Supercarborane Radical Anions with 2<i>n</i> + 3 Electron Counts: A Combined Experimental and Theoretical Study

The synthesis, structure, spectroscopic characterization, and density functional theory (DFT) calculations of several 13-vertex carborane radical anions were described. Reactions of 13-vertex <i>closo</i>-carboranes with 1 equiv of finely cut sodium metal in tetrahydrofuran at room temperature afforded the corresponding sodium salts of carborane radical anions as dark-red crystals in very good yield. They represent a rare class of clusters having 2<i>n</i> + 3 framework electrons, which lie between the two well-established and abundant closed 2<i>n</i> + 2 (<i>closo</i>) and open 2<i>n</i> + 4 (<i>nido</i>) skeleton systems. However, attempts to prepare the 12- or 14-vertex analogues failed. DFT calculations indicate that the Δ<i>G</i>_sol,1 – Δ<i>G</i>_sol,2 values, the difference in the solvation-corrected free-energy changes between the first and second one-electron-reduction processes, can be used as a measure for the stability of carborane radical anions. The relatively high stability of 13-vertex carborane radical anions is related to the unique structures of 13-vertex carboranes, which give rise to minimal structure disruption upon the first one-electron reduction