Very efficient organo-zinc scorpionates for CO fixation into a variety of cyclic carbonates: synthesis, coordination ability and catalytic studies

The fixation of CO 2 mediated by metal-based catalysts for the production of organic molecules of industrial interest such as cyclic carbonates is urgently required under green and eco-friendly conditions. Herein, we describe the easy preparation of sterically demanding scorpionate ligands bearing different electron-withdrawing groups, and their coordination ability for the preparation of robust zinc-based mononuclear complexes of the type [ZnMe(κ 3 -NNN′)] ( 4-6 ). These complexes, in combination with co-catalysts comprising larger ionic radius-based halides such as tetra- n -butylammonium, functioned as very active and selective catalysts for CO 2 fixation into five-membered cyclic carbonates. These studies have led to the development of sustainable, inexpensive, and low-toxicity systems formed by 4-5 and Bu 4 NBr for the cycloaddition of CO 2 into epoxides under very mild and solvent-free conditions, reaching very good to excellent conversions (TOF = 260 h −1 ). Moreover, these bicomponent systems show a broad substrate scope and functional group tolerance, including mono- and di-substituted epoxides, as well as bio-renewable diepoxides. Very interestingly, these are the first zinc-based systems reported to date for the successful transformation of the very challenging tri-substituted terpene-derived cis / trans -limonene oxide, whose reaction proceeds with high stereoselectivity to the formation of the bicyclic trans -limonene carbonate. Additionally, these bicomponents can be efficiently used up to six times without significant loss of activity. Kinetic investigations confirmed that the reaction shows an apparent first-order dependence on the catalyst and co-catalyst concentrations, which indicates an intramolecular monometallic mechanism. A series of [ZnMe(κ 3 -NNN′)] complexes combined with TBAB/TBAC show excellent performance (TOF up to 260 h −1 ) as catalysts for CO 2 fixation into cyclic carbonates under mild, solvent-free conditions, with good functional group tolerance.