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- i Pr 2 C 6 H 3 )] 2 ([ Dipp LMg] 2 ) and HC[C(Me)N(2,4,6-Me 3 C 6 H 2 )] 2 ([ Mes LMg] 2 )), towards the phosphaalkyne t BuCP. The steric profile of the magnesium(I) dimer results in selectivity for different products. The larger diisopropylphenyl derivative yields exclusively the monomeric dimagnesiated phosphaalkene [ Dipp LMg]PC( t Bu)([ Dipp LMg]) (1), while the mesityl derivative facilitates reductive coupling of two phosphaalkyne equivalents to give access to the 1,3-diphosphacyclobutadienediide [ Mes LMg] 2 [( t Bu) 2 C 2 P 2 ](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 Me 3 SiCN were carried out. In contrast to the reactions involving t BuCP, these afforded 1,3-diazabutadienediyl complexes via reductive coupling and silyl migration processes.