Synthetic Engineering in Na₂MSn₂(NCN)₆ (M = Mn, Fe, Co, and Ni) Based on Electronic Structure Theory

Abstract

Quaternary transition metal cyanamides Na2MSn2(NCN)6 with M = Mn, Fe, Co, and Ni were prepared via solid-state metathesis reactions between Na2Sn(NCN)3 and binary transition metal fluorides MF2 in a 2:1 molar ratio. All phases crystallize isotypically in [NiAs]-derived structures (P3̅1m) with inter- and intra-layer cation ordering over the octahedral sites. This leads to a highly asymmetric coordination of the NCN2– anion, resulting in a strong degree of cyanamide character, which is confirmed via IR measurements. Intriguingly, the optical properties of Na2MSn2(NCN)6 phases change markedly as the nature of the divalent transition metal is varied, and UV–vis measurements evidence a band gap reduction from Mn (3.43 eV) via Fe (1.90 eV) to Co (1.75 eV), which broadly mirrors the DFT+U calculated energetic interval from the Fermi level to the unoccupied 3d states. Mott–Schottky analysis then goes on to characterize Na2FeSn2(NCN)6 and Na2CoSn2(NCN)6 as n-type semiconductors with flat-band potentials of 0.46 and −0.24 eV, respectively, vs RHE. This study demonstrates the utility of transition metal substitutions, within a flexible cyanamide framework, to electronically tune this growing family of pseudo-oxides.

Computational Design and Theoretical Properties of WC3N6, an H-Free Melaminate and Potential Multifunctional Material

“…Besides binary C 3 N 4 , carbon–nitrogen-based complex anions such as carbodiimide − NCN 2– , guanidinate , CN 3 5– , or ortho-nitrido carbonate CN 4 8– also allow to generate three-dimensional materials in combination with (transition) metal cations, and these may be regarded as complex salts offering rich structural diversity and physicochemical properties. Molecular melamine, C 3 H 6 N 6 , the trimer of molecular cyanamide, H 2 NCN, plays a special role not only because it may be used for making g-C 3 N 4 (see above) but also due to the fact that it allows for deprotonation to prepare metal melaminate salts, − yet another way to arrive at high-performance photocatalysts.…”

Mechanistic Insights on the Formation of a Carbodiimide Ion from Urea in La2O2NCN Synthesis Based on the “Proanion” Strategy

“…15 A FT-IR spectrum of the sample displays two absorption bands: asymmetric stretching (ν as 1960 cm −1 ) and deformation (δ 681 cm −1 ) vibrations of NCN 2− (Figure S1b). 28,29 Judging from the absence of a symmetric stretching vibration (approximately 1250 cm −1 ), which is IR-forbidden for 24,25,30,31 UV−vis DRS revealed that the absorption edge of the obtained La 2 O 2 NCN exists at around 260 nm (Figure S1c). This value of the absorption edge position is comparable to that reported in a preceding paper by another group.…”

The [NCN]2– Carbodiimide Complex Anion and Its Compounds–A “Divalent Nitride” Unit for Nitridocarbonates in Materials and Applications

“…Moreover, systematic Ni substitution into this NiAs-derived structure type led to a pronounced increase in the current density, maximized in Co 0.9 Ni 0.1 NCN. In a similar manner varying the nature of the transition-metal species in Na 2 M Sn 2 (NCN) 6 ( M = Mn, Fe, Co and Ni) quaternaries, which also all crystallize isostructurally in NiAs-derived structures, has been shown as to be an efficient means of band gap tuning …”

“…In a similar manner varying the nature of the transition-metal species in Na 2 MSn 2 (NCN) 6 (M = Mn, Fe, Co and Ni) quaternaries, which also all crystallize isostructurally in NiAs-derived structures, has been shown as to be an efficient means of band gap tuning. 60 Meanwhile MnNCN has been successfully integrated as an electrocatalyst for CuWO 4 photoanodes increasing the photocurrent by a stunning 30% in a phosphate electrolyte. 146 Furthermore, in-depth structural analysis revealed that MnNCN is activated in situ to form a MnNCN@MnPO x core−shell structure thereby mimicking a manganese phosphate electrocatalyst and highlighting the importance of surface chemistry.…”

“…These analogues of the ubiquitous AM O 3 oxides crystallize in chiral structures ( P 6 3 22) with an intraplanar cation/vacancy ordering scheme that is very similar to that of ferroelectric LiNbO 3 ( R 3 c ). The final important group of [NiAs]-derived metal nitridocarbonates to mention are a series of isostructural quaternaries: Li 2 M Sn 2 (NCN) 6 ( M = Mn and Mg) and Na 2 M Sn 2 (NCN) 6 ( M = Mn, Fe, Co and Ni). − This structure-type is characterized by two distinct metal sheets, with corundum-like [Sn 2 (NCN) 3 ] 2+ layers alternating with [A 2 M (NCN) 3 ] 2– layers, resulting in a highly asymmetric coordination of [N–CN] 2– with a strong degree of cyanamide character. Furthermore, these are the first examples of metal nitridocarbonates to contain both alkali metal cations and redox-active transition metal cations, opening up the tantalizing prospect of oxidative (de)intercalation chemistry (see also electrochemistry section).…”