engleski

π-Interactions of semiquinone radicals: crystal design and tuning of the magnetic properties

A series of solvates of alkali salts of tetrachlolrosemiquinone (Cl4Q, chloranil) radical anion were prepared and their crystal structures and magnetic properties were studied. The crystals display a wide range of magnetic properties: they can be paramagnetic, 1D antiferromagnetic or diamagnetic, depending on the distance between ring planes of radicals, which can be modified by different size of alkali cations and solvent molecules. Therefore, their magnetic properties can be tuned by crystal engineering. Electron spins of the contiguous rings are coupled if the interplanar separation is shorter than 3.30 Å ; they are unpaired if the distance is larger. In acetone solvates, KCl4Q∙Me2CO and RbCl4Q∙Me2CO, strongly bound radical dimers in the singlet state exist ; therefore the crystals are diamagnetic. In acetonitrile solvate, NH4Cl4Q∙MeCN, infinite stacks of equidistant anions (interplanar separation 2.227 Å) occur ; therefore the crystals are 1D antiferromagnets. The most interesting example is ethylmethylketone solvate KCl4Q∙EtMeCO which displays a continuous, reversible, single crystal-to-single crystal phase transition in the range 120 - 170 K. In the low-temperature phase (100 K), strongly bound dimers with paired electrons occur, similar to those in KCl4Q∙Me2CO. However, in the high-temperatre phase (200 K), infinite stacks of equidistant radicals are detected ; electron spins are unpaired due to greater interplanar distance. Spin coupling was also confirmed by ab initio quantum chemical calculations.

semiquinone radicals; exchange coupling; X-ray structure; single crystal-to-single crystal; phase transition

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