Kinetic and quantum-chemical investigation of fac→mer isomerization of cobalt(III) trisaminoethoxide

Abstract

The kinetics of the fac®mer isomerization of cobalt(III) trisaminoethoxide were studied. The acceleration of the reaction with decrease of the pH of the solution is explained by the formation of a reactive monoprotonated form of the complex. The direction of isomerization agrees with quantum-chemical calculations performed at the B3LYP/6-31G** level. The rate constants and activation parameters of the process were obtained. The mechanism of isomerization is discussed.in the form of red-and blue-violet crystals. By the data from electronic and IR spectra it was possible to identify them as facial (fac) and meridional (mer) isomers [1]. An X-ray crystallographic analysis of the red-violet form confirmed its fac configuration [2]. In this isomer all three oxygen atoms are on one face of the octahedron in the cis position to each other, the three nitrogen atoms are on another face, and the O-Co-N atoms are located on all three diagonals of the octahedron. In the mer isomer two oxygen atoms are in the trans position while the third is in the cis position in relation to these atoms, and the O-Co-N, O-Co-O, and N-Co-N atoms are on the diagonals of the octahedron. The isomers of [Co(NH 2 C 2 H 4 O) 3 ] are used as the starting reagents for the synthesis of new mixed-ligand compounds of cobalt(III) and heterometallic complexes, and the configuration of the initial trisaminoethoxide substantially affects the composition of the polynuclear complexes and also the rate of intrasphere substitution of the aminoethoxide by other ligands and the structure of the (III) substitution products [3].Both isomers of [Co(NH 2 C 2 H 4 O) 3 ]·3H 2 O are not configurationally rigid and can under certain conditions be converted from one to the other. Thus, it was found [1-4] that moist crystals of the mer isomer change almost completely with time into the crystalline fac isomer. The process takes place in the same direction in a saturated aqueous solution. In a dilute aqueous solution, however, the reverse reaction is observed [4]: The fac isomer changes quickly and almost irreversibly into the mer isomer. The occurrence of the mer→fac isomerization in the saturated aqueous solution is in our opinion explained by a shift of the equilibrium toward the crystalline fac isomer, which has significantly lower solubility [1,4]. In order to explain the reason for the occurrence of the reverse fac→mer reaction in dilute aqueous solution, containing isolated molecules of [Co(NH 2 C 2 H 4 O) 3 ] quantum-chemical calculations on the stereochemical parameters of the structure and on energy characteristics were carried out for both isomers.The calculations were made by the Gaussian-98 software [5] for fac-and mer-[Co(NH 2 C 2 H 4 O) 3 ]. Both structures were preoptimized at the Hartree -Fock (HF) level and were then reoptimized at the higher level of density functional theory (DFT, B3LYP) in the 6-31G** basis set and also in the LANL2DZ basis set for the cobalt ion and 6-311+G** for the C, H, O, and N atoms. The obtained structures were characterized by h...

Mechanisms of reactions in solution

“…A quantum-chemical study of the fac-mer isomerism of cobalt(III) trisaminoethoxide reveals intervention of an intermediate protonated complex as a key feature of the acceleration as [H + ] is increased. 159 Kinetic measurements have been used to investigate reversible rearrangement of F and Ph ligands in [(Ph 3 P) 3 RhF] producing cis-[(Ph 3 P) 2 Rh(Ph)(Ph 2 PF)]. 160 An intramolecular process is identified and theoretical studies show that the most likely mechanism involves formation of a metallophosphorane via F transfer followed by movement of Ph to Rh.…”

Electronic insights into aminoquinoline-based PNHN ligands: protonation state dictates geometry while coordination environment dictates N–H acidity and bond strength

“…Fac to mer isomerization of transition metal complexes is not uncommon and has been studied in the literature. [22][23][24][25][26][27][28][29][30][31] As a result of our synthetic investigations, a series of complexes reported herein bearing coordinated secondary amines and their deprotonated analogues were found to be excellent candidates for the determination of the pK a and bond dissociation free energy (BDFE) of their N-H bonds. Upon coordination to a metal, amines with at least one hydrogen generally become more acidic and the N-H BDFE can be lowered.…”