The rates of racemization and dissociation of the tris(1,10-phenanthroline)iron(II) cation in various solvents

Abstract

Rates of racemization and dissociation of [Fe(phen)3I2+ were measured in water, methanol, acetone, formamide, N,N-dimethylformamide, acetonitrile, and acetic acid, and in mixtures of water with methanol, acetone, formamide, and N,N-dimethylformamide. The rate of racemization was also measured in mixtures of water with glycerol and ethylene glycol. In all cases the dissociation rate was much slower than the racemization rate, indicating that racemization is predominantly intramolecular in all these solvents. Ion association (with clod-), when it occurs, has a retarding effect on the rate of racemization. The effect of a solvent on the racemization rate appears to be due to solvation of the aromatic phenanthroline ligands in the transition state, and to the viscous resistance to reorganization of the solvent in the vicinity of the complex ion when the latter undergoes inversion. Supporting this view is the fact that the rate constants can be empirically represented by the equation log k = c -(0.26AClig,,,j/2.303RT) -0. 48 log ( q V , ) . where a c l i g a n d is the free energy of solvation of the aromatic ligands, q is the coefficient of viscosity of the solvent, and V , is the molar volume of the solvent. The effect of solvent on the dissociation rate appears to be due to solvation of the ligands in the transition state, and the solvent's ability to act as a replacing ligand

Investigation of the stereodynamics of molecules and catalyzed reactions by CE

“…While one study describes the investigation of the enantiomerization of ferroin by preparative enantiomer separation followed by a measurement of the racemization kinetics by polarimetry [155,156], other studies do not describe enantiomerizations for the higher homologs 1,10-phenanthrolin complexes of B VIII group elements [157,158].…”

Investigation of the stereodynamics of tris‐(α‐diimine)–transition metal complexes by enantioselective dynamic MEKC

“…It can be assumed that the transition state for the interconversion of tris(1,10-phenanthroline) complexes occurs via a dissociation-association mechanism, where at least one of the phenanthroline ligands completely dissociates from the central metal and leaves the complex sphere [46]. This process is indicated by positive activation entropies.…”

Dynamic HPLC of stereolabile iron(II) complexes on chiral stationary phases

“…In pure acetonitrile at T 5 258C the enantiomerization barrier was observed to be DG # 5 20.4 kcal/mol. 25 Positive values for the entropy of activation DS # 20-24 cal/K mol have also been found both in pure water and in mixed solvents. 24,25,31 Thus, small but sizeable solvent and temperature effect are known to affect the rate at which the D/L interconversion of 1 takes place.…”

“…Rats of racemization in pure water were measured at five temperatures between 12 and 35.68C: after conversion to enantiomerization rates, these data yield free energy barriers between 22.5 and 22.0 kcal/mol. 25 Recently, a value of DG # 5 21.8 kcal/mol for the same enantiomerization in water at T 5 258C was reported. 24 Enantiomerization barriers between 22.3 and 22.4 kcal/mol have been measured for 1 in 0.01 and 0.1 M HCl at temperatures between 15 and 258C.…”