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On The Lower Lewis Basicity of Siloxanes Compared to Ethers



1,3-Dimethyldisiloxane, O(SiH2Me)2, diethyl ether, OEt2, and their metal complexes (Li+,Ag+) have been used as model systems to uncover the reasons underlying the lower Lewis basicity of siloxanes compared to analogous ethers. Bonding in the metal complexes has been analyzed with quantum theory of atoms in molecules as well as natural bond orbital theory. The binding to Li+ by O(SiH2Me)2 and OEt2 is largely of monopole – dipole type whereas binding of Ag+ also shows small charge transfer components. The more ionic Si–O bonds in O(SiH2Me)2 compared to C–O bonds in OEt2 lead to more negative charge on the oxygen atom of O(SiH2Me)2 but the electrostatic attraction between metal cations and OEt2 is calculated to be slightly stronger by interacting quantum atoms energy decomposition analysis. The lower electrostatic attraction by O(SiH2Me)2 is due to repulsion between positively charged silicon atoms and the metal cations that compensate for the stronger attraction from the oxygen atom relative to OEt2. The total interaction of metal cations with O(SiH2Me)2 was calculated to be almost as strong as that with OEt2 and the lower stability of O(SiH2Me)2 complexes and the lower Lewis basicity of O(SiH2Me)2 is attributed to changes in the bonding and polarization of the ligands in the presence of metal cations. The polarization of O–Si bonds in O(SiH2Me)2 raises the energy of the molecule more than does the polarization O–C bonds in OEt2. Despite the lower stability of O(SiH2Me)2 metal complexes, solid state reaction enthalpies arising from a volume-based thermodynamics approach suggest that they could be stabilized with a sufficiently large anion such as [Al(ORF)4].