A Super-Oxidized Radical Cationic Icosahedral Boron Cluster

Abstract

While the icosahedral closo-[B<sub>12</sub>H<sub>12</sub>]<sup>2–</sup> cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all 12 B–H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. In this work, we evaluated the electrochemical behavior of the electron-rich B<sub>12</sub>(O-3-methylbutyl)<sub>12</sub> (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br<sub>2</sub>C<sub>6</sub>H<sub>3</sub>)<sub>3</sub>]<sup>•+</sup> afforded the isolable [1]<sup>•+</sup> cluster, which is the first example of an open-shell cationic B<sub>12</sub> cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1]<sup>•+</sup> with ferrocene resulted in its reduction back to 1. The identity of [1]<sup>•+</sup> is supported by EPR, UV–vis, multinuclear NMR (<sup>1</sup>H, <sup>11</sup>B), and X-ray photoelectron spectroscopic characterization.