Electronic Structure of Six-Coordinate Iron(III) Monoazaporphyrins

The electronic structures of six-coordinate iron(III) octaethylmonoazaporphyrins, [Fe(MAzP)L2]± (1), have been examined by means of 1H NMR and EPR spectroscopy to reveal the effect of meso-nitrogen in the porphyrin ring. The complexes carrying axial ligands with strong field strengths such as 1-MeIm, DMAP, CN−, and tBuNC adopt the low-spin state with the (d xy )2(d xz , d yz )3 ground state in a wide temperature range where the 1H NMR and EPR spectra are taken. In contrast, the complexes with much weaker axial ligands, such as 4-CNPy and 3,5-Cl2Py, exhibit the spin transition from the mainly S = 3/2 at 298 K to the S = 1/2 with the (d xy )2(d xz , d yz )3 ground state at 4 K. Only the THF complex has maintained the S = 3/2 throughout the temperature range examined. Thus, the electronic structures of 1 resemble those of the corresponding iron(III) octaethylporphyrins, [Fe(OEP)L2] ± (2). A couple of differences have been observed, however, in the electronic structures of 1 and 2. One of the differences is the electronic ground state in low-spin bis(tBuNC) complexes. While [Fe(OEP)(tBuNC)2]+ adopts the (d xz , d yz )4(d xy )1 ground state, like most of the bis(tBuNC) complexes reported previously, [Fe(MAzP)(tBuNC)2]+ has shown the (d xy )2(d xz , d yz )3 ground state. Another difference is the spin state of the bis(3,5-Cl2Py) complexes. While [Fe(OEP)(3,5-Cl2Py)2]+ has maintained the mixed S = 3/2 and 5/2 spin state from 298 to 4 K, [Fe(MAzP)(3,5-Cl2Py)2]+ has shown the spin transition mentioned above. These differences have been ascribed to the narrower N4 cavity and the presence of lower-lying π* orbital in MAzP as compared with OEP.