Many organometallic reactions are solvent-dependent, suggesting solvent molecules interact with reaction intermediates. Studies of the solvent exchange reaction of group 6 transition metal carbonyls with moderately binding ligands have provided insight into these interactions, however, studies of the mechanism for this reaction with weakly binding ligands have not been performed. Experiments were conducted on the nanosecond time scale in methylcyclohexane over the temperature range of 4 to 44 °C using Step Scan FTIR (SS FTIR) spectroscopy with weakly binding ligands benzene and mesitylene. Upon photolysis of the metal hexacarbonyls, the kinetically favored product (M(CO)5(solv)), decays following pseudo-first-order kinetics to the thermodynamical favored product, M(CO)5(L). The decay is fit using a single exponential decay with a single exponential instrument response function, time zero and an offset. An Arrhenius plot yielded activation energies of 23.7 kJ/mol (M = Mo, L = benzene), 35.1 kJ/mol (M = W, L = benzene) and 29.8 kJ/mol (M = Mo, L = mesitylene). DFT calculations using NWCHEM gave binding energies of 45.8 and 54.3 kJ mol-1 for Mo(CO)5C6H12 and W(CO)5C6H12. The experimental and computational results suggest the exchange mechanism proceeds through an associative pathway, were slightly negative values of the entropy of reaction denote that the transition state has greater metal solvent bond breaking character then the more moderately binding ligands in the literature. Density Functional theory was used to calculate C-O vibrational frequencies of metal carbonyl complexes measured in our work and other complexes from the literature, with density functionals B3LYP, M06 and M06-L. Measured and computational frequencies were compared to obtain both an overall vibrational shift and a scaling factor. Scaling factors were found to be 0.9519±0.0095 for B3LYP, 0.9429 ± 0.0087 for M06 and 0.9565 ± 0.0095 for M06-L with overall shifts of 102 ± 16, 121 ± 15, 93 ± 17 cm-1, respectively. The molybdenum mediated Pauson-Khand reaction, a [2+2+1] cyclo-addition begins very similarly to the solvent exchange reaction on molybdenum. The initial product, the solvated complex, decays away with pseudo-first-order kinetics as the solvent is replaced by the C-C triple bond in 2-(2-propen-1-yl)-2-(2-propyn-1-yl)-,1,3-diethyl ester. An Arrhenius plot over the temperature range of -8 to 20° C yielded an energy of activation of 15.6 kJ/mol.



College and Department

Physical and Mathematical Sciences; Chemistry and Biochemistry



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Kinetics, dynamics, molybdenum, tungsten, group six metal carbonyls