FeNCN and Fe(NCNH)2: synthesis, structure, and magnetic properties of a nitrogen-based pseudo-oxide and -hydroxide of divalent iron

The Mixed Transition-Metal Cyanamide MnCr2(NCN)4

The ternary transition-metal cyanamide MnCr2(NCN)4 was synthesized by a solid-state metathesis reaction between MnCl2, CrCl3, and ZnNCN. Powder X-ray diffraction reveals that MnCr2(NCN)4 adopts an orthorhombic [NiAs]-derived structure with Pbcn symmetry, featuring a hexagonally close-packed array of NCN2- with metal cations in 3/4 of the octahedral interstitial holes. The question of cation order was addressed via the combinatorial use of X-ray powder diffraction, neutron powder diffraction, electron diffraction, and HAADF-STEM measurements. These studies support an average structure with complete ordering of Mn2+ and Cr3+ in single and double corrugated rows of like cations, respectively. HAADF images, however, suggest locally a degree of cation disorder, especially at the manganese site. UV-vis and XANES measurements confirm the assignment of divalent manganese and trivalent chromium, whereas IR spectroscopy reveals the presence of cyanamide-type NCN2- moieties with a degree of single and triple bond character due to the asymmetric coordination that is distorted away from the regular trigonal prismatic coordination encountered in the CoNCN aristotype due to vacancy and cation order. Finally, SQUID magnetometry unearths predominantly antiferromagnetic interactions with a transition to what appears to be a ferrimagnetically ordered state below 160 K.

Synthesis, characterization and chemical bonding analysis of the quaternary cyanamides Li2MnHf2(NCN)6 and Li2MnZr2(NCN)6

Li2MnHf2(NCN)6 and Li2MnZr2(NCN)6 were prepared via solid-state metathesis reactions either via a more exothermic direct reaction between Li2NCN, MnCl2 and HfCl4 or a milder two-step reaction in which ternary Li2Zr(NCN)3 was first prepared and subsequently reacted with MnF2. Their crystal structures were determined from powder X-ray diffraction data and found to crystallize isotypically in low-symmetry variants of the [NiAs]-type MNCN structure with P3̄1m symmetry and comprise corundum-like [T2(NCN)3]2+ layers (T = Hf4+, Zr4+) alternating with [Li2Mn(NCN)3]2- layers. In-depth chemical bonding analysis was undertaken using LOBSTER to calculate the Löwdin charges which reveal significant differences in covalency between the two metal layers that is also reflected in the crystal orbital bond indices (COBI) of the metal-nitrogen bonds as well as the carbon-nitrogen bonds that show distinct single and triple bond character, which is also evident from infrared spectroscopy measurements. A geometric analysis of all known quaternary cyanamides with the general formula A2MT2(NCN)6 (A: an alkali metal, M: a divalent metal and T: a tetravalent metal) demonstrates the adaptation of the NCN unit to cation size differences, expressed as total distortion δtotal, by tilting away from the stacking axis. This tilting impacts the octahedral environment of the three different metal sites causing a distortion, quantified by means of the quadratic elongation λoct, revealing that the divalent and alkali metal sites are strongly dependent on δtotal whilst the tetravalent site is less influenced by the total distortion. Electronic structure calculations reveal Li2MnHf2(NCN)6 to have an indirect band gap with a wide band gap of approximately 2.4 eV, in good agreement with the measured value of 2.1 eV. Furthermore, SQUID magnetometry measurements reveal predominantly antiferromagnetic interactions, but no transition to a long-range ordered state, presumably as a result of the magnetic dilution of the octahedral site, in which only 1/6 of the interstices are occupied by paramagnetic cations.

Perovskite-Like Carbodiimides AB(NCN)3: Synthesis and Characterization of MnHf(NCN)3 and FeHf(NCN)3

Two novel ternary air-stable transition-metal carbodiimides, MnHf(NCN)3 and FeHf(NCN)3, were synthesized via solid-state metathesis using either ZnNCN or Na2NCN as the carbodiimide source and the corresponding binary metal chlorides. These two phases are the first examples of transition-metal carbodiimides with an AB(NCN)3 composition, akin to ubiquitous ABO3 perovskite oxides. The crystal structure of MnHf(NCN)3 was determined and refined from powder X-ray diffraction (XRD) data in the non-centrosymmetric space group P6322 allowing for chirality, the assignment of which is supported by second-harmonic generation (SHG) measurements. FeHf(NCN)3 was found to crystallize isotypically, and the presence of iron(II) in a high spin state was confirmed by 57Fe Mößbauer spectroscopy. The structures are revealed to be NiAs-derived and can be described as a hexagonal stack of NCN2- anions with metal cations occupying 2/3 of the octahedral voids. Both IR spectroscopic measurements and DFT calculations agree that the NCN2- unit is a bent carbodiimide with C2v symmetry, necessary to account for the size difference present in such a vacancy-ordered structure. Magnetic studies reveal predominantly strong antiferromagnetic interactions but no long-range order between the paramagnetic Mn2+ centers, likely due to the dilution of Mn2+ over the octahedral sites or perhaps even due to some degree of magnetic frustration. The optical and electrochemical properties of MnHf(NCN)3 were then studied, revealing a wide band gap of 3.04 eV and p-type behavior.

Electronic Structure and d-d Spectrum of Metal-Organic Frameworks with Transition-Metal Ions

The electronic structure of metal-organic frameworks (MOFs) containing transition metal (TM) ions represents a significant and largely unresolved computational challenge due to limited solutions to the quantitative description of low-energy excitations in open d-shells. These excitations underpin the magnetic and sensing properties of TM MOFs, including the observed remarkable spin-crossover phenomenon. We introduce the effective Hamiltonian of crystal field approach to study the d-d spectrum of MOFs containing TM ions; this is a hybrid QM/QM method based on the separation of crystal structure into d- and s,p-subsystems treated at different levels of theory. We test the method on model frameworks, carbodiimides, and hydrocyanamides and a series of M-MOF-74 (M = Fe, Co, Ni) and compare the computational predictions to experimental data on magnetic properties and Mössbauer spectra.

Synthesis and Characterization of Iron Bispyridine Bisdicyanamide, Fe[C5H5N]2[N(CN)2]2

Fe[C₅H₅N]₂[N(CN)₂]₂ (1) was synthesized from a reaction of stoichiometric amounts of NaN(CN)₂ and FeCl₂·4H₂O in a methanol/pyridine solution. Single-crystal and powder diffraction show that 1 crystallizes in the monoclinic space group I2/m (no. 12), different from Mn[C₅H₅N]₂[N(CN)₂]₂ (P2₁/c, no. 14) due to tilted pyridine rings, with a = 7.453(7) Å, b = 13.167(13) Å, c = 8.522(6) Å, β = 114.98(6)° and Z = 2. ATR-IR, AAS, and CHN measurements confirm the presence of dicyanamide and pyridine. Thermogravimetric analysis shows that π-stacking interactions of the pyridine rings play an important role in structural stabilization. Based on DFT-optimized structures, a chemical bonding analysis was performed using a local-orbital framework by projection from a plane-wave basis. The resulting bond orders and atomic charges are in good agreement with the expectations based on the structure analysis. SQUID magnetic susceptibility measurements show a high-spin state Fe^II compound with predominantly antiferromagnetic exchange interactions at lower temperatures.

NiAs-derived cyanamide (carbodiimide) structures – a group-theoretical view

The cyanamide and carbodiimide anions are complex nitrogen-derived one-dimensional species of the type NCN2− (hence, resembling O2− but more covalently bonding) that form a huge number of salt-like phases with a variety of metal cations stemming from the whole Periodic Table. Depending on the coloring (binary, ternary and quaternary salts are known), the cationic size and charge as well as covalent contributions, different distortion (tilting in particular) and/or vacancy ordering variants of cyanamides/carbodiimides occur. Herein we summarize those cyanamide/carbodiimide structures that derive from the aristotype NiAs. The crystal chemistry is discussed on the basis of group-subgroup schemes (Bärnighausen trees).

Controlled phase and crystallinity of FeNCN/NC dominating sodium storage performance

FeNCN is a potentially fast-charging sodium ion anode due to the presence of lots of broad tunnels and its high electronic conductivity. However, FeNCN has been rarely investigated due to its complicated synthetic process and unclear synthetic mechanism, which affect the precise control of its phase and crystallinity. In this work, phase- and crystallinity-controlled FeNCN polyhedrons grown on nitrogen-doped carbon (FeNCN/NC) are successfully fabricated by adjusting the growing time and temperature. Moreover, the synthesis mechanism is disclosed in this paper. High-crystallinity FeNCN grows along the [001] direction, which exposes sufficient broad channels on the {010} planes and significantly improves the diffusion rate of sodium ions. Moreover, high-crystallinity FeNCN exhibits higher mechanical strength, which reduces its pulverization rate and endows it with durable cycling stability. When applied as an anode in a sodium-ion battery, high-crystallinity FeNCN/NC exhibits a high rate capability of 332 mA h g^-1 at 5.0 A g^-1 and a stable cycling performance of 368 mA h g^-1 after 300 cycles at a high current density of 1.0 A g^-1. This work confirms that the sodium-ion storage performance of FeNCN can be further improved by tuning its crystallinity.

Metastable FeCN2@nitrogen-doped carbon with high pseudocapacitance as an anode material for sodium ion batteries

Pseudocapacitive materials are good candidates for fast charging anodes of sodium ion batteries (SIB). However, pseudocapacitive materials with a high surface area face the severe problem of low initial coulombic efficiency. In this work, micro-sized nitrogen-doped carbon (NC) coated and supported polyhedron FeCN₂ networks are designed and synthesized by a facile in situ gel-swelling technique. Impressively, FeCN₂@NC as an SIB anode exhibits excellent rate performance with highly reversible rate capacities of 466 and 303 mA h g^-1 at 0.2 and 10.0 A g^-1, respectively. Furthermore, the FeCN₂@NC anode shows a high initial coulombic efficiency (ICE) of 86% due to a low surface area. Electrochemical tests and density functional theory (DFT) calculation indicate that the metastable character enables the low intercalation/conversion reaction energy for FeCN₂ and further greatly promotes the fast pseudocapacitive storage mechanism for FeCN₂@NC. This work provides evidence that FeCN₂ is a new type of metastability induced pseudocapacitive material with high initial coulombic efficiency.

PbCN2 – an elucidation of its modifications and morphologies

Concerning the crystal structure of PbCN2 there exist two different descriptions in the literature, one based on the non-centrosymmetric structure, space group Pna21, another one on the centrosymmetric one in space group Pnma. To elucidate the conditions for their appearance, comprehensive preparative and structural investigations have been conducted which proved the existence of two distinct modifications of PbCN2. A detailed comparison of the two phases is provided. The growth conditions and crystallization processes of the two PbCN2 structures are reported with focus on the influence of the pH value on the products. Depending on the growth conditions several different morphologies arise, namely PbCN2 in needle-shaped and platelet-shaped crystals, as well as pompon-shaped and lance-shaped crystals.

First low-spin carbodiimide, Fe2(NCN)3, predicted from first-principles investigations

The structural stability and physical properties of the Fe(III) carbodiimide Fe2(NCN)3 were studied by use of density functional theory. The results indicate that Fe2(NCN)3 (space group R 3 ‾ c $R‾{3}c$ ) is stable both thermodynamically and mechanically. The electronic structure in combination with the phonon dispersion relations suggest that the title compound should be ferromagnetic and half-metallic, and that the Fe3+ ions are in the low-spin state.

Synthesis, Crystal Structure, Symmetry Relationships, and Electronic Structure of Bismuth Carbodiimide Bi2(NCN)3 and Its Ammonia Adduct Bi2(NCN)3·NH3

Bi₂(NCN)₃, the first binary pnictogen carbodiimide, and its ammonia derivative Bi₂(NCN)₃·NH₃ have been prepared via nonaqueous liquid-state low-temperature ammonolysis. The crystal structure of Bi₂(NCN)₃·NH₃ in space group Cc solved via single-crystal X-ray diffraction corresponds to a two-dimensional-like motif with layers of NCN^2- alternating with honeycomb-like layers of edge-sharing distorted BiN₆ octahedra, half of which are also coordinated by molecular ammonia occupying the octahedral holes. By contrast, Bi₂(NCN)₃ adopts a higher-symmetric C2/c structure with a single Bi position and stronger distortion but empty octahedral voids. In both cases, Bi³⁺ and its 6s² lone pair are well mirrored by antibonding Bi-N interactions below the Fermi level. Density functional theory calculations reveal an exothermic reaction for the intercalation of NH₃ into Bi₂(NCN)₃, consistent with the preferential formation of Bi₂(NCN)₃·NH₃ in the presence of ammonia. A Bärnighausen tree shows both compounds to be hettotypic derivatives of the R3̅c M₂(NCN)₃ corundum structure that express highly distorted hexagonal-close-packed layers of NCN^2- in order to accommodate the aspherical Bi³⁺ cations. Although Bi₂(NCN)₃ does not resemble the isovalent Bi₂Se₃ in forming two-dimensional layers and a topological insulator, theory suggests a driving force for the spontaneous formation of Bi₂Se₃/Bi₂(NCN)₃ sandwiches and a conducting surface state arising within the uppermost Bi₂(NCN)₃ layer.

Realizing Fast Charge Diffusion in Oriented Iron Carbodiimide Structure for High-Rate Sodium-Ion Storage Performance

Iron carbodiimide (FeNCN) belongs to a type of metal compounds with a more covalent bonding structure compared to common transition metal oxides. It could provide possibilities for various structural designs with improved charge-transfer kinetics in battery systems. Moreover, these possibilities are still highly expected for promoting enhancement in rate performance of sodium (Na)-ion battery. Herein, oriented FeNCN crystallites were grown on the carbon-based substrate with exposed {010} faces along the [001] direction (O-FeNCN/S). It provides a high Na-ion storage capacity with excellent rate capability (680 mAh g^-1 at 0.2 A g^-1 and 360 mAh g^-1 at 20 A g^-1), presenting rapid charge-transfer kinetics with high contribution of pseudocapacitance during a typical conversion reaction. This high rate performance is attributed to the oriented morphology of FeNCN crystallites. Its orientation along [001] maintains preferred Na-ion diffusion along the two directions in the entire morphology of O-FeNCN/S, supporting fast Na-ion storage kinetics during the charge/discharge process. This study could provide ideas toward the understanding of the rational structural design of metal carbodiimides for attaining high electrochemical performance in future.

Nanorod bundle-like silver cyanamide nanocrystals for the high-efficiency photocatalytic degradation of tetracycline

Silver cyanamide (Ag2NCN) is a type of functional semiconductor material with a visible-light response. Ag2NCN nanocrystals with nanorod bundle-like (RB) or straw bundle-like (SB) assemblies were successfully prepared, and it was found that the as-prepared Ag2NCN nanorod bundle (RB) samples had a narrower bandgap of 2.16 eV, which was lower than those reported. As a result, RB samples demonstrated a higher photocatalytic activity towards tetracycline (TC) degradation. The analyses of active species confirmed that both the photo-generated holes and ˙O2 - radicals of the RB sample played significant roles during the process of photocatalytic degradation of TC, and the holes were the main active species. These results indicated that effective charge separation could be achieved by adjusting the morphologies of Ag2NCN nanocrystals. This study provides a new approach to prepare Ag2NCN nanocrystals with a narrower bandgap and strong visible-light response towards antibiotic degradation.

Calculating and Characterizing the Charge Distributions in Solids

Accurate estimation of the partial atomic charges on metal centers is useful for understanding electronic and catalytic properties of materials. However, different methods of calculating these charges may give quite different results; this issue has been more widely studied for molecules than for solids. Here we study the charges on the metal centers of a test set of 18 solids containing transition metals by using density functional theory with several density functionals (PBE, PBE+U, TPSS, revTPSS, HLE17, revM06-L, B3LYP, B3LYP*, and other exchange-modified B3LYP functionals) and four charge models (Bader, Hirshfeld, CM5, and DDEC6). The test set contains 12 systems with nonmagnetic metal centers (eight metal oxides (MO₂), two metal sulfides (MS₂), and two metal selenides (MSe₂)) and six ferromagnetic transition metal complexes. Our study shows that, among the four types of charges, Bader charges are the highest and Hirshfeld charges are the lowest for all the systems, regardless of the functional being used. The CM5 charges are bigger than DDEC6 charges for MX₂ with M = Ti or Mo and X = S or Se, but for the other 14 cases they are lower. We found that the most of the systems are sensitive to the Hubbard U parameters in PBE+U and to the percentage X of Hartree-Fock exchange in exchange-modified B3LYP; as we increase U or X, the charges on the metal atoms in MX₂ increase steadily. Testing different density functionals shows charges calculated with higher Hubbard U parameters in PBE+U are comparable to B3LYP (with 20% Hartree-Fock exchange). Among four meta-GGA functionals studied, the charges with HLE17 have the closest agreement with B3LYP. The variation of charges with choice of charge model is greater than the variation with choice of density functional.

Quantum-Chemical Study of the FeNCN Conversion-Reaction Mechanism in Lithium- and Sodium-Ion Batteries

We report a computational study on 3d transition-metal (Cr, Mn, Fe, and Co) carbodiimides in Li- and Na-ion batteries. The obtained cell voltages semi-quantitatively fit the experiments, highlighting the practicality of PBE+U as an approach for modeling the conversion-reaction mechanism of the FeNCN archetype with lithium and sodium. Also, the calculated voltage profiles agree satisfactorily with experiment both for full (Li-ion battery) and partial (Na-ion battery) discharge, even though experimental atomistic knowledge is missing up to now. Moreover, we rationalize the structural preference of intermediate ternaries and their characteristic lowering in the voltage profile using chemical-bonding and Mulliken-charge analysis. The formation of such ternary intermediates for the lithiation of FeNCN and the contribution of at least one ternary intermediate is also confirmed experimentally. This theoretical approach, aided by experimental findings, supports the atomistic exploration of electrode materials governed by conversion reactions.

The Quasi‐Binary Acetonitriletriide Sr 3 [C 2 N] 2

The first quasi‐binary acetonitriletriide Sr₃[C₂N]₂ has been synthesised and characterised. The nearly colourless crystals were obtained from the reaction of Sr metal, graphite, and elemental N₂, generated by decomposition of Sr(N₃)₂, in a sealed Ni ampoule with the aid of an alkali metal flux. The structure of this compound was analysed via single‐crystal X‐ray diffraction and the identity of the [C₂N]³⁻ anion was confirmed by Raman spectroscopy and further investigated by quantum‐chemical methods. Computed interatomic distances within the [C₂N]³⁻ anion strikingly match the obtained experimental data.

Synthesis, Structure, and Electronic Properties of Sn9O5Cl4(CN2)2

The formation of the new compound Sn₉O₅Cl₄(CN₂)₂ is reported and placed in the context of several other recently discovered tin carbodiimide compounds (Sn(CN₂), Sn₂O(CN₂), and Sn₄Cl₂(CN₂)₃), all of which contain divalent tin. The crystal structure of Sn₉O₅Cl₄(CN₂)₂, as determined by X-ray powder diffraction, includes an unusual [Sn₈O₃] cluster, in which tin atoms form the motif of a hexagonal bipyramid. An additional tin atom and two oxygen atoms connect these clusters into chains. Mössbauer spectroscopy shows tin to predominantly adopt the +2 oxidation state, and electronic structure calculations predict Sn₉O₅Cl₄(CN₂)₂ to be a semiconductor.

A new tilt and an old twist on the nickel arsenide structure-type: synthesis and characterisation of the quaternary transition-metal cyanamides A2MnSn2(NCN)6 (A = Li and Na)

In this work, we describe the synthesis and structure of the quaternary transition-metal cyanamides Na2MnSn2(NCN)6 and Li2MnSn2(NCN)6. These phases crystallise isotypically in layered structures, with P3[combining macron]1m symmetry, that comprise hexagonal close-packed arrays of NCN2- anions with metal cations in 5/6 of the octahedral holes, thereby reflecting low-symmetry modifications of the hierarchical [NiAs]-type MNCN structure. The distinct coordination requirements of the metal cations template an ordered decoration across the octahedral sites with corundum-like [Sn2(NCN)3]2+ layers alternating with [A2Mn(NCN)3]2- layers which resemble a portion of the Li2Zr(NCN)3 structure. This motif is also mirrored in the form of the NCN2- anions which adopt N-C[triple bond, length as m-dash]N2- cyanamide shapes with clear single- and triple-bond character. Distortion-mode analysis reveals the importance of K1 octahedral twist and K2 cyanamide tilt displacements in stabilising these phases, the latter of which is only accessible because of the extended nature of the NCN2- anion. These are the first examples of non-binary transition-metal cyanamides to be discovered and this study highlights how the additional flexibility of the NCN2- anion affords a novel structure-type not observed in oxide chemistry.

Synthesis, Structure, and Electronic Properties of Sn(CN2) and Sn4Cl2(CN2)3

A solid-state metathesis reaction between equimolar amounts of Li₂(CN₂) and SnCl₂ revealed the formation of two new compounds, Sn₄Cl₂(CN₂)₃ and Sn(CN₂). Thermal analysis of this reaction indicated that Sn₄Cl₂(CN₂)₃ forms exothermically near 200 °C and subsequently transforms into Sn(CN₂) at higher temperatures. The crystal structures of both compounds are presented. According to optical measurements and band structure calculations, Sn(CN₂) can be considered as a semiconductor with a band gap on the order of 2 eV. The presence of Sn²⁺ ions in the structure of Sn(CN₂) with a toroidally shaped lone pair is indicated by electron localization function calculations. The structure of Sn(CN₂) is shown to be related to the structures of FeS₂ and CaC₂.

Chloride carbodiimide K12Pb51(CN2)30Cl54 with an unprecedented 45 Å unit cell axis and a large birefringence

K₁₂Pb₅₁(CN₂)₃₀Cl₅₄ exhibits a giant birefringence of 0.094 at 1064 nm and a large cell axis due to the rotation of Pb²⁺ cations.

Tuning the reactivity of nitriles using Cu(ii) catalysis - potentially prebiotic activation of nucleotides

A synergistic system was established involving activating nucleotides with nitriles using Cu(ii) and protecting RNA degradation by byproducts of alpha-aminonitriles.

During the transition from prebiotic chemistry to biology, a period of solution-phase, non-enzymatic activation of (oligo)nucleotides must have occurred, and accordingly, a mechanism for phosphate activation must have existed. Herein, we detail results of an investigation into prebiotic phosphate activation chemistry using simple, prebiotically available nitriles whose reactivity is increased by Cu²⁺ ions. Furthermore, although Cu²⁺ ions are known to catalyse the hydrolysis of phosphodiester bonds, we found this deleterious activity to be almost completely suppressed by inclusion of amino acids or dipeptides, which may suggest a productive relationship between protein and RNA from the outset.

Silver cyanamide nanoparticles decorated ultrathin graphitic carbon nitride nanosheets for enhanced visible-light-driven photocatalysis

Silver cyanamide nanoparticle decorated carbon nitride nanosheets are synthesized to build up a type-II semiconductor heterojunction for visible-light-driven photocatalysis.

Synthesis and thermoelastic properties of Zr(CN2)2and Hf(CN2)2

Metal carbodiimides with wine-rack topology exhibiting thermoelastic properties like those of flexible framework materials.

Carbodiimides as energy materials: which directions for a reasonable future?

Transition-metal carbodiimides have emerged as energy materials, both as anodes in rechargeable batteries and as catalysts in photochemical water oxidation.

Tin(ii) oxide carbodiimide and its relationship to SnO

Sn₂O(CN₂) was obtained from a solid-state metathesis. Its crystal structure incorporates a Sn²⁺ ion with a 5s² lone pair and was analyzed in relation to that of SnO by electronicstructure calculations and a COHP bonding analysis.

Nonaqueous synthesis of metal cyanamide semiconductor nanocrystals for photocatalytic water oxidation

Ag₂NCN nanorods synthesized in nonaqueous solution are used as novel visible-light-driven photocatalysts for water oxidation.