Hexacoordinate Silicon Compounds with a Dianionic Tetradentate (N,N′,N′,N)-Chelating Ligand

Abstract

In the context of our systematic investigations of penta- and hexacoordinate silicon compounds, which included dianionic tri- (O,N,O′; O,N,N′) and tetradentate (O,N,N,O; O,N,N′,O′) chelators, we have now explored silicon coordination chemistry with a dianionic tetradentate (N,N′,N′,N) chelator. The ligand [o-phenylene-bis(pyrrole-2-carbaldimine), H2L] was obtained by condensation of o-phenylenediamine and pyrrole-2-carbaldehyde and subsequently silylated with chlorotrimethylsilane/triethylamine. Transsilylation of this ligand precursor (Me3Si)2L with chlorosilanes SiCl4, PhSiCl3, Ph2SiCl2, (Anis)2SiCl2 and (4-Me2N-C6H4)PhSiCl2 afforded the hexacoordinate Si complexes LSiCl2, LSiPhCl, LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4), respectively (Anis = anisyl = 4-methoxyphenyl). 29Si NMR spectroscopy and, for LSiPh2, LSi(Anis)2 and LSiPh(4-Me2N-C6H4), single-crystal X-ray diffraction confirm hexacoordination of the Si atoms. The molecular structures of LSiCl2 and LSiPhCl were elucidated by computational methods. Despite the two different N donor sites (pyrrole N, X-type donor; imine N, L-type donor), charge delocalization within the ligand backbone results in compounds with four similar Si–N bonds. Charge distribution within the whole molecules was analyzed by calculating the Natural Charges (NCs). Although these five compounds carry electronically different monodentate substituents, their constituents reveal rather narrow ranges of their charges (Si atoms: +2.10–+2.22; monodentate substituents: −0.54–−0.56; L2−: −1.02–−1.11).

Keywords

• hypercoordination
• imine
• pyrrole
• 29Si NMR spectroscopy
• X-ray diffraction