Kinetic and Thermodynamic Investigations of CO sub(2) Insertion Reactions into Ru-Me and Ru-H Bonds - An Experimental and Computational Study

• Published in: European journal of inorganic chemistry 2013-08, Vol.2013 (22-23), p.4024-4031
• Main Authors: Darensbourg, Donald J, Kyran, Samuel J, Yeung, Andrew D, Bengali, Ashfaq A
• Format: Article
• Language: English
• Subjects: Ambient temperature; Barriers; Bonding; Carbon dioxide; Chemical bonds; Computation; Insertion; Ligands
• ISSN: 1434-1948; 1099-0682
• DOI: 10.1002/ejic.201300179

The rates of CO sub(2) insertion into trans-Ru(dmpe) sub(2)(Me)H [1, dmpe = 1,2-bis(dimethylphosphino)ethane] and trans-Ru(dmpe) sub(2)(Me) sub(2) (2) derivatives were monitored by in situ infrared and super(1)H NMR spectroscopy. The reactions are first order in both CO sub(2) and metal complex concentrations, and CO sub(2) insertion into the Ru-H bond of 1 occurs instantaneously at 0 degree C. The reverse process, decarboxylation, was observed to occur readily at ambient temperature as revealed by super(13)CO sub(2) exchange with subsequent CO sub(2) insertion into the Ru-Me bond at higher temperatures. No further CO sub(2) insertion into the Ru-H bond of the resulting acetate complex was observed. The activation barrier for CO sub(2) insertion into the first Ru-Me bond of 2 was determined to have Delta H super() and Delta S super() values of 12.7 plus or minus 0.6 kcalmol super(-1) and -31.9 plus or minus 2.0 e.u., respectively, which are indicative of a highly ordered transition state. The rate of CO sub(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) sub(2)(O sub(2)CMe) sub(2). In general, the insertion of CO sub(2) into the Ru-H or Ru-Me bonds of trans-Ru(dmpe) sub(2)(X)R (R = H or Me) was disvavored in the presence of poorly electron-donating X ligands. For example, the insertion of CO sub(2) into the Ru-H bond of trans-Ru(dmpe) sub(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 sub(2) activity and resultant complex stability if dmpe is replaced with tetramethylethylenediamine (tmeda). Kinetic measurements of CO sub(2) insertion into the Ru-H and Ru-Me bonds of trans-Ru(dmpe) sub(2)(X)R [R = H, Me; dmpe = 1,2-bis(dimethylphosphino)ethane] have shown that these processes are sensitive to the electron-donating ability of X, and poorly electron-donating X ligands have a negative impact. Computational studies of reaction barriers and reaction enthalpies support these observations.