Theoretical and experimental study on a spirocyclic diethyleneglycol silicon complex

In this paper a joint theoretical and experimental study examines the ability of diethyleneglycol as a ligand to form silicon complexes. Due to the known oxophilia of the silicon atom, it would be expected that in the reaction of this tridentate O,O,O-donor-ligand with a reagent such as SiCl4, the corresponding bis-chelate, hexacoordinated neutral silicon complex may be formed. However, a spirocyclic tetracoordinated silicon bis-chelate complex was isolated and no evidence of formation of any hypervalent alkoxysilane was observed. The tetracoordinated compound, 1,4,7,9,12,15-Hexaoxa-8-silaspiro[7.7]pentadecane (8CI,9CI), was crystallized in hexanes from an extract of the product of the aforementioned reaction in methylene chloride and its crystal structure has been determined by X-ray diffraction (C8H16O6Si; orthorhombic; α = 9.2892(3), b = 9.5845(3), c = 12.3748(4) A; space group P212121; Z = 4). From the same reaction, two other oligomeric tetracoordinated silicon compounds were detected spectroscopically. Furthermore, Density Functional calculations at the BP86/TZ2P level were performed for the bis-chelate compound. We obtained the condensed Fukui functions as well as theoretical NMR chemical shifts to rationalize why the diethyleneglycol acts only as a dianionic, bidentate ligand towards silicon. Our DFT results indicate that the tetracoordinated Si complex is a stable molecule (minimum energy point) whereas the hexacoordinated species is a first-order saddle point (transition state). These results are in agreement and rationalize the experimental findings.