Reaction of the monoanionic, pentacoordinate ligand lithium 1,4,8,11-tetraazacyclotetradecane-1-acetate, Li(cyclam-acetate), with FeCl3 yields, upon addition of KPF6, [(cyclam-acetato)FeCl]PF6 (1) as a red microcrystalline solid. Addition of excess NaN3 prior to addition of KPF6 yields the azide derivative [(cyclam-acetato)FeN3]PF6 (2a) as orange microcrystals. The X-ray crystal structure of the azide derivative has been determined as the tetraphenylborate salt (2b). Reaction of 1 with silver triflate yields [(cyclam-acetato)Fe(O3SCF3)]PF6 (3), which partially dissociates triflate in nondried solvents to yield a mixture of triflate and aqua bound species. Each of the iron(III) derivatives is low-spin (d5, S = 1/2) as determined by variable-temperature magnetic susceptibility measurements, Mössbauer and EPR spectroscopy. The low-spin iron(II) (d6, S = 0) complexes 1red and 2ared have been prepared by electrochemical and chemical methods and have been characterized by Mössbauer spectroscopy. Photolysis of 2a at 419 nm in frozen acetonitrile yields a nearly colorless species in approximately 80% conversion with an isomer shift delta = -0.04 mm/s and a quadrupole splitting delta EQ = -1.67 mm/s. A spin-Hamiltonian analysis of the magnetic Mössbauer spectra is consistent with an FeV ion (d3, S = 3/2). The proposed [(cyclam-acetato)FeV=N]+ results from the photooxidation of 2a via heterolytic N-N cleavage of coordinated azide. Photolysis of 2a in acetonitrile solution at -35 degrees C (300 nm) or 20 degrees C (Hg immersion lamp) results primarily in photoreduction via homolytic Fe-Nazide cleavage yielding FeII (d,6 S = 0) with an isomer shift delta = 0.56 mm/s and quadrupole splitting delta EQ = 0.54 mm/s. A minor product containing high-valent iron is suggested by Mössbauer spectroscopy and is proposed to originate from [((cyclam-acetato)Fe)2(mu-N)]2+ with a mixed-valent (FeIV(mu-N)FeIII))4+S = 1/2 core. Exposure of 3 to a stream of oxygen/ozone at low temperatures (-80 degrees C) in acetone/water results in a single oxidized product with an isomer shift delta = 0.01 mm/s and quadrupole splitting delta EQ = 1.37 mm/s. A spin-Hamiltonian analysis of the magnetic Mössbauer yields parameters similar to those of compound II of horseradish peroxidase which are consistent with an FeIV=O monomeric complex (S = 1).

The hexavalent state, considered to be the highest oxidation level accessible for iron, has previously been found only in the tetrahedral ferrate dianion, FeO4(2-). We report the photochemical synthesis of another Fe(VI) compound, an octahedrally coordinated dication bearing a terminal nitrido ligand. Mössbauer and x-ray absorption spectra, supported by density functional theory, are consistent with the octahedral structure having an FeN triple bond of 1.57 angstroms and a singlet d2(xy) ground electronic configuration. The compound is stable at 77 kelvin and yields a high-spin Fe(III) species upon warming.

Reaction of cis-[Fe III (cyclam)Cl 2 ]Cl in acidic H 2 O/CH 3 OH or CH 3 CN/H 2 O mixtures with NaN 3 at 50 °C produced upon addition of NaClO 4 or NaPF 6 the complex trans-[Fe III (cyclam)(N 3 ) 2 ]ClO 4 (1a) or the hexafluorophosphate salt 1b, whereas at -18 °C the same reaction produced cis-[Fe III 4,8,11-tetraazacyclotetradecane). The crystal structures of 1b and 2 were determined by singlecrystal X-ray crystallography. Complexes 1a, b, contain a low-spin (S ) 1 / 2 ) and 2, a high-spin ferric ion (S ) 5 / 2 ) as was established by variable-temperature magnetic susceptibility measurements and Mo ¨ssbauer and X-band EPR spectroscopy. The low-spin trans-[(cyclam)Fe II (N 3 ) 2 ] and high-spin cis-[(cyclam)Fe II (N 3 ) 2 ] species were generated electrochemically in CH 3 CN solution and were characterized by Mo ¨ssbauer spectroscopy. Photolysis of 1a in CH 3 CN at -35 °C and 20 °C with a Hg immersion lamp generated within 15 min a yellow solution. EPR and Mo ¨ssbauer spectra show that a single high-valent species with an S ) 1 / 2 ground state is formed: [{trans-(cyclam)Fe(N 3 )} 2 (µ-N)] 2+ . The Mo ¨ssbauer spectrum at 80 K exhibits two quadrupole doublets (ratio 1:1), indicating the presence of low-spin Fe IV (S ) 1) with isomer shift, δ, at 0.11 mm s -1 and quadrupole splitting, ∆E Q , at 0.97 mm s -1 and intermediate-spin Fe III (S ) 3 / 2 ) with δ ) 0.20 mm s -1 and ∆E Q ) 2.09 mm s -1 . The valencies in this mixed valent [Fe III (µ-N)Fe IV ] 4+ core (S t ) 1 / 2 ) are localized on the Mo ¨ssbauer time scale. In addition 18% of a low-spin Fe II complex (S ) 0) was found with δ ) 0.54 mm s -1 , ∆E Q ) 0.65 mm s -1 . Photolysis of 2 at -35 °C in CH 3 CN produced a yellow mixed valent dinuclear species with an S ) 3 / 2 ground state. For this species we propose the structure [{cis-(cyclam)Fe III (N 3 )}(µ-N){trans-(cyclam)Fe IV -(N 3 )}] 2+ with an [Fe III (µ-N)Fe IV ] 2+ core (S t ) 3 / 2 ). The Mo ¨ssbauer spectrum at 80 K clearly shows that the valencies are localized: two subspectra are detected (ratio 1:1) at (a) δ ) 0.14 mm s -1 , ∆E Q ) 0.81 mm s -1 (Fe IV ; S ) 1) and (b) δ ) 0.50 mm s -1 , ∆E Q ) 1.89 mm s -1 (Fe III ; S ) 5 / 2 ). Strong intramolecular antiferromagnetic coupling between the two iron sites produces the observed S t ) 3 / 2 ground state. The proposed spin states and spin-coupling schemes for both photolysis products were proven by magnetic Mo ¨ssbauer and EPR spectra. The formation of the dinuclear species requires the primary formation of a nitridoiron(V) species: trans-[NdFe V (cyclam)(N 3 )] + . This intermediate has been detected during photolysis of frozen CH 3 CN solutions of 1a at 4 and 77 K by EPR and Mo ¨ssbauer spectroscopy. The species contains an Fe V ion (d 3 , S ) 3 / 2 ) with an isomer shift δ ) -0.04 mm s -1 and a quadrupole splitting ∆E Q ) -1.90 mm s -1 at 80 K. In addition, a five-coordinate ferrous species has been identified as a primary photolysis product which is formed via photolytic homolysis of the Fe III -N 3 bond. Thus, simultaneous photooxidation and -re...

The iron-sulfur cluster-free hydrogenase (Hmd) from methanogenic archaea harbors an iron-containing, light-sensitive cofactor of still unknown structure as prosthetic group. The enzyme is reversibly inhibited by CO and cyanide and is EPR silent. We report here on Mössbauer spectra of the (57)Fe-labeled enzyme and of the isolated cofactor. The spectrum of the holoenzyme measured at 80 K revealed a doublet peak with an isomer shift delta = 0.06 mm.s(-)(1) and a quadrupole splitting of DeltaE(Q) = 0.65 mm.s(-)(1) (at pH 8.0). The signal intensity corresponded to the enzyme concentration assuming 1 Fe per mol active site. Upon addition of CO or cyanide to the enzyme, the isomer shift decreased to -0.03 mm.s(-)(1) and -0.00(1) mm.s(-)(1), and the quadrupole splitting increased to 1.38 mm.s(-)(1) and 1.75 mm.s(-)(1), respectively. The three spectra could be perfectly simulated assuming the presence of only one type of iron in Hmd. The low isomer shift is characteristic for Fe in a low oxidation state (0, +1, +2). When the spectra of the holoenzyme and of the CO- or cyanide-inhibited enzyme were measured at 4 K in a magnetic field of 4 and 7 T, the spectra obtained could be simulated assuming the presence of only the external magnetic field, which excludes that the iron in the active site of Hmd is Fe(I), high-spin Fe(0), or high-spin Fe(II). Mössbauer spectra of the isolated Hmd cofactor are also reported.

Low‐spin, high‐valent iron centers: A high‐valent iron–nitrido species containing a FeV center is accessible by the photolysis of [(cyclam‐ac)FeIIIN3]+. The identity of the resulting [(cyclam‐ac)FeV(N)]+ species was verified by spectroscopic methods, magnetic susceptibility data, and DFT calculations (see picture). Unexpectedly, the analysis provides strong evidence for a low‐spin d3 (S=1/2) ground‐state electron configuration at the FeV center.

Crystals of MIL-88B-Fe and NH2-MIL-88B-Fe were prepared by a new rapid microwave-assisted solvothermal method. High-purity, spindle-shaped crystals of MIL-88B-Fe with a length of about 2 μm and a diameter of 1 μm and needle-shaped crystals of NH2-MIL-88B-Fe with a length of about 1.5 μm and a diameter of 300 nm were produced with uniform size and excellent crystallinity. The possibility to reduce the as-prepared frameworks and the chemical capture of carbon monoxide in these materials was studied by in situ ultrahigh vacuum Fourier-transform infrared (UHV-FTIR) spectroscopy and Mössbauer spectroscopy. CO binding occurs to unsaturated coordination sites (CUS). The release of CO from the as-prepared materials was studied by a myoglobin assay in physiological buffer. The release of CO from crystals of MIL-88B-Fe with t(1/2) = 38 min and from crystals of NH2-MIL-88B-Fe with t(1/2) = 76 min were found to be controlled by the degradation of the MIL materials under physiological conditions. These MIL-88B-Fe and NH2-MIL-88B-Fe materials show good biocompatibility and have the potential to be used in pharmacological and therapeutic applications as carriers and delivery vehicles for the gasotransmitter carbon monoxide.

The cytoplasmic histidine sensor kinase NreB of Staphylococcus carnosus responds to O(2) and controls together with the response regulator NreC the expression of genes of nitrate/nitrite respiration. nreBC homologous genes were found in Staphylococcus strains and Bacillus clausii, and a modified form was found in some Lactobacillus strains. NreB contains a sensory domain with similarity to heme B binding PAS domains. Anaerobically prepared NreB of S. carnosus exhibited a (diamagnetic) [4Fe-4S](2+) cluster when assessed by Mossbauer spectroscopy. Upon reaction with air, the cluster was degraded with a half-life of approximately 2.5 min. No significant amounts of Mossbauer or EPR detectable intermediates were found during the decay, but magnetic Mossbauer spectra revealed formation of diamagnetic [2Fe-2S](2+) clusters. After extended exposure to air, NreB was devoid of a FeS cluster. Photoreduction with deazaflavin produced small amounts of [4Fe-4S](+), which were degraded subsequently. The magnetically perturbed Mossbauer spectrum of the [4Fe-4S](2+) cluster corroborated the S = 0 spin state and revealed uniform electric field gradient tensors of the iron sites, suggesting full delocalization of the valence electrons and binding of each of the Fe ions by four S ligands, including the ligand to the protein. Mutation of each of the four Cys residues inactivated NreB function in vivo in accordance with their role as ligands. [4Fe-4S](2+) cluster-containing NreB had high kinase activity. Exposure to air decreased the kinase activity and content of the [4Fe-4S](2+) cluster with similar half-lives. We conclude that the sensory domain of NreB represents a new type of PAS domain containing a [4Fe-4S](2+) cluster for sensing and function.

By employing the subcomponent self-assembly approach utilizing 5,10,15,20-tetrakis(4-aminophenyl)porphyrin or its zinc(II) complex, 1H-4-imidazolecarbaldehyde, and either zinc(II) or iron(II) salts, we were able to prepare O-symmetric cages having a confined volume of ca. 1300 Å . The use of iron(II) salts yielded coordination cages in the high-spin state at room temperature, manifesting spin-crossover in solution at low temperatures, whereas corresponding zinc(II) salts led to the corresponding diamagnetic analogues. The new cages were characterized by synchrotron X-ray crystallography, high-resolution mass spectrometry, and NMR, Mössbauer, IR, and UV/Vis spectroscopy. The cage structures and UV/Vis spectra were independently confirmed by state-of-the-art DFT calculations. A remarkably high-spin-stabilizing effect through encapsulation of C was observed. The spin-transition temperature T is lowered by 20 K in the host-guest complex.