Pressure-Induced Spin Crossover at Room Temperature in a Nanoporous Host-Guest Framework Structure.

The two-dimensional (2D) metal-organic framework compound Fe2 (azpy)4 (NCS)4 (azpy=trans-4,4'-azopyridine, NCS=thiocyanate) has been pressurized in isopropanol and ethanol that act as guest molecules and pressure-transmitting media. Room temperature conversion of metal-ion sites from high-spin to low-spin states under high-pressure-driven uptake of guest molecules was investigated. Upon isopropanol guest inclusion, spin conversion of ∼66 % low-spin abundance is attained by ∼3 GPa. Spin-crossover progression stimulated by ethanol guest inclusion is more sluggish and ∼56 % low-spin abundance is attained by ∼5.5 GPa. This pressure-facilitated guest-specific triggering of spin crossover is suggestive of guest-molecule interactions with the local coordination network of the Fe(II) core, which increases the ligand field. Appreciable low-spin abundances attained at room temperature upon pressurization, are comparable to maximal ∼50 % spin conversions only attainable below ∼150 K at ambient pressure.

(Phenoxyimidazolyl-salicylaldimine)iron complexes: synthesis, properties and iron catalysed ethylene reactions.

The reaction of 2-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L1), 2,4-di-tert-butyl-6-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L2) or 4-tert-butyl-2-{[2-(1H-imidazol-4-yl)-ethylimino]-methyl}-phenol (L3) with iron(ii) precursors produced either iron(ii) or iron(iii) complexes, depending on the nature of the anions in the iron(ii) precursor and the ligand. When the anion is chloride and the ligand L1, the product is [(L1)2Fe][FeCl4] (1), but when the anion is triflate (OTf(-)) and the ligand is L2, the product is [(L2)2Fe][OTf]2 (2). With iron(ii) halides and tert-butyl groups on the phenoxy ligands L2 and L3, the iron(iii) complexes [(L2)FeX2] {where X = Cl (3), Br (4) and I = (5)} and [(L3)FeCl2] (6) were formed. Complexes 1-6 were characterised by a combination of elemental analyses, IR spectroscopy and mass spectrometry; and in selected cases (3 and 4) by single crystal X-ray crystallography. The crystal structures of 3 and 4 indicated that the iron(ii) precursors oxidised to iron(iii) in forming complexes 3-6; an observation that was corroborated by the magnetic properties and the (57)Fe Mössbauer spectra of 3 and 4. The iron(iii) complexes 3-6 were used as pre-catalysts for the oligomerisation and polymerisation of ethylene. Products of these ethylene reactions depended on the solvent used. In toluene ethylene oligomerised mainly to 1-butene and was followed by the 1-butene alkylating the solvent to form butyl-toluenes via a Friedel-Crafts alkylation reaction. In chlorobenzene, ethylene oligomerised mainly to a mixture of C4-C12 alkenes. Interestingly small amounts of butyl-chlorobenzenes and hexyl-chlorobenzenes were also formed via a Friedel-Crafts alkylation with butenes and hexenes from the oligomerisation of ethylene.

On the physico-chemical and physiological requirements of hemozoin formation promoted by perimicrovillar membranes in Rhodnius prolixus midgut.

Triatomine insects are obligatory blood-feeders that detoxify most of the hemoglobin-derived heme through its crystallization into hemozoin (Hz). Previous evidence demonstrates the key role of midgut perimicrovillar membranes (PMVM) on heme crystallization in triatomines. Here, we investigated some of the physico-chemical and physiological aspects of heme crystallization induced by Rhodnius prolixus PMVM. Hz formation in vitro proceeded optimally at pH 4.8 and 28 degrees C, apparently involving three kinetically distinct mechanisms along this process. Furthermore, the insect feeding status and age affected PMVM-induced heme crystallization whereas pharmacological blockage of PMVM formation by azadirachtin, reduced hemoglobin digestion and Hz formation in vivo. Mössbauer spectrometry analyses of R. prolixus midgut showed that Hz represents the only measurable iron species found four days after a blood meal. Autocatalytic heme crystallization to Hz is revealed to be an inefficient process and this conversion is further reduced as the Hz concentration increases. Also, PMVM-derived lipids were able to induce rapid Hz formation, regardless of the diet composition. These results indicate that PMVM-driven Hz formation in R. prolixus midgut occurs at physiologically relevant physico-chemical conditions and that lipids derived from this structure play an important role in heme crystallization.

Thermally induced two-step, two-site incomplete 6A1<-->2T2 crossover in a mononuclear iron(III) phenolate-pyridyl Schiff-base complex: a rare crystallographic observation of the coexistence of pure S=5/2 and 1/2 metal centers in the asymmetric unit.

The six-coordinate mononuclear iron(III) complexes [Fe(salpm)2]ClO(4).0.5EtOH, [Fe(salpm)2]Cl, [Fe{(3,5-tBu2)-salpm}2]X (X=ClO4- or Cl-), and [Fe{(3,5-tBu2)-salpm}2]NO(3).2H2O [Hsalpm=N-(pyridin-2-ylmethyl)salicylideneamine; H(3,5-tBu2)-salpm=N-(pyridin-2-ylmethyl)-3,5-di-tert-butylsalicylideneamine] have been synthesized and isolated in crystalline form; their chemical identities have been ascertained by elemental analyses, FAB mass spectrometry, and infrared spectroscopy. The room-temperature effective magnetic moments [(8chiMT)1/2 approximately 5.85-5.90 microB] of these complexes are consistent with the high-spin (S=5/2) ground state. These complexes are intensely colored on account of the strong ppi-->dpi* LMCT visible absorptions. Definitive evidence for the structures of [Fe(salpm)2]ClO(4).0.5EtOH and [Fe{(3,5-tBu2)-salpm}2]NO(3).2H2O has been provided by single-crystal X-ray crystallography. The monomeric complex cations in both compounds comprise two uninegative phenolate-pyridyl tridentate Schiff-base ligands coordinated meridionally to the iron(III) to afford a distorted octahedral geometry with a trans,cis,cis-[FeO2N4] core. Whereas [Fe(salpm)2]ClO(4).0.5EtOH undergoes a thermally induced 6A1<-->2T2 crossover, [Fe{(3,5-tBu2)-salpm}2]NO(3).2H2O retains its spin state in the solid state down to 5 K. However, EPR spectroscopy reveals that the latter complex does exhibit a spin transformation in solution, albeit to a much lesser extent than does the former. The spin crossover in [Fe(salpm)2]ClO(4).0.5EtOH has resulted in an unprecedented crystallographic observation of the coexistence of high-spin and low-spin iron(III) complex cations in equal proportions around 100 K. At room temperature, the two crystallographically distinct ferric centers are both high spin; however, one [Fe(salpm)2]+ complex cation undergoes a complete spin transition over the temperature range approximately 200-100 K, whereas the other converts very nearly completely between 100 and 65 K; approximately 10% of the complex cations in [Fe(salpm)2]ClO(4).0.5EtOH remain in the high-spin state down to 5 K.

Fate of haem iron in the malaria parasite Plasmodium falciparum.

Chemical analysis has shown that Plasmodium falciparum trophozoites contain 61+/-2% of the iron within parasitized erythrocytes, of which 92+/-6% is located within the food vacuole. Of this, 88+/-9% is in the form of haemozoin. (57)Fe-Mössbauer spectroscopy shows that haemozoin is the only detectable iron species in trophozoites. Electron spectroscopic imaging confirms this conclusion.

Structural, Conformational, and Spectroscopic Studies of Primary Amine Complexes of Iron(II) Porphyrins.

Three novel bis(primary amine)iron(II) porphyrins [Fe(TPP)(RNH(2))(2)], where RNH(2) = 1-butylamine, benzylamine, and phenethylamine, have been synthesized and characterized by X-ray crystallography and IR, electronic, and Mössbauer spectroscopy. The compounds provide unprecedented structural data for the coordination of primary amines by iron(II) porphyrins. The Fe-N(ax) distances of [Fe(TPP)(1-BuNH(2))(2)], [Fe(TPP)(BzNH(2))(2)], and [Fe(TPP)(PhCH(2)CH(2)NH(2))(2)] are 2.039(3), 2.043(3), and 2.028(2) Å, respectively. The Fe-N(p) distances of the three complexes average 1.990(2) Å. The zero-field Mössbauer spectra (5-300 K) show comparable isomer shifts (0.393(1)-0.493(1) mm/s) and quadrupole splittings (1.144(6)-1.204(3) mm/s) that are consistent with an S = 0 iron(II) assignment in each case. The bis(primary amine) complexes are structurally and spectroscopically similar to [Fe(TPP)(Py)(2)] derivatives, where Py = an unsubstituted pyridine. Molecular mechanics (MM) calculations with a force field parametrized for primary and secondary amine complexes of iron(II) porphyrins show that stable conformations arise when the alpha-CH(2) and NH(2) protons of the coordinated ligands are staggered relative to the Fe-N(p) bonds of the porphyrin core. The lowest energy conformations of the three [Fe(TPP)(RNH(2))(2)] complexes therefore have the ligand alpha-carbons positioned directly over the Fe-N(p) bonds of the porphyrin core. The X-ray structure of [Fe(TPP)(PhCH(2)CH(2)NH(2))(2)] lies close to the global minimum (phi(1), phi(2) = 0, 180 degrees ) on the potential surface, while [Fe(TPP)(BzNH(2))(2)] and [Fe(TPP)(1-BuNH(2))(2)] show deviations that may be attributed to packing interactions in the solid and intrinsically low barriers to axial ligand rotation (<0.5 kcal/mol). Three types of minimum energy conformation are accessible for [Fe(TPP)(Pip)(2)]. The lowest energy conformation has an S(4)-ruffled porphyrin core. The conformation which matches the X-ray structure (Radonovich, L. J.; Bloom, A.; Hoard, J. L. J. Am. Chem. Soc. 1972, 94, 2073-2078) is a local minimum (1.6 kcal/mol higher in energy than the global minimum) with exact inversion symmetry. Higher in vacuo strain energy barriers ( approximately 2.2 kcal/mol) separate the potential minima of [Fe(TPP)(Pip)(2)], consistent with the increased bulk of the secondary amine axial ligands.