Pentacoordinate mono(β-diketonato)- and hexacoordinate bis-(β-diketonato)-silicon(IV) complexes obtained from (thiocyanato-N)hydridosilanes

Three novel neutral hypercoordinate silicon(IV) complexes containing the O,O′-donor deprotonated β-diketone ligands 1,3-diphenylpropane-1,3-dione (HL1) and 1-phenylbutane-1,3-dione (HL2) were synthesized starting from the hitherto unknown diphenyl(thiocyanato-N)silane [HPh2Si(NCS)] and methyldi(thiocyanato-N)silane [HMeSi(NCS)2], a class of silicon source with Si–NCS and Si–H reactive functionalities. The reaction of HL1 with the new starting material HPh2Si(NCS) only afforded a neutral pentacoordinate silicon complex containing one β-diketonato ligand coordinated in a bidentate fashion (complex 16). The reaction of HL1 and HL2 with HMeSi(NCS)2 yielded bis-β-diketonato neutral hexacoordinate silicon complexes (17 and 18, respectively). The three compounds 16–18 were characterized using solution 1H, 13C and 29Si INEPT NMR and FT-IR spectroscopies, and compounds 17 and 18 were also studied using solid-state 29Si CP/MAS NMR. Elemental analysis and single-crystal X-ray diffraction were used to establish the composition and the structure of all of the new complexes. The 29Si INEPT NMR solution studies confirmed that compounds 16–18 maintain the same hypercoordination in solution that they have in the solid state. Compound 16 has a SiO2NC2 coordinating framework and represents the first example of a pentacoordinate silicon β-diketonato whose X-ray single crystal structure has been elucidated, and compounds 17 and 18 are the first neutral, non-zwitterionic, hexacoordinate silicon complexes with a SiO4NC coordinating framework bearing only one –NCS functionality. The usefulness of the (thiocyanato-N)hydridosilanes HPh2Si(NCS) and HMeSi(NCS)2 as starting materials for the preparation of hypercoordinate organosilicon complexes is confirmed.

Graphical abstractA suitable starting material for the successful one-step synthesis of hypercoordinate organosilicon complexes must include good leaving groups and hypercoordination stabilizing ligands. The simultaneous presence of Si–H and Si–NCS functionalities on tetravalent silanes such as (thiocyanato-N)hydridosilanes make them suitable silicon sources to produce neutral penta- and hexacoordinate silicon compounds in those cases where anionic protonated ligands are involved, as shown here using β-diketones as silicon complexing agents.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► (Thiocyanato-N)hydridosilanes are useful to prepare hypercoordinate Si compounds. ► First X-ray single crystal structure of a pentacoordinate Si β-diketonate. ► First neutral, non zwitterionic, hexacoordinate Si complexes with a SiO4NC skeleton. ► NCS ligands cause a large high-field chemical shift of the 29Si NMR resonance.