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