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

Abstract

In contrast to cyclonona-3,5,7-trienylidene (1H) which turns out as a boat-shaped transition state for having a negative force constant, its heavier plumbylenic analogous (2X) where X = H, F, Cl, Br, and I emerge as boat-shaped minima. This unsubstituted carbene has a triplet ground state while exclusive of 2I which initially takes on a triplet multiplicity and eventually transforms to a less stable intramolecular ring opening product; all of the plumbylenes (2H, 2F, 2Cl, and 2Br) have a singlet ground state. Hence, stability anticipated by the singlet (S)–triplet (T) splitting (ΔES-T) decreases by going down in the group 17 column: 2Br > 2Cl > 2F > 2H > 1H > 2I. Also, the HOMO-LUMO gap (ΔEHOMO-LUMO) increases as a result of substituting. From a thermodynamic perspective, our scrutinized 2Br, 2Cl, and 2F 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 2Br shows that there is a mesomeric interaction between bromine lone-pair electron and the Pb divalent atom of 2Br 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 2I. This research signifies the thermodynamic and kinetic stabilities of the biggest unsaturated cyclic plumbylenes hoping to prompt the experimental attention toward them.