Macropolyhedral boron-containing cluster chemistry. Aspects of the S2B16H16 system. Preparation, structure, NMR spectroscopy and isomerism

Thermolysis of [arachno-4-SB₈H₁₂] (1) in boiling cyclohexane gives two isomers 2 and 3 of 18-vertex [S₂B₁₆H₁₆], together with known 12-vertex [closo-1-SB₁₁H₁₁] (4) and known 11-vertex [nido-7-SB₁₀H₁₂] (5). Compounds 2 and 3 are characterised by single-crystal X-ray diffraction analyses and single- and double-resonance ¹¹B- and ¹H-NMR spectroscopy. The [n-S₂B₁₆H₁₆] isomer 2 takes the form of nido ten-vertex: nido ten-vertex [anti-B₁₈H₂₂] with the 9 and 9′ positions occupied by S vertices, whereas the [iso-S₂B₁₆H₁₆] isomer 3 takes the form of a nido 11-vertex {SB₁₀} subcluster fused via a common two-boron edge to a nido-type {B₈} subcluster that is additionally linked exo to the {SB₁₀} subcluster by a bridging S atom that is held endo to the {B₈} unit. Isomer 2 is readily deprotonated and its monoanion 6 is characterised by NMR spectroscopy and by a single-crystal X-ray diffraction analysis of its [tmndH]⁺[n-S₂B₁₆H₁₅] salt 6b; deprotonation has occurred from an open-face B---H---B bridging site.