P‐Heterocyclic silylenes: a survey of stability with density functional theory

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. Copyright © 2011 John Wiley & Sons, Ltd.