Magnetic Susceptibility, EPR, Mössbauer, and X-ray Investigations of Heteropolynuclear Clusters Containing Iron(III) and Copper(II) Ions: {Cu(Mesalen)}₂Fe(acac)(NO₃)₂ and {Cu(Mesalen)}₃Fe(acac)(PF₆)₂

Magnetic susceptibility, EPR, Mössbauer, and X-ray studies have been performed on heteropolynuclear clusters containing iron(III) and copper(II) ions: {Cu(Mesalen)}₂Fe(acac)(NO₃)₂ and |Cu(Mesalen))₃Fe(acac)(PF₆)₂ (Mesalen = l,2-bis((methyl-salicylidene)amino)ethane; acac = acetylacetonato(l-)). The spectroscopic results obtained for the trinuclear system {2CuFe} suggest strong antiferromagnetic coupling of iron(III) to the two copper(II) ions to yield a S = ³/₂ ground-state system. X-ray studies reveal that the tetranuclear system {3CuFe} crystallizes in the hexagonal space group P6₁22 with Z = 6, a = b = 14.471 (3) Å, and c = 50.372 (2) Å; it consists of a trinuclear unit {2CuFe} plus a mononuclear Cu(Mesalen) molecule, arranged in helicoidal polymeric chains. Magnetic susceptibility, EPR, and Mössbauer studies under moderate fields consistently show that the {2CuFe} and Cu(Mesalen) moieties are weakly interacting. Under high-fíeld and low-temperature conditions, this interaction is not observable in Mössbauer spectroscopy. Since the Mössbauer spectra under these conditions are identical for the tri- and tetranuclear clusters, we conclude that the {2CuFe} moieties are very similar in both cluster types. The presence of the additional Cu(Mesalen) molecule in the tetranuclear cluster expresses itself in the Mössbauer spectra only at elevated temperatures by an enhancement of the spin-spin relaxation rates of nearly 1 order of magnitude larger compared to the trinuclear cluster.