Is Hexachloro-cyclo-triphosphazene Aromatic? Evidence from Experimental Charge Density Analysis

Area of Science:

• Inorganic Chemistry
• Quantum Chemistry
• Solid-State Chemistry

Background:

• Cyclophosphazenes are inorganic ring systems with unique electronic properties.
• Understanding electron delocalization is key to predicting their reactivity and applications.
• Previous studies suggested pseudoaromaticity in phosphazene rings.

Purpose of the Study:

• To experimentally determine the origin of electron delocalization in hexachlorocyclotriphosphazene (P3N3).
• To compare experimental findings with theoretical density functional theory (DFT) calculations.
• To investigate the electronic structure of octachlorocyclotetraphosphazene (P4N4).

Main Methods:

• Experimental charge density studies using X-ray diffraction.
• Analysis of electron density distribution and source functions.
• Comparison with DFT calculations in solid and gas phases.

Main Results:

• P-N bonds in phosphazene rings are highly polarized.
• The P3N3 and P4N4 rings exhibit a modular structure, separable into Cl2PN units.
• Experimental evidence confirms negative hyperconjugation involving nitrogen and chlorine electrons.
• Pseudoaromatic delocalization is ruled out in the P3N3 ring.

Conclusions:

• The electronic structure of cyclophosphazenes is best described by modular units and negative hyperconjugation.
• The concept of pseudoaromaticity is not applicable to the studied phosphazene systems.
• Experimental charge density provides unambiguous insights into bonding in inorganic ring systems.