Brominated [20]silafulleranes: pushing the limits of steric loading

Starting from the perhydrogenated silafullerane [nBu(4)N][Cl@Si-20(SiH3)(12)H-8], treatment with BBr3 leads to partially and exhaustively brominated clusters, [nBu(4)N][Cl@Si-20(SiBr2H)(12)Br-8] (120 eq. BBr3, room temperature, 30 min) and [nBu(4)N][Cl@Si-20(SiBr3)(12)Br-8] (300 eq. BBr3, 130 degrees C, 3 d). Perbromination is accompanied by a massively increased steric strain on the cluster surface, which explains why our approach achieves regioselective derivatization of the Si-32 framework when mild conditions are maintained. Partial Br/H exchange on [nBu(4)N][Cl@Si-20(SiBr2H)(12)Br-8] (30 eq. iBu(2)AlH, room temperature, 16 h) affords [nBu(4)N][Cl@Si-20(SiH3)(12)Br-8].