First Principles Studies toward the Design of Silylene Superbases: A Density Functional Theory Study

In this paper we have reported for the first time some designed silylene superbases using DFT calculations. These divalent Si(II) compounds can act as powerful neutral organic superbases in the gas phase and in the solvent phase. The DFT calculations performed with the B3LYP/6-311+G**//B3LYP/6-31+G* level of theory showed that one of the designed silylene derivatives :Si(N=PY3)(2) [Y = -N=C(NMe2)(2)] (8) can fall in the range of hyperbase with gas phase proton affinity similar to 310 kcal/mol. In THF the calculated proton affinity of 8 was found to be 327.5 kcal/mol. The proton affinities computed at the B3LYP/6-311+G**//B3LYP/6-31+G* level for some simple silylenes have been found to be good agreement with the corresponding experimentally measured values. Phosphazene groups attached to the divalent silicon center of silylenes enhanced the basicity of the Si center significantly and further acted as a second protonation site. The calculated second proton affinity of the silylene derivative, 8 in THF was found to be 285.5 kcal/mol. We have shown that the dimerization and cyclization of such silyene superbases were less likely and the monomeric forms would be more stable than their corresponding dimers. The calculated proton affinities also showed a good correlation with the HOMO-LUMO energy gap and energy difference between the singlet and triplet states (Delta ES-T) of the silylene systems. The isodesmic reactions have been employed to examine the stability of the silylene molecules by calculating the silylene stabilization energy (SiSE). The reactivity of silylene molecules has been presented in terms of the nucleophilicity, electronegativity, and hardness of such systems. The Lewis basic properties of these silylene systems have also been explored.