P-Heterocyclic silylenes: A survey of stability with density functional theory

摘要

The effects of phosphorous atom on the stability, multiplicity, and reactivity of six-member cyclic silylenes are investigated at B3LYP/AUG-cc-pVTZ//B3LYP/6-31+G* and MP2/6-311++G* *//B3LYP/6-31+G* coupled with appropriate isodesmic reactions. From a thermodynamic point of view, 1H-2-silaphosphinine-2-ylidene (1_a) and 1H-4-silaphosphinine-4-ylidene (2_a) are relatively the most stable with singlet-triplet energy gaps (δE_(S-T)) of 37.0 and 28.1 kcal/mol, respectively. The calculated energy barrier for the 1,2-H shift of 1_a to the corresponding 2-silapyridine (1) is 26.5 kcal/mol, which is lower than the 28.8 kcal/mol required for the 1,4-H shift of 2_a to the corresponding 4-silapyridine (2). In contrast to the previous reports, isodesmic reactions indicate that π-donor/σ-donor phosphorous destabilizes the singlet while stabilizes the triplet state. Both 1_a and 2_a silylenes appear invulnerable to the head-to-head as well as the head-to-tail dimerization, inviting experimental explorations.