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The treatment of the diethyl ester of o-phenylenebis(oxamic acid) (opbaH2Et2) with 2/3 of an equivalent of MeNH2 in MeOH does not result in the formation of the methyl ester of o-phenylene(N′-methyloxamide)(oxamic acid) (opooH3Me, 1) in pure state, as reported previously. The colourless crude material formed by this reaction was confirmed to be composed of 1 (89% content), the dimethyl ester of o-phenylenebis(oxamic acid) (opbaH2Me2, 2, 6%), 1,4-dihydro-2,3-quinoxalinedione (3, 3%) and o-phenylenebis(N′-methyloxamide) (opboH4Me2, 4, 1%), respectively. The identities of 1–4 have been verified by IR, 1H and 13C NMR spectroscopy as well as elemental analysis. In addition, the solid state structures of 1 and 2·2DMSO, respectively, were determined by single-crystal X-ray diffraction studies. Successive recrystallization of the crude material from MeOH and MeOH:THF (1:1), respectively, does not give pure 1, but a mixture of 1 and 2. It is shown further that out of this mixture pure bis(oxamato) complexes cannot be obtained, as previously reported. Instead, treatment of the mixture with NiII or CuII salts, followed by the addition of [nBu4N]OH, results in the formation of two mixtures of [nBu4N]2[Ni(opba)] (5) and [nBu4N]2[Ni(opooMe)] (6) as well as [nBu4N]2[Cu(opba)] (7) and [nBu4N]2[Cu(opooMe)] (8), respectively. The simultaneous formation of 5/6 and 7/8, respectively, has been verified by crystallization of the obtained mixtures and X-ray diffraction studies of the obtained single crystals. Co-crystallization of mixtures of 5/6 (99 mass%) and 7/8 (1 mass%), respectively, results in the formation of single-crystals of diamagnetically diluted 7 in the host lattice of 5 (7@5) accompanied by single-crystal formation of diamagnetically diluted 8 in the host lattice of 6 (8@6), as verified by EPR spectroscopy. It is finally shown that the ethyl ester of o-phenylene(N′-methyloxamide)(oxamic acid) (opooH3Et, 9), a homologue of 1, can be obtained in pure state by the treatment of opbaH2Et2 with 5/6 of an equivalent of MeNH2 in EtOH.
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