Thermal Isomerization of [Co(acac)2(N3)(py)] in Liquid Solution Studied by Time-Resolved Fourier-Transform Infrared Spectroscopy

Abstract

Abstract The thermally induced stereochemical interconversion between the trans and cis isomers of [Co(acac)2(N3)(py)] in liquid solution is investigated with time-resolved Fourier-transform infrared spectroscopy. The complex is synthesized stereo-selectively in its trans-form. Upon dissolution of the trans-form, the kinetic build-up of the cis-form is evidenced by the spectro-temporal evolution of the FTIR-spectrum. The individual isomer-specific component spectra are in good agreement with calculated spectra obtained from density functional theory. The rate constants of the forward and backward reactions responsible for the trans-cis isomerization equilibrium are derived from the kinetic traces in combination with existing thermochemical data from the literature. Moreover, the temperature-dependence of the rate constants are in line with Arrhenius activation energies of (122 ± 8) kJ/mol and (109 ± 8) kJ/mol for the forward and backward reactions, respectively. DFT-calculations suggest that the stereochemical rearrangement is caused by a pyridine rebound mechanism involving penta-coordinated square-pyramidal [Co(acac)2(N3)]-intermediates.