Kinetic and Thermodynamic Investigations of CO2 Insertion Reactions into Ru-Me and Ru-H Bonds - An Experimental and Computational Study
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The rates of CO2 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 CO2 and metal complex concentrations, and CO2 insertion into the Ru-H bond of 1 occurs instantaneously at 0 C. The reverse process, decarboxylation, was observed to occur readily at ambient temperature as revealed by 13CO2 exchange with subsequent CO2 insertion into the Ru-Me bond at higher temperatures. No further CO2 insertion into the Ru-H bond of the resulting acetate complex was observed. The activation barrier for CO2 insertion into the first Ru-Me bond of 2 was determined to have H and S 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(O2CMe)2. In general, the insertion of CO2 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 CO2 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 CO2 activity and resultant complex stability if dmpe is replaced with tetramethylethylenediamine (tmeda). 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.