Superstructures ortho

Carbon monoxide reacts with [Fe(TPP)] to form a five-coordinate complex [Fe(TPP)CO], which can be reduced electrochemically to the corresponding iron(I) species from which, however,245 CO spontaneously dissociates. The Fe—CO interaction is stabilized by the,presence of hydrocarbon chains bound by amide linkages to the ortho position of the TPP phenyl rings. Carbon monoxide adducts of iron(I) complexes of a number of these superstructured porphyrins have been reported.245 The chemistry of these highly reduced species is of relevance to understanding240 the reactions of cytochrome P-450 and the peroxidases. 

Fig. 7.3. The [001] diffraction pattern of the Ortho-I phase. Splitting of superstructure spots is caused by orthorhombic twinning (see text). Streaks appear when vacancies order in an irregular stacking of chains, as a precursor to Ortho-II or Ortho-III phases.

The Ortho-II phase is easily recognized in [001] ED patterns (Fig. 7.4) as well as in [001] or [010] zone HREM images (Fig. 7.5). In ED patterns superstructure spots appear at positions h- - kl, characteristic for a doubling of the a-parameter. Often however, as shown in Fig. 7.4, in such patterns also the h-parameter seems doubled by the appearance of spots at positions hk + l as well. These spots are due to twinning of the orthorhombic material, the resulting diffraction pattern being the overlap of the two orientation variants [7.29-7.31]. 

Dark-field images allow the visualization of the ordered domains in real space. Ortho-II domains will appear bright when selecting a superstructure reflection. Figures 7.6(a) and (b) show images corresponding to the diffraction... 

Fig. 7.4. (a) The [001] zone diffraction pattern with sharp superstructure reflections at positions h + jk I appearing in well-ordered Ortho-II material. Since the material is twinnecC spots seem to appear along both basic directions ... 

Electron microscopic observations of the Ortho-III phase have been reported far less than of the Ortho-II phase [7.8, 7.34, 7.35]. Clear evidence for the occurrence of this phase is given in the [001] diffraction of Fig. 7.4(d). Due to the short structural coherence length of this phase, the results could not be confirmed either by X-ray, or by neutron diffraction. The Ortho-III phase ideally appears at an oxygen deficiency 0 =, and compared to the Ortho-I phase, has one out of three CuiOi chains depleted of oxygen in an ordered succession. (See Fig. 7.2(c)). Lattice parameters are am 3ai, bm b and cm c. The tripling of the unit-cell is reflected in the diffraction pattern by the appearance of two superstructure spots at positions h+ kl and h + kl. Superstructure spots for the Ortho-III phase are always very broad, due to substantial disorder in the cell-tripling alternation, or to the limited size of the domains. 

Superstructures with longer periodicities of Aa and 5a, only appear occasionally on a very local scale they are hardly evidenced by any diffraction technique, but they have been observed in real space by high-resolution electron microscopy (Fig. 7.8). Other periodicities, have been reported in the literature and are also attributed to CuO-chain ordering, but these observations are not well-confirmed [7.8, 7.10, 7.36, 7.37]. Identifying these phases as CuiOi-chain ordered phases, they should probably be considered as metastable phases at oxygen contents intermediate to that of the stable Ortho-I, Ortho-II and Ortho-III phases. Theoretical studies seem to support this conclusion [7.38-7.39]. 

Fig. 7.12. (a) The [001] zone axis diffraction pattern with intense Ortho-111 superstructure reflections at positions h + kl and h + jkl. Superstructure reflections are marked hy short arrows (h) [010] zone axis diffraction pattern... 

Op Ortho-I Ortho-II Oxygen of the planes (02, 03) Orthorhombic phase of 123-0, Superstructure of the orthorhombic phase of 123-0, with only every second chain occupied XRD X-ray diffraction... 

Antiparallel atomic displacements in the 2oq (ortho-Il) and ia (ortho-in) superstructures of 123-0, ... 

Fig. 44a. The average crystallographic structures of ortho-Il and ortho-Hi phases. The arrows indicate the displacements of the various atoms due to the formation of the superstructures. After Plakhty et al. (1995).

Fig. 47a. Pseudo-binary T-x phase diagram of YBa2Cu30 calculated with the CVM approximation. The small solid circles are the experimental data of Andersen et al. (1990). The inserts are schematic illustrations of the 2D superstructures of the basal plane obtained from Monte Carlo simulations. Large solid circles, oxygen small solid circles, Cul (large and small solid circles form the black chains) open circles, vacant sites. After de Fontaine et al. (1992). The measurements of the phase boundary of the T-0 transition are in good agreement with the theoiy. However, comparison with ftg. 45 shows that the calculated critical temperature of the ortho-II phase (-570K) is much higher than the value recently found [-370 K, von Zimmermann et al. (1999)].

The 493 cm" mode has its maximum intensity at x = 6.47, which is consistent with figs. 35a and 64. It extends its maximum in a large range which is consistent with the ortho-II phase (2ao superstructure) in the phase diagram of fig. 44b, and with the lattice parameters (figs. 25a,b). 

Electrophilic Aromatic Substitution. Micellar SDS has been used as a reaction medium for the chlorination and bromlnatlon of alkyl phenyl ethers T gjj(j phenol by several halogenatlng agents (eq 1). Compared to reactions in H2O alone, theparar.ortho product ratio increased for pentyl, nonyl, and dodecyl phenyl ether, and decreased for anlsole. Enhanced ortho relative to para substitution was obtained with phenol. In each case the observed regios-electivity derived at least in part from alignment of the substrate at the micelle-H20 interface and resultant differential steiic shielding of the para and ortho positions by the micelle superstructure. 

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