Highly Efficient Alkane Oxidation Catalyzed by [MnV(N)(CN)4]2-. Evidence for [MnVII(N)(O)(CN)4]2- as an Active Intermediate

The oxidation of various alkanes catalyzed by [Mn-V(N)(CN)(4)](2-) using various terminal oxidants at room temperature has been investigated. Excellent yields of alcohols and ketones (>95%) are obtained using H2O2 as oxidant and CF3CH2OH as solvent. Good yields (>80%) are also obtained using (NH4)(2)[Ce(NO3)(6)] in CF3CH2OH/H2O. Kinetic isotope effects (KIEs) are determined by using an equimolar mixture of cyclohexane (c-C6H12) and cyclohexane-d(12) (c-C6D12) as substrate. The KIEs are 3.1 +/- 0.3 and 3.6 +/- 0.2 for oxidation by H2O2 and Ce(W), respectively. On the other hand, the rate constants for the formation of products using c-C6H12 or c-C6D12 as single substrate are the same. These results are consistent with initial rate-limiting formation of an active intermediate between [Mn(N)(CN)(4)](2-) and H2O2 or Ce-IV, followed by H-atom abstraction from cyclohexane by the active intermediate. When PhCH2C(CH3)(2)OOH (MPPH) is used as oxidant for the oxidation of c-C6H12, the major products are c-C6H11OH, c-C6H100, and PhCH2C(CH3)(2)OH (MPPOH), suggesting heterolytic cleavage of MPPH to generate a Mn=O intermediate. In the reaction of H2O2 with [Mn(N)(CN)(4))](2-) in CF3CH2OH, a peak at m/z 628.1 was observed in the electrospray ionization mass spectrometry, which is assigned to the solvated manganese nitrido oxo species, (PPh4)(Mn(N)(O)(CN)(4)](-)center dot CF3CH2OH. On the basis of the experimental results the proposed mechanism for catalytic alkane oxidation by [Mn-V(N)(CN)(4)](2-)/ROOH involves initial rate-limiting O-atom transfer from ROOH to [Mn(N)(CN)(4)](2-) to generate a manganese(VII) nitrido oxo active species, [Mn-VII(N)(O)(CN)(4)](2-), which then oxidizes alkanes (R'H) via a H-atom abstraction/O-rebound mechanism. The proposed mechanism is also supported by density functional theory calculations.