Chemical shift of meso-carbon: A powerful probe to determine the coordination structure and electron configuration of ferric porphyrin complexes

Article abstract of DOI:10.1039/b303785a

The meso-¹³C chemical shifts have been revealed to serve a powerful probe to determine the coordination structure and electron configuration of ferric porphyrin complexes.

Full text of DOI:10.1039/b303785a

Chemical shift of meso-carbon: a powerful probe to determine the
coordination structure and electron configuration of ferric porphyrin
complexes†
Mikio Nakamura,*ab Akito Hoshino,b Akira Ikezakia and Takahisa Ikeuea
a Department of Chemistry, School of Medicine, Toho University, Ota-ku, Tokyo 143-8540, Japan.
E-mail: mnakamu@med.toho-u.ac.jp
b Division of Biomolecular Science, Graduate School of Science, Toho University, Funabashi 274-8510,
Japan
Received (in Cambridge, UK) 4th April 2003, Accepted 28th May 2003
First published as an Advance Article on the web 18th June 2003
The meso-13C chemical shifts have been revealed to serve a
powerful probe to determine the coordination structure and
electron configuration of ferric porphyrin complexes.
with ca. 50% of the S = 3/2 contribution.5 The observed and
simulated spectra of 1a and 2a are given as ESI.† The result is
in accordance with our recent observation that the saddle
deformation stabilizes the S = 3/2 spin state.6
In the course of our research to produce high-valent iron–oxo
species by the reactions of pyridine N-oxide with ferric
porphyrin complexes such as [Fe(TPP)]ClO4 and [Fe(OETPP)]-
ClO4, we have found that the bis(pyridine N-oxide) complexes,
[Fe(TPP)L2]ClO4(1a–1c, Scheme 1) and [Fe(OETPP)L2]-
ClO4(2a–2c), showed unprecedented 13C NMR spectra.1 In this
paper, we report that the chemical shift of meso-13C can be a
powerful probe to determine not only the electron configura-
tion,2 but also the coordination structure of ferric porphyrin
complexes.
Most of the synthetic high-spin ferric porphyrins are
5-coordinate. There are, however, a limited number of examples
showing 6-coordination.3,4,7
A
typical example is
[Fe(OEP)(DMSO)2]+, whose meso-H signals appear fairly
downfield, at d 40.1 ppm. Interestingly, the meso-H signals of
5-coordinate high-spin [Fe(OEP)Cl] are observed in the
opposite direction, d 255.6 ppm.4b,8 We are very much
interested in the 13C NMR spectra of high-spin 6-coordinate
complexes because no 13C NMR spectra have ever been
reported for these complexes. Fig. 2 shows the 13C NMR spectra
of meso-13C enriched 1a and 2a together with those of some
relevant complexes. The high-spin 1a showed the meso signal at
27.8 ppm at 25 °C, which differs from that of typical high-spin
5-coordinate [Fe(TPP)Cl] by 460 ppm.9 Similarly, high-spin
1b–1f exhibited the meso signals at 25.5, 29.1, 10.0, 5.2, and
56.7 ppm, respectively. By contrast, the meso signal of 2a was
observed at d 255.1 ppm, which is between the meso signals of
high-spin 1a (d 27.8 ppm) and intermediate-spin
[Fe(OETPP)(THF)2]ClO4 (d 282.5 ppm).6 Therefore, the 13C
NMR results are consistent with the EPR results in the sense that
2a is an admixed intermediate-spin complex.
1
Fig. 1 shows the H NMR spectra of 1a–1c formed by the
addition of 4.0 equiv. of pyridine N-oxides (a–c) to the CD2Cl2
solutions of [Fe(TPP)]ClO4 at 250 °C. Bis-coordination of the
ligand is unambiguously verified from the integral intensities of
the coordinated ligand signals. The downfield shifts of the
pyrrole signals in 1a–1c, ca. 100 ppm at 250 °C, clearly
indicate that these complexes are high-spin (S = 5/2).3 The
ligand signals were observed at extremely far upfield and
downfield positions. Furthermore, the sign of the isotropic shift
was reversed by the methyl substitution; the chemical shift of
the 4-H in 1a is 275.5 ppm while that of the 4-CH3 in 1b is
+81.2 ppm. The results suggest that a considerable amount of
spin in the iron dp(dxz and dyz) orbitals is delocalized to the axial
ligands through dp–pp interactions.4 Similar p electron delocal-
ization was observed in 2. The 1 H NMR spectra, chemical shifts
and Curie plots of these complexes are given in the ESI.† To
determine the spin states of 2, EPR spectra were taken in frozen
CH2Cl2 solution at 4.2 K. The g4 and g∑ values of 2a were 5.05
and 2.00, while those of 1a were 5.95 and 2.00, respectively.
Thus, 2a is in an admixed intermediate-spin state (S = 5/2, 3/2)
The ferric ions in synthetic porphyrin complexes and
naturally occurring heme proteins usually show either the high-
spin or the low-spin state. In the previous papers, we have
reported that the meso-13C chemical shift is a good probe to
determine the electron configuration of low-spin complexes; the
ferric ions with (dxy)2 (dxz, dyz)3 electron configuration show the
meso signals at 50 to 100 ppm at 25 °C, while those with (dxz,
dyz)4 (dxy)1 exhibit them at much lower magnetic field.2 The
Scheme 1
† Electronic supplementary information (ESI) available:chemical shifts, 1H
NMR spectra, Curie plots and EPR spectra of 1 and 2. See http:
//www.rsc.org/suppdata/cc/b3/b303785a/
Fig. 1 1H NMR spectra of (a) 1a, (b) 1b, and (c) 1c taken in CD2Cl2
solutions at 250°C.
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respectively. Both the pyrrole-H and the meso-13C signals move
downfield as the (dxz, dyz)4 (dxy)1 character increases, giving a
semi-parabolic curve with a positive slope. In the high-spin
complexes, the plots for the 5-coordinate complexes are located
far above those for the 6-coordinate ones. Therefore, both the
coordination structure and electron configuration can be
determined by the meso-13C signals. La Mar and coworkers
pointed out the importance of the meso-H chemical shifts to
determine the coordination structure of high-spin ferric heme
proteins,10 though the observation of the meso-H signals is
sometimes hampered because of their extreme breadth. In this
regard, the meso-13C chemical shift could be a better probe to
determine the coordination structure of high-spin ferric heme
proteins if we utilize enzymes reconstituted with 13C labeled
heme; 13C labeled protoheme IX can be biosynthesized using
13C labeled d-aminolevulinic acid.11
This work was supported by the Grant in Aid for Scientific
Research (No 14540521) from Ministry of Education, Culture,
Sports, Science and Technology, Japan. Thanks are due to the
Research Center for Molecular-Scale Nanoscience, the Institute
for Molecular Science (IMS).
Fig. 2 13C NMR spectra of meso-13C enriched complexes taken in CD2Cl2
at 25°C. The meso-13C signals are signified by the arrow. (a) [Fe(TPP)Cl],
(b) [Fe(OETPP)Cl], (c) [Fe(TPP)(PyNO)2]ClO4 (1a), (d) [Fe(OETPP)(Py-
NO)2]ClO4 (2a), (e) [Fe(OETPP)(THF)2]ClO4. Curie plots of the meso-
carbons are given in the inset: Ω, [Fe(TPP)Cl]; 3, [Fe(OETPP)Cl]; “,
[Fe(TPP)F2]Bu4N; 5, [Fe(TPP)(PyNO)2]ClO4; 8, [Fe(TPP)(DMSO)2]-
ClO4; < , [Fe(TPP)(DMF)2]ClO4; 2, [Fe(OETPP)(PyNO)2]ClO4; +,
[Fe(OETPP)(THF)2]ClO4.
Notes and references
1 Abbreviations: TPP, TRP, OEP, OETPP, dianions of 5,10,15,20-tetra-
phenylporphyrin, 5,10,15,20-tetraalkylporphyrin, 2,3,7,8,12,13,17,18-
octaethylporphyrin, and 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetra-
phenylporphyrin, respectively.
2 (a) T. Ikeue, Y. Ohgo, T. Saitoh, M. Nakamura, H. Fujii and M.
Yokoyama, J. Am. Chem. Soc., 2000, 122, 4068; (b) T. Ikeue, Y. Ohgo,
T. Saitoh, T. Yamaguchi and M. Nakamura, Inorg. Chem., 2001, 40,
3423.
3 T. Mashiko, M. E. Kastner, K. Spartalian, W. R. Scheidt and C. A. Reed,
J. Am. Chem. Soc., 1978, 100, 6354.
4 (a) G. N. La Mar and F. A. Walker, in The Porphyrins, ed. D. Dolphin,
Academic Press, New York, 1979, vol. 4, pp. 57–161; (b) F. A. Walker,
in The Porphyrin Handbook, eds. K. M. Kadish, K. M. Smith and R.
Guilard, Academic Press, San Diego, 2000, vol. 5, pp. 81–183.
5 G. Palmer, in Iron Porphyrins, eds. A. B. Lever and H. B. Gray,
Addison-Wesley, Reading, 1983, part II, pp. 43–88.
present study further reveals that the meso-13C chemical shift
can indicate the coordination structure of high-spin complexes.
This is most explicitly demonstrated in Fig. 3, which shows the
correlation of chemical shifts between pyrrole-H and meso-13C.
Three types of ferric complexes, i.e. 6-coordinate low spin,
5-coordinate high-spin, and 6-coordinate high-spin, are clearly
classified. In the low spin complexes, the pyrrole-H and meso-
13C appear in the range 220 to 15 ppm and 50 to 800 ppm,
6 (a) T. Ikeue, T. Saitoh, T. Yamaguchi, Y. Ohgo, M. Nakamura, M.
Takahashi and M. Takeda, Chem. Commun., 2000, 1989; (b) T. Ikeue,
Y. Ohgo, T. Yamaguchi, M. Takahashi, M. Takeda and M. Nakamura,
Angew. Chem., Int. Ed., 2001, 40, 2617; (c) M. Nakamura, T. Ikeue, Y.
Ohgo, M. Takahashi and M. Takeda, Chem. Commun., 2002, 1198.
7 (a) M. Zobrist and G. N. La Mar, J. Am. Chem. Soc., 1978, 100, 1944;
(b) D. L. Budd, G. N. La Mar, K. C. Langry, K. M. Smith and R. Nayyir-
Mazhir, J. Am. Chem. Soc., 1979, 101, 6091; (c) W. R. Scheidt, Y. J.
Lee, S. Tamai and K. Hatano, J. Am. Chem. Soc., 1983, 105, 778; (d) A.
Malek, L. Latos-Grazynski, T. J. Bartczak and A. Zadlo, Inorg. Chem.,
1991, 30, 3222; (e) W. R. Scheidt, in The Porphyrin Handbook, eds. K.
M. Kadish, K. M. Smith and R. Guilard, Academic Press, San Diego,
2000, vol. 3, pp. 49–112.
8 E. P. Sullivan Jr., J. D. Grantham, C. S. Thomas and S. H. Strauss, J. Am.
Chem. Soc., 1991, 113, 5264.
9 J. Mispelter, M. Momenteau and J.-M. Lhoste, J. Chem. Soc., Dalton
Trans, 1981, 1729.
10 G. N. La Mar, J. D. Satterlee and J. S. De Ropp, in The Porphyrin
Handbook, eds. K. M. Kadish, K. M. Smith and R. Guilard, Academic
Press, San Diego, 2000, vol. 5, pp. 185–298.
11 G. A. Caignan, R. Deshmukh, A. Wilks, Y. Zeng, H.-W. Huang, P.
Moénne-Loccoz, R. A. Bunce, M. A. Eastman and M. Rivera, J. Am.
Chem. Soc., 2002, 124, 14879.
Fig. 3 Relation of chemical shifts between pyrrole-H and meso-13C nuclei
at 25°C: 2, 33 low spin complexes of the types [Fe(TArP)L2]± and
[Fe(TRP)L2]±; 8, 9 high-spin five-coordinate complexes of the types
[Fe(TPP)X] and [Fe(TRP)X]; 5, 6 high-spin six-coordinate complexes, 1a–
1f, determined in this study. The dxy and dp indicate the (dxz, dyz)4(dxy)1 and
(dxy)2(dxz, dyz)3 electron configuration, respectively.
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