Polyhedral monocarbaborane chemistry. Some C-phenylated seven, eight, nine, ten, eleven and twelve-vertex species

Some synthetic and structural systematics for monocarbaboranes, using the C-phenylated motif as the example, are investigated. The 10-vertex [6-Ph-nido-6-CB9H11]− anion 1, from reaction of PhCHO with B10H14 in KOH/H2O, is a useful entry synthon into C-phenyl monocarbaborane chemistry. Treatment of anion 1 with Na/thf yields the 10-vertex [1-Ph-closo-1-CB9H9]− anion 2a, whereas treatment of anion 1 with iodine in alkaline solution yields the isomeric 10-vertex [2-Ph-closo-2-CB9H9]− anion 2b, which isomerises quantitatively to 2a on heating under reflux in DME. Thermolysis of anion 1 yields the 9-vertex [4-Ph-closo-4-CB8H8]− anion 5, whereas treatment of anion 1 with FeCl3/HCl gives neutral 9-vertex [4-Ph-arachno-4-CB8H13] 3. Compound 3 gives neutral 9-vertex [1-Ph-nido-1-CB8H11] 4 in refluxing toluene, and gives the 7-vertex [2-Ph-closo-2-CB6H6]− anion 7 and the 8-vertex [1-Ph-closo-1-CB7H7]− anion 6 in refluxing toluene with NEt3. Reaction of 1 with [BH3(thf)] yields the 11-vertex [7-Ph-nido-7-CB10H12]− anion 8 which can be converted to the 12-vertex [1-Ph-closo-1-CB11H11]− anion 10 using [BH3(SMe2)]; alternatively, anion 1 yields anion 10 directly on treatment with [BH3(NEt3)]. Treatment of anion 8 with I2/KOH yields the 11-vertex [2-Ph-closo-2-CB10H10]− anion 9. The structures of anions 1, 2a, 2b, 5, 6, 7, 8, 9 and 10 have been established by single-crystal X-ray diffraction analyses of their [NEt4]+ salts, and those of neutral 3 and 4 estimated by DFT calculations at the B3LYP/6-31G* level; similar calculations have also been applied to the new anionic closo species 2a, 2b, 5, 6, 7, 9 and 10. Crystals of the [NEt4]+ salt of the [2-Ph-closo-2-CB6H6]− anion 7 required synchrotron X-radiation for sufficient diffraction intensity for molecular-structure elucidation. The syntheses are in principle generally applicable to give extensive derivative C-aryl and C-alkyl chemistries.