No abstract

Iron(II) octaethyltetraazaporphyrin has been discovered to exist as two polymorphs at room temperature, α- and β-Fe(OETAP). This has been established by X-ray powder diffraction and scanning electron microscopy. Both ac and dc SQUID magnetometry show that the α phase is a soft molecular ferromagnet with T Curie = 2.8 K. The β phase possesses the same number of unpaired spins and a similar g value, but it does not order above ∼1 K. Zero-field splitting is likely competing with ferromagnetic coupling in this compound. 57Fe Mössbauer spectroscopy has been used to support the local structural similarity between the two phases and to study the ferromagnetic transition. The isomorphous manganese analogue of α-Fe(OETAP), α-Mn(OETAP), expected to be a ferromagnet with a higher T Curie, exhibits many characteristics of ferromagnetism, notably hysteresis at 1.8 K with H coer = 2.5 kG and M rem = 4 kemu-G/mol. However, in contrast to ferromagnetic α-Fe(OETAP), α-Mn(OETAP) shows strongly frequency-dependent ac susceptibility in the range from 1 to 1000 Hz, indicating that it exists in a spin-glass-like state. This result underscores the need for frequency-dependent ac magnetometry to fully characterize ferromagnetic molecular materials.