Periodoannulenes: A Generalized Annulene-Within-An-Annulene Paradigm For Combined Sigma And Pi Ring Currents

Periodoannulene molecules and ions CxIxq in planar geometry offer examples of systems with the potential for outer sigma and inner pi ring-current double aromaticity, given a sufficient overlap of tangential p(sigma)-orbital manifolds on the large atoms of the outer cycle. Previous theoretical work indicated concentric diatropic currents in the dication C6I62+. Ab initio ipsocentric calculations support an account in terms of frontier-orbital selection rules for current contributions in C6I62+ (and radical C6I6+, implicated in recent experimental work on the oxidation of periodobenzene). A sigma/pi analogue of the annulene-within-an-annulene model is applied here to period() systems based on cyclooctatetraene. Model species C8I8q with charges q = 0, +1, +2, +4, -2 and structures constrained to a planar D-4h symmetry exhibit maps with all combinations of sigma/pi con- and counter-rotation, comprising global sigma ring currents on the iodine perimeter and central pi ring currents on the carbocycle. All can be rationalized by the separate application of the tropicity selection rules to the two subsystems, whether in singlet or triplet states.