| The zerovalent molybdenum trimethylphosphine complex, Mo(PMe 3)6, reacts with the ortho substituted phenols C6H2Me3OH and C6H2 i-Pr2OH to yield aryloxy-hydride complexes Mo(PMe 3)4(OAr)H. Unlike in the analogous tungsten system, C-H bond activation of the phenol substituents is not observed. Deuterium labeling and NMR experiments, however, demonstrate that C-H bond activation is kinetically accessible for the Mo system but not favored thermodynamically. The difference between the Mo and W systems is attributed to greater M-C and M-H bond strengths for tungsten. Density Functional Theory (DFT) calculations predict that the cyclometallation of M(PMe3)4(OC6H2Me 3)H is 12 kcal mol-1 more favorable for W than for Mo.;DFT calculations have also confirmed the experimental observations that the energetics of the oxidative addition of H2 to the six-coordinate trans-M(PMe3)4X2 complexes (M = Mo, W; X = F, Cl, Br, I) depend very strongly on the nature of both the metal and the halogen. Specifically, the exothermicity of the reaction increases in the sequences Mo < W and I < Br < Cl < F. Of most interest, this halogen dependence provides a striking contrast to that reported for oxidative addition of H2 to the Vaska system, trans-Ir(PPh 3)2(CO)X. The theoretical analysis suggests that pi-donation from X in the M(PMe3)4X2 system destabilizes the 6-coordinate complex relative to the H2 addition product and thus promotes the oxidative addition of hydrogen. Consequently, oxidative addition is most favored for the derivative of fluoride, the strongest pi-donor. Increased steric interactions upon forming the 8-coordinate complexes M(PMe 3)4H2X2 would also favor oxidative addition for the derivatives of the smallest halogen, fluoride. Thus the halide dependence for trans-M(PMe3)4X 2 is a result of both steric and electronic factors, the components of which serve to reinforce each other.;Sterically demanding diethers of p-tert-butylcalix[4]arene have been synthesized and used as ligands for divalent tin and germanium complexes. Two types of isomers have been observed in which the metal is located either exo or endo to the ligand cavity. For Ge, the endo isomer is the most thermodynamically stable, possibly due to the calixarene ligand providing a more favorable bite angle. The barrier to exo-endo isomerization is dependent on the ether group and is highest for the dimethyl ether. |
