DFT analysis of the activation and breaking of the Mo-N bond in a (mu-nitrido)dimolybdenum complex: is molybdenum tris(thiolate) an elusive intermediate?
The electronic structure, conformation, synthesis, and thermal decomposition pathways of the recently characterized dimolybdenum mu-nitrido complex (AdS)(3)Mo(mu-N)Mo(N[(t)Bu]Ph)(3) (1exp, Ad = adamantyl) are investigated by means of DFT calculations carried out on the model system (HS)(3)Mo(mu-N)Mo(NH(2))(3) (1). The observed asymmetry of the Mo(mu-N)Mo core is reproduced in the optimal conformation of 1 and assigned to the tendency for the electron density of the metal atoms to be preferably accommodated in the pi orbitals of Mo(thiolate). The balance in the metal-ligand and ligand-ligand interactions conditioning the flow of the electron density along the Mo-(mu-N)-Mo framework, and eventually the relative activation of the molybdenum-nitrido bonds, appears very sensitive to the nature of the ancillary substituents on both the thiolate and the amido sides. On one hand, replacing HS by AdS in 1 increases the calculated value of Delta(mu-N-Mo) from 0.053 to 0.094 A, close to the experimental value of 0.111 A. The mu-nitrido complex with bulky thiolates 1a is also less stable than 1 by 7.3 kcal x mol(-1) with respect to its monometallic constituents. On the other hand, substituting the bulky N[(t)Bu]Ph for NH(2) in the model complex 1b induces an important charge transfer toward the thiolate moiety resulting in structural and energetic consequences of similar magnitude. Even though these substitutional effects are not likely to be fully additive in the real complex, both should contribute to an increase of the mu-N-Mo(thiolate) bond activation in 1exp. The importance of this activation conditions the feasibility of the thermal decomposition of 1exp promoted by benzonitrile which eventually yields the molybdenum thiolate dimer (RS)(3)Mo [triple bond] Mo(SR)(3). The energy profile calculated for this reaction with model complex1 in the presence of one or two molecules of acetonitrile shows that the axial fixation of the promoter on one or both molecular ends forms intermediates in which the mu-N-Mo(thiolate) bond is further activated with respect to the original complex. The consequence is an important, but still insufficient, decrease of the barrier to Mo-N bond breaking, from 53 to 37 kcal x mol(-1). Furthermore, the thermodynamic balance of the reaction leading from the acetonitrile adducts of 1 to (HS)(3)Mo [triple bond] Mo(SH)(3) remains endothermic by 6.5 kcal x mol(-1) for the monoadduct, and more for the diadduct. It therefore appears that bulky substituents on both ends of the dinuclear complex are essential to the completion of the reaction, from both the thermodynamic and the kinetic viewpoints.
Rohmer
Marie-Madeleine
MM
Laboratoire de Chimie Quantique, UMR 7551, CNRS and Université Louis Pasteur, F-67000 Strasbourg, France.
Bénard
Marc
M