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 <i>N,N</i>′-ferrocenylenebis­(oxamic acid) (1,1′-fcbaH₂Et₂, <b>1</b>). <b>1</b> was converted readily to its saponificated form only, namely [^<i>n</i>Bu₄N]₂[(1,1′-fcbaH₂)] (<b>2</b>), when treated with 4 equiv of [^<i>n</i>Bu₄N]­OH followed by the subsequent addition of [Ni­(H₂O)₆]­Cl₂, whereas [^<i>n</i>Bu₄N]₂[Cu­(1,1′-fcba)] (<b>3</b>) was obtained in ca. 70% yield by using CuCl₂·2H₂O. Oxidation of <b>3</b> with I₂ led to the formation of [^<i>n</i>Bu₄N]­[Cu­(1,1′-fcba)] (<b>4</b>). A combined study of <b>4</b> by ESR and ⁵⁷Fe Mössbauer spectroscopy, supported by DFT calculations, revealed the iron atom of <b>4</b> to possess the oxidation state +3. Treatment of <b>1</b> with MeNH₂ resulted in the exclusive formation of the methyl ester of ferrocenylene-1-(<i>N</i>-methyloxamide)-1′-(oxamic acid) (1,1′-fcooH₃Me₂, <b>5</b>). Successive treatment of <b>5</b> with [Cu₂(OAc)₄(H₂O)₂] and [^<i>n</i>Bu₄N]­OH gave rise to the formation of [^<i>n</i>Bu₄N]₂[Cu­(fcooMe)]·2H₂O (<b>6A</b>·2H₂O and <b>6B</b>·2H₂O), for which single crystals of the compositions [^<i>n</i>Bu₄N]₂[Cu­(1,1′-fcooMe)]·¹/₂MeOH·¹/₂H₂O (<b>6A</b>·¹/₂MeOH·¹/₂H₂O) and [^<i>n</i>Bu₄N]₂[Cu­(1,1′-fcooMe)]·2.25H₂O (<b>6B</b>·2.25H₂O) were grown. Single-crystal X-ray diffraction studies revealed <b>6A</b>·¹/₂MeOH·¹/₂H₂O and <b>6B</b>·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 <b>3</b> and <b>4</b>, respectively, with [Cu­(pmdta)­(NO₃)₂] afforded the tetranuclear complexes [Cu₃(1,1′-fcba)­(pmdta)₂]­(NO₃)_<i>n</i> (<i>n</i> = 2 (<b>7</b>), 3 (<b>8</b>)). The solid-state structures of <b>1</b>–<b>3</b>, <b>7</b>, and <b>8</b> were determined by single-crystal X-ray diffraction studies. The magnetic properties of <b>3</b>, <b>4</b>, <b>7</b>, and <b>8</b> were studied by susceptibility measurements versus temperature. For <b>4</b> a weak antiferromagnetic coupling between the Cu^II and Fe^III ions has been obtained with <i>J</i> = −2 cm^–1, whereas <b>3</b> can be understood as a purely paramagnetic, Cu^II/Fe^II-containing complex only. For <b>7</b> a <i>J</i> parameter of −58 cm^–1 has been obtained due to an antiferromagnetic interaction between its Cu^II ions. This magnetic exchange interaction modifies in <b>8</b> to −64 cm^–1, and an additional ferromagnetic coupling of 4 cm^–1 between the Fe^III ion and the central Cu^II ion is obtained