Reversible metal-free carbon dioxide binding by frustrated Lewis pairs.

Carbon dioxide is a gas that is changing our environment. Its role as a greenhouse gas is clear with the onset of global warming. In efforts to address this issue, ambitious and creative schemes are targeting materials including zeolites, silica gels, aluminas, and activated carbons, as well as sophisticated metal–organic frameworks (MOFs) for the benign capture and storage of carbon dioxide. An alternative approach is focused on the potential use of carbon dioxide as a C1 chemical feedstock. [4] To this end, strategies typically target the potential of transition-metal-based chemistry and catalysis. For example, ruthenium-based catalysts have been shown to effect the hydrogenation of carbon dioxide to formic acid derivatives. The fundamental difficulty in addressing strategies to either sequester or chemically modify carbon dioxide is its remarkable thermodynamic stability and its limited reactivity. In the absence of water, carbon dioxide is known to react with strong nucleophiles and coordinatively unsaturated transition-metal species, while organic bases readily convert it to bicarbonate salts in the presence of hydroxide. On the other hand, reactions of carbon dioxide with main-group systems are poorly explored. While CO2 is known to insert into P N, As N, and Si N bonds, only recently have reports described the carboxylation of N-heterocyclic carbenes. Reactions of CO2 with main-group metal amides were also reported. We recently developed a new approach to main-group reactivity, derived from the concept of “frustrated Lewis pairs”, that is, systems in which steric congestion precludes neutralization. These systems offer latent Lewis acidity and basicity for reaction with small molecules. In this vein, we and others have exploited this notion for the heterolytic activation of dihydrogen and subsequent application of the resulting systems in metal-free catalytic hydrogenation and addition to olefins. Herein, we demonstrate that the concept of frustrated Lewis pairs can be exploited to effect the reversible binding of carbon dioxide under mild conditions. A solution of B(C6F5)3 and PtBu3 in C6H5Br was covered with an atmosphere of carbon dioxide, resulting in the immediate precipitation of a white solid (1), which was isolated in 87% yield (Scheme 1). This product exhibited