Regioselective Derivatization of Silylated [20]Silafulleranes

Abstract

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 [nBu 4 N][Cl@ Si 20 (SiCl 3 ) 12 Cl 8 ] ([nBu 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 [nBu 4 N][1] with excess iBu 2 AlH in ortho-difluorobenzene (oDFB) furnishes perhydrogenated [nBu 4 N][Cl@Si 20 (SiH 3 ) 12 H 8 ] ([nBu 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 oDFB to an oDFB/Et 2 O solvent mixture suppresses core hydrogenation and results in the formation of [nBu 4 N][Cl@Si 20 (SiH 3 ) 12 Cl 8 ] ([nBu 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 [nBu 4 N][1] with Me 2 AlH/Me 3 Al in oDFB ([nBu 4 N][Cl@ Si 20 (SiH 3 ) 12 Me 8 ], [nBu 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 iBu 2 AlH-OEt 2 adduct and its higher steric bulk compared to free iBu 2 AlH.

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

“…The siladodecahedranes [ n Bu 4 N][Cl@Si 20 (SiR 3 ) 12 R 8 ] (“silafulleranes”; Fig. 1) 1–8 are among the largest structurally authenticated oligosilanes and represent a new class of saturated silicon cages (along with, for example, silatetrahedranes, 9,10 silacubanes, 11,12 and silaadamantanes 13,14 ). Together with the unsaturated siliconoids 15–18 and the completely ligand-free Zintl ions, 19–21 these form the trinity of molecular silicon clusters.…”

“…22 Each of the [Cl@Si 20 (SiR 3 ) 12 R 8 ] − entities consists of an Si 20 core that encapsulates a Cl − ion and is decorated with 12 SiR 3 groups and 8 R units in perfect T h symmetry. 1,3 For future development, it would be desirable if the silafulleranes did not carry 44 of the same functional groups R, but at least two different types of substituents R and R′ with largely orthogonal reactivity so that R and R′ can be addressed individually. Considering that both the perhydrogenated silafullerane [ n Bu 4 N][Cl@Si 20 (SiH 3 ) 12 H 8 ] ([ n Bu 4 N][ 1 ]) 3 and its perchlorinated analog [ n Bu 4 N][Cl@Si 20 (SiCl 3 ) 12 Cl 8 ] ([ n Bu 4 N][ A ]) 1 exist and that SiH moieties provide highly diagnostic NMR handles, the most obvious choice would be R/R′ = H/Cl.…”

“…1,3 For future development, it would be desirable if the silafulleranes did not carry 44 of the same functional groups R, but at least two different types of substituents R and R′ with largely orthogonal reactivity so that R and R′ can be addressed individually. Considering that both the perhydrogenated silafullerane [ n Bu 4 N][Cl@Si 20 (SiH 3 ) 12 H 8 ] ([ n Bu 4 N][ 1 ]) 3 and its perchlorinated analog [ n Bu 4 N][Cl@Si 20 (SiCl 3 ) 12 Cl 8 ] ([ n Bu 4 N][ A ]) 1 exist and that SiH moieties provide highly diagnostic NMR handles, the most obvious choice would be R/R′ = H/Cl. Indeed we have recently succeeded in the synthesis of the mixed H/Cl-silafullerane [ n Bu 4 N][Cl@Si 20 (SiH 3 ) 12 Cl 8 ] ([ n Bu 4 N][ B ]) by partial Cl/H exchange on [ n Bu 4 N][ A ].…”

“…This approach was based on the higher reactivity of the sterically exposed silyl groups compared to the more protected core positions toward the H − donor iBu 2 AlH. 3 However, a reverse synthesis strategy for partially halogenated silafulleranes, starting from [ n Bu 4 N][ 1 ], also appears promising, since an increase in steric hindrance at the cluster surface as halogenation proceeds could keep some of the H substituents in place. Earlier quantum-chemical calculations have indicated that the perchlorinated framework of [ A ] − already probes the limits of steric bulk.…”

“…In the case of [ 4 ] − , one intense signal at 3.73 ppm ( 1 J (H,Si) = 203.4 Hz) arises from the chemically equivalent SiH 3 groups ( cf. [ 1 ] − : δ ( 1 H) = 3.47, 1 J (H,Si) = 191.8 Hz 3 ). The fact that the Si H 3 nuclei of [ 4 ] − are significantly more shielded than the Si H Br 2 nuclei of [ 2 ] − is consistent with the commonly observed deshielding effect of halogen substituents on geminally positioned protons in mixed hydrido(halogeno)silanes.…”