Regioselective Derivatization of Silylated [20]Silafulleranes.

Silafulleranes with endohedral Cl - ions are a unique, scarcely explored class of structurally well-defined silicon clusters and host-guest complexes. Herein, we report regioselective derivatization reactions on the siladodecahedrane [ n Bu 4 N][Cl@Si 20 (SiCl 3 ) 12 Cl 8 ] ([ n Bu 4 N][ 1 ]), which has its cluster surface decorated with 12 SiCl 3 and 8 Cl substituents in perfect T h symmetry. The room-temperature reaction of [ n Bu 4 N][ 1 ] with excess i Bu 2 AlH in ortho -difluorobenzene ( o DFB) furnishes perhydrogenated [ n Bu 4 N][Cl@Si 20 (SiH 3 ) 12 H 8 ] ([ n Bu 4 N][ 2 ]) in 50% yield; the non-pyrophoric [ 2 ] - is the largest structurally authenticated (by X-ray diffraction) hydridosilane known to date. A simple switch from pure o DFB to an o DFB/Et 2 O solvent mixture suppresses core hydrogenation and results in the formation of [ n Bu 4 N][Cl@Si 20 (SiH 3 ) 12 Cl 8 ] ([ n Bu 4 N][ 3 ]). In addition to the exhaustive Cl/H exchange at all 44 Si-Cl bonds of [ 1 ] - and the regioselective 36-fold silyl group hydrogenation, we achieved the simultaneous introduction of Me substituents at all 8 SiCl vertices along with the conversion of all 12 SiCl 3 to SiH 3 groups by treating [ n Bu 4 N][ 1 ] with Me 2 AlH/Me 3 Al in o DFB ([ n Bu 4 N][Cl@Si 20 (SiH 3 ) 12 Me 8 ], [ n Bu 4 N][ 4 ]; 73%). Quantum-chemical free-energy calculations find an S N 2-Si-type hydrogenation of the exohedral SiCl 3 moieties in [ 1 ] - (trigonal-bipyramidal intermediate) slightly preferred over metathesis-like S N i-Si substitutions (four-membered transition state). Cage hydrogenation likely occurs via S N i-Si processes. The experimentally demonstrated influence of an Et 2 O co-solvent, which drastically increases the respective reaction barriers, is attributed to the increased stability of the resulting i Bu 2 AlH-OEt 2 adduct and its higher steric bulk compared to free i Bu 2 AlH.