Conjugation, Resonance, and Stability in N-Heterocyclic Silylenes and in Phosphorus Ylide Substituted Silylenes

Abstract

The role of π conjugation in the thermodynamic stabilization of N-heterocyclic silylenes and phosphorus ylide substituted silylenes is analyzed using the block localized wave function (BLW) method. This method enables the direct calculation of the resonance stabilization energies caused by π-electron delocalization and/or by cyclic 6-π-electron delocalization (aromaticity). The major advantage of the BLW method is that there is no need for reference compounds, as the fully conjugated molecule itself serves as the reference compound. The observed high stability of the C═C unsaturated N-heterocyclic silylenes 1(E = Si; silaimidazol-2-ylidene) and the C–C saturated 2 (E = Si; silaimidazolin-2-ylidene) is rationalized by their high resonance energies (REs) of 79.4 and 53.4 kcal/mol, respectively. The nuclear independent chemical shift (NICSzz(1.0)) value of −22.0 for 1 (E = Si) indicates significant diatropic ring current, implying the existence of aromaticity. The additional stabilization of 1 due to aromaticity (6-π-electron delocalization) is 14 kcal/mol, which is only 18% of the total RE of 1 but is 55% of the aromatic stabilization of benzene. Thus, aromaticity contributes to the stability of 1, but it is not a prerequisite for the isolation of N-heterocyclic silylenes. Cyclic unsaturated silylenes substituted by exocyclic phosphorus ylide substituents, i.e., 12, have calculated cyclic resonance energies and thermodynamic stabilization energies similar to those of 1 (E = Si), and they are significantly larger than those of saturated 2 (E = Si); i.e., the REs of 12 (R″ = CH3, SiH3) are 73.4 and 75.6 kcal/mol, respectively. These data suggest that 12 is sufficiently stable to be isolated, as indeed was recently reported. The energy of the lone pair orbital on Si in 12 (R″ = Ph) is higher by 1.9 eV than that of 1 (E = Si, R = t-Bu), suggesting that 12 are better σ donors and thus may exhibit higher activity in transition-metal catalysis.