Structurally simple complexes of CO2

Abstract

The ability to bind CO₂ through the formation of low-energy, easily-broken, bonds could prove invaluable in a variety of chemical contexts. For example, weak bonds to CO₂ would greatly decrease the cost of the energy-intensive sorbent-regeneration step common to most carbon capture technologies. Furthermore, exploration of this field could lead to the discovery of novel CO₂ chemistry. Reduction of complexed carbon dioxide might generate chemical feedstocks for the preparation of value-added products, particularly transportation fuels or fuel precursors. Implementation on a large scale could help to drastically reduce CO₂ concentrations in the atmosphere. However, literature examples of weakly bonded complexes of CO₂ are relatively few and true coordination complexes to a ‘naked’ CO₂ fragment are nearly unheard of. In this review article, a variety of complexes of CO₂ featuring diverse binding modes and reactivity will be examined. Topics covered include: (A) inclusion complexes of CO₂ in porous materials. (B) Zwitterionic carbamates produced from the reaction of CO₂ with polyamines. (C) Carbamate salts produced from reaction of CO₂ with two equivalents of an amine. (D) Insertion products of CO₂ into acid-base adducts (e.g., metal complexes). (E) Lewis acid–base activated CO₂, such as frustrated Lewis pair complexes. (F) Simple base–CO₂ adducts, wherein the base–CO₂ bond is the only interaction formed. Complexes in the last category are of particular interest, and include imidazol-2-carboxylates (N-heterocyclic carbene adducts of CO₂) as well as a few other examples that lie outside NHC chemistry.