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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Abstract

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
Pages (from-to)8035-8045
Number of pages11
JournalChemical Science
Volume9
Issue number42
DOIs
Publication statusPublished - 14 Nov 2018

Keywords

  • carbon dioxide
  • carboxylic acid
  • rare earth complexes

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