# Stabilization of the [C2N5]7– Anion in Recoverable High-Pressure Eu4Fe0.864(6)(C2N5)2 Pyronitridocarbonate

## TL;DR

Scientists created a new inorganic compound with unique nitrogen-rich anions using high-pressure synthesis and confirmed its structure with advanced techniques.

## Contribution

The first synthesis and characterization of the hydrogen-free pyronitridocarbonate Eu4Fe0.864(6)(C2N5)2 with novel [C2N5]7– anions.

## Key findings

- Eu4Fe0.864(6)(C2N5)2 was synthesized at 50(3) GPa and features [C2N5]7– anions.
- The compound is recoverable at near-ambient pressure and partially decomposes into [CN3]5– and [CN2]2– anions.
- Crystal structures were confirmed using synchrotron X-ray diffraction and DFT calculations.

## Abstract

Synthesis at extreme conditions enables access to nitrogen-rich
carbon–nitrogen anions that cannot be obtained at ambient conditions.
Here, through a direct reaction between Eu­(N3)2 and EuC2 with Fe in a laser-heated diamond anvil cell
(DAC) at 50(3) GPa, we synthesized the first inorganic hydrogen-free
pyronitridocarbonate, Eu4Fe
x
(C2N5)2, x = 0.864(6),
featuring novel highly charged [C2N5]7– anions, along with the first stoichiometric oxygen-free rare-earth
metal guanidinate Eu5(CN3)3. The
crystal structures of both compounds were determined via synchrotron
single-crystal X-ray diffraction (SCXRD) and fully corroborated by
density functional theory (DFT) calculations. Eu4Fe
x
(C2N5)2 was
found to be recoverable at pressures close to ambient. Keeping the
sample at ambient conditions for 1 day leads to splitting of half
of the [C2N5]7– units in Eu4Fe
x
(C2N5)2 into the guanidinate [CN3]5– and carbodiimide [CN2]2– anions. The
statistical analysis of the multigrain SCXRD data and DFT-based electronic
structure analysis well defined the chemical nature of the bonding
in [C2N5]7– and [CN3]5– anions. This study provides a clear synthetic
pathway to a new class of inorganic nitridocarbonates.