Full P4 to P3- 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: Pm 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.