High-pressure synthesis of lead compounds featuring new carbon-nitrogen anion

The challenge

Carbon–nitrogen compounds play an essential role in many areas of chemistry and materials science. Their versatile bonding enables applications ranging from catalysis and energy storage to the synthesis of ultrahard materials. Among these, melamine – a small molecule based on a triazine ring (C₃N₃) with three amino groups (NH₂) – has long served as a model and precursor for nitrogen-rich solids such as graphitic carbon nitride, noted for its photocatalytic activity. Carbon−nitrogen species are also relevant to prebiotic chemistry, as their simple structures facilitate the synthesis of key biomolecules such as amino acids and nucleotides.

Despite this broad interest, a hydrogen-free derivative of melamine – composed solely of carbon and nitrogen atoms – had never been synthesised. Theoretical studies predicted that such a species, the [C₃N₆]⁶⁻ melaminate anion, could exist in combination with metallic cations [1]. These melaminate units were expected to confer promising photocatalytic and optoelectronic properties, providing strong motivation for experimental realisation. However, synthesising such a compound has long remained a challenge.

The experiment

This study investigated the use of high-pressure synthesis to stabilise new carbon–nitrogen frameworks. Lead and nitrogen-rich precursors were compressed and laser-heated in diamond anvil cells, generating the extreme conditions – pressures up to 48 GPa and temperatures above 2000 K – required to drive the formation of new chemical bonding configurations.

The resulting samples were analysed by X-ray diffraction at the ID11 and ID15B beamlines, producing high-quality single-crystal diffraction data from submicron crystallites (< 1 µm³) under high-pressure conditions. These data enabled the determination and refinement of two previously unknown compounds – tP48–Pb₃(C₃N₆) and hP72–Pb₃(C₃N₆) – both sharing the same stoichiometry but differing in crystal structure (Figure 1). Complementary measurements were carried out at the Petra III synchrotron in Hamburg.
 

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Fig. 1: Atomic structures of the two newly discovered Pb₃(C₃N6) polymorphs. (a) The tP48-Pb₃(C₃N₆) phase and (b) the hP72-Pb₃(C₃N₆) phase. The planar hydrogen-free [C₃N₆]⁶⁻ melaminate anions (brown = C, light blue = N) are coordinated by Pb²⁺ cations (grey).

Both phases contain the long-sought [C₃N₆]⁶⁻ anion (Figure 2) – the first experimental realisation of a hydrogen-free melaminate. In this anion, carbon and nitrogen atoms alternate around a fully conjugated triazine ring (C₃N₃), with nearly identical C–N bonds lengths and three terminal C-N bonds.
 

Click figure to enlarge

Fig. 2: (a) A representation of the deprotonated melaminate (C₃N₆)^6- anion found in both tP48-Pb₃(C₃N₆) and hP72- Pb₃(C₃N₆). (b) Resonance form of the deprotonated melaminate anion. 

The two polymorphs differ in the arrangements of these melaminate units: in tP48–Pb₃(C₃N₆), the anions exhibit an offset π-stacking pattern, whereas in hP72–Pb₃(C₃N₆) they adopt a chiral, helical stacking pattern – an architecture not previously observed in melamine-derived materials. Both structures are non-centrosymmetric and stable at ambient pressure and temperature.

Density functional theory calculations confirmed the experimental structures and provided further insights into their electronic and optical properties. Both Pb₃(C₃N₆) polymorphs are predicted to be direct, narrow-gap semiconductors, with calculated bandgaps around » 2 eV.

They also show strong ultraviolet absorption, suggesting potential optoelectronic applications. Their non-centrosymmetric nature could give rise to piezoelectric, pyroelectric, or nonlinear optical effects. Although practical applications remain distant, these findings open a new avenue in the search for exotic, stable, and electronically tunable materials formed under extreme conditions.

On a fundamental level, this discovery fills a missing link in the family of carbon–nitrogen anions. The formation of the fully deprotonated melaminate represents a crucial intermediate between small molecular anions such as cyanamide ([CN]⁻) or guanidinate ([CN₃]⁵⁻), and extended carbon–nitrogen frameworks [2,3]. This work also demonstrates how high-pressure, high-temperature synthesis, coupled with state-of-the-art synchrotron techniques, can reveal entirely new regions of chemical space. 

Principal publication
High-Pressure Synthesis of Two Pb₃(C₃N₆) Polymorphs Featuring Fully Deprotonated [C₃N₆]⁶⁻ Melaminate Anions, U. Ranieri et al., J. Am. Chem. Soc. 147, 35431-35437 (2025); https://pubs.acs.org/doi/10.1021/jacs.5c09198

References
[1] D. Chen et al., J. Am. Chem. Soc. 145, 6986-6993 (2023). 
[2] A. Aslandukov et al., J. Am. Chem. Soc. 146, 18161-18171 (2024). 
[3] L. Brüning et al., Angew. Chem. Int. Ed. 62, e202311519 (2023).