Density functional studies of dicobalt octacarbonyl-mediated azobenzene formation from 4-ethynylaniline.

Two adiabatic potential-energy surfaces are employed for probing the processes of [Co2(CO)8]-mediated C6H5N=NC6H5 formation from NH2C6H5. Elementary steps, including oxidative addition of the coordinated amine proton to the cobalt center, reductive elimination of H2, CO association, and the coupling process of the diamino fragments, are modeled and examined by using DFT methods at the B3 LYP/631 LAN level. The formation of C6H5N=NC6H5 from NH2C6H5 through reductive coupling is a thermodynamically unfavorable process. Three hydride-migration processes, from the proton of N-H to the cobalt center, are established as the most energy-demanding steps. The activation energies (deltaG*) are calculated as 49.4, 55.4, and 33.3 kcal mol(-1), respectively, for the proposed reaction route 1. These large activation energies might be reduced slightly by purposely adding small protic molecules, such as H2O, or by changing the active metal from Co to a heavier metal, such as Rh or Ir. An alternative pathway, route 2, is also proposed, in which transition states with four-membered rings are formed. By this route, severe strain caused by the formation of three-membered rings during the hydride-migration processes in route 1 can be avoided. Route 2 is established as the more energy-feasible reaction pathway.