Acyclic 1,2-dimagnesioethanes/-ethene derived from magnesium(i) compounds: multipurpose reagents for organometallic synthesis

Reactions of three magnesium(I) dimers, [{((Ar)Nacnac)Mg-}(2)] ((Ar)Nacnac = [(ArNCMe)(2)CH](-); Ar = xylyl mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), alpha-methylstyrene (MS), trans-stilbene (TS) or diphenylacetylene (DPA) led to the 1,2 -addition of the Mg -Mg bond across the substrate, giving rise to the 1,2-dimagnesioethanes, [{((Xyl)Nacnac)Mg}(2)(mu-DPE)], [{((DeP)Nacnac)Mg}(2)(mu-MS)], [{((Ar)Nacnac)Mg}(2)(mu-TS)] (Ar = Mes or Dep); and a 1,2-dimagnesioethene, [{((Mes)Nacnac)Mg}(2)(mu-DPA)]. The reactions involving the 1,1 -substituted alkenes are shown to be readily redox reversible, in that the reaction products are in equilibrium with a significant proportion of the starting materials at room temperature. Variable temperature NMR spectroscopy and a van't Hoff analysis point to low kinetic barriers to these weakly exergonic reactions. [{((Mes)Nacnac)Mg}(2)(mu-DPE)] and [{((Mes)Nacnac)Mg}(2)(mu-DPA)] behave as 1,2-di-Grignard reagents in their reactions with very bulky amido-zinc bromides, yielding the first examples of a 1,2-dizincioethane, [(L*Zn)(2)(mu-DPE)] (L* = -N(Ar*)(SiPr3i); Ar* = C6H2Me{C(H)Ph-2}(2)-4,2,6), and a 1,2-dizincioethene, [((TBo)17n)(2)(mu-DPA)] (L-TBo = -N(SiMe3){B(DipNCH)(2)}, Dip = 2,6-diisopropylphenyl), respectively. Divergent reactivity is shown for [{((Mes)Nacnac)Mg}(2)(mu-DPE)], which behaves as a two -electron reducing agent when treated with amido-cadmium and amido-magnesium halide precursors, yielding the cadmium(i) and magnesium(i) dimers, [(LCdCdL)-L-PhBo-L-PhBo] (L-PhBo = -N(SiPh3) {B(DipNCH)(2)}) and [(LMgMgL dagger)-Mg-dagger] (L-dagger = -N(Ar-dagger)(SiMe3); Ar-dagger = C6H2Pri{C(H)Ph-2}(2)-4,2,6), respectively. A further class of reactivity for [{(mesNacnac)Mg}(2)(mu-DPE)] derives from its reaction with the bulky amido-germanium chloride, L*GeCI, which gives a magnesio-germane, presumably via intramolecular C-H activation of a highly reactive magnesiogermylene intermediate, [:Ge(L*){Mg((Mes)Nacnac)}]. [{((Mes)Nacnac) Mg}(2)(mu-DPE)] can be considered as acting as a two -electron reducing, magnesium transfer reagent in this reaction.