Synthesis, stereoisomerism and crystal structures of neutral hexacoordinate silicon(IV) complexes with Salen-O,N,N,O and thiocyanato-N ligands

• Three stereoisomers of (NCS)2Si(MeSalen) in DMSO solution.
• Quantitative 29Si NMR spectroscopy gave ratio of stereoisomers.
• X-ray crystal structures of hexacoordinate Salen-type silicon.
• Intermolecular interactions at silicon center.

The reaction of methyldi(thiocyanato-N)silane (HMeSi(NCS)2) with the Salen-type ligands 2,2′-{ethane-1,2-diylbis[nitrile(E)methylylidene]}diphenol (H2Salen) and 2,2′-{ethane-1,2-diylbis[nitrile(1E)eth-1-yl-1-ylidene]}diphenol (H2MeSalen) leads to the formation of the neutral hexacoordinate silicon(IV) complexes (Salen)SiMe(NCS) (4) and (MeSalen)SiMe(NCS) (5) with one –NCS functional group and a SiO2N3C coordination framework. The reaction of Si(NCS)4 with the H2MeSalen ligand leads to the formation of (MeSalen)Si(NCS)2 (6), a neutral silicon(IV) complex bi-functionalized with –NCS. 29Si NMR solution studies confirmed that silicon complexes 4–6 are hexacoordinated and that complex 6 exists as a mixture of the three possible stereoisomers trans-6, all-cis-6 and cis-6 in approximately a 2.25/1.75/1.00 ratio, respectively. The assignments of the stereoisomers of 6 in dmso-d6 solution were achieved by quantitative 29Si NMR analysis and their relative energies (ΔH0298 values) obtained from density functional theory (DFT) calculations. All of the complexes were characterized by FT-IR spectroscopy, solid-state 29Si CP/MAS and solution 1H, 13C and 29Si NMR. Single-crystal X-ray diffraction and elemental analysis were used to confirm the structures and compositions of all of the complexes.

Three new neutral hexacoordinate silicon(IV) complexes containing O,N,N,O donor Salen-type ligand and one or two –NCS functional group were synthesized and structural characterized by X-ray diffraction analysis. For the first time, the three possible stereoisomers (trans, cis and all-cis) of the complex (MeSalen)Si(NCS)2 were detected and quantified by 29Si NMR spectroscopy in dmso-d6 solution. Complex (Salen)Si(Me)(NCS) showed interesting H⋯Si intermolecular interactions in the solid state crystal structure.Figure optionsDownload as PowerPoint slide