Magnetic behavior of copper(II) complexes of a nitronyl-nitroxide-substituted pyrimidine

Abstract

Two Cu(II) complexes of nitronyl-nitroxide-substituted pyrimidine ( 1) were prepared, and their structures and magnetic properties were clarified. The reaction of 1 with Cu(hfac) 2 gave a dark-green complex 2 with the formula [ 1 2·{Cu(hfac) 2} 3], whereas the reaction with Cu(ClO 4) 2 in the presence of dien (diethylenetriamine) gave a dark-blue complex 3 [ 1·Cu(ClO 4) 2·dien(H 2O) 3]. Both structures were solved by X-ray structure analysis. The complex 2 consists of a dimer with the formula [ 1·{Cu(hfac) 2}] 2 and an almost magnetically free Cu(hfac) 2. In the dimer structure, the Cu(II) ion is coordinated to the oxygen atom of nitronyl nitroxide 1 and is weakly bound to the nitrogen atom of the pyrimidine of the second molecule of 1. The magnetic behavior of this dimer structure is shown to be strongly antiferromagnetic ( J=−1600 K) by efficient orbital overlap between the Cu(II) magnetic orbital and the nitroxide p-orbital. The spin of the dimer moiety disappears owing to this strong antiferromagnetic interaction in the temperature region below 200 K, so that the overall magnetic property of 2 is governed by the remaining Cu(hfac) 2 in the low-temperature region. For complex 3, the Cu(II) ion was shown to be strongly bound to the pyrimidine nitrogen atom and the dien ligand. Coordination between the nitronyl nitroxide and the Cu(II) ion was not observed. The magnetic behavior of 3 was explained by the ferromagnetic interaction ( J=+16.4 K) between the Cu(II) ion and the nitronyl nitroxide and by an antiferromagnetic interaction ( J=−3.4 K) probably due to the possible intermolecular interaction.