Synthesis, Reactions, and Electronic Properties of 16 π-Electron Octaisobutyltetraphenylporphyrin

The reaction of the doubly oxidized β-octaisobutyl-meso-tetraphenylporphyrin (OiBTPP, 4), which has a 16 π-electronic structure at the porphyrin core, with a variety of metal reagents was investigated. The reaction of 4 with SnCl2 followed by ethanolysis afforded an 18 π-electron tin complex, (OiBTPP)Sn(OEt)2 (5), in a redox manner. No reactions were observed using zerovalent metals (Zn, Cu, and Pd). However, the reaction of 16π [(OiBTPP)Li]+[BF4]− (6), which was easily derived from 4, with Zn, Cu, and Pd2(dba)3 gave the corresponding 18π metalloporphyrins (OiBTPP)M (7, M = Zn(EtOH); 8, M = Cu; and 9, M = Pd). One-electron oxidation of the copper complex 8 by AgSbF6 afforded a 17 π-electron cation radical complex, [(OiBTPP)Cu]•+[SbF6]− (10). The UV−visible and electron spin resonance spectra of 10 were quite similar to those of previously reported β-octaethyl-meso-tetraphenylporphyrin (OETPP) derivatives, [(OETPP)Cu]•+X− (X = ClO4, I). In contrast to the reaction of 6 with Zn to give the 18π complex 7, the reaction of 4 with divalent ZnCl2 enabled us to isolate a new 16π porphyrin−zinc(II) complex, [(OiBTPP)Zn(Cl)]+[ZnCl3]− (11), in 92% yield. The solid-state structures of 5 and 7−11 were unambiguously determined by single-crystal X-ray crystallography. The porphyrin cores of 10 (17π) and 11 (16π) are much more distorted than those of the 18π derivatives 5 and 7−9. Furthermore, the bond distances of 10 and 11 clearly showed the presence of bond alternation in contrast to aromatic 18π species 8 and 7, respectively. Nucleus-independent chemical shift calculations of 4 and some metalated porphyrins indicated that the highly distorted 16π porphyrins are essentially nonaromatic, with only weak antiaromaticity. Magnetic circular dichroism studies in conjunction with ZINDO/S calculations assisted in identifying the electronic transitions of the UV−vis spectra of key porphyrins. Electrochemical and thin-layer UV−vis spectroelectrochemical experiments on 4 (16π) and 11 (16π) indicated that both compounds can be electroreduced to give the 18π species, with the 16π/18π transition being reversible in the case of [(OiBTPP)Zn(Cl)]+[ZnCl3]− (11).