A quest for stable 2,5-bis(halobora)cyclopentenylidene and its Si, Ge, Sn and Pb analogs at theoretical levels

Replacing the two nitrogen atoms of Arduengo's N-heterocyclic carbenes (NHCs) with electron deficient boron atoms forms B-heterocyclic carbenes (BHCs) which may appear destabilizing at first glance. Yet, among the 40 optimized singlet (s) and triplet (t) BHCs and their Si, Ge, Sn and Pb homologues (BHEs), eight species are found that show higher stability than their corresponding NHEs for exhibiting wider singlet-triplet energy gaps (Delta E-st), at B3LYP/TZ2P, as well as CBS-QB3 and G4MP2 ab initio levels. Moreover, triplet BHEs assume a planar geometry with a dihedral angle (D1) of about zero degrees. In contrast, their corresponding singlets show a high tendency for puckering with D1 congruent to 66 degrees. The preference of the latter for puckered nonplanar geometries is evidenced by NBO calculations and visually through their frontier molecular orbitals. The main stabilizing interactions appear to be pi- and sigma-bond hyperconjugation across the ring. The resulting eight species that demonstrate higher stability are: 2,5-bis(iodobora) cyclopentensilylene, 2,5-bis(Z-bora) cyclopenten-germylene and -stannylene, for Z = Br and I; as well as 2,5-bis(Z(2)-bora) cyclopentenplumbylene, for Z(2) = Cl, Br and I.