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Peter Höhn
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Scientific Report

Nitrides, Nitridometalates, and Related Compounds

The investigation of structures and properties of compounds containing nitrogen like nitrides, nitridometallates, cyanamides, and cyanides is still a fast-growing area of solid state science, although great progress has been made in recent years. In contrast to the vast numbers of oxo-, thio-, and higher pnictide compounds, there is still only limited knowledge about the related nitrides [1]. 

This may be attributed to the two following reasons:

  1. due to the high bonding energy of molecular nitrogen (941 kJ/mol) and the unfavorable electron affinity (N –> N3– + 2300 kJ/mol), nitrides are inherently thermodynamically less stable than comparable oxides and
  2. the synthesis of many if not most nitrides and nitridometalates is experimentally challenging. Many starting materials and reaction products are sensitive towards air and moisture, the temperature ranges of phase formation are quite small, and especially in low-valency systems the N2 partial pressure in the reaction chamber plays an important part in phase formation.

Nitridometalates of d metals M represent an interesting class of solid state phases, which feature isolated complex anions or low-dimensional anionic frameworks [MxNy]n– with M coordination numbers by nitrogen between two and four. Whereas the bonding within these complex anions and frameworks is essentially covalent, the structures are stabilized by predominantly ionic bonding through counterions like alkali (A) or alkaline-earth (AE) metal cations. Structural data for the majority of phases reported [1] were derived from X-ray single crystal data, whereas single phase powder samples had to be employed for physical properties investigations, since no suitably large single crystals were available until recently.

Besides the investigation of these new multinary systems, the preparation of phase pure samples for physical properties measurements is a major goal of our research. The systematic development and refinement of new reaction paths besides the use of established methods like the “classical” solid state synthesis (binary nitride + metal + nitrogen) leads to innovative approaches like the synthesis via intermetallic precursors [2] and nitrogen or ammonia, low-temperature methods sharing soft chemistry characteristics [3] , spark plasma sintering (SPS), or crucible free methods like levitation melting at elevated pressures. Recently, crystal growth experiments in lithium melts led to impressive results (Figures 1, 2).

Figure 1. Crucible with millimeter-size single crystals of Li2[Li1–xMnxN]. Zoom Image
Figure 1. Crucible with millimeter-size single crystals of Li2[Li1–xMnxN].
Figure 2. Millimeter-size single crystals of Ba3[CrN3] grown from lithium melts. Zoom Image
Figure 2. Millimeter-size single crystals of Ba3[CrN3] grown from lithium melts.
Figure 3. The hierarchical construction of the crystal structure of Ba9Ca[Co2N3]3 (Ba: red, Ca: orange, Co: turquoise, N: green) is based on NCa6 octahedra (top left) which are connected by [Co2N3] units (top right) forming large cubes (center). The barium species (red spheres / red squares) complete the octahedral coordination of N, thereby forming a slightly distorted cubic primitive arrangement of Ba. The trigonal unit cell (black, bottom) generates three interpenetrating sets of equivalent cubes (red, green, blue).  Zoom Image

Figure 3. The hierarchical construction of the crystal structure of Ba9Ca[Co2N3]3 (Ba: red, Ca: orange, Co: turquoise, N: green) is based on NCa6 octahedra (top left) which are connected by [Co2N3] units (top right) forming large cubes (center). The barium species (red spheres / red squares) complete the octahedral coordination of N, thereby forming a slightly distorted cubic primitive arrangement of Ba. The trigonal unit cell (black, bottom) generates three interpenetrating sets of equivalent cubes (red, green, blue). 

Crystal chemical characteristics of nitridometalates, which are usually synthesized with excess nitrogen, are best classified by coordination number and oxidation state of the transition metal in the nitridometalate anion [MNx]n. High oxidation states and coordination numbers 4-6 are generally observed, if M corresponds to an early transition element, whereas late transition metals exhibit low valence states (0-1.17) and linear coordination by N. Our investigations into phase formation in the systems AE-M-N(-C) and into the stabilisation of low valency states of transition metals [1], especially in manganese, iron, cobalt, and nickel containing compounds, show a wide variety of new phases. One especially noteworthy highlight is the crystal structure of Ba9Ca[Co2N3]3, a complex defect variant of the perovskite type: Ba9(CaCo6O2)(N9O18) (Fig. 3).

Figure 4. The crystal structure of (Ca7N4)[Mx] [4]. (Ca7N4) matrix in polyhedral rendition; the charge assignment of Ca2+ (orange) and Ca1.5+ (red) is emphasized. [Mx] chains are resembled by black cylindrical rods within the channels of the matrix. Table: lattice parameters (space group Pbam) and occupation x for (Ca7N4)[Mx]. Zoom Image
Figure 4. The crystal structure of (Ca7N4)[Mx] [4]. (Ca7N4) matrix in polyhedral rendition; the charge assignment of Ca2+ (orange) and Ca1.5+ (red) is emphasized. [Mx] chains are resembled by black cylindrical rods within the channels of the matrix. Table: lattice parameters (space group Pbam) and occupation x for (Ca7N4)[Mx].

In reducing conditions binary nitrides do not react with certain metals to form nitridometalates, but form nitride metalides, which are composed of a matrix of a binary (sub-)nitride with the metal species occupying the interstitial voids available (Fig. 4).

The presence of carbon in the reaction mixtures is another topic of our research. Depending on the conditions during synthesis, in oxidizing N2 atmospheres nitridometalate-carbodiimides result. In these phases [MN2]5– dumb-bells besides [CN2]2– ions are observed, and these complex ions may be either ordered [5], or they substitute each other on a common position [6]. In contrast, in “reducing” Ar atmospheres nitrido-cyano-metalates [7] are formed (Fig. 5).

Figure 5. Top: quaternary compounds in the system Sr-Ni-C-N [5-7]. In oxidizing atmospheres nitridometalate-carbodiimides result, whereas under reducing conditions, nitrido-cyano-nickelates(0) are formed. Bottom: The complex anions observed in these compounds contain different nitrogen species as emphasized by differently colored circles. Zoom Image

Figure 5. Top: quaternary compounds in the system Sr-Ni-C-N [5-7]. In oxidizing atmospheres nitridometalate-carbodiimides result, whereas under reducing conditions, nitrido-cyano-nickelates(0) are formed. Bottom: The complex anions observed in these compounds contain different nitrogen species as emphasized by differently colored circles.

Figure 6. Crystal structure of AE3[Co(CN)3] [8]. View along [001] (left) and interconnection of neighbouring AE9 polyhedra along [001] (right). Zoom Image

Figure 6. Crystal structure of AE3[Co(CN)3] [8]. View along [001] (left) and interconnection of neighbouring AE9 polyhedra along [001] (right).

Upon further reduction of the nitrogen content, highly reduced metalates (e. g. Ba3[Co(CN)3] [8]) are formed (Fig. 6). A consistent picture of these unusual highly reduced compounds is based on the presence of complex anions [M(CN)3]6– (M = Co, Ru, Ir) with a closed-shell configuration for M1– (d10, as determined from XPS data) and significantly reduced CN1.67– ligands (consistent with QTAIM effective charges), which are unprecedented in inorganic chemistry.

 

Figure 7. Mesomeric forms of [Co(CN)3]6– [8] Zoom Image

Figure 7. Mesomeric forms of [Co(CN)3]6– [8]

Transition metals in negative oxidation states are a common occurrence in carbonyl metalates, whereas related metalates containing CN-ligands were hitherto unknown. An appropriate basic description implies the stabilization of the complex anion by mesomeric forms [Co1–(CN)3–(CN)2]6– (Fig. 7), including the (CN)3– ligand, which looses its innocence with respect to the length of the CN-bond and corresponds to a 12 e system, isoelectronic with the O2 molecule, and called a percyano-group. With this, the complex anion can be described as a monopercyano-dicyano-cobaltate (1–). Formally, [Co(CN)3]6–, as well as [Co(CO)3]3– and [Co(NO)3], represent exceptionally rare examples of 3-coordinate, 18-electron systems which are isoelectronic to Krypton.

As a selective probe to distinguish different nitrogen species in nitrides and nitridometalates, to a certain extent IR and Raman spectroscopy may be employed. 15N enriched samples are best investigated by 15N-MAS-NMR-spectroscopy; quantitative analysis of N is achieved via LA-ICP-MS [9]. Our investigations in binary reference compounds for this method of analysis led to the synthesis of a new metastable modification of Ca3N2 (Fig. 8).

Figure 8. Left: Crystal structure of ß-Ca3N2 [3]. Unit cell and coordination spheres (octahedra) around N are emphasized. Right: Reaction conditions for the synthesis of binary nitrides from the elements and transitions between the binary phases in the system Ca-N. Zoom Image

Figure 8. Left: Crystal structure of ß-Ca3N2 [3]. Unit cell and coordination spheres (octahedra) around N are emphasized. Right: Reaction conditions for the synthesis of binary nitrides from the elements and transitions between the binary phases in the system Ca-N.

The crystal structures of nitrides and nitridometalates are commonly solved and refined via X-ray diffraction experiments on single crystals or powders; information on oxidation states and bonding conditions are deduced from X-ray absorption spectroscopic (XAS) investigations and corresponding calculations (ELI-D, QTAIM). Even today the physical properties of most ternary nitrides and nitridometalates are only insufficiently investigated; data already analyzed support the hope that in the near future new and interesting optical, electric, electronic, and magnetic properties in these compounds may be discovered.

References:

[1] 2.06 – Low-Valency Nitridometalates, R. Kniep, P. Höhn, Reference Module in Chemistry, Molecular Sciences and Chemical Engineering, Comprehensive Inorganic Chemistry II (Second Edition), Volume 2 (2013) 137-160.

[2] Sr2Ni3 – A Strontium Subnickelide? P. Höhn, S. Agrestini, A. Baranov, Alexey, et al., Chem. Eur. J. 17 (2011) 3347-3351.

[3] ß-Ca3N2, a Metastable Nitride in the System Ca-N, P. Höhn, S. Hoffmann, J. Hunger, S. Leoni, F. Nitsche, W. Schnelle, R. Kniep, Chem. Eur. J. 15 (2009) 3419-3425.

[4] (Ca7N4)[Mx] (M = Ag, Ga, In, Tl): Linear metal chains as guests in a subnitride host, P. Höhn, G. Auffermann, R. Ramlau, H. Rosner, W. Schnelle, R. Kniep, Angew. Chem. Int. Ed. 45 (2006) 6681-6685.

[5] (Sr6N)[CoN2][CN2](2): The first low-valency nitridometalate carbodiimide, J. K. Bendyna, P. Höhn, W. Schnelle, R. Kniep, Sci. Tech. Adv. Mat. 8 (2007) 393-398.

[6] New nitridocobaltates(I): Crystal structures and physical properties, J. K. Bendyna, P. Höhn, A. Ormeci,W. Schnelle, R. Kniep, J. Alloys Compd. 480 (2009) 138–140

[7] Low valency nickelates in the system Sr-Ni-C-N, P. Höhn, J. K. Bendyna, F. Nitsche, W. Schnelle, R. Kniep, Book of Abstracts, p. 296, SCTE 2008, 16th International Conference on Solid Compounds of Transition Elements, Dresden, Germany, 2008; Ba2[(NC)NiN]: Ein Cyano-Nitrido-Nickelat(0), P. Höhn, R. Kniep, Z. anorg. allg. Chem. 636 (2010)2104.

[8] Sr3[Co(CN)3] and Ba3[Co(CN)3]: Crystal Structure, Chemical Bonding, and Conceptional Considerations of Highly Reduced Metalates, P. Höhn, F. Jach, B. Karabiyik, et al., Angew. Chem. Int. Ed. 50 (2011) 9361-9364.

[9] Quantitative determination of nitrogen by LA-ICP-MS using N-15 enriched binary calcium nitrides, M. Böhme, P. Höhn, D. Günther, et al., J. Anal. At. Spectrom. 25 (2010) 856-860.

Cooperations

F. Haarmann, RWTH Aachen

Investigation of the environment of N in nitrides and nitridometalates employing 15N enriched samples and 15N-MAS-NMR-spectroscopy

M. Somer, Koç University, Istanbul, Turkey

Structure and properties of Azideometalates, Nitridoborates, and Amide-Borohydrides

 
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