Synthesis and reactivity of 7,9-diphenyl-nido-carbaundecaboranes: Rearrangement processes in carbaplatinaboranes revisited

A number of synthetic routes towards diphenyl nido-carbaborane [7,9-Ph-2-7,9-nido-C2B9H10](-) have been elucidated. Base-induced degradation of 1,7-Ph-2-1,7-closo-C2B10H10 afforded two isolable products [NEt3H]-[7,9-Ph-2-x-OEt-7,9-nido-C2B9H9] (x = 3 1a or 2 1b), both of which contain an ethoxide-substituted cage. The structural identity of la was established by a single-crystal X-ray diffraction study. Fluoride-induced degradation gave a product assigned as [NBu4][7,9-Ph-2-10-OH-7,9-nido-C2B9H9] 2, also studied crystallographically. Heating a solid sample of Cs[7,8-Ph-2-7,8-nido-C2B9H10] in a sealed tube under vacuum for 4 h at 300 degrees C afforded the target species [NEt3H][7,9-Ph-2-7,9-nido-C2B9H10] 3, after cation metathesis. Deprotonation of 3 and reaction with [PtCl2(PMe2Ph)(2)] afforded the new compound 1,1-(PMe2Ph)(2)-2,4-Ph-2-1,2,4-closo-PtC2B9H9 4. Compound 4 has been characterised by multinuclear (H-1, B-11 and P-31-{H-1}) NMR spectroscopy as well as a single-crystal X-ray diffraction study. The isolation of this stable (in refluxing toluene) compound having a 1,2,4-MC2B9 cage architecture indicates that a hextuple-concerted diamond-square-diamond mechanism cannot be operating in the spontaneous isomerisation of 1,2-Ph-2-3,3-(PMe2Ph)(2)-3,1,2-closo-PtC2B9H9 below ambient temperature to afford 1,1-(PMe2Ph)(2)-2,8-Ph-2-1,2,8-closo-PtC2B9H9. The new compounds 1,1-(PR3)(2)-2,4-Ph-2-7-OEt-closo-1,2,4-PtC2B9H8 (R-3 = Me2Ph 5a or Et-3 5b), both fully characterised by multinuclear NMR spectroscopy; have also been synthesized.