Bi- and Trinuclear Copper(II) Complexes with a Bridging Pyrazole/Oxime Ligand: Structures and Magnetic Properties

The pyrazole-based asymmetric ligand 3-[(1E)-N-hydroxyethanimidoyl]-4-methyl-1H-pyrazole-5-carboxylic acid (L) is shown to serve as a versatile ligand in copper(II) coordination chemistry with both the pyrazole and the oxime groups as potential bridges upon deprotonation. Three complexes [Cu-2(L-2H)(2)(DMSO)(H2O)].H2O (1a), [Cu-2(L-2H)(2)(DMSO)(2)].H2O (1b) and [Cu-3(L-3H)(2)(Py)(5)] (2) have been isolated and their crystal structures determined by X-ray single crystal analysis. The former feature a bis(pyrazolato)-bridged dicopper core, whereas adjacent copper ions are spanned by a pyrazolate and an oximate in the latter complex. The presence of different species in solution depending on the pH has been demonstrated by EPR. Alterations in the protonation state are accompanied by changes in nuclearity., which cause pronounced changes in color, magnetic and EPR properties. Magnetic susceptibility measurements in the temperature range 1.8 - 300 K have revealed anti ferromagnetic exchange coupling between copper(II) ions both in the bi- and trinuclear complexes (J = -160 cm(-1) for 1 and -191cm(-1) for 2). The binuclear species [Cu-2(L-2H)(2)] is almost silent in X-band EPR at 77 K due to antiparallel alignment of the unpaired Cu-II electrons, while the EPR signatures of species at alkaline condition suggest a Cu-centered S = 1/2 state.