Aromaticity in group 14 metalloles: Structural, energetic, and magnetic criteria

Various structural (C-C bond length equalization, D), energetic (isodesmic stabilization energies, ISE), and magnetic (diamagnetic susceptibility exaltations, Lambda and nucleus-independent chemical shifts, NICS) criteria are employed (using B3LYP, CSGT, and GIAO ab initio methods) to assess the aromaticity and antiaromaticity of a variety of group 14 (E = C, Si, Ge, Sn, Pb) metalloles: C4H4EH2 (C-2v), C4H4EH- (C-s and C-2v; C, D-5h), C4H4EH+ (singlet, C-2v), C4H4EHLi (C-s; C, C-5v), and C4H4ELi2(C-2v). In addition, structural trends are established for C4H4ELi- (C-s) and for C4H4E2- (C-2v) as well as for the singlet and triplet C4H4E (C-2v) sets. The increased pyramidality at E down group 14 results in strongly decreased aromaticity of metallolyl anions C4H4EH- (C-s). In contrast, all planar C4H4EH-(C-2v) geometries are significantly more aromatic. Although all C4H4EH+ (C-2v) structures are planar, the antiaromaticity in singlet C5H5+ is much higher than that of the heavier congeners (E = Si to Pb). The four-pi-electron singlets C4H4E exhibit nearly as localized geometries as the C4H4EH+ ions, but the C4H4E triplets are more delocalized. As in the free anions, pyramidally coordinated E's lead in C4H4EHLi (C,) to reduced aromaticity, but stabilizing Li-H interactions are apparent in these structures. The metallole dianions and their Li+ complexes (e.g. C4H4ELi2, C-2v) are the most aromatic among the species studied. The aromaticity in these dianionic metalloles is remarkably constant in going from E = C to E = Pb.