Tunable CO 2 Capture and Release Using Redox-Switchable Carboranes

We report the use of redox-switchable carboranes for the tunable and reversible capture of CO2. Two substituted 1-PR2-2-BR'2-ortho-carboranes (denoted R/R'Cb) with Lewis acid (LA) and Lewis base (LB) groups were synthesized. These were Ph/CyCb containing weaker LB/LA groups, and tBu/C6F5Cb with stronger LB/LA groups. While closo forms of these were unreactive to CO2, reduction using KC8 yielded reactive nido species. The reduced compound, K(18-c-6)2Ph/CyCb (18-c-6 = 18-crown-6), captured and reduced CO2 to generate the cage-appended formate species, K(18-c-6)2Ph/CyCb-HCO2. In contrast, the nido compound, K(18-c-6)2tBu/C6F5Cb, weakly captured CO2 to generate the species K(18-c-6)2tBu/C6F5Cb-CO2, wherein CO2 capture was thought to occur between the P and K+ centers. This was structurally confirmed by performing the reaction with added Li+ to generate the strongly activated P-C(O)O-Li species, Li1.5K0.5tBu/C6F5Cb-CO2. This also provided an avenue to tune CO2 binding constants by controlling the addition of added alkali cations to the solution. Using the encapsulated nido species, K(kryp)2tBu/C6F5Cb (kryp = kryptofix 2,2,2), binding constants (K) were calculated upon the addition of 1 equiv of MTFSI (M = Li, Na, K; TFSI = bis(trifluoromethanesulfonyl)imide) and resulted in log K values of: 5.5 (Li+), 4.5 (Na+), and 3.4 (K+). Binding constants with >1.0 equiv. M+ were qualitatively similar, whereas lower values were observed with +. Chemical oxidation of the CO2-captured products also led to CO2 release. Together, these results open avenues to redox-controlled CO2 capture/release coupled with tunable CO2 capture binding constants, significant areas of interest in carbon capture technologies.