On the Mechanism of Re(I)–Carboxylate Bond Cleavage by Perchloric Acid: A Kinetic and Spectroscopic Study

We have studied the reaction between pz–CO2–Re(CO)3(bpy) and perchloric acid in acetonitrile by following the UV–vis and IR spectral changes in the reaction mixture. A fast equilibrium was found to be established between solvated protons, pz–CO2–Re(CO)3(bpy), and the protonated intermediate [pz–C(OH)O–Re(CO)3(bpy)]+ which finally yields pz–COOH and Re(CO)3(bpy)(CH3CN)+ as reaction products. This intermediate has been characterized by UV–vis and IR spectroscopies and by DFT calculations. The fully optimized DFT/CPCM structures for pz–CO2–Re(CO)3(bpy) and [pz–C(OH)O–Re(CO)3(bpy)]+ were compared with the X-ray structure of pz–CO2–Re(CO)3(bpy). The structural parameters associated with the carboxyl group in the protonated intermediate are between those of pz–CO2–Re(CO)3(bpy) and pz–COOH. Multivariate curve resolution methods were employed to obtain the spectrum of the protonated intermediate and the concentration profiles from the full matrix of time-resolved UV–vis spectra. The proposed mechanism was numerically simulated by using Runge–Kutta methods. Model parameters were estimated by nonlinear regression fitting of the concentration profiles, yielding values of log(K) = 4.9 ± 0.3 and k = 0.16 ± 0.03 min–1 for the formation equilibrium constant and the decay rate constant of the protonated intermediate, respectively.