Low temperature NMR spectroscopic investigation of a zinc bicarbonate compound: Thermodynamics of bicarbonate formation by insertion of CO2 into the zinc hydroxide bond of [TpBut,MeTpBut,Me]ZnOH

Low temperature 1H and 13C NMR spectroscopic studies on solutions of the tris(3-t  -butyl-5-methylpyrazolyl)hydroborato zinc hydroxide complex [TpBut,MeTpBut,Me]ZnOH in the presence of CO2 have allowed for the identification of the bicarbonate moiety of [TpBut,MeTpBut,Me]ZnOCO2H. For example, the bicarbonate ligand is characterized by 1H and 13C NMR spectroscopic signals at 13.2 and 161.9 ppm, respectively, in d2-dichloromethane solution at 189 K. Both [TpBut,MeTpBut,Me]ZnOH and [TpBut,MeTpBut,Me]ZnOCO2H may be observed in equilibrium in the presence of <1 atm of CO2 at 217 K, thereby allowing measurement of the equilibrium constant for insertion of CO2 into the Zn–OH bond. At 217 K, the equilibrium constant for formation of the bicarbonate complex is 6 ± 2 × 103 M−1, corresponding to a value of ΔG = −3.8 ± 0.2 kcal mol−1.

Graphical abstractLow temperature 1H and 13C NMR spectroscopic studies on solutions of the tris  (3-tbutyl-5-methylpyrazolyl)hydroborato zinc hydroxide complex [TpBut,MeTpBut,Me]ZnOH in the presence of CO2 have allowed for the identification of the bicarbonate moiety of [TpBut,MeTpBut,Me]ZnOCO2H. At 217 K, the equilibrium constant for formation of the bicarbonate complex is 6 ± 2 × 103 M−1, corresponding to a value of ΔG = −3.8 ± 0.2 kcal mol−1.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The bicarbonate ligand of [TpBut,MeTpBut,Me]ZnOCO2H exhibits a 1H NMR signal at 13.2 ppm. ► The bicarbonate ligand of [TpBut,MeTpBut,Me]ZnOCO2H exhibits a 13C NMR signal at 161.9 ppm. ► K for insertion of CO2 into [TpBut,MeTpBut,Me]ZnOH is 6 ± 2 × 103 M−1 at 217 K.