Partial spin crossover behaviour in a dinuclear iron(II) triple helicate

Reported herein are the synthesis, structural, magnetic and M?ssbauer spectroscopic characterisation of a dinuclear Fe(II) triple helicate complex [Fe(2)(L)(3)](ClO(4))(4).xH(2)O (x = 1-4), 1(H(2)O), where L is a bis-bidentate imidazolimine ligand. Low temperature structural analysis (150 K) and M?ssbauer spectroscopy (4.5 K) are consistent with one of the Fe(II) centres within the helicate being in the low spin (LS) state with the other being in the high-spin (HS) state resulting in a [LS:HS] species. However, M?ssbauer spectroscopy (295 K) and variable temperature magnetic susceptibility measurements (4.5-300 K) reveal that 1(H(2)O) undergoes a reversible single step spin crossover at one Fe(II) centre at higher temperatures resulting in a [HS:HS] species. Indeed, the T(1/2)(SCO) values at this Fe(II) centre also vary as the degree of hydration, x, within 1(H(2)O) changes from 1 to 4 and are centred between ca. 210 K-265 K, respectively. The dehydration/hydration cycle is reversible and the fully hydrated phase of 1(H(2)O) may be recovered on exposure to water vapour. This magnetic behaviour is in contrast to that observed in the related compound [Fe(2)(L)(3)](ClO(4))(4)·2MeCN, 1(MeCN), whereby fully reversible SCO was observed at each Fe(II) centre to give [LS:LS] species at low temperature and [HS:HS] species at higher temperatures. Reasons for this differing behaviour between 1(H(2)O) and 1(MeCN) are discussed.     

Enhanced bistability by guest inclusion in Fe(II) spin crossover porous coordination polymers

Inclusion of thiourea guest molecules in the tridimensional spin crossover porous coordination polymers {[Fe(pyrazine)[M(CN)(4)]} (M = Pd, Pt) leads to novel clathrates exhibiting unprecedented large thermal hysteresis loops of ca. 60 K wide centered near room temperature.     

Photomagnetic properties of iron(II) spin crossover complexes of 2,6-dipyrazolylpyridine and 2,6-dipyrazolylpyrazine ligands

The photomagnetic properties of the following iron(II) complexes have been investigated: [Fe(L1)2][BF4]2, [Fe(L2)2][BF4]2, [Fe(L2)2][ClO4]2, [Fe(L3)2][BF4]2, [Fe(L3)2][ClO4]2 and [Fe(L4)2][ClO4]2 (L1 = 2,6-di{pyrazol-1-yl}pyridine; L2 = 2,6-di{pyrazol-1-yl}pyrazine; L3 = 2,6-di{pyrazol-1-yl}-4-{hydroxymethyl}pyridine; and L4 = 2,6-di{4-methylpyrazol-1-yl}pyridine). Compounds display a complete thermal spin transition centred between 200-300 K, and undergo the light-induced excited spin state trapping (LIESST) effect at low temperatures. The T(LIESST) relaxation temperature of the photoinduced high-spin state for each compound has been determined. The presence of sigmoidal kinetics in the HS --> LS relaxation process, and the observation of LITH hysteresis loops under constant irradiation, demonstrate the cooperative nature of the spin transitions undergone by these materials. All the compounds in this study follow a previously proposed linear relation between T(LIESST) and their thermal spin-transition temperatures T(1/2): T(LIESST) = T(0)- 0.3T(1/2). T(0) for these compounds is identical to that found previously for another family of iron(II) complexes of a related tridentate ligand, the first time such a comparison has been made. Crystallographic characterisation of the high- and low-spin forms, the light-induced high-spin state, and the low-spin complex [Fe(L4)2][BF4]2, are described.     

Lattice-solvent controlled spin transitions in iron(II) complexes

A series of spin transition (ST) iron(II) compounds of the type [FeII2](X)2.{S}2 (where is 4'-(4'-cyanophenyl)-1,2':6'1'-bispyrazolylpyridine, X=ClO4- or BF4-, and S is acetonitrile) was synthesized and magnetically investigated. The effects of the removal of the lattice-solvent molecules and of their different positions relative to the iron(II) cations on the ST process were investigated. Crystallization yields orange block (A.{S}2) crystals of the composition [FeII()2](ClO4)2.{S}2, and two polymorphic compounds of the stoichiometry [FeII()2](BF4)2.{S}2 as red coffin (B.{S}2) and orange block (C.{S}2) crystals. The Fe-N bond distances of A.{S}2 (from 1.921(9) to 1.992(3) A; at 150 K), B.{S}2 (from 1.943(2) to 2.017(2) A; at 180 K) and C.{S}2 (from 1.883(3) to 1.962(3) A; at 180 K) indicate low spin (LS) states of the respective iron(II) ions. Notably, the observed small difference in the Fe-N distances at 180 K for the two polymorphs B.{S}2and C.{S}2 are due to different positions of the acetonitrile molecules in the crystal lattices and illustrate the sensitivity of the spin transition properties on lattice-solvent effects. Variable-temperature single crystal X-ray studies display single-crystal thermochroism (red (LS)<-->orange (HS)) for A.{S}2 and B.{S}2 and ca. 3.6% decrease in the unit cell volume of A.{S}2 from 4403 A3 at 300 K to 4278 A3 at 150 K. The temperature dependent magnetic susceptibilities of A.{S}2 and B.{S}2 demonstrate systematic increase of the spin transition temperatures (T1/2) and continuous decreases of the hysteresis loop width (DeltaT1/2) upon slow lattice-solvent exclusion.     

Thermal and light induced polymorphism in iron(II) spin crossover compounds

The spin crossover complexes [Fe[H(2)B(pz)(2)](2)L]([H(2)B(pz)(2)](-)= dihydrobis(pyrazolyl)borate, L = 2,2[prime or minute]-bipyridine (1), bipy and 1,10-phenanthroline, phen (2)) undergo both thermal and light induced spin crossover, but the structure of the low spin and light induced high spin states for are different from that of the thermally induced high spin state and from those of.     

Difference in spin crossover pathways among saddle-shaped six-coordinated iron(III) porphyrin complexes

The electronic states of a series of saddle-shaped porphyrin complexes [Fe(OMTPP)L(2)](+) and [Fe(TBTXP)L(2)](+) have been examined in solution by (1)H NMR, (13)C NMR, and EPR spectroscopy and by magnetic measurements. While [Fe(OMTPP)(DMAP)(2)](+) and [Fe(TBTXP)(DMAP)(2)](+) maintain the low-spin (S = (1)/(2)) state, [Fe(OMTPP)(THF)(2)](+) and [Fe(TBTXP)(THF)(2)](+) exhibit an essentially pure intermediate-spin (S = (3)/(2)) state over a wide range of temperatures. In contrast, the Py and 4-CNPy complexes of OMTPP and TBTXP exhibit a spin transition from S = (3)/(2) to S = (1)/(2) as the temperature was decreased from 300 to 200 K. Thus, the magnetic behavior of these complexes is similar to that of [Fe(OETPP)Py(2)](+) reported in our previous paper (Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.; Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617-2620) in the context that all these complexes exhibit a novel spin crossover phenomenon in solution. Close examination of the NMR and EPR data of [Fe(OMTPP)L(2)](+) and [Fe(TBTXP)L(2)](+) (L = Py, 4-CNPy) revealed, however, that these complexes adopt the less common (d(xz), d(yz))(4)(d(xy))(1) electron configuration at low temperature in contrast to [Fe(OETPP)Py(2)](+) which shows the common (d(xy))(2)(d(xz), d(yz))(3) electron configuration. These observations have been attributed to the flexible nature of the OMTPP and TBTXP cores as compared with that of OETPP; the relatively flexible OMTPP and TBTXP cores can ruffle the porphyrin ring and adopt the (d(xz), d(yz))(4)(d(xy))(1) electron configuration at low temperature. Therefore, this study reveals that the rigidity of porphyrin cores is an important factor in determining the spin crossover pathways.     

Relevance of supramolecular interactions, texture and lattice occupancy in the designer iron(II) spin crossover complexes

New Fe^II complexes of formula [Fe(3-Br-phen)₂(NCS)₂]·Solvent (Solvent=0.5 CH₃OH (1), 2 CH₂Cl₂ (2), desolvation of 2 (3), 0.5 CH₃COCH₃ (4) and 0 (5)) have been synthesized. ⁵⁷Fe Mössbauer and magnetic investigation reveal unique features atypical of classic [Fe(phen)₂(NCS)₂] polymorphs. Complex 1, prepared by precipitation in MeOH, undergoes upon cooling below room temperature an incomplete and gradual thermally induced spin conversion, while 4 prepared by an extraction method remains mostly in the low-spin state. The non solvated compounds 3 and 5, display a more abrupt spin crossover on cooling around T_1/2=175 K and T_1/2=198 K, respectively. Defects/soft lattice inclusion due to different methods of material synthesis, extent of aging, reaction medium and associated solvent molecules have enormous influence on the particle size and magnetic properties of these complexes. Scanning electron micrographs helps to establish a logical relationship among methods employed for synthesis, texture of materials and their effect on magnetic properties. The crystal structure of 2 determined in the monoclinic space group P2/c (100 K) reveals a mononuclear complex consisting of a distorted FeN₆ octahedron in the low-spin state, constructed from two 3-bromo-1, 10-phenanthroline and two isothiocyanato anions in cis position. Intermolecular interactions between mononuclear units of the S?Br, S?C(H) and π-π type afford a 2D supramolecular network. DFT calculations for the single molecule 2 reveals an energy difference between high-spin and low-spin isomers of 7 kJ/mol suggesting a slight destabilization of the low-spin state compared to [Fe(phen)₂(NCS)₂]. Normal co-ordinate analysis was also carried out for 3 and compared with experimental temperature dependent Raman spectra for 5.     

Iron(II) spin crossover complexes with branched long alkyl chain

The bzimpy iron(II) complexes, 1-3, containing branched long alkyl chains were synthesized and characterized in detail. The temperature-dependant magnetic susceptibility of 1 showed gradual spin crossover behavior from low spin to high spin state, while 2 retained only low spin state in the same condition. Interestingly, 3 displayed an abrupt spin transition in temperature range from T_1/2↑ = 236 K to T_1/2↓ = 230 K with the thermal hysteresis loop about 6 K. The differential scanning calorimetric analysis of 3 revealed two species of liquid crystal phase transitions at 236 K and 351 K, respectively.     

Picosecond X-ray absorption spectroscopy of a photoinduced iron(II) spin crossover reaction in solution

In this study, we perform steady-state and time-resolved X-ray absorption spectroscopy (XAS) on the iron K-edge of [Fe(tren(py)3)](PF6)2 dissolved in acetonitrile solution. Static XAS measurements on the low-spin parent compound and its high-spin analogue, [Fe(tren(6-Me-py)3)](PF6)2, reveal distinct spectroscopic signatures for the two spin states in the X-ray absorption near-edge structure (XANES) and in the X-ray absorption fine structure (EXAFS). For the time-resolved studies, 100 fs, 400 nm pump pulses initiate a charge-transfer transition in the low-spin complex. The subsequent electronic and geometric changes associated with the formation of the high-spin excited state are probed with 70 ps, 7.1 keV, tunable X-ray pulses derived from the Advanced Light Source (ALS). Modeling of the transient XAS data reveals that the average iron-nitrogen (Fe-N) bond is lengthened by 0.21+/-0.03 A in the high-spin excited state relative to the ground state within 70 ps. This structural modification causes a change in the metal-ligand interactions as reflected by the altered density of states of the unoccupied metal orbitals. Our results constitute the first direct measurements of the dynamic atomic and electronic structural rearrangements occurring during a photoinduced FeII spin crossover reaction in solution via picosecond X-ray absorption spectroscopy.     

Spin crossover behavior in a family of iron(II) zigzag chain coordination polymers

The paper reports the synthesis and detailed characterization of two new Fe(II) compounds: [Fe(pyim)(2)(bpen)](ClO(4))(2).2C(2)H(5)OH (2) and [Fe(pyim)(2)(bpe)](ClO(4))(2).C(2)H(5)OH (3) (pyim = 2-(2-pyridyl)imidazole, bpen = 1,2-bis(4-pyridyl)ethane, and bpe = 1,2-bis(4-pyridyl)ethene). Both compounds and the earlier synthesized [Fe(pyim)(2)(bpy)](ClO(4))(2).2C(2)H(5)OH (1) (bpy = 4,4'-bipyridine) form a family of one-dimensional spin crossover coordination polymers. Variable-temperature magnetic susceptibility measurements and M?ssbauer spectroscopy have revealed rather gradual spin transitions centered at 176 and 198 K for 2 and 3, respectively. The fitting of magnetic properties with the regular solution model leads to the enthalpy and entropy of spin transitions and the cooperativity parameter equal to DeltaH = 12.3 kJ mol(-1), DeltaS = 68.5 J mol(-1) K(-1), Gamma = 1.80 kJ mol(-1) for 2 and DeltaH = 13.6 kJ mol(-1), DeltaS = 68.1 J mol(-1) K(-1), Gamma = 2.05 kJ mol(-1) for 3. The crystal structures of 2 and 3, resolved by X-ray diffraction at 293 K, belong to the monoclinic space group C2/c (Z = 4). Both compounds display a one-dimensional infinite zigzag-chain structure. The polymer chains are stacked into two-dimensional sheets through intermolecular pi-interactions. The crystal packing of both compounds encloses two kinds of channels in which the counter ions and ethanol molecules are inserted. The DFT calculations of binuclear fragments extracted from three polymers resulted in the energy gaps between the LS and HS states being ordered as the observed transition temperatures. The influence of bridging ligands in the studied family of compounds was found in the modulation of the energy gap between the LS and HS states, leading to different transition temperatures.     

Spin crossover in tetranuclear cyanide-bridged iron(II) square complexes: a theoretical study

Spin crossover in a series of six cyanide-bridged iron(II) tetranuclear square complexes was analyzed using density functional theory (DFT) methods. As the spin crossover between the low-spin (LS) and high-spin (HS) states can occur only for two of four iron ions, we characterized energetically and structurally the [LS-LS], [HS-LS], and [HS-HS] spin-state isomers. For all studied complexes, the energy of the mixed [HS-LS] spin state does not deviate essentially from the halfway point between the energies of homogeneous spin states, thereby satisfying the conditions for an one-step transition between the [LS-LS] and [HS-HS]. This fact reflects the weak elastic coupling between the environments of transiting centers. The two-step spin transition observed in one complex can appear only due to the crystal packing effects. We also evaluated the strength of exchange coupling between the paramagnetic ions in the [HS-HS] state.     

Synthesis and characterization of a dinuclear iron(II) spin crossover complex with wide hysteresis

The magnetic properties and results from X-ray structure analysis for a new pair of iron(II) spin-crossover complexes [FeL1(meim) 2](meim) ( 1(meim)) and [Fe 2L2(meim) 4](meim) 4 ( 2(meim) 4), with L1 being a tetradentate N 2O 2 (2-) coordinating Schiff-base-like ligand [([3,3']-[1,2-phenylenebis(iminomethylidyne)]bis(2,4-pentane-dionato)(2-)N,N',O (2),O (2)'], L2 being an octadentate, dinucleating N 2O 2 (2-) coordinating Schiff-base-like ligand [3,3',3',3']-[1,2,4,5-phenylenetetra(iminomethylidyne)]tetra(2,4-pentanedionato)(2-) N, N', N', N', O (2), O (2) ', O (2) ', O (2) '], and meim being N-methylimidazole, are discussed in this work. Crystalline samples of both complexes show a cooperative spin transition with an approximately 2-K-wide thermal hysteresis loop in the case of 1(meim) ( T 1/2 increase = 179 K and T 1/2 decrease = 177 K) and an approximately 21-K-wide thermal hysteresis loop in the case of dinuclear complex 2(meim) 4 ( T 1/2 increase= 199 K and T 1/2 decrease= 178 K). For a separately prepared powder sample of 2, a gradual spin transition with T 1/2 = 229 K is observed that was additionally followed by Mossbauer spectroscopy. The results from X-ray structure analysis give a deeper insight into the molecule packing in the crystal and, by this, help to explain the increase of cooperative interactions during the spin transition when going from the mononuclear to the dinuclear complex. Both compounds crystallize in the triclinic space group P1, and the X-ray structure was analyzed before and after the spin transition. The change of the spin state at the iron center is accompanied by a change of the O-Fe-O angle, the so-called bite of the equatorial ligand, from about 109 degrees in the high-spin state to 89 degrees in the low-spin state. The cooperative interactions responsible for the thermal hysteresis loop are due to elastic interactions between the complex molecules in both cases. However, due to the higher symmetry of the dinucleating ligand in 2(meim) 4, a 3D network of short contacts is formed, while for mononuclear complex 1(meim), a 2D layer of linked molecules is observed. The spin transition was additionally followed in solution using (1)H NMR spectroscopy for both complexes. In both cases, a gradual spin transition is observed, and the increase of cooperative interactions when going from the mononuclear to the dinuclear system is solely attributed to the extended network of intermolecular contacts.     

A DFT computational study of spin crossover in iron(III) and iron(II) tripodal imidazole complexes. A comparison of experiment with calculations

B3LYP* functionals were used to model the sixteen iron(II) (1A, LS and 5T, HS) and iron(III) (2T, LS and 6A, HS) complexes of the 1:3 Schiff base condensate of tris(2-aminoethyl)amine and imidazole-4-carboxaldehyde, H3L1, and its deprotonated forms, [H2L1]1-, [HL1]2-, and [L1]3-. This ligand system is unusual in that [FeH3L1]3+, [FeH3L1]2+ and [FeL1]- all exhibit a spin crossover between 100-300 K. This makes these complexes ideal for a hybrid DFT computational approach and provides an opportunity to refine the value of the exact exchange admixture parameter, c3, and to predict properties of partially protonated complexes that are not experimentally available. The accepted value of 0.20 is larger than the value of approximately 0.13 that was found to best reproduce experimental data in terms of spin state predictions. With iron(III) B3LYP calculations showed that all of the complexes were low spin at 298 K with the exception of [FeH3L1]3+ which is spin crossover in agreement with experimental results. It was also shown for iron(III) that the ligand field increased as the number of protons decreased. In contrast all of the iron(II) complexes were close to the spin crossover region regardless of protonation state. Experimental structures are fairly well modeled by this system in regard to the key structural indicators of spin state, which are the bite and trans angles. The calculated iron to nitrogen atom distances are always larger in the high spin form than the low spin form but all iron to nitrogen bond distances are larger than the experimental values. In general non-bonded interactions are not well modeled by this methodology.     

A DFT computational study of spin crossover in iron(III) and iron(II) tripodal imidazole complexes. A comparison of experiment with calculations

B3LYP* functionals were used to model the sixteen iron(II) (1A, LS and 5T, HS) and iron(III) (2T, LS and 6A, HS) complexes of the 1 : 3 Schiff base condensate of tris(2-aminoethyl)amine and imidazole-4-carboxaldehyde, H3L1, and its deprotonated forms, [H2L1]1-, [HL1]2-, and [L1]3-. This ligand system is unusual in that [FeH3L1]3+, [FeH3L1]2+ and [FeL1]- all exhibit a spin crossover between 100-300 K. This makes these complexes ideal for a hybrid DFT computational approach and provides an opportunity to refine the value of the exact exchange admixture parameter, c3, and to predict properties of partially protonated complexes that are not experimentally available. The accepted value of 0.20 is larger than the value of approximately 0.13 that was found to best reproduce experimental data in terms of spin state predictions. With iron(III) B3LYP calculations showed that all of the complexes were low spin at 298 K with the exception of [FeH3L1]3+ which is spin crossover in agreement with experimental results. It was also shown for iron(III) that the ligand field increased as the number of protons decreased.In contrast all of the iron(II) complexes were close to the spin crossover region regardless of protonation state. Experimental structures are fairly well modeled by this system in regard to the key structural indicators of spin state, which are the bite and trans angles. The calculated iron to nitrogen atom distances are always larger in the high spin form than the low spin form but all iron to nitrogen bond distances are larger than the experimental values. In general non-bonded interactions are not well modeled by this methodology.     

Theoretical and experimental investigation of novel iron(II)-based spin crossover compounds

Two novel spin crossover (SCO) compounds, namely [Fe(INMe)(pyN₄)]Br₂, and [Fe(IMMe)(pyN₄)](OTf)₂, where pyN₄ = 2,6-Bis(1′,3′-diamino-2′methyl-prop-2′yl)pyridine, INMe = isonicotinic acid methyl ester, IMMe = N-methyl-imidazole, and OTf = triflate, are characterized here both from experimental and theoretical viewpoints. In particular, we apply various density functionals and basis sets to obtain optimized geometries for low- (LS) and high-spin (HS) states, vibrational spectra, LS–HS splittings, and temperature-dependent UV/vis spectra. While geometries and spectra are in good agreement with experimental data, the well-known spin pairing problem makes it difficult to compute accurate LS–HS splitting energies and enthalpies. Based on TD–DFT calculations, the capacity of the compounds for use as reversibly photo-switchable molecules is discussed.     

Dinuclear Fe(III) complexes with spin crossover

Abstract  A series of dinuclear Fe(III) complexes was synthesized in which the Schiff-base blocking ligand L⁵ coordinates each of the centers which are linked by a bidentate, bipyridine-type ligand. For these systems, [L⁵Fe^III{bridge}Fe^IIIL⁵](BPh₄)₂, thermally induced spin crossover is observed. The corollary of the systems is that the spin crossover interferes with the magnetic exchange interaction. The overlap of the energy bands of the LL and HH reference states (L, low-spin; H, high-spin) causes the exchange interaction to act against the spin crossover (leading to incompleteness or gradual behavior). Graphical abstract        

Spin crossover in iron(II) complexes with tris(pyrazol-1-yl)methane

Mononuclear iron(II) coordination compounds with tris(pyrazol-1-yl)methane (HC(Pz)₃) described as [Fe{HC(Pz)₃}₂]A₂ × nH₂O, where A = Cl⁻, Br⁻, I⁻, 1/2 SO₄²⁻, n = 0–7, were synthesized. The compounds were studied by static magnetic susceptibility measurements, IR and UV/Vis spectroscopy, and powder X-ray diffraction. The crystal and molecular structures of all compounds were determined by single crystal X-ray diffraction.     

Spin crossover in a four-coordinate iron(II) complex

The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)(3)Fe-N═PPh(3) undergoes an S = 0 to S = 2 spin transition with T(C) = 81 K, as determined by variable-temperature magnetic measurements and Mo?ssbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.