Environmental effects on the rate of intramolecular electron transfer in trinuclear mixed-valence transition metal carboxylate complexes in the solid state

Jang, Ho Gyeom

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https://hdl.handle.net/2142/23159 Copy

Description

Title

Environmental effects on the rate of intramolecular electron transfer in trinuclear mixed-valence transition metal carboxylate complexes in the solid state

Author(s)

Jang, Ho Gyeom

Issue Date

1989

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

Chemistry, Inorganic

Language

eng

Abstract

• 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.
• Interestingly, solid-state $\sp2$H NMR studies of (Fe$\sb3$O(O$\sb2$CCH$\sb3$)$\sb6$(Py)$\sb3$) (CDCl$\sb3$) and (Fe$\sb3$O(O$\sb2$CCH$\sb3$)$\sb6$(4-Me-Py)$\sb3$) (CDCl$\sb3$) show that the C$\sb3$-symmetry CHCl$\sb3$ molecule synchronously moves with the changes of the vibronic coordinates in neighboring Fe$\sb3$O molecules in the lattice. Thus, we can suggest that another important factor in controlling the rate of electron transfer may be the van der Waals interactions between a solvate molecule and neighboring Fe$\sb3$O complexes. This van der Waals interactions may be large enough to modify the ground state potential-energy surface for a Fe$\sb3$O complex to affect the rate at which such a complex can tunnel from one vibronic minimum to another.
• Finally, we have discovered the first trinuclear iron acetate complex (Fe$\sb3$O(O$\sb2$CCH$\sb3$)$\sb6$(3-Et-Py)$\sb3$) (C$\sb7$H$\sb8$)$\sb{0.5}$ which exhibits an isosceles Fe$\sb3$O triangular plane at room temperature, i.e., completely valence-trapped on the X-ray time scale. However, the analogous mixed-valence (Fe$\sb3$O(O$\sb2$CCH$\sb3$)$\sb6$(3-Et-Py)$\sb3$) (CH$\sb3$CCl$\sb3$) shows a valence detrapping phenomenon due to the adoption of a symmetric solvate molecule configuration. Thus, one really can turn on and off the intramolecular electron transfer in the mixed-valence complexes by controlling the lattice environments.

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Copyright 1989 Jang, Ho Gyeom

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