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 n-(triphenylphosphonio)-7,8-dicarba-nido-undecaboranes (2a, n = 1; 2b, n = 3; 2c, n = 5; 2d, n = 9) were prepared via a cross-coupling reaction between the tetrabutylammonium iodo-7,8-dicarba-nido-undecaborates (1a–d) 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- (3) and 9,11-diiodo-7,8-dicarba-nido-undecaborate (5) anions exclusively yielded the monosubstitution products 5-iodo-6-(triphenylphosphonio)-7,8-dicarba-nido-undecaborane (4) and 9-iodo-11-(triphenylphosphonio)-7,8-dicarba-nido-undecaborane (6), respectively. The reaction of tetrabutylammonium 6,9-diiodo-7,8-dicarba-nido-undecaborate (7) exclusively produced the phosphine substitution product in the open face of the nido-carborane, 6-iodo-9-triphenylphosphonio-7,8-dicarba-nido-undecaborane (8). The addition of a base (Cs2CO3, NaH) to the reactions of 3 and 5 with PPh3 afforded the corresponding bis­(triphenylphosphonio)-7,8-dicarba-nido-undecaboranes, 9 and 10. Compound 10 was also prepared from 6 using the general procedure. The reaction of the triiodocarborane tetrabutylammonium 5,6,9-triiodo-7,8-dicarba-nido-undecaborate (11) with excess PPh3 in the presence of Cs2CO3 and Pd­(PPh3)4 only produced neutral 5-iodo-6,9-bis­(triphenylphosphonio)-7,8-dicarba-nido-undecaborane (12); 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 2c, 6, 8, 9, and 12 were established by the X-ray diffraction analysis of single crystals.本研究通过碘代7,8-二碳巢式十一硼酸四丁基铵（1a–d）与三苯基膦(PPh3)在四(三苯基膦)合钯(Pd(PPh3)4)催化剂作用下的交叉偶联反应，制备了一系列单取代n-(三苯基鏻鎓(triphenylphosphonio))-7,8-二碳巢式十一硼烷（2a，n=1；2b，n=3；2c，n=5；2d，n=9）。研究发现，取代反应速率取决于碘原子在碳硼烷笼中的位置。在相似条件下，5,6-二碘代-7,8-二碳巢式十一硼酸盐阴离子（3）与9,11-二碘代-7,8-二碳巢式十一硼酸盐阴离子（5）分别发生反应，专一得到单取代产物5-碘代-6-(三苯基鏻鎓)-7,8-二碳巢式十一硼烷（4）与9-碘代-11-(三苯基鏻鎓)-7,8-二碳巢式十一硼烷（6）。6,9-二碘代-7,8-二碳巢式十一硼酸四丁基铵（7）的反应则专一生成位于巢式碳硼烷开口面的膦鎓取代产物6-碘代-9-三苯基鏻鎓-7,8-二碳巢式十一硼烷（8）。向3、5与三苯基膦的反应体系中加入碱（碳酸铯Cs2CO3、氢化钠NaH），可得到对应的双(三苯基鏻鎓)-7,8-二碳巢式十一硼烷9和10。化合物10亦可通过通用合成方法由6制备得到。三碘代碳硼烷底物5,6,9-三碘代-7,8-二碳巢式十一硼酸四丁基铵（11）与过量三苯基膦在碳酸铯和四(三苯基膦)合钯存在下反应，仅得到中性产物5-碘代-6,9-双(三苯基鏻鎓)-7,8-二碳巢式十一硼烷（12），未生成带正电荷的三(鏻鎓)类物种。所有合成产物的组成均通过多核核磁共振波谱法与高分辨质谱法进行表征，化合物2c、6、8、9及12的分子结构则通过单晶X射线衍射分析得以确证。