Novel Synthetic Approach to Charge-Compensated Phosphonio-<i>nido</i>-Carboranes. Synthesis and Structural Characterization of Neutral Mono and Bis(Phosphonio) <i>nido</i>-<i>ortho</i>-Carboranes

A number of monosubstituted <i>n</i>-(triphenylphosphonio)-7,8-dicarba-<i>nido</i>-undecaboranes (<b>2a</b>, <i>n</i> = 1; <b>2b</b>, <i>n</i> = 3; <b>2c</b>, <i>n</i> = 5; <b>2d</b>, <i>n</i> = 9) were prepared via a cross-coupling reaction between the tetrabutylammonium iodo-7,8-dicarba-<i>nido</i>-undecaborates (<b>1a</b>–<b>d</b>) and PPh₃ in the presence of a Pd­(PPh₃)₄ catalyst. The substitution rate was found to depend on the iodine position in the carborane cage. Under similar conditions, the reaction of 5,6-diiodo- (<b>3</b>) and 9,11-diiodo-7,8-dicarba-<i>nido</i>-undecaborate (<b>5</b>) anions exclusively yielded the monosubstitution products 5-iodo-6-(triphenylphosphonio)-7,8-dicarba-<i>nido</i>-undecaborane (<b>4</b>) and 9-iodo-11-(triphenylphosphonio)-7,8-dicarba-<i>nido</i>-undecaborane (<b>6</b>), respectively. The reaction of tetrabutylammonium 6,9-diiodo-7,8-dicarba-<i>nido</i>-undecaborate (<b>7</b>) exclusively produced the phosphine substitution product in the open face of the <i>nido</i>-carborane, 6-iodo-9-triphenylphosphonio-7,8-dicarba-<i>nido</i>-undecaborane (<b>8</b>). The addition of a base (Cs₂CO₃, NaH) to the reactions of <b>3</b> and <b>5</b> with PPh₃ afforded the corresponding bis­(triphenylphosphonio)-7,8-dicarba-<i>nido</i>-undecaboranes, <b>9</b> and <b>10</b>. Compound <b>10</b> was also prepared from <b>6</b> using the general procedure. The reaction of the triiodocarborane tetrabutylammonium 5,6,9-triiodo-7,8-dicarba-<i>nido</i>-undecaborate (<b>11</b>) with excess PPh₃ in the presence of Cs₂CO₃ and Pd­(PPh₃)₄ only produced neutral 5-iodo-6,9-bis­(triphenylphosphonio)-7,8-dicarba-<i>nido</i>-undecaborane (<b>12</b>); no positively charged tris­(phosphonio) species formed. The compositions of all prepared compounds were determined by multinuclear NMR spectroscopy and high-resolution mass spectrometry. The structures of compounds <b>2c</b>, <b>6</b>, <b>8</b>, <b>9</b>, and <b>12</b> were established by the X-ray diffraction analysis of single crystals