C-N and C-H Activation of an N-Heterocyclic Carbene by Magnesium(II) Hydride and Magnesium(I) Complexes.

Reactions of the hindered N-heterocyclic carbene, :C{(MesNCH)} (IMes; Mes = mesityl), with a series of β-diketiminatomagnesium(II) hydride and dimagnesium(I) complexes were carried out at 80 °C. The reactions involving the magnesium hydrides, [{(Nacnac)Mg(μ-H)}] [Nacnac = [(ArNCMe)CH], where Ar = 2,6-diethylphenyl (Dep) or 2,6-diisopropylphenyl (Dip)], proceeded via activation of an exocyclic C-N bond of IMes, giving magnesium imidazolyl compounds [(Nacnac)Mg(μ-H)(μ-Imid)Mg(Nacnac)] (Imid = [NCHN(Mes)C]) and mesitylene. A low-yield IMes methyl C-H activation product, [(Nacnac)Mg(IMes)], was also obtained, via H elimination, from the reaction between IMes and [{(Nacnac)Mg(μ-H)}]. Reactions between IMes and dimagnesium(I) compounds [{(Nacnac)Mg}] [Ar = 2,6-dimethylphenyl (Xyl) or Mes] afforded isostructural C-H activation products [(Nacnac)Mg(IMes)] but in higher yields. Density functional theory calculations suggest that the reactions do not progress via stable adduct complex intermediates, which are sterically inaccessible.