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)]^3- (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 Ga³⁺ 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)]^3- (n = 1-3) and [Ga(SBT)]³⁺ (m = 1, 2) complexes, the negative charge transfer from ligands to the Ga³⁺-center plays an important role in their stability, because of the strong polarization of the Ga³⁺ cation. For [Ga(OTf)(SBT)]^3- (n = 1, 2; m = 1, 2) complexes, the negative charge transfer from ligands to the Ga³⁺-center plays an essential role in their stability, accompanied by an electrostatic interaction between the Ga³⁺-center and ligands and/or spatial inclusion of ligands toward the Ga³⁺-center.