Proton-Initiated Reversible Chalcogen-Vertex Extrusion in Macropolyhedral Chalcogenaboranes

Protonation of the macropolyhedral chalcogenaborane anions [E₂B₁₇H₁₈]^- (E = S, <b>1a</b>^<b>-</b> Se, <b>1b</b>^<b>-</b>) induces a radical cluster rearrangement, yielding the new metastable isoelectronic neutral species E₂B₁₇H₁₉ (<b>2a</b> and <b>2b</b>). This transformation converts the original structure comprising two identical "<i>arachno</i>" 10-vertex subclusters into a new architecture with two distinct "<i>nido</i>" 10-vertex subclusters. Notably, one of the chalcogen atoms, which originally was a triconnected cluster vertex in anions <b>1</b>^<b>-</b>, is extruded, forming a bridging μ₂-{EH} unit. Remarkably, deprotonation of compounds <b>2</b> facilitates a boronotropic dehydrogenation, giving reintegration of the extruded {EH} unit and regeneration of the original [E₂B₁₇H₁₈]^- anions <b>1</b>^<b>-</b>. Compound <b>2a</b> is characterized by a single-crystal X-ray diffraction (SCXRD) study, and by agreement between experimental and density functional theory (DFT)-calculated multielement NMR spectra. Also, for <b>2b</b>, the measured and calculated δ(⁷⁷Se) values for the two widely separated selenium resonances add further support. Metastable compounds <b>2</b> ultimately eliminate dihydrogen to yield the known neutral E₂B₁₇H₁₇ compounds <b>3</b>. For <b>2b</b>, a competing pathway leads to loss of selenium, to give known SeB₁₇H₁₉ <b>4</b>.