1. π -INTERACTIONS OF SEMIQUINONE RADICALS: CRYSTAL DESIGN AND TUNING OF THE MAGNETIC PROPERTIES Krešimir Molčanov and Biserka Kojić-Prodić Rudjer Bošković Institute, Bijenička c. 54, Zagreb, Croatia; E-mail: kmolcano@irb.hr A series of solvates of alkali salts of tetrachlolrosemiquinone (Cl 4 Q, chloranil) radical anion were prepared [1] 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, KCl 4 Q∙Me 2 CO and RbCl 4 Q ∙Me 2 CO, strongly bound radical dimers in the singlet state exist; therefore the crystals are diamagnetic [2]. In acetonitrile solvate, NH 4 Cl 4 Q ∙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 KCl 4 Q ∙EtCOMe 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 KCl 4 Q ∙Me 2 CO. 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. Nucleus Independent Chemical Shift (NICS) values calculated along the axes perpendicular to the quinone ring for the neutral quinone, radical anion - semiquinone, hydroquinone dianion and the benzene are shown. The NICS profiles for neutral quinone species are typical for nonaromatic molecules, while those for hydroquinone dianions are typical for aromatic species, although notably less aromatic than the prototype benzene. [2] p-Tetrachlorosemiquinone aromaticity is in between the bordering cases, benzene and quinone. Conclusions π -Interactions of semiquinone radical anions observed and described for the first time. Multi-atom centered bonding occurring in radical dimers with π-systems offers unique characteristics. The reversibility and dynamic nature of this bonding provide the possibility to design swichable molecular systems. KCl 4 Q ∙MeCOEt: Structural changes induced by increased temperature are accompanied by changes of magnetic properties [1] H. A. Torrey, W. H. Hunter, J. Am. Chem. Soc., 1912, 34, 702-716. [2] K. Molčanov, B. Kojić-Prodić, D, Babić, D. Žilić, B. Rakvin, CrystEngComm. 2011, 13, 5170-5178. 3.58 Å 3.27 Å 3.46 Å 100 K diamagnetic 200 K paramagnetic SQUID aromatic quinoid Cl 4 Q - 300th anniversary of Ruđer Bošković KCl 4 Q ∙Me 2 CO and RbCl 4 Q ∙Me 2 CO: diamagnetic crystals NH 4 Cl 4 Q ∙MeCN: 1D antiferromagnet heating cooling Reversible transition! heating cooling Crystals decompose Equidistant anions: 3.23 Å 3.50 Å 3.10 Å