Selective and catalytic carbon dioxide and heteroallene activation mediated by cerium N-heterocyclic carbene complexes

A series of rare earth complexes of the form Ln(L-R)(3) supported by bidentate ortho-aryloxide-NHC ligands are reported (L-R = 2-O-3,5-Bu-t(2)-C6H2(1-C{N(CH)(2)N(R)})); R = Pr-i, Bu-t, Mes; Ln = Ce, Sm, Eu). The cerium complexes cleanly and quantitatively insert carbon dioxide exclusively into all three cerium carbene bonds, forming Ce(L-R center dot CO2)(3). The insertion is reversible only for the mesityl-substituted complex Ce(L-Mes)(3). Analysis of the capacity of Ce(L-R)(3) to insert a range of heteroallenes that are isoelectronic with CO2 reveals the solvent and ligand size dependence of the selectivity. This is important because only the complexes capable of reversible CO2-insertion are competent catalysts for catalytic conversions of CO2. Preliminary studies show that only Ce(L-Mes center dot CO2)(3) catalyses the formation of propylene carbonate from propylene oxide under 1 atm of CO2 pressure. The mono-ligand complexes can be isolated from reactions using LiCe((NPr2)-Pr-i)(4) as a starting material; LiBr adducts [Ce(L-R)(NiPr2)Br center dot LiBr(THF)](2) (R = Me, iPr) are reported, along with a hexanuclear N-heterocyclic dicarbene [Li2Ce3(OArCMe-H)(3)(NiPr2)(5)(THF)(2)](2) by-product. The analogous para-aryloxide-NHC proligand (p-L-Mes = 4-O-2,6-tBu2-C6H2(1-C{N(CH)(2)NMes}))) has been made for comparison, but the rare earth tris-ligand complexes Ln(p-L-Mes)(3)(THF)(2) (Ln = Y, Ce) are too reactive for straightforward Lewis pair separated chemistry to be usefully carried out.