The R<sub>3</sub>O<sup>+</sup>···H<sup>+</sup> Hydrogen Bond: Toward a Tetracoordinate
Oxadionium(2+) Ion
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Abstract
Oxatriquinanes are tricyclic oxonium ions which are known
to possess
remarkable solvolytic stability compared to simple alkyl oxonium salts.
Their rigid, hemispherical structure presents an oxygen at the apex
of three fused five-membered rings. While trivalent oxygen species
like these have been well described in the literature, the ability
of oxygen to enter into a fourth covalent bonding relationship has
been visited in theory and suggested by the outcome of certain reactions
conducted in superacidic media, but has never been established by
the characterization of a stable, persistent R<sub>3</sub>OH<sup>2+</sup> or R<sub>4</sub>O<sup>2+</sup> ion. In this study, the nucleophilicity
of the oxatriquinane oxygen was evaluated first by a series of protonation
studies using the Brønsted superacid H(CHB<sub>11</sub>Cl<sub>11</sub>) both in the solid state and in liquid HCl solution. The
interaction of the oxatriquinane oxygen with a bridging carbocation
was also examined. A strong case could be made for the occurrence
of hydrogen bonding between H(CHB<sub>11</sub>Cl<sub>11</sub>) and
oxatriquinane using IR spectroscopy. Under the most forcing protonation
conditions, the oxatriquinane ring is cleaved to give a bridged, dicationic,
protonated tetrahydrofuran–carbenium ion