A Distorted Trigonal Antiprismatic Cationic Silicon Complex with Ureato Ligands: Syntheses, Crystal Structures and Solid State 29Si NMR Properties

Abstract

AbstractInsertion of phenyl isocyanate into the Si–N bond of N‐(trimethylsilyl)diethylamine yields the N′‐silylated N,N‐diethyl‐N′‐phenylurea 1, which undergoes transsilylation with SiCl4 to yield the C3‐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.