Spin-State Versatility in a Series of Fe4 [2 × 2] Grid Complexes: Effects of Counteranions, Lattice Solvent, and Intramolecular Cooperativity

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.

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.

Short- vs. long-range elastic distortion: structural dynamics of a [2 × 2] tetrairon(II) spin crossover grid complex observed by time-resolved X-Ray crystallography

Spin crossover complexes (SCO) are among the most studied molecular switches due to their potential use in displays, sensors, actuators and memory components. A prerequisite to using these materials is the understanding of the structural changes following the spin transition at out-of-equilibrium conditions. So far, out-of-equilibrium studies in SCO solids have been focused on mononuclear complexes, though a growing number of oligonuclear SCO complexes showing cooperative effects are being reported. Here, we use time-resolved pink Laue crystallography to study the out-of-equilibrium dynamics of a [2 × 2] tetranuclear metallogrid of the form [FeII4LMe4](BF4)4·2MeCN ([LMe]- = 4-methyl-3,5-bis{6-(2,2'-bipyridyl)}pyrazolate). The out-of-equilibrium spin state switching induced by a ps laser pulse demonstrates that the metallogrid exhibits a multi-step response similar to that reported for mononuclear complexes. Contrary to the mononuclear complexes, the metallogrid shows two types of elastic distortions at different time scales. The first is a short-range distortion that propagates over the entire Fe4 grid complex during the ps time scale, and it is caused by the rearrangement of the coordination sphere of the photo-switching ion and the constant feedback between strongly linked metal ions. The second is a long-range distortion caused by the anisotropic expansion of the lattice during the ns time scale, observed in mononuclear materials. The structural analysis demonstrates that the long-range prevails over the short-range distortion, inducing the largest deformation of both the entire grid and the coordination sphere of each metal ion. The present study sheds light on the out-of -equilibrium dynamics of a non-cooperative oligonuclear complex.

Metal-to-metal communication during the spin state transition of a [2 × 2] Fe(II) metallogrid at equilibrium and out-of-equilibrium conditions

Spin crossover (SCO) complexes are prototypes of materials with bi- or multi-stability in the solid state. The structural evolution during their spin transition is a key feature to establish the foundations of how to utilize this type of material. So far, ultrafast time-resolved structural investigations of SCO solids have been focused on monometallic complexes, though an increasing number of oligometallic SCO complexes showing cooperativity effects are being reported. Here, we used single crystal X-ray crystallography and time-resolved pink Laue photocrystallography to study the molecular reorganisation during the thermal and photoinduced SCO of a [2 × 2] tetranuclear metallogrid of the form [FeII4LMe4](BF4)4·2MeCN ([LMe]- = 4-methyl-3,5-bis{6-(2,2'-bipyridyl)}pyrazolate). A multitemperature crystallographic investigation on single crystals reveals an effective communication between the metal centres during thermal SCO, observed by the simultaneous transformation of the coordination polyhedra of both crystallographic-symmetry independent metal atoms accompanying the SCO in only one of them. Time-resolved photocrystallography results reveal the different molecular responses between mononuclear and oligonuclear complexes, after light irradiation with a picosecond laser pulse. While mononuclear SCO complexes reorganise once during the first nanosecond after excitation, the tetranuclear metallogrid exhibits a multiple structural rearrangement in the same span of time. Such behaviour is attributed to the elastic communication between metal atoms, which allows the propagation of a short-range elastic distortion over the entire Fe4 grid complex. The present study sheds light on the importance of strong elastic coupling of metal atoms during the correlated spin transition of oligometallic complexes.

Cooperative Spin Transitions Triggered by Phonons in Metal Complexes Coupled to Molecular Vibrations

The present article is a short overview of the theoretical modeling of spin transitions in polymetallic compounds. As distinguished from many insightful reviews on this topic, the present work is focused on the nature of cooperative interaction of the metal clusters in molecular crystals with emphasis at the physical role of molecular vibrations and phonons. The underlying model assumes that the cooperativity is triggered by phonons while the metal centers are coupled to molecular vibrations. It is demonstrated that the suggested model gives a satisfactory description of the observed spin transitions in mono-, bi- and tetranuclear compounds. In the framework of the described approach, we discuss the experimental data on spin crossover in the mononuclear [Fe(ptz)6](BF4)2, binuclear [{Fe(bt)(NCS)2}2bpym] and tetranuclear [Fe(tpa){N(CN)2}]4·(BF4)4·(H2O)2 compounds containing iron ions. The approach is also applied to the description of the charge-transfer-induced spin transition in the [{(Tp)Fe(CN)3}{Co-(PY5Me2)}](CF3SO3) complex.

Light actuated single-chain magnet with magnetic coercivity

A cyanide-bridged {Fe2Co}-based coordination polymer was synthesized. It showed photo-induced slow relaxation of magnetization and a coercive field of 400 Oe.

Tuning of spin-crossover behavior in two cyano-bridged mixed-valence FeIII2FeII trinuclear complexes based on a TpR ligand

With the use of TpR derivatives, we have demonstrated the modulation in the SCO behavior in two analogous trinuclear complexes. Moreover, a change in the spin transition temperature via solvent loss is observed.

Macrocycle based dinuclear dysprosium(III) single molecule magnets with local D5h coordination geometry

Targeted approaches for manipulating the coordination geometry of lanthanide ions are a promising way to synthesize high-performance single-molecule magnets (SMMs), but most of the successful examples reported to date focus on mononuclear complexes. Herein, we describe a strategy to assemble dinuclear SMMs with Dy^III ions in approximate D_5h coordination geometry based on pyrazolate-based macrocyclic ligands with two binding sites. A Dy4 complex with a rhomb-like arrangement of four Dy^III as well as two dinuclear complexes having axial chlorido ligands (Dy2·Cl and Dy2*·Cl) were obtained; in the latter case, substituting Cl^- by SCN^- gave Dy2·SCN. Magneto-structural studies revealed that the μ-OH bridges with short Dy-O bonds dominate the magnetic anisotropy of the Dy^III ions in centrosymmetric Dy4 to give a vortex type diamagnetic ground state. Dynamic magnetic studies of Dy4 identified two relaxation processes under zero field, one of which is suppressed after applying a dc field. For complexes Dy2·Cl and Dy2*·Cl, the Dy^III ions feature almost perfect D_5h environment, but both complexes only behave as field-induced SMMs (U_eff = 19 and 25 K) due to the weak axial Cl^- donors. In Dy2·SCN additional MeOH coordination leads to a distorted D_2d geometry of the Dy^III ions, yet SMMs properties at zero field are observed due to the relatively strong axial ligand field provided by SCN^- (U_eff = 43 K). Further elaboration of preorganizing macrocyclic ligands appears to be a promising strategy for imposing a desired coordination geometry with parallel orientation of the anisotropy axes of proximate Dy^III ions in a targeted approach.

A ring of grids: a giant spin-crossover cluster

Mononuclear and icosanuclear spin-crossover complexes, [Fe^II(HL)₂](BF₄)₂ (1) and [FeII20(L)₂₄](BF₄)₁₆ (2), were synthesized using an asymmetric multidentate ligand (HL). 1 has a bis-chelate structure with two protonated ligands, while 2 has a ring-shape structure comprising four [2 × 2] grid moieties and four mononuclear units.

A Family of Heterobimetallic Cubes Shows Spin-Crossover Behaviour Near Room Temperature

Using 4-(4'-pyridyl)aniline as a simple organic building block in combination with three different aldehyde components together with metal(II) salts gave three different Fe8 Pt6 -cubes and their corresponding Zn8 Pt6 analogues by employing the subcomponent self-assembly approach. Whereas the use of zinc(II) salts gave rise to diamagnetic cages, iron(II) salts yielded metallosupramolecular cages that show spin-crossover behaviour in solution. The spin-transition temperature T1/2 depends on the incorporated aldehyde component, giving a construction kit for the deliberate synthesis of spin-crossover compounds with tailored transition properties. Incorporation of 4-thiazolecarbaldehyde or N-methyl-2-imidazole-carbaldehyde yielded cages that undergo spin-crossover around room temperature whereas the cage obtained using 1H-4-imidazolecarbaldehyde shows a spin-transition at low temperatures. Three new structures were characterized by synchrotron X-ray diffraction and all structures were characterized by mass spectrometry, NMR and UV/Vis spectroscopy.

Electron-Induced Spin-Crossover in Self-Assembled Tetramers

The spin crossover compound Fe(H₂B(pyrazole)(pyridylpyrazole))₂ was investigated in detail on Ag(111) with scanning tunneling microscopy (STM). A large fraction of the deposited molecules condenses into gridlike tetramers. Two molecules of each tetramer may be converted between two states by current injection. We attribute this effect to a spin transition. This interpretation is supported by control experiments on the analogous, magnetically passive Zn compound that forms virtually identical tetramers but exhibits no switching. The switching yields were studied for various electron energies, and the resulting values exceed those reported from other SCO systems by 2 orders of magnitude. The other two molecules of a tetramer were immutable. However, they may be used as contacts for current injection that leads to conversion of one of their neighbors. This "remote" switching is fairly efficient with yields reduced by only one to two orders of magnitude compared to direct excitation of a switchable molecule. We present a model of the tetramer structure that reproduces key observations from the experiments. In particular, sterical blocking prevents spin crossover of two molecules of a tetramer. Density functional theory calculations show that the model indeed represents a minimum energy structure. They also reproduce STM images and corroborate a remote-switching mechanism that is based on electron transfer between molecules.

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.

Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(formazanate) Iron(II) Complexes

The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Δ_O) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T_1/2 above room temperature (300–368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the π-acceptor properties of the formazanate ligand, resulting in an “inverted” ligand field, with an approximate “two-over-three” splitting of the d-orbitals and a high degree of metal–ligand covalency due to metal → ligand π-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C–Ar or N–Ar rings, which is ascribed to the opposing effect on metal–ligand σ- and π-bonding.

Guest-Dependent Pressure-Induced Spin Crossover in FeII 4 [MIV (CN)8 ]2 (M=Mo, W) Cluster-Based Material Showing Persistent Solvent-Driven Structural Transformations

Discrete molecular species that can perform certain functions in response to multiple external stimuli constitute a special class of multifunctional molecular materials called smart molecules. Herein, cyanido-bridged coordination clusters {[Fe^II (2-pyrpy)₂ ]₄ [M^IV (CN)₈ ]₂ }⋅4 MeOH⋅6 H₂ O (M=Mo (1 solv), M=W (2 solv) and 2-pyrpy=2-(1-pyrazolyl)pyridine are presented, which show persistent solvent driven single-crystal-to-single-crystal transformations upon sorption/desorption of water and methanol molecules. Three full desolvation-resolvation cycles with the concomitant change of the host molecules do not damage the single crystals. More importantly, the Fe₄ M₂ molecules constitute a unique example where the presence of the guests directly affects the pressure-induced thermal spin crossover (SCO) phenomenon occurring at the Fe^II centres. The hydrated phases show a partial SCO with approximately two out-of-four Fe^II centres undergoing a gradual thermal SCO at 1 GPa, while in the anhydrous form the pressure-induced SCO effect is almost quenched with only 15 % of the Fe^II centres undergoing high-spin to low-spin transition at 1 GPa.

Exploring the light-induced dynamics in solvated metallogrid complexes with femtosecond pulses across the electromagnetic spectrum

Oligonuclear complexes of d⁴-d⁷ transition metal ion centers that undergo spin-switching have long been developed for their practical role in molecular electronics. Recently, they also have appeared as promising photochemical reactants demonstrating improved stability. However, the lack of knowledge about their photophysical properties in the solution phase compared to mononuclear complexes is currently hampering their inclusion into advanced light-driven reactions. In the present study, the ultrafast photoinduced dynamics in a solvated [2 × 2] iron(II) metallogrid complex are characterized by combining measurements with transient optical-infrared absorption and x-ray emission spectroscopy on the femtosecond time scale. The analysis is supported by density functional theory calculations. The photocycle can be described in terms of intra-site transitions, where the Fe^II centers in the low-spin state are independently photoexcited. The Franck-Condon state decays via the formation of a vibrationally hot high-spin (HS) state that displays coherent behavior within a few picoseconds and thermalizes within tens of picoseconds to yield a metastable HS state living for several hundreds of nanoseconds. Systematic comparison with the closely related mononuclear complex [Fe(terpy)₂]²⁺ reveals that nuclearity has a profound impact on the photoinduced dynamics. More generally, this work provides guidelines for expanding the integration of oligonuclear complexes into new photoconversion schemes that may be triggered by ultrafast spin-switching.

Construction of SCO-Active Fe(II) Mononuclear Complexes from the Thio-pybox Ligand

The development of new spin-crossover complexes provides novel promising switching materials with significant potential at the molecular level. Ter-imine-type molecules represent one of the important classes of ligands in creating SCO-active complexes. Herein we report a family of mononuclear Fe(II) SCO-active compounds constructed from a new type of ter-imine ligand named the thio-pybox ligand (2,6-bis(4,4-dimethyl-4,5-dihydrothiazol-2-yl)pyridine, L¹). Through the variation of counteranions, some cases display complete SCO and with T_1/2 near ambient temperature. Among them, annealed [Fe^II(L¹)₂](ClO₄)₂ [1(ClO₄)] shows T_1/2↓ and T_1/2↑ as 319 and 349 K, respectively. The wide thermal hysteresis of ΔT = 30 K originated from the weak interaction between complex cations and counteranions in the crystal lattice. Impressively, its high-spin population can be increased considerably by annealing at high temperature. The metastable high-spin phase is stable in the successive magnetic measurements and would gradually relax to its initial state with high population of low-spin configuration at ambient temperature. In acetonitrile-diluted solution, 1(ClO₄) still maintains SCO with an estimated T_1/2 at 240 K. Differential scanning calorimetry discloses the structural phase at around 355 K in the first heating process and the increase in the high-spin population concomitant with annealing was also corroborated by ⁵⁷Fe Mössbauer measurements. Additionally, the influences on SCO by counteranion and ligand structure are investigated, which show that the fine tuning of complex structures can affect the behavior of the spin state significantly. Finally, magneto-structural correlation studies were performed on the structures of 1(ClO₄) and its oxygen analogue at multiple temperatures. The analyses of some structural parameters, including terminal N···N donor separation, bite angle, patulous angle, and the root mean squared deviation indicate that the replacement of the oxygen atom with a sulfur atom can effectively improve the flexibility and release the steric strain and thus tune the SCO toward ambient temperature. Our research demonstrates the rational design of the ligand can lead to new SCO-active compounds with high performance.

Revealing Hot and Long-Lived Metastable Spin States in the Photoinduced Switching of Solvated Metallogrid Complexes with Femtosecond Optical and X-ray Spectroscopies

An atomistic understanding of the photoinduced spin-state switching (PSS) within polynuclear systems of d⁴-d⁷ transition metal ion complexes is required for their rational integration into light-driven reactions of chemical and biological interest. However, in contrast to mononuclear systems, the multidimensional dynamics of the PSS in solvated molecular arrays have not yet been elucidated due to the expected complications associated with the connectivity between the metal centers and the strong interactions with the surroundings. In this work, the PSS in a solvated triiron(II) metallogrid complex is characterized using transient optical absorption and X-ray emission spectroscopies on the femtosecond time scale. The complementary measurements reveal the photoinduced creation of energy-rich (hot) and long-lived quintet states, whose dynamics differ critically from their mononuclear congeners. This finding opens major prospects for developing novel schemes in solution-phase spin chemistry that are driven by the dynamic PSS process in compact oligometallic arrays.

Two azido-bridged [2×2] cobalt(ii) grids featuring single-molecule magnet behaviour

Two Co(ii) [2 × 2] grid-like clusters containing both pyridazine and azido bridges were reported to exhibit overall intramolecular ferromagnetic coupling and field-induced single-molecule magnet behavior with the effective energy barriers up to 56 K.

Site-Selective Photoswitching of Two Distinct Magnetic Chromophores in a Propeller-Like Molecule To Achieve Four Different Magnetic States

Magnetic photoswitching is a highly important but relatively rare phenomenon for enabling optical writing/reading of the magnetic state of a molecule. In this work, an unprecedented site-selective double photoswitching is reported from the assembly of two different "photomagnetic chromophores" into a single hexanuclear molecule: namely, a spin-crossover Fe(II) center exhibiting light-induced excited spin state trapping (LIESST) and a photochemically active octacyanometalate(IV) unit. Four different magnetization levels are accessible through the appropriate combination of violet/red light and temperature, results that highlight the potential of photomagnetic molecules as future molecular memory cells.

Heterometallic Ru2Co2 [2 × 2] Grid with Localized Single Molecule Magnet Behavior

Metal complexes with a [n × n] gridlike structure are discussed as attractive building blocks for various materials chemistry applications in molecular nanotechnology and electronics, which often rely on the grids' magnetic and redox properties. Most of the known metallogrids are homometallic, though heterometallic systems that comprise two or more different metals promise higher level functionalities. However, heterometallic [n × n] grids are relatively rare, mostly because of the more challenging synthetic strategies. To that end a new heterometallic [2 × 2] grid complex [L₄Ru₂Co₂](BF₄)₄ (2) based on a known pyrazolate-bridged bis(tridentate) compartmental N-donor ligand [L]^- is presented in this work, along with its doubly oxidized congener [L₄Ru₂Co₂](BF₄)₆ (3). In order to prevent scrambling of the different metal ions, a stepwise synthetic approach was implemented in which an inert Ru^II "corner complex" [(HL)₂Ru](BF₄)₂ (1) was isolated first, followed by addition of the more labile Co^II. This exclusively yields the desired [L₄Ru₂Co₂]⁴⁺ with anti-topology, viz., with the Ru^II and Co^II ions situated at opposite corners of the [2 × 2] grid, as confirmed by single crystal X-ray diffraction. 2 can be sequentially oxidized four times, first at the Co vertices and then at the Ru vertices. ¹H NMR spectroscopy as well as ESI mass spectrometry evidenced integrity of the [L₄Ru₂Co₂]^4+/6+ grids in solution. Structural and magnetic analyses revealed that paramagnetic 2 features LS-Ru^II and HS-Co^II ions (LS = low-spin, HS = high-spin) whereas LS-Ru^II and LS-Co^III ions are present in diamagnetic 3. The LS-Ru^II ions in 2 serve to magnetically isolate the HS-Co^II whose coordination geometry is strongly distorted from octahedral. A large and negative axial zero-field splitting value (D = -64 cm^-1) for the local S = 3/2 ions is shown to lead to single molecule magnetic (SMM) properties characterized by a barrier to spin inversion of U_eff = 8.8 cm^-1 and a single relaxation process with τ_o = 3.1 × 10^-5 s. Transition metal [2 × 2] grid complexes showing SMM behavior are extremely rare, and this is the first heterometallic 3d/4d grid system featuring such a magnetic signature.

Tetranuclear MnII, CoII, CuII and ZnII grid complexes of an unsymmetrical ditopic ligand: synthesis, structure, redox and magnetic properties

Synthesis towards the preparation of an unsymmetrical binucleating Schiff-base ligand, (H₂L) is discussed. It has been synthesized by reacting methyl 6-(hydrazinecarbonyl)picolinate with 2-hydroxy acetophenone. The ligand (H₂L) comprises two different asymmetric binding pockets; however, when reacted with Mn(ii), Co(ii), Cu(ii) and Zn(ii) salts, very stable self-assembled [2 × 2] grid complexes form regardless of the employed metal-to-ligand ratio. The obtained complexes [Mn₄L₄]·(CH₃CN) (1), [Co₄L₄]·(CHCl₃) (2), [Cu₄L₄]·(CHCl₃) (3) and [Zn₄L₄] (4) have been fully characterized by physicochemical methods, including ESI mass spectrometry and X-ray crystallographic analyses, and their EPR, magnetic and redox properties are discussed. All discussed complexes self-assembled in a 'head-to-tail' fashion leading to [2 × 2] grid architectures. Mn, Co and Cu grid complexes show weak to moderate antiferromagnetic coupling among the four metal centers. The high stability of the grid structures is in line with the lack of any observable dissociation or exchange between metal ions in solution. Complexes 1 and 2 show four quasi-reversible to irreversible oxidative responses in cyclic voltammograms at a glassy carbon working electrode in 1,2-dichlorobenzene and at a platinum working electrode in dichloromethane, respectively.

Dicyanometalates as Building Blocks for Multinuclear Iron(II) Spin-Crossover Complexes

A synthetic strategy featuring dicyanometalates [M(CN)₂]^- (M = Ag, Au) as N-coordinating ditopic linkers connecting partially blocked Fe^II centers has been employed to produce heterometallic hexanuclear complexes, which exhibit spin-crossover (SCO) behavior at the Fe^II sites. The reaction between tris(2-pyridylmethyl)amine (tpma)-capped Fe^II ions and [Ag(CN)₂]^- proceeded with partial decomposition of the dicyanoargentate and led to the formation of {[Fe(tpma)]₄(μ-CN)₂[μ-Ag(CN)₂]₂}(ClO₄)₄·3H₂O (1), in which both [Ag(CN)₂]^- and CN^- act as bridging ligands, and the opposite [Ag(CN)₂]^- bridges are engaged in a pronounced argentophilic d¹⁰-d¹⁰ interaction. In an analogous synthesis, the more stable [Au(CN)₂]^- species remained intact and furnished the complex {[Fe(tpma)]₂[μ-Au₂(CN)₄]₂} (2), which features two Fe^II centers bridged by two [Au₂(CN)₄]^2- dimers. The use of S,S'-bis(2-pyridylmethyl)-1,2-thioethane (bpte) as a mixed-donor, N₂S₂-coordinating capping ligand yielded {[Fe(bpte)]₂[μ-Au₂(CN)₄]₂} (3), with a structure analogous to that of 2. Variable-temperature magnetic susceptibility measurements revealed that complexes 1-3 exhibit an onset of SCO above 350 K. Measurements above 400 K further confirmed the occurrence of a gradual spin-state conversion for complex 2.

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.

Heterometallic grids: synthetic strategies and recent advances

Supramolecular metallogrid complexes are metalloclusters involving metal ions in planar array arrangements and organic ligands in perpendicular arrangements at each corner of the metal sites. Such essentially metal ion arrays have attracted great attention in the last three decades owing to their variety of interesting optical, electronic and magnetic properties. Among them, metallogrids containing more than one type of metal, that is heterometallic grids, are rare but have more potential to engineer a higher level functionality into one molecule. However, until now, only dozens of heterometallic grids have been reported without any specific review. Herein, we aim to give an overview of the assembly strategies, the physicochemical properties, and finally some perspectives on the future development of heterometallic grids.

A Cuprous [4 × 4] Grid: Single-Crystal to Single-Crystal Transformation and Fading of Luminescence by Solvent Inclusion

The subtle balance of the metrics of the highly supple 5,5'-pyridyl-3,3'-bis-1 H-pyrazolate and the variable coordination numbers of Cu(I) favor the stabilization of a [4 × 4] grid complex with all Cu(I). It has numerous cuprophilic bonds (Cu···Cu = 2.56-3.02 Å), and consequently, exhibits strong orange luminescence that fades upon insertion of solvents without loss of crystallinity.

Solvent Effects on the Spin-Transition in a Series of Fe(II) Dinuclear Triple Helicate Compounds

This work explores the effect of lattice solvent on the observed solid-state spin-transition of a previously reported dinuclear Fe(II) triple helicate series 1–3 of the general form [FeII2L3](BF4)4(CH3CN)n, where L is the Schiff base condensation product of imidazole-4-carbaldehyde with 4,4-diaminodiphenylmethane (L1), 4,4′-diaminodiphenyl sulfide (L2) and 4,4′-diaminodiphenyl ether (L3) respectively, and 1 is the complex when L = L1, 2 when L = L2 and 3 when L = L3 (Craze, A.R.; Sciortino, N.F.; Bhadbhade, M.M.; Kepert, C.J.; Marjo, C.E.; Li, F. Investigation of the Spin Crossover Properties of Three Dinuclear Fe(II) Triple Helicates by Variation of the Steric Nature of the Ligand Type. Inorganics. 2017, 5 (4), 62). Desolvation of 1 and 2 during measurement resulted not only in a decrease in T1/2 and completeness of spin-crossover (SCO) but also a change in the number of steps in the spin-profile. Compounds 1 and 2 were observed to change from a two-step 70% complete transition when fully solvated, to a single-step half complete transition upon desolvation. The average T1/2 value of the two-steps in the solvated materials was equivalent to the single T1/2 of the desolvated sample. Upon solvent loss, the magnetic profile of 3 experienced a transformation from a gradual SCO or weak antiferromagnetic interaction to a single half-complete spin-transition. Variable temperature single-crystal structures are presented and the effects of solvent molecules are also explored crystallographically and via a Hirshfeld surface analysis. The spin-transition profiles of 1–3 may provide further insight into previous discrepancies in dinuclear triple helicate SCO research reported by the laboratories of Hannon and Gütlich on analogous systems (Tuna, F.; Lees, M. R.; Clarkson, G. J.; Hannon, M. J. Readily Prepared Metallo-Supramolecular Triple Helicates Designed to Exhibit Spin-Crossover Behaviour. Chem. Eur. J. 2004, 10, 5737–5750 and Garcia, Y.; Grunert, C. M.; Reiman, S.; van Campenhoudt, O.; Gütlich, P. The Two-Step Spin Conversion in a Supramolecular Triple Helicate Dinuclear Iron(II) Complex Studied by Mössbauer Spectroscopy. Eur. J. Inorg. Chem. 2006, 3333–3339).

Halogenated Alkyltetrazoles for the Rational Design of FeII Spin-Crossover Materials: Fine-Tuning of the Ligand Size

1-(3-Halopropyl)-1H-tetrazoles and their corresponding Fe^II spin-crossover complexes have been investigated in a combined experimental and theoretical study. Halogen substitution was found to positively influence the spin transition, shifting the transition temperature about 70 K towards room temperature. Halogens located at the ω position were found to be too far away from the coordinating tetrazole moiety to have an electronic impact on the spin transition. The subtle variation of the steric demand of the ligand in a highly comparable series was found to have a comparatively large impact on the spin-transition behavior, which highlights the sensitivity of the effect to subtle structural changes.

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.

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.

Tuning the spin crossover behavior of the polyanion [(H2O)6Fe3(μ-L)6]6–: the case of the cesium salt

Cation exchange transforms the gradual spin transition of the trimer [(H₂O)₆Fe₃(μ-L)₆]^6– into an abrupt transition with a wide hysteresis above room temperature.

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.

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.