The important role of the Mo-Mo quintuple bond in catalytic synthesis of benzene from alkynes. A theoretical study.

The Mo-Mo quintuple bond was recently applied to catalytic synthesis of benzene from alkynes, which is the first example of the catalytic reaction of the metal-metal multiple bond. This new reaction was studied using DFT and CASSCF/CASPT2 methods. The entire catalytic cycle consists of four steps: [2 + 2], [4 + 2], and [6 + 2] cycloadditions, and reductive elimination of benzene. The symmetry-forbidden [2 + 2] cycloaddition and asymmetric [2 + 2] cycloaddition are two possible pathways for the reaction between an alkyne and the Mo-Mo quintuple bond. Though the barrier of the former pathway is moderate because of the presence of the multi-reference character of the Mo-Mo quintuple bond, the asymmetric pathway is much more favorable because of its symmetry-allowed feature. The C-C bond formation in the next [4 + 2] cycloaddition occurs through charge transfer (CT) from the π orbital of the incoming alkyne to the π* orbital of another alkyne coordinating with the Mo center to afford a novel dimolybdenacyclic species 3. In 3, the δ(d(xz)) and δ(d(xz))* orbitals of the Mo-Mo moiety and four π orbitals of the [C4H4] moiety construct the π and π* orbitals in the six-membered ring. The next [6 + 2] cycloaddition between 3 and one more alkyne affords an eight-membered ring compound 4 which has a Mo-Mo quadruple bond. This is the rate-determining step of the entire catalytic cycle, the ΔG(0‡) value of which is 22.4 kcal mol(-1). The subsequent reductive elimination of benzene easily occurs to yield a μ2-η(2):η(2)-benzene dinuclear Mo complex with a Mo-Mo quintuple bond. On the other hand, further [8 + 2] cycloaddition between 4 and one more alkyne is much more unfavorable than the reductive elimination of benzene. The similar [4 + 2] process between alkyne and a Cr-Cr quadruple bond is calculated to be difficult, which is consistent with the experimental result that only the Mo-Mo quintuple bond was successfully applied to this reaction. It is likely that the crowded coordination environment and the much more stable π(d(yz)) orbital in the Cr-Cr quadruple bond are responsible for the difficulty in the reaction.