Redox-Active Ferrocene as a Tuning Functionality for Magnetic Superexchange Interactions of Bis(oxamato) Type Complexes

Abstract

The reaction of 1,1′-diaminoferrocene with two equivalents of ethyloxalyl chloride in THF afforded the diethyl ester of N,N′-ferrocenylenebis(oxamic acid) (1,1′-fcbaH2Et2, 1). 1 was converted readily to its saponificated form only, namely [nBu4N]2[(1,1′-fcbaH2)] (2), when treated with 4 equiv of [nBu4N]OH followed by the subsequent addition of [Ni(H2O)6]Cl2, whereas [nBu4N]2[Cu(1,1′-fcba)] (3) was obtained in ca. 70% yield by using CuCl2·2H2O. Oxidation of 3 with I2 led to the formation of [nBu4N][Cu(1,1′-fcba)] (4). A combined study of 4 by ESR and 57Fe Mössbauer spectroscopy, supported by DFT calculations, revealed the iron atom of 4 to possess the oxidation state +3. Treatment of 1 with MeNH2 resulted in the exclusive formation of the methyl ester of ferrocenylene-1-(N-methyloxamide)-1′-(oxamic acid) (1,1′-fcooH3Me2, 5). Successive treatment of 5 with [Cu2(OAc)4(H2O)2] and [nBu4N]OH gave rise to the formation of [nBu4N]2[Cu(fcooMe)]·2H2O (6A·2H2O and 6B·2H2O), for which single crystals of the compositions [nBu4N]2[Cu(1,1′-fcooMe)]·1/2MeOH·1/2H2O (6A·1/2MeOH·1/2H2O) and [nBu4N]2[Cu(1,1′-fcooMe)]·2.25H2O (6B·2.25H2O) were grown. Single-crystal X-ray diffraction studies revealed 6A·1/2MeOH·1/2H2O and 6B·2.25H2O to contain [Cu(fcooMe)]2– fragments in the form of two different conformers, having a significant difference in their total energy, as found by DFT calculations. Treatment of 3 and 4, respectively, with [Cu(pmdta)(NO3)2] afforded the tetranuclear complexes [Cu3(1,1′-fcba)(pmdta)2](NO3)n (n = 2 (7), 3 (8)). The solid-state structures of 1–3, 7, and 8 were determined by single-crystal X-ray diffraction studies. The magnetic properties of 3, 4, 7, and 8 were studied by susceptibility measurements versus temperature. For 4 a weak antiferromagnetic coupling between the CuII and FeIII ions has been obtained with J = −2 cm–1, whereas 3 can be understood as a purely paramagnetic, CuII/FeII-containing complex only. For 7 a J parameter of −58 cm–1 has been obtained due to an antiferromagnetic interaction between its CuII ions. This magnetic exchange interaction modifies in 8 to −64 cm–1, and an additional ferromagnetic coupling of 4 cm–1 between the FeIII ion and the central CuII ion is obtained.