(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 PtBu3, 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 PtBu3.
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 160oC.
Finally, variable temperature 13C, 31P and 19F NMR studies of mixtures of both PtBu3 and B(C6F5)3 with 13CO2 in CD2Cl2 were conducted. VT NMR analysis indicates that both PtBu3 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.