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 Akira Ikezaki † and Mikio Nakamura* ,†,‡ Department of Chemistry, Toho UniVersity School of Medicine, Ota-ku, Tokyo 143-8540, Japan, and DiVision of Biomolecular Science, Graduate School of Science, Toho UniVersity, Funabashi 274-8510, Japan Received August 6, 2001 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 13 C- and 1 H-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 configura- tion, the (d xz ,d yz ) 4 (d xy ) 1 ground state, in methanol, because the d π orbitals 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 d π orbitals 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 π 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. Introduction Two types of electron configurations, (d xy ) 2 (d xz ,d yz ) 3 and (d xz ,d yz ) 4 (d xy ) 1 , exist in low-spin iron(III) porphyrin com- plexes as shown in Scheme 1. 1-3 Major factors determining the ground-state electron configurations are (i) the ligand field strength of axial ligands, (ii) the deformation mode of porphyrin rings, (iii) the electronic effects at peripheral substituents, etc. 3 Thus, while most of the low-spin com- plexes show the electronic ground state with the (d xy ) 2 (d xz , * Author to whom correspondence should be addressed. E-mail: mnakamu@med.toho-u.ac.jp. † Toho University School of Medicine. ‡ Graduate School of Science, Toho University. (1) Walker, F. A.; Simonis, U. Proton NMR Spectroscopy of Model Hemes. In NMR of Paramagnetic Molecules; Berliner, L. J., Reuben, J., Eds.; Biological Magnetic Resonance; Plenum Press: New York, 1993; Vol. 12, pp 133-274. (2) Walker, F. A. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard, R., Eds.; Academic Press: San Diego, CA, 2000; Chapter 36, Vol. 5, pp 81-183. (3) Ikeue, T.; Ohgo, Y.; Saitoh, T.; Yamaguchi, T.; Nakamura, M. Inorg. Chem. 2001, 40, 3423-3434. Inorg. Chem. 2002, 41, 2761-2768 10.1021/ic0108383 CCC: $22.00 © 2002 American Chemical Society Inorganic Chemistry, Vol. 41, No. 10, 2002 2761 Published on Web 04/23/2002