Macropolyhedral thiaboranes with unusual arachno subclusters—a computational investigation

The cluster structures of experimentally characterized macropolyhedral thiaboranes [S2B17H17·SMe2] (1) and [S2B18H19]− (2) were investigated at the B3LYP/6-311+G**//B3LYP/6-31G* density functional theory level. Their conjuncto structures combine a usual nido-{SB10} subcluster and isoarachno 10 and 11 vertex subclusters, respectively. The latter are exceptional in having a sixfold coordinated vertex, not known in arachno borane chemistry. While for isolated [SB9H12]− and [SB10H13]− the expected structures are more stable by 31.2 and 17.2 kcal mol−1 than the structures realized in the conjuncto compounds, the stability order is reversed in the conjuncto structures: the unexpected clusters are preferred by 4.2 and 14.0 kcal mol−1 over alternatives incorporating regular arachno subclusters. Compounds 1 and 2 do not form for kinetic reasons only. Large geometrical distortions of the nido subcluster occur, when it is attached to regular arachno subclusters but not with the iso subclusters. Nido subcluster opening may be due to transfer of electron density from the arachno to the nido part, which is prevented in the presence of the more electron deficient sixfold coordinated vertex of the iso subclusters.