Guest Tunable Structure and Spin Crossover Properties in a Nanoporous Coordination Framework Material

Phase Transition Control in Molecular Solids via Complementarity of Hydrogen-Bond Strength

Phase transitions in molecular solids involve synergistic changes in chemical and electronic structures, leading to diversification in physical and chemical properties. Despite the pivotal role of hydrogen bonds (H-bonds) in many phase-transition materials, it is rare and challenging to chemically regulate the dynamics and to elucidate the structure-property relationship. Here, four high-spin CoII compounds were isolated and systematically investigated by modifying the ligand terminal groups (X=S, Se) and substituents (Y=Cl, Br). S-Cl and Se-Br undergo a reversible structural phase transition near room temperature, triggering the rotation of 15-crown-5 guests and the swing between syn- and anti-conformation of NCX- ligands, accompanied by switchable magnetism. Conversely, S-Br and Se-Cl retain stability in ordered and disordered phases, respectively. H-bonds geometric analysis and ab initio calculations reveal that the electronegativity of X and Y affects the strength of NY-ap-H⋅⋅⋅X interactions. Entropy-driven structural phase transitions occur when the H-bond strength is appropriate; otherwise, the phase stays unchanged if it is too strong or weak. This work highlights a phase transition driven by H-bond strength complementarity - pairing strong acceptor with weak donor and vice versa, which offers a straightforward and effective approach for designing phase-transition molecular solids from a chemical perspective.

Guest-Induced Reversible Single-Crystal-to-Single-Crystal Transformation Involving Displacement of 2D Layers and Spin Crossover Behavior Change in a Hofmann-Type Coordination Polymer

A novel two-dimensional (2D) Hofmann-type coordination polymer, {FeII(PyHbim)2[Pd(CN)4]}·2CH3OH [1·2CH3OH, PyHbim = 2-(4-pyridyl)benzimidazole], has been synthesized, which can undergo a spontaneous guest exchange, transforming to 1·2H2O in a single-crystal-to-single-crystal (SCSC) manner, shifting from orthorhombic Cmmm to monoclinic C2/m involving the displacement of 2D layers. The solvent-induced SCSC transformation process was reversible and verified through powder X-ray diffraction (PXRD) and single-crystal X-ray crystallography analyses. Both 1·2CH3OH and 1·2H2O exhibit complete and abrupt spin crossover (SCO) behaviors in two steps, while their SCO temperature ranges drastically shift by ca.100 K, spanning room temperature, owing to different intermolecular interactions resulting from diverse interlayer packing manners and host-guest interactions. Besides, a structural phase transition is observed in 1·2CH3OH, contributing to the two-step spin transition.

Hofmann-Type Cyanide Bridged Coordination Polymers for Advanced Functional Nanomaterials

Since the discovery of Hofmann clathrates of inorganic cyanide bridged coordination polymers (Hofmann-type CN-CPs), extensive research is done to understand their behavior during spin transitions caused by guest molecules or external stimuli. Lately, research on their nanoscale architectures for sensors and switching devices is of interest. Their potential is reported for producing advanced functional inorganic materials in two-dimensional (2D) morphology using a scalable solid-state thermal treatment method. For instance, but not restricted to, alloys, carbides, chalcogenides, oxides, etc. Simultaneously, their in situ crystallization at graphene oxide (GO) nanosheet surfaces, followed by a subsequent self-assembly to build layered lamellar structures, is reported providing hybrid materials with a variety of uses. Hence, an overview of the most recent developments is presented here in the synthesis of nanoscale structures, including thin films and powders, using Hofmann-type CN-CPs. Also thoroughly demonstrated are the most recent synthetic ideas with the modest control over the size and shape of nanoscale particles. Additionally, in order to create new functional hybrid materials for electrical and energy applications, their thermal decomposition in various environments and hybridization with GO and other guest molecules is examined. This review article also conveyed their spin transition, astounding innovative versatile adhesives, and structure features.

Electronic Structure and d-d Spectrum of Metal-Organic Frameworks with Transition-Metal Ions

The electronic structure of metal-organic frameworks (MOFs) containing transition metal (TM) ions represents a significant and largely unresolved computational challenge due to limited solutions to the quantitative description of low-energy excitations in open d-shells. These excitations underpin the magnetic and sensing properties of TM MOFs, including the observed remarkable spin-crossover phenomenon. We introduce the effective Hamiltonian of crystal field approach to study the d-d spectrum of MOFs containing TM ions; this is a hybrid QM/QM method based on the separation of crystal structure into d- and s,p-subsystems treated at different levels of theory. We test the method on model frameworks, carbodiimides, and hydrocyanamides and a series of M-MOF-74 (M = Fe, Co, Ni) and compare the computational predictions to experimental data on magnetic properties and Mössbauer spectra.

Highly Porous Cyanometallic Spin-Crossover Frameworks Employing Pyridazino[4,5-d]pyridazine Bridge

Single crystals of two spin-crossover (SCO) cyanometallic coordination polymers based on the pyridazino[4,5-d]pyridazine ligand (pp) of the composition [Fe(pp)M(CN)4]∙G (where M = Pd, Pt; G = guest molecules) were obtained by a slow diffusion technique. A single-crystal X-ray analysis showed that both compounds adopted the structure of porous 3D frameworks, consisting of heterometallic cyano-bridged layers and interlayer pillar pp ligands, with a total solvent accessible volume of ca. 160 Å3 per iron(II) ion (about 37% of the unit cell volume). These frameworks displayed hysteretic SCO behaviour with T1/2 of 150/190 K (heating/cooling) for Pd complex and 135/170 K (heating/cooling) for Pt complex, which was confirmed by variable-temperature SCXRD experiments. This research shows the perspective of using pp ligand for building porous MOFs with spin transitions.

Redox-Programmable Spin-Crossover Behaviors in a Cationic Framework

Metal-organic frameworks (MOFs) provide versatile platforms to construct multi-responsive materials. Herein, by introducing the neutral tetradentate ligand and the linear dicyanoaurate(I) anion, we reported a rare cationic MOF [Fe^II(TPB){Au^I(CN)₂}]I·4H₂O·4DMF (TPB = 1,2,4,5-tetra(pyridin-4-yl)benzene) with hysteretic spin-crossover (SCO) behavior near room temperature. This hybrid framework with an open metal site (Au^I) exhibits redox-programmable capability toward dihalogen molecules. By means of post-synthetic modification, all the linear [Au^I(CN)₂]^- linkers can be oxidized to square planar [Au^III(CN)₂X₂]^- units, which results in the hysteretic SCO behaviors switching from one-step to two-step for Br₂ and three-step for I₂. More importantly, the stepwise SCO behaviors can go back to one-step via the reduction by l-ascorbic acid (AA). Periodic DFT calculations using various SCAN-type exchange-correlation functionals have been employed to rationalize the experimental data. Hence, these results demonstrate for the first time that switchable one-/two-/three-stepped SCO dynamics can be manipulated by chemical redox reactions, which opens a new perspective for multi-responsive molecular switches.

Solvatomorphs of Iron(II) Complex with N,N'-Disubstituted 2,6-Bis(pyrazol-3-yl)pyridine with a Temperature-Induced Spin Transition in Solution

Abstract

The reaction of the tridentate ligand 4-(2,6-bis(5-tert-butyl-1-(2,6-dichlorophenyl)-1H-pyrazol-3-yl)pyridin-4-yl)benzonitrile (L) with iron(II) salt gave the complex [Fe(L)2](BF4)2, which was isolated in a pure state and characterized by elemental analysis, NMR spectroscopy, and X-ray diffraction as two crystal polymorphs differing in the nature of the solvent molecule in the crystal (solvatomorphs I and II). According to the results of X-ray diffraction study (CCDC nos. 2104367 (I), 2104368 (II)), the iron(II) ion in these compounds occurs in different spin states and does not undergo a temperature-induced spin transition, which was previously observed for this complex in solution. The details of supramolecular organization of two solvatomorphs that prevent this transition were studied using the Hirshfeld surface analysis.

Local Coordination and Electronic Structure Ramifications of Guest-Dependent Spin Crossover in a Metal-Organic Framework: A Combined X-ray Absorption and Emission Spectroscopy Study

The porous Hoffman-type 3D lattice Fe(pz)[Ni^II(CN)₄] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

Synthesis, X-ray, Hirshfeld, and AIM Studies on Zn(II) and Cd(II) Complexes with Pyridine Ligands

The synthesis and crystal structures of three heteroleptic complexes of Zn(II) and Cd(II) with pyridine ligands (ethyl nicotinate (EtNic), N,N-diethylnicotinamide (DiEtNA), and 2-amino-5-picoline (2Ampic) are presented. The complex [Zn(EtNic)2Cl2] (1) showed a distorted tetrahedral coordination geometry with two EtNic ligand units and two chloride ions as monodentate ligands. Complexes [Zn(DiEtNA)(H2O)4(SO4)]·H2O (2) and [Cd(OAc)2(2Ampic)2] (3) had hexa-coordinated Zn(II) and Cd(II) centers. In the former, the Zn(II) was coordinated with three different monodentate ligands, which were DiEtNA, H2O, and SO42−. In 3, the Cd(II) ion was coordinated with two bidentate acetate ions and two monodentate 2Ampic ligand units. The supramolecular structures of the three complexes were elucidated using Hirshfeld analysis. In 1, the most important interactions that governed the molecular packing were O···H (15.5–15.6%), Cl···H (13.6–13.8%), Cl···C (6.3%), and C···H (10.3–10.6%) contacts. For complexes 2 and 3, the H···H, O···H, and C···H contacts dominated. Their percentages were 50.2%, 41.2%, and 7.1%, respectively, for 2 and 57.1%, 19.6%, and 15.2%, respectively, for 3. Only in complex 3, weak π-π stacking interactions between the stacked pyridines were found. The Zn(II) natural charges were calculated using the DFT method to be 0.8775, 1.0559, and 1.2193 for complexes 1–3, respectively. A predominant closed-shell character for the Zn–Cl, Zn–N, Zn–O, Cd–O, and Cd–N bonds was also concluded from an atoms in molecules (AIM) study.

Halobenzene Clathrates of the Porous Metal-Organic Spin-Crossover Framework [Fe(tvp)2(NCS)2]n. Stabilization of a Four-Step Transition

Here we show that the porous metal–organic spin crossover (SCO) framework [Fe(tvp)₂(NCS)₂]@4(CH₃CN·H₂O) [1@4(CH₃CN·H₂O)] is an excellent precursor material for the systematic synthesis, via single-crystal to single-crystal transformation, of a series of halobenzene clathrates. Immersion of samples constituted of single crystals of 1@4(CH₃CN·H₂O) in the liquid halobenzenes PhX_n, X = F (n = 1–6), X = Cl (n = 1, 2), and X = Br (n = 1) at room temperature induces complete replacement of the guest molecules by PhX_n to afford 1@2PhX_n. Single-crystal analyses of the new clathrates confirm the integrity of the porous framework with the PhX_n guests being organized by pairs via π-stacking filling the nanochannels. The magnetic and calorimetric data confirm the occurrence of practically complete SCO behavior in all of the clathrates. The characteristic SCO equilibrium temperatures, T_1/2, seem to be the result of a subtle balance in the host–guest interactions, which are temperature- and spin-state-dependent. The radically distinct supramolecular organization of the PhCl₂ guests in 1@2PhCl₂ affords a rare example of four-step SCO behavior following the sequence [HS₁:LS₀] ↔ [HS_2/3:LS_1/3] ↔ [HS_1/2:LS_1/2] ↔ [HS_1/4:LS_3/4] ↔ [HS₀:LS₁], which has been structurally characterized.

Guest exchange via single-crystal to single-crystal transformation has been revealed to be an excellent approach to systematically stow halobenzene molecules in the nanochannels of the porous spin-crossover (SCO) metal−organic framework [Fe^II(tvp)₂(NCS)₂]_n. This has enabled the unveiling of key structural factors leading to a four-step SCO behavior.

A complementary characterisation technique for spin crossover materials; the application of X-ray photoelectron spectroscopy for future device applications

Spin crossover (SCO) materials have long been studied for their inherent electronic switchability, which has been well investigated for potential application in electronic and switching devices. As the technologies for the fabrication of thin films and monolayers continue to develop at an exceedingly rapid pace, an emerging challenge for the SCO community has become the characterisation of spin transitions in the surface layers of a material, as well as understanding the origins of discrepancies observed between SCO in thin films and that of the bulk material. For the manufacture of such devices to become a reality, it is crucial to understand how spin crossover is affected by interactions with the substrate material and within thin films. As such, detailed analysis of the surface layers without interference from the substrate material emerged as a critical area of characterisation for future developments in SCO devices. In this regard, X-ray Photoelectron Spectroscopy (XPS) has emerged as a complementary technique for the analysis of SCO in the surface layers of a material, becoming an essential part of a multi-technique protocol that is driving advances in the field. Here we describe the complementary application of XPS to a variety of SCO materials, review major developments and provide illustrative examples of innovations made through surface analysis with XPS.

Enhanced Interplay between Host-Guest and Spin-Crossover Properties through the Introduction of an N Heteroatom in 2D Hofmann Clathrates

Controlled modulation of the spin-crossover (SCO) behavior through the sorption–desorption of invited molecules is an extensively exploited topic because of its potential applications in molecular sensing. For this purpose, understanding the mechanisms by which the spin-switching properties are altered by guest molecules is of paramount importance. Here, we show an experimental approach revealing a direct probe of how the interplay between SCO and host–guest chemistry is noticeably activated by chemically tuning the host structure. Thus, the axial ligand 4-phenylpyridine (4-PhPy) in the 2D Hofmann clathrates {Fe(4-PhPy)₂[M(CN)₄]} (PhPyM; M = Pt, Pd) is replaced by 2,4-bipyridine (2,4-Bipy), resulting in the isomorphous compounds {Fe(2,4-Bipy)₂[M(CN)₄]} (BipyM; M = Pt, Pd), which basically differ from the former in that they have a noncoordinated N heteroatom in the ancillary aromatic substituent, i.e., 2-pyridyl instead of phenyl. Our chemical, magnetic, calorimetric, and structural characterizations demonstrate that this subtle chemical composition change provokes outstanding modifications not only in the capability to adsorb small guests as water or methanol but also in the extent to which these guests affect the SCO characteristics.

The introduction of an N-heterocyclic atom in the aromatic interdigitated axial ligands of a 2D Hofmann-type framework provokes dramatic changes on its affinity to solvent guests. Sorption−desorption of these guests induces drastic structural changes, affecting dramatically the hysteretic spin-crossover properties of the framework.

Neutral Nitrogen Donor Ligand-based MOFs for Sensing Applications

Neutral nitrogen donor (N-donor) ligand-based MOFs, with their enticing features inclusive of facile synthesis, labile metal-ligand bond, framework flexibility, atomic level tunability renders them appealing in molecular recognition-based studies. Intriguingly, the flexibility in such systems (owing to weaker metal-nitrogen bonds) promote maximization of host-analyte interactions, which is critical for the manifestation of a signaling response. Such host-analyte interactions can be tapped by discerning any change in the physical properties associated with the system, such as optical, fluorometric, chemiresistive, magnetic, dielectric constant, mass. This minireview presents a brief discussion on the various types of signal transduction pathways unveiled hitherto using neutral N-donor ligand-based MOFs and the fundamental insight into the signal's origin. Moreover, an elaborate compilation of the recent examples in this field has been presented. Also, the untapped prospects have been highlighted, which may serve as a beacon to drive future research.

Controlling the spin-crossover behavior of the [Cr(indenyl)2] family via ligand functionalization

In this work, a computational method to study the spin-crossover (SCO) behavior of the [Cr(n-Meindenyl)2] family was carried out. Using the TPSSh/Def2TZVP method with the GD3BJ dispersion correction scheme, we computed the thermochemistry and transition temperatures (T1/2) for all members of this family, which are in excellent agreement with the available experimental data. Moreover, the computed data allow us to build a model that describes the effect of functionalizing the indenyl ligand in different positions on the spin-state energy gap and transition temperature. Our results show that the C4 and C7 positions of the indenyl ligand have a greater effect on tuning the SCO properties of such complexes. The model quantitatively reproduces the DFT calculations, thus providing a powerful tool to analyze and predict the SCO properties in any member of the [Cr(n-Meindenyl)2] family.

Slight ligand modifications within multitopic linear hydroxamates promotes connectivity differences in Cu(ii) 1-D coordination polymers

Two novel hydroxamic acids have been used in constructing two 1-D coordination polymers. Slight structural differences promote connectivity changes upon Cu(ii) metalation as rationalised using DFT calculations and Hirschfeld surface analysis.

A metal-organic framework that exhibits CO2-induced transitions between paramagnetism and ferrimagnetism

With adequate building blocks, metal-organic frameworks (MOFs) can combine magnetic ordering and porosity. This makes MOFs a promising platform for the development of stimuli-responsive materials that show drastically different magnetic properties depending on the presence or absence of guest molecules within their pores. Here we report a CO₂-responsive magnetic MOF that converts from ferrimagnetic to paramagnetic on CO₂ adsorption, and returns to the ferrimagnetic state on CO₂ desorption. The ferrimagnetic material is a layered MOF with a [D⁺-A^--D] formula, produced from the reaction of trifluorobenzoate-bridged paddlewheel-type diruthenium(II) clusters as the electron donor (D) with diethoxytetracyanoquinodimethane as the electron acceptor (A). On CO₂ uptake, it undergoes an in-plane electron transfer and a structural transition to adopt a [D-A-D] paramagnetic form. This magnetic phase change, and the accompanying modifications to the electronic conductivity and permittivity of the MOF, are electronically stabilized by the guest CO₂ molecules accommodated in the framework.

Pressure-Induced Spin Crossover at Room Temperature in a Nanoporous Host-Guest Framework Structure

The two-dimensional (2D) metal-organic framework compound Fe₂ (azpy)₄ (NCS)₄ (azpy=trans-4,4'-azopyridine, NCS=thiocyanate) has been pressurized in isopropanol and ethanol that act as guest molecules and pressure-transmitting media. Room temperature conversion of metal-ion sites from high-spin to low-spin states under high-pressure-driven uptake of guest molecules was investigated. Upon isopropanol guest inclusion, spin conversion of ∼66 % low-spin abundance is attained by ∼3 GPa. Spin-crossover progression stimulated by ethanol guest inclusion is more sluggish and ∼56 % low-spin abundance is attained by ∼5.5 GPa. This pressure-facilitated guest-specific triggering of spin crossover is suggestive of guest-molecule interactions with the local coordination network of the Fe(II) core, which increases the ligand field. Appreciable low-spin abundances attained at room temperature upon pressurization, are comparable to maximal ∼50 % spin conversions only attainable below ∼150 K at ambient pressure.

CO2 -Induced Spin-State Switching at Room Temperature in a Monomeric Cobalt(II) Complex with the Porous Nature

CO₂ -responsive spin-state conversion between high-spin (HS) and low-spin (LS) states at room temperature was achieved in a monomeric cobalt(II) complex. A neutral cobalt(II) complex, [Co^II (COO-terpy)₂ ]⋅4 H₂ O (1⋅4 H₂ O), stably formed cavities generated via π-π stacking motifs and hydrogen bond networks, resulting in the accommodation of four water molecules. Crystalline 1⋅4 H₂ O transformed to solvent-free 1 without loss of porosity by heating to 420 K. Compound 1 exhibited a selective CO₂ adsorption via a gate-open type of the structural modification. Furthermore, the HS/LS transition temperature (T_1/2 ) was able to be tuned by the CO₂ pressure over a wide temperature range. Unlike 1 exhibits the HS state at 290 K, the CO₂ -accomodated form 1⊃CO₂ (P CO 2 =110 kPa) was stabilized in the LS state at 290 K, probably caused by a chemical pressure effect by CO₂ accommodation, which provides reversible spin-state conversion by introducing/evacuating CO₂ gas into/from 1.

Assembling Homometallic Sb6 and Heterometallic Ti4Sb2 Oxo Clusters

Isolation and structural characterization of novel organoantimony(V)-based oxo clusters are reported. (RSb)₄(OH)₄(t-BuPO₃)₆ and (RSb)₂(O)(t-BuPO₃H)₆ independently in the presence of pyridine under solvothermal conditions afford the hexanuclear organoantimonate clusters [(RSb)₆(μ₃-O)₂(μ₂-O)₆(t-BuPO₃)₄], where R = p-i-PrC₆H₄ (1), p-ClC₆H₄ (2). Further, reaction of organostibonate phosphonate with Ti(OⁱPr)₄ in the presence of pyridine under solvothermal conditions afforded the mixed-metal titanium stibonate hexanuclear clusters [(RSb)₂Ti₄(μ₃-O)₂(μ₂-O)₂(t-BuPO₃)₄(μ-OCH₃)₄(OCH₃)₄], where R = p-i-PrC₆H₄ (3), p-ClC₆H₄ (4). Band gap measurements were performed on 1-4. They reveal a remarkable reduction in the band gap on moving from the heavier main-group-based oxo cages (1 and 2) to the titanium-incorporated oxo cages (3 and 4).

Seeing Is Believing: Experimental Spin States from Machine Learning Model Structure Predictions

Determination of ground-state spins of open-shell transition-metal complexes is critical to understanding catalytic and materials properties but also challenging with approximate electronic structure methods. As an alternative approach, we demonstrate how structure alone can be used to guide assignment of ground-state spin from experimentally determined crystal structures of transition-metal complexes. We first identify the limits of distance-based heuristics from distributions of metal–ligand bond lengths of over 2000 unique mononuclear Fe(II)/Fe(III) transition-metal complexes. To overcome these limits, we employ artificial neural networks (ANNs) to predict spin-state-dependent metal–ligand bond lengths and classify experimental ground-state spins based on agreement of experimental structures with the ANN predictions. Although the ANN is trained on hybrid density functional theory data, we exploit the method-insensitivity of geometric properties to enable assignment of ground states for the majority (ca. 80–90%) of structures. We demonstrate the utility of the ANN by data-mining the literature for spin-crossover (SCO) complexes, which have experimentally observed temperature-dependent geometric structure changes, by correctly assigning almost all (>95%) spin states in the 46 Fe(II) SCO complex set. This approach represents a promising complement to more conventional energy-based spin-state assignment from electronic structure theory at the low cost of a machine learning model.

Contactless Spin Switch Sensing by Chemo-Electric Gating of Graphene

Direct electrical probing of molecular materials is often impaired by their insulating nature. Here, graphene is interfaced with single crystals of a molecular spin crossover complex, [Fe(bapbpy)(NCS)₂ ], to electrically detect phase transitions in the molecular crystal through the variation of graphene resistance. Contactless sensing is achieved by separating the crystal from graphene with an insulating polymer spacer. Next to mechanical effects, which influence the conductivity of the graphene sheet but can be minimized by using a thicker spacer, a Dirac point shift in graphene is observed experimentally upon spin crossover. As confirmed by computational modeling, this Dirac point shift is due to the phase-dependent electrostatic potential generated by the crystal inside the graphene sheet. This effect, named as chemo-electric gating, suggests that molecular materials may serve as substrates for designing graphene-based electronic devices. Chemo-electric gating, thus, opens up new possibilities to electrically probe chemical and physical processes in molecular materials in a contactless fashion, from a large distance, which can enhance their use in technological applications, for example, as sensors.

A homochiral 3D framework of mechanically interlocked 1D loops with solvent-dependent spin-state switching behaviors

An atypical homochiral spin-crossover (SCO) framework (1) constructed from mechanically interlocked 1D molecular loops was prepared. Due to the flexibility of the interlocked structure, the guest solvent molecules can be reversibly exchanged. Consequently, its SCO behavior was capable of modulating between one- and two-stepped transitions in response to acetonitrile and methanol.

Large-scale comparison of 3d and 4d transition metal complexes illuminates the reduced effect of exchange on second-row spin-state energetics

The origin of distinct 3d vs. 4d transition metal complex sensitivity to exchange is explored over a large data set.

Heat-Triggered Crystallization of Liquid Crystalline Macrocycles Allowing for Conductance Switching through Hysteretic Thermal Phase Transitions

A polymesomorphic thermal phase-transition of a macrocyclic amphiphile consisting of aromatic groups and oligoethylene glycol (OEG) chains is reported. The macrocyclic amphiphile exists in a highly-ordered liquid crystal (LC) phase at room temperature. Upon heating, this macrocycle shows phase-transition from columnar-lamellar to nematic LC phases followed by crystallization before melting. Spectroscopic studies suggest that the thermally induced crystallization is triggered by a conformational change at the OEG chains. Interestingly, while the macrocycle returns to the columnar-lamellar phase after cooling from the isotropic liquid, it retains the crystallinity after cooling from the thermally-induced crystal. Thanks to this bistability, conductance switching was successfully demonstrated. A different macrocyclic amphiphile also shows an analogous phase-transition behavior, suggesting that this molecular design is universal for developing switchable and memorizable materials, by means of hysteretic phase-transition processes.

Spin switching in tris(8-aminoquinoline)iron(ii)(BPh4)2: quantitative guest-losing dependent spin crossover properties and single-crystal-to-single-crystal transformation

As a derivative of 2-picolylamine, which contains rich protons favouring hydrogen bond formation to assemble a variety of valuable spin crossover (SCO) compounds, 8-aminoquinoline (aqin) will be a good candidate for constructing new mononuclear bistable state compounds. With the guidance of this view, two solvated compounds [Fe(aqin)3](BPh4)2·2(CH3CN) (1·2CH3CN) and [Fe(aqin)3](BPh4)2·1.5(CH3COCH3) (2·1.5CH3COCH3) were synthesized. The structural characterization and magnetic studies demonstrate that this strategy has been successful. Single-crystal diffraction reveals that both the mononuclear compounds have facial (fac-)-configuration cations, which form hydrogen bonds using -NH2 groups with solvent molecules (acetonitrile or acetone). Subsequent magnetic measurement shows the highly sensitive solvent-dependent occurrence of a spin transition above room temperature for both compounds. Interestingly, for compound 1·2CH3CN, in the successively repeated heating and cooling process, by monitoring the loss of solvent molecules by TGA, the shifting of the spin transition curve is found to be linearly dependent on the fraction of the residual solvent content. Additionally, the desolvated sample can re-solvate with CH3CN and recover the magnetic response reproducibly. Furthermore, after losing the acetonitrile molecules, the single-crystal-to-single-crystal transformation occurred to give 1.

Increasing spin crossover cooperativity in 2D Hofmann-type materials with guest molecule removal

Ambient temperature spin crossover with wide hysteresis has been achieved in 2D Hofmann-type materials, where removal of guest molecules optimises ligand–ligand interactions, resulting in increased cooperativity.

Molecule-based spin state switching materials that display ambient temperature transitions with accompanying wide thermal hysteresis offer an opportunity for electronic switching, data storage, and optical technologies but are rare in existence. Here, we present the first 2D Hofmann-type materials to exhibit the elusive combination of ambient temperature spin crossover with wide thermal hysteresis (ΔT = 50 and 65 K). Combined structural, magnetic, spectroscopic, and theoretical analyses show that the highly cooperative transition behaviours of these layered materials arise due to strong host–host interactions in their interdigitated lattices, which optimises long-range communication pathways. With the presence of water molecules in the interlayer pore space in the hydrated phases, competing host–host and host–guest interactions occur, whilst water removal dramatically increases the framework cooperativity, thus affording systematic insight into the structural features that favour optimal spin crossover properties.

Covalent and Ionic Capacity of MOFs To Sorb Small Gas Molecules

In this work, the aim is to characterize how an Fe-based metal-organic framework (MOF) behaves when gases, like carbon dioxide, are inserted through their channels and to characterize the nature and strength of those interactions. Despite the computational nature of the project, it is based on the experimental results obtained in 2016 by Mı́nguez-Espallargas and co-workers ( J. Am. Chem. Soc. 2013, 135, 15986 - 15989 ). Those MOFs were found to selectively allocate/adsorb CO₂, having as a drawback that apparently each cavity allocates only one CO₂ molecule. Despite truncating the MOF to its unitary cell, the whole cavity of the MOF can be described in detail by precise ab initio calculations. Another computational goal is to unravel why experimentally CO₂ was preferred with respect to N₂, and for the sake of consistency, a list of common gases will be further studied, such as H₂, O₂, H₂O, CH₄, C₂H₆, N₂O, or NO.

Control of the Speed of a Light-Induced Spin Transition through Mesoscale Core-Shell Architecture

The rate of the light-induced spin transition in a coordination polymer network solid dramatically increases when included as the core in mesoscale core-shell particles. A series of photomagnetic coordination polymer core-shell heterostructures, based on the light-switchable Rb _aCo _b[Fe(CN)₆] _c· mH₂O (RbCoFe-PBA) as core with the isostructural K _jNi _k[Cr(CN)₆] _l· nH₂O (KNiCr-PBA) as shell, are studied using temperature-dependent powder X-ray diffraction and SQUID magnetometry. The core RbCoFe-PBA exhibits a charge transfer-induced spin transition (CTIST), which can be thermally and optically induced. When coupled to the shell, the rate of the optically induced transition from low spin to high spin increases. Isothermal relaxation from the optically induced high spin state of the core back to the low spin state and activation energies associated with the transition between these states were measured. The presence of a shell decreases the activation energy, which is associated with the elastic properties of the core. Numerical simulations using an electro-elastic model for the spin transition in core-shell particles supports the findings, demonstrating how coupling of the core to the shell changes the elastic properties of the system. The ability to tune the rate of optically induced magnetic and structural phase transitions through control of mesoscale architecture presents a new approach to the development of photoswitchable materials with tailored properties.

Accelerating Chemical Discovery with Machine Learning: Simulated Evolution of Spin Crossover Complexes with an Artificial Neural Network

Machine learning (ML) has emerged as a powerful complement to simulation for materials discovery by reducing time for evaluation of energies and properties at accuracy competitive with first-principles methods. We use genetic algorithm (GA) optimization to discover unconventional spin-crossover complexes in combination with efficient scoring from an artificial neural network (ANN) that predicts spin-state splitting of inorganic complexes. We explore a compound space of over 5600 candidate materials derived from eight metal/oxidation state combinations and a 32-ligand pool. We introduce a strategy for error-aware ML-driven discovery by limiting how far the GA travels away from the nearest ANN training points while maximizing property (i.e., spin-splitting) fitness, leading to discovery of 80% of the leads from full chemical space enumeration. Over a 51-complex subset, average unsigned errors (4.5 kcal/mol) are close to the ANN's baseline 3 kcal/mol error. By obtaining leads from the trained ANN within seconds rather than days from a DFT-driven GA, this strategy demonstrates the power of ML for accelerating inorganic material discovery.

Three-dimensional iron(ii) porous coordination polymer exhibiting carbon dioxide-dependent spin crossover

A three-dimensional iron(ii) porous coordination polymer has been observed spin crossover behaviours through the sorption of H₂O and CO₂, respectively.

Effect of coordination geometry on the magnetic properties of a series of Ln2 and Ln4 hydroxo clusters

Tetra and di-nuclear Lanthanide hydroxo clusters have been synthesized and characterized magnetically.

Water molecule induced reversible single-crystal-to-single-crystal transformation between two trinuclear Fe(ii) complexes with different spin crossover behaviour

Water molecule induced reversible single-crystal-to-single-crystal transformation between two trinuclear Fe(ii) complexes with different spin crossover behaviour.

Structure and Magnetic Studies on a Series of Two-Dimensional Iron(II) Framework Materials with Varying Ligand Characteristics

Targeting the general (4,4)-grid structural motif, we have prepared seven new coordination polymers in the general family [Fe(NCX)2(L)2]·(guest) (L = bis-pyridyl-type bridging ligands; X = S, Se) as an extension of the well-established spin crossover framework (SCOF) family. In all cases, the (4,4)-grid topology is formed by the bridging of octahedral iron(ii) sites in the equatorial plane by bis-pyridyl ligands of varying length, flexibility, and intermolecular interaction capacity. In particular, the six ligands n-(4-pyridyl)-isonicotinamide (pin), trans-1,2-bis(4′-pyridyl)ethane (tvp), 1,2-dibromo-1,2-bis(4′-pyridyl)ethane (dbbpe), bis(4-pyridyl)-1,2,4,5-tetrazine (bptz), 4,4′-bis(pyridyl)acetylene (bpac), and 1,4-bis(4-pyridylethynyl)benzene (bpeben) have been utilised. The seven new materials [Fe(NCS)2(pin)2]·2(MeCN) (pin-S), [Fe(tvp)2(NCS)2]·1/2(tvp)·(CH3CH2OH) (tvp-S), [Fe(dbbpe)2(NCS)2]·6(CH3CN) (dbbpe-S), [Fe(NCS)2(bptz)2]·2(CHCl3)·6(EtOH) (bptz-S), [Fe(NCSe)2(bptz)2]·4(CHCl3)·(EtOH)·(H2O) (bptz-Se), [Fe(NCS)2(bpac)2]·2(PrOH) (bpac-S), and [Fe(NCS)2(bpeben)2]·2(CHCl3) (bpeben-S) all form (4,4)-grids of varying size that are arranged in a parallel stacked topology. Despite being in the [FeN6] coordination environment known to be conducive to spin crossover, these materials all remain high-spin with thermal variation. These results are discussed in context with the large family of SCOFs that show varied spin crossover behaviours.

Reversible crystal-to-crystal transformation from a trinuclear cluster to a 1D chain and the corresponding spin crossover (SCO) behaviour change

Reversible crystal-to-crystal transformation between a linear trinuclear Fe(ii) complex and a 1D chain, and the SCO behaviour change are reported.

An updated roadmap for the integration of metal–organic frameworks with electronic devices and chemical sensors

This review highlights the steps needed to bring the properties of MOFs from the chemical lab to the microelectronics fab.

Near Room-Temperature Memory Devices Based on Hybrid Spin-Crossover@SiO2 Nanoparticles Coupled to Single-Layer Graphene Nanoelectrodes

The charge transport properties of SCO [Fe(Htrz)2 (trz)](BF4 ) NPs covered with a silica shell placed in between single-layer graphene electrodes are reported. A reproducible thermal hysteresis loop in the conductance above room-temperature is evidenced. This bistability combined with the versatility of graphene represents a promising scenario for a variety of technological applications but also for future sophisticated fundamental studies.