Chemical Science · 2019 · 38 citations · 35 references
Reactions of three magnesium(i) dimers, [{(<sup>Ar</sup>Nacnac)Mg-}<sub>2</sub>] (<sup>Ar</sup>Nacnac = [(ArNCMe)<sub>2</sub>CH]<sup>-</sup>; Ar = xylyl (Xyl), mesityl (Mes) or 2,6-diethylphenyl (Dep)), with either 1,1-diphenylethylene (DPE), α-methylstyrene (MS), <i>trans</i>-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, [{(<sup>Xyl</sup>Nacnac)Mg}<sub>2</sub>(μ-DPE)], [{(<sup>Dep</sup>Nacnac)Mg}<sub>2</sub>(μ-MS)], [{(<sup>Ar</sup>Nacnac)Mg}<sub>2</sub>(μ-TS)] (Ar = Mes or Dep); and a 1,2-dimagnesioethene, [{(<sup>Mes</sup>Nacnac)Mg}<sub>2</sub>(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. [{(<sup>Mes</sup>Nacnac)Mg}<sub>2</sub>(μ-DPE)] and [{(<sup>Mes</sup>Nacnac)Mg}<sub>2</sub>(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)<sub>2</sub>(μ-DPE)] (L* = -N(Ar*)(SiPr<sup>i</sup> <sub>3</sub>); Ar* = C<sub>6</sub>H<sub>2</sub>Me{C(H)Ph<sub>2</sub>}<sub>2</sub>-4,2,6), and a 1,2-dizincioethene, [(<sup>TBo</sup>LZn)<sub>2</sub>(μ-DPA)] (<sup>TBo</sup>L = -N(SiMe<sub>3</sub>){B(DipNCH)<sub>2</sub>}, Dip = 2,6-diisopropylphenyl), respectively. Divergent reactivity is shown for [{(<sup>Mes</sup>Nacnac)Mg}<sub>2</sub>(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, [<sup>PhBo</sup>LCdCd<sup>PhBo</sup>L] (<sup>PhBo</sup>L = -N(SiPh<sub>3</sub>){B(DipNCH)<sub>2</sub>}) and [L<sup>†</sup>MgMgL<sup>†</sup>] (L<sup>†</sup> = -N(Ar<sup>†</sup>)(SiMe<sub>3</sub>); Ar<sup>†</sup> = C<sub>6</sub>H<sub>2</sub>Pr<sup>i</sup>{C(H)Ph<sub>2</sub>}<sub>2</sub>-4,2,6), respectively. A further class of reactivity for [{(<sup>Mes</sup>Nacnac)Mg}<sub>2</sub>(μ-DPE)] derives from its reaction with the bulky amido-germanium chloride, L*GeCl, which gives a magnesio-germane, presumably <i>via</i> intramolecular C-H activation of a highly reactive magnesiogermylene intermediate, [:Ge(L*){Mg(<sup>Mes</sup>Nacnac)}]. [{(<sup>Mes</sup>Nacnac)Mg}<sub>2</sub>(μ-DPE)] can be considered as acting as a two-electron reducing, magnesium transfer reagent in this reaction.
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