A Ferromagnetic Carbodiimide: Cr₂(NCN)₃

Abstract

Extended pseudochalcogenides such as the solid-state carbodiimides (incorporating complex À N = C = N À units with D 1h symmetry) or cyanamides (with a less symmetrical N C À N 2À unit) have been extensively investigated, and a large number including alkali [1] and alkaline-earth metals, [2] main-group elements, [3] d 10 transition metals, [4] and also rare-earth metals [5] have been reported. The synthesis of the complete set of MNCN (M = Mn-Cu) phases with only partially filled 3d orbitals, however, looked far more difficult, because firstprinciples calculations [6] had predicted them to be unstable in terms of formation enthalpy DH f and Gibbs formation energy DG f , with the instability continuously rising from MnNCN to CuNCN, thereby mirroring the gradual filling of antibonding levels from 3d 5 to 3d 9 . Nonetheless, we have succeeded in finding new routes to synthesize the MNCN series of the divalent 3d metals as a prerequisite to determine their crystal structures. MnNCN, the first carbodiimide of a magnetic transition metal ever realized, is made by a metathesis around 600 8C involving ZnNCN and MnCl 2 [7] but this method is unsuitable for the later, more unstable compounds. FeNCN, [8] CoNCN, and NiNCN [9] are synthesized, instead, by a two-step route via the corresponding hydrogencyanamides M(HNCN) 2 at about 400 8C.[10] Finally, the most delicate carbodiimide, CuNCN, is obtained by the oxidation of a copper(I) cyanamide precursor under aqueous conditions at room temperature. [11] The magnetic properties of the carbodiimides involving divalent transition metals are similar to those of the isolobal oxides, in particular by indicating antiferromagnetic interactions. For example, the magnetic structure of MnNCN based on spin-polarized neutron diffraction reveals frustration between the high-spin (S = 5/2) Mn 2+ ions as a function of temperature [12] but also shows that the carbodiimide unit ensures a strong magnetic communication. Likewise, experimental as well as theoretical studies on the magnetic structure of CuNCN have shown a fascinating interplay between geometrical packing and exchange couplings. [13] Finally, UV/Vis measurements on MnNCN [14] stress the importance of the more covalent MnÀN bond and higher ligand-field splitting compared to Mn-O chromophores but a smaller nephelauxetic ratio. Also, the Mn À N bond lacks significant p interaction.In light of this information, the design of a carbodiimide that is not antiferromagnetic requires an alternative composition, for example, by moving towards a different oxidation state. The hypothetical chromium(III) carbodiimide, Cr 2 -(NCN) 3 , is such a synthetic target although its isolobal oxide Cr 2 O 3 is, in fact, antiferromagnetic. Because the electron configuration (3d 3 ) is even lower than in the MnNCN case, theory [6] implies that the synthetic route for MnNCN should also be applicable for Cr 2 (NCN) 3 . In addition, there is related information with respect to existing rare-earth metal(III) carbodiimides. Recent research on such Ln 2 (NCN) 3 ...

Reversible High Capacity and Reaction Mechanism of Cr2(NCN)3 Negative Electrodes for Li‐Ion Batteries

“…The refinement (Figure ) of the X‐ray diffraction (XRD) pattern of pristine Cr 2 (NCN) 3 powder confirms that it crystallizes in the rhombohedral space group R m , in agreement with Tang et al The crystal packing of alternating layers of cations and anions along the c ‐axis is similar to that of other divalent transition‐metal carbodiimides. Because of the larger cationic charge, however, the packing of the Cr layers is less dense than in the latter carbodiimides, with each Cr atom surrounded by only three Cr neighbors.…”

“…The bending contributions are visible at 633 and 695 cm −1 . As reported previously and as expected from the D ∞ h point‐group symmetry of the N═C═N 2− carbodiimide moiety, its symmetric stretching is generally inactive in IR, and active only in Raman spectroscopy . In the case of the N≡C—N 2− cyanamide unit, conversely, such vibrations are active in IR‐providing bands also in the range 1200–1400 cm −1 but they are absent in this case and rule out the presence of such ligands.…”

“…The Fourier transform infrared spectroscopy (FT‐IR) and Raman spectra are shown in Figure a. The FT‐IR results are comparable with those of Tang et al, with the asymmetric stretching modes of the (NCN) 2− units visible as a strong peak at 2003 cm −1 and a broad peak between 2050 and 2150 cm −1 . The bending contributions are visible at 633 and 695 cm −1 .…”

“…To understand the reaction pathway during a discharge process, one needs to conduct a thorough survey of the possible intermediate compounds, that is, Li–Cr–NCN ternaries. Here, four structural bases were adopted, i.e., Cr 2 (NCN) 3 , Li 2 NCN, CrNCN, and LiCr(NCN) 2 , as shown in Figure SI‐2, Supporting Information. A 3 × 3 × 1 supercell was used for Li 2 NCN (or CrNCN) and 2 × 1 × 1 for LiCr(NCN) 2 .…”

Synthesis, Structure, and Properties of SrC(NH)3, a Nitrogen‐Based Carbonate Analogue with the Trinacria Motif

“…[5] N-based inorganic compounds of the second generation may be classified as those containing an additional non-metal atom, for example,t he nitridoborates, [6] oxidonitrides, [7] or carbodiimides, [8] all of which gave rise to novel findings.F ollowing our own contributions concerning (correlated) 3d carbodiimides with an NCN 2À unit, our interest has broadened to include at hird class of complex N-based compounds which relate to the biomolecule guanidine, CN 3 H 5 ,and which all exhibit atriangular motif with acentral CN 3 core.…”