An interplay between the spin density distribution and magnetic superexchange interactions: a case study of mononuclear [nBu4N]2[Cu(opooMe)] and novel asymmetric trinuclear [Cu3(opooMe)(pmdta)2](NO3)2·3MeCN

Abstract

Treatment of the diethyl ester of o-phenylenebis(oxamic acid) (opbaH₂Et₂, 1) with 5/6 equivalent of MeNH₂ in abs. EtOH results in the exclusive formation of the ethyl ester of o-phenylene(N′-methyl oxamide)(oxamic acid) (opooH₃EtMe, 2) in ca. 50% yield. Treatment of 2 with four equivalents of [Me₄N]OH followed by the addition of Cu(ClO₄)₂·6H₂O gave [Me₄N]₂[Cu(opooMe)]·H₂O (3A) in ca. 80% yield. As 3A appears to be a hygroscopic solid, the related [ⁿBu₄N]⁺ salts [ⁿBu₄N]₂[M(opooMe)]·H₂O (M = Cu (3B), Ni (4)) have been synthesized. By addition of two equivalents of [Cu(pmdta)(NO₃)₂] to a MeCN solution of 3B the novel asymmetric trinuclear complex [Cu₃(opooMe)(pmdta)₂](NO₃)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 [Me₄N]₂[Cu(opooMe)]·MeOH (3A′), 3B in the form of [ⁿBu₄N]₂[Cu(opooMe)] (3B′), 4 in the form of [ⁿBu₄N]₂[Ni(opooMe)]·1.25H₂O (4′) and 5 in the form of [Cu₃(opooMe)(pmdta)₂] (NO₃)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 ^CuA and transferred ^NA hyperfine interaction. Out of these studies the spin density distribution of the [Cu(opooMe)]²⁻ complex fragment could be determined. The magnetic properties of 5 were studied by susceptibility measurements versus temperature. An intramolecular J parameter of −65 cm⁻¹ has been obtained, unexpectedly, as 5 should possess two different J values due to its two different spacers between the adjacent Cu^II ions, namely an oxamate (C₂NO₃) and an oxamidate (C₂N₂O₂) 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)]²⁺ fragments of 5 determined by X-ray crystallographic studies and accompanying quantum chemical calculations of the spin density distribution of the mononuclear [Cu(opooMe)]²⁻ and of the magnetic exchange interactions of trinuclear [Cu₃(opooMe)(pmdta)₂]²⁺ complex fragments.