Tuning Magnetic Interactions Between Triphenylene Radicals by Variation of Crystal Packing in Structures with Alkali Metal Counterions.

The monoanion of triphenylene (C18H12, 1) was generated in THF using several alkali metals (Na, K, Rb, and Cs) as reducing agents and crystallized with the corresponding cations in the presence of 18-crown-6 ether. The UV-vis spectroscopy points to the metal-dependent coordination environment of the triphenylene monoanion-radicals, 1·-, in solution. The X-ray diffraction characterization confirmed the formation of a solvent-separated ion pair (SSIP) with sodium ions, [{Na+(18-crown-6)(THF)2}(1·-)] (2), and three contact-ion pair (CIP) complexes formed by larger alkali metal ions, [{K+(18-crown-6)}(1·-)] (3), [{Rb+(18-crown-6)}(1·-)] (4), and [{Cs+(18-crown-6)}(1·-)] (5). Structural analysis of the series reveals a notable geometry perturbation of the triphenylene framework in 2 caused by one-electron acquisition, which is further enhanced by direct metal binding in 3-5. This has been correlated with the aromaticity changes and charge redistribution upon one-electron reduction of 1, as revealed by the computational studies. The EPR spectroscopy and magnetic susceptibility measurements confirm antiferromagnetic interactions corresponding to an S = 1/2 system in the solid state. The magnetic behavior of 3-5 correlates with the arrangement of triphenylene radicals in the crystal structures. All three compounds exhibit antiferromagnetic (AFM) interactions between S = 1/2 radicals in the solid state, but the exchange coupling in 4 and 5 is notably stronger than that in 3, which leads to AFM ordering at 3.8 K in 4 and at 2.0 K in 5. The magnetic phase transitions in 4 and 5 can be interpreted as originating from interactions between the chains of the AFM-coupled S = 1/2 radicals.