Stable silylenes with acyclic, cyclic, and unsaturated cyclic structures: Effects of heteroatoms and cyclopropyl α-substituents at DFT

The effects of alkyl, amino, phosphino, oxy, and thio moieties on the stability, multiplicity and reactivity of α-cyclopropylsilylenes with acyclic, cyclic, and unsaturated cyclic structures are compared and contrasted at B3LYP/AUG-cc-pVTZ//B3LYP/6-31G level. The simultaneous stabilizing effects of heteroatom and cyclopropyl α-substituents on the above silylenes create the following trend of singlet-triplet energy gaps (E_S-T): amino ≈ oxy > thio > alkyl > phosphino. The ΔE_S-T values for all the above silylenes increase through cyclization, because cyclization stabilizes singlet states of all our α-heteroatom substitued silylenes while destabilizes their corresponding triplet states. The ΔE _S-T values for all the heteroatom substituted cyclic silylenes decrease upon unsaturation, due to the higher stabilization of triplet compared to the singlet state. Interestingly, isodesmic reactions showed the phosphino substituents stabilize the triplet more than the corresponding singlet states, not only in the unsaturated form but also in the acyclic and cyclic structures. This is in contrast to cyclopropyl which stabilizes singlet considerably more than the corresponding triplet states. The reactivity of the above silylenes is discussed in terms of nucleophilicity, and electrophilicity; showing phosphino- and aminocyclopropylsilylenes more nucleophilic than oxy- and thiocyclopropylsilylenes.