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 (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), α-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)(μ-DPE)], [{((Dep)Nacnac)Mg}(2)(μ-MS)], [{((Ar)Nacnac)Mg}(2)(μ-TS)] (Ar = Mes or Dep); and a 1,2-dimagnesioethene, [{((Mes)Nacnac)Mg}(2)(μ-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)(μ-DPE)] and [{((Mes)Nacnac)Mg}(2)(μ-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)(μ-DPE)] (L* = –N(Ar*)(SiPr(i)(3)); Ar* = C(6)H(2)Me{C(H)Ph(2)}(2)-4,2,6), and a 1,2-dizincioethene, [((TBo)LZn)(2)(μ-DPA)] ((TBo)L = –N(SiMe(3)){B(DipNCH)(2)}, Dip = 2,6-diisopropylphenyl), respectively. Divergent reactivity is shown for [{((Mes)Nacnac)Mg}(2)(μ-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, [(PhBo)LCdCd(PhBo)L] ((PhBo)L = –N(SiPh(3)){B(DipNCH)(2)}) and [L(†)MgMgL(†)] (L(†) = –N(Ar(†))(SiMe(3)); Ar(†) = C(6)H(2)Pr(i){C(H)Ph(2)}(2)-4,2,6), respectively. A further class of reactivity for [{((Mes)Nacnac)Mg}(2)(μ-DPE)] derives from its reaction with the bulky amido-germanium chloride, L*GeCl, 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)(μ-DPE)] can be considered as acting as a two-electron reducing, magnesium transfer reagent in this reaction.