Cyclooctatetraenes Tetrakis-Annelated with α-Dithio- or α-Diselenocarbonyl Groups: Diradicals Predicted To Have Ground States with 10 π Electrons in the Eight-Membered Ring and Two-Center, Three-Electron, σ Bonds between Two Pairs of Chalcogen Atoms

Abstract

(U)B3LYP calculations with the 6-31+G(d) and 6-311+G(2df) basis sets have been carried out on cyclooctatetraenes 6 and 7, in which the COT ring is tetrakis-annelated with alpha-dithio- or alpha-diselenocarbonyl groups. Transferring two electrons from the high-lying b(1g) and e(u) sigma MOs in 6 and 7 into the unoccupied, nonbonding, COT pi orbital is computed to be energetically favorable. The lowest D(4h) electronic state is calculated to be (3)E(u), which formally contains 10 pi electrons in the eight-membered ring and has two unpaired electrons in sigma MOs. The (3)E(u) state undergoes a first-order Jahn-Teller distortion to form 6d and 7d, in which the pair of one-electron holes in the sigma MOs is stabilized by the formation of two, two-center, three-electron bonds between pairs of chalcogen atoms that are diagonally across the eight-membered ring from each other. The corresponding open-shell singlets are computed to be about 1 kcal/mol lower in energy than the Jahn-Teller distorted triplets. Molecules 6i and 7i, in which the C-C bond in one four-membered ring is cleaved, are computed to be lower in energy than 6d and 7d. However, a substantial barrier is predicted to separate each of the two pairs of isomers so that 6d and 7d should, at least in principle, be isolable.