Anomalous difference in magnetic behavior between highly saddled iron(III) porphyrin complexes in the solid state

The spin states of microcrystalline samples of [Fe(OMTPP)L ₂]⁺ (L = DMAP or Py) have been examined and interpreted using Mössbauer spectroscopy, SQUID magnetometry and X-ray crystallography. The Mössbauer spectra of [Fe(OMTPP)(DMAP)₂]⁺ and [Fe(OMTPP)Py₂]⁺ showed that both of these complexes maintain the low-spin (S = 1/2) state over the 77-300 K temperature range. The spin states of these complexes were further confirmed by SQUID magnetometry. Thus, the magnetic behavior of [Fe(OMTPP)Py₂]⁺ is quite different from that of the structurally related species [Fe(OETPP)Py ₂]⁺. The latter complex exhibits a novel spin crossover between the S = 3/2 and S = 1/2 states as revealed by the spectroscopic and magnetic measurements. In order to understand the reasons for the absence of the spin crossover process in [Fe(OMTPP)Py₂]⁺, we have compared the crystal and molecular structures of [Fe(OMTPP)Py₂] ⁺ with those of the recently reported species [Fe(OETPP)Py ₂]⁺. In the case of [Fe(OMTPP)Py₂]⁺, the Fe-N_axial bond lengths hardly change with temperature and are 2.058(6) and 2.024(4) Å at 298 and 80 K, respectively. These results are in sharp contrast to those of the spin crossover complex [Fe(OETPP)Py ₂]⁺, in which the Fe-N_axial bonds contract from 2.201(3) Å at 298 K to 1.993(3) Å at 80 K. We have ascribed the difference in magnetic behavior between [Fe(OMTPP)Py₂]⁺ and [Fe(OETPP)Py₂]⁺ to the difference in molecular packing; the former adopts a densely packed cubic crystal system while the latter shows a less condensed monoclinic system. A cavity calculation has further confirmed the above mentioned assumption. While the cavity sizes around the pyridine ligands in [Fe(OETPP)Py₂]⁺ are 32.08 and 28.88 ų at 298 K, that in [Fe(OMTPP)Py₂] ⁺ is only 19.81 ų. Furthermore, the cavities contract by 17.7% in [Fe(OETPP)Py₂]⁺ when the temperature is lowered from 298 to 80 K whereas the contraction is only 5.3% in the case of [Fe(OMTPP)Py₂]⁺. On the basis of these results, we have concluded that the loosely packed crystal system and the wide cavities around the axial ligands are the important requirements for the occurrence of the spin crossover process in the solid state.