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

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.