Guest Binding Subtly Influences Spin Crossover in an FeII4L4Capsule

Proton-Induced Reversible Spin-State Switching in Octanuclear FeIII Spin-Crossover Metal-Organic Cages

Responsive spin-crossover (SCO) metal-organic cages (MOCs) are emerging dynamic platforms with potential for advanced applications in magnetic sensing and molecular switching. Among these, FeIII-based MOCs are particularly noteworthy for their air stability, yet they remain largely unexplored. Herein, we report the synthesis of two novel FeIII MOCs using a bis-bidentate ligand approach, which exhibit SCO activity above room temperature. These represent the first SCO-active FeIII cages and feature an atypical {FeN6}-type coordination sphere, uncommon for FeIII SCO compounds. Our study reveals that these MOCs are sensitive to acid/base variations, enabling reversible magnetic switching in solution. The presence of multiple active proton sites within these SCO-MOCs facilitates multisite, multilevel proton-induced spin-state modulation. This behavior is observed at room temperature through 1H NMR spectroscopy, capturing the subtle proton-induced spin-state transitions triggered by pH changes. Further insights from extended X-ray absorption fine structure (EXAFS) and theoretical analyses indicate that these magnetic alterations primarily result from the protonation and deprotonation processes at the NH active sites on the ligands. These processes induce changes in the secondary coordination sphere, thereby modulating the magnetic properties of the cages. The capability of these FeIII MOCs to integrate magnetic responses with environmental stimuli underscores their potential as finely tunable magnetic sensors and highlights their versatility as molecular switches. This work paves the way for the development of SCO-active materials with tailored properties for applications in sensing and molecular switching.

Self-assembly of a supramolecular spin-crossover tetrahedron

A new mononuclear iron(II) SCO compound featuring H-bonding donor and acceptor units has been synthesized and exploited to produce a purely supramolecular switchable [Fe4] tetrahedron. Magnetic and crystallographic measurements evidence a singular magnetic behavior for each of the four Fe(II) centers of the generated architecture and underscore the potential of this strategy to develop novel SCO materials.

54 K Spin Transition Temperature Shift in a Fe6L4 Octahedral Cage Induced by Optimal Fitted Multiple Guests

Spin-crossover (SCO) coordination cages are at the forefront of research for their potential in crafting next-generation molecular devices. However, due to the scarcity of SCO hosts and their own limited cavities, the interplay between the SCO host and the multiple guests binding has remained elusive. In this contribution, we present a family of pseudo-octahedral coordination cages (M6L4, M = ZnII, CoII, FeII, and NiII) assembled from a tritopic tridentate ligand L with metal ions. The utilization of FeII ion leads to the successful creation of the Fe6L4-type SCO cage. Host-guest studies of these M6L4 cages reveal their capacity to encapsulate four adamantine-based guests. Notably, the spin transition temperature T1/2 of Fe6L4 is dependent on the multiple guests encapsulated. The inclusion of adamantine yields an unprecedented T1/2 shift of 54 K, a record shift in guest-mediated SCO coordination cages to date. This drastic shift is ascribed to the synergistic effect of multiple guests coupled with their optimal fit within the host. Through a straightforward thermodynamic cycle, the binding affinities of the high-spin (HS) and low-spin (LS) states are separated from their apparent binding constant. This result indicates that the LS state has a stronger binding affinity for the multiple guests than the HS state. Exploring the SCO thermodynamics of host-guest complexes allows us to examine the optimal fit of multiple guests to the host cavity. This study reveals that the T1/2 of the SCO host can be manipulated by the encapsulation of multiple guests, and the SCO cage is an ideal candidate for determining the multiple guest fit.

Spin-state versatility in FeII4L6 supramolecular cages with a pyridyl-hydrazone ligand scaffold modulated by solvents and counter anions

Discrete spin crossover (SCO) tetranuclear cages are a unique class of materials that have potential use in next-generation molecular recognition and sensing. In this work, two new edge-bridged SCO FeII4L6 (L = 2,7-bis(((E)-pyridin-2-ylmethylene)amino)benzo[lmn] [3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) supramolecular cages with different counter anions: ClO4- (2) and CF3SO3- (3) were constructed via subcomponent self-assembly to investigate both solvent and anion influences on their magnetic properties and compare them to cage 1 with a BF4- anion. Pyridyl-hydrazone bidentate ligand scaffolds were employed to replace the 'classical' imidazole/thiazolyl-imine coordination units to induce SCO behaviour in these cages. 2 and 3 were structurally characterized by single-crystal X-ray diffraction analysis and electrospray ionization time-of-flight mass spectrometry. Magnetic susceptibilities of 1-3 and 1-3·desolvated indicate that the solvents' presence is in favor of the low-spin (LS) state. While different counter anions in 1-3·desolvated affect the spin-state configurations of the four FeII metal centers. According to the 57Fe Mössbauer spectral analysis, the spin-state distributions in 1-3 at 80 K are [2 high-spin (HS)-2LS], [1HS-3LS] and [2HS-2LS], respectively and density functional theory calculations were employed to investigate the reasons. These findings provide insights to regulate the spin-state versatility of SCO FeII cage systems in the solid state.

Heterometallic cages: synthesis and applications

High symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis and drug delivery. Recently there have been increasing efforts to enhance those applications by generating reduced symmetry MSAs. While there are several emerging methods for generating lower symmetry MSAs, this tutorial review examines the general methods used for synthesizing heterometallic MSAs with a particular focus on heterometallic cages. Additionally, the intrinsic properties of the cages and their potential emerging applications as host-guest systems and reaction catalysts are described.

Tuning the spin-crossover properties of FeII4L6 cages via the interplay of coordination motif and linker modifications

Despite the increasing number of spin-crossover FeII-based cages, the interplay between ligand modifications (e.g. coordination motif substituents and linker) is not well-understood in these multinuclear systems, limiting rational design. Here, we report a family of FeII4L6 spin-crossover cages based on 2,2'-pyridylbenzimidazoles where subtle ligand modifications lowered the spin crossover temperature in CD3CN by up to 186 K. Comparing pairs of cages, CH3 substituents on either the coordination motif or phenylene linker lowered the spin-crossover temperature by 48 K, 91 K or 186 K, attributed to electronic effects, steric effects and a combination of both, respectively. The understanding of the interplay between ligand modifications gained from this study could be harnessed on the path towards the improved rational design of spin-crossover cages.

A supramolecular helicate with two independent Fe(II) switchable centres and a [Fe(anilate)3]3- guest

A biphenyl-spaced bis-pyrazolylpyridine ligand interacts with ferrous ions to engender a dimetallic helical coordination cage that encapsulates an Fe3+ tris-anilate complex. The host-guest interaction breaks the symmetry of the Fe2+ centers causing a differential spin crossover behavior in them that can be followed in great detail crystallographically.

Binuclear spin-crossover [Fe(bt)(NCS)2]2(bpm) complex: A study using first principles calculations

The spin-crossover [Fe(bt)(NCS)₂]₂(bpm) complex is studied using spin-polarized density functional theory within the generalized gradient approximation, the Hubbard U and the weak van der Waals interactions in conjunction with the projector augmented wave method in its molecular and periodic arrangements. It is shown that the considered complex has three magnetic configurations [high spin state (HS)-HS, HS-low spin state (LS), and LS-LS] corresponding to those observed experimentally after two transition temperatures T_c ⁽¹⁾ of 163 K and T_c ⁽²⁾ of 197 K. For the HS-HS magnetic state, we found that the two Fe centers are antiferromagnetically coupled for both molecular and periodic structures in good agreement with the experimental observations. Our results show that the computed total energy difference between the magnetic state configurations of the considered Fe₂ complex is significantly smaller compared to those reported in the literature for other mono- or binuclear compounds.

Exploiting Supramolecular Interactions to Control Isomer Distributions in Reduced-Symmetry [Pd2L4]4+ Cages

High-symmetry metallosupramolecular architectures (MSAs) have been exploited for a range of applications including molecular recognition, catalysis, and drug delivery. Recently, there have been increasing efforts to enhance those applications by generating reduced-symmetry MSAs. Here we report our attempts to use supramolecular (dispersion and hydrogen-bonding) forces and solvophobic effects to generate isomerically pure [Pd₂(L)₄]⁴⁺ cage architectures from a family of new reduced-symmetry ditopic tripyridyl ligands. The reduced-symmetry tripyridyl ligands featured either solvophilic polyether chains, solvophobic alkyl chains, or amino substituents. We show using NMR spectroscopy, high-performance liquid chromatography, X-ray diffraction data, and density functional theory calculations that the combination of dispersion forces and solvophobic effects does not provide any control of the [Pd₂(L)₄]⁴⁺ isomer distribution with mixtures of all four cage isomers (HHHH, HHHT, cis-HHTT, or trans-HTHT, where H = head and T = tail) obtained in each case. More control was obtained by exploiting hydrogen-bonding interactions between amino units. While the cage assembly with a 3-amino-substituted tripyridyl ligand leads to a mixture of all four possible isomers, the related 2-amino-substituted tripyridyl ligand generated a cis-HHTT cage architecture. Formation of the cis-HHTT [Pd₂(L)₄]⁴⁺ cage was confirmed using NMR studies and X-ray crystallography.

Iron(II) Mediated Supramolecular Architectures with Schiff Bases and Their Spin-Crossover Properties

Supramolecular architectures, which are formed through the combination of inorganic metal cations and organic ligands by self-assembly, are one of the techniques in modern chemical science. This kind of multi-nuclear system in various dimensionalities can be implemented in various applications such as sensing, storage/cargo, display and molecular switching. Iron(II) mediated spin-crossover (SCO) supramolecular architectures with Schiff bases have attracted the attention of many investigators due to their structural novelty as well as their potential application possibilities. In this paper, we review a number of supramolecular SCO architectures of iron(II) with Schiff base ligands exhibiting varying geometrical possibilities. The structural and SCO behavior of these complexes are also discussed in detail.

Spin Crossover Induced by Changing the Identity of the Secondary Metal Ion from PdII to NiII in a Face-Centered FeII 8 MII 6 Cubic Cage

Discrete spin crossover (SCO) heteronuclear cages are a rare class of materials which have potential use in next-generation molecular transport and catalysis. Previous investigations of cubic cage [Fe₈ Pd₆ L₈ ]²⁸⁺ constructed using semi-rigid metalloligands, found that Fe^II centers of the cage did not undergo spin transition. In this work, substitution of the secondary metal center at the face of the cage resulted in SCO behavior, evidenced by magnetic susceptibility, Mössbauer spectroscopy and single crystal X-ray diffraction. Structural comparisons of these two cages shed light on the possible interplay of inter- and intramolecular interactions associated with SCO in the Ni^II analogue, 1 ([Fe₈ Ni₆ L₈ (CH₃ CN)₁₂ ]²⁸⁺ ). The distorted octahedral coordination environment, as well as the occupation of the CH₃ CN in the Ni^II axial positions of 1, prevented close packing of cages observed in the Pd^II analogue. This led to offset, distant packing arrangements whereby important areas within the cage underwent dramatic structural changes with the exhibition of SCO.

A spin crossover FeII4L6 cage based on pyridyl-hydrazone sites

Reported here is the first Fe^II based supramolecular cage with pyridyl-hydrazone ligand scaffolds that exhibits temperature induced spin crossover behaviour. Density functional theory calculations were employed to investigate the cause of the occurrence of this phenomenon based on the ligand structure. These results indicate that the reported low-spin cages with pyridyl-imine sites could be reconsidered for spin crossover by carefully manipulating the functional groups in the ligand system.

Self-Assembly of a Rare High Spin FeII/PdII Tetradecanuclear Cubic Cage Constructed via the Metalloligand Approach

Polynuclear heterobimetallic coordination cages in which different metal cations are connected within a ligand scaffold are known to adopt a variety of polyhedral architectures, many of which display interesting functions. Within the extensive array of coordination cages incorporating Fe(II) centres reported so far, the majority contain low-spin (LS) Fe(II), with high-spin (HS) Fe(II) being less common. Herein, we present the synthesis and characterisation of a new tetradecanuclear heterobimetallic [Fe8Pd6L8](BF4]28 (1) cubic cage utilising the metalloligand approach. Use of the tripodal tris-imidazolimine derivative (2) permitted the formation of the tripodal HS Fe(II) metalloligand [FeL](BF4)2·CH3OH (3) that was subsequently used to form the coordination cage 1. Magnetic and structural analyses gave insight into the manner in which the HS environment of the metalloligand was transferred into the cage architecture along with the structural changes that accompanied its occupancy of the eight corners of the discrete cubic structure.

Correlations between ligand field Δ o, spin crossover T 1/2 and redox potential E pa in a family of five dinuclear helicates

A family of five new bis-bidentate azole-triazole Rat ligands (1,3-bis(5-(azole)-4-isobutyl-4H-1,2,4-triazol-3-yl)benzene), varying in choice of azole (2-imidazole, 4-imidazole, 1-methyl-4-imidazole, 4-oxazole and 4-thiazole), and the corresponding family of spin-crossover (SCO) and redox active triply bridged dinuclear helicates, [FeII 2L3]4+, has been prepared and characterised. X-ray crystal structures show all five Fe(ii) helicates are low spin at 100 K. Importantly, DOSY NMR confirms the intactness of these SCO-active dinuclear helicates in D3-MeCN solution, regardless of HS fraction: γ HS(298 K) = 0-0.81. Variable temperature 1H NMR Evans and UV-vis studies reveal that the helicates are SCO-active in MeCN solution. Indeed, the choice of azole in the Rat ligand used in [Fe2L3]4+ tunes: (a) solution SCO T 1/2 from 247 to 471 K, and (b) reversible redox potential, E m(FeII/III), from 0.25 to 0.67 V for four helicates, whilst one has an irreversible redox process, E pa = 0.78 V, vs. 0.01 M AgNO3/Ag. For the four reversible redox systems, a strong correlation (R 2 = 0.99) is observed between T 1/2 and E pa. Finally, the analogous Ni(ii) helicates have been prepared to obtain Δ o, establishing: (a) the ligand field strength order of the ligands: 4-imidazole (11 420) ∼ 1-methyl-4-imidazole (11 430) < 2-imidazole (11 505) ∼ 4-oxazole (11 516) < 4-thiazole (11 804 cm-1), (b) that Δ o ([NiII 2L3]4+) strongly correlates (R 2 = 0.87) with T 1/2 ([FeII 2L3]4+), and (c) interestingly that Δ o strongly correlates (R 2 = 0.98) with E pa for the four helicates with reversible redox, so the stronger the ligand field strength, the harder it is to oxidise the Fe(ii) to Fe(iii).

Controlling dynamic magnetic properties of coordination clusters via switchable electronic configuration

Large-sized coordination clusters have emerged as a new class of molecular materials in which many metal atoms and organic ligands are integrated to synergize their properties. As dynamic magnetic materials, such a combination of multiple components functioning as responsive units has many advantages over monometallic systems due to the synergy between constituent components. Understanding the nature of dynamic magnetism at an atomic level is crucial for realizing the desired properties, designing responsive molecular nanomagnets, and ultimately unlocking the full potential of these nanomagnets for practical applications. Therefore, this review article highlights the recent development of large-sized coordination clusters with dynamic magnetic properties. These dynamic properties can be associated with spin transition, electron transfer, and valence fluctuation through their switchable electronic configurations. Subsequently, the article also highlights specialized characterization techniques with different timescales for supporting switching mechanisms, chemistry, and properties. Afterward, we present an overview of coordination clusters (such as cyanide-bridged and non-cyanide assemblies) with dynamic magnetic properties, namely, spin transition and electron transfer in magnetically bistable systems and mixed-valence complexes. In particular, the response mechanisms of coordination clusters are highlighted using representative examples with similar transition principles to gain insights into spin state and mixed-valence chemistry. In conclusion, we present possible solutions to challenges related to dynamic magnetic clusters and potential opportunities for a wide range of intelligent next-generation devices.

Exploiting host-guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages

Reaction of Ni(OTf)₂ with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [Ni^II₄L₆]⁸⁺. By selectively exchanging the bound triflate from [OTf⊂Ni^II₄L₆](OTf)₇ (1), we have been able to prepare a series of host–guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [M^IIX₄⊂Ni^II₄L₆](OTf)₆, where M^IIX₄²⁻ = MnCl₄²⁻ (2), CoCl₄²⁻ (5), CoBr₄²⁻ (6), NiCl₄²⁻ (7), and CuBr₄²⁻ (8) or [M^IIIX₄⊂Ni^II₄L₆](OTf)₇, where M^IIIX₄⁻ = FeCl₄⁻ (3) and FeBr₄⁻ (4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host–guest complexes 1–8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN₆} vertexes within a single Ni₄L₆ unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [Ni^II₄] complex, and between the host and the MX₄ⁿ⁻ guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX₄ⁿ⁻ guest molecules lead to stronger host–guest magnetic exchange interactions.

The tetrahedral [Ni^II₄L₆]⁸⁺ cage can reversibly bind paramagnetic MX₄^1/2− guests, inducing magnetic exchange interactions between host and guest.

A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages

The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. As a result, NMR analysis of paramagnetic complexes is limited in comparison to diamagnetic compounds and often relies on theoretical models. We report a toolbox of 1D (1H, proton-coupled 13C, selective 1H-decoupling 13C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods that enables unprecedented structural characterisation and in some cases, provides more structural information than would be observable for a diamagnetic analogue. We demonstrate the toolbox's broad versatility for fields from coordination chemistry and spin-crossover complexes to supramolecular chemistry through the characterisation of CoII and high-spin FeII mononuclear complexes as well as a Co4L6 cage.

Paramagnetic Organocobalt Capsule Revealing Xenon Host-Guest Chemistry

We investigated Xe binding in a previously reported paramagnetic metal-organic tetrahedral capsule, [Co4L6]4-, where L2- = 4,4'-bis[(2-pyridinylmethylene)amino][1,1'-biphenyl]-2,2'-disulfonate. The Xe-inclusion complex, [XeCo4L6]4-, was confirmed by 1H NMR spectroscopy to be the dominant species in aqueous solution saturated with Xe gas. The measured Xe dissociation rate in [XeCo4L6]4-, koff = 4.45(5) × 102 s-1, was at least 40 times greater than that in the analogous [XeFe4L6]4- complex, highlighting the capability of metal-ligand interactions to tune the capsule size and guest permeability. The rapid exchange of 129Xe nuclei in [XeCo4L6]4- produced significant hyperpolarized 129Xe chemical exchange saturation transfer (hyper-CEST) NMR signal at 298 K, detected at a concentration of [XeCo4L6]4- as low as 100 pM, with presaturation at -89 ppm, which was referenced to solvated 129Xe in H2O. The saturation offset was highly temperature-dependent with a slope of -0.41(3) ppm/K, which is attributed to hyperfine interactions between the encapsulated 129Xe nucleus and electron spins on the four CoII centers. As such, [XeCo4L6]4- represents the first example of a paramagnetic hyper-CEST (paraHYPERCEST) sensor. Remarkably, the hyper-CEST 129Xe NMR resonance for [XeCo4L6]4- (δ = -89 ppm) was shifted 105 ppm upfield from the diamagnetic analogue [XeFe4L6]4- (δ = +16 ppm). The Xe inclusion complex was further characterized in the crystal structure of (C(NH2)3)4[Xe0.7Co4L6]·75 H2O (1). Hydrogen bonding between capsule-linker sulfonate groups and exogenous guanidinium cations, (C(NH2)3)+, stabilized capsule-capsule interactions in the solid state and also assisted in trapping a Xe atom (∼42 Å3) in the large (135 Å3) cavity of 1. Magnetic susceptibility measurements confirmed the presence of four noninteracting, magnetically anisotropic high-spin CoII centers in 1. Furthermore, [Co4L6]4- was found to be stable toward aggregation and oxidation, and the CEST performance of [XeCo4L6]4- was unaffected by biological macromolecules in H2O. These results recommend metal-organic capsules for fundamental investigations of Xe host-guest chemistry as well as applications with highly sensitive 129Xe-based sensors.

Paramagnetic Shifts and Guest Exchange Kinetics in ConFe4-n Metal-Organic Capsules

We investigate the magnetic resonance properties and exchange kinetics of guest molecules in a series of hetero-bimetallic capsules, [Co_nFe_4-nL₆]^4- (n = 1-3), where L^2- = 4,4'-bis[(2-pyridinylmethylene)amino]-[1,1'-biphenyl]-2,2'-disulfonate. H bond networks between capsule sulfonates and guanidinium cations promote the crystallization of [Co_nFe_4-nL₆]^4-. The following four isostructural crystals are reported: two guest-free forms, (C(NH₂)₃)₄[Co_1.8Fe_2.2L₆]·69H₂O (1) and (C(NH₂)₃)₄[Co_2.7Fe_1.3L₆]·73H₂O (2), and two Xe- and CFCl₃-encapsulated forms, (C(NH₂)₃)₄[(Xe)_0.8Co_1.8Fe_2.2L₆]·69H₂O (3) and (C(NH₂)₃)₄[(CFCl₃)Co_2.0Fe_2.0L₆]·73H₂O (4), respectively. Structural analyses reveal that Xe induces negligible structural changes in 3, while the angles between neighboring phenyl groups expand by ca. 3° to accommodate the much larger guest, CFCl₃, in 4. These guest-encapsulated [Co_nFe_4-nL₆]^4- molecules reveal ¹²⁹Xe and ¹⁹F chemical shift changes of ca. -22 and -10 ppm at 298 K, respectively, per substitution of low-spin Fe^II by high-spin Co^II. Likewise, the temperature dependence of the ¹²⁹Xe and ¹⁹F NMR resonances increases by 0.1 and 0.06 ppm/K, respectively, with each additional paramagnetic Co^II center. The optimal temperature for hyperpolarized (hp) ¹²⁹Xe chemical exchange saturation transfer (hyper-CEST) with [Co_nFe_4-nL₆]^4- capsules was found to be inversely proportional to the number of Co^II centers, n, which is consistent with the Xe chemical exchange accelerating as the portals expand. The systematic study was facilitated by the tunability of the [M₄L₆]^4- capsules, further highlighting these metal-organic systems for developing responsive sensors with highly shifted ¹²⁹Xe resonances.

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.

Qualitative Guest Sensing via Iron(II) Triazole Complexes

The pyridazine-pyridine triazole-based Rat ligand, L^pydzpy [4-(4-methylphenyl)-3-(3-pyridazinyl)-5-(2-pyridinyl)-1,2,4-triazole], is potentially ditopic. Nevertheless, L^pydzpy is shown herein to exclusively form mononuclear iron(II) complexes, [Fe^II(L^pydzpy)₂(NCE)₂]·solvent, in the presence of coordinating NCE anions (E = S or Se). Specifically, a new family of 10 mononuclear complexes, in which L^pydzpy binds in a monotopic bidentate manner, has been made: two solvent-free complexes, [Fe^II(L^pydzpy)₂(NCS)₂] (1) and [Fe^II(L^pydzpy)₂(NCSe)₂] (2); six solvatomorphs, 1·4CH₃CN, 2·4CH₃CN, 1·2.25CH₃CN, 2·3CH₃CN, 2·tetrahydrofuran, and 2·CHCl₃; and a pair of desolvated polymorphs, 1' and 2'. Seven of them are spin crossover-active, the exceptions being 1, 2, and 2'. This is confirmed by single-crystal X-ray diffraction (XRD) for 1, 2, 1·4CH₃CN, and 2·4CH₃CN and is consistent with variable-temperature optical microscopy observations on single crystals of 1·4CH₃CN and 2·4CH₃CN and on samples of 1' and 2'. Powder XRD, thermogravimetric analysis, and solid-state magnetometry reveal that desolvated 1' and 2' are capable of absorbing and desorbing a range of volatile guests: CH₃CN in both cases and also tetrahydrofuran and CHCl₃ in the case of 2'.

Dinuclear helicate and tetranuclear cage assembly using appropriately designed ditopic triazole-azine ligands

Architecture, helicate or cage, is controlled by choice of meta vs. para phenylene linker in new, robust, ditopic triazole-pyrimidine ligands.

A mixed-spin spin-crossover thiozolylimine [Fe4L6]8+ cage

A mixed-spin spin-crossover thiozolylimine [Fe₄L₆]⁸⁺ tetrahedral cage is reported.

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.

Carboxylic Acid Functionalized Spin-Crossover Iron(II) Grids for Tunable Switching and Hybrid Electrode Fabrication

Two carboxyl-substituted iron(II) grids, one protonated, [Fe₄(HL)₄](BF₄)₄·4MeCN·AcOEt (1), and the other deprotonated, [Fe₄(L)₄]·DMSO·EtOH (2), where H₂L = 4-{4,5-bis[6-(3,5-dimethylpyrazol-1-yl)pyrid-2-yl]-1 H-imidazol-2-yl}benzoic acid, were synthesized. Single-crystal X-ray structure analyses reveal that both complexes have a tetranuclear [2 × 2] grid structure. 1 formed one-dimensional chains through intermolecular hydrogen bonds between the carboxylic acid units of neighboring grids, while 2 formed two-dimensional layers stabilized by π-π-stacking interactions. 1 showed spin transition between the 3HS-1LS and 1.5HS-2.5LS states around 200 K, while 2 showed spin-crossover between the 4LS and 2LS-2HS states above 300 K. A modified indium-tin oxide (ITO) electrode was fabricated by soaking the ITO in a solution of 1. The resultant electrode showed reversible redox waves attributed to the original redox processes of iron(II)/iron(III).

Tetrahedral metal–organic cages with cube-like cavities for selective encapsulation of fullerene guests and their spin-crossover properties

Selective encapsulation of fullerene guests and solid state spin-crossover behaviors were observed in iron(ii) tetrahedral metal–organic cages with cube-like cavities.

Spin crossover in discrete polynuclear iron(ii) complexes

A comprehensive review of 127 dinuclear to octanuclear complexes, mostly 2012-present, reveals key design features and future directions for spin crossover active supramolecular assemblies.

Subcomponent Exchange Transforms an FeII4L4 Cage from High- to Low-Spin, Switching Guest Release in a Two-Cage System

Subcomponent exchange transformed new high-spin Fe^II₄L₄ cage 1 into previously-reported low-spin Fe^II₄L₄ cage 2: 2-formyl-6-methylpyridine was ejected in favor of the less sterically hindered 2-formylpyridine, with concomitant high- to low-spin transition of the cage’s Fe^II centers. High-spin 1 also reacted more readily with electron-rich anilines than 2, enabling the design of a system consisting of two cages that could release their guests in response to combinations of different stimuli. The addition of p-anisidine to a mixture of high-spin 1 and previously-reported low-spin Fe^II₄L₆ cage 3 resulted in the destruction of 1 and the release of its guest. However, initial addition of 2-formylpyridine to an identical mixture of 1 and 3 resulted in the transformation of 1 into 2; added p-anisidine then reacted preferentially with 3 releasing its guest. The addition of 2-formylpyridine thus modulated the system’s behavior, fundamentally altering its response to the subsequent signal p-anisidine.

Heteroleptic iron(iii) Schiff base spin crossover complexes: halogen substitution, solvent loss and crystallite size effects

New heteroleptic iron(iii) spin crossover complexes, [Fe(qsal-X)(thsa)]·nMeCN, have been investigated. Halogen substitution, solvent and crystallite size effects are discussed.

Electron-deficient trifluoromethyl-substituted sub-components affect the properties of M4L4 tetrahedral cages

Two supramolecular cages based on a trifluoromethyl-substituted pyridylimine ligand have been synthesised where the iron(ii) complex shows host–guest chemistry and complex-to-complex transformations.

Guest-tuned spin crossover in flexible supramolecular assemblies templated by a halide (Cl−, Br− or I−)

A supramolecular dimer of spin-crossover [FeL₃]²⁺ complexes, L, being a polyheterocyclic ligand, encapsulates halides to form [X@[FeL₃]₂]³⁺ cations with tunable magnetic properties.

Co–Co and Co–Fe cyano-bridged pentanuclear clusters based on a methylpyrazinyl-diamine tetradentate ligand: spin crossover and metal substitution effects

A pentanuclear [Co^II₃Co^III₂] cluster complex has been developed by a solvothermal synthesis. Its highly stable metal-mixed Fe–Co derivatives display robust spin crossover (T_1/2 = 268 K) controlled by the degree of substitution.

Reversible quantitative guest sensing via spin crossover of an iron(ii) triazole

A new phenyl-triazole-pyrazine ligand, 4-p-tolyl-3-(phenyl)-5-(2-pyrazinyl)-1,2,4-triazole (tolpzph), was prepared in order to enforce pyrazine coordination of the iron(ii) centre in the resulting complex, [FeII(tolpzph)2(NCS)2]·THF (1·THF). Structure determinations carried out on this discrete mononuclear complex, 1·THF, at 273 K (mostly high spin) and 100 K (mostly low spin) demonstrate this was successful, and that spin crossover (SCO) occurred on cooling. Subsequent magnetic measurements on 1·THF revealed that it shows highly sensitive and reversible solvent-dependent SCO, with T1/2(1·THF) = 255 K vs. T1/2(1) = 212 K (with SCO of 1 more abrupt and occurring with a 4 K hysteresis loop), a drop of 43 K due to THF loss. This is reversible over at least 10 cycles of re-solvating with THF followed by re-drying, so 1 ↔ 1·THF can be considered an 'on-off' THF sensor, monitored by the T1/2 reversibly shifting (by 43 K). Furthermore, quantitative sensing of the fractional amount of THF present in 1·nTHF, 0 ≤ n ≤ 1, is demonstrated. Monitoring the T1/2 and using TGA to quantify n(THF) revealed a linear dependence (25 data points; Pearson r2 = 0.93): T1/2 = 41.1n(THF) + 219. Finally, 1 is also shown to take up CHCl3 [T1/2(1·CHCl3) = 248 K], with a logarithmic T1/2 dependence on the fractional amount of CHCl3 present (10 data points; Pearson r2 = 0.98): T1/2 = 27.0 log10[n(CHCl3)] + 243. This study is a proof of principle that a (multi-use) quantitative sensor material based on spin crossover is feasible.

Spin state switching of metal complexes by visible light or hard X-rays

Different kinds of stimuli to induce spin crossover in iron compounds are covered, focusing on electromagnetic irradiation-induced spin state switching phenomena in mono- as well as multinuclear coordination compounds.

Enhanced kinetic stability of [Pd2L4]4+ cages through ligand substitution

[Pd₂(tripy)₄]⁴⁺ cage architectures (where tripy = 2,6-bis(pyridin-3-ylethynyl)pyridine) were made more kinetically robust in the presence of range of nucleophiles by the addition of amino groups in either the 2-(2A-tripy) or 3-(3A-tripy) positions of the tripy ligands' terminal pyridines, with the [Pd₂(2A-tripy)₄]⁴⁺ cage proving the most stable.