Light metal pyrazolates excel in carbon dioxide uptake

Abstract

The development of new carbon (CO₂) capture materials has emerged as a top-priority transdisciplinary research field. Ideally, CO₂ is not only captured and stored (CCS), but also transformed into more valuable organic compounds, because CO₂ itself is a cheap, abundant, non-flammable gas and thus an attractive C1 building block. However, activation of this thermodynamically rather stable molecule requires high activation energies. To overcome this energy barrier, activation of the CO double bond is routinely achieved by exploiting a synergetic metal–ligand cooperativity. The most promising candidates from academia or industry revolve around amino-functionalized materials or components featuring metal–nitrogen bonds. Given their natural abundance, low prices and nontoxicity, environmentally friendly materials should ultimately involve light metals. Recently, we found that the cerium pyrazolate [Ce^+IV(pz^Me₂)₄]₂ is able to insert CO₂ exhaustively and reversibly. In general, such nitrogen-rich azolato ligands comprising pyrazolato, triazolato and tetrazolato derivatives exhibit five-membered aromatic ring systems with nucleophilic nitrogen coordination sites. Azolato ligands adopt a wide variety of coordination modes and especially light metal pyrazolates are a well-established class of compounds. Aiming at higher CO₂ uptake capacities, the conceptual approach, developed for the heavy metal cerium, has been consequently adapted to the light metals magnesium, aluminium, scandium and titanium. This review gives an overview of light metal pyrazolates and their CO₂ insertion behaviour as well as their catalytic activity in the cycloaddition reaction of CO₂ and epoxides to cyclic carbonates. In addition, consideration is given to immobilized variants as well as exemplary complexes and metal–organic framework materials derived from nitrogen-richer azoles/azolates.