Iron complexes Mossbauer spectroscopy

Since many metallobiochemicals can be considered complexes, the Mossbauer spectra of these chemicals can be interpreted using the results previously discussed for inorganic complexes. An important class of metallobiochemicals is that in which the metal is iron. The Mossbauer spectroscopy of a number of these has been studied, and the results have been tabulated by Gonser and Grant (20). The hemoprotein, hemo-... 

The complexes obtained were characterized by x-ray diffraction and magnetic measurements. Iron-57 Mossbauer spectroscopy data for Fe(SQ)3 complexes are consistent with the iron ion being described as high-spin iron(III) [227,228]. 

A particular method for studying complexes of iron is Mossbauer spectroscopy [7]. For instance, a- and p modifications of Fe(Pc) were studied by this method [8], and the dioxygen derivatives, as well. It was found that the oxygen molecule interacts with the complex, and (p-Oxo)-bis-phthalocyaninato-iron(lll) is formed [9]. The stability of the complex depends strongly on the solvents, e.g. in presence of strong N-bases the Fe ll) form is restored at room temperature, even in presence of oxygen [10]. Further, the Y-zeolite encaged Fe(Pc) was also studied, namely stabilization of the pyridine complex, Fe(Pc)(Py)2, as well as the effects of preparation conditions and presence of various counterions in the framework (Na, K, Rb) on the formation of Fe(Pc)(Py)2 are reported [11-13]. 

For diiron complexes Mossbauer spectroscopy allows to asses (1) oxidation and spin states of the iron atoms, (2) diamagnetism and ferromagnetism of the groimd state for diferric and mixed-valent oxidation levels and (3) valence (de)localisation in the solid state for mixed-valence complexes [2,3]. Isomer shifts (IS) in the range 0.35-0.60 mm/s are characteristic of 5- or 6-coordinate high-spin diferric p-hydroxo complexes [2,3], Tetrahedral high-spin ferric iron has lower isomeric shifts in the range of 0.22 mm/s [2,3]. For isolated ferric iron with S =... 

Bis(imino)pyridine iron complex 5 as a highly efficient catalyst for a hydrogenation reaction was synthesized by Chirik and coworkers in 2004 [27]. Complex 5 looks like a Fe(0) complex, but detailed investigations into the electronic structure of 5 by metrical data, Mossbauer parameters, infrared and NMR spectroscopy, and DFT calculations established the Fe(ll) complex described as 5 in Fig. 2 to be the higher populated species [28]. 

Of special Interest as O2 reduction electrocatalysts are the transition metal macrocycles In the form of layers adsorptlvely attached, chemically bonded or simply physically deposited on an electrode substrate Some of these complexes catalyze the 4-electron reduction of O2 to H2O or 0H while others catalyze principally the 2-electron reduction to the peroxide and/or the peroxide elimination reactions. Various situ spectroscopic techniques have been used to examine the state of these transition metal macrocycle layers on carbon, graphite and metal substrates under various electrochemical conditions. These techniques have Included (a) visible reflectance spectroscopy (b) laser Raman spectroscopy, utilizing surface enhanced Raman scattering and resonant Raman and (c) Mossbauer spectroscopy. This paper will focus on principally the cobalt and Iron phthalocyanlnes and porphyrins. 

A unique situation is encountered if Fe-M6ssbauer spectroscopy is applied for the study of spin-state transitions in iron complexes. The half-life of the excited state of the Fe nucleus involved in the Mossbauer experiment is tj/2 = 0.977 X 10 s which is related to the decay constant k by tj/2 = ln2/fe. The lifetime t = l//c is therefore = 1.410 x 10 s which value is just at the centre of the range estimated for the spin-state lifetime Tl = I/Zclh- Thus both the situations discussed above are expected to appear under suitable conditions in the Mossbauer spectra. The quantity of importance is here the nuclear Larmor precession frequency co . If the spin-state lifetime Tl = 1/feLH is long relative to the nuclear precession time l/co , i.e. Tl > l/o) , individual and sharp resonance lines for the two spin states are observed. On the other hand, if the spin-state lifetime is short and thus < l/o) , averaged spectra with intermediate values of quadrupole splitting A q and isomer shift 5 are found. For the intermediate case where Tl 1/cl , broadened and asymmetric resonance lines are obtained. These may be the subject of a lineshape analysis that will eventually produce values of rate constants for the dynamic spin-state inter-conversion process. The rate constants extracted from the spectra will be necessarily of the order of 10 -10 s"F... 

Mossbauer spectroscopy is particularly suitable to study ST since (1) the spectral parameters associated with the HS and LS states of iron(II) clearly differ and (2) the time-scale of the technique ( 10 s) allows the detection of the separate spin states in the course of the transition. Typically, Mossbauer spectra of HS iron(II) show relatively high quadrupole splitting (AEq 2-3 mm s ) and isomer shift (3 1 mm s ), while for LS iron(II), these parameters are generally smaller (AEq < 1 mm s 3 < 0.5 mm s ). Among the early applications of Mossbauer spectroscopy to study ST phenomena in iron(II) complexes is the work of Dezsi et al. [7] on [Fe (phen)2(NCS)2] (phen = 1,10-phenanthroline) as a function of temperature (Fig. 8.2). The transition from the HS ( 12) state (quadrupole doublet of outer two lines with AEq 3 mm s ) to the LS CAi) state (quadrupole... 

Four-coordinate, planar iron(II)-dithiolate complexes also exhibit intermediate spin. The first example described was the tetraphenylarsonium salt of the square-planar bis(benzene-l,2-dithiolate)iron(II) dianion, (AsPh4)2[Fe(II)bdt2], which showed 5 = 0.44 mm s and AEq = 1.16 mm s at 4.2 K [157]. The electronic structure of a different salt was explored in depth by DFT calculations, magnetic susceptibility, MCD measurements, far-infra red spectroscopy and applied-field Mossbauer spectroscopy [158]. 

Mossbauer spectroscopy can be used for in situ study of electrodes containing nuclei capable of resonance absorption of y radiation for practical systems, primarily the 57Fe isotope is used (passivation layers on iron electrodes, adsorbed iron complexes, etc.). It yields valuable information on the electron density on the iron atom, on the composition and symmetry of the coordination sphere around the iron atom and on its oxidation state. 

About twenty years ago we reported on the di-isothiocyanato iron(II) complex of the tetradentate ligand tpa (tris(2-pyridylmethyl)amine) [7] (6). It was shown that this complex exhibits the spin crossover phenomenon with a critical temperature Tm of about 170 K. Several different solvated phases of the same system have since been characterized by Chansou et al. [8]. The unsolvated phase which can be isolated from an aqueous solution has been investigated by nuclear forward scattering (NFS), nuclear inelastic scattering (NIS) [9], extended x-ray absorption fine structure (EXAFS) spectroscopy, conventional Mossbauer spectroscopy, and by measurements of the magnetic susceptibility (SQUID) [10-13]. The various measurements consistently show that the transition is complete and abrupt and it exhibits a hysteresis loop between 102 and 110 K. 

For iron and tin the magnitude of 8 has been found to be related to the oxidation state of the metal (15, 28) (Table I). In iron complexes, the spin of the 3d electrons of the iron atoms can be paired (low spin) or unpaired (high spin). In low spin ferrous and ferric complexes, 8 and AEq values are similar, but the value of AEq differs greatly for the high spin complexes (Figure 1). Mossbauer spectroscopy has been used to... 

Recently, molecular orbital calculations on some iron complexes have been made by Gray and his co-workers (2, 30). These values for ferrous complexes are plotted in Figure 2 as the dashed line [3d 4s (MO)]. Hence, Mossbauer spectroscopy provides estimates for the 4s electron contribution for molecular orbital calculations. This correlation does not hold for high spin complexes such as FeCb" and FeFe ". Bersuker (3, 4) has attempted to relate both S and AEq directly to molecular orbital parameters. Using the equations developed from this approach, Bersuker, Gordanskii, and Makarov (5) have concluded that in tin tetrahalides the role of dx-Px bonding is significant. 

MOssbauer spectroscopy has been used to investigate the oxidation state of iron in complexes such as [Fe(CN)6] " " intercalated in LDHs [176]. By means of MOssbauer spectroscopy, it has also been demonstrated that ferrocene sulfonates intercalated in LDHs decompose on attempted ion-exchange with sodium carbonate solutions and that the liberated Fe " ions become incorporated in the layers [189]. 

The Fe(II)-NO complexes of porphyrins 66-68) and heme proteins 24, 49, 53, 69-76) have been studied in detail by EPR spectroscopy, which allows facile differentiation between five-coordinate heme—NO and six-coordinate heme—NO(L) centers. However, only a few reports of the Mossbauer spectra of such complexes have been published 68, 77-82), and the only Fe(III)-NO species that have been studied by Mossbauer spectroscopy include the isoelectronic nitroprusside ion, [FeCCNlsCNO)] (7S), the five-coordinate complexes [TPPFe(NO)]+ 68) and [OEPFe(NO)]+ 82), and two reports of the nitro, nitrosyl complexes of iron(III) tetraphenylporphjrrins, where the ligand L is NO2 82, 83). 

There are four naturally occurring isotopes of iron ( Fe 5.82%, Fe 91.66%, Fe 2.19%, Fe 0.33%), and nine others are known. The most abundant isotope ( Fe) is the most stable nuclear configuration of all the elements in terms of nuclear binding energy per nucleon. This stability, in terms of nuclear equilibrium established in the last moments of supernova events, explains the widespread occurrence of iron in the cosmos. The isotope Fe has practical applications, most notably in Mossbauer spectroscopy, which has been widely exploited to characterize iron coordination complexes. 

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