Multitemperature Resonance-Diffraction and Structural Study of the Mixed-Valence Complex [Fe₃O(OOCC(CH₃)₃)₆(C₅H₅N)₃]

Abstract

The Fe-O and Fe-N bond lengths at two iron sites of the mixed-valence complex [Fe(3)O(OOCC(CH(3))(3))(6)(C(5)H(5)N)(3)] show a pronounced temperature dependence; the bonds from two of the Fe atoms to the central oxygen atoms vary by more than 0.10 Å on cooling to 10 K whereas the bond from the third iron atom is essentially invariant. The variation is such that the longest Fe-O bonds at ambient temperature are the shorter ones at 10 K, with the crossover occurring at about 90 K. The bonds to the axial pyridine ligand show the opposite dependence. The variation is attributed to an equilibrium between different configurations, which interconvert through vibronic coupling, a process that involves electron transfer between the metal atoms. The position of the absorption edge for each of the iron atoms has been determined by resonance-diffraction experiments at the Fe K edge, performed at four different temperatures. At each temperature, the order of the absorption edges corresponds to that of the experimentally determined bond lengths. The crossover near 90 K is confirmed by the resonance experiments. The absorption-edge positions are related to the formal oxidation state by calibration with reference complexes of known oxidation state. The experiments demonstrate the close relation between the changes in coordination geometry and the oxidation states of the iron atoms.

Verwey Transition under Oxygen Loading in RBaFe2O5+w (R=Nd and Sm)

“…A formal replacement of the tetrahedral Fe 3þ in magnetite by yttrium (or similar rare-earth elements) leads to rhombohedral YFe 2 O 4 which exhibits Verwey transition around 200 K, this time on the background of an antiferromagnetic (AFM) arrangement. It is interesting to note that molecular iron oxides where various organic ligands stabilize the same valence states in the same ratio as in magnetite (trinuclear iron complexes) undergo an analogous transition where two Fe-O bonds become unequal at low temperatures whereas the third bond does not participate (15,16 ossbauer spectroscopy (17). It occupies identical square-pyramidally coordinated sites created by an oxygen vacancy residing in the Sm layer of the doublecell perovskite-type structure (18).…”

Site‐Specific Oxidation State Assignments of the Iron Atoms in the [4Fe:4S]2+/1+/0 States of the Nitrogenase Fe‐Protein

“…[17] Af ull understanding of the nitrogenase mechanism must include the detailed oxidation state assignments of the component metalloclusters.A ssigning oxidation states to individual metals in ametallocluster is difficult using conventional spectroscopic techniques,d ue to the ambiguity in assigning spectral features to specific sites.T oc ircumvent these limitations,w eh ave developed SpReAD (Spatially Resolved Anomalous Dispersion) refinement, acombination of X-ray crystallography and X-ray absorption spectroscopy to determine site-specific X-ray absorption spectra, [22,23] building on pioneering work by Coppens. [24] By refining structures against diffraction data collected at as eries of energies across an absorption edge,t he X-ray absorption spectra of each individual metal site can be obtained from the corresponding Df"values.F or the Fe-protein, oxidation leads to ashift of the main edge position to higher energies by circa 1eVp er 0.5 change in oxidation state. [25] It should be noted that edge positions are sensitive to many factors including the ligand sphere.…”

Resonant elastic X-ray scattering in chemistry and materials science

“…As well as single crystal and powder applications, these are now extended to other states of matter, such as solutions and thin films. Improved ease of measurement has allowed multiple data sets to be measured, so as to study variation of temperature to assess redox changes, in a site specific way [20]. Dichroism applications are increasingly underway, utilising the SR X-ray beam polarisation [18,19].…”

“…Another interesting case study [20], mentioned above, is the investigation of the mixed valence complex Fe 3 O(OOCC(CH 3 ) 3 ) 6 (C 5 H 5 N) 3 (Fig. 4(a)), which shows a temperature dependence of the bond lengths and oxidation state, with a cross-over of the Fe (1) (Fig.…”

“…19]. Chemical changes induced by temperature and/or time-resolved studies should also increase the range and scope of these REXS applications as per the case study above [20]. As the speed of measuring improves, as more instruments are available and as analysis is more routine then increasing numbers of samples can be studied per molecular system and perhaps even sample to sample variations allowed for, although unlikely to be a major concern.…”