An Experimental Solution to the “Missing Hydrogens” Question Surrounding the Macropolyhedral 19-Vertex Boron Hydride Monoanion [B₁₉H₂₂]⁻, a Simplification of Its Synthesis, and Its Use As an Intermediate in the First Example of <i>syn</i>-B₁₈H<sub>22</sub

The macropolyhedral [B19H22]− monoanion 1 and the dianion [B19H21]2− 2 are synthesized in consistent 86−92% yields by the reaction of [PSH]+[syn-B18H21]− with BH3(SMe2) in 1,2-Cl2C2H4 at 72 °C. [‘PS’ is an abbreviation for ‘Proton Sponge’, 1,8-bis-(dimethylamino)naphthalene. ‘PSH’ is its protonated derivative.] The molecular structures of 1 and 2 were elucidated as their [PS{BH2}]+ and [PS{BH2}]2+ salts 1a and 2a by single-crystal X-ray diffraction studies, in which all atoms were located, and supported by mass spectrometric analyses together with calculations of the cluster molecular geometries (ab ignitio and/or DFT) and of 11B chemical shifts based on GIAO−DFT shielding tensors. Acidification of dianion 2 with CF3COOH in acetonitrile, H2SO4 in dichloromethane, or aqueous HCl results in the clean formation of the monoanion [B19H22]− 1. Conversely, shaking a concentrated acetonitrile solution of 1 in 0.5 M aqueous NaOH cleanly yields the [B19H21]2− dianion 2. Reaction of a dichloromethane solution of 1 with a 36% aqueous solution of HCHO in the presence of H2SO4 quantitatively converts 1 at room temperature to a 1:1 mixture of the syn- and anti-isomers of B18H22. This cluster dismantling process is the first example of a syn- to anti-B18H22 isomer conversion.