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

Abstract

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.