Solid-state deuterium NMR, iron-57 Moessbauer, and x-ray structural characteristics of .mu.3-oxo-bridged mixed-valence [Fe3O(O2CCH3)6(4-Me-py)3](C6H6): dynamics of the benzene solvate molecules influencing intramolecular electron transfer

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Scott E., Woehler; R. J., Wittebort; Oh, Seung M.; Hendrickson, Z David N.; Inniss, Daryl; Charles E., Strouse
Issue Date
American Chemical Society
J. Am. Chem. Soc. 1986, 108, 2938
Intramolecular electron transfer is investigated in the mixed-valence complex [Fe30(02CCH3)6(4-Me-py)3] (C6H6),
where 4-Me-py is 4-methylpyridine. Rotational motion of the benzene solvate molecule and the possible influence of this motion
on the rate of intramolecular electron transfer are studied crystallographically and spectroscopically. The compound crystallizes
in the rhombohedral space group R32; a = b = 18.552 (3) A, c = 10.556 (2) A at 133 K with 2 = 3. The final discrepancy
factors are R = 0.048 and R, = 0.061 for 1209 reflections with I > 340. Complex molecules and disordered benzene solvate
molecules are stacked in alternate sites of 32 symmetry along the 3-fold c axis. The unit cell contains three such stacks related
by a 31 axis. The 4-Me-py ligands are nearly coplanar with the Fe30 moiety. The 3l axis passes through the three nearly
parallel 4-Me-py ligands of three adjacent stacks. The interligand separation, c/3 = 3.51 A, along the 3, axes probably controls
the size of the solvate cavity along the 3-fold axis. Electron density maps indicate a preferred orientation for the benzene
solvate molecule with its 6-fold axis perpendicular to both the crystallographic 3-fold and 2-fold axes. The large thermal parameter
observed for the solvate molecule is consistent with a dynamic disorder of this group. Two doublets of area ratio 2:l (Fe"':Fe")
are present in the MGssbauer spectrum at temperatures approaching liquid helium. As the sample temperature is increased
above -60 K, the spectrum changes to eventually become a single average-valence doublet at temperatures above -200 K.
The complex [Fe,0(02CCH3),(4-Me-py)3](Ci6nD w6h)i,ch perdeuteriobenzene is the solvate molecule, gives high-resolution
ZH NMR spectra. Spectra are readily obtained from three sample types: random powders, magnetically oriented microcrystals,
and single crystals. The spectral properties are determined both by the motionally averaged ZH quadrupolar coupling and
by dipolar interactions of the deuterons with the unpaired electrons of the neighboring trinuclear complexes. These two types
of interactions are readily separated in the ZH NMR experiment. Single-crystal 2H NMR data were obtained at room temperature
by rotating a -1 X 1 X 1 mm crystal about three mutually orthogonal axes. Temperature studies were also carried out for
powdered and magnetically oriented microcrystalline samples in the range of - 150-293 K. From the orientation and magnitude
of the residual 2H quadrupolar coupling, it was concluded that the benzene solvate molecules are not only ring rotating about
their c6 axes, but they are also rotating about the C3 stacking axes. An appreciable through-space dipolar interaction of the
magnetic dipole of the deuterons with the magnetic dipoles of the nearby paramagnetic Fe30 complexes is present.
0002-7863 (print)1520-5126 (online)
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Chemical and Biological Engineering (화학생물공학부)Journal Papers (저널논문_화학생물공학부)
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