Planar macrocycles from nature

A Practical Guide to Supramolecular Chemistry Peter J. Cragg 2005 John Wiley Sons, Ltd ISBN 0-470-86653-5 

Pyrrole [HARMFUL] Propanoic acid [CORROSIVE] Benzaldehyde [TOXIC] Methanol [FLAMMABLE] 

Small-scale distillation apparatus Heating/stirring mantles (25 and 250 mL) and stirring bars Thermometer 

Note The distillation and porphyrin synthesis must be carried out in a fume hood as several of the reagents have a pungent smell. 

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X-ray structure analysis showed that macrocycle 57 was essentially planar, with the twist angle of the benzene rings from the plane of the macrocycle being less than 2°. Most of the strain was seemingly contained in the triple bonds, as these were bent from linearity by 10.1° to 12.3°. Despite its strained nature, the macrocycle showed remarkable stability. Decomposition occurred above 300°C on attempted melting. No reaction was observed between 57 and cyclopenta-diene at room temperature. 

Figure 3 Single crystal X-ray diffraction structure of the gadolinium texaphyrin complex PCI-0101 (2) (bis-nitrate form) showing the planar nature of the basic monoanionic texaphyrin macrocycle and the four putative inner sphere coordination sites for water (occupied by two apical methanol molecules and a bidentate nitrate anion in this structure). The Gd(III) ion is nine-coordinate and lies 0.60 A out of the plane through the five nitrogen atoms of the macrocycle. Most hydrogen atoms have been omitted for clarity. Thermal ellipsoids have been scaled to the 30% probability level. Reproduced from [22] with permission. 1993 American Chemical Society...

A single crystal X-ray diffraction analysis has been carried out on both the chloro- and the phenyl-substituted subphthalocyanines 2.284 and 2.287 (Figures 2.3.2 and 2.3.3). These analyses served to show that subphthalocyanines lie in a bowl-shaped conformation. This is, of course, very different from the near-planar conformation of the parent phthalocyanines. Presumably, this bowl-shaped structure accounts, in part, for the decreased molar absorptivities of the subphthalocyanines relative to their phthalocyanine parents . Nevertheless, despite the non-planar nature of these macrocycles, the subphthalocyanines are capable of supporting an induced diamagnetic ring current (as judged by NMR spectroscopy). Thus, they may appropriately be considered as being aromatic. 

One example of a dimeric subphthalocyanine has recently been reported. This macrocycle (2.332) was prepared by condensing of an excess of 4-r-butylphtha-lonitrile 2.278 with 1,2,4,5-tetracyanobenzene (2.331) in the presence of Ph2BBr. The resulting laterally bridged system 2.332 is obtained in 2.8% yield (along with a 24% yield of the monomeric subphthalocyanine 2.286) (Scheme 3.14). Despite the presumed non-planar nature of the subphthalocyanine units, H NMR data collected for this system indicate that it, like the monomeric systems discussed above, is aromatic. On the other hand, compound 2.332 displays spectral properties that differ dramatically from those of the monomeric macrocycle 2.286. For instance, compared to that of 2.286, the UV-vis absorption spectrum of 2.332 is rather featureless, although absorption bands are observed in nearly the same spectral region as in the case of 2.286. The emission spectrum of 2.332 also resembles that of the mono-... 

To illustrate the vectorial nature of transition dipoles for a larger molecule, Fig. 4.9 shows the two highest occupied and two lowest unoccupied molecular orbitals of bacteriochlorophyll-a. These four wavefunctions are labeled — 04 in order of increasing energy. The products of the wavefunctions for the four possible excitations (0j 03, 0j 04, 02 —> 03, and 02 04) are shown in Fig. 4.10. The conjugated atoms of bacteriochlorophyll-a form an approximately planar jt system, and the wavefunctions and their products all have a plane of reflection symmetry that coincides with the plane of the macrocyclic ring. Because the wavefunctions have opposite signs on the two sides of this plane (z), their products are even functions of z. The z component of the transition dipole for excitation from... 

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