Supercarborane radical anions with 2n + 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 closo-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 2n + 3 framework electrons, which lie between the two well-established and abundant closed 2n + 2 (closo) and open 2n + 4 (nido) skeleton systems. However, attempts to prepare the 12- or 14-vertex analogues failed. DFT calculations indicate that the ΔGsol,1 - ΔGsol,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.