Role of N-donor groups on the stability of hydrazide based hypercoordinate silicon(IV) complexes: Theoretical and experimental perceptions

Abstract

A triphenylphosphinimino donor group is illustrated as a ligand in pentacoordinate siliconium halide dichelates, [YSiL2]+X−, where L is the bidentate ligand –OC(R)NNPPh3)– (R=t-Bu-Ph or Ph), Y=Me, Ph, CH2Cl, CHCl2, Cl or Br, and X=Cl or Br. All the new complexes were characterized by NMR spectroscopy and elemental analysis. The remote substituent, the t-Bu-phenyl or phenyl group, imparts more pentacoordinate character, i.e. more ionization to the complexes, compared to the PhCH2 group. DFT calculations indicate that the central silicon atom, due to the more positive charge, demands greater electron density. As a result of this, shorter Si–O, Si–N and Si–Cl bonds were observed. Both theoretical and experimental analysis indicate that the phosphinimino ligand is a stronger donor than the previously studied dimethylamino and isopropylidenimino ligands, causing all of the complexes to be pentacoordinate siliconium-halide salts in solution. The hypercoordinate silicon dichelates undergo unique intermolecular chelate exchange reactions: (i) complete ligand transfer from the dichelates to PhSiCl3 by a ligand priority order and (ii) bidentate ligand interchange between the dichelates and a trimethylsilyl-hydrazide precursor. Thermolysis of some selected hypercoordinated silicon(IV) complexes containing a silicon–carbon σ-bond significantly undergo a two step decomposition, while other complexes with silicon–halogen σ-bonds follow three steps. The thermal decomposition strongly depends on the nature of the substituents directly attached to the central silicon atom.