Low-spin iron complexes

The reaction in Eq. (9.34) is also faster because the bpy ligand is a strong field ligand and there is no longer any need for electronic rearrangement upon change in oxidation state. The process is now comparable to those discussed earlier for low spin iron complexes. 

A very simple concept of partial quadrupole splitting (PQS) was developed in order to explain the peculiarities of QS in various compounds [275-277]. For example, the substitution of one or two ligands B in an octahedral low-spin iron complex FeBe (QS=0) results in the following QS values ... 

ESR of heme proteins has characterized both the high-spin (S = 5/2) and the low-spin (S=l/2) forms. The review by Palmer [226] on ESR of heme proteins is particularly useful, in which the model for low-spin iron complexes (for which the d y, and dy are split from the d 2 and orbitals) is clearly explained. The five... 

The best known of the low-spin iron complexes are undoubtedly the cyanides, and considerable Mossbauer data have now been collected. The diamagnetic... 

The ° values for two low-spin iron complexes in acidic solution are as follows ... 

Also, many new aryl porphyrine high- and low-spin iron complexes were obtained. The aryl group may migrate from the iron metal to the nitrogen atom. 

Alkyliron(lll) porphyrin complexes are air. sensitive, and when exposed to oxygen under ambient conditions the products are the very stable iron(IIl) /t-oxo dimers, [Fe(Por)]20. A more careful investigation revealed that the reaction of the alkyl complexes with oxygen proceeds via insertion of O2 into the Fe—C bond. " When a solution of Fe(Por)R (R = Me, Et, i-Pr) is exposed to O2 at —70 C, the characteristic H NMR spectrum of the low spin iron alkyl complex disappears and is replaced by a new, high spin species. The same species can be generated from the reaction of an alkyl hydroperoxide with Fe(Por)OH, and is formulated as... 

The low spin aryliron complexes Fe(Por)Ar behave differently than their alkyl counterparts toward oxygen. In chloroform the products of the reaction of Fe(Por)Ar with O2 are the iron(IV) alkyl Fe(Por)Ar] and Fe(Por)Cl, while in toluene the aryloxide complexes Fe(Por)OAr are formed with no direct evidence for the formation of arylperoxo intermediates."" The high spin iron(lll) alkynyl complexes which have more ionic character in the Fe—C bond do not react with oxygen, and the order of reactivity of organoiron(lll) complexes toward O2 is given by Fe(Por)(alkyl) > Fe(Por)(aryl) > Fe(Por)(C=CR). ... 

Treatment via chelation has been observed for 2-acetylpyridine thiosemi-carbazone derivatives, which have been found to possess inhibitory activity for the RNA-polymerases of the influenza virus [133]. The iron(III) complexes were shown to be 3 to 6 times more active as inhibitors of partially purified ribonucleotide reductase (no added iron) compared to uncomplexed thiosemi-carbazone [128]. Raina and Srivastava [134] prepared and characterized low spin iron(III) complexes of 2-acetylpyridine thiosemicarbazone, [Fe(8-H)2A] (A = NO3, OH, Cl, N3, NCS or NO2), which were proposed as being seven-coordinate. However, all but the azide complex are 1 1 electrolytes in DMF and their solid ESR spectra are rhombic with the g-values being about 2.20,2.15 and 2.00. Of the six complexes, the azide ion seems to interact ihost strongly with the iron(III) center. 

The same effect is observed for the substituted pyridyl-pyrazole and -imidazole systems. While 2-(pyrazol-l-yl)pyridine 24 gives a low spin iron(II) complex a continuous spin transition is observed centred just above room temperature in solid salts of [Fe (31)3]2+ and just below in solution [39]. Spin crossover occurs in the [Fe N6]2+ derivative of 2-(pyridin-2-yl)benzimidazole 32 (Dq(Ni2+)=1050 cm"1) but not in that of the 6-methyl-pyridyl system 33 (Dq(Ni2+)=1000 cm"1). Although the transition in salts of [Fe 323]2+ is strongly influenced by the nature of the anion and the extent of hydration, suggesting an influence of hydrogen-bonding, in all instances it is continuous [40]. 

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