Molecular Manipulations of a Utility Nitrogen–Heterocyclic Carbene by Sodium Magnesiate Complexes and Transmetallation Chemistry with Gold Complexes

Expanding the scope and applications of anionic N-heterocyclic carbenes (NHCs), a novel series of magnesium NHC complexes is reported using a mixed sodium–magnesium approach. Sequential reactivity of classical imidazol- 2-ylidene carbene IPr with NaR and MgR₂ (R=CH₂SiMe₃) affords [(THF)₃Na(μ-IPr⁻)MgR₂(THF)] (2) [IPr⁻=:C{[N(2,6-iPr₂C₆H₃)]₂CHC] containing an anionic NHC ligand, whereas surprisingly sodium magnesiate [NaMgR₃] fails to deprotonate IPr affording instead the redistribution coordination adduct [IPr₂Na₂MgR₄] (1). Compound 2 undergoes selective C2-methylation when treated with MeOTf furnishing novel abnormal NHC complex [{aIPr^MeMgR₂}₂] (3). Dissolving 3 in THF led to the dissociation of this complex into MgR₂ and aIPr^Me with the latter isomerizing to the olefinic NHC IPr=CH₂. The ability of 2 and 3 to transfer their anionic and abnormal NHC ligands, respectively to Au^I metal fragments has been investigated allowing the isolation and structural characterization of [RAu(μ-IPr⁻)MgR(THF)₂] (4) and [aIPr^MeAuR] (5) respectively. In both cases transfer of an alkyl R group is observed. However while 3 can also transfer its abnormal NHC ligand to give 5, in 4 the anionic NHC still remains coordinated to Mg via its C4 position, whereas the {AuR} fragment occupies the C2 position previously filled by a donor-solvated {Na(THF)₃}⁺ cation.