Carbon rings decorated with group 14 elements: new aromatic clusters containing planar tetracoordinate carbon

Abstract

A simple and chemically intuitive approach is used to design ptC-containing E–C clusters (E = Si–Pb). This approach consists in replacing three or two consecutive protons from an aromatic hydrocarbon by one E₂⁴⁺ or one E²⁺ fragment, respectively. In the model, electrons from E are removed from the p_z orbitals, emptying them. Si–Pb favors the formation of a 3c-2e (E–C–E) σ-bond, which involves the ptC. Additionally, the π-electronic cloud is delocalized through the E-p_z orbitals allowing the E atoms to effectively take part in the electronic delocalization, preserving the 4n + 2 Hückel's rule from the parent hydrocarbon. Two aromatic monocycles and one aromatic bicycle – benzene (C₆H₆), cyclopentadienyl anion (C₅H₅⁻) and pentalene dianion (C₈H₆²⁻) – have been transformed into C–E systems. After an extensive exploration of their potential energy surfaces, four new global minima with ptC are identified, resulting from the substitution of the protons by Si and Ge cations in C₅H₅⁻ and C₈H₆²⁻ (E₃C₅ and E₄C₈). The analysis of both the chemical bonding and the magnetic response to an external magnetic field confirms the aromatic character of these species.