Full P₄ to P^3- Reduction with a Redox-Active Metal Crown Complex

Traditional bulk syntheses of phosphorus compounds start with P₄ to PCl₃ oxidation but more sustainable methods cleave P─P bonds reductively. This generally results in larger polyphosphide Zintl anions: P_m ⁿ-. We report a relatively selective full reduction of P₄ at room temperature to give a unique hydrocarbon-soluble s-block metal complex of the P³- anion. Key to this chemistry is a recently reported redox-active metal crown complex: (BDI*)MgNa₃N″₂ (VI); N″ = N(SiMe₃)₂ and BDI* = HC[(tBu)C═N-DIPeP]₂, DIPeP = 2,6-CHEt₂-phenyl. The reduction of P₄ according to 2 VI + 0.25 P₄ → (BDI*)MgNa₅N″₃P (1) + 0.5 [(BDI*)Mg]₂ + 0.33 (NaN″)₃ is calculated to be exothermic (ΔH = -40.5 kcal mol^-1). The crystal structure of 1 shows a strongly bound (BDI*)MgP²- anion with two chelating [Na-N″-Na⁺] and [Na-N″-Na-N″-Na⁺] arms of unequal length. Although these arms are highly fluxional and rapidly exchange ions, they effectively stabilize the P³- anion. DFT calculations confirm the highly ionic nature of the complex and describe P³- as full valence-shell anion with four lone-pairs of electrons. Reactivity studies show that the P³- anion can react as a triple Brønsted base, a three-fold nucleophile or as a reducing agent.