Pre- and post-synthetic modulation of the ordering temperatures in a family of anilato-based magnets

Abstract

We report the synthesis and characterization of six novel heterometallic molecule-based 2D magnets with the bromanilato ligand (C₆O₄Br₂²⁻ = 1,3-dibromo-2,5-dihydroxy-1,4-benzoquinone dianion) and six different benzene derivative molecules. The compounds, formulated as (NBu₄)[MnCr(C₆O₄Br₂)₃]·1.75C₆H₅Br (1), (NBu₄)[MnCr(C₆O₄Br₂)₃]·C₆H₅X with X = Cl (2), I (3) and CH₃ (4) and (NBu₄)[MnCr(C₆O₄Br₂)₃]·2C₆H₅X with X = CN (5) and NO₂ (6), present the classical hexagonal honeycomb-(6,3) lattice with alternating Mn(II) and Cr(III) ions. The layers are packed in an eclipsed way along the a direction giving rise to hexagonal channels where the benzene derivative molecules are located with π–π interactions between the benzene and anilato rings. The interlayer space contains the NBu₄⁺ cations needed to compensate the anionic charge of the [Mn^IICr^III(C₆O₄Br₂)₃]⁻ layers. The Mn–Cr exchange coupling through the bromanilato ligands is antiferromagnetic, leading to a long range ferrimagnetic order in the six compounds with ordering temperatures around 10 K. These ordering temperatures can be slightly modified in the range 9.5–11.4 K by simply changing the benzene-derivative solvent molecule. Here we discuss the possible structural and electronic reasons for this tuning effect of the solvent molecule and the important structural role played by the solvent molecules. We also show that it is possible to exchange the solvent molecules inside the hexagonal channels post-synthetically causing a tiny change in the ordering temperature and coercive field. Furthermore, we also show that it is possible to further change the ordering temperatures by simply removing the solvent molecules by heating the sample at low pressures to obtain a de-solvated phase.