, resulting in the generation of a new phase, {[(Tp4‐Me)FeII(CN)3][(Tp4‐Me)FeIII(CN)3]8 [FeIIIFeII3(trz‐ph)6]}⋅ [Ph3PMe]2⋅[(Tp4‐Me)FeIII(CN)3] (1 a). Structural and magnetic studies reveal that MMCT can tune the two‐step SCO behavior of 1 into one‐step SCO behavior of 1 a. Our work demonstrates that the integration of MMCT and SCO can provide a new alternative for manipulating functional spin‐transition materials with accessible multi‐electronic states. [ABSTRACT FROM AUTHOR]

The effect of high pressure on the electronic characteristics of CoO is analyzed in the framework of the multielectron model of transition metal oxides. The specific features of spin crossover for d7 terms of Co2+ are compared to those characteristic of d5 and d6 configurations. A pressure-induced transition between antiferromagnetic and ferromagnetic states at the spin crossover point is predicted. A model is proposed to describe the pressure-induced variation of the electrical resistance exhibiting a stepwise change by eight orders of magnitude at the structural transition point observed at 43 GPa and the subsequent metallization above 133 GPa. [ABSTRACT FROM AUTHOR]

With the new semi‐rigid V‐shaped bidentate pyridyl amide compound 5‐methyl‐N,N′‐bis(pyridin‐4‐yl)benzene‐1,3‐dicarboxamide (L) as an auxiliary ligand and the FeII ion as the metal centre, one mononuclear complex, bis(methanol‐κO)bis[5‐methyl‐N,N′‐bis(pyridin‐4‐yl)benzene‐1,3‐dicarboxamide‐κN]bis(thiocyanato‐κN)iron(II), [Fe(SCN)2(C19H16N4O2)2(CH3OH)2] (1), and one two‐dimensional coordination polymer, catena‐poly[[[bis(thiocyanato‐κN)iron(II)]‐bis[μ‐5‐methyl‐N,N′‐bis(pyridin‐4‐yl)benzene‐1,3‐dicarboxamide‐κ2N:N′]] methanol disolvate dihydrate], {[Fe(SCN)2(C19H16N4O2)2]·2CH3OH·2H2O}n (2), were prepared by slow evaporation and H‐tube diffusion methods, respectively, indicating the effect of the method of crystallization on the structure type of the target product. Both complexes have been structurally characterized by elemental analysis, IR spectroscopy and single‐crystal X‐ray crystallography. The single‐crystal X‐ray diffraction analysis shows that L functions as a monodentate ligand in mononuclear 1, while it coordinates in a bidentate manner to two independent Fe(SCN)2 units in complex 2, with a different conformation from that in 1 and the ligands point in two almost orthogonal directions, therefore leading to a two‐dimensional grid‐like network. Investigation of the magnetic properties reveals the always high‐spin state of the FeII centre over the whole temperature range in 1 and a gradual thermally‐induced incomplete spin crossover (SCO) behaviour below 150 K in 2, demonstrating the influence of the different coordination fields on the spin properties of the metal ions. The current results provide useful information for the rational design of functional complexes with different structure dimensionalities by employing different conformations of the ligand and different crystallization methods. [ABSTRACT FROM AUTHOR]

[Display omitted] • Fe(II) metal complexes exhibit spin-crossover transition useful in molecular devices. • Surface decoupling can enhance the switching properties of individual molecule. • Charge transport is dominated by electron tunneling and bulk-like hopping mechanisms. • SCO composites through the support of conductive matrix offers more applications. • Translation of SCO properties into mechanical distortion is useful in MEMS and NEMS. Metal complexes involving d4-d7 first-row transition metals exhibit spin-crossover (SCO) behaviour between low-spin and high-spin states in response to external stimuli such as temperature, pressure, voltage control and light irradiation. They are of interest in functional devices such as sensors, molecular electronics, spintronic devices, and well as memory and information processing applications. In particular, many electronic devices exploiting the SCO phenomena contain Fe-based SCO materials, which have shown significant performance. This paper reviews the state of the art and configurations of such functional devices on a range of scales. Utilisation of SCO functionality ranging from single molecule junctions to nanoparticles and thin films incorporated into horizontal and vertical junction device architectures, will be elaborated. This review provides a systematic overview in the characteristics of Fe-based SCO functional device and their corresponding performance. This understanding provides a pathway for further optimization of Fe-based SCO technologies in a wide range of electronic applications. [ABSTRACT FROM AUTHOR]

Combined IR spectroscopic/quantum-chemical analysis of a 4-propyl-1,2,4-triazole trinuclear Fe(II) complex capable of reversible thermal spin crossover has revealed mid-IR bands of the ligand sensitive to the Fe(II) spin state. The character of the correlations found between the intensity and peak position of the triazole bands and the spin state of the metal center depends neither on the identity of the metal nor on the nuclearity of the complex. The found spectral correlations therefore allow analysis of various similar complexes. This is illustrated by the example of experimental IR spectra reported earlier for Fe(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with triazole ligands. Quantum-chemical IR spectral simulations further suggest that certain ligand bands vary between the states with the same total molecular spin, but different distribution of the spin density between the metal centers. However these variations are too subtle to discriminate between the spin transitions of the central and peripheral Fe(II) ions. The experimentally revealed mid-IR markers are therefore conclusive only for the total molecular spin. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis of the new cyclotriphosphazene (CTP) ligand substituted with a pendant 2,6-bis(benzimidazole-2-yl)pyridine (bbp), namely (pentaphenoxy)[4-{2,6-bis(benzimidazole-2-yl)pyridine-4-yl}phenoxy]cyclotriphosphazene L is reported. The single crystal structure of L shows that the bbp group is attached to the CTP via the oxygen. L reacts with FeX2 (X=ClO4 − or BF4 -) salts forming the [FeL 2]X2 complexes 1 and 2 respectively. For [FeL 2](BF4)2 (2), the single crystal structure shows an ‘N6’ coordination sphere around the iron atom. UV–Vis, resonance Raman and Mössbauer spectroscopies and magnetic susceptibility measurements, aided by density functional theory (DFT) calculations, determine the complexes are low spin below 300K but display spin crossover (SCO) behavior above this temperature, hence showing that the addition of a phosphazene to a SCO moiety does not prevent SCO. [Copyright &y& Elsevier]