Porphyrin saddled structure

Figure 3.4 Mechanism of porphyrin metallation. (a) Out-of-plane saddle structure in which two pyrrole rings with unprotonated nitrogens (blue spheres) point upwards, while the other two, protonated (blue and white spheres) point downwards, (b) Steps in the mechanism for incorporation of the metal ion (red) into the porphyrin (pyrrole rings in green), described in the text. (From Al-Karadaghi et al., 2006. Copyright 2006, with permission from Elsevier.)...

INDO calculations predict the experimental trends, with calculated red shifts of 1900 cm" for a conformational change from a planar Zn porphyrin to one with the saddle structure of ZnTPOEP, to be compared with observed shifts of 1370 and 1880 cm relative to ZnTPP and ZnOEP, respectively. The calculations also predict that the puckered porphyrin will be easier to oxidize by 0.12 eV, whereas reduction is insensitive to the distortion. Similar calculations for puckered chlorins and bacteriochlorins predict red shifts of 1200 and 820 cm , respectively [32]. ... 

An important class of porphyrins is that constituted by confor-mationally distorted porphyrins, which mimic the non-planar geometry of the porphyrins present in photosynthetic systems.89 Obtainment of such non-planar distortions is associated with the introduction into the macrocyclic frame of proper crowding substituents, which therefore not only cause structural distortion but also affect, through their electronic effects, the redox potentials. A typical case is that constituted by [Cun(OETPP)] (OETPP = 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin), the saddle-distorted molecular structure of which is illustrated in Figure 56.102... 

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... 

The examples cited above represent part of an increasing body of structural information on chlorophylls, chlorins, bacteriochlorins and isobacteriochlorins (10-14 and references therein) that points to the remarkable flexibility of these molecules This ability of the macrocycle to adjust is not limited to hydroporphyrins but is also observed in porphyrins 5,10,15,20-tetra-n-propylporphinato lead (II) assumes a "roof" shape by folding along an axis defined by two opposite methine carbons with the two planes of the "roof" inclined at 22 to one another (15) In contrast, triclinic 5,10,15, 20-tetraphenylporphinato cobalt (II) is distinctly saddle shaped with the 3 carbons of adjacent pyrrole rings lying 40 66 and -0 66A above and below the plane of the four nitrogens (16) ... 

Recent structural studies of single crystals of meso tetraaryl porphyrin cation radicals reveal that their Zn(II), Cu(li), Fe(III)Cl and Mg(II) complexes are all saddle shaped (17-20) Since the unoxidized species are either planar or slightly domed, oxidation to the radicals results in a major conformational change An example of this effect is shown in Fig 6 which presents the displacements of the atoms that comprise the skeleton of the cation radical of Mg... 

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. 

Structural Aspects. Four-coordinate iron(II) porphyrins are of intermediate spin state (5=1) and are in general square planar, with the iron placed in the center of the porphyrin core. However, the porphyrin ring is not always planar, as is observed for the OEPFe species. It can be ruffled as was found for TPPFe or OECFe derivatives, or saddled as was reported for a second crystalline form of TPPFe All complexes are characterized by relatively short Fe-Npor bond lengths (1.95-1.97 A), which are indicative of the depopulation of the d S = 1 spin state. 

L. A. Yatsimyk, Ph.D. dissertation, Synthesis, Structures, Redox and Magnetic Resonance Spectroscopy of Saddle-Distorted Porphyrin Complexes of Iron, University of Arizona, 2003. 

Porphyrin 83 is substantially twisted due to unfavorable steric interactions between bromine atoms and the hydrogen atoms of the phenyl substituents.281-283 Like complex 16, 83 is saddle-shaped. The nonplanarity of the LCo structure reduces the hydrogen atom abstraction capability of 83, making it very good for capping propagating radicals without side reactions. 

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