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-(t)Bu(2)-C(6)H(2)(1-C{N(CH)(2)N(R)})); R = (i)Pr, (t)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(R)·CO(2))(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 CO(2) reveals the solvent and ligand size dependence of the selectivity. This is important because only the complexes capable of reversible CO(2)-insertion are competent catalysts for catalytic conversions of CO(2). Preliminary studies show that only Ce(L(Mes)·CO(2))(3) catalyses the formation of propylene carbonate from propylene oxide under 1 atm of CO(2) pressure. The mono-ligand complexes can be isolated from reactions using LiCe(N(i)Pr(2))(4) as a starting material; LiBr adducts [Ce(L(R))(N(i)Pr(2))Br·LiBr(THF)](2) (R = Me, (i)Pr) are reported, along with a hexanuclear N-heterocyclic dicarbene [Li(2)Ce(3)(OArC(Me)–H)(3)(N(i)Pr(2))(5)(THF)(2)](2) by-product. The analogous para-aryloxide–NHC proligand (p-L(Mes) = 4-O-2,6-(t)Bu(2)-C(6)H(2)(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.