Full P4 to P3− Reduction with a Redox‐Active Metal Crown Complex

TL;DR

A new method uses a metal crown complex to fully reduce P4 to P3− at room temperature, enabling the stable formation and reactivity of the P3− anion.

Contribution

The paper introduces a selective room-temperature reduction of P4 to P3− using a redox-active metal crown complex, enabling the isolation of a unique phosphido anion.

Findings

The P4 reduction to P3− is exothermic and results in a stable s-block metal complex.

The P3− anion is stabilized by a two-armed pincer-like ligand with highly fluxional ion exchange.

The P3− anion exhibits reactivity as a base, nucleophile, and 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: Pm
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.

Breaking the crown: Reduction of P4 generally results in larger polyphosphide Zintl anions: Pm
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.

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