A New Approach to the Design of Neutral 10-C-5 Trigonal-Bipyramidal Carbon Compounds: A "π-Electron Cap" Effect

• Published in: Chemistry : a European journal 2013-07, Vol.19 (31), p.10302-10311
• Main Authors: Sidorkin, Valery F., Doronina, Evgeniya P., Belogolova, Elena F.
• Format: Article
• Language: English
• Subjects: ab initio calculations; cage compounds; Carbon; Chemical bonds; Compressing; Covalence; density functional calculations; Fragmentation; hypervalent carbon compounds; Optimization; Silicon compounds; Tetradecane
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201300461

DFT (B3LYP, M06‐2X) and MP2 methods are applied to the design of a wide series of the potentially 10‐C‐5 neutral compounds based on 6‐azabicyclotetradecanes: XC1(YCH2CH2CH2)3N 1–3, XC1(YC6H4CH2)3N 4–6, XC1[Y(tBuC6H3)CH2]3N 7–9 and carbatranophanes 10–25 (X=Me, F, Cl; Y=O, NH, CH2, SiH2; Z=O, CH2, (CH2)2, (CH2)3). Carbatranophanes 10–25 are characterized by a sterical compression of their axial 3c–4e XC1←N fragment with respect to that in the parent molecules 4–6. A magnitude of the revealed effect depends on a valence surrounding of the central carbon atom C1, the size and the nature of the side chains (Z) that link the “π‐electron cap” with a tetradecane backbone. This circumstance allowed us to obtain 10‐C‐5 structures with the configuration of the bonds around the C1 atom, which corresponds to practically an ideal trigonal bipyramid. In these compounds, the values of the covalence ratio χ C 1←N of approximately 0.6 for the coordination C1←N contacts with a covalent contribution (atoms in molecules (AIM) and natural bond orbital (NBO)) are record in magnitude. These values lie close to a low limit of the interval of the χSi←D change (0.6–0.9) being characteristic of the dative and ionic‐covalent (by nature) Si←D bond (D=N, O) in the known 10‐Si‐5 silicon compounds. Neutral 10‐C‐5 compounds are feasible! By using a revealed effect of steric compression of the 3c–4e axial XC1←N fragment of the carbatranophanes (see figure), an increase in the coordination number of the carbon atom C1 of five was achieved (i.e., the optimization of the box size for C1 in the framework of the “ball‐in‐a‐box” model).