Synthesis, spectroscopic, and structural studies of extremely short chain basket handle porphyrins and their zinc(II) complexes

The synthesis of extremely short chain basket handle porphyrins is described. These molecules are derived from 5,10,15,20-tetraphenyl-21H,23H-porphine bearing alkoxy chains (handles) of 4-, 5-, and 6-carbon atoms covalently attached at the o-phenyl positions. For the n = 5 and 6 free base porphyrins three different isomers are formed: cross-trans-linked (I), adjacent-trans-linked (II) and adjacent-cis-linked (III) isomers. For the butyl-bridged compound only the latter two could be obtained. Attempts to insert zinc(II) ion into the porphinato cores have been successful only for isomers II and III; isomer I could not be metalated. The structural assignment of the three different isomers is based on UV-vis and /sup 1/H NMR spectroscopy as well as on X-ray structure analysis of the Zn(II) complex of the hexyl-bridged isomer II. The Zn hexyl II complex crystallizes in the monoclinic system, space group P2/sub 1//n, and cell dimensions at 296 K of a = 8.608 (2) A, b = 22.212 (4) A, c = 13.774 (2) A, and ..beta.. = 91.90 (1)/sup 0/ with two molecules per unit cell. The average Zn-N bond distance is 2.031 (3) A with the zinc atom precisely centered in the porphinato plane. On the basis of the spectroscopic and preliminary structural data, it is shown that the porphyrin ring of isomer I is permanently distorted from planarity. Furthermore, from analysis of spectroscopic data for isomers II and III it appears likely that the porphyrin rings of the butyl-bridged compounds are also distorted significantly, while those of the pentyl- and hexyl-bridged porphyrins are planar.