(485d) Extension of Reversible Carbon Dioxide Binding by Frustrated Lewis Pairs to Other Phosphine and Amine Bases

Recently, a new approach to metal-free activation of CO₂ using sterically congested Lewis acid/base pairs was discovered which can reversibly bind CO₂ under mild conditions.  This reaction was limited to the Lewis base P^tBu₃, which is pyrophoric and difficult to handle in air.  A series of alternative Lewis bases, comprised of less-reactive phosphines and amines, was studied in an attempt to extend the CO₂-complexation reaction to other systems.  Among the reactions studied, amines tended to form direct acid-base adducts, despite their steric bulk. One system using the amine DBU led to the isolation of crystals which were studied using single crystal X-ray diffraction.  Four of the phosphines studied, rather than complexing CO₂, reacted at the para position of one of the borane phenyl rings to form a bridging-C₆F₄ complex, with the lost F atom bonded to B.  None of the Lewis bases studied was capable of complexing CO₂ with B(C₆F₅)₃, presumably due to the diminished basicities compared to P^tBu₃.

The air and vacuum stability of the [^tBu₃P-(m-CO₂)-B(C₆F₅)₃] complex was studied at ambient conditions.  A film of [^tBu₃P-(m-CO₂)-B(C₆F₅)₃] was cast from CH₂Cl₂ solution on CaF₂ and stored both in a vacuum chamber for 6 weeks and in the laboratory air for 12 weeks, both at ambient temperature.  Neither film exhibited any change over the span of the experiment, suggesting that the bridging-CO₂ complex is remarkably stable and suggests that [^tBu₃P-(m-CO₂)-B(C₆F₅)₃] could be used as a basis for CO₂-capture and storage.  Heating the [^tBu₃P-(m-CO₂)-B(C₆F₅)₃] film under vacuum led to the disappearance of the n(C=O) band at 1694 cm^-1 in the FTIR spectrum.  Heating the [^tBu₃P-(m-CO₂)-B(C₆F₅)₃] film under vacuum in a sealed system led to free CO₂ with a band at 2349 cm^-1 in the FTIR spectrum.  TGA-MS analyses of CO₂-containing products show thermal release of CO₂ between 100 and 160^oC.

Finally, variable temperature ¹³C, ³¹P and ¹⁹F NMR studies of mixtures of both P^tBu₃ and B(C₆F₅)₃  with ¹³CO₂ in CD₂Cl₂ were conducted.   VT NMR analysis indicates that both P^tBu₃ and B(C₆F₅)₃  form complexes with ¹³CO₂ that can be detected and whose relative amounts vary with temperature.  The results from variable temperature scans permit the calculation of the enthalpies of formation for these complexes.