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-Melm, 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-Cl ₂Py)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.