Mössbauer Spectral Study of Two Layered Honeycomb Molecular Magnets: PPh4FeIIFeIII(ox)3 and NBu4FeIIFeIII(ox)3

The Mossbauer spectra of the layer ferrimagnets PPh4FeIIFeIII(ox)3 and NBu4FeIIFeIII(ox)3 have been measured between 1.9 and 315 K. The paramagnetic spectra exhibit both high-spin iron(II) and iron(III) doublets. The iron(II) quadrupole splittings are negative and decrease substantially upon cooling as a result of a low-symmetry crystal field splitting. In contrast, the iron(III) splittings are small, positive, and virtually independent of temperature. The respective magnetic ordering temperatures of 30 and 42.5 K agree well with the phase observed by bulk magnetometry and the magnetic sextets yield hyperfine fields of ca. 60 kOe for iron(II) and 540 kOe for iron(III). The unusually low iron(II) field results from the exceptionally large orbital and dipolar contributions, while the iron nuclei experience a static hyperfine field at 1.9 K and a relaxing hyperfine field between ca. 10 and 30 K.