Kinetic and mechanistic studies of low-pressure ion–molecule association reactions of unsaturated Ru(II) complexes with CO

Abstract

The reactions of 14 electron unsaturated ruthenium complexes, [Ru(bipy)(X)]2+, where (X=bipyridine, 2-(pyridin-4-yl)-1,3-benzothiazole, and 5-aminophenathroline), with CO and O2 were studied at low pressures using Fourier transform ion cyclotron resonance mass spectrometry (FTICR MS). These studies were aimed at examining the mechanism of these association reactions occurring at very low pressures. All observed bimolecular association rate constants were found to be independent of pressure, indicating that collisions within the ICR were not responsible for cooling of the initially formed excited adducts. Experimental rate constants for the association of CO and O2 to [Ru(bipy)(X)]2+ complexes were determined to be very similar to the theoretical ion/molecule collisions rate constants. The reactions were further probed by theoretical calculations. Radiative stabilization rate constants and RRKM unimolecular back dissociation rate constants of the nascent complexes were calculated. According to the rate constant expression for the radiative association mechanism, significantly larger radiative rate constants compared to unimolecular dissociation rate constants results in the association rate constant, kass, being equal to the collision rate constant, kf. The experimental and theoretical results presented are consistent with the association reactions of CO occurring at the collision rate. The association complexes between [Ru(bipy)(X)]2+ and CO observed in the ICR cell are, therefore, chemically activated species which can only dissipate the energy of association by radiative means. Due to their high binding energies and the size of the nascent ion–molecule complexes, they are expected to have very long unimolecular dissociation lifetimes.