Coordination of sorbitol to Ga(OTf) 3 in the liquid phase: an experimental and theoretical study.

In a solution of sorbitol (SBT) and Ga(OTf) ₃ compounds, the coordination of sorbitol (SBT) to [Ga(OTf) _n ] ^{3- n} ( n = 0-3) has been investigated, using both ESI-MS spectra and density functional theory (DFT) calculations at the M06/6-311++g(d,p), aug-cc-pvtz level using a polarized continuum model (PCM-SMD). In sorbitol solution, the most stable conformer of sorbitol includes three intramolecular H-bonds, i.e. , O2H⋯O4, O4H⋯O6, and O5H⋯O3. Through ESI-MS spectra, in a tetrahydrofuran solution of both SBT and Ga(OTf) ₃ compounds, five main species are observed, i.e. , [Ga(SBT)] ³⁺ , [Ga(OTf)] ²⁺ , [Ga(SBT) ₂ ] ³⁺ , [Ga(OTf)(SBT)] ²⁺ , and [Ga(OTf)(SBT) ₂ ] ²⁺ . Through DFT calculations, in a solution of sorbitol (SBT) and Ga(OTf) ₃ compounds, the Ga ³⁺ cation tends to form five six-coordination complexes, i.e. , [Ga(η2O,O-OTf) ₃ ], [Ga(η3O2-O4-SBT) ₂ ] ³⁺ , [(η2O,O-OTf)Ga(η4O2-O5-SBT)] ²⁺ , [(η1O-OTf)(η2O2,O4-SBT)Ga(η3O3-O5-SBT)] ²⁺ , and [(η1O-OTf)(η2O,O-OTf)Ga(η3O3-O5-SBT)] ⁺ , which are in good agreement with the experimental observation of the ESI-MS spectra. For both [Ga(OTf) _n ] ^{3- n} ( n = 1-3) and [Ga(SBT) _m ] ³⁺ ( m = 1, 2) complexes, the negative charge transfer from ligands to the Ga ³⁺ -center plays an important role in their stability, because of the strong polarization of the Ga ³⁺ cation. For [Ga(OTf) _n (SBT) _m ] ^{3- n} ( n = 1, 2; m = 1, 2) complexes, the negative charge transfer from ligands to the Ga ³⁺ -center plays an essential role in their stability, accompanied by an electrostatic interaction between the Ga ³⁺ -center and ligands and/or spatial inclusion of ligands toward the Ga ³⁺ -center.