Electronic nature of zwitterionic alkali metal methanides, silanides and germanides - a combined experimental and computational approach

This project was funded in part by the NSF (grant no. 1407681; Project SusChEM: IUPAC) as part of the IUPAC International Funding Call on “Novel Molecular and Supramolecular Theory and Synthesis Approaches for Sustainable Catalysis”. Support was also provided by the TTU Department of Chemistry & Biochemistry cluster Robinson whose purchase was funded by the NSF (CRIF-MU CHE-0840493).Zwitterionic group 14 complexes of the alkali metals of formula [C(SiMe2OCH2CH2OMe)3M], (M- 1 ), [Si(SiMe2OCH2CH2OMe)3M], (M- 2 ), [Ge(SiMe2OCH2CH2OMe)3M], (M- 3 ), where M = Li, Na or K, have been prepared, structurally characterized and their electronic nature was investigated by computational methods. Zwitterions M- 2 and M- 3 were synthesized via reactions of [Si(SiMe2OCH2CH2OMe)4] ( 2 ) and [Ge(SiMe2OCH2CH2OMe)4] ( 3 ) with MOBut (M = Li, Na or K), resp., in almost quantitative yields, while M- 1 were prepared from deprotonation of [HC(SiMe2OCH2CH2OMe)3] (1) with LiBut, NaCH2Ph and KCH2Ph, resp. X-ray crystallographic studies and DFT calculations in the gas-phase, including calculations of the NPA charges confirm the zwitterionic nature of these compounds, with the alkali metal cations being rigidly locked and charge separated from the anion by the internal OCH2CH2OMe donor groups. Natural bond orbital (NBO) analysis and the second order perturbation theory analysis of the NBOs reveal significant hyperconjugative interactions in M- 1 -M- 3 , primarily between the lone pair and the antibonding Si-O orbitals, the extent of which decreases in the order M- 1 > M- 2 > M- 3 . The experimental basicities and the calculated gas-phase basicities of M- 1 -M- 3 reveal the zwitterionic alkali metal methanides M- 1 to be significantly stronger bases than the analogous silanides M- 2 and germanium M- 3 .Publisher PDFPeer reviewe