Breaking the crown: Reduction of P₄ generally results in larger polyphosphide Zintl anions: P_m ⁿˉ. However, using a hydrocarbon-soluble Mg⁰ complex, P₄ can be fully reduced to mononuclear P³ˉ. The metal crown opens to a two-armed pincer-like ligand that efficiently stabilizes the phosphido anion. Latter P³ˉ anion reacts as a base, a three-fold nucleophile or a reducing agent.

Abstract

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 ⁿˉ. We report a relatively selective full reduction of P₄ at room temperature to give a unique hydrocarbon-soluble s-block metal complex of the P³ˉ 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*)MgP²ˉ 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 P³ˉ anion. DFT calculations confirm the highly ionic nature of the complex and describe P³ˉ as full valence-shell anion with four lone-pairs of electrons. Reactivity studies show that the P³ˉ anion can react as a triple Brønsted base, a three-fold nucleophile or as a reducing agent.