Alternant Bond Distances in Octaethylporphyrin π-Cation Radicals

Abstract

The possible appearance and nature of alternant bond distance patterns in the inner 16-membered ring of several octaethylporphyrin [Formula: see text]-cation radicals has been investigated. This study was made possible by recognizing an unexpected solvent system, namely dichloromethane/chloroform, even though the [Formula: see text]-cation species have extremely limited solubility in chloroform. A total of six [M(OEP[Formula: see text]][Formula: see text] derivatives were studied by single-crystal X-ray structure determinations. Two new zinc derivatives display, quantitatively, the same alternant pattern observed previously. A new nickel complex shows a smaller but now probably significant alternant pattern. However, a copper derivative, independently analyzed twice, shows no evidence for an alternant pattern. The importance of ring–ring interactions on the energy of the top two orbitals is shown by two distinct magnesium derivatives. The derivative with strongly overlapped rings displays an alternant bond distance pattern, whereas the other, with a modestly overlapped ring pair, does not. This suggests the importance of strong ring–ring interactions in leading to a pseudo-Jahn-Teller state; this hypothesis is also supported by other prior results.