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

Abstract

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.