Electronic structures of six-coordinate ferric porphyrin complexes with weak axial ligands: Usefulness of 13C NMR chemical shifts

Akito Hoshino, Yoshiki Ohgo, Mikio Nakamura

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1H NMR, 13C NMR, and EPR spectra of six-coordinate ferric porphyrin complexes [Fe(Por)L2]ClO4 with different porphyrin structures are presented, where porphyrins (Por) are planar 5,10,15,20-tetraphenylporphyrin (TPP), ruffled 5,10,15,20- tetraisopropylporphyrin (TiPrP), and saddled 2,3,7,8,12,13,17,18- octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP), and axial ligands (L) are weak oxygen ligands such as pyridine-N-oxide, substituted pyridine-N-oxide, DMSO, DMF, MeOH, THF, 2-MeTHF, and dioxane. These complexes exhibit the spin states ranging from an essentially pure high-spin (S = 5/2) to an essentially pure intermediate-spin (S = 3/2) state depending on the field strength of the axial ligands and the structure of the porphyrin rings. Reed and Guiset reported that the pyrrole-H chemical shift is a good probe to determine the spin state in the spin admixed S = 5/2,3/2 complexes (Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282). In this paper, we report that the chemical shifts of the α- and β-pyrrole carbons can also be good probes to determine the spin state because they have shown good correlation with those of the pyrrole-H or pyrrole-Cα. By putting the observed or assumed pyrrole-H or pyrrole-Cα chemical shifts of the pure high-spin and pure intermediate-spin complexes into the correlation equations, we have estimated the carbon chemical shits of the corresponding complexes. The orbital interactions between iron(III) and porphyrin have been examined on the basis of these chemical shifts, from which we have found that both the d xy-a2u interaction in the ruffled Fe(TiPrP) L2+ and dxy-a1u interaction in the saddled Fe(OETPP)L2+ are quite weak in the high-spin and probably in the intermediate-spin complexes as well. Close inspection of the correlation lines has suggested that the electron configuration of an essentially pure intermediate-spin Fe(TiPrP)L2+ changes from (dxy, dyz)3(dxy) 1(dz2)1 to (dxy)2(d xz, dyz)2(dz2)1 as the axial ligand (L) changes from DMF to MeOH, THF, 2-MeTHF, and then to dioxane. Although the DFT calculation has indicated that the highly saddled intermediate-spin Fe(OETPP)-(THF)2+ should adopt (d xy, dyz)3(dxy)1(d z2)1 rather than (dxy)2(d xz, dyz)2(dz2)1 because of the strong dxy-aiu interaction (Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han, Y.-P.; Chang, C.-C. Chem. Commun. 2005, 1312-1314), our 13C NMR study again suggests that Fe(OETPP)(THF)2 + should be represented as (dxy)2(d xz, dyz)2(dz2)1 because of the weak dxy-aiu interaction. The contribution of the S = 3/2 state in all types of the spin admixed S = 5/2,3/2 six-coordinate complexes has been determined on the basis of the 13C NMR chemical shifts.

Original languageEnglish
Pages (from-to)7333-7344
Number of pages12
JournalInorganic Chemistry
Issue number21
Publication statusPublished - 2005 Oct 17
Externally publishedYes

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry


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