A distorted trigonal antiprismatic cationic silicon complex with ureato ligands: syntheses, crystal structures and solid state²⁹Si NMR properties

Insertion of phenyl isocyanate into the Si-N bond of N-(tri methylsilyl)diethylamine yields the N'-silylated N,N-diethylN'-phenylurea 1, which undergoes transsilylation with SiCl₄ to yield the C ₃-symmetric cationic hexacoordinate silicon complex 3s⁺ [tris-Κ-O,N'-(N,N-diethyl-N'-phenylureato)siliconium] as chloride salt, which was characterized crystallographically. The cationic complex. 3s ⁺ exhibits a distorted trigonal antiprismatic coordination sphere about the silicon atom with fac arrangement of the three N-atoms (and the three O-atoms) relative to one another. This C₃-symmetric complex undergoes isomerization into its asymmetric isomer 3a⁺ (mer arrangement of NNN or OOO relative to one another) in. CDCl₃ solution. Hence, two ²⁹Si NMR signals appear and four sets of signals emerge in the ¹H and ¹³C NMR spectra. Despite its pronounced axial symmetry, the ²⁹Si NMR shielding tensor of the cation 3s⁺ in its chloride salt exhibits an unusually small span (less than 20 ppm), which was analyzed CP/MAS NMR spectroscopically and by computational methods.