First principle study on the mechanism of O2 activation by Pt(II) monomethyl complex

The Shilov system is the first successful case to activate methane C-H bonds and convert it to Me derivs. In this system, the kinetics of the oxidn. of Pt(II) is important because the oxidn. competes with the protonolysis of Pt(II)-Me complex, the reverse reaction of C-H activation. Several oxidants, such as Pt(IV), chlorine, hydroperoxide and Cu(I) are effective. However, the direct utilization of dioxygen as stoichiometric oxidant without electron-transfer reagent is not achieved yet. Vedernikov et al demonstrated that Pt(II) monomethyl complex with facially chelating ligands such as di(2-pyridine)methanesulfonate ligand (dpms) contg. the semilabile sulfonate donor facilitates the kinetics of oxygen activation, making it an attractive route to achieve aerobic oxidn. in C-H activation.We carried out DFT calcns. with B3LYP functional to study the mechanism of Pt(II) oxidn. Starting with Pt(II) and O2(gas), the reaction first goes through the triplet-singlet crossing point to form Pt(IV)-OOH with 25.4 kcal/mol barrier at pH=7. Because the semilabile sulfonate ligand is ready to coordinate on the metal, the free energy cost to transit from 4-coordinate Pt(II) to 6-coorinate Pt(IV) is reduced, making the kinetics fast. The next step is to cleave the O-O bond in Pt(IV)-peroxide through a bimol. reaction which oxidizes a second Pt(II). Lower pH accelerates this step because the formation of Pt(IV)-OOH takes one proton from solvent. At pH=7 the barrier height is calcd. as 27.1 kcal/mol. We also examd. the methyl-transfer between Pt(II) and Pt(IV) obsd. exptl. at higher pH, where the oxidn. is hindered for the scarcity of proton. Pt(II) complex first isomerizes to make the Me group trans to the sulfonate ligand and then forming Pt(IV)-OOH. With a better leaving group trans to Me group, the nucleophilic substitution by Pt(II) has much lower barrier (17.0 kcal/mol v.s. 32.5 kcal/mol for the unisomerized case)