Electrochemical Oxidation and Radical Cations of Structurally Non-rigid Hypervalent Silatranes: Theoretical and Experimental Studies

Abstract

Using 18 silatranes XSi(OCH₂ CH₂ )₃ N (1) as examples, the potentials of electrochemical oxidation E⁰ of the hypervalent compounds of Si were calculated for the first time at the ab initio and DFT levels. The experimental peak potentials E_p (acetonitrile) show an excellent agreement (MAE=0.03) with the MP2//B3PW91 calculated E⁰ (C-PCM). Radical cations of 1 reveal a stretch isomerism of the N→Si dative bond. Localization of the spin density (SD) on the substituent X and the short (s) coordination contact Si⋅⋅⋅N (d_SiN <2.13 Å) along with the high five-coordinate character of Si are typical for the first isomer 1^+.(s) , whereas the second one, 1^+.(l) , has a longer (l) Si⋅⋅⋅N distance (d_SiN >3.0 Å), the four-coordinate Si and the SD localized on the silatrane nitrogen atom N_s . The vertical model of adiabatic ionization (1→1^+.(s) or 1→1^+.(l) ) was developed. It allows, in accordance with an original experimental test (electrooxidation of 1 in the presence of ferrocene), a reliable prediction of the most probable pathways of the silatrane oxidation. The reliable relationships of E⁰ (1) with the strength characteristics of the dative contact N→Si were revealed.

Keywords: Group 14 elements; cyclic voltammetry; density functional calculations; hypervalent compounds; silatranes.