Promotion of antiferromagnetic exchange interaction in multinuclear copper(II) complexes via fused oxamato/oxamidato ligands

Treatment of N,N′-bis(2-aminophenyl)oxalamide (bapoxH6, 1) with ethyl oxalyl chloride in THF afforded oxamide-N,N′-bis(o-phenylene oxamic acid ethyl ester) (L1H4Et2, 2), which was converted to 3 (oxamide-(o-phenylene oxamic acid ethyl ester)(o-phenylene-N1-methyloxalamide); L2H5EtMe) and 4 (oxamide-N,N′-bis(o-phenylene-N1-methyloxalamide); L3H6Me2) by the addition of appropriate equivalents of MeNH2. Successive treatment of 2–4 with six equivalents of [nBu4N]OH and two equivalents of CuII salt resulted in the formation of the binuclear complexes [nBu4N]2[Cu2(L1)] (5), [nBu4N]2[Cu2(L2Me)] (6) and [nBu4N]2[Cu2(L3Me2)] (7). Upon addition of one equivalent of [Cu(pmdta)(NO3)2] (pmdta = N,N,N′,N′′,N′′-pentamethyldiethylenetriamine) to 5–7, the trinuclear complexes [Cu3(L1)(pmdta)] (8), [Cu3(L2Me)(pmdta)] (9) and [Cu3(L3Me2)(pmdta)] (10) were obtained, while the addition of two equivalents of [Cu(pmdta)(NO3)2] gave rise to the tetranuclear complexes [Cu4(L1)(pmdta)2](NO3)2 (11), [Cu4(L2Me)(pmdta)2](NO3)2 (12) and [Cu4(L3Me2)(pmdta)2](NO3)2 (13). The identities of compounds 2–4 were established by elemental analyses, NMR and IR spectroscopy, and ESI-MS measurements. For the multinuclear complexes 5–13, elemental analysis and IR spectroscopy were applied to confirm their identities. Furthermore, the solid-state structures of 5–8 and 11–13 were determined by single crystal X-ray diffraction studies. The magnetic behavior of 5–8 and 11–13 was studied by direct current susceptibility measurements as a function of temperature. For the binuclear complexes 5–7, exceptionally large antiferromagnetic exchange interactions with J1,2 values of −378, −397 and −419 cm−1 were determined. Astonishingly, the tendency of these series of magnetic exchange interactions can be smoothly associated with the d–d transitions of 5–7 measured through UV-Vis spectroscopy. The J1,2 value of 8 amounts to −507 cm−1, while for the tetranuclear complexes 11–13, this value is significantly smaller and amounts to −294, −292 and −370 cm−1. The second antiferromagnetic exchange interactions J2,3 and J1,4 are equal due to inversion symmetry and are −100, −135, −80 and −108 cm−1 for 8 and 11–13, respectively, in the expected range. Magneto-structural correlations are used to discuss the variable magnetic exchange interactions of all here reported multinuclear complexes.