Surprising Insights in the Various Molecular Structures of Hypercoordinate Bis(oxinato)silicon Complexes

Abstract

The syntheses of two cyclic diorganosilicon enamines =CH2) [R = Ph (2a), Me (2b)] are described. These compounds react with 8-oxyquinoline leading to bis(oxinato)silicon complexes RPhSi(oxinate)2 [R = Ph (5a), Me (5b)]. Their X-ray structures reveal hexacoordination of the Si atom with the monodentate substituents in cis-positions and N atoms as well as O atoms in trans-positions. In crystalline dimethylbis(oxinato)silicon, Me2Si(oxinate)2 (7), the silicon atom is only bicapped tetrahedrally coordinated, while for dichlorobis(oxinato)silicon, Cl2Si(oxinate)2 (8), there is an octahedral coordination of the Si atom with chlorine atoms in trans-positions. This conclusion is based on the results of spectroscopic analysis (IR, 29Si CP/MAS NMR) as well as quantum chemical calculations. The first example of a silicon-bis-oxinate with the N→Si dative bonds in a trans-arrangement has been detected in the hexacoordinate silicon tris-chelate (oxinate)2Si(PhN-CH2CH2-NPh) (11). Its configuration was proven by X-ray structure analysis. Thus, for hexacoordinate bis(oxinato)silicon compounds three new architectures were found which complement the previously established building pattern of the N,N’-cis-O,O’-trans-bis(oxinato)silicon complexes. The mer-tris(oxinato)siliconium cation (9+) (its configuration being proven by 1H and 13C NMR spectroscopy) features at least three coordination patterns with (O,O;N,N)-cis,cis-, -cis,trans- as well as -trans,cis-arrangements of two oxinate ligands.