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

Abstract

The catalyst-free hydrosilylation of CO 2 under mild conditions remains limited. Herein, we report the synthesis, characterization, and reactivity of 5,10,15,20-tetraphenylporphyrinato(dihydrido)silicon(IV)(1)a sas ix-coordinate silicon dihydride. The Si-H moiety of 1 reacts with polar double bonds and CO 2 in the absence of ac atalyst to afford hydrosilylated products. Combining the hydrosilylation with subsequentt ransformation furnishes formic acidf rom CO 2 .C omputational studies indicate that the hydride-donorp roperties of 1 are exceptionally high for an eutrals ilicon hydride, and that the directh ydride transfer from silicon to carbon is ap ivotal step in the hydrosilylation of CO 2 with 1.

Trans Element-Hydrogen Bonds: A Distinctive Difference Between Transition Metals and Main Group Elements

“…This would require a combined experimental and theoretical study that is beyond the scope of this study. , Both calculations provide very similar transition state geometries with modes of 314 i and 778 i cm –1 , respectively. These values are in line with previous DFT calculations for SiH 2 (porphyrin), which has an activation barrier of 7.7 kcal mol –1 …”

“…The hydride transfer from the octahedral porphyrin compounds in tetrahydrofuran (THF) were calculated with three different functionals corrected for the effects of dispersion. This solvent was chosen in order to compare results with the SiH 2 (porphyrin) work . The qst3 method of Gaussian was used to locate the transition states for hydride transfer to CO 2 .…”

“…Finally, the transition state geometry of a pair of porphyrin dihydride complexes for CO 2 hydrogenation are explored. While many examples of well-defined main group hydride complexes are shown to insert into unsaturated C–E bonds in studies by the groups of Vanka, Corriu, and Cantat, to the best of our knowledge there is only one example incorporating a trans -dihydride motif. − In a recent paper by Ishida and co-workers, the six-coordinate compound SiH 2 (TPP) (TPP = 5,10,15,20-tetraphenylporphyrin) was demonstrated to be active for the hydrogenation of CO 2 to formate in THF by nucleophilic attack of the hydride . A transition state for hydride delivery was calculated by DFT, but the motion of the trans -hydride was not examined in detail.…”

“…This solvent was chosen in order to compare results with the SiH 2 (porphyrin) work. 36 The qst3 method of Gaussian was used to locate the transition states for hydride transfer to CO 2 .…”

“…This motion resembles the infrared active antisymmetric stretching mode, which itself is coupled strongly to the Raman active symmetric stretching mode. We wondered what would be the nature of the interaction between modes of main group elements, especially in a transition state such as the attack on CO 2 by a main group trans -dihydride as reported by Ishida et al While there are vibrational data for octahedral silicon dihydrides, − the lack of combined infrared and Raman studies left these questions unresolved.…”