Copper(II) complexes with pyrazolyl-substituted nitronyl and imino nitroxides

Abstract

It was established that the reactions of pyrazol-3-yl-substituted nitronyl nitroxide ( HL 1) and pyrazol-3-yl-substituted imino nitroxide ( HL 3) with Cu(II) acetate lead to self-assembly of the Cu 4(OH) 2(OAc) 4(DMF) 2(L 1) 2 tetranuclear and Cu 2(OAc) 2(H 2O) 2(L 3) 2 dinuclear complexes, respectively. The reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted nitronyl nitroxide ( HL 2) gave unexpected solid Cu 2(H 2O) 2(L 6) 2 · 2DMF, in which L 6 is a deprotonated 5-carboxy-pyrazol-3-yl-substituted nitronyl nitroxide, formed as a result of cleavage of an ester bond in the starting HL 2. A similar transformation of the paramagnetic ligand was observed in the reaction of Cu(II) acetate with 5-ethoxycarbonyl-pyrazol-3-yl-substituted imino nitroxide ( HL 4). It led to the formation of Cu 2(DMF) 2(L 7) 2, where L 7 is deprotonated 2-(5-carboxy-1 H-pyrazol-3-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1 H-imidazole 3-oxide. An X-ray diffraction study indicated that in Cu 4(OH) 2(OAc) 4(DMF) 2(L 1) 2 and Cu 2(OAc) 2(H 2O) 2(L 3) 2, the L 1 and L 3 paramagnetic ligands perform the bridging cyclic tridentate function, while in Cu 2(H 2O) 2(L 6) 2 · 2DMF and Cu 2(DMF) 2(L 7) 2, the paramagnetic L 6 and diamagnetic L 7 are bridging bicyclic tetradentate ligands. The magnetic behavior of complexes with coordinated nitronyl nitroxide – Cu 4(OH) 2(OAc) 4(DMF) 2(L 1) 2 and Cu 2(H 2O) 2(L 6) 2 · 2DMF is dictated by the dominant antiferromagnetic exchange interactions, which is confirmed by quantum-chemical data. The magnetic susceptibility of Cu 2(OAc) 2(H 2O) 2(L 3) 2 reflects the competition between the antiferromagnetic and ferromagnetic components, of which the latter is due to electron coupling in the Cu(II) ← N=C–N ∸ O exchange channels. EPR data confirm the results received from static magnetic measurements for multispin solids.