Lipid-packaged linear iron(II) triazole complexes in solution: controlled spin conversion via solvophobic self-assembly

Linear Fe(II) 1,2,4-triazole complexes with lipid counteranions are newly developed. These complexes show sharp and reversible spin conversion in toluene, with temperatures significantly higher (by 20-100 K) than the spin crossover temperatures observed in the crystalline states. This is accounted for in terms of increased metal-ligand interactions in organic media, which is caused by solvophobic compaction of charged coordination chains. In atomic force microscopy, developed nanowires are observed for low spin (LS) complexes. On the other hand, fragmented nanostructures are seen for high spin (HS) complexes, indicating that the spin conversion in solution is governed by a self-assembly process. The lipid packaging of charged coordination chains thus provides powerful means to improve and regulate their functions via solvophobic self-assembly.     

The influence of ligand conformation on the thermal spin transitions in iron(III) saltrien complexes

We report the syntheses of six new salts of the iron(III) complexes of four hexadentate Schiff bases of the saltrien-type derived from 1,8-diamino-3,6-diazaoctane and 2 equiv. of the appropriate salicylaldehyde derivative. Three of these new compounds undergo gradual spin-transitions centred between 155-204 K in the solid state, that proceed to 35-50% completeness. One of the other compounds remains high-spin on cooling, another is low-spin, while the last undergoes spin-crossover centred above room temperature. This spin-state variability is typical of solid compounds of this type. As an aid to understanding this behaviour, the crystal structures and magnetochemistry of the known spin-crossover salts [Fe(saltrien)]PF6, [Fe(saltrien)]ClO4 and [Fe(saltrien)]BPh4 have also been reexamined. The structural chemistry of all three salts was found to differ significantly from what has been previously reported. Correlation of the crystal structures and magnetic behaviour of the compounds in this and previous work suggests that the disposition of the two ligand phenoxy groups plays an important role in determining whether high-spin [Fe(saltrien)]+ complexes can undergo thermal spin-transitions.     

Solid-state ligand-driven light-induced spin change at ambient temperatures in bis(dipyrazolylstyrylpyridine)iron(II) complexes

We previously reported that an Fe(II) complex ligated by two (Z)-2,6-di(1H-pyrazol-1-yl)-4-styrylpyridine ligands (Z-H) presented a solid state ligand-driven light-induced spin change (LD-LISC) upon one-way Z-to-E photoisomerization, although modulation of the magnetism was trivial at ambient temperatures (Chem. Commun.2011, 47, 6846). Here, we report the synthesis of new derivatives of Z-H, Z-CN and Z-NO(2), in which electron-withdrawing cyano and nitro substituents are introduced at the 4-position of the styryl group to attain a more profound photomagnetism at ambient temperatures. Z-CN and Z-NO(2) undergo quantitative one-way Z-to-E photochromism upon excitation of the charge transfer band both in acetonitrile and in the solid state, similar to the behavior observed for Z-H. In solution, these substituents stabilized the low-spin (LS) states of Z-CN and Z-NO(2), and the behavior was quantitatively analyzed according to the Evans equation. The photomagnetic properties in the solid state, on the other hand, cannot be explained in terms of the substituent effect alone. Z-CN displayed photomagnetic properties almost identical to those of Z-H. Z-CN preferred the high-spin (HS) state at all temperatures tested, whereas photoirradiated Z-CN yielded a lower χ(M)T at ambient temperatures. The behavior of Z-NO(2) was counterintuitive, and the material displayed surprising photomagnetic properties in the solid state. Z-NO(2) occupied the LS state at low temperatures and underwent thermal spin crossover (SCO) with a T(1/2) of about 270 K. The photoirradiated Z-NO(2) displayed a higher value of χ(M)T and the modulation of χ(M)T exceeded that of Z-H or Z-CN. Z-NO(2)·acetone, in which acetone molecules were incorporated into the crystal lattice, further stabilized the LS state (T(1/2) > 300 K), thereby promoting large modulations of the χ(M)T values (87% at 273 K and 64% at 300 K) upon Z-to-E photoisomerization. Single crystal X-ray structure analysis revealed that structural factors played a vital role in the photomagnetic properties in the solid state. Z-H and Z-CN favored intermolecular π-π stacking among the ligand molecules. The coordination sphere around the Fe(II) nucleus was distorted, which stabilized the HS state. In contrast, Z-NO(2)·acetone was liberated from such intermolecular π-π stacking and coordination distortion, resulting in the stabilization of the LS state.     

Spin transition in iron(III) and iron(II) complexes

The main stages of the studies on the spin transitions in iron(III) and iron(II) complexes are considered. The types of the spin transitions and the factors responsible for the latter are reported. The problems arising during experiments in this field are discussed.     

Thermally induced spin transitions in nitroxide-copper(II)-nitroxide spin triads studied by EPR

Thermally induced spin transitions in a family of heterospin polymer chain complexes of Cu2+ hexafluoroacetylacetonate with two pyrazole-substituted nitronyl nitroxides are studied using electron paramagnetic resonance (EPR) spectroscopy. The structural rearrangements at low temperatures induce spin transitions in exchange-coupled spin triads of nitroxide-copper(II)-nitroxide. The values of exchange interactions in spin triads of studied systems are typically on the order of tens to hundreds of inverse centimeters. The large magnitude of exchange interaction determines the specific and very informative peculiarities in EPR spectra due to the predominant population of the ground state of a spin triad and spin exchange processes. The variety of these manifestations depending on structure and magnetic properties of spin triads are described. EPR is demonstrated as an efficient tool for the characterization of spin transitions and for obtaining information on the temperature-dependent sign and value of the exchange interaction in strongly coupled spin triads.     

Anion-solvent dependence of bistability in a family of meridional N-donor-ligand-containing iron(II) spin crossover complexes

Five mononuclear spin crossover iron(II) bis-meridional ligand complexes of the general formula [Fe(L)(2)](X)(2).solvent, have been synthesized, where X = BF(4)- or ClO(4)-; L = 2-(1-pyridin-2-ylmethyl-1H-pyrazol-3-yl)-pyrazine (picpzpz) or 2-(3-(2-pyridyl)pyrazol-1-ylmethyl)pyridine) (picpypz); solvent = MeOH or EtOH. The magnetic and structural consequences of systematic variation of meridional ligand, solvent, and anion, including a desolvated species, have been investigated. The complex [Fe(picpzpz)(2)](BF(4))(2).MeOH, 1.MeOH, displays several unique properties including a two-step spin transition with a gradual higher-temperature step ((1)T(1/2) = 197 K) and an abrupt low-temperature step with hysteresis ((2)T(1/2) = 91/98 K) and a metastable intermediate spin state below 70 K with quench-cooling. Removal of the solvent methanol results in the loss of the abrupt step and associated hysteresis (T(1/2) = 150 K). The complexes [Fe(picpzpz)(2)](BF(4))(2).EtOH (1.EtOH), [Fe(picpzpz)(2)](ClO(4))(2).MeOH (2.MeOH), [Fe(picpzpz)(2)](ClO(4))(2).EtOH (2.EtOH), and [Fe(picpypz)(2)](BF(4))(2).MeOH (3.MeOH) all show gradual one-step spin transitions with T(1/2) values in the range 210-250 K. Photomagnetic LIESST measurements on 1.MeOH reveal a near-quantitative excitation of high-spin sites and a unique two-step relaxation process related to the two-step thermal spin transition ((1)T(LIESST) = 49 K and (2)T(LIESST) = 70 K). The structural consequences of the unusual spin transition displayed by 1.MeOH have been investigated by single-crystal X-ray diffraction structural analyses between 25 and 293 K. Detailed characterization of the unit cell parameter evolution vs temperature reflects both the gradual high-temperature step and abrupt low-temperature step, including the thermal hysteresis, observed magnetically.     

Alkyl isomerisation in three-coordinate iron(II) complexes

The tertiary to iso-butyl isomerisation of three-coordinate iron(II) diketiminate complexes is reported and a hydride intermediate is proposed on the basis of exchange experiments.     

The nature of spin-state transitions in Fe(II) complexes

The nature of the spin-state transition for three complexes of Fe(II), namely [Fe(phy)₂] (ClO₄)₂, [Fe(phy)₂] (BF₄)₂ and [Fe(bts)₂(NCS)₂] (where phy = 1,10-phenantroline-2-carbaldehyde phenyihydrazone and bts = 2.2′-bi-5-methyl-2-thiazoline) has been investigated by differential scanning calorimetry. For [Fe(phy)₂] (ClO₄)₂ and [Fe(phy)₂] (BF₄)₂, the spin transition is essentially of first order with ΔH = 15.7 ± 1. ΔS = 64 ± 4 and ΔH = 24.2 = 1kJ/mole. ΔS = 86 ± 5 J mol^?1 K^?1, respectively. For [Fe(bts)₂(NCS)₂] the DSC studies do not suggest a first-order transition. The observations conform to the conclusions drawn from previous studies. The relevance of ΔH and ΔS derived from ln K versus l/T plots is discussed.     

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.     

Gaseous iron(II) and iron(III) complexes with BINOLate ligands

Electrospray ionization (ESI) of dilute solutions of 1,1'-bi-2-naphthol (BINOL) and iron(II) or iron(III) sulfate in methanol/water allows the generation of monocationic complexes of iron and deprotonated BINOL ligands with additional methanol molecules in the coordination sphere, and the types of complexes formed can be controlled by the valence of the iron precursors used in ESI. Thus, iron(II) sulfate leads to [(BINOLate)Fe(CH3OH)n]+ complexes (n=0-3), whereas usage of iron(III) sulfate allows the generation of [(BINOLdiate)-Fe(CH3OH)n]+ cations (n=0-2); here, BINOLate and BINOLdiate stand for singly and doubly deprotonated BINOL, respectively. Upon collision-induced dissociation, the mass-selected ions with n>0 first lose the methanol ligands and then undergo characteristic fragmentations. Bare [(BINOLdiate)Fe]+, a formal iron(III) species, undergoes decarbonylation, which is known as a typical fragmentation of ionized phenols and phenolates either as free species or as the corresponding metal complexes. The bare [(BINOLate)Fe]+ cation, on the other hand, preferentially loses neutral FeOH to afford an organic C20H12O+* cation radical, which most likely corresponds to ionized 1,1'-dinaphthofurane.     

Chemistry of iron complexes. Part V. Electron spin resonance spectra of some iron(III) complexes and X-Ray diffraction (powder) data of iron(II) complexes employing tertiary phosphine/arsine oxides

Summary Electron spin resonance spectra (X-band, 9.3 GHz) of iron(III) chloride complexes with tri-p-tolylarsine oxide (T₃AO), methylenebis(diphenylphosphine oxide) (mdpo) and 1,4-tetramethylenebis(diphenylphosphine oxide) (tmdpo) support their structures as [Fe(L-L)₂Cl₂][FeCl₄] (L-L=mdpo or tmdpo) and [Fe(T₃AO)₂Cl₂(OH₂)₂][FeCl₄]2H₂O. The x-ray powder diffraction patterns of some iron(II) iodide complexes with mdpo, tmdpo, dmdpo, dmdao and tmdao [dmdpo-1,2-dimethylenebis(diphenylphosphine oxide); dmdao and tmdao are the arsine analogs of dmdpo and tmdpo] show that the complexes are crystalline but not isomorphous.     

Solid complexes of iron(II) and iron(III) with rutin

Solid complex compounds of Fe(II) and Fe(III) ions with rutin were obtained. On the basis of the elementary analysis and thermogravimetric investigation, the following composition of the compounds was determined: (1) FeOH(C₂₇H₂₉O₁₆)·5H₂O, (2) Fe₂OH(C₂₇H₂₇O₁₆)·9H₂O, (3) Fe(OH)₂(C₂₇H₂₉O₁₆)·8H₂O, (4) [Fe₆(OH)₂(4H₂O)(C₁₅H₇O₁₂)SO₄]·10H₂O. The coordination site in a rutin molecule was established on the basis of spectroscopic data (UV–Vis and IR). It was supposed that rutin was bound to the iron ions via 4C=O and 5C—oxygen in the case of (1) and (3). Groups 5C–OH and 4C=O as well as 3′C–OH and 4′C–OH of the ligand participate in binding metals ions in the case of (2). At an excess of iron(III) ions with regard to rutin under the synthesis conditions of (4), a side reaction of ligand oxidation occurs. In this compound, the ligands’ role plays a quinone which arose after rutin oxidation and the substitution of Fe(II) and Fe(III) ions takes place in 4C=O, 5C–OH as well as 4′C–OH, 3′C–OH ligands groups. The magnetic measurements indicated that (1) and (3) are high-spin complexes.     

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.     

Radiation synthesis of iron(II) and ruthenium(II) alkylnitroso complexes

The radiolysis of deoxygenated aqueous solutions of Ru(NH₃)₅NO³⁺ and Fe(CN)₅NO²⁻ in the presence of organic compounds (RH) generates alkylnitroso complexes of the form Ru(NH₃)₅N(O)R²⁺ and Fe(CN)₅N(O)R³⁻ where RH = tert-butyl alcohol, tert-butyl amine, N,N-dimethylacetamide, α-aminoisobutyric acid, pivalic acid, and α-hydroxyisobutyric acid. The products form from the rapid combination of the β-carbon radical derived from the reaction of the organic compound with OH radicals (OH + RH → R· + H₂O) and the one-electron reduced metal complex formed by interaction with e_aq⁻: Ru(NH₃)₅NO³⁺ + e_aq⁻ → Ru(NH₃)₅NO²⁺; Fe(CN)₅NO²⁻ + e_aq⁻ → Fe(CN)₅NO³⁻. The alkylnitroso complexes are moderately O₂-insensitive but display varying degrees of thermal stability. Stability permitting, these complexes have been characterized by ion-exchange chromatography and UV-vis-IR spectroscopy. The green ruthenium complexes exhibit λ_max 740 and 342 nm (ϵ 22 and 4.5 × 10³ M⁻¹ cm⁻¹, respectively) and ν_NO in the 1365–1405 cm⁻¹ region. The less stable red iron analogues absorb at 475 and ∼ 250 nm (ϵ 5.0 × 10³ and ∼ 9 × 10³ M⁻¹ cm⁻¹, respectively).