Application of the Redox-Transmetalation Procedure to Access Divalent Lanthanide and Alkaline-Earth NHC Complexes**

Divalent lanthanide and alkaline-earth complexes supported by N-heterocyclic carbene (NHC) ligands have been accessed by redox-transmetalation between air-stable NHC-AgI complexes and the corresponding metals. By using the small ligand 1,3-dimethylimidazol-2-ylidene (IMe), two series of isostructural complexes were obtained: the tetra-NHC complexes [LnI(2)(IMe)(4)] (Ln=Eu and Sm) and the bis-NHC complexes [MI2(IMe)(2)(THF)(2)] (M=Yb, Ca and Sr). In the former, distortions in the NHC coordination were found to originate from intermolecular repulsions in the solid state. Application of the redox-transmetalation strategy with the bulkier 1,3-dimesitylimidazol-2-ylidene (IMes) ligand yielded [SrI2(IMes)(THF)(3)], while using a similar procedure with Ca metal led to [CaI2(THF)(4)] and uncoordinated IMes. DFT calculations were performed to rationalise the selective formation of the bis-NHC adduct in [SrI2(IMe)(2)(THF)(2)] and the tetra-NHC adduct in [SmI2(IMe)(4)]. Since the results in the gas phase point towards preferential formation of the tetra-NHC complexes for both metal centres, the differences between both arrangements are a result of solid-state effects such as slightly different packing forces.

Automated summary

Divalent lanthanide and alkaline-earth complexes supported by N-heterocyclic carbene (NHC) ligands were synthesized through redox-transmetalation, with the size of NHC ligands playing a key role in determining the product structures, likely due to intermolecular repulsions in the solid state. Computational studies suggest solid-state effects contribute to differences in the formation of these complexes.