An interplay between the spin density distribution and magnetic superexchange interactions: a case study of mononuclear [ super()nu sub(4)N] sub(2)[Cu(opooMe)] and novel asymmetric trinuclear [Cu sub(3)(opooMe)(pmdta) sub(2)] (NO sub(3)) sub(2).3MeCN

Treatment of the diethyl ester of o-phenylenebis(oxamic acid) (opbaH sub(2)Et sub(2), 1) with 5/6 equivalent of MeNH sub(2) in abs. EtOH results in the exclusive formation of the ethyl ester of o-phenylene(N'-methyl oxamide)(oxamic acid) (opooH sub(3)EtMe, 2) in ca.50% yield. Treatment of 2 with four equivalents of [Me sub(4)N]OH followed by the addition of Cu(ClO sub(4)) sub(2).6H sub(2)O gave [Me sub(4)N] sub(2)[Cu(opooMe)].H sub(2)O (3A) in ca.80% yield. As 3A appears to be a hygroscopic solid, the related [ super()nu sub(4)N] super(+) salts [ super()nu sub(4)N] sub(2)[M(opooMe)].H sub(2)O (M = Cu (3B), Ni (4)) have been synthesized. By addition of two equivalents of [Cu(pmdta)(NO sub(3)) sub(2)] to a MeCN solution of 3B the novel asymmetric trinuclear complex [Cu sub(3)(opooMe)(pmdta) sub(2)] (NO sub(3)) sub(2) (5) could be obtained in ca.90% yield. Compounds 2, 3A, 3B, 4 and 5 have been characterized by elemental analysis and NMR/IR spectroscopy. Furthermore, the solid state structures of 3A in the form of [Me sub(4)N] sub(2)[Cu(opooMe)].MeOH (3A'), 3B in the form of [ super()nu sub(4)N] sub(2)[Cu(opooMe)] (3B'), 4 in the form of [ super()nu sub(4)N] sub(2)[Ni(opooMe)].1.25H sub(2 )O (4') and 5 in the form of [Cu sub(3)(opooMe)(pmdta) sub(2)] (NO sub(3)) sub(2).3MeCN (5'), respectively, have been determined by single-crystal X-ray diffraction studies. By controlled cocrystallization, diamagnetically diluted 3B (1%) in the host lattice of 4 (99%) in the form of single crystals have been made available, allowing single crystal EPR studies to extract all components of the g-factor and the tensors of onsite super(Cu)Aand transferred super(N)Ahyperfine interaction. Out of these studies the spin density distribution of the [Cu(opooMe)] super(2-) complex fragment could be determined. The magnetic properties of 5 were studied by susceptibility measurements versustemperature. An intramolecular Jparameter of -65 cm super(-1) has been obtained, unexpectedly, as 5 should possess two different Jvalues due to its two different spacers between the adjacent Cu super(II) ions, namely an oxamate (C sub(2)NO sub(3)) and an oxamidate (C sub(2)N sub(2)O sub(2)) fragment. This unexpected result is explained by a summarizing discussion of the experimentally obtained EPR results (spin density distribution) of 3B, the geometries of the terminal [Cu(pmdta)] super(2+) fragments of 5 determined by X-ray crystallographic studies and accompanying quantum chemical calculations of t