Spin Crossover Process Research Articles

Enhancing Magnetic Cooperativity in Fe(II) Triazole-based Spin-crossover Nanoparticles by Pluronic Matrix Confinement.

Polymeric one-dimensional (1D) triazole-based FeII spin crossover nanoparticles have been entrapped in pluronic P123 matrix, forming nanorods in which the interaction between host (P123) and guest (FeII complex) promoted high reproducibility of the spin crossover process, significant shifts of the transition temperatures (T↑=370 K, T↓=338 K for the P123 entrapped material vs the literature values of T↑=358 K, T↓=341 K for the neat/polymer free system) and larger magnetic hysteresis width.

Spin crossover properties of Fe(III) complexes in [Fe (bzacen)(tvp)]BPh4 ·nSolv chain structures: EPR study.

Two types of Fe(III) polynuclear iron(III) 1D-chain coordination compounds of the general formula [Fe (L)(tvp)]BPh4 nSolv, where L = dianion of N,N'-ethylenebis (benzoylacetylacetone)2,2'-imine (bzacen), tvp = 1,2-di(4-pyridyl)ethylene were synthesized and studied by the electron paramagnetic resonance (EPR) and magnetic susceptibility methods in the temperature range (100-300) К. Two types of spin-variable complexes are formed depending on the time of precipitation of the complexes from the same solution leading to differently solvated species. They have different characteristics of the local ligand field and the spin transition behavior. The thermodynamic parameters of spin transitions were determined from the temperature dependence of the EPR signals integral intensity. The energy levels splitting values obtained by analyzing g-factors of low-spin Fe(III) centers evidenced not only on the crucial role of low-symmetry distortions on the principal possibility of spin-crossover processes, but also on the temperature peculiarities of spin transitions.

Multiresponsive Spin Crossover Driven by Rotation of Tetraphenylborate Anion in an Iron(III) Complex

Dynamic molecular materials, which involve mechanical motions in crystals, usually exhibit tunable properties related to conformational polymorphs. Herein, we describe a solvent-free molecular crys...

Monitoring Fe(II) Spin-State Equilibria via Eu(III) Luminescence in Molecular Complexes: Dream or Reality?

The modulation of light emission by Fe(II) spin-crossover processes in multifunctional materials has recently attracted major interest for the indirect and noninvasive monitoring of magnetic information storage. In order to approach this goal at the molecular level, three segmental ligand strands, L4-L6, were reacted with stoichiometric mixtures of divalent d-block cations (M(II) = Fe(II) or Zn(II)) and trivalent lanthanides (Ln(III) = La(III) or Eu(III)) in acetonitrile to give C3-symmetrical dinuclear triple-stranded helical [LnM(Lk)3]5+ cations, which can be crystallized with noncoordinating counter-anions. The divalent metal M(II) is six-coordinate in the pseudo-octahedral sites produced by the facial wrapping of the three didentate binding units, the ligand field of which induces variable Fe(II) spin-state properties in [LnFe(L4)3]5+ (strictly high-spin), [LnFe(L5)3]5+ (spin-crossover (SCO) around room temperature), and [LnFe(L6)3]5+ (SCO at very low temperature). The introduction of the photophysically active Eu(III) probe in [EuFe(Lk)3]5+ results in europium-centered luminescence modulated by variable intramolecular Eu(III) → Fe(II) energy-transfer processes. The kinetic analysis implies Eu(III) → Fe(II) quenching efficiencies close to 100% for the low-spin configuration and greater than 95% for the high-spin state. Consequently, the sensitivity of indirect luminescence detection of Fe(II) spin crossover is limited by the resulting weak Eu(III)-centered emission intensities, but the dependence of the luminescence on the temperature unambiguously demonstrates the potential of indirect lanthanide-based spin-state monitoring at the molecular scale.

Spin Crossover in Iron(II) Porphyrazine Induced by Noncovalent Interactions Combined with Hybridization of Iron(II) Porphyrazine and Ligand’s Orbitals: CASPT2, CCSD(T), and DFT Studies

Spin crossover processes invoked by external stimuli, e.g., temperature, pressure, irradiation with light, electric or magnetic field, as well as formation of dative bond, are well-known phenomena....

Spin Interconversion of Heme-Peroxo-Copper Complexes Facilitated by Intramolecular Hydrogen-Bonding Interactions.

Synthetic peroxo-bridged high-spin (HS) heme-(μ-η2:η1-O22-)-Cu(L) complexes incorporating (as part of the copper ligand) intramolecular hydrogen-bond (H-bond) capabilities and/or steric effects are herein demonstrated to affect the complex's electronic and geometric structure, notably impacting the spin state. An H-bonding interaction with the peroxo core favors a low-spin (LS) heme-(μ-η1:η1-O22-)-Cu(L) structure, resulting in a reversible temperature-dependent interconversion of spin state (5 coordinate HS to 6 coordinate LS). The LS state dominates at low temperatures, even in the absence of a strong trans-axial heme ligand. Lewis base addition inhibits the H-bond facilitated spin interconversion by competition for the H-bond donor, illustrating the precise H-bonding interaction required to induce spin-crossover (SCO). Resonance Raman spectroscopy (rR) shows that the H-bonding pendant interacts with the bridging peroxide ligand to stabilize the LS but not the HS state. The H-bond (to the Cu-bound O atom) acts to weaken the O-O bond and strengthen the Fe-O bond, exhibiting ν(M-O) and ν(O-O) values comparable to analogous known LS complexes with a strong donating trans-axial ligand, 1,5-dicyclohexylimidazole, (DCHIm)heme-(μ-η1:η1-O22-)-Cu(L). Variable-temperature (-90 to -130 °C) UV-vis and 2H NMR spectroscopies confirm the SCO process and implicate the involvement of solvent binding. Examining a case of solvent binding without SCO, thermodynamic parameters were obtained from a van't Hoff analysis, accounting for its contribution in SCO. Taken together, these data provide evidence for the H-bond group facilitating a core geometry change and allowing solvent to bind, stabilizing a LS state. The rR data, complemented by DFT analysis, reveal a stronger H-bonding interaction with the peroxo core in the LS compared to the HS complexes, which enthalpically favors the LS state. These insights enhance our fundamental understanding of secondary coordination sphere influences in metalloenzymes.

Light-induced spin crossover—Solution and solid-state processes

Abstract This article overviews the different light-induced spin-crossover processes that are induced using ligand photoreaction, photothermal effects, or optical absorption of the center metallic ion. A particular attention is drawn on the light-induced excited spin-state trapping effect, the most studied phenomenon, by discussing the recent developments in terms of description of the initial photoswitching process and the mechanisms developing at different time scales, as well as the lifetime of the photoinduced state.

Monitoring Ultrafast Spin Crossover Intermediates in an Iron(II) Complex by Broad Band Stimulated X-ray Raman Spectroscopy.

The photoinduced spin crossover dynamics of transition metal complexes is of fundamental scientific importance and is used for sensor device applications and solar energy harvesting. Current X-ray and optical spectroscopy experiments for [FeII(bpy)3], an archetypal earth-abundant metal complex, show conflicting spin dynamics. We have simulated the broad band transient X-ray absorption and hybrid (broad + narrow band) X-ray stimulated Raman signals at the N and Fe K-edges of the key excited state intermediates involved in the spin crossover process of this complex. We find that these signals are much more sensitive to electron and spin populations than transition absorption and may be useful in the design of photovoltaic and artificial photosynthetic systems.

Monitoring the Spin States of Ferrous Ions by Fluorescence Spectroscopy in Spin-Crossover-Fluorescent Hybrid Materials.

The grafts of fluorophores 9-anthraldehyde (AD) and 9-phenanthrenecarboxaldehyde (PD), respectively, on the one-dimensional spin-crossover compound [Fe(L)3 ](ClO4 )2 (FeL, L=4-amino-1,2,4-triazole) by post-synthetic aldimine condensation reactions produced two spin-crossover (SCO)-fluorescent hybrid materials, that is, FeL-AD and FeL-PD. The spin-crossover critical temperatures of the two materials both centered at Tc ↓=254 and Tc ↑=256 K, whereas the fluorescence intensities of the two materials featured functions of the temperature that strictly synchronized with the spin-crossover processes, which showed that the ligand-centered fluorescence was dominated by the spin states of the ferrous ions. The bifunctional entities (spin-crossover centers and fluorophores) in FeL-AD or FeL-PD showed spectral band overlap that purported the Förster resonance energy transfer mechanism of such spin-crossover-fluorescence correlation. The post-synthetic modification of SCO materials and the relationship between the fluorescence and the SCO may be helpful in the development of multifunctional materials that can be sensitive to multiple stimuli.

Spin crossover Fe(ii) complexes of a cross-bridged cyclam derivative.

A cross-bridged cyclam derivative containing two 2-pyridylmethyl pendant arms (L = 4,11-bis((pyridin-2-yl)methyl)-1,4,8,11-tetraaza-bicyclo[6.6.2]hexadecane) was synthesized by dialkylation of the cross-bridged cyclam with 2-chloromethylpyridine. A series of Fe(ii) complexes with L and different counter-anions of the formulas [Fe(L)][FeCl4]·H2O (1·H2O), [Fe(L)]Cl2·4H2O (2·4H2O), [Fe(L)](BF4)2·0.5CH3CN (3·0.5CH3CN) and [Fe(L)](BPh4)2·CH3OH (4·CH3OH) was prepared and thoroughly characterized. In all the cases, the [Fe(L)]2+ cation adopts a cis-V configuration with a distorted octahedral geometry, and with the FeN6 donor set. The magnetic measurements within the temperature interval of 5-400 K revealed the spin crossover (SCO) behaviour of all the complexes with the transition temperature T1/2 increasing with the counter anion in the order BF4- (3) < [FeCl4]2- (1) < BPh4- (4). However, the SCO process was complete in the case of compound 3 only, with T1/2 = 177 K, which proceeded after removal of co-crystallized CH3CN molecules accompanied by a change of the crystallographic phase. The SCO behaviour of 3 was also confirmed by a single crystal X-ray analysis providing the average Fe-N distances of 2.086 Å at 120 K and 2.197 Å at 293 K typical of low-spin, and high-spin FeII complexes, respectively. The obtained results clearly showed that the nature of the counter anion and the presence/absence of co-crystallized solvent molecule(s) significantly affected the temperature as well as the abruptness of the spin transition. This is the first report of SCO behaviour observed for iron complexes containing a cross-bridged cyclam derivative.

Theoretical Study of the Light-Induced Spin Crossover Mechanism in [Fe(mtz)6]2+ and [Fe(phen)3]2.

The deactivation pathway of the light-induced spin crossover process in two Fe(II) complexes has been studied by combining density functional theory calculations for the geometries and the normal vibrational modes and highly correlated wave function methods for the energies and spin-orbit coupling effects. For the two systems considered, the mechanism of the photoinduced conversion from the low-spin singlet to the high-spin quintet state implies two intersystem crossings through intermediate triplet states. However, for the [Fe(mtz)6]2+ complex, the process occurs within few picoseconds and involves uniquely metal-centered electronic states, whereas for the [Fe(phen)3]2+ system the deactivation channel involves both metal to ligand charge transfer and metal-centered states and takes place in a femtosecond time scale.

Polymorphs of spin-crossover iron(II) complex fac-[FeII(HLn-Pr)3]Cl·PF6 (HLn-Pr = 2-methylimidazol-4-yl-methylideneamino-n-propyl): Assembly structures and scan rate dependent spin-crossover properties with thermal hysteresis

Two polymorphs of iron(II) complex fac-[FeII(HLn-Pr)3]Cl·PF6, 1 and 2, were obtained (HLn-Pr=2-methylimidazole-4-yl-methylideneamino-n-propyl ligand). Complex cations, fac-[FeII(HLn-Pr)3]2+, of 1 and 2 had similar octahedral and facial geometries coordinated with three bidentate HLn-Pr ligands. The three imidazole groups in fac-[FeII(HLn-Pr)3]2+ with Δ- and Λ-enantiomorphs are hydrogen-bonded with three Cl− ions, forming one-dimensional (1D) and two-dimensional (2D) network structures in 1 and 2, respectively. Complex 1 had a ladder-like 1D structure formed by imidazole⋯chloride hydrogen bonds, with a circular binuclear net unit of {⋯fac-[FeII(HLn-Pr)3]2+⋯Cl−⋯}2, and showed two successive scan rate dependent spin crossover (SCO) processes with thermal hysteresis. Complex 2 had a 2D network structure formed by imidazole⋯chloride hydrogen bonds with circular binuclear and tetranuclear net units of {⋯fac-[FeII(HLn-Pr)3]2+⋯Cl−⋯}2,4, and showed a one-step scan rate dependent SCO profile centered at 143K with thermal hysteresis. The relaxation property was identified from the scan rate dependence of magnetic susceptibility and temperature dependent Mössbauer spectra and X-ray diffraction studies. The crystal structure of 2 was analyzed at 12 temperatures in a cooling cycle. Upon lowering the temperature, the crystal system transformed from a monoclinic to a triclinic lattice at 210K in a high-spin state, and then transformed to a low-spin state at 150K in the triclinic system.

Ultrafast Spin-State Dynamics in Transition-Metal Complexes Triggered by Soft-X-Ray Light.

Recent advances in attosecond physics provide access to the correlated motion of valence and core electrons on their intrinsic timescales. For valence excitations, processes related to the electron spin are usually driven by nuclear motion. For core-excited states, where the core hole has a nonzero angular momentum, spin-orbit coupling is strong enough to drive spin flips on a much shorter time scale. Here, unprecedented short spin crossover is demonstrated for L-edge (2p→3d) excited states of a prototypical Fe(II) complex. It occurs on a time scale, which is faster than the core-hole lifetime of about 4fs and can be manipulated by the excitation conditions. A detailed analysis of such phenomena will help to gain a fundamental understanding of spin-crossover processes and establish the basis for their control by light.

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

Manganese(iii) hexadentate Schiff base complexes ([Mn(sal-N-1,5,8,12)]Y·S, Y = AsF6 (1); Y = SbF6 (2); Y = NO3, S = C2H5OH (3) and Y = Cl (4)) have been investigated to determine the impact of anion effects, intramolecular ligand distortions, and intermolecular supramolecular structures on the spin crossover (SCO) behavior. The crystal structure of [Mn(sal-N-1,5,8,12)]PF6, a complex known to exhibit an abrupt SCO behavior with an 8 K hysteresis window, reveals that this complex has a temperature-dependent anion order-disorder transition that disrupts the hydrogen-bonding chain upon SCO, indicating that hydrogen bonds between cations and anions greatly influence the magnetic properties. The SCO in 1 is mediated by intermolecular hydrogen-bonding interactions. The subtle balance of these hydrogen bonds induces a cooperative SCO process with a hysteresis width of 18 K, which is the largest one reported in the d4 SCO chemistry. For 2, crystal structural analysis indicates that changing the anion from AsF6- to SbF6- led to close stackings between phenyl groups from ligands. These stackings preclude the spin transition of the [Mn(sal-N-1,5,8,12)]+ cations. With NO3- and Cl- as counterions, the [Mn(sal-N-1,5,8,12)]+ cations are arranged more loosely and exhibit gradual SCO in the temperature range of 300-100 K. Careful evaluation of the supramolecular structures of these complexes and similar complexes reported previously revealed strong correlation between the supramolecular packing forces and their magnetic properties.

Direct crystallographic evidence of the reversible photo-formation and thermo-rupture of a coordination bond inducing spin-crossover phenomenon.

A detailed crystallographic study of [FeII(LN5)(CN)2]·MeOH (LN5 = 2,13-dimethyl-3,6,9,12,18-pentaazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene) shows the light-induced formation and thermally-assisted rupture of a metal-ligand coordination bond concomitantly with a spin-crossover process occurring at the Fe(ii) center between its low spin (LS) and high spin (HS) states. These results illustrate the remarkable reversibility and non-destructive character of this photo-induced magneto-structural process in a single crystal.

Mechanism of Vanadium-Catalyzed Deoxydehydration of Vicinal Diols: Spin-Crossover-Involved Processes

Vanadium-catalyzed deoxydehydration (DODH) reactions provide a cost-effective approach for the conversion of vicinal diols to olefin and polycyclic aromatic hydrocarbons. In this paper, density functional theory (DFT) calculations employing M06 and M06-L methods were conducted to clarify the mechanism of V-catalyzed DODH. Three types of mechanisms generally proposed for transition-metal-catalyzed DODH, associated with the previously omitted spin crossover processes, were considered herein. As a result, a different catalytic cycle including a new olefin-formation mechanism was located, which is in contrast to the findings of a recent study. We found that the favorable mechanism involves the condensation of diols to form vanadium(V) diolate, reduction of the vanadium(V) diolate by PPh3, and spin-crossover steps to form a triplet vanadium(III) diolate. Thereafter, single C–O bond cleavage occurs followed by another spin crossover to form a singlet alkylvanadium(V) intermediate. The final concerted V–O/C–O bo...

Oxotris(oxalato)niobate(V) as counterion in cobalt(II) spin-crossover systems

This work is devoted to the investigation of the thermally induced spin-crossover behavior from a high-spin state (HS, S=3/2) at higher temperatures to a low-spin phase (LS, S=1/2) at lower temperatures of the six-coordinate cobalt(II) complex in the compound [Co(terpy)2]3[NbO(C2O4)3]2·3CH​3OH·4H2O (2). The crystal structure of 2 together with that of its counterion as tetraphenylarsonium(V) salt (AsPh4)3[NbO(C2O4)3]·9H2O (1) are also included. The spin-crossover process was followed by the thermal variation of the χMT product between 2.0 and 400K under the warming mode, with the LS configuration being achieved at T⩽200K and the LS→HS interconversion being incomplete at 400K. X-band EPR measurements both in solution and in the solid state were used to characterize the LS state. In order to substantiate the phenomenon in 2, an analytical expression was presented based on the combination of the spin–orbit coupling and both first- and second-order Zeeman effects for a d7 electronic configuration. The best-fit parameters were Cvib=0.0249, λ=−163cm−1, α=1.06, Δ=1532cm−1, with a fixed value of gLS=2.11 determined from the fit of the EPR spectrum at 10K.

A Model Study on the Carbon‐Halogen Activation in Perhalonated Arenes with Isonitrile Under Photoirradiation

AbstractThe mechanisms of the photochemical carbon‐halogen bond activation reactions were investigated theoretically using a model system of hexahalobenzene C6X6 (X=F, Cl, and Br) and phenyl isonitrile (PhN=C:), with the M06‐2X method and the 6–311G(d) basis set. The theoretical findings suggest the photoactivation mechanism should proceed as follows: Rea‐X‐S0 + hν → FC‐X‐T1 → PhNC‐T1 + C6X6 → Int‐X‐T1 → TS‐X‐T1 → T1/S0‐X → Pro‐X‐S0. The present theoretical evidence confirms the reported results by Studer and co‐workers that using UV irradiation, a simple and efficient transition‐metal‐free C‐F activation in various perfluoroarenes with aryl and alkyl isonitriles obtain imidoyl fluorides. Nevertheless, due to the highly vertical Frank‐Condon excitation energy, our theoretical study demonstrates N‐heterocyclic carbene cannot undergo the C‐F bond activation reaction with hexafluorobenzene under photoirradiation, which closely agrees with the available experimental observations. The present theoretical research reveals the spin crossover processes, which lead to a concerted reaction pathway, play a key role in the regioselecitivities of the final photoinsertion products.

Theoretical Study of Spin Crossover in 30 Iron Complexes.

Iron complexes are important spin crossover (SCO) systems with vital roles in oxidative metabolism and promising technological potential. The SCO tendency depends on the free energy balance of high- and low-spin states, which again depends on physical effects such as dispersion, relativistic effects, and vibrational entropy. This work studied 30 different iron SCO systems with experimentally known thermochemical data, using 12 different density functionals. Remarkably general entropy-enthalpy compensation across SCO systems was identified (R = 0.82, p = 0.002) that should be considered in rational SCO design. Iron(II) complexes displayed higher ΔH and ΔS values than iron(III) complexes and also less steep compensation effects. First-coordination sphere ΔS values computed from numerical frequencies reproduce most of the experimental entropy and should thus be included when modeling spin-state changes in inorganic chemistry (R = 0.52, p = 3.4 × 10(-3); standard error in TΔS ≈ 4.4 kJ/mol at 298 K vs 16 kJ/mol of total TΔS on average). Zero-point energies favored high-spin states by 9 kJ/mol on average. Interestingly, dispersion effects are surprisingly large for the SCO process (average: 9 kJ/mol, but up to 33 kJ/mol) and favor the more compact low-spin state. Relativistic effects favor low-spin by ∼9 kJ/mol on average, but up to 24 kJ/mol. B3LYP*, TPSSh, B2PLYP, and PW6B95 performed best for the typical calculation scheme that includes ZPE. However, if relativistic and dispersion effects are included, only B3LYP* remained accurate. On average, high-spin was favored by LYP by 11-15 kJ/mol relative to other correlation functionals, and by 4.2 kJ/mol per 1% HF exchange in hybrids. 13% HF exchange was optimal without dispersion, and 15% was optimal with all effects included for these systems.

Spin-crossover molecule based thermoelectric junction

Using ab-initio numerical methods, we explore the spin-dependent transport and thermoelectric properties of a spin-crossover molecule (i.e., iron complex of 2-(1H-pyrazol-1-yl)-6-(1H-tetrazole-5-yl)pyridine) based nano-junction. We demonstrate a large magnetoresistance, efficient conductance-switching, and spin-filter activity in this molecule-based two-terminal device. The spin-crossover process also modulates the thermoelectric entities. It can efficiently switch the magnitude as well as spin-polarization of the thermocurrent. We find that thermocurrent is changed by ∼4 orders of magnitude upon spin-crossover. Moreover, it also substantially affects the thermopower and consequently, the device shows extremely efficient spin-crossover magnetothermopower generation. Furthermore, by tuning the chemical potential of electrodes into a certain range, a pure spin-thermopower can be achieved for the high-spin state. Finally, the reasonably large values of figure-of-merit in the presence and absence of phonon demonstrate a large heat-to-voltage conversion efficiency of the device. We believe that our study will pave an alternative way of tuning the transport and thermoelectric properties through the spin-crossover process and can have potential applications in generation of spin-dependent current, information storage, and processing.