Synthesis and Structural Characterization of Novel Neutral Hexacoordinate Silicon(IV) Complexes with SiO₂N₄ Skeletons Containing Cyanato-N or Thiocyanato-N Ligands

Abstract

A series of novel hexacoordinate silicon(IV) complexes with an SiO2N4 skeleton (compounds (OC-6-12)-3, (OC-6-12)-4, (OC-6-12)-5, (OC-6-12)-6, and (OC-6-2'2)-7) were synthesized, starting from Si(NCO)4 or Si(NCS)4. These compounds contain (i) two bidentate O,N-chelate ligands (or one tetradentate O,N,N,O-chelate ligand) derived from 4-aminopent-3-en-2-ones of the formula type Me-C(NRH)=CH-C(O)-Me (R = organyl) and (ii) two monodentate cyanato-N or thiocyanato-N ligands. Formally, the bidentate singly negatively charged O,N-chelate ligands (tetradentate 2-fold negatively charged O,N,N,O-chelate ligand) behave as ligands of the imino-enolato type. In addition, the adduct trans-8 was synthesized by reaction of Si(NCS)4 with 2 molar equiv of Me-C(Ni-PrH)=CH-C(O)-Me. This hexacoordinate silicon(IV) complex contains (i) four monodentate thiocyanato-N ligands and (ii) two neutral monodentate ligands of the iminio-enolato type. All compounds synthesized were structurally characterized by single-crystal X-ray diffraction and solid-state and solution NMR spectroscopy. To get more information about the stereochemistry of these compounds, the experimental investigations were complemented by computational studies.

Photo‐Driven Si‐C Bond Cleavage in Hexacoordinate Silicon Complexes

“…Furthermore, the difference in energy between SN and SN' (4.8 kcal/mol) proves the structure of 17 an unexpected case, less favorable than its trans-C-Sn-C isomer. This finding is well in accord with a computational study by Tacke et al, which proved a mer-mer coordinated silicon complex within a tetradentate ONNO ligand system more stable than its mer-fac isomer [23]. The rather small difference in energy, however, shows that the special ligand arrangement in 17 in the solid state may arise for reasons such as crystal packing effects and revert into the more favorable arrangement upon dissolution (as found in NMR spectra).…”

Synthesis and Structural Characterization of Neutral Higher‐Coordinate Silicon(iv) Complexes with Tridentate Dianionic Chelate Ligands

“…and N-C-X angles have also been reported for the Si-NCX moieties of 1-7. [1][2][3] To the best of our knowledge, compound 8 is the first pentacoordinate silicon compound with cyanato-N ligands Scheme 1. Syntheses of compounds 8, 9, and 10.…”

“…[ [1][2][3] These silicon(iv) complexes contain two monodentate NCX À (X = O, S) ligands each and two bidentate monoanionic chelate ligands (1)(2)(3)(4)(5)(6) or one tetradentate dianionic chelate ligand (7). We have now succeeded in synthesizing the pentacoordinate silicon compounds 8 and 9 and the hexacoordinate silicon compound 10·1/2 CH 3 CN, starting from Si(NCO) 4 or Si(NCS) 4 .…”

The E. coli Siderophores Enterobactin and Salmochelin Form Six‐Coordinate Silicon Complexes at Physiological pH

“…The report of a transient hexavalent silicon complex in the diatom Navicula pelliculosa [11] is an exception. Silicon complexes of Ent (1) Si NMR spectra of Si-Sal ( Figure 1) and Si-Ent (see the Supporting Information) each exhibit a single peak at À140 ppm which, in the majority of studied cases, is characteristic of hexaoxo silicon complexes, [12][13][14] (d( 29 Si) = À135 to À145 ppm), whereas chemical shifts of d = À71 to À110 ppm are typical for tetraoxo silicon complexes and values of d = À98 to À110 ppm are common for pentaoxo silicon complexes. Accordingly, the NMR data suggest hexaoxo-coordinated silicon in the siderophore complexes.…”

“…In the last years inorganic chemists have made considerable progress in the synthesis of hypervalent silicon complexes, [14,16] and the formation and structure of these complexes is an area of continuing interest. [7] Inorganic chemists have reported on silicon complexation with catechols, [17] organosilanes, [16] thiocyanates, [18] and a large number of other ligands.…”