Researchers have synthesised two new lead carbonitrides under high pressure, each containing a fully deprotonated melaminate anion. Using X-ray diffraction at beamlines ID11 and ID15B, they determined the single-crystal structure from submicrometre-sized samples formed under extreme conditions. The resulting solids remain stable at ambient pressure, and their distinctive chemistry offers promising routes to new functional materials.
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 (CN) with three amino groups (NH) - 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 [CN] 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.
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 µm) under high-pressure conditions. These data enabled the determination and refinement of two previously unknown compounds - tP48-Pb(CN) and hP72-Pb(CN) - both sharing the same stoichiometry but differing in crystal structure (Figure 1). Complementary measurements were carried out at the Petra III synchrotron in Hamburg.
Both phases contain the long-sought [CN] 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 (CN), with nearly identical C-N bonds lengths and three terminal C-N bonds.
The two polymorphs differ in the arrangements of these melaminate units: in tP48-Pb(CN), the anions exhibit an offset π-stacking pattern, whereas in hP72-Pb(CN) 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 Pb(CN) 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 ([CN]), 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.