MOSSBAUER, MAGNETIC-SUSCEPTIBILITY, EPR, AND EXAFS INVESTIGATIONS OF THE VIBRATIONALLY-INDUCED LOW-SPIN HIGH-SPIN TRANSITION IN A BIOMIMETIC FE(III) COMPLEX

Magnetic susceptibility measurements from 2 to 520 K, Mossbauer measur ements from 1.2 to 450 K, and EPR measurements at 10 K have been perfo rmed on the monomeric Fe-III complex t-butyl-2-mercaptobenzyl)-1-4-7-t riaza-cyclononan) Fe. The complex exhibits a low-spin/high-spin transi tion at temperatures above 250 K. This behavior is quantitatively expl ained on the basis of a crystal-field model, which explicitly includes the vibrational properties of iron ligands. The EPR spectrum at 10 K yields a pure Fe-III low-spin signal with g values 2.58(5), 2. 12(5), 1.45(5). The values are consistently described by a crystal-field mode l, which explicitly includes spin-orbit coupling within the t(2g), sub space. The temperature dependence of the quadrupole splitting indicate s a phase transition at approximately 100 K. The existence of the phas e transition is corroborated by the temperature dependence of the effe ctive thickness. The observation of only one quadrupole doublet up to 450 K indicates that the relaxation time between the high-spin and the low-spin configurations is shorter than the quadrupole precession tim e. X-ray structure analysis on single crystals at RT and temperature-d ependent EXAFS investigation of powder material between 30 and 200 K i ndicate that the observed phase transition induces only changes of bon d angles, while the low-spin/high-spin transition most likely induces changes of metal-ligand bond distances.