A quest for stable 2,2,9,9-tetrahaloplumbacyclonona-3,5,7-trienylidenes at density functional theory

In contrast to cyclonona-3,5,7-trienylidene (1_H) which turns out as a boat-shaped transition state for having a negative force constant, its heavier plumbylenic analogous (2_X) where X = H, F, Cl, Br, and I emerge as boat-shaped minima. This unsubstituted carbene has a triplet ground state while exclusive of 2_I which initially takes on a triplet multiplicity and eventually transforms to a less stable intramolecular ring opening product; all of the plumbylenes (2_H, 2_F, 2_Cl, and 2_Br) have a singlet ground state. Hence, stability anticipated by the singlet (S)–triplet (T) splitting (ΔE_S-T) decreases by going down in the group 17 column: 2_Br > 2_Cl > 2_F > 2_H > 1_H > 2_I. Also, the HOMO-LUMO gap (ΔE_HOMO-LUMO) increases as a result of substituting. From a thermodynamic perspective, our scrutinized 2_Br, 2_Cl, and 2_F are found 1.5–2 times more stable than that of the reported cyclopenta-2,4-dienplumbylene and 2,5-bis(halobora)-cyclopentenplumbylenes analogues, respectively. From a kinetic perspective, these nine-membered plumbylenes are found 20–26 kcal/mol more stable than that of their corresponding five-membered congeners. The NBO analysis on stable singlet 2_Br shows that there is a mesomeric interaction between bromine lone-pair electron and the Pb divalent atom of 2_Br with bonding (σ) and anti-bonding (σ^*) orbitals of carbon–bromine and carbon–lead. The main stabilizing effect appears to be π- and σ-bond hyperconjugation among the iodine heteroatom and divalent center of triplet 2_I. This research signifies the thermodynamic and kinetic stabilities of the biggest unsaturated cyclic plumbylenes hoping to prompt the experimental attention toward them.