Environmental effects on the rate of intramolecular electron transfer in trinuclear mixed-valence transition metal carboxylate complexes in the solid state
In a series of the isostructural (R32 space group) mixed-valence (Fe$\sb3$O(O$\sb2$CCH$\sb3$)$\sb6$(4-Me-Py)$\sb3$) $\cdot$ S complexes, where (4-Me-Py) is 4-methylpyridine and S is a solvate molecule, we have found that systematic changes of solvate molecules have a pronounced impact on the phase transitions at which a given complex valence detraps. This sensitivity is a reflection of the fact that the lowest energy electronic states of Fe$\sb3$O complexes are vibronic and as a result these complexes are very sensitive to their environment. It is also found that the CHCl$\sb3$ solvate complex exhibits a very abrupt phase transition at low temperature (95K) and the CH$\sb3$CCl$\sb3$ solvate complex exhibits a phase transition at 125K. $\sp{57}$Fe Mossbauer spectra of this CHCl$\sb3$ solvate complex show that this complex valence-detraps at $\sim$95K. However, the complex with the less symmetric CH$\sb3$CHCl$\sb2$ solvate molecule becomes valence-detrapped at $\sim$45 degrees higher than for the CH$\sb3$CCl$\sb3$ complex and $\sim$75 degrees higher than for the CHCl$\sb3$ complex. Changing the solvate molecules may lead to changes in the intermolecular interactions propagated via the pyridine-pyridine overlaps between neighboring Fe$\sb3$O molecules. The introduction of the bulky solvate (CH$\sb3$CCl$\sb3$) and less symmetric solvate (CH$\sb3$CHCl$\sb2$) gives rise to less intermolecular interactions between neighboring Fe$\sb3$O molecules and, consequently, gives higher transition temperature than that of the C$\sb3$ symmetry CHCl$\sb3$ solvate. In fact, the results of CNDO/2 molecular orbital calculations show that an important factor is the intermolecular interactions between the 4-Me-Py$\cdots$4-Me-Py ligands for controlling the intramolecular electron transfer rate in addition to the onset of solvate molecules dynamic motion.
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