A Variety of Spin-Crossover Behaviors Depending on the Counter Anion: Two-Dimensional Complexes Constructed by NH⋅⋅⋅Cl− Hydrogen Bonds, [FeIIH3LMe]Cl⋅X (X = PF6−, AsF6−, SbF6−, CF3SO3−; H3LMe = Tris[2-{[(2-methylimidazol-4-yl)methylidene]amino}ethyl]amine)

Magnetoelectric and MIESST effects in spin crossover materials exhibiting symmetry-breaking

Giant magnetoelectric coupling and magnetic-field-induced spin state trapping (MIESST) were recently reported in spin crossover materials with polar phases. We discuss these phenomena considering the distinct contributions of the change of the molecular spin state, driven by the magnetic field, and the coupled structural symmetry-breaking during the stepwise change of electric polarisation or MIESST.

Spin crossover of a Fe(II) mononuclear complex induced by intermolecular factors involving chloride and solvent ordering

Two new salts of a mononuclear tripodal Fe(II) complex were prepared, using ClO4- and Cl-. The ClO4- sample (1) remained HS at low temperatures, similar to the previously reported BF4- analogue. Crystallising with the Cl- anion (2) led to a markedly different crystal packing arrangement, and engendered SCO activity. This has been correlated to the lower crystal packing density in 2 and the coordination complex conformational differences arising due to the packing motifs of 1 and 2. Further, solvent ordering effects have been proposed to facilitate spin transition behaviour in 2.

Iron(II) Complexes of 4-(Alkyldisulfanyl)-2,6-di(pyrazolyl)pyridine Derivatives. Correlation of Spin-Crossover Cooperativity with Molecular Structure Following Single-Crystal-to-Single-Crystal Desolvation

The complex salts [Fe(L 1)2]X2 (1X 2 ; L 1 = 4-(isopropyldisulfanyl)-2,6-bis(pyrazolyl)pyridine; X- = BF4 -, ClO4 -) form solvated crystals from common organic solvents. Crystals of 1X 2 ·Me2CO show abrupt spin transitions near 160 K, with up to 22 K thermal hysteresis. 1X 2 ·Me2CO cocrystallizes with other, less cooperative acetone solvates, which all transform into the same solvent-free materials 1X 2 ·sf upon exposure to air, or mild heating. Conversion of 1X 2 ·Me2CO to 1X 2 ·sf proceeds in a single-crystal to single-crystal fashion. 1X 2 ·sf are not isomorphous with the acetone solvates, and exhibit abrupt spin transitions at low temperature with hysteresis loops of 30-38 K (X- = BF4 -) and 10-20 K (X- = ClO4 -), depending on the measurement method. Interestingly, the desolvation has an opposite effect on the SCO temperature and hysteresis in the two salts. The hysteretic spin transitions in 1X 2 ·Me2CO and 1X 2 ·sf do not involve a crystallographic phase change but are accompanied by a significant rearrangement of the metal coordination sphere. Other solvates 1X 2 ·MeNO2, 1X 2 ·MeCN, and 1X 2 ·H2O are mostly isomorphous with each other and show more gradual spin-crossover equilibria near room temperature. All three of these lattice types have similar unit cell dimensions and contain cations associated into chains through pairwise, intermolecular S···π interactions. Polycrystalline [Fe(L 2)2][BF4]2·MeNO2 (2[BF 4 ] 2 ·MeNO2; L 2 = 4-(methyldisulfanyl)-2,6-bis(pyrazolyl)pyridine) shows an abrupt spin transition just above room temperature, with an unsymmetrical and structured hysteresis loop, whose main features are reversible upon repeated thermal scanning.

Cation modulated spin state and near room temperature transition within a family of compounds containing the same [FeL2]2- center

Spin-crossover (SCO) active compounds have received much attention due to their potential application in molecular devices. Herein, a family of solvent-free Fe^II compounds, formulated as (A)₂[FeL₂], (H₂L = pyridine-2,6-bi-tetrazolate, A = (Me)₄N⁺1, Et₂NH₂⁺2, ⁱPr₂NH₂⁺3 and ⁱPrNH₃⁺4), were synthesized and characterized. Single-crystal X-ray diffraction studies reveal that 1-4 are all supramolecular frameworks containing the same [FeL₂]^2- center, which is arranged into two packing modes via inter-molecular interactions, that is, a 3D architecture in 1 and 1D chain in 2-4. The spin states of 1-4 at different temperatures are assigned on the basis of the single-crystal X-ray diffraction data. Solid state magnetic investigations indicate that 1 and 4 exhibit a low spin state (below 350 K) and high spin state (2-400 K), respectively. 2 and 3 display clear SCO behavior in the measured temperature, but with different profiles and critical temperatures. 2 undergoes a complete gradual SCO with a critical temperature of T_1/2 = 260 K. 3 has an abrupt near room temperature transition between T_{1/2 cooling} = 278 K and T_{1/2 warming} = 286, centered at 282 K (9 °C). This study reveals the importance of organic cations in the modulation of SCO behavior and offers a new insight for the design of SCO compounds with near room temperature spin transitions.

Steric Quenching of Mn(III) Thermal Spin Crossover: Dilution of Spin Centers in Immobilized Solutions

Structural and magnetic properties of a new spin crossover complex [Mn(4,6-diOMe-sal2323)]+ in lattices with ClO4−, (1), NO3−, (2), BF4−, (3), CF3SO3−, (4), and Cl− (5) counterions are reported. Comparison with the magnetostructural properties of the C6, C12, C18 and C22 alkylated analogues of the ClO4− salt of [Mn(4,6-diOMe-sal2323)]+ demonstrates that alkylation effectively switches off the thermal spin crossover pathway and the amphiphilic complexes are all high spin. The spin crossover quenching in the amphiphiles is further probed by magnetic, structural and Raman spectroscopic studies of the PF6− salts of the C6, C12 and C18 complexes of a related complex [Mn(3-OMe-sal2323)]+ which confirm a preference for the high spin state in all cases. Structural analysis is used to rationalize the choice of the spin quintet form in the seven amphiphilic complexes and to highlight the non-accessibility of the smaller spin triplet form of the ion more generally in dilute environments. We suggest that lattice pressure is a requirement to stabilize the spin triplet form of Mn3+ as the low spin form is not known to exist in solution.

Co-crystallisation as a Modular Approach to the Discovery of Spin-Crossover Materials

Herein we present co-crystallisation as a strategy for materials discovery in the field of switchable spin crossover (SCO) systems. Using [Fe(3-bpp)2]·2A (where 3-bpp = 2,6-bis(pyrazol-3-yl)pyridine, A = BF4- / PF6-)...

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn³⁺ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn^III(3,5-diCl-sal₂(323))]BPh₄ reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P1 → P1_(1/2). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn³⁺ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P1 and P1 → P1_(1/2) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1→ P1_(1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

Cation- and Anion-Mediated Supramolecular Assembly of Bismuth and Antimony Tris(3-pyridyl) Complexes

The use of antimony and bismuth in supramolecular chemistry has been largely overlooked in comparison to the lighter elements of Group 15, and the coordination chemistry of the tripodal ligands [Sb(3-py)₃] and [Bi(3-py)₃] (L) containing the heaviest p-block element bridgehead atoms has been unexplored. We show that these ligands form a common hybrid metal–organic framework (MOF) structure with Cu(I) and Ag(I) (M) salts of weakly coordinating anions (PF₆^–, SbF₆^–, and OTf^–), composed of a cationic substructure of rhombic cage (M)₄(L)₄ units linked by Sb/Bi–M bonding. The greater Lewis acidity of Bi compared to Sb can, however, allows anion···Bi interactions to overcome Bi–metal bonding in the case of BF₄^–, leading to collapse of the MOF structure (which is also seen where harder metals like Li⁺ are employed). This study therefore provides insight into the way in which the electronic effects of the bridgehead atom in these ligand systems can impact their supramolecular chemistry.

The Lewis acidity of the Group 15 bridgehead atoms (E = Sb vs Bi) proves to be a decisive structural directing factor in the coordination of tris(3-pyridyl) ligands E(3-py)₃, being responsible for promoting or disfavoring E−metal or E···anion interactions.

Thermal and Magnetic Field Switching in a Two-Step Hysteretic MnIII Spin Crossover Compound Coupled to Symmetry Breakings

A Mn^III spin crossover complex with atypical two-step hysteretic thermal switching at 74 K and 84 K shows rich structural-magnetic interplay and magnetic-field-induced spin state switching below 14 T with an onset below 5 T. The spin states, structures, and the nature of the phase transitions are elucidated via X-ray and magnetization measurements. An unusual intermediate phase containing four individual sites, where 1 / 4 are in a pure low spin state, is observed. The splitting of equivalent sites in the high temperature phase into four inequivalent sites is due to a structural reorganization involving a primary and a secondary symmetry-breaking order parameter that induces a crystal system change from orthorhombic→monoclinic and a cell doubling. Further cooling leads to a reconstructive phase transition and a monoclinic low-temperature phase with two inequivalent low-spin sites. The coupling between the order parameters is identified in the framework of Landau theory.

The number and shape of lattice solvent molecules controls spin-crossover in an isomorphous series of crystalline solvate salts

Crystals of [FeL2][BF4]2·nMeCN (L = N-(2,6-di{pyrazol-1-yl}pyrid-4-yl)acetamide; n = 1 or 2) and [FeL2][ClO4]2·MeCN are isomorphous. When n = 1 the compounds exhibit an abrupt, hysteretic spin-transition below 200 K, but when n = 2 the material remains high-spin on cooling. [FeL2]X2·EtCN (X- = BF4- or ClO4-) are isomorphous with the MeCN solvates and undergo their spin-transition at almost the same temperature. However this now occurs in two-steps via a re-entrant mixed-spin intermediate phase, which correlates with crystallographic ordering of the bent propionitrile molecule.

Modulation of Mn3+ Spin State by Guest Molecule Inclusion

Spin state preferences for a cationic Mn3+ chelate complex in four different crystal lattices are investigated by crystallography and SQUID magnetometry. The [MnL1]+ complex cation was prepared by complexation of Mn3+ to the Schiff base chelate formed from condensation of 4-methoxysalicylaldehyde and 1,2-bis(3-aminopropylamino)ethane. The cation was crystallized separately with three polyatomic counterions and in one case was found to cocrystallize with a percentage of unreacted 4-methoxysalicylaldehyde starting material. The spin state preferences of the four resultant complexes [MnL1]CF3SO3·xH2O, (1), [MnL1]PF6·xH2O, (2), [MnL1]PF6·xsal·xH2O, (2b), and [MnL1]BPh4, (3), were dependent on their ability to form strong intermolecular interactions. Complexes (1) and (2), which formed hydrogen bonds between [MnL1]+, lattice water and in one case also with counterion, showed an incomplete thermal spin crossover over the temperature range 5-300 K. In contrast, complex (3) with the BPh4-, counterion and no lattice water, was locked into the high spin state over the same temperature range, as was complex (2b), where inclusion of the 4-methoxysalicylaldehyde guest blocked the H-bonding interaction.

Alternative Route Triggering Multistep Spin Crossover with Hysteresis in an Iron(II) Family Mediated by Flexible Anion Ordering

Multistep spin crossover (SCO) compounds have attracted much attention, since they can be great candidates for high-density multinary memory devices. The introduction of substituents, such as methyl (Me), chloro (Cl), bromo (Br), and methoxy (MeO) groups, at para positions to the phenyl-substituted tripodal N₆ ligand-coordinated SCO Fe^II material, [FeL^Ph](NTf₂)₂ [where L^Ph = tris(2-{[(1-phenyl-1H-1,2,3-triazol-4-yl)methylidene]amino}ethyl)amine and NTf₂ = bis(trifluoromethanesulfonyl)imide], affords a new family of solvent-free Fe^II complexes: [FeL^4-R-Ph](NTf₂)₂ {where L^4-R-Ph = tris[2-({[1-(4-R-phenyl)-1H-1,2,3-triazol-4-yl]methylidene}amino)ethyl]amine, where R = Me (1), Cl (2), Br (3), and MeO (4)}. 1 shows temperature invariant high-spin (HS) state, whereas the others show spin transitions with different characteristics, such as half-SCO (4), two-step SCO (3), and unusual three-step SCO with hysteresis (2). Mössbauer and X-ray absorption fine structure (XAFS) spectroscopic studies of them support the magnetic susceptibilities results. Density functional theory calculations indicate that the electronic effect of different substituents on magnetic properties is negligible in this Fe^II family. Single-crystal X-ray diffraction studies reveal that 1-4 has a similar packing arrangement with three-dimensional supramolecular network via intermolecular π-π and CH···π interactions between complex cations, and CH···X (X = O, N, and F) hydrogen bonding interactions between cations and inherently frustrated NTf₂ anions. Variable-temperature structural studies unveil a variety of stepped SCO behaviors of 2-4 and deactivation of SCO in 1 are governed by the regulation of ordering of NTf₂ counteranions through the subtle modification of terminal substituents of complex cations. Quantitative light-induced excited spin-state trapping (LIESST) effect was observed for 2-4 via green light irradiation (532 nm) at 10 K. This study opens up a new way for systematic control of magnetic response from no SCO to half-, two-step, and finally three-step SCO with hysteresis by precise tuning of the ordering of flexible NTf₂ anions included in the supramolecular network with potentially SCO-active complex cations.

Structural Evidence of Spin State Selection and Spin Crossover Behavior of Tripodal Schiff Base Complexes of tris(2-aminoethyl)amine and Related Tripodal Amines

A review of the tripodal Schiff base (SB) complexes of tris(2-aminoethyl)amine, Nap(CH2CH 2NH2)3 (tren), and a few closely related tripodal amines with Cr(II), Mn(III) (d4), Mn(II), Fe(III) (d5), Fe(II) (d6), and Co(II) (d7) is provided. Attention is focused on examination of key structural features, the M-Nimine, M-Namine, or M-O and M-Nap bond distances and Nimine-M-N(O) bite and C-Nap-C angles and how these values correlate with spin state selection and spin crossover (SCO) behavior. A comparison of these experimental values with density functional theory calculated values is also given. The greatest number, 132, of complexes is observed with cationic mononuclear iron(II) in a N6 donor set, Fe(II)N6. The dominance of two spin states, high spin (HS) and low spin (LS), in these systems is indicated by the bimodal distribution of histogram plots of Fe(II)-Nimine and Fe(II)-Nazole/pyridine bond distances and of Nimine–Fe(II)-Nazole/pyridine and C-Nap-C bond angles. The values of the two maxima, corresponding to LS and HS states, in each of these histograms agree closely with the theoretical values. The iron(II)-Nimine and iron(II)-Nazole/pyridine bond distances correlate well for these complexes. Examples of SCO complexes of this type are tabulated and a few of the 20 examples are discussed that exhibit interesting features. There are only a few mononuclear iron(III) cationic complexes and one is SCO. In addition, a significant number of supramolecular complexes of these ligands that exhibit SCO, intervalence, and chiral recognition are discussed. A summary is made regarding the current state of this area of research and possible new avenues to explore based on analysis of the present data.

Elucidating the Structural Chemistry of a Hysteretic Iron(II) Spin-Crossover Compound From its Copper(II) and Zinc(II) Congeners

Annealing [FeL₂ ][BF₄ ]₂ ⋅2 H₂ O (L=2,6-bis-[5-methyl-1H-pyrazol-3-yl]pyridine) affords an anhydrous material, which undergoes a spin transition at T_1/2 =205 K with a 65 K thermal hysteresis loop. This occurs through a sequence of phase changes, which were monitored by powder diffraction in an earlier study. [CuL₂ ][BF₄ ]₂ ⋅2 H₂ O and [ZnL₂ ][BF₄ ]₂ ⋅2 H₂ O are not perfectly isostructural but, unlike the iron compound, they undergo single-crystal-to-single-crystal dehydration upon annealing. All the annealed compounds initially adopt the same tetragonal phase but undergo a phase change near room temperature upon re-cooling. The low-temperature phase of [CuL₂ ][BF₄ ]₂ involves ordering of its Jahn-Teller distortion, to a monoclinic lattice with three unique cation sites. The zinc compound adopts a different, triclinic low-temperature phase with significant twisting of its coordination sphere, which unexpectedly becomes more pronounced as the crystal is cooled. Synchrotron powder diffraction data confirm that the structural changes in the anhydrous zinc complex are reproduced in the high-spin iron compound, before the onset of spin-crossover. This will contribute to the wide hysteresis in the spin transition of the iron complex. EPR spectra of copper-doped [Fe_0.97 Cu_0.03 L₂ ][BF₄ ]₂ imply its low-spin phase contains two distinct cation environments in a 2:1 ratio.

The Role of Vibrational Anharmonicity in the Computational Study of Thermal Spin Crossover

Spin crossover in transition metal complexes can be studied in great detail with computational chemistry. Over the years, the understanding has grown that the relative stability of high-spin (HS) versus low-spin (LS) states is a subtle balance of many factors that all need to be taken into account for a reliable description. Among the different contributions, the zero-point energy (ZPE) and the entropy play key roles. These quantities are usually calculated assuming a harmonic oscillator model for the molecular vibrations. We investigated the impact of including anharmonic corrections on the ZPE and the entropy and indirectly on the critical temperature of spin crossover. As test systems, we used a set of ten Fe(II) complexes and one Fe(III) complex, covering different coordination modes (mono-, bi-, and tri-dentate ligands), decreasing coordination number upon spin crossover, coordination by second- and third-row atoms, and changes in the oxidation state. The results show that the anharmonicity has a measurable effect, but it is in general rather small, and tendencies are not easily recognized. As a conclusion, we put forward that for high precision results, one should be aware of the anharmonic effects, but as long as computational chemistry is still struggling with other larger factors like the influence of the environment and the accurate determination of the electronic energy difference between HS and LS, the anharmonicity of the vibrational modes is a minor concern.

Solvent Dependent Spin-Crossover and Photomagnetic Properties in an Imidazolylimine FeII Complex

The synthesis and physico-chemical characterization of an Fe^II complex [Fe(L1)₃ ](ClO₄ )₂ ⋅CH₃ CN⋅0.5H₂ O, 1, incorporating a bidentate imidazolylimine-based ligand are reported. Complex 1 crystallises as the mer-isomer and the crystal lattice is replete with hydrogen bonding interactions between ClO₄ ^- anions, solvent molecules and imidazole N-H groups. Variable-temperature structural, magnetic, photomagnetic and optical reflectivity techniques have been deployed to fully characterise the spin-crossover (SCO) behaviour in 1 along with its desolvated phase, 1⋅desolv. Variable-temperature (1.8-300 K) magnetic-susceptibility measurements reveal a broad two-step full SCO for 1 (T_1/2 =158 and 184 K) and photomagnetic experiments at 10 K under white-light irradiation revealed complete photo-induced SCO. 1⋅desolv displays considerably different magnetic behaviour with sharp single-step SCO accompanied by a thermal hysteresis (T_1/2↑ =105 K, T_1/2↓ =95 K) in addition to full photo-induced SCO at lower temperatures.

Directing self-assembly in solution towards improved cooperativity in Fe(iii) complexes with amphiphilic tridentate ligands

An amphiphilic iron(iii) complex with a tridentate Schiff-base ligand was prepared by condensation of a hexadecyloxy functionalised salycylaldehyde with a diamine followed by complexation with FeCl2 and anion methathesis with NaClO4. The complex shows spin crossover both in the solid state and solution. However in solution self-assembly and consequently aggregation of individual molecules form concentration dependent particles with sizes of 300 nm for higher concentrations, or 5 nm for lower concentrations. Aggregate formation was confirmed by NANO-flex 180° DLS Size, scan-rate dependent cyclic voltammetry and scanning electron microscopy. Molecular simulations were used to investigate the self-assembly of the complex in solution, including the role of residual water molecules. The simulations showed the self-assembly of reverse micelle-like structures when a small water cluster is inserted in solution, whereas no large aggregates formed in dehydrated environments. The perchlorate anions were found near the metal centres, stabilizing the aggregates around the water pool. Simulations of pre-assembled structures further showed the lack of stability of large aggregates in the absence of water. The larger aggregates promoted efficient communication between the iron(iii) centres and the compound displayed spin crossover in solution at around 220 K with a 10 K hysteresis window, as measured by NMR and SQUID magnetometry.

Substituent dependence on the spin crossover behaviour of mononuclear Fe(ii) complexes with asymmetric tridentate ligands

Three mononuclear iron(ii) complexes of the formula [Fe^II(H₂L^1-3)₂](BF₄)₂·x(solv.) (H₂L^1-3 = 2-[5-(R-phenyl)-1H-pyrazole-3-yl] 6-benzimidazole pyridine; H₂L¹: R = 4-methylphenyl, H₂L², R = 2,4,6-trimethylphenyl, H₂L³, R = 2,3,4,5,6-pentamethylphenyl) (1, H₂L¹; 2, H₂L²; 3, H₂L³) with asymmetric tridentate ligands (H₂L^1-3) were synthesized and their structures and magnetic behaviour investigated. Significant structural distortions of the dihedral angles between phenyl and pyrazole groups were observed and found to depend on the nature of the substituent groups. Cryomagnetic studies reveal that 1 and 2 show gradual spin crossover behavior, while 3 remains in the high spin state between 1.8 and 300 K.

Solvent Effects on the Spin Crossover Properties of Iron(II) Imidazolylimine Complexes

A series of Fe(II) complexes, fac-[Fe(4-ima-Bp)3](Y)2⋅sol (Y = ClO4; sol = 3EtOH 1, 3MeOH 2; Y= BF4; sol = EtOH⋅4H2O 3, 4H2O 4 and 3.5MeCN 5) have been prepared and structurally and magnetically characterized. The low temperature structures of 1, 2 and 5 have been determined by X-ray crystallography with LS Fe(II) centres found in all cases. Extensive C–H···π interactions between the cations form 2D layers, which are linked to one another through N-H···O and O-H···O hydrogen bonds, resulting in high cooperativity. Despite 5 containing MeCN, N-H···O/F hydrogen bonds, and C–H···π and π-π interactions combine to give similar 2D layers. Magnetic measurements reveal moderately abrupt spin crossover for 1-4; becoming more gradual and only 50% complete in 1 due to solvent loss. The MeCN solvate shows more gradual SCO and reinforces how subtle changes in packing can significantly influence SCO behaviour.

High-temperature Spin Crossover of a Solvent-Free Iron(II) Complex with the Linear Hexadentate Ligand [Fe(L2-3-2Ph)](AsF6)2 (L2-3-2Ph = bis[N-(1-Phenyl-1H-1,2,3-triazol-4-yl)methylidene-2-aminoethyl]-1,3- propanediamine)

A novel mononuclear iron(II) complex with a linear hexadentate N6 ligand, containing two 1,2,3-triazole moieties, [Fe(L2-3-2Ph)](AsF6)2 (1), was synthesized (L2-3-2Ph = bis[N-(1-Phenyl-1H-1,2,3-triazol-4-yl)methylidene-2-aminoethyl]-1,3-propanediamine). Variable-temperature magnetic susceptibility measurements revealed a gradual one-step spin crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states above room temperature (T1/2 = 468 K). The spin transition was further confirmed by differential scanning calorimetry (DSC). A single-crystal X-ray diffraction study showed that the complex was in the LS state (S = 0) at room temperature (296 K). In the crystal lattice, a three-dimensional (3D) supramolecular network was formed by intermolecular CH⋯ and – interactions of neighboring complex cations [Fe(L2-3-2Ph)]2+. AsF6− ions were located interstitially in the 3D network of complex cations, with no solvent-accessible voids. The crystal structure at 448 K (mixture of HS and LS species) was also successfully determined thanks to the thermal stability of the solvent-free crystal.

Oxide versus Nonoxide Cathode Materials for Aqueous Zn Batteries: An Insight into the Charge Storage Mechanism and Consequences Thereof

Aqueous Zn-ion batteries, which are being proposed as large-scale energy storage solutions because of their unparalleled safety and cost advantage, are composed of a positive host (cathode) material, a metallic zinc anode, and a mildly acidic aqueous electrolyte (pH ≈ 3-7). Typically, the charge storage mechanism is believed to be reversible Zn²⁺ (de)intercalation in the cathode host, with the exception of α-MnO₂, for which multiple vastly different and contradicting mechanisms have been proposed. However, our present study, combining electrochemical, operando X-ray diffraction, electron microscopy in conjunction with energy-dispersive X-ray spectroscopy, and in situ pH evolution analyses on two oxide hosts-tunneled α-MnO₂ and layered V₃O₇·H₂O vis-à-vis two nonoxide hosts-layered VS₂ and tunneled Zn₃[Fe(CN)₆]₂, suggests that oxides and nonoxides follow two dissimilar charge storage mechanisms. While the oxides behave as dominant proton intercalation materials, the nonoxides undergo exclusive zinc intercalation. Stabilization of H⁺ on the hydroxyl-terminated oxide surface is revealed to facilitate the proton intercalation by a preliminary molecular dynamics simulation study. Proton intercalation for both oxides leads to the precipitation of layered double hydroxide (LDH)-Zn₄SO₄(OH)₆·5H₂O with a ZnSO₄/H₂O electrolyte and a triflate anion (CF₃SO₃^-)-based LDH with a Zn(SO₃CF₃)₂/H₂O electrolyte-on the electrode surface. The LDH precipitation buffers the pH of the electrolytes to a mildly acidic value, sustaining the proton intercalation to deliver large specific capacities for the oxides. Moreover, we also show that the stability of the LDH precipitate is crucial for the rechargeability of the oxide cathodes, revealing a critical link between the charge storage mechanism and the performance of the oxide hosts in aqueous zinc batteries.

Spin crossover and structural phase transition in homochiral and heterochiral Fe[(pybox)2]2+ complexes

Spin crossover and structural phase transition were discovered in three pairs of homochiral and heterochiral [Fe(pybox)₂]²⁺ diastereomers.

Synthesis and characterization of six-coordinate iron(II/III) 5,10,15,20-tetrakis(pentafluorophenyl) porphyrinato complexes with non-hindered imidazole ligands

Four bis-imidazole iron(II/III) 5,10,15,20-tetrakis(pentafluorophenyl)porphyrinato (TFPP) complexes, [Fe(TFPP)(1-MeIm)[Formula: see text]], [Fe(TFPP)(1-VinylIm)[Formula: see text]], [Fe(TFPP)(4-MeHIm)[Formula: see text]]Cl and [Fe(TFPP)(1-EtIm)[Formula: see text]]BF[Formula: see text] (1-MeIm [Formula: see text] 1-methylimidazole, 1-VinylIm [Formula: see text] 1-vinylimidazole, 4-MeHIm [Formula: see text] 4-methylimidazole and 1-EtIm [Formula: see text] 1-ethylimidazole) were synthesized and characterized by single-crystal X-ray and UV-vis spectroscopy. A negative correlation is found between the absolute imidazole orientation ([Formula: see text] and the Fe–N[Formula: see text] distance for the [Fe(II)(Porph)(Im)[Formula: see text]] (Im [Formula: see text] 1-MeIm or 4-MeHIm) complexes where the smaller [Formula: see text] angle corresponds to a longer axial distance. Hydrogen bonding, which might affect the orientations of the axial imidazoles is found for Fe(TFPP)(4-MeHIm)[Formula: see text]]Cl (A and B). The autoreduction of [Fe(III)(TFPP)]Cl to [Fe(II)(TFPP)(1-MeIm)[Formula: see text]] with 1-methylimidazole has been monitored by UV-vis spectroscopic titration.

Varied spin crossover behaviour in a family of dinuclear Fe(ii) triple helicate complexes

Dinuclear triple-helicate complexes of the formula [Fe2L3](BF4)4·solv (solv = CH3CN, CHCl3, H2O) have been synthesised and structurally characterised. The bis-bidentate ligands, L, present either strong-field 2-pyridylimine (1) or weaker-field 2-imidazolylimine (2) and 4-imidazolylimine (3) coordination spheres about Fe(ii) centres in an octahedral geometry. Whereas 1 is pervasively diamagnetic, spin crossover (SCO) behaviour is observed in 2 and 3 and has been studied using variable-temperature structural, UV-visible spectroscopic, magnetic and photo-magnetic techniques. Variable-temperature (1.8-400 K) magnetic-susceptibility measurements reveal the T1/2 values of 2 and 3 to be strongly dependent upon the solvent and degree of solvation. Photomagnetic studies at 10 K under white-light irradiation revealed an inefficient photo-induced SCO in 2, but full switching in 3.

An Incomplete Spin Transition Associated with a Z'=1→Z'=24 Crystallographic Symmetry Breaking

Crystalline [FeL2 ][BF4 ]2 ⋅Me2 CO (L=N-[2,6-di{pyrazol-1-yl}pyrid-4-yl]acetamide) is high-spin at room temperature, and undergoes an abrupt, hysteretic spin-crossover at T1/2 =137 K (ΔT1/2 =14 K) that proceeds to about 50 % completeness. This is associated with a crystallographic phase transition, from phase 1 (P21 /c, Z=4) to phase 2 (P21 , Z=48). The cations associate into chains in the crystal through weak intermolecular π⋅⋅⋅π interactions. Phase 2 contains a mixture of high-spin and low-spin molecules, which are grouped into triads along these chains. The perchlorate salt [FeL2 ][ClO4 ]2 ⋅Me2 CO also adopts phase 1 at room temperature but undergoes a different phase transition near 135 K to phase 3 (P21 /c, Z=8) without a change in spin state.

Synthesis, structure, and spin crossover above room temperature of a mononuclear and related dinuclear double helicate iron(ii) complexes

A mononuclear iron(ii) complex, [Fe^II(L1^Me)₂](PF₆)₂ (1), and a related dinuclear one, [Fe(L2^C2)₂](PF₆)₄·5H₂O·MeCN (2), were prepared, where L1^Me is a tridentate N₃ coordinating Schiff-base ligand, 1-methyl-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and L2^C2 is a dimeric form ligand of L1^Me, 1,1'-(1,2-ethanediyl)bis-1,2,3-triazol-4-yl-methylideneamino-2-ethylpyridine, and their magnetostructural relationships were investigated. Mononuclear 1 shows a gradual one-step spin crossover (SCO) from the low-spin (LS) state (S = 0) at room temperature (RT) to the mostly high-spin (HS) state (S = 2) at 470 K spanning a temperature range of more than 170 K, while dinuclear 2 shows an anomalous two-step SCO with a gradual transition in a first step from the LS-LS state at RT to around 432 K and then an abrupt spin transition centred at 432 K with 11 K wide hysteresis in a second step (T_1/2↑ = 437 K and T_1/2↓ = 426 K) reaching the mostly HS-HS state. Single crystal X-ray structure analyses revealed that the iron(ii) centre of mononuclear 1 has an N6 octahedral coordination environment with two tridentate L1^Me ligands, while the complex-cation of 2 has a dinuclear double helicate architecture as though a dimeric form of two complex-cations of 1 replaced their methyl group with a bridging ethylene chain. Both complexes have a cationic two-dimensional (2D) structure in which the 2D layer of mononuclear 1 is constructed of intermolecular CHN hydrogen bonds between adjacent complex-cations, while that of dinuclear 2 is constructed of both intermolecular π-π and CH/π interactions between neighboring helicates.

Stabilisation of small mono- and bimetallic gold–silver nanoparticles using calix[8]arene derivatives

We report a facile approach to synthesise small Au–Ag alloyed nanoparticles using a new cali[8]arene derivative as a stabiliser.

Pybox-Iron(II) Spin-Crossover Complexes with Substituent Effects from the 4-Position of the Pyridine Ring (Pybox = 2,6-Bis(oxazolin-2-yl)pyridine)

Spin-crossover (SCO) behavior of a series of [Fe(X-pybox)2](ClO4)2 was investigated, where X-pybox stands for 4-X-substituted 2,6-bis(oxazolin-2-yl)pyridine with X = H, Cl, Ph, CH3O, and CH3S. We confirmed that the mother compound [Fe(H-pybox)2](ClO4)2 underwent SCO above room temperature. After X was introduced, the SCO temperatures (T1/2) were modulated as 310, 230, and 330 K for X = Cl, Ph, and CH3S, respectively. The CH3O derivative possessed the high-spin state down to 2 K. Crystallographic analysis for X = H, Cl, CH3O, and CH3S was successful, being consistent with the results of the magnetic study. Distorted coordination structures stabilize the HS (high-spin) state, and the highest degree of the coordination structure distortion is found in the CH3O derivative. A plot of T1/2 against the Hammett substituent constant σp showed a positive relation. Solution susceptometry was also performed to remove intermolecular interaction and rigid crystal lattice effects, and the T1/2’s were determined as 260, 270, 240, 170, and 210 K for X = H, Cl, Ph, CH3O, and CH3S, respectively, in acetone. The substituent effect on T1/2 became very distinct, and it is clarified that electron-donating groups stabilize the HS state.

Formation of local spin-state concentration waves during the relaxation from a photoinduced state in a spin-crossover polymer

The complex relaxation from the photoinduced high-spin phase (PIHS) to the low-spin phase of the bimetallic two-dimensional coordination spin-crossover polymer [Fe[(Hg(SCN)₃)₂](4,4'-bipy)₂]_n is reported. During the thermal relaxation, commensurate and incommensurate spin-state concentration waves (SSCWs) form. However, contrary to the steps forming at thermal equilibrium, associated with long-range SSCW order, the SSCWs forming during the relaxation from the PIHS phase correspond to short-range order, revealed by diffuse X-ray scattering. This is interpreted as resulting from the competition between the two types of SSCW order and another structural symmetry breaking, due to ligand ordering, occurring at low temperature and precluding long-range SSCW order.

The molecular and supramolecular aspects in mononuclear manganese(iii) Schiff-base spin crossover complexes

A cooperative SCO process with a hysteresis width of 18 K is the largest one reported in d⁴ SCO chemistry.

The role of symmetry breaking in the structural trapping of light-induced excited spin states

Six isostructural compounds show differing agreement with the correlation T(LIESST) = T₀ − 0.3T_1/2 (red line; LIESST = Light-Induced Excited Spin-State Trapping). This reflects crystallographic symmetry breaking under LIESST conditions, exhibited by one of the compounds but not by two others.

Hydroxo-bridged diiron(iii) and dimanganese(iii) bisporphyrins: modulation of metal spins by counter anions

A brief account has been presented on how the inter-heme interactions in μ-hydroxo diiron(iii) bisporphyrins and counter anions can induce significant change in the structure and properties including the iron spin state without affecting the overall topology.

Remarkable Anion-Dependent Spin-State Switching in Diiron(III) μ-Hydroxo Bisporphyrins: What Role do Counterions Play?

Addition of 2,4,6-trinitrophenol (HTNP) to an ethene-bridged diiron(III) μ-oxo bisporphyrin (1) in CH₂ Cl₂ initially leads to the formation of diiron(III) μ-hydroxo bisporphyrin (2⋅TNP) with a phenolate counterion that, after further addition of HTNP or dissolution in a nonpolar solvent, converts to a diiron(III) complex with axial phenoxide coordination (3⋅(TNP)₂ ). The progress of the reaction from μ-oxo to μ-hydroxo to axially ligated complex has been monitored in solution by using ¹ H NMR spectroscopy because their signals appear in three different and distinct spectral regions. The X-ray structure of 2⋅TNP revealed that the nearly planar TNP counterion fits perfectly within the bisporphyrin cavity to form a strong hydrogen bond with the μ-hydroxo group, which thus stabilizes the two equivalent iron centers. In contrast, such counterions as I₅ , I₃ , BF₄ , SbF₆ , and PF₆ are found to be tightly associated with one of the porphyrin rings and, therefore, stabilize two different spin states of iron in one molecule. A spectroscopic investigation of 2⋅TNP has revealed the presence of two equivalent iron centers with a high-spin state (S=5/2) in the solid state that converts to intermediate spin (S=3/2) in solution. An extensive computational study by using a range of DFT methods was performed on 2⋅TNP and 2⁺ , and clearly supports the experimentally observed spin flip triggered by hydrogen-bonding interactions. The counterion is shown to perturb the spin-state ordering through, for example, hydrogen-bonding interactions, switched positions between counterion and axial ligand, ion-pair interactions, and charge polarization. The present investigation thus provides a clear rationalization of the unusual counterion-specific spin states observed in the μ-hydroxo bisporphyrins that have so far remained the most outstanding issue.

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 γHS . Between the fully HS (γHS =1) and LS (γHS =0) phases, two steps associated with different ordering appear in terms of spin-state concentration waves (SSCW). On the γHS ≈0.5 step, a periodic SSCW forms with a HS-LS-HS-LS sequence. On the γHS ≈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.