Calix[4]pyrrole Hydridosilicate: The Elusive Planar Tetracoordinate Silicon Imparts Striking Stability to Its Anionic Silicon Hydride

Abstract

Anionic hydridosilicates are highly reactive and strong hydride donors. In contrast, calix[4]pyrrole hydridosilicate is an entirely water-stable, anionic silicon hydride, which does not show hydridic reactivity. However, it still acts as an electron donor and enables the detection of a single electron transfer process in the reduction chemistry with hydridosilicates. Most important, these unusual properties are imparted by the unique planar structure of its elusive parent neutral silanesubstantiating the effect of planar tetracoordinate silicon for the first time.

Flat crown ethers with planar tetracoordinate carbon atoms

“…Stabilization of ptC by hydrogenation, in the case of CAl 4 H −/0 , has also been reported recently. [ 22 ] The effect of “pt” silicon (ptSi) [ 23–27 ] atom in the stability of calix[4]pyrrole hydrdosilicate [ 28 ] serves as the first experimental proof for ptSi, which further motivates the study of these molecules. Experimentally, it was also demonstrated in the past that molecules with a ptC atom can be enormously stabilized by the cooperative influence of metal pairs (Zr/Al or Zr/Zr + ) of atoms.…”

Transition between [R]- and [S]-stereoisomers without bond breaking

“…In summary, the energetic and dynamical considerations of this novel chiral system clearly point to stereoinversion upon rotation about the asymmetric atom and hence provides a fundamental model that exhibit such phenomenon. Experimental determination of molecules containing planar tetracoordinate Si 68 and P + 26,69 have been reported before and hence we hope that it is possible to realize these and/or related molecular systems.…”

A Six‐Coordinate Silicon Dihydride Embedded in a Porphyrin: Enhanced Hydride‐Donor Properties and the Catalyst‐Free Hydrosilylation of CO2

“…In addition, this value of 1 opt stands in sharp contrast to that of the structurally related calix[4]pyrrole hydridosilicate E [Eq. ], which exhibits very weak hydride‐donor properties (517 kJ mol −1 ) and, as a five‐coordinate silicon hydride, outstanding stability toward water [21] …”

“…(2)], which exhibits very weak hydride-donor properties (517 kJ mol À1 )a nd,a safive-coordinate silicon hydride, outstandings tability towardw ater. [21] To obtain furtheri nsight into the reactivity of 1 towardC O 2 , theoretical studies using model compounds without phenyl groups were carried out at the B3PW91-D3/6-31 + G(d) (PCM: THF) level of theory. [14] The resultss uggest as tepwise mechanism that includes ad irect hydride transfer from 1m to CO 2 as the initial and pivotal step, followed by the formation of an SiÀObond (Figure 4).…”