A density functional theory perspective on 2,2,9,9‐tetrahalostannacyclonona‐3,5,7‐trienylidenes

Abstract

AbstractCompared with cyclonona‐3,5,7‐trienylidene (1H), which turns out as a boat‐shaped singlet transition state for having a negative force constant, its heavier stanylenic analogues (2X), where X = H, F, Cl, Br, and I, emerge as boat‐shaped minima. This unsubstituted carbene has triplet ground state while exclusive of 2I, which initially take on a triplet multiplicity and eventually transforms to a less stable intramolecular ring opening product; other stanylenes (2H, 2F, 2Cl, and 2Br) have singlet ground state. Hence, stability anticipated by the singlet‐triplet splitting (ΔES‐T) decreases by going down in group 17 column: 2Br > 2Cl > 2F > 2H > 2I > 1H. Also, the HOMO‐LUMO energy separation (ΔEHOMO‐LUMO) increases as a result of substituting. The main stabilizing effect appears to be π– and σ–bond hyperconjugation among the iodine hetero atom and divalent center of triplet 2I. From a thermodynamic perspective, our scrutinized 2Br, 2Cl, and 2F are found 1.5 to 2 times more stable than that of the reported cyclopenta‐2,4‐dienstanylene and 2,5‐bis (halobora)‐cyclopentenstanylene analogues. From a kinetic perspective, these nine‐membered stanylenes are found 10 to 18 kcal/mol more stable than that of their corresponding five‐membered congeners. The NBO analysis on stable singlet 2Br‐S shows that there is a three‐type interaction including the first interaction between its bromine's lone pairs and tin's 5pπ orbital (mesomeric effect), second interaction between its bonding orbital (σCα‐Br) and tin's 5pπ orbital (positive hyperconjugation), and third interaction between its σ2Sn occupied orbital and antibonding orbitals of σ*Cα‐Br (negative hyperconjugation).