Tunable CO₂ 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)]₂[^Ph/CyCb] (18-c-6 = 18-crown-6), captured and reduced CO₂ to generate the cage-appended formate species, [K(18-c-6)]₂[^Ph/CyCb-HCO₂]. In contrast, the nido compound, [K(18-c-6)]₂[^tBu/C6F5Cb], weakly captured CO₂ to generate the species [K(18-c-6)]₂[^tBu/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, [Li]_1.5[K]_0.5[^tBu/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)]₂[^tBu/C6F5Cb] (kryp = kryptofix 2,2,2), binding constants (K) were calculated upon the addition of 1 equiv of [M][TFSI] (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 <1.0 equiv of M⁺. 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.