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 [nBu4N][Cl@Si20(SiCl3)12Cl8] ([nBu4N][1]), which has its cluster surface decorated with 12 SiCl3 and 8 Cl substituents in perfect Th symmetry. The room-temperature reaction of [nBu4N][1] with excess iBu2AlH in ortho-difluorobenzene (oDFB) furnishes perhydrogenated [nBu4N][Cl@Si20(SiH3)12H8] ([nBu4N][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 oDFB to an oDFB/Et2O solvent mixture suppresses core hydrogenation
and results in the formation of [nBu4N][Cl@Si20(SiH3)12Cl8] ([nBu4N][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 SiCl3 to SiH3 groups by treating [nBu4N][1] with Me2AlH/Me3Al
in oDFB ([nBu4N][Cl@Si20(SiH3)12Me8], [nBu4N][4]; 73%). Quantum-chemical free-energy
calculations find an SN2-Si-type hydrogenation of the exohedral
SiCl3 moieties in [1]− (trigonal-bipyramidal
intermediate) slightly preferred over metathesis-like SNi-Si substitutions (four-membered transition state). Cage hydrogenation
likely occurs via SNi-Si processes. The experimentally
demonstrated influence of an Et2O co-solvent, which drastically
increases the respective reaction barriers, is attributed to the increased
stability of the resulting iBu2AlH-OEt2 adduct and its higher steric bulk compared to free iBu2AlH.