Zwitterionic spirocyclic lambda Si-5-silicates with two bidentate acetohydroximato(2-) or benzohydroximato(2-) ligands: Synthesis, structure, and dynamic stereochemistry

The syntheses of the zwitterionic spirocyclic lambda(5)Si-silicates 7-14 ar e described. The chiral zwitterions contain a pentacoordinate (formally neg atively charged) silicon atom and a tetracoordinate (formally positively ch arged) nitrogen atom, the ate and onium center being connected by an alkyle ne group. The zwitterions each contain two identical bidentate diolato(2-) ligands that formally derive from acetohydroximic acid or benzohydroximic a cid. The stereochemistry and dynamic behavior of these compounds were inves tigated by experimental and theoretical methods. For this purpose, the zwit terionic lambda(5)Si-silicates 7-14 were studied by solution (H-1,C-13, Si- 29) and solid-state (C-13,N-15, and Si-29 CP/MAS) NMR experiments. In addit ion, compounds 7, 8, 10, 11, and 13 were structurally characterized by sing le-crystal X-ray diffraction. The dynamic behavior (intramolecular enantiom erization) of 7 and 13 in solution was studied by VT 'H NMR experiments. Th ese experimental studies were completed by ab initio investigations of the related anionic model species 15. The chiral compounds 7-14 exist as(Lambda )-and (Delta)-enantiomers in the solid state and in solution. The trigonal- bipyramidal structure of the respective Si-coordination polyhedra, with the two carbon-linked oxygen atoms in the axial sites, is the energetically mo st favorable one. The (Lambda)- and (Delta)-enantiomers of 7-14 are configu rationally stable in solution on the NMR time scale ([D-6]DMSO, room temper ature). They undergo an intramolecular (Lambda)/(Delta)-enantiomerization ( twist-type mechanism), with an activation free enthalpy of Delta G double d agger = 72-73 kJ mol(-1) (experimentally established for 7 and 13; calculat ed energy barrier for the model species 15. 66.0 kJ mol(-1)).