Ligand Field Theoretical Considerations

Unravelling Kinetics of Intramolecular NdIII → FeII Energy Transfer in Spin Crossover Single Molecules: Dotting the i's and Crossing the t's

Compared with the ripple of visible EuIII-based emission intensity induced by appended [FeIIN6] spin crossover (SCO) units, as detected in the triple-stranded [EuFe(L1)3]5+ helicate, the lanthanide-based luminescent detection of FeII spin-state equilibria could be improved significantly if the luminophore emission is shifted toward the near-infrared (NIR) domain. Replacing EuIII with NdIII in [NdFe(L1)3]5+ (i) maintains the favorable SCO properties in acetonitrile [critical temperature T1/2 = 322(2) K], (ii) saturates nonradiative vibrational relaxation processes in the 233-333 K range, and (iii) boosts the crucial intramolecular NdIII → FeII energy transfer rate processes, which are sensitive to the spin state of the FeII metallic center. Consequently, the steady-state NIR Nd(4F3/2 → 4IJ) emission of the luminophore is amplified and linearly correlated with the low-spin-[FeIIN6] and high-spin-[FeIIN6] mole fractions controlled by the SCO equilibrium. This paves the way for a straightforward and direct NIR luminescent reading/sensing of the FeII spin state in single molecules.

The role of intermolecular interactions in [Fe(X-salEen)2]ClO4 spin crossover complexes

Two new spin crossover (SCO) Fe(III) compounds were prepared, their structures were analysed and their magnetic properties were investigated. An exhaustive analysis of the effects of halogen substitution and aromatic ring functionalisation on the magnetic properties of non-solvated Fe(III) perchlorate complexes has been performed. Through comparative analysis, different magnetic profiles were found for the compounds studied, namely F (1), Cl (2), H (3), Br (4a, 4b), and I (5). Using tools like Hirshfeld analysis, the study revealed patterns in octahedral distortions and deviations from the ideal octahedral geometry. The SCO phenomenon as the conducting wire in this study, emphasises the influence of intermolecular interactions on the low spin (LS) to high spin (HS) transitions in these halogen-substituted complexes. The prevalence of H⋯H contributions has been demonstrated, albeit being the weakest and an inverse strength relationship in H⋯X interactions ranging from F to I. The findings not only interpret the intricate balance between halogen substitution, functionalisation, and intermolecular interactions in modulating magnetic properties but also direct future works in designing similar molecular systems.

Exploring spin states and ligand field splitting in metal complexes: a theoretical analysis of spin-orbital interactions and magnetic properties

Metal complexes are pivotal in diverse fields due to their wide array of functionalities, including magnetism, conductivity, and photoresponsiveness. These functionalities are intricately linked to the phenomenon of ligand field splitting, yet controlling the magnitude of this splitting within metal complexes presents a significant challenge. This study aims to address this challenge by developing a novel 2D spectrochemical series, integrating two critical parameters: metal ions and ligands. Employing the DV-Xα molecular orbital method, we directly calculated ligand field splitting width, enabling a detailed assessment of energy splitting trends. Our results reveal that the magnitude of ligand field splitting, encompassing 17 metal types and 29 ligand types, can be precisely controlled. This represents a significant advancement over traditional spectrochemical series, such as those proposed by R. Tsuchida, which predominantly focus on either ligands or metals in isolation. Additionally, our study extends to the calculation of spin states in these metal complexes, contributing valuable insights for the development of magnetic materials. We demonstrate that the relative ligand field splitting and spin polarization can be used to predict spin states, offering a new perspective in material design and functionality. These findings not only enhance our understanding of ligand field splitting in metal complexes but also provide a comprehensive framework for predicting their electronic and magnetic properties, paving the way for innovative applications in material science and coordination chemistry.

Detecting Fe(II) Spin-Crossover by Modulation of Appended Eu(III) Luminescence in a Single Molecule

Multifunctionality in spin-crossover (SCO) devices is limited to macroscopic or nanoscopic materials because of the need for long-range effects for inducing favorable cooperativity, efficient energy migration processes, and detectable magnetization transfer. The difficult reproducibility, control, and rational design of doped materials offer some place to SCO processes, modulating the optical properties of neighboring luminescent probes in single molecules. We report here on the combination of a [FeN6] chromophore, the SCO temperature and absorption spectra of which have been tuned to induce unprecedented room-temperature modulation of Eu(III)-based line-like luminescence in the molecular triple-helical [EuFe(L2)3]5+ complex in solution.

Symmetry Breaking and Cooperative Spin Crossover in a Hofmann-Type Coordination Polymer Based on Negatively Charged {FeII(μ2-[MII(CN)4])2}n2n- Layers (MII = Pd, Pt)

We report herein the synthesis and characterization of two unprecedented isomorphous spin-crossover two-dimensional coordination polymers of the Hofmann-type formulated {FeII(Hdpyan)2(μ2-[MII(CN)4])2}, with MII = Pd, Pt and Hdpyan is the in situ partially protonated form of 2,5-(dipyridin-4-yl)aniline (dpyan). The FeII is axially coordinated by the pyridine ring attached to the 2-position of the aniline ring, while it is equatorially surrounded by four [MII(CN)4]2- planar groups acting as trans μ2-bidentate ligands defining layers, which stack parallel to each other. The other pyridine group of Hdpyan, being protonated, remains peripheral but involved in a strong [MII-C≡N···Hpy+] hydrogen bond between alternate layers. This provokes a nearly 90° rotation of the plane defined by the [MII(CN)4]2- groups, with respect to the average plane defined by the layers, forcing the observed uncommon bridging mode and the accumulation of negative charge around each FeII, which is compensated by the axial [Hdpyan]+ ligands. According to the magnetic and calorimetric data, both compounds undergo a strong cooperative spin transition featuring a 10-12 K wide hysteresis loop centered at 220 (Pt) and 211 K (Pd) accompanied by large entropy variations, 97.4 (Pt) and 102.9 (Pd) J/K mol. The breaking symmetry involving almost 90° rotation of one of the two coordinated pyridines together with the large unit-cell volume change per FeII (ca. 50 Å3), and subsequent release of significantly short interlayer contacts upon the low-spin → high-spin event, accounts for the strong cooperativity.

Spin Crossover and Thermochromism in Iron(II) Complexes with 2,6-Bis(1H-imidazol-2-yl)-4-methoxypyridine

New iron(II) complexes with 2,6-bis(1H-imidazol-2-yl)-4-methoxypyridine (L) of the composition [FeL₂]A_n∙mH₂O (A = SO₄^2-, n = 1, m = 2 (I); A = ReO₄^-, n = 2, m = 1 (II); A = Br^-, n = 2, m = 2 (III)) have been synthesized and investigated. To determine the coordination ability of the ligand, a single crystal of a copper(II) complex of the composition [CuLCl₂] (IV) was obtained and studied by X-ray technique. Compounds I-III were studied using methods of X-ray phase analysis, electron (diffuse reflection spectra), infrared and Mössbauer spectroscopy, static magnetic susceptibility. The study of the µ_eff(T) dependence showed that the ¹A₁ ↔ ⁵T₂ spin crossover manifests itself in the compounds. The spin crossover is accompanied by thermochromism: there is a distinct color change orange ↔ red-violet.

Incorporation and Deposition of Spin Crossover Materials into and onto Electrospun Nanofibers

We synthesized iron(II)-triazole spin crossover compounds of the type [Fe(atrz)₃]X₂ and incorporated and deposited them on electrospun polymer nanofibers. For this, we used two separate electrospinning methods with the goal of obtaining polymer complex composites with intact switching properties. In view of possible applications, we chose iron(II)-triazole-complexes that are known to exhibit spin crossover close to ambient temperature. Therefore, we used the complexes [Fe(atrz)₃]Cl₂ and [Fe(atrz)₃](2ns)₂ (2ns = 2-Naphthalenesulfonate) and deposited those on fibers of polymethylmethacrylate (PMMA) and incorporated them into core-shell-like PMMA fiber structures. These core-shell structures showed to be inert to outer environmental influences, such as droplets of water, which we purposely cast on the fiber structure, and it did not rinse away the used complex. We analyzed both the complexes and the composites with IR-, UV/Vis, Mössbauer spectroscopy, SQUID magnetometry, as well as SEM and EDX imaging. The analysis via UV/Vis spectroscopy, Mössbauer spectroscopy, and temperature-dependent magnetic measurements with the SQUID magnetometer showed that the spin crossover properties were maintained and were not changed after the electrospinning processes.

Modifications of Tanabe-Sugano d6 Diagram Induced by Radical Ligand Field: Ab Initio Inspection of a Fe(II)-Verdazyl Molecular Complex

Quantum entanglement between the spin states of a metal center and radical ligands is suggested in an iron(II) [Fe(dipyvd)₂]²⁺ compound (dipyvd = 1-isopropyl-3,5-dipyridil-6-oxoverdazyl). Wave function ab initio (Difference Dedicated Configuration Interaction, DDCI) inspections were carried out to stress the versatility of local spin states. We named this phenomenon excited state spinmerism, in reference to our previous work (see Roseiro et al., ChemPhysChem 2022, e202200478) where we introduced the concept of spinmerism as an extension of mesomerism to spin degrees of freedom. The construction of localized molecular orbitals allows for a reading of the wave functions and projections onto the local spin states. The low-energy spectrum is well-depicted by a Heisenberg picture. A 60 cm^-1 ferromagnetic interaction is calculated between the radical ligands with the S_total = 0 and 1 states largely dominated by a local low-spin S_Fe = 0. In contrast, the higher-lying S_total = 2 states are superpositions of the local S_Fe = 1 (17%, 62%) and S_Fe = 2 (72%, 21%) spin states. Such mixing extends the traditional picture of a high-field d⁶ Tanabe-Sugano diagram. Even in the absence of spin-orbit coupling, the avoided crossing between different local spin states is triggered by the field generated by radical ligands. This puzzling scenario emerges from versatile local spin states in compounds which extend the traditional views in molecular magnetism.

High-Temperature Spin Crossover in Iron(II) Complexes with 2,6-Bis(1H-imidazol-2-yl)pyridine

Novel iron(II) coordination compounds containing a ligand 2,6-bis(1H-imidazol-2-yl)pyridine (L), having such a composition as [FeL₂]SO₄·0.5H₂O, [FeL₂]Br₂·H₂O, [FeL₂](ReO₄)₂, [FeL₂]B₁₀H₁₀∙H₂O, [FeL₂]B₁₂H₁₂∙1.5H₂O had been synthesized and studied using UV-Vis (diffuse reflectance), infrared, extended X-ray absorption fine structure (EXAFS), and Mössbauer spectroscopy methods, as well as X-ray diffraction and static magnetic susceptibility methods. The analysis of the μ_eff(T) dependence in the temperature range of 80–600 K have shown that all the obtained complexes exhibit a high-temperature spin crossover ¹A₁ ↔ ⁵T₂.

Study of Spin-Crossover in Iron(II) Complexes with 2,6-Bis(4,5-Dimethyl-1H-Imidazol-2-yl)Pyridine and closo-Borate Anions

Abstract

New iron(II) coordination compounds with 2,6-bis(4,5-dimethyl-1H-imidazol-2-yl)pyridine (L) and closo-borate(2–) anions [FeL2]B10H10⋅2H2O and [FeL2]B12H12⋅H2O have been synthesized. The compounds have been identified and studied by CHN analysis, electron spectroscopy (diffuse reflection spectroscopy), IR, Mössbauer, and EXAFS spectroscopies, X-ray powder diffraction, and static magnetic susceptibility. The structures of the coordination knots of complexes [FeL2]B10H10⋅2H2O and [FeL2]B12H12⋅H2O has been obtained by modeling the EXAFS spectra. The ligand is coordinated by the iron(II) ion in a tridentate-cyclic manner by two nitrogen atoms of imidazole cycles and a nitrogen atom of pyridine to form the FeN6 coordination knot. The study of the temperature dependence of the magnetic susceptibility in the range of 80–500 K has showed that the high-temperature spin-crossover 1А1 ↔ 5Т2 manifests itself in the obtained compounds.

Thermal hysteresis of stress and strain in spin-crossover@polymer composites: towards a rational design of actuator devices

Polymer composites of molecular spin crossover complexes have emerged as promising mechanical actuator materials, but their effective thermomechanical properties remain elusive. In this work, we investigated a series of iron(ii)-triazole@P(VDF-TrFE) particulate composites using a tensile testing stage with temperature control. From these measurements, we assessed the temperature dependence of the Young's modulus as well as the free deformation and blocking stress, associated with the thermally-induced spin transition. The results denote that the expansion of the particles at the spin transition is effectively transferred to the macroscopic composite material, providing ca. 1-3% axial strain for 25% particle load. This strain is in excess of the 'neat' particle strain, which we attribute to particle-matrix mechanical coupling. On the other hand, the blocking stress (∼1 MPa) appears reduced by the softening of the composite around the spin transition temperature.

Record Chemical-Shift Temperature Sensitivity in a Series of Trinuclear Cobalt Complexes

Designing spins that exhibit long-lived coherence and strong temperature sensitivity is central to designing effective molecular thermometers and a fundamental challenge in the chemistry/quantum-information space. Herein, we provide a new pathway to both properties in the same molecule by designing a nuclear spin, which possesses a robust spin coherence, to mimic the strong temperature sensitivity of an electronic spin. This design strategy is demonstrated in the group of trinuclear Co(III) spin-crossover compounds [(CpCo(OP(OR)₂)₃)₂Co](SbCl₆) where Cp = cyclopentadienyl and R = Me (1), Et (2), i-Pr (3), and t-Bu (4). Nuclear magnetic resonance analyses of the ⁵⁹Co nuclear spins reveal ⁵⁹Co chemical-shift temperature sensitivity (Δδ/ΔT) values that span from 101(1) ppm/°C in 1 to 149(1) ppm/°C in 2 and 150(2) ppm/°C in 4, where the latter two are record temperature sensitivities for any nuclear spin. Additionally, complexes 2 and 4 have T₂^* values of 74 and 78 μs in solution at ambient temperatures surpassing those from electron-spin-based complexes, which typically display long coherence times only at extremely low temperatures. Our results suggest that spin-crossover phenomena can enable electron-spin-like temperature sensitivities in nuclear spins while retaining robust coherence times at room temperature.

Toward High-Temperature Light-Induced Spin-State Trapping in Spin-Crossover Materials: The Interplay of Collective and Molecular Effects

Spin-crossover (SCO) materials display many fascinating behaviors including collective phase transitions and spin-state switching controlled by external stimuli, e.g., light and electrical currents. As single-molecule switches, they have been fêted for numerous practical applications, but these remain largely unrealized-partly because of the difficulty of switching these materials at high temperatures. We introduce a semiempirical microscopic model of SCO materials combining crystal field theory with elastic intermolecular interactions. For realistic parameters, this model reproduces the key experimental results including thermally induced phase transitions, light-induced spin-state trapping (LIESST), and reverse-LIESST. Notably, we reproduce and explain the experimentally observed relationship between the critical temperature of the thermal transition, T_1/2, and the highest temperature for which the trapped state is stable, T_LIESST, and explain why increasing the stiffness of the coordination sphere increases T_LIESST. We propose strategies to design SCO materials with higher T_LIESST: optimizing the spin-orbit coupling via heavier atoms (particularly in the inner coordination sphere) and minimizing the enthalpy difference between the high-spin (HS) and low-spin (LS) states. However, the most dramatic increases arise from increasing the cooperativity of the spin-state transition by increasing the rigidity of the crystal. Increased crystal rigidity can also stabilize the HS state to low temperatures on thermal cycling yet leave the LS state stable at high temperatures following, for example, reverse-LIESST. We show that such highly cooperative systems offer a realistic route to robust room-temperature switching, demonstrate this in silico, and discuss material design rationale to realize this.

On-Surface Synthesis: A New Route Realizing Single-Layer Conjugated Metal-Organic Structures

Recently, both experimental and theoretical advances have demonstrated that two-dimensional conjugated metal-organic frameworks (2D-cMOFs) exhibit interesting electronic and magnetic properties, such as high conductivity and ferromagnetism. Theoretical studies have predicted that exotic quantum states, including topological insulating states and superconductivity, emerge in some 2D-MOFs. The high design tunability of MOFs' structure and composition provides great opportunities to realize these structures. However, most conventional synthesis methods yield multilayer structures of the 2D-cMOFs, in which the predicted exotic quantum phases are often quenched because of interlayer interactions. It is highly desirable to synthesize single-layer cMOFs. On-surface synthesis represents a novel strategy toward this goal. In this Perspective, we discuss the recent developments in on-surface synthesis of 1D- and 2D-cMOFs.

Panchromatic Absorption and Oxidation of an Iron(II) Spin Crossover Complex

In order to expand and exploit the useful properties of d⁶-iron(II) and d⁵-iron(III) complexes in potential magnetic, photophysical, or magnetooptical applications, crucial ligand-controlled parameters are the ligand field strength in a given coordination mode and the availability of suitable metal and ligand frontier orbitals for charge-transfer processes. The push-pull ligand 2,6-diguanidylpyridine (dgpy) features low-energy π* orbitals at the pyridine site and strongly electron-donating guanidinyl donors combined with the ability to form six-membered chelate rings for optimal metal-ligand orbital overlap. The electronic ground states of the pseudo-octahedral d⁶- and d⁵-complexes mer-[Fe(dgpy)₂]²⁺, cis-fac-[Fe(dgpy)₂]²⁺, and mer-[Fe(dgpy)₂]³⁺ as well as their charge-transfer (CT) and metal-centered (MC) excited states are probed by variable temperature UV/vis absorption, NMR, EPR, and Mössbauer spectroscopy, magnetic susceptibility measurements at variable temperature as well as quantum chemical calculations.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

Pressure Tunable Electronic Bistability in Fe(II) Hofmann-like Two-Dimensional Coordination Polymer [Fe(Fpz)2Pt(CN)4]: A Comprehensive Experimental and Theoretical Study

A comprehensive experimental and theoretical study of both thermal-induced spin transition (TIST) as a function of pressure and pressure-induced spin transition (PIST) at room temperature for the two-dimensional Hofmann-like SCO polymer [Fe(Fpz)₂Pt(CN)₄] is reported. The TIST studies at different fixed pressures have been carried out by magnetic susceptibility measurements, while PIST studies have been performed by means of powder X-ray diffraction, Raman, and visible spectroscopies. A combination of the theory of elastic interactions and numerical Monte Carlo simulations has been used for the analysis of the cooperative interactions in TIST and PIST studies. A complete (T, P) phase diagram for the compound [Fe(Fpz)₂Pt(CN)₄] has been constructed. The critical temperature of the spin transition follows a lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions. The analysis shows the exceptional role of the total entropy and phonon contribution in setting the temperature of the spin transition and the width of the hysteresis. The anomalous behavior of the thermal hysteresis width under pressure in [Fe(Fpz)₂Pt(CN)₄] is a direct consequence of a local distortion of the octahedral geometry of the Fe(II) centers for pressures higher than 0.4 GPa. Interestingly, there is not a coexistence of the high- and low-spin (HS and LS, respectively) phases in TIST experiments, while in PIST experiments, the coexistence of the HS and LS phases in the metastable region of the phase transition induced by pressure is observed for a first time in a first-order gradual spin transition with hysteresis.

A comprehensive experimental and theoretical study of both thermal-induced spin transition as a function of pressure and pressure-induced spin transition at room temperature for the two-dimensional Hofmann-like SCO polymer [Fe(Fpz)₂Pt(CN)₄] is reported. A complete (T, P) phase diagram for compound [Fe(Fpz)₂[Pt(CN)₄] has been constructed. The critical temperature of the spin transition follows a lineal dependence with pressure, meanwhile the hysteresis width shows a nonmonotonic behavior contrary to theoretical predictions.

Optimization of crystal packing in semiconducting spin-crossover materials with fractionally charged TCNQ δ- anions (0 < δ < 1)

Co-crystallization of the prominent Fe(ii) spin-crossover (SCO) cation, [Fe(3-bpp)₂]²⁺ (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine), with a fractionally charged TCNQ^δ− radical anion has afforded a hybrid complex [Fe(3-bpp)₂](TCNQ)₃·5MeCN (1·5MeCN, where δ = −0.67). The partially desolvated material shows semiconducting behavior, with the room temperature conductivity σ_RT = 3.1 × 10⁻³ S cm⁻¹, and weak modulation of conducting properties in the region of the spin transition. The complete desolvation, however, results in the loss of hysteretic behavior and a very gradual SCO that spans the temperature range of 200 K. A related complex with integer-charged TCNQ⁻ anions, [Fe(3-bpp)₂](TCNQ)₂·3MeCN (2·3MeCN), readily loses the interstitial solvent to afford desolvated complex 2 that undergoes an abrupt and hysteretic spin transition centered at 106 K, with an 11 K thermal hysteresis. Complex 2 also exhibits a temperature-induced excited spin-state trapping (TIESST) effect, upon which a metastable high-spin state is trapped by flash-cooling from room temperature to 10 K. Heating above 85 K restores the ground-state low-spin configuration. An approach to improve the structural stability of such complexes is demonstrated by using a related ligand 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) to obtain [Fe(bzimpy)₂](TCNQ)₆·2Me₂CO (4) and [Fe(bzimpy)₂](TCNQ)₅·5MeCN (5), both of which exist as LS complexes up to 400 K and exhibit semiconducting behavior, with σ_RT = 9.1 × 10⁻² S cm⁻¹ and 1.8 × 10⁻³ S cm⁻¹, respectively.

Co-crystallization of the cationic complex [Fe(3-bpp)2]²⁺ with fractionally charged TCNQ^δ− anions (0 < δ < 1) affords semiconducting spin-crossover (SCO) materials. The abruptness of SCO is strongly dependent on the interstitial solvent content.

Giant and Reversible Barocaloric Effect in Trinuclear Spin-Crossover Complex Fe3 (bntrz)6 (tcnset)6

A giant barocaloric effect (BCE) in a molecular material Fe₃ (bntrz)₆ (tcnset)₆ (FBT) is reported, where bntrz = 4-(benzyl)-1,2,4-triazole and tcnset = 1,1,3,3-tetracyano-2-thioethylepropenide. The crystal structure of FBT contains a trinuclear transition metal complex that undergoes an abrupt spin-state switching between the state in which all three Fe^II centers are in the high-spin (S = 2) electronic configuration and the state in which all of them are in the low-spin (S = 0) configuration. Despite the strongly cooperative nature of the spin transition, it proceeds with a negligible hysteresis and a large volumetric change, suggesting that FBT should be a good candidate for producing a large BCE. Powder X-ray diffraction and calorimetry reveal that the material is highly susceptible to applied pressure, as the transition temperature spans the range from 318 at ambient pressure to 383 K at 2.6 kbar. Despite the large shift in the spin-transition temperature, its nonhysteretic character is maintained under applied pressure. Such behavior leads to a remarkably large and reversible BCE, characterized by an isothermal entropy change of 120 J kg^-1 K^-1 and an adiabatic temperature change of 35 K, which are among the highest reversible values reported for any caloric material thus far.

Tuning spin-crossover transition temperatures in non-symmetrical homoleptic meridional/facial [Fe(didentate)3]2+ complexes: what for and who cares about it?

The [FeN₆] chromophores found in [Fe(didentate)₃]²⁺ complexes, where didentate is a non-symmetrical 2-(6-membered-heterocyclic ring)-benzimidazole ligand (Lk), exist as mixtures of two geometrical mer (C₁-symmetry) and fac (C₃-symmetry) isomers. Specific alkyl-substituted six-membered heterocyclic rings connected to the benzimidazole unit (pyridines in ligands L1-L3, pyrazines in L4-L5 and pyrimidines in L6-L7) control the ligand field strength and the electron delocalization so that [Fe^II(Lk)₃]²⁺ display tunable thermally-induced spin transitions in solution. Thermodynamic, spectroscopic (UV-Vis, NMR) and magnetic studies in solution demonstrate that [Fe(L6)₃]²⁺ (L6 = 1-methyl-2-(pyrimidin-2-yl)-1H-benzo[d]imidazole) exhibits a close to room temperature spin transition (T_1/2 = 273(3) K) combined with a high stability formation constant ( in acetonitrile), which makes this complex suitable for the potential modulation of lanthanide-based luminescence in polymetallic helicates. A novel method is proposed for assigning specific thermodynamic spin crossover parameters to fac-[Fe(L6)₃]²⁺ and mer-[Fe(L6)₃]²⁺ isomers in solution. The observed difference relies mainly on the entropic content ΔS-ΔS = 11(1) J mol^-1 K^-1, which favors the spin transition for the meridional isomer. Intermolecular interactions occurring in the crystalline state largely overcome minor thermodynamic trends operating in diluted solutions and a single configurational isomer is usually observed in the solid state. Among the thirteen solved crystal structures 1-13 containing the [M(Lk)₃]²⁺ cations (M = Fe, Ni, Zn, Lk = L6-L7), pure meridional isomers are observed six times, pure facial isomers also six times and a mixture (44% mer and 56% fac) is detected only once. Solid-state magnetic data recorded for the Fe^II complexes show the operation of slightly cooperative spin transitions in 7 (fac-[Fe(L6)₃]²⁺) and 12 (mer-[Fe(L7)₃]²⁺). For the meridional isomer in 6, a two-step spin state transition curve, associated with two phase transitions, is detected.

Navigating Transition-Metal Chemical Space: Artificial Intelligence for First-Principles Design

The variability of chemical bonding in open-shell transition-metal complexes not only motivates their study as functional materials and catalysts but also challenges conventional computational modeling tools. Here, tailoring ligand chemistry can alter preferred spin or oxidation states as well as electronic structure properties and reactivity, creating vast regions of chemical space to explore when designing new materials atom by atom. Although first-principles density functional theory (DFT) remains the workhorse of computational chemistry in mechanism deduction and property prediction, it is of limited use here. DFT is both far too computationally costly for widespread exploration of transition-metal chemical space and also prone to inaccuracies that limit its predictive performance for localized d electrons in transition-metal complexes. These challenges starkly contrast with the well-trodden regions of small-organic-molecule chemical space, where the analytical forms of molecular mechanics force fields and semiempirical theories have for decades accelerated the discovery of new molecules, accurate DFT functional performance has been demonstrated, and gold-standard methods from correlated wavefunction theory can predict experimental results to chemical accuracy.The combined promise of transition-metal chemical space exploration and lack of established tools has mandated a distinct approach. In this Account, we outline the path we charted in exploration of transition-metal chemical space starting from the first machine learning (ML) models (i.e., artificial neural network and kernel ridge regression) and representations for the prediction of open-shell transition-metal complex properties. The distinct importance of the immediate coordination environment of the metal center as well as the lack of low-level methods to accurately predict structural properties in this coordination environment first motivated and then benefited from these ML models and representations. Once developed, the recipe for prediction of geometric, spin state, and redox potential properties was straightforwardly extended to a diverse range of other properties, including in catalysis, computational "feasibility", and the gas separation properties of periodic metal-organic frameworks. Interpretation of selected features most important for model prediction revealed new ways to encapsulate design rules and confirmed that models were robustly mapping essential structure-property relationships. Encountering the special challenge of ensuring that good model performance could generalize to new discovery targets motivated investigation of how to best carry out model uncertainty quantification. Distance-based approaches, whether in model latent space or in carefully engineered feature space, provided intuitive measures of the domain of applicability. With all of these pieces together, ML can be harnessed as an engine to tackle the large-scale exploration of transition-metal chemical space needed to satisfy multiple objectives using efficient global optimization methods. In practical terms, bringing these artificial intelligence tools to bear on the problems of transition-metal chemical space exploration has resulted in ML-model assessments of large, multimillion compound spaces in minutes and validated new design leads in weeks instead of decades.

Widening the Window of Spin-Crossover Temperatures in Bis(formazanate)iron(II) Complexes via Steric and Noncovalent Interactions

Bis(formazanate)iron(II) complexes undergo a thermally induced S = 0 to S = 2 spin transition in solution. Here we present a study of how steric effects and π-stacking interactions between the triarylformazanate ligands affect the spin-crossover behavior, in addition to electronic substituent effects. Moreover, the effect of increasing the denticity of the formazanate ligands is explored by including additional OMe donors in the ligand (7). In total, six new compounds (2–7) have been synthesized and characterized, both in solution and in the solid state, via spectroscopic, magnetic, and structural analyses. The series spans a broad range of spin-crossover temperatures (T_1/2) for the LS ⇌ HS equilibrium in solution, with the exception of compound 6 which remains high-spin (S = 2) down to 210 K. In the solid state, 6 was shown to exist in two distinct forms: a tetrahedral high-spin complex (6a, S = 2) and a rare square-planar structure with an intermediate-spin state (6b, S = 1). SQUID measurements, ⁵⁷Fe Mössbauer spectroscopy, and differential scanning calorimetry indicate that in the solid state the square-planar form 6b undergoes an incomplete spin-change-coupled isomerization to tetrahedral 6a. The complex that contains additional OMe donors (7) results in a six-coordinate (NNO)₂Fe coordination geometry, which shifts the spin-crossover to significantly higher temperatures (T_1/2 = 444 K). The available experimental and computational data for 7 suggest that the Fe···OMe interaction is retained upon spin-crossover. Despite the difference in coordination environment, the weak OMe donors do not significantly alter the electronic structure or ligand-field splitting, and the occurrence of spin-crossover (similar to the compounds lacking the OMe groups) originates from a large degree of metal–ligand π-covalency.

A series of Fe(II) complexes with formazanate ligands are reported, and ligand substituent effects on structure and spin-crossover properties are examined. These ligand modifications allow isolation of compounds with tetrahedral geometries in both low- and high-spin ground states as well as an intermediate-spin square-planar complex. Steric properties, π-stacking interactions, and additional donor substituents lead to a wide range of spin-crossover temperatures (T_1/2) in this class of compounds.

Spin-Crossover Properties of an Iron(II) Coordination Nanohoop

Addition of the bipyridyl-embedded cycloparaphenylene nanohoop bipy[9]CPP to [Fe{H₂ B(pyz)₂ }] (pyz=pyrazolyl) produces the distorted octahedral complex [Fe(bipy[9]CPP){H₂ B(pyz)₂ }₂ ] (1). The molecular structure of 1 shows that the nanohoop ligand contains a non-planar bipy unit. Magnetic susceptibility measurements indicate spin-crossover (SCO) behaviour with a T_1/2 of 130 K, lower than that of 160 K observed with the related compound [Fe(bipy){H₂ B(pyz)₂ }₂ ] (2), which contains a conventional bipy ligand. A computational study of 1 and 2 reveals that the curvature of the nanohoop leads to the different SCO properties, suggesting that the SCO behaviour of iron(II) can be tuned by varying the size and diameter of the nanohoop.

Theoretical Investigations of Electronic Structure and Magnetic and Optical Properties of Transition-Metal Dinuclear Molecules

In this work, we have reported the electronic structure, spin state, and optical properties of a new class of transition-metal (TM) dinuclear molecules (TM = Cr, Mn, Fe, Co, and Ni). The stability of these molecules has been analyzed from the vibration spectra obtained by using density functional theory (DFT) calculations. The ground-state spin configuration of the tetra-coordinated TM atom in each molecule has been predicted from the relative total energy differences in different spin states of the molecule. The DFT + U method has been used to investigate the precise ground-state spin configuration of each molecule. We further performed time-dependent DFT calculations to study the optical properties of these molecules. The planar geometric structure remains intact in most of the cases; hence, these molecules are expected to be well adsorbed and self-assembled on metal substrates. In addition, the optical characterization of these molecules indicates that the absorption spectra have a large peak in the blue-light wavelength range; therefore, it could be suitable for advanced optoelectronic device applications. Our work promotes further computational and experimental studies on TM dinuclear molecules in the field of molecular spintronics and optoelectronics.

Variable Cooperative Interactions in the Pressure and Thermally Induced Multistep Spin Transition in a Two-Dimensional Iron(II) Coordination Polymer

Two types of experiments conducted to investigate the effect of pressure on the spin crossover (SCO) properties of the 2D Fe(II) coordination polymer formulated {Fe[bipy(ttr)₂]}_n are reported, namely, (1) magnetic measurements performed at variable temperature and at fixed pressure and (2) visible spectroscopy at variable pressure and fixed temperature. The magnetic experiments carried out under a hydrostatic pressure constraint of 0.04, 0.08, and 0.8 GPa reveal a two-step spin transition behavior. The characteristic critical temperatures of the spin transition are shifted upward in temperature as pressure increases. The slope of the straight-line of the T_c vs P plot, dT_c/dP, is 775 K/GPa and 300 K/GPa, for the high temperature and the low temperature steps, respectively. These values are remarkably large and denote the extreme sensitivity of the material to the application of pressure. Indeed, the visible spectroscopic measurements performed at 293 K show that a complete spin transition is induced at pressures as low as 0.4 GPa. Moreover, the pressure-induced spin transition is reversible and shows an asymmetric hysteresis. An analysis of the cooperative interactions of the thermal- and pressure-induced spin transition in the framework of the model of elastic interactions reveals that the elastic energy of the lattice as well as the interaction parameter between the SCO centers change during the course of the spin transition. Consequently, the character of the spin transition varies from abrupt for the high temperature step to continuous for the low temperature step.

The Tyranny of Arm-Wrestling Methyls on Iron(II) Spin State in Pseudo-Octahedral [Fe(didentate)3] Complexes

The connection of a sterically constrained 3-methyl-pyrazine ring to a N-methyl-benzimidazole unit to give the unsymmetrical α,α’-diimine ligand L5 has been programmed for the design of pseudo-octahedral spin-crossover [Fe(L5)3]2+ units, the transition temperature (T1/2) of which occurs in between those reported for related facial tris-didentate iron chromophores fitted with 3-methyl-pyridine-benzimidazole in a LaFe helicate (T1/2 ~ 50 K) and with 5-methyl-pyrazine-benzimidazole L2 ligands (T1/2 ~350 K). A thorough crystallographic analysis of [Fe(L5)3](ClO4)2 (I), [Ni(L5)3](ClO4)2 (II), [Ni(L5)3](BF4)2∙H2O (III), [Zn(L5)3](ClO4)2 (IV), [Ni(L5)3](BF4)2∙1.75CH3CN (V), and [Zn(L5)3](BF4)2∙1.5CH3CN (VI) shows the selective formation of pure facial [M(L5)3]2+ cations in the solvated crystals of the tetrafluoroborate salts and alternative meridional isomers in the perchlorate salts. Except for a slightly larger intra-strand interannular twist between the aromatic heterocycles in L5, the metric parameters measured in [Zn(L5)3]2+ are comparable to those reported for [Zn(L2)3]2+, where L2 is the related unconstrained ligand. This similitude is reinforced by comparable ligand-field strengths (∆oct) and nephelauxetic effects (as measured by the Racah parameters B and C) extracted from the electronic absorption spectra recorded for [Ni(L5)3]2+ and [Ni(L2)3]2+. In this context, the strictly high-spin behavior observed for [Fe(L5)3]2+ within the 5–300 K range contrasts with the close to room-temperature spin-crossover behavior of [Fe(L2)3]2+ (T1/2 = 349(5) K in acetonitrile). This can be unambiguously assigned to an intraligand arm wrestling match operating in bound L5, which prevents the contraction of the coordination sphere required for accommodating low-spin FeII. Since the analogous 3-methyl-pyridine ring in [Fe(L3)3]2+ derivatives are sometimes compatible with spin-crossover properties, the consequences of repulsive intra-strand methyl–methyl interactions are found to be amplified in [Fe(L5)3]2+ because of the much lower basicity of the 3-methyl-pyrazine ring and the resulting weaker thermodynamic compensation. The decrease of the stability constants by five orders of magnitude observed in going from [M(L2)3]2+ to [M(L5)3]2+ (M = NiII and ZnII) is diagnostic for the operation of this effect, which had been not foreseen by the authors.

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.

Spin-crossover in iron(ii) coordination compounds with 2,6-bis(benzimidazol-2-yl)pyridine

Novel iron(ii) bromide, nitrate and dicyanamide complexes with 2,6-bis(benzimidazol-2-yl)pyridine possessing high-temperature ¹A₁ ↔ ⁵T₂ spin crossover were obtained.

Structural Insights into the Two-Step Spin-Crossover Compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2

The crystal structure of the polymeric spin crossover compound Fe(3,4-dimethyl-pyridine)2[Ag(CN)2]2 has been solved and its temperature dependence followed by means of single-crystal and powder X-ray diffraction. This compound presents a two-step spin transition with relatively abrupt steps centred at ca. 170 K and 145 K and a plateau at around 155 K. The origin of the two-step transition is discussed in light of these structural studies. The observations are compatible with a mostly disordered state between the two steps, consisting of mixing of high-spin and low-spin species, while weak substructure reflections in the mixed phase could indicate some degree of long-range order of the high-spin and low-spin sites.

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.

Solution-State Spin Crossover in a Family of [Fe(L)2(CH3CN)2](BF4)2 Complexes

We report herein on five new Fe(II) complexes of general formula [Fe(L)2(NCCH3)2](BF4)2•xCH3CN (L = substituted 2-pyridylimine-based ligands). The influence of proximally located electron withdrawing groups (e.g., NO2, CN, CF3, Cl, Br) bound to coordinated pyridylimine ligands has been studied for the effect on spin crossover in their Fe(II) complexes. Variable-temperature UV-visible spectroscopic studies performed on complexes with more strongly electronegative ligand substituents revealed spin crossover (SCO) in the solution, and thermodynamic parameters associated with the spin crossover were estimated.

Hysteretic thermal spin-crossover in heteroleptic Fe(ii) complexes using alkyl chain substituted 2,2′-dipyridylamine ligands

The complexes trans-[Fe^II(LC₄)₂(NCS)₂] (1C4) and trans-[Fe^II(LC₁₀)₂(NCS)₂] (1C10) undergo thermally hysteretic spin-crossover with T_1/2 = 127.5 K and 119.0 K respectively.

Abrupt spin crossover in iron(iii) complexes with aromatic anions

Two iron(iii) complexes, [Fe(qsal-X)₂]OTs·nH₂O, are found to exhibit abrupt spin crossover with the spin transition temperature substituent dependent, and X⋯O halogen bonds linking the spin centres.

Slow spin crossover in bis-meridional Fe2+ complexes through spin-state auto-adaptive N6/N8 coordination

Fe²⁺ spin crossover (SCO) complexes with long-lived excited high-spin (HS) states are promising molecular switches. An enhanced kinetic stability of spin-state isomers can be expected to foster applications beyond the limits of cooperative SCO. In this study, we describe a new approach to slow down the spin-state exchange by simple commutation of a phenyl substituent by a pyridyl substituent. To this end, N4 ligand 6-(3-(pyridin-2-yl)-1H-pyrazol-1-yl)-2,2'-bipyridine (3b) is synthesized as an N4 homologue of the well-established meridional N3 ligands motif. Phenyl-substituted 6-(3-phenyl-1H-pyrazol-1-yl)-2,2'-bipyridine (3a) serves as an intrinsic N3 reference throughout. 3b offers variable coordination numbers, N3 versus N3(+1) and N4, reflecting the preferences of the metal center. As is shown herein through an extended solid-state structure-chemical and solution-state NMR study, which is augmented by density-functional theory modeling, both the coordination geometry and its structural dynamics are indeed highly sensitive towards the expansion of the nominal donor number. The additional donors in 3b introduced through the phenyl-pyridine commutation actually give rise to a rich and diverse stereochemistry of the derived Zn²⁺ and Fe²⁺ complexes. Notably, even within a single complex unit coordination of 3b ranges from strongly distorted N3 coordination with a long assisting additional contact (Zn²⁺ and Fe²⁺) to a more symmetric N2(+2) or N4 situation in Fe²⁺. DFT modeling unravels that the additional donors are hemi-labile and coordinate to the Fe²⁺ only in HS state, leaving the elusive low-spin (LS) state in a fairly undisturbed octahedral environment with 3b being N3 coordinate. That is, the coordination number of the complex autogeneously responds to the altered spin-state. Necessarily this switch in coordination number requires strong structural changes upon SCO. This leads to increased activation barriers for SCO as could be deduced from a temperature-dependent analysis of the dynamic ¹H NMR-line broadening and corroborated by accompanying theoretical analysis of the SCO reaction coordinate. For [Fe(3b)₂]²⁺ long spin-state lifetimes τ > 1 ms prevail below the characteristic temperature T (1 ms) = 235 K; this value should be compared with a lifetime of only 150 ns derived for the close analogue [Fe(3a)₂]²⁺. The principle applied herein is general and allows transferring of LS Fe²⁺ complexes with suitably placed phenyl substituents into SCO complexes with spin-state adaptive coordination number and hence long-lived HS excited states.

Mechanism Underlying the Effectiveness of Deferiprone in Alleviating Parkinson's Disease Symptoms

Elevation in iron content as well as severe depletion of dopamine (DA) as a result of iron-induced loss of dopaminergic neurons has been recognized to accompany the progression of Parkinson's disease (PD). To better understand the mechanism of the mitigating effect of the iron chelator deferiprone (DFP) on PD, the interplay between iron and DFP was investigated both in the absence and presence of DA. The results show that DFP was extremely efficient in scavenging both aqueous iron and iron that was loosely bound to DA with the entrapment of iron in Fe-DFP complexed form critical to halting the iron catalyzed degradation of DA and associated generation of toxic metabolites. The DFP related scavenging of dopamine semiquinone (DA^•-) and superoxide (O₂^•-) may also contribute to its positive effects in the treatment of PD.

Spin Crossover Nanomaterials: From Fundamental Concepts to Devices

Nanoscale spin crossover materials capable of undergoing reversible switching between two electronic configurations with markedly different physical properties are excellent candidates for various technological applications. In particular, they can serve as active materials for storing and processing information in photonic, mechanical, electronic, and spintronic devices as well as for transducing different forms of energy in sensors and actuators. In this progress report, a brief overview on the current state-of-the-art of experimental and theoretical studies of nanomaterials displaying spin transition is presented. Based on these results, a detailed analysis and discussions in terms of finite size effects and other phenomena inherent to the reduced size scale are provided. Finally, recent research devices using spin crossover complexes are highlighted, emphasizing both challenges and prospects.