Kinetic and Thermodynamic Investigations of CO_2 Insertion Reactions into Ru-Me and Ru-H Bonds - An Experimental and Computational Study

摘要

The rates of CO_2 insertion into trans-Ru(dmpe)_2(Me)H [1, dmpe = 1,2-bis(dimethylphosphino)ethane] and trans- Ru(dmpe)_2(Me)_2 (2) derivatives were monitored by in situ infrared and ~1H NMR spectroscopy. The reactions are first order in both CO_2 and metal complex concentrations, and CO_2 insertion into the Ru-H bond of 1 occurs instantaneously at 0 ℃. The reverse process, decarboxylation, was observed to occur readily at ambient temperature as revealed by ~(13)CO_2 exchange with subsequent CO_2 insertion into the Ru-Me bond at higher temperatures. No further CO_2 insertion into the Ru-H bond of the resulting acetate complex was observed. The activation barrier for CO_2 insertion into the first Ru-Me bond of 2 was determined to have ΔH~? and ΔS~? values of 12.7±0.6 kcalmol~(~(-1)) and -31.9±2.0 e.u., respectively, which are indicative of a highly ordered transition state. The rate of CO_2 insertion into the second Ru-Me bond was two orders of magnitude slower at ambient temperature and resulted in the formation of trans-Ru(dmpe)_2(O_2CMe)_2. In general, the insertion of CO_2 into the Ru-H or Ru-Me bonds of trans-Ru(dmpe)_2(X)R (R = H or Me) was disfavored in the presence of poorly electron-donating X ligands. For example, the insertion of CO_2 into the Ru-H bond of trans-Ru(dmpe)_2- (Cl)H was not observed even under forcing conditions. Computational results were in excellent agreement with these observations and predict a significant enhancement in CO_2 activity and resultant complex stability if dmpe is replaced with tetramethylethylenediamine (tmeda).