Computational studies of transition metal complexes relevant to catalytic alkane functionalization.

摘要

Oxidative addition by one or two molecules of dihydrogen (H₂) to the coordinatively unsaturated M(PH₃)₂Cl, M = Rh and Ir were investigated using first principles computational methods based on the traditional molecular orbital approach and on density functional theory with a focus on thermodynamic and kinetic parameters. All three computational methods used for geometry optimizations (BLYP, B3LYP, and MP2) produced comparable structures for all the isomers. A general agreement among the methods regarding the relative energies of isomers was also observed.; The thermodynamics of small molecule (H₂, arene, alkane, and CO) addition to pincer-ligated iridium complexes of several different configurations have been investigated. The substituent para to the iridium (Y) has been varied in complexes containing the (Y-PCP)Ir unit (Y-PCP = η 3-1,3,5-C₆H₂[CH₂PR₂] ₂Y; R = methyl). In general, increasing electron-donation by the substituent Y in the 16-electron complexes (Y-PCP)Ir(CO) or (Y-PCP)Ir(H)₂ disfavors addition of H-H or C-H bonds, in contradiction with the idea of such additions being “oxidative”. By contrast, addition of H-H and C-H bonds or CO to the three-coordinate parent species (Y-PCP)Ir is favored by increased electron-donation. The trends can be fully rationalized in terms of simple MO interactions, but not in terms of concepts related to oxidation, such as charge-transfer or electronegativity differences.; The mechanism of acceptorless dehydrogenation of alkanes by (PCP)Ir and (PCP)IrH₂, (PCP = η3-1,3-C₆H ₃(CH₂PR′₂)₂, R ′ = Me and t-Bu) has been investigated using DFT/ECP methods. Two possible pathways for (PCP)IrH₂ reactions with model linear (propane) and cyclic (cyclohexane) alkanes may proceed through associative A or dissociative D pathways. At experimental conditions (high temperature, low pressure of H₂, high alkane concentration) the overall free energy barrier of A is calculated to be higher than that of D by ca. 9 kcal/mol when R′ = Me and 17 kcal/mol when R′ = t-Bu.; The factors that govern the thermodynamic selectivity of C-H bond addition have been investigated. The stability of M-C complexes decreases with increasing substitution on a carbon bonded directly to the metal. The results of our calculations indicate that the electronic effects play a major role in the selectivity of late transition metals for different C-H bonds.