Difference in Spin Crossover Pathways among Saddle-Shaped Six-Coordinated Iron(III) Porphyrin Complexes

The electronic states of a series of saddle-shaped porphyrin complexes [Fe(OMTPP)L2]+ and [Fe(TBTXP)L2]+ have been examined in solution by 1H NMR, 13C NMR, and EPR spectroscopy and by magnetic measurements. While [Fe(OMTPP)(DMAP)2]+ and [Fe(TBTXP)(DMAP)2]+ maintain the low-spin (S = 1/2) state, [Fe(OMTPP)(THF)2]+ and [Fe(TBTXP)(THF)2]+ exhibit an essentially pure intermediate-spin (S = 3/2) state over a wide range of temperatures. In contrast, the Py and 4-CNPy complexes of OMTPP and TBTXP exhibit a spin transition from S = 3/2 to S = 1/2 as the temperature was decreased from 300 to 200 K. Thus, the magnetic behavior of these complexes is similar to that of [Fe(OETPP)Py2]+ reported in our previous paper (Ikeue, T.; Ohgo, Y.; Yamaguchi, T.; Takahashi, M.; Takeda, M.; Nakamura, M. Angew. Chem., Int. Ed. 2001, 40, 2617−2620) in the context that all these complexes exhibit a novel spin crossover phenomenon in solution. Close examination of the NMR and EPR data of [Fe(OMTPP)L2]+ and [Fe(TBTXP)L2]+ (L = Py, 4-CNPy) revealed, however, that these complexes adopt the less common (d xz , d yz )4(d xy )1 electron configuration at low temperature in contrast to [Fe(OETPP)Py2]+ which shows the common (d xy )2(d xz , d yz )3 electron configuration. These observations have been attributed to the flexible nature of the OMTPP and TBTXP cores as compared with that of OETPP; the relatively flexible OMTPP and TBTXP cores can ruffle the porphyrin ring and adopt the (d xz , d yz )4(d xy )1 electron configuration at low temperature. Therefore, this study reveals that the rigidity of porphyrin cores is an important factor in determining the spin crossover pathways.