Selective and catalytic carbon dioxide and heteroallene activation mediated by cerium N-heterocyclic carbene complexes† †Electronic supplementary information (ESI) available. CCDC 1856101–1856106, 1868204–1868209. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c

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).


Using a modification of general procedure 1 -To a suspension of [o-H 2 L iPr ][Br]
(276 mg, 0.75 mmol) in DME (7.5 mL) was added KN(SiMe 3 ) 2 (300 mg, 1.5 mmol) and the resulting mixture was stirred for 5 minutes at room temperature. SmI 2 (thf) 2 (205 mg, 0.38 mmol) was added, and the resulting mixture instantly turned blue and was allowed to stir at room temperature for 2 hours by which time the solution was dark yellow. Volatiles were removed under reduced pressure, the crude product was extracted three times with hexane and the combined filtrates were concentrated to saturation, and cooled to -30 ᵒC overnight. The resulting suspension was glassfibre filtered and the solid collected and dried under vacuum to give Sm(III)(O(o-C 6 H 2 -t Bu 2 -2,6-CN(C 2 H 2 )N i Pr) 3 (1Sm iPr ) (40 %, 327 mg, 0.3 mmol).

and ethylphenylketene
To a solution of Ce(L iPr ) 3 1Ce iPr (109 mg, 0.1 mmol) in C 6 H 6 (2 mL) was added ethylphenylketene 1 (14 mg, 0.1 mmol) in a capped vial and stirred for 5 minutes at RT. The solution was analysed by 1 H NMR spectroscopy which showed that all starting material was consumed and multiple products had formed which could not be isolated.
The reaction was also carried out at lower temperature (-20 °C), in different solvents (THF, DME) and at different ketene loadings (0.05 mmol, 0.3 mmol) with no obvious improvement in product distribution.
Ethylphenylketene was synthesised using a previously reported procedure by Smith et al. 1

Using 2CeL iPr
To a Youngs tap NMR tube charged with propylene oxide (68 µL, 1 mmol) in THF (0.6 mL) and C 6 D 6 (2 drops) was added 2Ce iPr (12 mg, 0.01 mmol). The reaction mixture was freeze-pump-thaw degassed three times then exposed to an atmosphere of CO 2 . The reaction was then heated to 80 °C for 7 days while monitoring by 1 H NMR spectroscopy at 24h intervals; no changes were observed.

Using 2CeL Mes
To a Youngs tap NMR tube charged with propylene oxide (68 µL, 1 mmol) in THF (0.6 mL) and C 6 D 6 (2 drops) was added 2Ce Mes (14 mg, 0.01 mmol). The reaction mixture was freeze-pump-thaw degassed three times then exposed to an atmosphere of CO 2 . The reaction was then heated to 80 °C for 7 days while monitoring by 1

Ce
Ce

An NMR tube was loaded with [o-H 2 L Me ][Br]
(15.0 mg, 40.8 μmol) and Ce( i Pr 2 N) 4 Li(THF) (32 mg, 51.6 μmol) and ca. 0.5 mL of C 6 D 6 . After ca. 7 days, the formation of bright orange crystals, which were suitable for X-ray diffraction, was observed as 5Ce Me but upon repeated attempts could never isolated on a larger scale. 7 Li NMR (194.4 MHz, C 6 D 6 ) δ Li : 38.01.

[p-H 2 L Mes ][Br] [HO(p-C 6 H 2 -t Bu 2 -2,6-CHN(C 2 H 4 )NMes][Br]
An ampoule was charged with 4-bromo-2,4,6-tri-tert-butyl-2,5-cyclohexadien-1-one (6.82 g, 20 mmol), N-mesitylimidazoline (7.4285g, 40 mmol) and ethylene glycol (  In addition, the thermal parameters were set to be similar by use of the SIMU command, and the bond angles were restrained through the use of the SADI command. Two solvent accessible voids of 80 electrons were excluded from the lattice using the olex solvent mask feature, which was believed to be highly disordered heptane. 4Ce Me -The THF ligand, diisopropyl amide ligand, and one tert-butyl group were positionally disordered and a split layer refinement with the sum of occupancies set to 1 was applied. In addition, thermal parameters were constrained by use of the SAME, SIMU and DELU command. 5Ce Me -The THF ligands, tert-butyl groups and benzene lattice solvent molecule were positionally disordered and a split layer refinement with the sum of occupancies set to 1 was applied. In addition, thermal parameters were constrained by use of the SAME, SADI, SIMU and RIGU command. (coordinated THF) was disordered and constrained using the DELU, SIMU and RIGU commands. 1Ce iPr .HBr -Single crystals were once grown from a concentrated solution of 1Ce iPr in benzene stored at room temperature overnight. R 1 = 10.55%, the structure is included for connectivity only. In addition 34 electrons were excluded from the lattice using the olex solvent mask feature, which was believed to be highly disordered benzene.
1Eu iPr .HBr -Single crystals were once grown from a concentrated solution of 1Eu iPr in toluene stored at room temperature overnight. C(183) is positionally disordered over two positions, and was spilt into C(183) and C (184) and refined with an occupany ratio of 0.6:0.4.  Crystals of 4Ce iPr were obtained by slow evaporation of a solution in benzene. 4Ce iPr crystallises in the triclinic spacegroup P as a dimer with an internal centre of inversion (see Figure S1). The compound consists of two ̅ 1 Ce(III) ions, two ligands L1 iPr , four bromides, two diisopropylamide ligands and two lithium ions, which are each coordinated by a THF molecule. Ce(III) is coordinated in a distorted octahedral fashion. Two bromides exhibit a μ 3 -bridging mode between two Ce(III) and one Li(I) with one Ce-Br distance significantly longer than the other  Crystals of 4Ce Me were obtained by slow evaporation of a solution in benzene. The compound crystallizes in the monoclinic spacegroup P2 1 /n as a dimer with an internal centre of inversion (see Figure S2) and with half an equivalent of co-crystallized benzene. The molecular structure is analogous to Ce 2 Br 4 L1 iPr 2 ( i Pr 2 N) 2 Li 2 (THF) 2 . Bond lengths are also comparable (e.g. Ce-C carbene = 2.639(4)), only the angles between coordinating atoms around the central cluster of the structure show small differences due to the different wingtip. The degree of distortion of the Ce-NHC coordination is higher in-plane (Ce1-C1-N1 = 135.7(3)° vs. Ce1-C1-N2 119.0(3)°) and lower out-of-plane (Ce1-C1-centroid(NHC) = 165.45°) as compared to the isopropyl derivative. Similar to the isopropyl derivative, a tilted diisopropylamide ligand is observed, indicating an interaction of Ce(III) with the H atom on C19. Due to the disorder of this ligand, however, this is less certain than in the isopropyl derivative.