Trimethylsilylmethyl complexes of the rare-earth metals with sterically hindered N-heterocyclic carbene ligands: adduct formation and C–H bond activation

Abstract

Tris(trimethylsilylmethyl) complexes of yttrium and lutetium [LnR₃(THF)₂] (R = CH₂SiMe₃) were treated with sterically bulky N-heterocyclic carbenes (NHC) 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) and 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes). IPr gave labile mono-adducts [LnR₃(NHC)], isolated as thermally robust crystals and fully characterized by NMR spectroscopy and X-ray diffraction. IMes gave a similar lutetium mono-adduct [LuR₃(IMes)] with the lutetium alkyl [LuR₃(THF)₂], whereas the yttrium alkyl [YR₃(THF)₂] resulted in the formation of an ortho-metalated product. This compound, isolated as a crystalline bis(THF) adduct, contains a strained six-membered chelate ring that has been formed by the C–H bond activation of one of the ortho-methyl groups of the mesityl group. In contrast [LuR₃(IMes)] only slowly underwent a similar C–H bond activation.