Stretched porphycene

Oxoiron(IV) tefraphenylchlorin complexes have been prepared as the first models of a reaction intermediate in the catalytic cycle of cytochrome d Optical absorption spectra show a characteristic red-shified band at 630 nm as observed in the oxoferryl intermediate of cytochrome d, and the proton NMR spectra of the N-Melm complex exhibit very small hyperfine shifts of the pyrrole protons, as is true for oxoferryl porphyrin complexes. The pyrroline protons of the saturated pyrrole ring show unusual splitting into upheld and downfield resonances. The N-Melm complex also shows normal Fe =0 stretching frequencies as compared to the corresponding oxoferryl porphyrin complexes. And finally, for iron porphycenes, both peroxo and ferryl intermediates have been detected by H NMR spectroscopy during the oxygenation of the Fe complexes. ... 

Vinylogous porphyrins are formally built up of odd-numbered carbon spacers separating four pyrrole rings. There are, however, several expanded porphyrins that contain four pyrrolic or pyrrole-like subunits that contain only even-numbered carbon spacers. These macrocycles will be grouped under the general heading stretched porphycenes , since they are typically prepared via the low-valent titanium mediated coupling of pyrrolic aldehydes just as are the porphycenes (cf. Chapter 3). 

The first example of a stretched porphycene was reported in 1990 by Vogel and coworkers. - It was the acetylene-cumulene macrocycle 4.115 ([22]tetra-dehydroporphyrin-(2.2.2.2)) prepared via the McMurry coupling of the diformyl dipyrryldimethyne derivative 4.113. As proved true for porphycene, the presumed intermediate species 4.114 could not be isolated. Instead it was allowed to undergo spontaneous oxidation during workup to afford the aromatic macrocycle 4.115 in 18% yield (Scheme 4.3.1). 

The stretched porphycenes have shown some promise as ligands for the com-plexation of transition metal cations. For instance, when [22]tetradehydroporphyrin-(2.2.2.2) 4.115 is treated with di-p-chlorobis[dicarbonylrhodium(I)] in dichloro-methane using potassium carbonate as base, a bis[Rh(CO)2] complex is formed. ... 

Vogel and coworkers have also prepared other stretched porphycenes derived from 2,2 -bipyrrolic building blocks. The smallest of these are the [22]porphyrins-(4.0.4.0) 4.135 and 4.136. These macrocycles were prepared from their corres-... 

The stretched porphycene 4.142 ([26]-porphyrin-(6.0.6.0)) was also recently reported by Vogel, et al. This macrocycle was prepared in 50% yield via a McMurry-type dimerization of dialdehyde 4.140, followed by a spontaneous dehydrogenation of the presumed non-aromatic intermediate (Scheme 4.3.11). The free-base form of [26]-porphyrin-(6.0.6.0) (4.142a) proved to be rather fluctional in a variety of solvents over the temperature range of 25 to -50 °C. This hindered... 

A single crystal X-ray structural analysis of the alkyne-containing product 6.26a revealed that the macrocycle adopts a near-planar conformation in the solid state (Figure 6.3.3). Deviation from planarity results, presumably, from the methyl-methyl steric interactions within the terpyrrolic subunit. The crystallographic data also served to support the contention that the macrocycle does indeed contain a true an alkyne-like (C-C=C-C) fragment. Such a bond isolation, although expected in the present instance, stands in contrast to what is seen in the case of Vogel s stretched porphycene precursor. In this latter instance cumulene-type (C = C = C = C) bond delocalization is observed. 

The differences in barrier heights are even more dramatic. This is best exemplified by calculations on porphycenes 1 and la the inclusion of zero point energy results in the transition state energy being lower than that of the cis form in 1 and of both forms in la. These results show that the harmonic approximation is not appropriate for the vibrations involved in the hydrogen transfer path in porphycenes (NH stretch, in particular) and that the barriers to tautomerization must be very low. 

The assignment of the N-H (N-D) stretching mode in the d//dV spectra of a porphycene molecule is also supported byDFT calculations The bottom inset of Figure 7.16 shows the calculated vibrational density of states for a porphycene molecule on Cu(llO) (i.e., their projection on in-plane displacement of the inner H (D) atoms and N atoms). The N-H (N-D) stretching mode is found to be at 376 (279) meV, which is in very good agreement with the experimental values. 

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