Perovskite-Like Carbodiimides <i>AB</i>(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 <i>AB</i>(NCN)₃ composition, akin to ubiquitous <i>AB</i>O₃ 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 <i>P</i>6₃22 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 NCN^2- anions with metal cations occupying 2/3 of the octahedral voids. Both IR spectroscopic measurements and DFT calculations agree that the NCN^2- unit is a bent carbodiimide with <i>C</i>_2<i>v</i> 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.