Full P to P Reduction with a Redox-Active Metal Crown Complex.

Traditional bulk syntheses of phosphorus compounds start with P to PCl oxidation but more sustainable methods cleave P─P bonds reductively. This generally results in larger polyphosphide Zintl anions: P -. We report a relatively selective full reduction of P at room temperature to give a unique hydrocarbon-soluble s-block metal complex of the P- anion. Key to this chemistry is a recently reported redox-active metal crown complex: (BDI*)MgNaN″ (VI); N″ = N(SiMe) and BDI* = HC[(tBu)C═N-DIPeP], DIPeP = 2,6-CHEt-phenyl. The reduction of P according to 2 VI + 0.25 P → (BDI*)MgNaN″P (1) + 0.5 [(BDI*)Mg] + 0.33 (NaN″) is calculated to be exothermic (ΔH = -40.5 kcal mol). The crystal structure of 1 shows a strongly bound (BDI*)MgP- 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 P- anion. DFT calculations confirm the highly ionic nature of the complex and describe P- as full valence-shell anion with four lone-pairs of electrons. Reactivity studies show that the P- anion can react as a triple Brønsted base, a three-fold nucleophile or as a reducing agent.