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

Polly L. Arnold, Ryan W. F. Kerr, Catherine Weetman, Scott R. Docherty , Julia Rieb, Faye L. Cruickshank, Kai Wang, Christian Jandl, Max W. McMullon, Alexander Pöthig, Fritz E. Kühn, Andrew D. Smith

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A series of rare earth complexes of the form Ln(LR)3 supported by bidentate ortho-aryloxide–NHC ligands are reported (LR = 2-O-3,5-tBu2-C6H2(1-C{N(CH)2N(R)})); R = iPr, tBu, Mes; Ln = Ce, Sm, Eu). The cerium complexes 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 under 1 atm of CO2 pressure. 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 = 4-O-2,6-tBu2-C6H2(1-C{N(CH)2NMes}))) 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.
Original languageEnglish
Number of pages11
JournalChemical Science
VolumeAdvance Article
Early online date10 Sept 2018
Publication statusE-pub ahead of print - 10 Sept 2018


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