Cyclo-P₅ ^- Revisited: The Surprisingly Stable Uncoordinated Pentaphospholide Anion.

Area of Science:

• Solid-state inorganic chemistry
• Main group element chemistry
• Coordination chemistry

Background:

• The cyclo-P5 anion, known spectroscopically since the 1980s, was previously only handled in solution.
• Alkali metal cations typically require strong coordination, limiting the isolation of reactive anions.
• The [2.2.2]cryptand ligand is known for its ability to encapsulate alkali metal cations.

Purpose of the Study:

• To synthesize and isolate the cyclo-P5 anion in a stable, solid form.
• To characterize the structure and properties of the uncoordinated cyclo-P5 anion.
• To explore the reactivity of the stabilized cyclo-P5 anion in forming new compounds.

Main Methods:

• Reaction of alkali metal heptaphosphides (M3P7, M=Na, K) with [2.2.2]cryptand.
• Single-crystal X-ray diffraction for crystallographic characterization.
• UV/Vis spectroscopy, Raman spectroscopy, and 31P MAS NMR spectroscopy for characterization.

Main Results:

• Formation of stable solid salts [M([2.2.2]cryptand)][cyclo-P5] (M=Na, K).
• First crystallographic characterization of the uncoordinated cyclo-P5 anion, revealing planar D5h symmetry.
• Synthesis of a ferrocene derivative, [Cp*Fe(cyclo-P5)], demonstrating the anion's utility.

Conclusions:

• Complexation with [2.2.2]cryptand enables the isolation and solid-state handling of the cyclo-P5 anion.
• The cyclo-P5 anion exhibits planar D5h symmetry in its uncoordinated state.
• This work opens new avenues for exploring phosphorus-based inorganic materials and organometallic compounds.