Multi-Temperature Crystallographic Studies of Mixed-Valence Polynuclear Complexes; Valence Trapping Process in the Trinuclear Oxo-Bridged Iron Compound, [Fe3O(O2CC(CH3)3)6(C5H5N)3

Abstract

Single-crystal X-ray diffraction data have been collected on five different crystals at 12 different temperatures (10, 28, 35, 60, 85, 100, 118, 135, 160, 200, 240, 295 K) on a trinuclear, oxo-bridged, mixed-valence iron complex, Fe3O(O2CC(CH3)3)6(C5H5N)3, using both synchrotron and conventional radiation sources. The present study for the first time provides structural information for an oxo-bridged trinuclear compound below the boiling point of nitrogen (77 K). The use of very low-temperature crystallographic data is crucial for understanding the physical properties of the complex. No change of space group is observed in the whole temperature range, although a reversible broadening of the Bragg peaks is observed around 85 K. The structure has ordering processes involving the tert-butyl groups, and above 85 K, four tert-butyl groups become disordered. Around 150 K, a fifth tert-butyl becomes disordered, whereas the last tert-butyl is ordered at all temperatures. Very significant temperature-dependent changes in the Fe-ligand bond lengths are observed which are interpreted as being due to dynamic disorder caused by intramolecular electron transfer (ET) between the metal sites. The ET process is significantly affected by changes in the molecular potential energy surface (PES) caused by the dynamic behavior of the tert-butyls. The dynamic disorder of the Fe3O core resulting from the ET process is examined through analysis of the atomic displacement parameters. The ET process involves only two of the three iron sites, with the third site appearing to be valence-trapped at all temperatures. The trapping of this iron site at all temperatures appears to be related to the asymmetry caused by the different dynamic behaviors of the tert-butyls. At very low temperatures (<10 K), the system becomes valence-trapped and consists of a single configuration without disorder. Boltzmann population models are used to estimate the energy difference between the two lowest-lying minima on the PES (ΔE < 100 cm-1) and between two disordered configurations of each of the tert-butyls (ΔE = 217, 212, 255, 359, and 345 cm-1).