Crystal Structures of the Carborane Dianions [1,4-(PhCB10H10C)2C6H4]2− and [1,4-(PhCB10H10C)2C6F4]2− and the Stabilizing Role of the para-Phenylene Unit on 2 n+3 Skeletal Electron Clusters

While carboranes with 2 n+2 and 2 n+4 (n=number of skeletal atoms) skeletal electrons (SE) are widely known, little has been reported on carboranes with odd SE numbers. Electrochemical measurements on two-cage assemblies, where two C-phenyl-ortho-carboranyl groups are linked by a para-phenylene or a para-tetrafluorophenylene bridge, revealed two well separated and reversible two-electron reduction waves indicating formation of stable dianions and tetraanions. The salts of the dianions were isolated by reduction with sodium metal and their unusual structures were determined by X-ray crystallography. The diamagnetic dianions contain two 2 n+3 SE clusters where each cluster has a notably long carborane C–carborane C distance of ca 2.4 Å. The π conjugation within the phenylene bridge plays an important role in the stabilization of these carboranes with odd SE counts.

extracted with hot DME (5 mL). The combined filtrates were freed from solvent in vacuo.

Cyclic voltammetry
Electrochemical measurements were carried out using an EcoChemie Autolab PG-STAT 30 potentiostat at 298 K with a glassy carbon working electrode and platinum wires as counter and reference electrodes with 0.1 M n Bu 4 NPF 6 in dichloromethane under nitrogen. Scan rates of 100 mV s -1 and analyte concentrations of 10 -3 M were used. The ferrocene/ferrocenium FcH/FcH + couple served as internal reference at 0.0 V for potential measurements.  Table S1. Detailed electrochemical data for 1, [3] 3 and 4.  Table S2. Detailed electrochemical data for 2 based on square wave measurements.

Crystallography
Single crystals were coated with a layer of hydrocarbon oil and attached to a glass fiber.
Crystallographic data were collected with a Bruker AXS X8 Prospector Ultra with APEX II for ([Na(dme) 3 ] + ) 2 [3] 2with Cu-K α radiation (multilayer optic, λ = 1.54178 Å) and a Nonius Kappa CCD diffractometer with Mo-K α radiation (graphite monochromator, λ = 0.71073 Å) at 100 (2) K for ([Na(dme) 3 ] + ) 2 [4] 2-. Crystallographic programs used for structure solution and refinement were from SHELX-97. [4] The structures were solved by direct methods and were refined by using full-matrix least squares on F 2 of all unique reflections with anisotropic thermal parameters for all non-hydrogen atoms. The hydrogen atoms bonded to boron atoms were refined isotropically, all other hydrogen atoms were refined using a riding model with U(H) = 1.2 U eq . Crystallographic data for the compounds are listed in

Computations
All computations were carried out with the Gaussian 03 package. [5] The geometries of 1, [1] -, 2-were fully optimized at the ground states (S 0 ) with the B3LYP functional [6] with no symmetry constraints using the 6-31G* basis set [7] for all atoms. Frequency calculations on these optimized geometries revealed no imaginary frequencies. Natural