Proton-Initiated Reversible Chalcogen-Vertex Extrusion in Macropolyhedral Chalcogenaboranes

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