Yonezo Maeda

Photo‐Induced Spin Transition of Iron(III) Compounds with π–π Intermolecular Interactions

Iron(III) spin-crossover compounds [Fe(pap)(2)]ClO(4) (1), [Fe(pap)(2)]BF(4) (2), [Fe(pap)(2)]PF(6) (3), [Fe(qsal)(2)]NCS (4), and [Fe(qsal)(2)]NCSe (5) (Hpap=2-(2-pyridylmethyleneamino)phenol and Hqsal=2-[(8-quinolinylimino)methyl]phenol) were prepared and their spin-transition properties investigated by magnetic susceptibility and Mössbauer spectroscopy measurements. The iron(III) compounds exhibited spin transition with thermal hysteresis. Single crystals of the iron(III) compounds were obtained as suitable solvent adducts for X-ray analysis, and structures in high-spin (HS) and low-spin (LS) states were revealed. Light-induced excited-spin-state trapping (LIESST) effects of the iron(III) compounds were induced by light irradiation at 532 nm for 1-3 and at 800 nm for 4 and 5. The activation energy E(a) and the low-temperature tunneling rate k(HL)(T-->0) of iron(III) LIESST compound 1 were estimated to be 1079 cm(-1) and 2.4x10(-8) s(-1), respectively, by HS-->LS relaxation experiments. The Huang-Rhys factor S of 1 was also estimated to be 50, which was similar to that expected for iron(II) complexes. It is thought that the slow relaxation in iron(III) systems is achieved by the large structural distortion between HS and LS states. Introduction of strong intermolecular interactions, such as pi-pi stacking, can also play an important role in the relaxation behavior, because it can enhance the structural distortion of the LIESST complex.

Photoinduced spin transition for Iron(II) compounds with liquid-crystal properties.

The iron(II) compounds [Fe(3Cn-L)2(NCS)2] (n = 6 (1), n = 8 (2), n = 10 (3), n = 12 (4), n = 14 (5), n = 16 (6), n = 18 (7), n = 20 (8), and n = 22 (9)) were synthesized and their physical properties characterized by polarizing optical microscopy, differential scanning calorimetry, and powder X-ray analysis, where 3Cn-L denotes bidentate Schiff-base ligands formed from the corresponding aniline derivatives and pyridine-2-carboxyaldehyde. The iron(II) compounds 4-8 exhibited crystal to liquid-crystal transitions at 318, 334, 345, 338, and 347 K, respectively. Variable-temperature magnetic susceptibility measurements revealed that the compounds 1-9 exhibit spin-crossover behavior between the high-spin and low-spin states and a photoinduced spin transition from a low-spin state to a metastable high-spin state. Therefore, the iron(II) compounds 4-8 can undergo spin-crossover and photoinduced spin transition as well as have liquid-crystal properties all in a single molecule. Compounds with multifunctions are important in the development of molecular switches and optical materials.

Unusual heat capacity of the ferric spin-crossover complex, [Fe(acpa)2]PF6, showing a gradual but complete spin-state interconversion at the fast spin-flipping rate†

Abstract The heat capacity of the iron(III) spin-crossover complex [Fe(acpa)2]PF6[Hacpa = N−(1-acetyl-2-propylidene)(2-pyridylmethyl)amine], which shows fast electronic relaxation between S = 1 2 ( 2 T 2g ) and S = 5 2 ( 6 A 1g ) in comparison with the 57Fe Mossbauer lifetime (10−7s), was measured with an adiabatic calorimeter in the 15–320 K range. Variable temperature IR and Raman spectra were recorded between 84 and 300 K. An unusually broad heat-capacity peak starting from ∼ 120K, culminating at ∼ 190K, and terminating at ∼ 280 K was observed. A normal heat capacity curve which separates the excess heat capacity from the observed values was determined. The enthalpy and entropy arising from the spin-crossover phenomenon were 7025 J mol−1 and 36.19 J K−1 mol−1, respectively. The entropy gain was well accounted for in terms of the sum (37.69 J K−1mol−1) of the contribution from a change in the spin-manifold R 1n( 6 2 ) (= 9.13 J K −1 mol −1 ) and from a change in the skeletal normal modes of vibration detected by IR and Raman spectra (28.56 J K−1 mol−1). To elucidate the question, Why does the spin-crossover occur so gradually for the complexes of the fast electronic relaxation type?, the Frenkel theory of heterophase fluctuation in liquids was applied. As a result, the number of molecules in a domain was proved to be as small as five. This makes a large fluctuation possible between the low and high spin states, eventually leading to the spin-equilibrium type transition taking place over a wide temperature range.

Mössbauer studies of FeNiP and related compounds

Abstract Phosphides of the type FeMeP, where Me is a transition metal, have been studied by Mossbauer spectroscopy. This work is mainly concerned with the site preference of metal atoms in many ternary systems in which Me is Ni, Co, Mn, Cr, Zr, Nb and Ta. Iron atoms in alloys FeMnP, FeCrP, FeNbP, FeTaP and FeZrP occupy the smaller lattice position exclusively. Iron atoms in FeCoP are randomly distributed among the available sites. A temperature dependence of site preference in the disordered system, (Fe, Ni) 2 P, was not observed.

Identification of the contamination source of plutonium in environmental samples with isotopic ratios determined by inductively coupled plasma mass spectrometry and alpha-spectrometry

Concentrations of239+249Pu in environmental samples were detemined by ICP-MS and α spectrometry, showing consistent results, which suggests an applicability of ICP-MS to239Pu and240Pu measurement. The activity ratios of238Pu/239+240Pu and240Pu/239Pu were significantly different in samples from the general environment and near Sellafield reprocessing plants, indicating the usefulness of these ratios for identification of the Pu contamination source.

Occupation of tungsten site by iron in sodium tungstate glasses

Abstract 57 Fe Mossbauer spectra of x Na 2 O · (99 − x )WO 3 · 57 Fe 2 O 3 glasses (30 ≤ x ≤ 42) comprised of a doublet due to octahedral iron, Fe 3+ (O h ), and a weak doublet due to Fe 2+ (O h ); a large Debye temperature of 580 K was obtained at low temperatures. Fourier transform infrared spectra of the sodium tungstate glasses showed a gradual increase in the fraction of WO 4 tetrahedra (T d ) when Na 2 O content was increased. Mossbauer spectra of the tungstate glasses irradiated with 60 Co γ-rays showed an increase in the fraction of Fe 2+ , due to electron transfer (scattering) from WO and FeO bonds to Fe 3+ , as the electron spin resonance spectra of iron-free sodium tungstate glass indicated a simultaneous formation of W 5+ and the ‘hole’ trapped on the oxygen atom. From the linear relationship between glass transition temperature, T g , and quadrupole splitting, Δ, of Fe 3+ (‘ T g − Δ rule’), the slope of the straight line was estimated to be 260°C/(mm s −1 ). Mossbauer spectra of heat-treated tungstate glasses showed a gradual change from Fe 3+ (O h ) to Fe 3+ (T d ), along with the precipitation of the Na 2 W 2 O 7 phase composed of W 6+ (O h ) and W 6+ (T d ). These results indicate that iron occupies W 6+ (O h ) sites in glasses, while it occupies W 6+ (T d ) sites in glass-ceramics.

SYNTHESIS AND PROPERTIES OF NEW DIBENZOTETRAAZA[14] ANNULENES AND ITS METAL CHELATES

Abstract New tetraaza-macrocylic ligands, 6,13-diaryl-1,8-dihydrodibenzo[b,i] [1,4,8,11] tetraazacyclotetradecenes, and its copper(II), nickel(II), cobalt(II) and iron(III) complexes were prepared and characterized. The diimidazole complexes of iron(III) with these N4-macrocyclic ligands are of low-spin type. ESR and Mossbauer spectral studies led to the conclusion that one unpaired electron lies in the dxy orbital of iron atom for the iron(III) complexes.

Spectroscopic and magnetic properties of [FeL2(salacen)]PF6(L = imidazole or N-methylimidazole): new examples of intermediate electronic relaxation between S=½ and S=5//2 states. X-Ray crystal structure of [Fe(Him)2(salacen)]PF6

The iron(III) complexes [FeL2(salacen)]PF6[L = imidazole (Him) or N-methylimidazole (mim), salacen = ethylene(N-acetylacetonylideneiminate)(N′-α-methylsalicylideneiminate)] were synthesized, and the phenomenon of the spin transition between high-spin (S=5//2) and low-spin (S=½) states depending on temperature was confirmed by temperature-dependent magnetism and Mossbauer spectra. The time-averaged Mossbauer spectra between the high-spin and low-spin states were observed for [Fe(mim)2(salacen)]PF6. A stochastic model was proposed to estimate a spin-interconversion rate from the Mossbauer spectra of spin-crossover complexes. The spin-interconversion rates of 6.7 × 106 s–1 at 298 K and 3.1 × 106 s–1 at 221 K were estimated using the model. Crystals of [Fe(Him)2(salacen)]PF6 are triclinic, space group P, with a= 10.642(2), b= 12.771(3), c= 9.430(1)A, α= 89.69(1), β= 104.03(1), γ= 96.97(2)°, and Z= 2. The average bond distances Fe–N 1.970 and Fe–O 1.895 A are in good agreement with the corresponding values reported for low-spin isomers of other spin-crossover iron(III) complexes.

Biologically induced Po emission from fresh water

Behavior of Po in fresh waters was examined in laboratory culture experiments using fresh water collected from a small pool, Xi river and Xiqing lake, showing formation of volatile Po compounds followed by emission to air. Addition of tryptone to the fresh water cultures increased the emission of Po considerably along with a growth of microorganisms, suggesting a connection of chemoheterotrophs to Po emission. Participation of photoautotrophs was also considered because Po emission was increased when NaHCO3 was added to the fresh water cultures. The emission behavior of Po and S in these experiments appeared in different ways. The quantity of Po emitted was comparable to the previous culture experiments (Momoshima, Song, Osaki & Maeda, Environ. Sci. Technol., 35, 2956-2960, 2001) in which artificial culture medium containing 3% NaCl was used and inoculated with sea sediment extract. The biological support for Po emission, thus, would be a general phenomenon in fresh water as well as a seawater environment and is possibly a source for atmospheric Po.

TRITIUM CONCENTRATION IN OCEAN

Surface seawater and water vapor about 10 m above the sea level were collected in the Pacific and Indian Oceans during the expedition of KH-96-5 to examine tritium concentrations in open sea. The tritium concentration in the water vapor was one order of magnitude higher than that in the surface seawater, attributed to downward movement of naturally occurring tritium from stratosphere to troposphere.