ESRF beamlines uncover new class of carbon-nitrogen anions

Inorganic ternary metal-C-N compounds with covalently bonded C-N anions are a significant class of solids with diverse applications. Among these compounds, cyanides (CN^-) and carbodiimides (CN₂^2-) are extensively studied and used in various fields. While these are well-known, the next member of the series – the CN₃^5- anion, a completely deprotonated guanidine molecule – has remained elusive, despite numerous attempts to synthesise it using strong bases to deprotonate guanidine molecules. So far, only partially deprotonated guanidine has been successfully stabilised.

In a recent breakthrough, research teams from the University of Bayreuth, the University of Cologne and Goethe University Frankfurt in Germany have successfully synthesised the first representatives of the guanidinates (SbCN₃) and oxo-guanidinates (a series of Ln₃O₂(CN₃) compounds, where Ln = La, Eu, Gd, Tb, Ho, and Yb). These compounds contain [CN₃]^5- anions, which are analogous to the well-known carbonate anions [CO₃]^2- but with each oxygen atom replaced by a nitrogen atom. The researchers successfully stabilised the CN₃^5- guanidinate anion in the compounds via solid-state synthesis under extreme conditions in laser-heated diamond anvil cells.

Utilising laser-heated diamond anvil cells, the researchers subjected the samples to extreme conditions of high pressure (up to half a million atmospheres) and high temperature (up to three thousand degrees Celsius). Several synthetic pathways worked. SbCN₃ could be synthesised from its constituent elements – antimony (Sb) and nitrogen (N₂) – by loading them into a diamond anvil cell, and diamond itself providing the carbon source; alternatively, cyanuric triazide, C₃N₁₂, could be the source of both the carbon and nitrogen. Similarly, Ln₃O₂(CN₃) could be synthesised either from its constituent elements or from azides, such as Eu(N₃)₂ or Yb(N₃)₂.

Fig. 1: Crystal structures of La₃O₂(CN₃) and SbCN₃.

Upon successfully synthesising the compounds, the researchers employed the ESRF beamlines ID27 and ID15B to conduct pressure-dependent single-crystal X-ray diffraction to elucidate their crystal structures and properties (Figure 1). Remarkably, SbCN₃ crystallises in a simple calcite (CaCO₃) structure type, demonstrating a direct analogy to well-known carbonates. Ln₃O₂(CN₃) possess more complex structures and do not have direct structural analogues. The researchers also performed experiments at two other synchrotrons, Petra III in Hamburg, Germany, and the Advanced Photon Source (APS) in Chicago, USA. Additionally, they used X-ray absorption near-edge structure (XANES) spectroscopy at the ESRF’s ID12 beamline to determine the oxidation state of La, +3, which is in excellent agreement with the crystal chemical analysis.

Despite their high-pressure synthesis, the compounds remained stable at ambient conditions, which offers potential for further upscaling and applications. One of the seven discovered novel compounds, SbCN₃, exhibits direct band-gap semiconductor properties, making it a promising candidate for optical devices. Furthermore, this novel synthesis approach paves the way for the development of other guanidinates and ortho-nitridocarbonates with promising properties, such as enhanced photochemical water-splitting and non-linear optical devices. The discovery of this new C-N anion could even hold implications for planetary science, as compounds containing such an anion may exist within the interiors of exoplanets.

Principal publications and authors
Stabilization of the CN₃^5- anion in recoverable high-pressure Ln₃O₂(CN₃) (Ln = La, Eu, Gd, Tb, Ho, Yb) oxoguanidinates, A. Aslandukov (a), P.L. Jurzick (b), M. Bykov (b,c), A. Aslandukova (a), A. Chanyshev (a), D. Laniel (d), Y. Yin (a), F.I. Akbar (a), S. Khandarkhaeva (a), T. Fedotenko (d), K. Glazirin (d), S. Chariton (e), V. Prakapenka (e), F. Wilhelm (f), A. Rogalev (f), D. Comboni (f), M. Hanfland (f), N. Dubrovinskaia (a), L. Dubrovinsky (a), Angew. Chem. Int. Ed. 62, 47, e202311516 (2023); https://doi.org/10.1002/anie.202311516

Stabilization of Guanidinate Anions [CN₃]^5- in Calcite-Type SbCN₃, L. Brüning (b), N. Jena (g), E. Bykova (c), P.L. Jurzick (b), N.T. Flosbach (b), M. Mezouar (f), M. Hanfland (f), N. Giordano (d), T. Fedotenko (d), B. Winkler (c), I.A. Abrikosov (g), M. Bykov (b,c), Angew. Chem. Int. Ed. 62, 47, e202311519 (2023); https://doi.org/10.1002/anie.202311519

(a) University of Bayreuth, Bayreuth (Germany)
(b) University of Cologne, Cologne (Germany)
(c) Goethe University Frankfurt, Frankfurt (Germany)
(d) DESY, Hamburg (Germany)
(e) University of Chicago, Chicago, Germany
(f) ESRF
(g) Linköping University, Linköping (Sweden)