Observation of a 331 K (58 °C) spin transition for bis[hydrotris(1,2,4-triazol-1-yl)borate]iron(II) by variable temperature infrared spectroscopy and magnetic susceptibility measurements

Fe{HB(CHN)} is observed by variable temperature infrared and magnetic studies to have a spin transition between the low spin S = 0 and high spin S = 2 states at 331 K (58 °C) with thermal hysteresis of ~1.5 K. Changes in the triazole ligand IR absorptions demonstrate that distant non-metal-ligand vibrations are altered upon the change in electronic structure associated with the spin-crossover can be used to monitor the the spin-crossover transition.

Structure and physical properties of [µ-Tris(1,4-bis(tetrazol-1-yl)butane-N4,N4‘')iron(II)] Bis(hexafluorophosphate), a new Fe(II) spin-crossover compound with a three-dimensional threefold interlocked crystal lattice

[μ-Tris(1,4-bis(tetrazol-1-yl)butane-N4,N4‘)iron(II)] bis(hexafluorophosphate), [Fe(btzb)3](PF6)2, crystallizes in a three-dimensional 3-fold interlocked structure featuring a sharp two-step spin-crossover behavior. The spin conversion takes place between 164 and 182 K showing a discontinuity at about T1/2 = 174 K and a hysteresis of about 4 K between T1/2 and the low-spin state. The spin transition has been independently followed by magnetic susceptibility measurements, 57Fe-Mössbauer spectroscopy, and variable temperature far and midrange FTIR spectroscopy. The title compound crystallizes in the trigonal space group P30¯(No. 147) with a unit cell content of one formula unit plus a small amount of disordered solvent. The lattice parameters were determined by X-ray diffraction at several temperatures between 100 and 300 K. Complete crystal structures were resolved for 9 of these temperatures between 100 (only low spin, LS) and 300 K (only high spin, HS), Z = 1 [Fe(btzb)3](PF  6)2:  300 K (HS), a = 11.258(6) Å, c = 8.948(6) Å, V = 982.2(10) Å3; 100 K (LS), a = 10.989(3) Å, c = 8.702(2) Å, V = 910.1(4) Å3. The molecular structure consists of octahedral coordinated iron(II) centers bridged by six N4,N4‘ coordinating bis(tetrazole) ligands to form three 3-dimensional networks. Each of these three networks is symmetry related and interpenetrates each other within a unit cell to form the interlocked structure. The Fe−N bond lengths change between 1.993(1) Å at 100 K in the LS state and 2.193(2) Å at 300 K in the HS state. The nearest Fe separation is along the c-axis and identical with the lattice parameter c.

catena-[μ-tris(1,2-bis(tetrazol-1-yl)ethane-N4,N4')iron(II)] bis(tetrafluoroborate): Synthesis, structure, spectroscopic and magnetic characterisation of a chain-type coordination polymer spin-crossover compound.

In analogy to a common synthesis of 1-substituted 5-H tetrazoles (Tetrahedron Lett. 36 (1995)1759; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 11 (1985) 1521; Beloruss. Gos. Univ., Minsk, USSR. Khim. Geterotsikl. Soedin. 1 (1991) 66; BGU, Belarus. Vestsi Akad. Navuk Belarusi, Ser. Khim. Navuk 1 (1992) 73), the new bidentate ligand 1,2-bis(tetrazol-1-yl)ethane [endi] was synthesized and characterized by X-ray diffraction, NMR, IR and UV–Vis spectroscopy. By using iron(II) tetrafluoroborate hexahydrate the complexation with this ligand yields a 1-dimensional linear coordination polymer similar to the recently published chain compound (Inorg. Chem. 39 (2000) 1891) exhibiting a thermally induced spin-crossover phenomenon. Similar to the 1,2-bis(tetrazol-1-yl)propane-bridged compound, our 1,2-bis(tetrazol-1-yl)ethane-bridged compound shows a gradual spin transition, but the spin-crossover temperature T1/2≈140 K is found to be 10 K above the other T1/2. The T1/2 was determined by temperature-dependent 57Fe-Mössbauer, far FT-IR and UV–Vis spectroscopy as well as by temperature-dependent magnetic susceptibility measurements. Single crystals of the complex were grown in situ from a solution of the ligand and iron(II) tetrafluoroborate. The X-ray structure determinations of both the high spin as well as the low spin state of the compound revealed a solid state structure, which is comparable to that of catena-[Fe(1,2-bis(tetrazole-1-yl)propane)3](ClO4)2 (Inorg. Chem. 39 (2000) 1891; 2nd TMR-TOSS Meeting, 4th Spin Crossover Family Meeting, Lufthansa Training Center, Seeheim/Germany, April 30–May 2, 1999). Both the 1,2-bis(tetrazol-1-yl)propane-bridged and our compound do not show a thermal hysteresis effect (J. Am. Chem. Soc. 115 (1993) 9810; Inorg. Chim. Acta 37 (1979) 169; Chem. Phys. Lett. 93 (1982) 567). The synthesis of the complex described in the experimental section yielded a fine powdered product being poorly soluble in most common solvents. The single crystal measurements were done with crystals obtained by various diffusion methods. Most of them yielded either thin needles or small hexagonal prism crystals depending on the specific conditions.

The missing link:Analogous human and primate cortical gamma oscillations

Recent animal studies highlighting the relationship between functional imaging signals and the underlying neuronal activity have revealed the potential capabilities of non-invasive methods. However, the valuable exchange of information between animal and human studies remains restricted by the limited evidence of direct physiological links between species. In this study we used magnetoencephalography (MEG) to investigate the occurrence of 30-70 Hz (gamma) oscillations in human visual cortex, induced by the presentation of visual stimuli of varying contrast. These oscillations, well described in the animal literature, were observed in retinotopically concordant locations of visual cortex and show striking similarity to those found in primate visual cortex using surgically implanted electrodes. The amplitude of the gamma oscillations increases linearly with stimulus contrast in strong correlation with the gamma oscillations found in the local field potential (LFP) of the macaque. We demonstrate that non-invasive magnetic field measurements of gamma oscillations in human visual cortex concur with invasive measures of activation in primate visual cortex, suggesting both a direct representation of underlying neuronal activity and a concurrence between human and primate cortical activity. © 2005 Elsevier Inc. All rights reserved.

Development of a novel magnetic resonance imaging contrast agent for pressure measurements using lipid-coated microbubbles

The aim of this study was to prepare gas-filled lipid-coated microbubbles as potential MRI contrast agents for imaging of fluid pressure. Air-filled microbubbles were produced with phospholipid 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC) in the presence or absence of cholesterol and/or polyethylene-glycol distearate (PEG-distearate). Microbubbles were also prepared containing a fluorinated phospholipid, perfluoroalkylated glycerol-phosphatidylcholine, F-GPC shells encompassing perfluorohexane-saturated nitrogen gas. These microbubbles were evaluated in terms of physico-chemical characteristics such as size and stability. In parallel to these studies, DSPC microbubbles were also formulated containing nitrogen (N2) gas and compared to air-filled microbubbles. By preventing advection, signal drifts were used to assess their stability. DSPC microbubbles were found to have a drift of 20% signal change per bar of applied pressure in contrast to the F-GPC microbubbles which are considerably more stable with a lower drift of 5% signal change per bar of applied pressure. By increasing the pressure of the system and monitoring the MR signal intensity, the point at which the majority of the microbubbles have been damaged was determined. For the DSPC microbubbles this occurs at 1.3 bar whilst the F-GPC microbubbles withstand pressures up to 2.6 bar. For the comparison between air-filled and N2-filled microbubbles, the MRI sensitivity is assessed by cycling the pressure of the system and monitoring the MR signal intensity. It was found that the sensitivity exhibited by the N2-filled microbubbles remained constant, whilst the air-filled microbubbles demonstrated a continuous drop in sensitivity due to continuous bubble damage.