Benzoannelation stabilizes the d _xy ¹ state of low-spin iron(III) porphyrinates

A series of low-spin, six-coordinate complexes [Fe(TBzTArP)L ₂]X (1) and [Fe(TBuTArP)L ₂]X (2) (X=Cl ^-, BF ₄ ^-, or Bu ₄N ⁺), where the axial ligands (L) are HIm, 1-MeIm, DMAP, 4-MeOPy, 4-MePy, Py, and CN ^-, were prepared. The electronic structures of these complexes were examined by ¹H NMR and electron paramagnetic resonance (EPR) spectroscopy as well as density functional theory (DFT) calculations. In spite of the fact that almost all of the bis(HIm), bis(1-MeIm), and bis(DMAP) complexes reported previously (including 2) adopt the (d _xy) ²(d _xz, d _yz) ³ ground state, the corresponding complexes of 1 show the (d _xz, d _yz) ⁴(d _xy) ¹ ground state at ambient temperature. At lower temperature, the electronic ground state of the HIm, 1-MeIm, andDMAP complexes of 1 changes to the common (d _xy) ²(d _xz, d _yz) ³ ground state. All of the other complexes of 1 and 2 carrying 4-MeOPy, 4-MePy, Py, and CN ^- maintain the (d _xz, d _yz) ⁴(d _xy) ¹ ground state in the NMR temperature range, i.e., 298-173 K. The EPR spectra taken at 4.2 K are fully consistent with the NMRresults because theHImand 1-MeIm complexes of 1 and 2 adopt the (d _xy) ²(d _xz, d _yz) ³ ground state, as revealed by the rhombic-type spectra. The DMAP complex of 1 exists as a mixture of two electron-configurational isomers. All of the other complexes adopt the (dxz, dyz)4(dxy)1 ground state, as revealed by the axialtype spectra. Among the complexes adopting the (d _xz, d _yz) ⁴(d _xy) ¹ ground state, the energy gap between the dxy and dp orbitals in 1 is always larger than that of the corresponding complex of 2. Thus, it is clear that the benzoannelation of the porphyrin ring stabilizes the (d _xz, d _yz) ⁴(d _xy) ¹ ground state. The DFT calculation of the bis(Py) complex of analogous iron(III) porphyrinate, [Fe(TPTBzP)(Py) ₂] ⁺, suggests that the (d _xz, d _yz) ⁴(d _xy) ¹ state is more stable than the (d _xy) ²(d _xz, d _yz) ³ state in both ruffled and saddled conformations. The lowest-energy states in the two conformers are so close in energy that their ordering is reversed depending on the calculation methods applied.On the basis of the spectroscopic and theoretical results, we concluded that 1, having 4-MeOPy, 4-MePy, and Py as axial ligands, exists as an equilibrium mixture of saddled and ruffled isomers both of which adopt the (d _xz, d _yz) ⁴(d _xy) ₁ ground state. The stability of the (d _xz, d _yz) ⁴(d _xy) ¹ ground state is ascribed to the strong bonding interaction between the iron d _xy and porphyrin a1u orbitals in the saddled conformer caused by the high energy of the a1u highest occupied molecular orbital in TBzTArP. Similarly, a bonding interaction occurs between the dxy and a2u orbitals in the ruffled conformer. In addition, the bonding interaction of the dp orbitals with the low-lying lowest unoccupied molecular orbital, which is an inherent characteristic of TBzTArP, can also contribute to stabilization of the (dxz, dyz)4(dxy)1 ground state.