Perovskite-Like Carbodiimides <i>AB</i>(NCN)<sub>3</sub>: Synthesis and Characterization of MnHf(NCN)<sub>3</sub> and\nFeHf(NCN)<sub>3</sub>

Two novel ternary air-stable transition-metal carbodiimides,\nMnHf(NCN)<sub>3</sub> and FeHf(NCN)<sub>3</sub>, were synthesized\nvia solid-state\nmetathesis using either ZnNCN or Na<sub>2</sub>NCN as the carbodiimide\nsource and the corresponding binary metal chlorides. These two phases\nare the first examples of transition-metal carbodiimides with an <i>AB</i>(NCN)<sub>3</sub> composition, akin to ubiquitous <i>AB</i>O<sub>3</sub> perovskite oxides. The crystal structure\nof MnHf(NCN)<sub>3</sub> was determined and refined from powder X-ray\ndiffraction (XRD) data in the non-centrosymmetric space group <i>P</i>6<sub>3</sub>22 allowing for chirality, the assignment\nof which is supported by second-harmonic generation (SHG) measurements.\nFeHf(NCN)<sub>3</sub> was found to crystallize isotypically, and the\npresence of iron(II) in a high spin state was confirmed by <sup>57</sup>Fe Mößbauer spectroscopy. The structures are revealed\nto be NiAs-derived and can be described as a hexagonal stack of NCN<sup>2–</sup> anions with metal cations occupying 2/3 of the octahedral\nvoids. Both IR spectroscopic measurements and DFT calculations agree\nthat the NCN<sup>2–</sup> unit is a bent carbodiimide with <i>C</i><sub>2<i>v</i></sub> symmetry, necessary to account\nfor the size difference present in such a vacancy-ordered structure.\nMagnetic studies reveal predominantly strong antiferromagnetic interactions\nbut no long-range order between the paramagnetic Mn<sup>2+</sup> centers,\nlikely due to the dilution of Mn<sup>2+</sup> over the octahedral\nsites or perhaps even due to some degree of magnetic frustration.\nThe optical and electrochemical properties of MnHf(NCN)<sub>3</sub> were then studied, revealing a wide band gap of 3.04 eV and p-type\nbehavior.