Metal-centered monocyclic carbon wheel clusters with record coordination numbers in planar species

Abstract

The highest coordination number identified to date in planar species is CN = 10 in metal-centered monocyclic boron wheel clusters D_10h M©B₁₀⁻ (M = Ta and Nb) (Galeev et. al., Angew. Chem. Int. Ed., 2012, 51, 2101). Extensive global minimum searches and first-principles theory calculations performed herein indicate that the experimentally observed LaC₁₃⁺ and LaC₁₄⁺ possess the well-defined global minima of perfect metal-centered monocyclic carbon wheel D_13h La©C₁₃⁺ (1) and slightly off-centered C_2v La©C₁₄⁺ (4) (¹A₁) with record coordination numbers of CN = 13 and 11 in planar structures, respectively, further pushing the boundary of our understanding of chemical structures and bonding. Detailed molecular orbital, nucleus-independent chemical shift, and ring current analyses indicate that D_13h La©C₁₃⁺ (1) is σ + π dually aromatic in nature, with 14 totally delocalized in-plane σ electrons and 14 totally delocalized out-of-plane π electrons each matching the 4N + 2 aromatic rule (N_σ = N_π = 3). Similar σ + π dually aromatic metal-centered monocyclic wheel clusters D_13h Ca©C₁₃ (2), C_13v Ac©C₁₃⁺ (3), C_2v Y©B₆C₆⁺ (5), and C_2v Sc©B₅C₆ (6) have also been obtained with CN = 13, 13, 12, and 11, respectively. The results obtained in this work effectively enrich the chemical structures and bonding patterns of planar hypercoordinated complexes.