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

Abstract

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 P1m 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 δ_total, 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 λ_oct, revealing that the divalent and alkali metal sites are strongly dependent on δ_total 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.