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) (opbaH(2)Et(2), 1) with 5/6 equivalent of MeNH(2) in abs. EtOH results in the exclusive formation of the ethyl ester of o-phenylene(N′-methyl oxamide)(oxamic acid) (opooH(3)EtMe, 2) in ca. 50% yield. Treatment of 2 with four equivalents of [Me(4)N]OH followed by the addition of Cu(ClO(4))(2)·6H(2)O gave [Me(4)N](2)[Cu(opooMe)]·H(2)O (3A) in ca. 80% yield. As 3A appears to be a hygroscopic solid, the related [(n)Bu(4)N](+) salts [(n)Bu(4)N](2)[M(opooMe)]·H(2)O (M = Cu (3B), Ni (4)) have been synthesized. By addition of two equivalents of [Cu(pmdta)(NO(3))(2)] to a MeCN solution of 3B the novel asymmetric trinuclear complex [Cu(3)(opooMe)(pmdta)(2)](NO(3))(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(4)N](2)[Cu(opooMe)]·MeOH (3A′), 3B in the form of [(n)Bu(4)N](2)[Cu(opooMe)] (3B′), 4 in the form of [(n)Bu(4)N](2)[Ni(opooMe)]·1.25H(2)O (4′) and 5 in the form of [Cu(3)(opooMe)(pmdta)(2)] (NO(3))(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 (Cu)A and transferred (N)A 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 (C(2)NO(3)) and an oxamidate (C(2)N(2)O(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)](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 [Cu(3)(opooMe)(pmdta)(2)](2+) complex fragments.

Probing the magnetic superexchange couplings between terminal CuII ions in heterotrinuclear bis(oxamidato) type complexes

“…1 and Table 2. A related observation was made recently for the asymmetric trinuclear complex [Cu 3 (opooMe)(pmdta) 2 ](NO 3 ) 2 ( 13 , opooMe = o -phenylene( N’ -methyl oxamidato)(oxamato)) [13] and has been compared to observations made for bis(oxamato) type entities. As observed for the CuN 3 O 2 units of 1A – 3A , even in the case of 13 , the largest O–Cu–N bond angle involves the O donor atom of the function for the oxamidato side, whereas in case of the oxamato side the largest bond angle involves the O donor atom of the function.…”

“…As observed for the CuN 3 O 2 units of 1A – 3A , even in the case of 13 , the largest O–Cu–N bond angle involves the O donor atom of the function for the oxamidato side, whereas in case of the oxamato side the largest bond angle involves the O donor atom of the function. Consequences of this observation to magnetic exchange couplings have been discussed [13]. Thus, it seems that for polynuclear complexes comprising one or two oxamidato groups, cf.…”

“…Thus, it seems that for polynuclear complexes comprising one or two oxamidato groups, cf. [13] and [15], this specific feature of the terminal CuN 3 O 2 units is of broader validity.…”

“…As the sample preparation was performed under aerobic conditions, it is imaginable that air moisture had an impact on these measured as it is shown for the IR spectroscopically characterized 3 . Attempts to model this contribution with some specific models invoking possible exchange interactions between the two Cu centers (e.g., [13,15,27]) were not successful.…”

“…One could expect that due to the lower electronegativity of the nitrogen atoms of type III (compared to the oxygen atoms of type IV complexes), the magnetic exchange couplings should increase [1]. These are studies to which we have already contributed [13–18]. …”