Guest‐Switchable Multi‐Step Spin Transitions in an Amine‐Functionalized Metal–Organic Framework

Materials with hysteretic multi‐step spin‐crossover (SCO) have potential application in high‐order data storage. Here, an unprecedented hysteretic four‐step SCO behavior with the sequence of LS↔HS_0.25LS_0.75↔HS_0.5LS_0.5↔ HS_0.75LS_0.25↔HS is found in a three‐dimensional (3D) Hofmann‐type metal–organic framework (MOF), which is evidenced by magnetic, differential scanning calorimetry, and crystal data. Further experiments involving guest exchange leads to the first reversible modulation of four‐, two‐, and one‐stepped SCO behaviors, which provides a new strategy for developing multi‐step SCO materials.     

Deposition of the Spin Crossover FeII–Pyrazolylborate Complex on Au(111) Surface at the Molecular Level

The interaction at the molecular level of the spin-crossover (SCO) Fe^II((3,5-(CH₃)₂Pz)₃BH)₂ complex with the Au(111) surface is analyzed by means of rPBE periodic calculations. Our results show that the adsorption on the metallic surface enhances the transition energy, increasing the relative stability of the low spin (LS) state. The interaction indeed is spin-dependent, stronger for the low spin than the high spin (HS) state. The different strength of the Fe ligand field at low and high temperature manifests on the nature, spatial extension and relative energy of the states close to the Fermi level, with a larger metal–ligand hybridization in the LS state. This feature is of relevance for the differential adsorption of the LS and HS molecules, the spin-dependent conductance, and for the differences found in the corresponding STM images, correctly reproduced from the density of states provided by the rPBE calculations. It is expected that this spin dependence will be a general feature of the SCO molecule–substrate interaction, since it is rooted in the different ligand field of Fe site at low and high temperatures, a common hallmark of the Fe^II SCO complexes. Finally, the states involved in the LIESST phenomenon has been identified through NEVPT2 calculations on a model reaction path. A tentative pathway for the photoinduced LS→HS transition is proposed, that does not involve the intermediate triplet states, and nicely reproduces both the blue laser wavelength required for the activation, and the wavelength of the reverse HS → LS transition.     

Hysteretic Four‐Step Spin Crossover within a Three‐Dimensional Porous Hofmann‐like Material

Materials that display multiple stepped spin crossover (SCO) transitions with accompanying hysteresis present the opportunity for ternary, quaternary, and quinary electronic switching and data storage but are rare in existence. Herein, we present the first report of a four‐step hysteretic SCO framework. Single‐crystal structure analysis of a porous 3D Hofmann‐like material showed long‐range ordering of spin states: HS, HS_0.67LS_0.33, HS_0.5LS_0.5, HS_0.33LS_0.67, and LS. These detailed structural studies provide insight into how multistep SCO materials can be rationally designed through control of host–host and host–guest interactions.     

The First Observation of Hidden Hysteresis in an Iron(III) Spin‐Crossover Complex

Molecular magnetic switches are expected to form the functional components of future nanodevices. Herein we combine detailed (photo‐) crystallography and magnetic studies to reveal the unusual switching properties of an iron(III) complex, between low (LS) and high (HS) spin states. On cooling, it exhibits a partial thermal conversion associated with a reconstructive phase transition from a [HS‐HS] to a [LS‐HS] phase with a hysteresis of 25 K. Photoexcitation at low temperature allows access to a [LS‐LS] phase, never observed at thermal equilibrium. As well as reporting the first iron(III) spin crossover complex to exhibit reverse‐LIESST (light‐induced excited spin state trapping), we also reveal a hidden hysteresis of 30 K between the hidden [LS‐LS] and [HS‐LS] phases. Moreover, we demonstrate that Fe^III spin‐crossover (SCO) complexes can be just as effective as Fe^II systems, and with the advantage of being air‐stable, they are ideally suited for use in molecular electronics.     

Direct Observation of Ordered High‐Spin–Low‐Spin Intermediate States of an Iron(III) Three‐Step Spin‐Crossover Complex

A neutral mononuclear Fe^III complex [Fe^III(H‐5‐Br‐thsa‐Me)(5‐Br‐thsa‐Me)]?H₂O ( 1 ; H₂‐5‐Br‐thsa‐Me=5‐bromosalicylaldehyde methylthiosemicarbazone) was prepared that exhibited a three‐step spin‐crossover (SCO) with symmetry breaking and a 14 K hysteresis loop owing to strong cooperativity. Two ordered intermediate states of 1 were observed, 4HS–2LS and 2HS–4LS, which exhibited reentrant phase‐transition behavior. This study provides a new platform for examining multistability in SCO complexes.     

Bidirectional Chemo‐Switching of Spin State in a Microporous Framework

The ins and outs of spin : Using the microporous coordination polymer {Fe(pz)[Pt(CN)₄]} ( 1 , pz=pyrazine), incorporating spin‐crossover subunits, two‐directional magnetic chemo‐switching is achieved at room temperature. In situ magnetic measurements following guest vapor injection show that most guest molecules transform 1 from the low‐spin (LS) state to the high‐spin (HS) state, whereas CS₂ uniquely causes the reverse HS‐to‐LS transition.

     

Anomalous Light‐Induced Spin‐State Switching for Iron(II) Spin‐Crossover Molecules in Direct Contact with Metal Surfaces

Light‐induced spin‐state switching is one of the most attractive properties of spin‐crossover materials. In bulk, low‐spin (LS) to high‐spin (HS) conversion via the light‐induced excited spin‐state trapping (LIESST) effect may be achieved with a visible light, while the HS‐to‐LS one (reverse‐LIESST) requires an excitation in the near‐infrared range. Now, it is shown that those phenomena are strongly modified at the interface with a metal. Indeed, an anomalous spin conversion is presented from HS state to LS state under blue light illumination for Fe^II spin‐crossover molecules that are in direct contact with metallic (111) single‐crystal surfaces (copper, silver, and gold). To interpret this anomalous spin‐state switching, a new mechanism is proposed for the spin conversion based on the light absorption by the substrate that can generate low energy valence photoelectrons promoting molecular vibrational excitations and subsequent spin‐state switching at the molecule–metal interface.     

Coupled crystallographic order-disorder and spin state in a bistable molecule: multiple transition dynamics

A novel bispyrazolylpyridine ligand incorporating lateral phenol groups, H₄L, has led to an Fe^II spin‐crossover (SCO) complex, [Fe(H₄L)₂][ClO₄]₂ ? H₂O ? 2 (CH₃)₂CO ( 1 ), with an intricate network of intermolecular interactions. It exhibits a 40 K wide hysteresis of magnetization as a result of the spin transition (with T_0.5 of 133 and 173 K) and features an unsymmetrical and very rich structure. The latter is a consequence of the coupling between the SCO and the crystallographic transformations. The high‐spin state may also be thermally trapped, exhibiting a very large T_TIESST (≈104 K). The structure of 1 has been determined at various temperatures after submitting the crystal to different processes to recreate the key points of the hysteresis cycle and thermal trapping; 200 K, cooled to 150 K and trapped at 100 K (high spin, HS), slowly cooled to 100 K and warmed to 150 K (low spin, LS). In the HS state, the system always exhibits disorder for some components (one ClO₄^? and two acetone molecules) whereas the LS phases show a relative ≈9 % reduction in the Fe? N bond lengths and anisotropic contraction of the unit cell. Most importantly, in the LS state all the species are always found to be ordered. Therefore, the bistability of crystallographic order–disorder coupled to SCO is demonstrated here experimentally for the first time. The variation in the cell parameters in 1 also exhibits hysteresis. The structural and magnetic thermal variations in this compound are paralleled by changes in the heat capacity as measured by differential scanning calorimetry. Attempts to simulate the asymmetric SCO behaviour of 1 by using an Ising‐like model underscore the paramount role of dynamics in the coupling between the SCO and the crystallographic transitions.     

Spin‐Übergänge in supramolekularen Strukturen. Für Speicherbausteine von morgen?

We reported about octahedrally coordinated Fe2+‐complexes, which are able to switch between two stable spin states (LS and HS) with different magnetic properties. This phenomenon is called spin crossover (SCO). The interaction between metal ion and ligand determines the actual spin state and whether an extern stimulus can trigger a spin crossover. Due to this fact it is possible for the chemist through the choice of the ligands to manipulate the character and the temperature region of the SCO. Some metal complexes assemble into highly ordered structures on graphite by molecular self assembly. The substitution of the metal complexes with alkyl chains and the interaction of these chains with the highly ordered graphite is crucial for a periodic arrangement of the complexes on the surface. For the future we are curious to see whether through the cooperative effort of coordination chemistry (SCO phenomenon) and surface science (self assembly of SCO complexes on a surface) the vision of a molecular memory will turn into a reality.     

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.     

Enantioselective Guest Effect on the Spin State of a Chiral Coordination Framework

The diversity of spin crossover (SCO) complexes that, on the one hand, display variable temperature, abruptness and hysteresis of the spin transition, and on the other hand, are spin‐sensitive to the various guest molecules, makes these materials unique for the detection of different organic and inorganic compounds. We have developed a homochiral SCO coordination polymer with a spin transition sensitive to the inclusion of the guest 2‐butanol, and these solvates with (R)‐ and (S)‐alcohols demonstrate different SCO behaviours depending on the chirality of the organic analyte. A stereoselective response to the guest inclusion is detected as a shift in the temperature of the transition both from dia‐ to para‐ and from para‐ to diamagnetic states in heating and cooling modes respectively. Furthermore, the Mössbauer spectroscopy directly visualizes how the metallic centres in a chiral coordination framework differently sense the interaction with guests of different chiralities.     

Synthesis, magnetic and photomagnetic study of new iron(II) spin-crossover complexes with N₄O₂ coordination sphere

A new family of neutral mononuclear iron(II) spin crossover (SCO) compounds, Fe(L1??)? (L1?? = N'-((pyridin-2-yl)methylene)benzohydrazide (HL1), N'-(1-(pyridin-2-yl)ethylidene)-benzohydrazide (HL2), N'-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL3), 2-hydroxy-N'-((pyridin-2-yl)methylene)benzohydrazide (HL?), 2-hydroxy-N'-(1-(pyridin-2-yl)ethylidene)benzohydrazide (HL?), 2-hydroxy-N'-(phenyl(pyridin-2-yl)methylene)benzohydrazide (HL?)) with N?O? donor sets have been synthesized from series tridentate Schiff base ligands with N,N,O donor sets. The investigation of magnetic properties of these compounds reveal that in the measured temperature range, compound 1 is in the high-spin (HS) state, and compound 3 and 6 are mainly in the low-spin (LS) state, whereas the other compounds exhibit various SCO properties: compound 2 undergoes a gradual incomplete SCO with characteristic temperature T(1/2) higher than 350 K; compound 4 exhibits a special stepwise thermally induced SCO occurring at ~150 K (smooth) and 200 K (two-steps, with T(S1↑/↓) = 204/202 K and T(S2↑/↓) = 227/219 K) with a mixture of the HS and LS states yielded below 100 K; compound 5 shows a gradual and complete LS?HS SCO with characteristic temperature T(1/2) = 273 K. All the three SCO compounds show the LIESST (light induced exited spin state trapping) effect with different levels of photoconversion. To thoroughly analyze these behaviours, M?ssbauer spectra and DSC of 4 and 5, crystal structures of all the compounds at 290 K and 5 in the LS state at 110 K were carried out, which confirmed the structural changes accompanying the spin transition. In addition, alkyl substitution effect on the ligand field was suggested for this system.     

Cobalt complexes with "Click"-derived functional tripodal ligands: spin crossover and coordination ambivalence

We demonstrate the use of a Cu(I) catalyzed "Click" reaction in the synthesis of novel ligands for spin crossover complexes. The reaction between azides and alkynes was used to synthesize the reported tripodal ligand tris[(1-benzyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBTA, and the new ligands tris[(1-cyclohexyl-1H-1,2,3-triazol-4-yl)methyl]amine, TCTA, and tris[(1-n-butyl-1H-1,2,3-triazol-4-yl)methyl]amine, TBuTA. Reactions of TBTA with Co(ClO(4))(2) lead to complexes of the form [Co(TBTA)(CH(3)CN)(3)](ClO(4))(2), 1, and [Co(TBTA)(2)](ClO(4))(2), 2, where complex formation can be controlled by the metal/ligand ratio and the complexes 1 and 2 can be chemically and reversibly switched from one form to another in solution resulting in coordination ambivalence. The benzyl substituents of TBTA in 2 show intramolecular C-H-π T-stacking that generates a chemical pressure to stabilize the low spin (LS) state at lower temperatures. The structural parameters of 2 are consistent with a Jahn-Teller active LS Co(II) (elongation) ion showing four short and two long bonds. 2 shows spin-crossover (SCO) behavior in the solid state and in solution with a high T(0) close to room temperature which is driven by the T-stacking. 1 remains high spin (HS) between 2 and 400 K. Reversible chemical switching is observed between 1 and 2 at room temperature, with an accompanying change in the spin state from HS to LS. The importance of the intramolecular T-stacking in driving the SCO behavior is proven by comparison with two analogous compounds that lack an aromatic substituent and remain HS down to very low temperatures.     

Understanding the two-step spin-transition phenomenon in Iron(II) 1D chain materials

Three analogous one dimensional (1D) polymeric iron(II) spin crossover (SCO) materials containing the new ligand 4,6-bis(2',2'-pyridyl)pyrazine (bdpp) have been comprehensively characterised magnetically (thermal and light-induced) and structurally. Within this series are two polymorphs of the formula [Fe(NCS)(2)(bdpp)], 1 and 2 a, which differ magnetically in that phase 1 undergoes a full two-step SCO (T(1/2(1))=135 K and T(1/2(2))=90 K) whereas phase 2 a remains high spin (HS) over all temperatures. The central distinction between these two materials lies in the presence of intermolecular pi-pi interactions generated by the crystal packing in 1, which are absent in 2 a. The isostructural selenocyanate analogue of 2 a, [Fe(NCSe)(2)(bdpp)], 2 b, undergoes a full two-step SCO (T(1/2(1))=200 K and T(1/2(2))=125 K). Structural analyses of 1 and 2 b at a range of temperatures provide deep insight into their two-step SCO nature. Structural analysis of 1 at 25 K (1(LS-LS)), 123 K (1(LS-HS)) and 250 K (1(HS-HS)) reveals two distinct iron(II) centres at each temperature, with ordered, alternating HS and LS (low spin) sites at the intermediate plateau (IP) temperatures. In contrast, structural analysis of 2 b at 90 K (2 b(LS)), 150 K (2 b(LS/HS)) and 250 K (2 b(HS)) reveals one unique iron(II) centre at each temperature with an "averaged" LS/HS character at the IP temperature. Weak planes of diffuse scattering in the single-crystal X-ray diffraction patterns were observed for this phase at 90 and 150 K, indicating that 1D long range ordering of alternating HS/LS iron(II) centres occurs along the 1D coordination chains, but that there is no correlation between chains. The lack of observable diffuse scattering at 250 K suggests that the onset of the 1D structural ordering in the chain direction corresponds to the first step of the SCO and that this structural transition is electronically driven. The photomagnetic properties of both 1 and 2 b have been investigated and show approximately 62 and 53 % photo-excitation of a HS metastable state at low temperatures and T(LIESST) values of 55 and 49 K, respectively. Relaxation studies on the HS fraction in 2 b fitted well to a stretched exponential model with kinetic parameters indicative of weak cooperativity.     

Nanoporosity,Inclusion Chemistry,and Spin Crossover in Orthogonally Interlocked Two‐Dimensional Metal–Organic Frameworks

[Fe(tvp)₂(NCS)₂] ( 1 ) (tvp=trans‐(4,4′‐vinylenedipyridine)) consists of two independent perpendicular stacks of mutually interpenetrated two‐dimensional grids. This uncommon supramolecular conformation defines square‐sectional nanochannels (diagonal≈2.2 nm) in which inclusion molecules are located. The guest‐loaded framework 1@guest displays complete thermal spin‐crossover (SCO) behavior with the characteristic temperature T_1/2 dependent on the guest molecule, whereas the guest‐free species 1 is paramagnetic whatever the temperature. For the benzene–guest derivatives, the characteristic SCO temperature T_1/2 decreases as the Hammet σ_p parameter increases. In general, the 1@guest series shows large entropy variations associated with the SCO and conformational changes of the interpenetrated grids that leads to a crystallographic‐phase transition when the guest is benzonitrile or acetonitrile/H₂O.     

Coherent Spin Transport Through a Six-Coordinate FeN6 Spin-Crossover Complex with Two Di erent Spin Con gurations

Due to the magnetic bistability, single-molecule spin-crossover (SCO) complexes have been considered to be the most promising building blocks for molecular spintronic devices. Here, we explore the SCO behavior and coherent spin transport properties of a six-coordinate FeN₆ complex with the low-spin (LS) and high-spin (HS) states by performing extensive first-principles calculations combined with non-equilibrium Green’s function technique. Theoretical results show that the LS?HS spin transition via changing the metal-ligand bond lengths can be realized by external stimuli, such as under light radiation in experiments. According to the calculated zero-bias transmission coefficients and density of states as well as the I-V curves under small bias voltages of FeN₆ SCO complex with the LS and HS states sandwiched between two Au electrodes, we find that the examined molecular junction can act as a molecular switch, tuning from the OFF (LS) state to the ON (HS) state. Moreover, the spin-down electrons govern the current of the HS molecular junction, and this observed perfect spin-filtering effect is not sensitive to the detailed anchoring structure. These theoretical findings highlight this examined six-coordinate FeN₆ SCO complex for potential applications in molecular spintronics.     

Structural Investigation of the High Spin→Low Spin Relaxation Dynamics of the Porous Coordination Network [Fe(pz)Pt(CN)4]⋅2.6 H2O

The Hoffman‐type coordination compound [Fe(pz)Pt(CN)₄] ? 2.6 H₂O (pz=pyrazine) shows a cooperative thermal spin transition at around 270 K. Synchrotron powder X‐Ray diffraction studies reveal that a quantitative photoinduced conversion from the low‐spin (LS) state into the high‐spin (HS) state, based on the light‐induced excited spin‐state trapping effect, can be achieved at 10 K in a microcrystalline powder. Time‐resolved measurements evidence that the HS→LS relaxation proceeds by a two‐step mechanism: a random HS→LS conversion at the beginning of the relaxation is followed by a nucleation and growth process, which proceeds until a quantitative HS→LS transformation has been reached.     

Partial spin crossover behaviour in a dinuclear iron(II) triple helicate

Reported herein are the synthesis, structural, magnetic and M?ssbauer spectroscopic characterisation of a dinuclear Fe(II) triple helicate complex [Fe(2)(L)(3)](ClO(4))(4).xH(2)O (x = 1-4), 1(H(2)O), where L is a bis-bidentate imidazolimine ligand. Low temperature structural analysis (150 K) and M?ssbauer spectroscopy (4.5 K) are consistent with one of the Fe(II) centres within the helicate being in the low spin (LS) state with the other being in the high-spin (HS) state resulting in a [LS:HS] species. However, M?ssbauer spectroscopy (295 K) and variable temperature magnetic susceptibility measurements (4.5-300 K) reveal that 1(H(2)O) undergoes a reversible single step spin crossover at one Fe(II) centre at higher temperatures resulting in a [HS:HS] species. Indeed, the T(1/2)(SCO) values at this Fe(II) centre also vary as the degree of hydration, x, within 1(H(2)O) changes from 1 to 4 and are centred between ca. 210 K-265 K, respectively. The dehydration/hydration cycle is reversible and the fully hydrated phase of 1(H(2)O) may be recovered on exposure to water vapour. This magnetic behaviour is in contrast to that observed in the related compound [Fe(2)(L)(3)](ClO(4))(4)·2MeCN, 1(MeCN), whereby fully reversible SCO was observed at each Fe(II) centre to give [LS:LS] species at low temperature and [HS:HS] species at higher temperatures. Reasons for this differing behaviour between 1(H(2)O) and 1(MeCN) are discussed.     

First Step Towards a Devil's Staircase in Spin‐Crossover Materials

The unprecedented bimetallic 2D coordination polymer {Fe[(Hg(SCN)₃)₂](4,4′‐bipy)₂}_n exhibits a thermal high‐spin (HS)?low‐spin (LS) staircase‐like conversion characterized by a multi‐step dependence of the HS molar fraction γ_HS. Between the fully HS (γ_HS=1) and LS (γ_HS=0) phases, two steps associated with different ordering appear in terms of spin‐state concentration waves (SSCW). On the γ_HS≈0.5 step, a periodic SSCW forms with a HS‐LS‐HS‐LS sequence. On the γ_HS≈0.34 step, the 4D superspace crystallography structural refinement reveals an aperiodic SSCW, with a HS‐LS sequence incommensurate with the molecular lattice. The formation of these different long‐range spatially ordered structures of LS and HS states during the multi‐step spin‐crossover is discussed within the framework of “Devil's staircase”‐type transitions. Spatially modulated phases are known in various types of materials but are uniquely related to molecular HS/LS bistability in this case.     

Guest Modulation of Spin‐Crossover Transition Temperature in a Porous Iron(II) Metal–Organic Framework: Experimental and Periodic DFT Studies

The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin‐crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)₄]} ( 1 ) with five‐membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin‐crossover transition with hysteresis loops 14–29 K wide and average critical temperatures T_c=201 K ( 1?fur ), 167 K ( 1?pyr ), and 114.6 K ( 1?thio ) well below that of the parent compound 1 (T_c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1?fur , while 1?pyr and 1?thio show 50 % spin transition. For 1?fur the transformation between the HS and IS (middle of the plateau) phases occurs concomitantly with a crystallographic phase transition between the tetragonal space groups P4/mmm and I4/mmm, respectively. The latter space group is retained in the subsequent transformation involving the IS and the LS phases. 1?pyr and 1?thio display the tetragonal P4/mmm and orthorhombic Fmmm space groups, respectively, in both HS and IM phases. Periodic calculations using density functional methods for 1?fur , 1?pyr , 1?thio , and previously reported derivatives 1?CS₂ , 1?I, 1?bz (benzene), and 1?pz (pyrazine) have been carried out to investigate the electronic structure and nature of the host–guest interactions as well as their relationship with the changes in the LS–HS transition temperatures of 1?Guest . Geometry‐optimized lattice parameters and bond distances in the empty host 1 and 1?Guest clathrates are in general agreement with the X‐ray diffraction data. The concordance between the theoretical results and the experimental data also comprises the guest molecule orientation inside the host and intermolecular distances. Furthermore, a general correlation between experimental T_c and calculated LS–HS electronic energy gap was observed. Finally, specific host–guest interactions were studied through interaction energy calculations and crystal orbital displacement (COD) curve analysis.