Iron complexes electron spin resonance spectroscopy

Jezowska-Trezebiatowska, B., and Jezierski, A. (1973). Electron spin resonance spectroscopy of iron nitrosyl complexes with organic ligands. J. Mol. Struct. 19, 635-640. 

Senesi et al. (1977), using the methods of electron spin resonance and Mossbauer spectroscopy in conjunction with chemical methods, established that at least two and possibly three forms of binding of Fe occur in humic materials. Ferric iron is firmly bound and protected in tetrahedral or octahedral coordination this form of binding of iron is resistant to chemical complexing and reduction. Fe adsorbed on the outer surfaces of humic materials is less firmly bound. The iron-fulvic acid complexes studied contain from 5.5 to 50.1% Fe, but a large part of the iron is bound to the surficial octahedral position. 

The conclusion that the cobalt and iron complexes 2.182 and 2.183 are formally TT-radical species is supported by a wealth of spectroscopic evidence. For instance, the H NMR spectrum of the cobalt complex 2.182 indicated the presence of a paramagnetic system with resonances that are consistent with the proposed cobalt(III) formulation (as opposed to a low-spin, paramagnetic cobalt(IV) corrole). Further, the UV-vis absorption spectrum recorded for complex 2.182 was found to be remarkably similar to those of porphyrin 7r-radicals. In the case of the iron complex 2.183, Mdssbauer spectroscopy was used to confirm the assignment of the complex as having a formally tetravalent metal and a vr-radical carbon skeleton. Here, measurements at 120 K revealed that the formal removal of one electron from the neutral species 2.177 had very little effect on the Mdssbauer spectrum. This was interpreted as an indication that oxidation had occurred at the corrole ligand, and not at the metal center. Had metal oxidation occurred, more dramatic differences in the Mdssbauer spectrum would have been observed. 

A number of transition metal nuclei can be studied by Mossbauer techniques (e.g., Fe, Ni, Ru, W, Os, Ir, and Pt). Of these, only Ir, Ru, and Fe have been used to study nitrosyl bonding. The most detailed studies have been on the well-known iron complexes [Fe(CN)5(NO)] - (87. 89) and [Fe(NO)(dtc)z] (88-90) (dtc is N,N-dialkyldithiocarbamato). In the latter, high-spin/low-spin equilibria can be followed by Fe Mossbauer spectroscopy, and the Mossbauer parameters agree well with data from electron spin resonance (ESR) spectroscopy in determining the ground states of these complexes. 

The third principal application of the electron spin resonance technique is to the study of paramagnetic transition metal ions in biochemical systems. Most examples are complexes of copper, iron, manganese, chromium, cobalt and molybdenum. Other metals such as titanium, vanadium and nickel are sometimes employed as structural probes. Only four of these ions, Cu ", Mn, Gd " and VO ", are seen in ESR spectroscopy at room temperature under virtually all conditions. Therefore, they are of special importance. 

Recently, iron(V) nitride complex [PhB( Bulm)3Fe =N]BArF24 (PhB( Bulm)3 = phenyltris(3-tert-butylimidazol-2-ylidene)borato BArp24 = B(3,5-(CF3)2QH3)4 ) has been prepared [100]. This complex has a four-coordinate iron ion with a terminal nitride ligand. The characterization of the complex by Mossbauer and electron paramagnetic resonance spectroscopy demonstrated d iron(V) metal center in a low-spin (S= 1/2) electron configuration. The decomposition of the complex by water at low temperature formed ammonia and iron(ll) species. 

Cross-conjugated dihydroacepentalene complexes 156 (NR2 = NEt2, piperidino, 3,5-dimethylpiperidino, morpholino) and 159 were prepared by de Meijere and Butenschon [179, 180] by treatment of the ligands with either Fe2(CO)9 or CpCo(H2CCH2)2 in yields up to 70%. When complex 156 was reduced with sodium, the persistent radical anion was detected with spin density on the iron atom as indicated by electron spin resonance (ESR) spectroscopy. Further reduction with sodium afforded the ferrate(-2) 158, which is diamagnetic and could be characterized by NMR investigations (Scheme 10.56). 

The [Fe(terpy)2] cation is low-spin, as demonstrated by Mossbauer, electronic, H NMR, and resonance Raman spectroscopy and magnetic measurements (20,184,187, 228, 266). Similarly, spectroscopic studies of the iron(III) cation have indicated a low-spin ( B) ground term (382). There have been numerous electrochemical studies of the bis complexes (177, 200, 256, 298, 332, 344, 373, 378, 379, 397, 398). Ligand-centered reductions to formal oxidation states of iron(I), iron(O), and iron( — 1) and oxidations to iron(III) are observed. The complex [Fe(terpy)L][C104]2 [L = tris(2 -pyridyl)l,3,5-triazine (Fig. 15)] has been prepared (399, 442). 

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