Molecular structures of iron

Structural parameters for some azocompounds reported in the literature from electron or X-ray diffraction studies are collected in Table 1. Trans- and cis-NjFj have planar structures and the carbon-nitrogen skeleton of azomethane is also planar. In N2F2 the cis isomer is 3 kcal.mole" more stable than the trans and in N2H2 the cis isomer is probably also the more stable . In alkyl or aryl substituted derivatives, however, for steric reasons the trans form lies below the cis in energy. The molecular structure of Irons-azobenzene is known to be planar in the solid state while c/j-azobenzene deviates from planarity and a propeller shaped conformation has been proposed, wherein the phenyl rings are rotated approximately 30° out of plane . 

Scheme 3.4 Molecular structures of iron (III) tetrakis(N-methyl-4-pyridyl)porphyrin, Fe(l Il)TMPyP (A) and trans-[Co([14]aneN4)...

Fig. 6.3 Molecular structures of Iron centers In the oxyhemoglobins from mammalians and the nematode (A), and descriptions of the active sites for oxyhemerythrin (B) and oxyhemocyanin (C).

Although heme is absent in Clostridia, it was early recognized that anaerobic bacteria may contain substantial levels of iron (44). To date the best characterized iron compounds from this source are the iron-sulfur proteins termed ferredoxins and rubredoxins. Molecular structures of representatives of both types of protein have been worked out by Jensen and his colleagues by X-ray diffraction analysis (see below). 

The pentanuclear carbido species Ms(CO)lsC (M = Fe, Ru, Os) have been prepared. The iron compound has been known for some considerable time (209), but the ruthenium and osmium complexes were prepared recently by pyrolysis reactions (210). The ruthenium adduct was only isolated in low yield (—1%), while the osmium complex was obtained in higher yield (—40%). The infrared spectrum and mass spectral breakdown pattern indicate a common structure to these compounds. The molecular structure of the iron complex is shown in Fig. 46. 

Let us consider the mixed valent Feni2Fen oxo-centred complex [Fe3OL3], the molecular structure of which is illustrated in Figure 77.121 It is evident that all the iron centres have the same coordination sphere. 

Figure 20 illustrates the molecular structure of [Fe3(CO)i2]ld It consists of a triangle of iron atoms. Two CO molecules assume a bridging position with respect to one Fe-Fe side (Fe2-Fe3), rendering this side shorter by about 0.1 A with respect to the other two sides bearing only terminal carbonyl groups. 

Figure 24 shows the molecular structure of the dianion [Fe4(CO)i3]2-. ld The iron atoms are disposed in a tetrahedral arrangement and all the carbonyl groups are terminally coordinated, except the one triply bridging the Fe3 base. 

The last class of iron-sulfur proteins to be considered is that of the bacterial 7Fe ferredoxins, which contain both an Fe3S4 and an Fe4S4 cluster. Figure 28 shows the molecular structure of that from Azotobacter vinelandii (FW — 12 700).53... 

We define "oxo-lron aggregates" as a collection of three or more iron ions linked continuously by bridging oxo-, hydroxo- or alkoxo groups. The aggregates are labelled according to their nuclearlty. Undoubtedly the molecular structures of the aggregates represent the key to understanding their properties and therefore they will be discussed first. 

The chemistry of the iron core is most interesting, in the light of the experiments mentioned above with respect to the hydrolytic polymers of Fe (III). The molecular structure of the iron core obtained from crystalline ferritin by treatment with concentrated sodium hydroxide has recently been investigated using low angle X-ray scattering patterns (44). These authors have proposed a structure quite similar to that of Green rust II ... 

Aminoboranes have been used as ligands in complexes with transition metals (66) in one instance giving a rare example of two-coordinate, non-/0 transition-metal complexes. The molecular structure of the iron complex Fe[N(Mes)B(Mes)2]2 where Mes = 2,4,6-(CH3)3C5H2 is shown in Figure 1. The less sterically demanding lithium borylamide, LiN(CH3)B(CH3)2, used to prepare mercury and tin complexes, has also been prepared (67). 

The iron-selenium cubane-type cluster [Fe4Se4(NO)4] has been shown (52) to adopt a molecular structure of almost exact Td symmetry, whereas in the corresponding anion [Fe4Se4(N0)4] the symmetry is lowered to D2a, just as for the sulfur analogs (37). Upon one-electron reduction the size of the Fe4 cage increases in the neutral molecule the Fe-Fe and Fe-Se distances are 2.703 and 2.342 A, but in the anion the... 

Many investigations of the molecular structure of thin films formed by y-APS deposited onto inorganic substrates from aqueous solutions have been carried out. Ondrus and Boerio [2] used reflection-absorption infrared spectroscopy (RAIR) to determine the structure of y-APS films deposited on iron, 1100 aluminum, 2024 aluminum, and copper substrates from aqueous solutions at pH 10.4. They found that the as-formed films absorbed carbon dioxide and water vapor to form amine bicarbonate salts which were characterized by absorption bands near 1330, 1470, 1570, and 1640 cm-1. y-APS films had to be heated to temperatures above about 90°C in order to dissociate the bicarbonates, presumably to free amine, carbon dioxide, and water. Since the amine bicarbonates failed to react with epoxies, the strength of adhesive joints prepared... 

Ondrus and Boerio also found that metallic substrates had a significant effect on the molecular structure of y-APS films cured against them [2], When y-APS films were dried against iron substrates at 110°C for 30 min, the extent of polymerization increased and the bicarbonates dissociated. Similar behavior was observed for films formed on commercially pure 1100 aluminum substrates. However, when y-APS films were dried against 2024 aluminum substrates which contain about 4.6% copper, a new band attributed to an imine formed by copper-catalyzed oxidation of the propylamine group appeared near 1660 cm 1. 

Figure 2.3 Molecular structure of [Fe(Me3TACN)(DBC)CI], a model complex for a catechol dioxygenase coordinated to its substrate molecule [28]. Hydrogen atoms have been omitted for clarity. The l,4,7-trimethyl-l,4,7-triazacyclononane (Me3TACN) ligand coordinates facially to the iron center. The remaining three coordination sites are occupied by 3,5-di-tert-butylcatecholate (DBC) and a chlorido ligand.

A basic understanding of the electronic structures of iron bearing clays and oxides is needed before one can understand the mechanisms of electron transfer and photochemical reactions associated with these minerals. This chapter will discuss the electronic structures of iron bearing clays and oxides (primarily from cluster molecular orbital calculations) and compare theoretical results with experiment. The discussion will be... 

The theoretical results described here give only a zeroth-order description of the electronic structures of iron bearing clay minerals. These results correlate well, however, with the experimentally determined optical spectra and photochemical reactivities of these minerals. Still, we would like to go beyond the simple approach presented here and perform molecular orbital calculations (using the Xo-Scattered wave or Discrete Variational method) which address the electronic structures of much larger clusters. Clusters which accomodate several unit cells of the crystal would be of great interest since the results would be a very close approximation to the full band structure of the crystal. The results of such calculations may allow us to address several major problems ... 

Fig. 7. Molecular structure of the cation in the bridged iron(II) nitro/nitrito complex [(l)2Fe2 r 1-lV r 1-0)-N02 ]Br2PF6 (37) intramolecular hydrogen bonds are shown as broken lines.

Figure 2.5 Molecular structures of zinc-iron porphyrin complexes across (a) hydrogen, (b) aliphatic and (c) aromatic bridges, and the corresponding rates of photoinduced electron transfer for each species, as reported by Rege et al. [12]...

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