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(LR)3 supported by bidentate ortho-aryloxide-NHC ligands are reported (LR = O(o-C6H2-tBu2-2,6-CN(C2H2)NR); R = iPr, tBu, Mes; Ln = Ce, Sm, Eu). The cerium complexes react cleanly and quantitatively insert carbon dioxide exclusively into all three cerium carbene bonds, forming Ce(LR.CO2)3. The insertion is reversible only for the mesityl-substituted complex Ce(LMes)3. Analysis of the capacity of Ce(LR)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(LMes.CO2)3 catalyses the formation of propylene carbonate from propylene oxide and 1 atmosphere pressures of carbon dioxide. The mono-ligand complexes can be isolated from reactions using LiCe(NiPr2)4 as a starting material; LiBr adducts [Ce(LR)(NiPr2)Br.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-LMes = O(p-C6H2tBu2-2,6-CN(C2H4)NMes) has been made for comparison, but the rare earth tris-ligand complexes Ln(p-LMes)3(THF)2 (Ln = Y, Ce) are too reactive for straightforward Lewis pair separated chemistry to be usefully carried out.