Synthesis, reactions, and electronic properties of 16 pi-electron octaisobutyltetraphenylporphyrin.

The reaction of the doubly oxidized beta-octaisobutyl-meso-tetraphenylporphyrin (OiBTPP, 4), which has a 16 pi-electronic structure at the porphyrin core, with a variety of metal reagents was investigated. The reaction of 4 with SnCl(2) followed by ethanolysis afforded an 18 pi-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 16pi [(OiBTPP)Li](+)[BF(4)](-) (6), which was easily derived from 4, with Zn, Cu, and Pd(2)(dba)(3) gave the corresponding 18pi metalloporphyrins (OiBTPP)M (7, M = Zn(EtOH); 8, M = Cu; and 9, M = Pd). One-electron oxidation of the copper complex 8 by AgSbF(6) afforded a 17 pi-electron cation radical complex, [(OiBTPP)Cu](*+)[SbF(6)](-) (10). The UV-visible and electron spin resonance spectra of 10 were quite similar to those of previously reported beta-octaethyl-meso-tetraphenylporphyrin (OETPP) derivatives, [(OETPP)Cu](*+)X(-) (X = ClO(4), I). In contrast to the reaction of 6 with Zn to give the 18pi complex 7, the reaction of 4 with divalent ZnCl(2) enabled us to isolate a new 16pi porphyrin-zinc(II) complex, [(OiBTPP)Zn(Cl)](+)[ZnCl(3)](-) (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 (17pi) and 11 (16pi) are much more distorted than those of the 18pi derivatives 5 and 7-9. Furthermore, the bond distances of 10 and 11 clearly showed the presence of bond alternation in contrast to aromatic 18pi species 8 and 7, respectively. Nucleus-independent chemical shift calculations of 4 and some metalated porphyrins indicated that the highly distorted 16pi 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 (16pi) and 11 (16pi) indicated that both compounds can be electroreduced to give the 18pi species, with the 16pi/18pi transition being reversible in the case of [(OiBTPP)Zn(Cl)](+)[ZnCl(3)](-) (11).