Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

7C, 9A/2002G251

Local Structure Analysis of Cr Complexes with Distorted Porphyrins
Yasuhiro INADA1*, Taro ISHIBASHI2, Masahiko INAMO2
1

KEK-PF, Tsukuba, Ibaraki 305-0801, Japan
Aichi University of Education, Igaya, Kariya 448-8542, Japan

2

Introduction
The low reactivity for ligand substitutions of Cr(III) isimproved by the coordination of electron donating ligands.
For example, the water exchange rate constant of theCr(III) tetraphenylporphyrin complex ([Cr(tpp)Cl(H2O)])is larger by ca. 6 orders of magnitude than that of3+Cr(H2O)6 , and this is interpreted to be due to theenhanced electron density on Cr(III), which assists thedissociation of coordinating water molecules because of
the increased electronic repulsion with donating electronsof water. The improvement of substitution reactivity ismuch enhanced for the Cr complexes with a deformed
porphyrin, but the origin of its enhancement is unclear
because the structure of such Cr complex is not resolved
and the electronic state of the Cr center is not clarified. In
this study, the local structure and the valence state of theCr center are clarified by the XAFS measurements for theCr complex of bulky dodecaphenylporphyrin (DPP).
Experiments
The XAFS spectra were measured by the conventional
method at BL-7C and 9A. The higher order harmonics
were removed by the double-mirror system of SiO2 (3.5mrad) or Ni-coated SiO2 (6 mrad) at BL-7C or 9A,respectively. The ionization chambers were used tomeasure the incident and transmitted X-ray intensities.
The first crystal of Si(111) double-crystal monochromator
was sagittally focused to get higher X-ray flux at BL-7C.
The XANES and EXAFS spectra of Cr–DPP complex(1) was measured together with those of K2Cr2O7 (2)Na[CrO(hmp)2] (3: hmp = 2-hydroxy-2-methylpropionate), [CrO(tpp)] (4: tpp = 5,10,15,20-tetraphenylporphyrin), [CrCl(tpp)(H2O)] (5). All powder samples werediluted using BN powder.
Results and Discussion
The XANES spectrum of the Cr–DPP complex is very
similar to that of 5 with the Cr(III) center as shown inFigure 1. The other complexes with the different valenceof Cr show a clear and intense pre-edge peak, which is
assigned to the transition from 1s to 3d, and the pre-edge
peak is shifted to the higher energy with the increasing
valence. In the cases of Cr–DPP and 5, the corresponding
pre-edge peak is quite weak and their energies are very
similar. It is thus concluded that the valence state of Cr is+3 in the Cr–DPP complex.
The EXAFS spectra were analyzed to determine thelocal structure parameters around the Cr center inFigure 1: XANES spectra of Cr foil (a), [CrCl(tpp)(H2O)] (b), [CrO(tpp)] (c), Na[CrO(hmp)2] (d), K2Cr2O7(e), and Cr–DPP.
considering all the contributions including multiple
scatterings. The number (N) and distance (R) of somedirect interactions are summarized in Table 1 for Cr–DPPand 5. Two O atoms are found to bind to the Cr center in
Cr–DPP. One is assigned to a coordinating water
2
1
according to its Cr–O distance, and the other (Cr–O ) is
considered to be the OH group. This coordination
environment is consistent with the assignment of Cr(III)on the basis of XANES. The shorter Cr–C interactions ofCr–DPP relative to those of 5 indicate that the porphyrin
core is largely deformed in the Cr–DPP complex.
Table 1: Structure parameters––––––––––––––––––––––––––––––––––––––––[CrCl(tpp)(H2O)]
Cr–DPP–––––––––––––––––––––––––
interaction N
R /Å
N
R /Å
––––––––––––––––––––––––––––––––––––––––
Cr–N
42.04
42.03
1Cr–O
11.92
2
Cr–O
1
2.03
1
2.03
Cr–Cl
12.32
1
Cr–C
8
3.02
8
2.98
2
Cr–C
43.46
4
3.38
3
Cr–C
84.30
84.22
––––––––––––––––––––––––––––––––––––––––*

yasuhiro.inada@kek.jp

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