Experimental and Quantum-Chemical Investigations of the UV/Vis Absorption Spectrum of Manganese Carbodiimide, MnNCN

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.

A Halomanganates(II) with P,P'-Diprotonated Bis(2-Diphenylphosphinophenyl)ether: Wavelength-Excitation Dependence of the Quantum Yield and Role of the Non-Covalent Interactions

A [H2DPEphos][MnX4] [X = Br, Cl] tetrahalomanganates(II) with P,P'-diprotonated bis[2-(diphenylphosphino)phenyl]ether cation has been designed and investigated in photophysics and EPR terms. The complexes exhibit a green luminescence resulted from the Mn(II) d-d transitions (4T1→6A1) with the wavelength-excitation dependence of the quantum yield. The solid [H2DPEphos][MnBr4] complex exhibits a bright green phosphorescence (λmax = 515 nm) with the high luminescence quantum yield depending on the excitation energy whereas the solid [H2DPEphos][MnCl4] complex exhibits a very weak phosphorescence (λmax = 523 nm). The unexpected shorter luminescence lifetime for the [H2DPEphos][MnCl4] than for the [H2DPEphos][MnBr4] at 300 K can be a result of the higher non-radiative relaxation contribution. On the one hand, the non-covalent PH…X(Mn) interactions quench the manganese(II) luminescence. On the other hand, the PH…X(Mn) interactions are a pathway of the excitation transfer from [H2DPEphos]2+ to [MnX4]2-.

Manganese(ii) bromo- and iodo-complexes with phosphoramidate and phosphonate ligands: synthesis, characterization and photoluminescence

Phosphoramidates and phosphonates are suitable ligands for the preparation of tetrahedral mono- and polynuclear Mn(ii) complexes characterized by green luminescence, influenced by the symmetry of the compounds and by the P-bonded substituents.

Increased photocurrent of CuWO4 photoanodes by modification with the oxide carbodiimide Sn2O(NCN)

Tin(ii) oxide carbodiimide is a novel prospective semiconductor material with a band gap of 2.1 eV and lies chemically between metal oxides and metal carbodiimides. We report on the photochemical properties of this oxide carbodiimide and apply the material to form a heterojunction with CuWO₄ thin films for photoelectrochemical (PEC) water oxidation. Mott-Schottky experiments reveal that the title compound is an n-type semiconductor with a flat-band potential of -0.03 V and, as such, the position of the valence band edge would be suitable for photochemical water oxidation. Sn₂O(NCN) increases the photocurrent of CuWO₄ thin films from 32 μA cm^-2 to 59 μA cm^-2 at 1.23 V vs. reversible hydrogen electrode (RHE) in 0.1 M phosphate buffer (pH 7.0) under backlight AM 1.5G illumination. This upsurge in photocurrent originates in a synergistic effect between the oxide and oxide carbodiimide, because the heterojunction photoanode displays a higher current density than the sum of its individual components. Structural analysis by powder X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) reveals that Sn₂O(NCN) forms a core-shell structure Sn₂O(NCN)@SnPO_x during the PEC water oxidation in phosphate buffer. The electrochemical activation is similar to the behavior of Mn(NCN) but different from Co(NCN).

An MnNCN-Derived Electrocatalyst for CuWO4 Photoanodes

CuWO₄ is a photoanode candidate in neutral pH, and manganese-based oxygen evolution reaction electrocatalysts are of high interest due to their low price and low toxicity. Considering the unexplored chemistry of transition-metal carbodiimides/cyanamides for the PEC water oxidation, we investigated MnNCN as an electrocatalyst for CuWO₄ under AM 1.5G illumination in potassium phosphate electrolyte (pH 7). Surface functionalization of CuWO₄ photoanodes with MnNCN increased the photocurrent from 22 to 30 μA cm^-2 at 1.23 V vs RHE. Complementary structural analysis by means of XRD and XPS revealed that MnNCN forms a core-shell structure MnNCN@MnPO _x in phosphate electrolyte and mimics a manganese phosphate electrocatalyst. As such, the surface chemistry of MnNCN significantly differs from previous studies on the cobalt analogue (CoNCN). A separately prepared MnNCN electrode developed a small but detectable photocurrent due to photogenerated holes inside the semiconducting carbodiimide core of the MnNCN@MnPO _x structure.

Cr2(NCN)3, a ferromagnetic carbodiimide with an unusual two-step magnetic transition

Crystallographic and magnetic characterization of a ferromagnetic transition-metal carbodiimide with an unusual two-step magnetic phase transition.

d-d Spectra of transition-metal carbodiimides and hydrocyanamides as derived from many-particle effective Hamiltonian calculations

We apply the local many-particle method of the Effective Hamiltonian of Crystal Field (EHCF) to analyze the magnetic ground state and the low-energy excitation spectra of the transition-metal carbodiimides MNCN with M = Fe-Ni. Experimentally, these materials represent a uniform group of (high-spin) antiferromagnetic, optically transparent, colored insulators with absorption lines in the visible spectrum. These findings are fully supported by the EHCF numerical modeling. In all three cases, we arrive at high-spin ground states in agreement with the results of previous magnetic measurements as well as the presence of the d-d intrashell transitions for the visible absorption spectra. Remarkably enough, the EHCF approach resolves the controversial case of FeNCN which was earlier predicted to be metallic by density-functional theory even when including explicit electronic correlation (GGA+U). We also address the ground state and the low-energy excitation spectra of the transition-metal hydrocyanamides of the general formula M(NCNH)(2) with M = Fe-Ni, another uniform group of optically transparent colored insulators. EHCF also arrives at high-spin ground states and visible d-d intrashell transitions.