Reduction of tert-butylphosphaalkyne and trimethylsilylnitrile with magnesium(i) dimers

We report on the reactivity of magnesium(I) dimers, [Mg(nacnac)]2 (nacnac = HC[C(Me)N(2,6-iPr2C6H3)]2 ([DippLMg]2) and HC[C(Me)N(2,4,6-Me3C6H2)]2 ([MesLMg]2)), towards the phosphaalkyne tBuCP. The steric profile of the magnesium(I) dimer results in selectivity for different products. The larger diisopropylphenyl derivative yields exclusively the monomeric dimagnesiated phosphaalkene [DippLMg]P[double bond, length as m-dash]C(tBu)([DippLMg]) (1), while the mesityl derivative facilitates reductive coupling of two phosphaalkyne equivalents to give access to the 1,3-diphosphacyclobutadienediide [MesLMg]2[(tBu)2C2P2](2). The reactivity differs in coordinating solvents such as THF, which allowed for the observation of C–P coupled products. For sake of comparison, reactions of magnesium(I) compounds with Me3SiCN were carried out. In contrast to the reactions involving tBuCP, these afforded 1,3-diazabutadienediyl complexes via reductive coupling and silyl migration processes.