In iron complexes

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... 

Drickamer and Frank (1973) Spin changes in iron complexes [208]. 

Giitlich (1997) Spin crossover, LIESST and NIESST—fascinating electronic games in iron complexes [239]. 

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... 

The rate of the spin state change for the octahedral cobalt(II) complexes is expected to be faster than that observed for the iron(II) and iron(III) complexes. In the cobalt(II) case the spin state change involves only one electron, that is AS = 1. The 2E and 4T states are directly mixed by spin-orbit coupling (10, 163). The spin state transition should be adiabatic, with k = 1, without any spin-forbidden barrier. Furthermore, the coordination sphere reorganization involves a change in bond length of 21 pm along only two bonds, instead of all six bonds as in iron complexes. Both of these factors lead to the prediction of rapid spin state interconversion. 

Here the reversible formal potential, E° of the redox couple of interest has been measured at a series of temperatures versus a reference electrode maintained at constant temperature. Values of AShr have been interpreted in terms of differences in water-ligand hydrogen bonding [1] and have been compared to the entropy change for spin crossover in iron complexes [2]. 

All substrates were considered to compete as ligands in iron complexes and to modify the reaction characteristics of each other and of the complex. Reaction 6.34 yields hydroxyl radicals, so the free-radical mechanism proposed by Walling appeared to be possible however, Equation (6.35) to Equation (6.38) involve electron transfer and do not lead to formation of hydroxyl radicals. Equation (6.37) and Equation (6.38) involve ionic mechanisms ... 

Hydrothermal methods, for molecuarlar precursor transformation to materials, 12, 47 Hydrotris(3,5-diisopropylpyrazolyl)borate-containing acetylide, in iron complex, 6, 108 Hydrotris(3,5-dimethylpyrazolyl)borate groups, in rhodium Cp complexes, 7, 151 Hydrotris(pyrazolyl)borates in cobalt(II) complexes, 7, 16 for cobalt(II) complexes, 7, 16 in rhodium Cp complexes, 7, 151 Hydrovinylation, with transition metal catalysts, 10, 318 Hydroxides, info nickel complexes, 8, 59-60 Hydroxo complexes, with bis-Cp Ti(IV), 4, 586 Hydroxyalkenyl complexes, mononuclear Ru and Os compounds, 6, 404-405 a-Hydroxyalkylstannanes, preparation, 3, 822 y-Hydroxyalkynecarboxylate, isomerization, 10, 98 Hydroxyalkynes, in hexaruthenium carbido clusters, 6, 1015 a-Hydroxyallenes... 

Comparison of the tropone ring geometry in iron complex 282 with that of the parent tropone shows that the double bond character of the carbonyl group and the bond between C-2 and C-3 in 282 is even higher, probably as a result of dominant contributions from two cyclobutadiene resonance... 

Further work on human serum transferrin by Aasa et al. (1) did not corroborate the earlier work of Davis et al. (32) and showed that there were no differences between the ions in iron complexes of human serum transferrin. Aasa et al. (1) believed that the earlier work of Davis et al. (32) was incorrect in that the competitive experiments with ethylene-diaminetetraacetic acid were not dialyzed sufficiently long for equilibrium to be obtained. 

Fig. 8. Energy diagram (kcal/mol) of H20-mediated 1,3-migration of CH3 from SnH2 to phosphenium P in iron complex.

Williams (28) suggested that the band found at about 16 kK in high-spin systems was a charge transfer transition, in which the excited state arises from a configuration in which an electron has been transferred from a porphyrin 71-orbital to a metal -orbital. Such transitions frequently occur in iron complexes. His alternative suggestion that the band could be an intra-metal d—d transition mixed with charge transfer was considered most unlikely on intensity grounds (88). 

Electrophilic attack on the coordinated C02 in iron complex resulting in the C-O bond cleavage has been reported (Eq. 60) [136,137]. 

In view of the current interest in iron complexes for organic synthesis, we have employed dimethyldioxirane for epoxidation purposes. For example, the quite reluctant iron tricarbonyl diene complex with the alkenyl side chain led to the desired epoxide in moderate yield [35]. Also the isoprenyl-substituted ferrocene could be oxyfunctionalized without any difficulties [36],... 

In order to translate the relative values of I.S, in terms of total s-electron density a calibration has been made by assuming a negligible occupation of the 4s orbitals in iron complexes with F, H O (1). However, even for the more ionic Fe(III) complexes the N.E. is large, indicating a relatively strong perturbation of the central ion by the ligands. 

In the case of Fe complexes, however, the situation is reversed whereas the oxidation of chelates as well as nonchelates occurs at similar potentials, the reduction of chelates is more difficult than reduction of nonchelates by about 100 mV. The question is, why (in contradiction to the Cr derivatives), in iron complexes (chel) = (nonchelate), whereas l redl (chel) > Ifij-edl (nonchelate). 

These data from ergosterol and ergosterol-type compounds prove very clearly that approach control in iron-complex formation differs markedly from the one in Diels-Alder cycloadditions. While here all examples investigated so far call for an a-approach of the dienophile to form products like 8 with remarkable regioselectivity, complex formation does at least to some extent take place from the P-side of the molecule. 

---