Ruffled Deformation

The introduction of all l groups at the porphyrin meso positions induces ruffled deformation [9,10,27,28]. A typical example is meso-tetra(te/t-butyl)porphyrin prepared by Smith and coworkers [27,51]. The meso carbon atoms deviate from the mean porphyrin plane upward and downward alternately. The average deviation of the meso carbons in the zinc complex Zn(T BuP) reaches as much as 0.899 A [28], Thus, the iron complexes of this porphyrin such as Fe(T BuP)X and [Fe(T BuP)L2] should be the best model to elucidate the effect of ruffled porphyrin ring on the electronic structure of the iron(lll) complexes. However, the synthesis of Fe(T BuP)X has been unsuccessful until now. For this reason, meso-tetraisopropylporphyrin complexes Fe(T PrP)X, where the meso te/t-butyl groups are replaced by the less bullqr isopropyl groups, have been examined [52,53]. 

ORTEP diagrams of (a) Fe(T PrP)CI, (Reproduced from Ref. [53] with permission from the American Chemical Society), and (b) [Fe(f PrP)(THF)2l+, (Reproduced from Ref. [54] with permission from the International Union of Crystallography). 

10 SPIN CROSSOVER IN IRON(III) PORPHYRINS INVOLVING THE INTERMEDIATE-SPIN STATE 

TABLE 10.4 HNMR, CNMR, and EPR Data of Five-Coordinate Fe(TEtPrP)X and Fe(T PrP)X with Anionic Axial Ligand (X)  

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Table 10.5 also lists the EPR g values of [Fe(T PrP)L2] and [Fe(TPP)L2]. The axial ligands are classified into two groups. The [Fe(TPrP)L2] complexes carrying pyridine N-oxide and substituted pyridine N-oxide exhibit the high-spin state while all the other ligands located below 4-CIPyNO in Table 10.5 show an essentially pure intermediate-spin state the Int (%) values are larger than 90%. In contrast, all the TPP complexes cept [Fe(TPP)(THF)2] adopt the high-spin state. These results clearly indicate that the ruffled deformation does stabilize the intermediate-spin state [15-18,26]. 

One of the two most common nonplanar deformations of the porphyrin ligand is the saddle conformation in which the pyrrole Cp—C bonds are displaced alternately above and below the mean 24-atom plane. The other is the ruffled conformation in which the Cmeso carbon atoms are displaced alternately above and below the mean 24-atom plane with concomitant twisting of the pyrrole rings. There are various measures for the extent of mffling for example, the C eso cross-ring distance decreases and the mean displacement of Cmeso from the mean 24-atom plane increases as the structures become more ruffled. The average M—N... 

This reinforces a view that the thermal isomerization, while perhaps assisted by deformations originating near the pyrrole nitrogens, does not require severe ruffling of the porphyrin macrocycle. 

The porphyrinic complexes are all essentially planar. However, the simple porphyrin and tbp macrocycles are somewhat flexible, and can adopt a ruffled, or saddle-shaped distortion through twisting at the methine carbon atoms. These deformations cause almost negligible changes in electronic structure, but the conformational mobility offers an added element of subunit variability. 

Figure 4 Six out-of-plane vibrational coordinates of a Z)4h metalloporphyrin used in normal structural decompositions to reveal compositions of equilibrium out-of-plane heme deformations. Mulliken S5nnbols are of the I>4h point group and the abbreviations sad, ruf, dom, wav, and pro indicate saddled, ruffled, domed, wave, and propeller distortions, respectively. (Note that wav and waVy derive from a doubly degenerate pair of Eg vibrational modes of the porph5rin.) The dark and light circles indicate atoms on opposite sides of the mean heme plane. Adapted from ref. 11.

A red blood cell is atypical because its surface is smooth. More typically, a phagocyte cell has a ruffled surface with more than twice the surface area of an equivalent sphere. This cell has no resistance to deformation until the ruffles are pulled flat. After that the cell is stiff and ruptures when extended more, as in the experiment of Fig, 12.6. 

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