The Polyiodide Anions

Chapter 19 Some Electron-Excess o Bonded Systems 

The lengths of their I-I bonds are longer than the I-l single bond length of 2.67 A for free. We can account for these observations by inspection of the increased-valence structures (30), (32) and (34), which we may derive from the Lewis or non-paired spatial oibital stmctures (29), (31) and (33). For the latter three structures, we have subdivided the ions into + r + Ij, Ij +1, + Ij, and Ij + r + Ij + r +12 components, and used the non-paired spatial orbital stmctuie (8) for I3. All of the bonds are o-bonds. 

The penta-atomic ions Xe Fj and H Fj have the bond-lengths shown in (35) and (36). 

We may generate the increased-valence structures (37) and (38) from the Lewis structures for XeF + F + XeF and HF + F + HF. These increased-valence structures are similar to structure (30) for Ij, and imply that the two terminal bonds of each ion should be shorter than the two bridging bonds, and that all bonds should be longer than the single bond lengths of 1.81 and 0.92 A for XeF andHF. 

Evidence has been provided for the existence of a near-linear I the appropriate increased-valence stmcture (41) for it is similar to stmcture (40) for 

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I3 and I5" anions are linear with symmetrical charge population. There arises a question of the orientation of polyiodide ions with respect to polyacetylene chains. Mossbauer studies carried out on stretched polyacetylene showed a dependence of the spectra on the angle between the incident y-rays and the stretching direction revealing the alignment of the polyiodide anions in the stretched film [111]. 

In the purple film, both inclination of EDT-TTF(SCig)2 and disorder in the alkyl chains occurred. The value of the inclination angle of the TTF part was determined to be 24° by IR analysis. Isolated triiodide ions changed to chains, which was confirmed by resonance Raman spectra. The polyiodide anions exist parallel to the substrate from the X-ray near-edge structure, which was consistent with the short interlayer periodicity. 

Figure 12 The polyiodide network determined by I- I cation/anion contacts in 4-ferrocenium-ethylene-l,3-dithiole-2-thione I2 adducts balanced by V-shaped pentaiodides...

The slight steric differences between the alkyl groups of the cations lead to quite different far-orders of their polyiodide anion structures. In solid tBu2iPrPSeI7 [Figure 16(c)], one iodide anion bridges two of the iPr3PSeI+ cations and is in further contact with three I2 molecules the other I anion is in contact with five I2 molecules.59... 

In conclusion, structures containing polyiodide anions, with cationic aromatic ligands as counter parts of formulae [(L)(HL+)] (I ) are known to be synthesized by the treatment of the appropriate amide with HI [26-28], In contrast, the complexes with PYOH, in the present case, were formed by the direct reaction of 2-hydroxypyridine with di-iodine in a molar ratio of 2 1 and 1 2. This is a redox reaction, where 2-hydroxy-pyridine firstly is oxidized to pyridinone-2 radical cation. In the case of 2-hydroxy-pyridine however, peroxide structures are not formed like disulphides in the case of PYSH. Polyiodide anions are simultaneously produced in this case This should be a consequence of redox differences between -SH and OH groups and may be proven a useful pathway for the synthesis of polyiodide materials. 

In the ion-exchange method, brine solution is passed through an anion-exchange resin. Iodide (and polyiodide) anions from the solution adsorb onto the resin from which they are desorbed by treatment with caustic soda solution. The resin is treated with sodium chloride solution to regenerate its activity for reuse. The iodide solution (also rich in iodate, IO3 ions) is acidified with sulfuric acid. The acid solution is oxidized to precipitate out iodine. Iodine is purified by sublimation. 

The homopolyhalogen anions are formed mainly by iodine, which exhibits the highest tendency to form stable catenated anionic species. Numerous examples of small polyiodides, such as 1, I% and IJ, and extended discrete oligomeric anionic polyiodides, such as I7, Ig-, Ij7, Ijg, I4g, I42 and I29, and polymeric (17 ) networks have been reported. These polyiodides are all formed by the relatively loose association of several I2 molecules with several I- and/or 1 ... 

Figure 17.1.2 shows some polyiodides, which have been characterized structurally. All polyiodine anions consist of units of I-, I2, and 1 ". The bond length of the structural components of the polyiodides are often characteristic 267 to 285 pm in I2 molecular fragments, whereas those of symmetrical triiodide IJ are about 292 pm. 

The first report of high conductivity with small-ring metallomacrocycle is found in the study of the iodine oxidation products of metallooctaethylporphyrins, M(oep)134. Iodine oxidation of Ni(oep), Cu(oep), and H2(oep) yields polycrystalline materials with a range of stoichiometries. Resonance Raman spectroscopy indicates that the iodine occurs as the polyiodide IJ species, and thus the materials present an interesting contrast with the large-ring M(L)IX compounds, where, to date, iodine occurs only as the I3 anion. 

The earliest XPS work on modem electroactive polymers appears to be that of Hsu et al. [63] on the chemical states of the dopant in iodine-doped (CH)X films. In this work, combined XPS and Raman scattering studies revealed the presence of I3 and IJ species. The latter species resulted from the equilibrium process of the type I2 + IJ = I5. Similar findings were also made in at least two separate studies [64, 16]. The presence of polyiodide anion species was also observed... 

The treatment of 3-phenyl- and 3-ferrocenyl-3-chloropropenal with sodium sulfide nonahydrate gave rise to intermediate 3-phenyl- and 3-ferrocenyl-3-thiopropenal, which produced with aniline the 1-phenyl- (29a) and l-ferrocenyl-3-phenylaminothioprope-non 29h. The reaction of 29a,h with iodine in methanol yielded the oxidized bis(2,5-diphenylisothiazolium) octaiodide 30a (85%) and 2-phenyl-5-ferrocenylisothiazolium pentaiodide 30h (91%) (99ZAAC511). The structures of these iodides 30a (X = 17 ) and 30h were confirmed by X-ray diffraction (99ZAAC511, Scheme 8). 30a (I8 ) also formed a layer structure with isothiazolium cations and polyiodide anions. 

Richter et al. demonstrated that the crystal structure of 2-phenyl-5-ferrocenyl-isothiazolium pentaiodide 30h is a layer structure arranged toward z-axis and bis (2,5-diphenylisothiazolium) octaiodide 30a toward x-axis with changed layer of isothiazolium cation and polyiodide anion (99ZAAC511). 

In general, the vibrations originating from the doping anions are not observed by vibrational spectroscopies (IR, Raman). There are, however, a few exceptions. The coexistence of I3 and I5 in iodine doped conjugated polymers can be conveniently verified by Raman spectroscopy since both polyiodide anions give rise to resonantly enhanced lines at 107 and 162 cm" respectively [71,72]. 

In conclusion, structures containing polyiodide anions, with cationic aromatic ligands as counter parts of formulae [(L)(HL+)] (1 ) are known to be synthesized by the treatment of the appropriate amide with HI [26-28], In contrast, the complexes with PYOH, in the present case, were formed by the direct reaction of... 

Raman spectroscopy should help to characterize the nature of the incorporated anionic units (l3 or 15"). Fig. 3 shows representative solid-state resonance Raman spectra in the polyiodide scattering region for the [(Ga(Por)FIx]n materials. There is no evidence of free I2... 

It may be noted that the free halogens will not react with the respective liquid hydrogen halides. Not even iodine reacts with hydrogen iodide, where the formation oi a polyiodide anion might be expected. On the other hand iodine monochloride will react with chloride ion donors in hydrogen chloride just as well as in the absence of a solvent32... 

The propensity for iodine to catenate is well illustrated by the numerous polyiodides which crystallize from solutions containing iodide ions and iodine. The symmetrical and unsymmetrical 13 ions (Table 17.15) have already been mentioned as have the I5- and anions and the extended networks of stoichiometry (Fig. 17.12). The stoichiometry of the crystals and the detailed geometry of the polyhalide depend sensitively on the relative concentrations of the components and the nature of the cation. For example, the linear ion may have the following dimensions ... 

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