Twofold Deprotonated Citric Acid as a Bidentate Ligand of Pentacoordinate Silicon: Synthesis and Structural Characterization of the Zwitterionic λ 5 Si ‐Spirosilicate Bis[citrato(2–)‐ O 3 , O 4 ][(dimethylammonio)methyl]silicate Hydrate

The zwitterionic λ 5 Si ‐spirosilicate bis[citrato(2–)‐ O 3 , O 4 ][(dimethylammonio)methyl]silicate ( 4 ) was synthesized by reaction of (MeO) 3 SiCH 2 NMe 2 ( 3 ) with citric acid (molar ratio 1:2) in acetonitrile at room temperature and isolated, after crystallization from water, as the hydrate 4 . H 2 O (yield 81%). The crystal structure of 4 . H 2 O was studied by single‐crystal X‐ray diffraction. The alcoxide oxygen atoms and central carboxylate oxygen atoms of two citrato(2–) ligands and one carbon atom coordinate to the silicon atom of 4 . H 2 O. The coordination polyhedron around the pentacoordinate silicon atom ( Si O 4 C framework) can be described as a distorted trigonal bipyramid, the two carboxylate oxygen atoms occupying the axial sites. The λ 5 Si ‐silicon(IV) complex 4 also exists in solution (DMSO, H 2 O).