Magnetic superexchange interactions: trinuclear bis(oxamidato) versus bis(oxamato) type complexes

Abstract

The diethyl ester of o-phenylenebis(oxamic acid) (opbaH2Et2) was treated with an excess of RNH2 in MeOH to cause the exclusive formation of the respective o-phenylenebis(N(R)-oxamides) (opboH4R2, R = Me , Et , (n)Pr ) in good yields. Treatment of with half an equivalent of [Cu2(AcO)4(H2O)2] or one equivalent of [Ni(AcO)2(H2O)4] followed by the addition of four equivalents of [(n)Bu4N]OH resulted in the formation of mononuclear bis(oxamidato) type complexes [(n)Bu4N]2[M(opboR2)] (M = Ni, R = Me , Et , (n)Pr ; M = Cu, R = Me , Et , (n)Pr ). By addition of two equivalents of [Cu(pmdta)(NO3)2] to MeCN solutions of , novel trinuclear complexes [Cu3(opboR2)(L)2](NO3)2 (L = pmdta, R = Me , Et , (n)Pr ) could be obtained. Compounds have been characterized by elemental analysis and NMR/IR spectroscopy. Furthermore, the solid state structures of and have been determined by single-crystal X-ray diffraction studies. By controlled cocrystallization, diamagnetically diluted and (1%) in the host lattice of and (99%) (@ and @), respectively, in the form of single crystals have been made available, allowing single crystal ESR studies to extract all components of the g-factor and the tensors of onsite (Cu)A and transferred (N)A hyperfine (HF) interaction. From these studies, the spin density distribution of the [Cu(opboEt2)](2-) and [Cu(opbo(n)Pr2)](2-) complex fragments of and , respectively, could be determined. Additionally, as a single crystal ENDOR measurement of @ revealed the individual HF tensors of the N donor atoms to be unequal, individual estimates of the spin densities on each N donor atom were made. The magnetic properties of were studied by susceptibility measurements versus temperature to give J values varying from -96 cm(-1) () over -104 cm(-1) () to -132 cm(-1) (). These three trinuclear Cu(II)-containing bis(oxamidato) type complexes exhibit J values which are comparable to and slightly larger in magnitude than those of related bis(oxamato) type complexes. In a summarizing discussion involving experimentally obtained ESR results (spin density distribution) of and , the geometries of the terminal [Cu(pmdta)](2+) fragments of determined by crystallographic studies, together with accompanying quantum chemical calculations, an approach is derived to explain these phenomena and to conclude if the spin density distribution of mononuclear bis(oxamato)/bis(oxamidato) type complexes could be a measure of the J couplings of corresponding trinuclear complexes.

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

“…It should be emphasized that the findings described in the following have been made analogously for our previously reported homotrinuclear Cu II Cu II Cu II complexes as described in [15]. Thus, the terminal Cu II ions of 1A – 3A are each coordinated by two O donor atoms of the oxamidato groups as well as three N donor atoms of the pmdta ligands to form CuN 3 O 2 coordination units closer to the ideal square-pyramidal compared to the ideal trigonal-bipyramidal coordination geometry with respect to their τ parameters [26], 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.…”

“…In our earlier work, we previously reported on the magnetic characterization of homotrinuclear Cu II Cu II Cu II type IV complexes [15]. We noticed, unexpectedly, that the central Cu II ions of these complexes were not coordinated by any counter ions or solvents.…”

“…Elemental analysis for C, H and N were performed on a Thermo FlashAE 1112 series. The mononuclear Ni II -containing complexes [ n -Bu 4 N] 2 [Ni(opboR 2 )] (R = Me, Et, n- Pr) were synthesized according to the literature [15]. Static magnetization measurements at T = 1.8 K and in magnetic fields µ 0 H up to 7 T were carried out with a 7 T VSM-SQUID magnetometer from Quantum Design.…”