Effects of solvents on the electron configurations of the low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato]iron(III) complex: Importance of the C-H···N weak hydrogen bonding

There are two types of electron configurations, (d_xy)²(d_xz, d_yz)³ and (d_xz, d_yz)⁴(d_xy)¹, in low-spin iron(III) porphyrin complexes. To reveal the solvent effects on the ground-state electron configurations, we have examined the ¹³C-and ¹H-NMR spectra of low-spin dicyano[meso-tetrakis(2,4,6-triethylphenyl)porphyrinato] ferrate(III) in a variety of solvents, including protic, dipolar aprotic, and nonpolar solvents. On the basis of the NMR study, we have reached the following conclusions: (i) the complex adopts the ground state with the (d_xz, d_yz)⁴(d_xy)¹ electron configuration, the (d_xz, d_yz)⁴(d_xy)¹ ground state, in methanol, because the dorbitals are stabilized due to the O-H···N hydrogen bonding between the coordinated cyanide and methanol; (ii) the complex also exhibits the (d_xz, d_yz)⁴(d_xy)¹ ground state in nonpolar solvents, such as chloroform and dichloromethane, which is ascribed to the stabilization of the dorbitals due to the C-H···N weak hydrogen bonding between the coordinated cyanide and the solvent molecules; (iii) the complex favors the (d_xz, d_yz)⁴(d_xy)¹ ground state in dipolar aprotic solvents, such as DMF, DMSO, and acetone, though the (d_xz, d_yz)⁴(d_xy)¹ character is less than that in chloroform and dichloromethane; (iv) the complex adopts the (d_xy)²(d_xz, d_yz)³ ground state in nonpolar solvents, such as toluene, benzene, and tetrachloromethane, because of the lack of hydrogen bonding in these solvents; (v) acetonitrile behaves like nonpolar solvents, such as toluene, benzene, and tetrachloromethane, though it is classified as a dipolar aprotic solvent. Although the NMR results have been interpreted in terms of the solvent effects on the ordering of the d_xy and d orbitals, they could also be interpreted in terms of the solvent effects on the population ratios of two isomers with different electron configurations. In fact, we have observed the unprecedented EPR spectra at 4.2 K which contain both the axial- and large g_max-type signals in some solvents such as benzene, toluene, and acetonitrile. The observation of the two types of signals has been ascribed to the slow interconversion on the EPR time scale at 4.2 K between the ruffled complex with the (d_xz, d_yz)⁴(d_xy)¹ ground state and, possibly, the planar (or nearly planar) complex with the (d_xy)²(d_xz, d_yz)³ ground state.