Theoretical elucidation of the platinum-mediated arene C-H activation reactions

The complete reaction mechanism of the platinum-mediated arene [(N-N)PtPh2 (N-N = diimine ArNCMe-CMeNAr with Ar = 2,6-Me2C6H3)] C-H activation has been investigated by using the B3LYP density functional theory method. The calculations show that the protonation should occur at the metal center, giving a coordinately unsaturated five-coordinate Pt(IV) hydride which is trapped by acetonitrile; otherwise monophenyl solvento cations (N-N)Pt(Ph)(NCMe)(+) should be isolated and characterized spectroscopically. Of the H exchange process, oxidative addition-reductive elimination and sigma-bond metathesis mechanisms (including direct sigma-bond metathesis and indirect sigma-bond metathesis) are considered. The indirect sigma-bond metathesis, which goes through a two-step H migration, is dramatically higher in energy than that of the direct one, so we can safely rule it out. The barrier associated with direct sigma-bond metathesis is 14.70 kcal/mol, to be compared with 20.56 kcal/mol found in oxidative addition. So, unlike the methane C-H activation, the direct sigma-bond metathesis pathway is predicted to take place predominantly. Furthermore, the solvent-induced associative elimination of benzene is also confirmed by the current theoretical studies for the first time. The trigonal bipyramidal transition states offer substantial evidence for this process, and the principle of microscopic reversibility then implies that benzene coordination will also be a solvent-assisted, associative process for a C-H activation reaction. For the different rates of benzene elimination, our calculations show that nucleophiles with higher electron-donating abilities and small steric effects of the diimine ligands will favor elimination greatly