Ferrimagnetic Mixed-Valency and Mixed-Metal Tris(oxalato)iron(III) Compounds: Synthesis, Structure, and Magnetism

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Abstract
The synthesis and structural and magnetic characterization of 16 compounds AMIIFeIII(C2O4)3 (A = N(n-C3H7)4, N(n-C4H9)4, N(n-C5H11)4, P(n-C4H9)4, P(C6H5)4, N(n-C4H9)3(C6H5CH2), (C6H5)3PNP(C6H5)3, As(C6H5)4; MII = Mn, Fe) are reported. X-ray powder diffraction profiles are indexed in R3c or its subgroup P6522 or P6/mmm to derive unit cell constants. The structures of all the compounds consist of two-dimensional honeycomb networks [MIIFeIII(C2O4)3-]∞. The MII = Fe compounds behave as ferrimagnets with Tc between 33 and 48 K, but five exhibit a crossover from positive to negative magnetization near 30 K when cooled in a field of 10 mT. The compounds exhibiting this unusual magnetic behavior are those that have the highest Tc. Within the set N(n-CnH2n+1)4FeIIFeIII(C2O4)3 (n = 3−5), Tc increases with interlayer separation and the low-temperature magnetization changes from positive (n = 3) to negative (n = 4, 5). In the M = MnII compounds, the in-plane cell parameter a0 is ∼0.03 Å greater than in the corresponding M = FeII ones while the interlayer separation (c0/6) is on average 0.08 Å smaller. All members of the MII = Mn series have magnetic susceptibilities showing broad maxima at 55 K characteristic of two-dimensional antiferromagnetism, but the magnetization of several of the salts increases sharply below 27 K due to the onset of spin canting, the magnitude of which varies significantly with A.

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