Rate Constants and Thermodynamic Parameters of Rotation of Axial Ligands in a Bisligated Ferric Tetramesitylporphyrinate Complex Measured from the Temperature Dependence of ^1H Transverse Relaxation Rates
The rate constant of the four-site cyclic chemical exchange between pyrrole protons in tetramesitylporphyrinatoiron(III)bis(2-methylimidazole) has been measured in the temperature range 236-255 K from 1H transverse relaxation times (T_2) observed at the 500 MHz field strength. The values of the rate constant were obtained through the simulartion of the observed T_2's under the assumption that their values are dominated by the rate of the chemical exchange. The measurement of the exchange rate constant was also attempted at a lower temperature (230 K), but the use of extrapolated intrinsic T_2's at that temperature introduces a significant error into the simulated rate constant. The thermodynamic activation parameters of the cyclic exchange were determined as Delta H++ = 48 +- 1 kJ/mol and Delta S = -10 +- 6 J/K mol. The value of k varied from 30 to 270 s^-1 in the temperature range 236-255 K. The near-zero exchange activation entropy is consistent with two previous studies but contradicts another study based on saturation-transfer measurements previously carried out by the authors of this work. A comparison of all available studies shows that the saturation-transfer experiments overestimate the value of the exchange rate constant by up to a factor of 2. It is showt that equilibrium with the high-spin form of the porphyrin complex does not affect the observed T_2's in the temperature range used in this study but may do so at temperatures above 270 K. It is also shown that neither the linearity of the Curie plot nor the behavior of the T_1 versus temperature plot should be considered indicators of the absence of the low-spin <-> high spin equilibrium.