Using Reduced Catalysts for Oxidation Reactions: Mechanistic Studies of the “Periana-Catalytica” System for CH₄ Oxidation

Abstract

Designing oxidation catalysts based on CH activation with reduced, low oxidation state species is a seeming dilemma given the proclivity for catalyst deactivation by overoxidation. This dilemma has been recognized in the Shilov system where reduced PtII is used to catalyze methane functionalization. Thus, it is generally accepted that key to replacing PtIV in that system with more practical oxidants is ensuring that the oxidant does not over-oxidize the reduced PtII species. The “Periana-Catalytica” system, which utilizes (bpym)PtIICl2 in concentrated sulfuric acid solvent at 200 °C, is a highly stable catalyst for the selective, high yield oxy-functionalization of methane. In lieu of the over-oxidation dilemma, the high stability and observed rapid oxidation of (bpym)PtIICl2 to PtIV in the absence of methane would seem to contradict the originally proposed mechanism involving CH activation by a reduced PtII species. Mechanistic studies show that the originally proposed mechanism is incomplete and that while CH activation does proceed with PtII there is a solution to the over-oxidation dilemma. Importantly, contrary to the accepted view to minimize PtII overoxidation, these studies also show that increasing that rate could increase the rate of catalysis and catalyst stability. The mechanistic basis for this counterintuitive prediction could help to guide the design of new catalysts for alkane oxidation that operate by CH activation.

New Trends in Oxidative Functionalization of Carbon–Hydrogen Bonds: A Review

“…Another route that uses as oxidants dioxygen or even concentrated sulfuric acid [305][306][307][308] that require high temperatures seems to be more attractive from the point of view of industrial applications. It is noteworthy that the Periana reaction occurring under severe conditions employs a metal complex catalyst (but not a simple salt).…”

“…Platinum complexes derived from the bidiazine ligand family, discovered by Periana and colleagues [305][306][307], catalyze oxidation of methane by sulfuric acid to a methanol derivative in >70% one-pass yield based on methane. These complexes are very stable in concentrated sulfuric acid at relatively high temperature and are among the most effective catalysts for methane conversion.…”

Methyl Complexes of the Transition Metals

“…In accordance with theoretical and kinetic studies which also involve the synthesis of mono-and dimethylated Pt(II) complexes bearing 2,2′-bipyrimidine as the ancillary ligand, a relevant role of Pt(IV) species for reaction with methane is discarded, being C-H activation performed by Pt(II) to generate Pt(II)-CH3 intemediates. 121 In 2014, a challenging 1,2-selective catalytic hydrosilylation of conjugated dienes was accomplished by means of an anionic cyclometallated Pt(II) monomethyl compound resulting from the reaction of the corresponding dimethyl derivative with HSi(OEt)3 (Scheme 20a). 122 A series of square-planar Pt(II) methyl complexes have been prepared by Perutz et al by oxidative addition of O-CH3 bonds of highly fluorinated arylethers at a linear platinum(0) centre (Scheme 20b).…”

Steigerung der Katalysatoreffizienz in der C‐H‐Aktivierungskatalyse

“…Der pK a -Wert von Methan beträgt etwa 50 und die Bindungsdissoziationsenthalpie 105 kcal mol À1 .Dennoch haben Prozesse für die Umsetzung von Methanrohstoffen in funktionalisierte Derivate im großen Maßstab enorme çkonomische und çkologische Potentiale. [71] Das Produkt Methylsulfat wird dann zu Methanol hydrolysiert, sodass im Gesamtprozess Methan zu Methanol oxidiert wird. [70] Im vorgeschlagenen Mechanismus aktiviert der Pt II -Katalysator eine C-H-Bindung durch elektrophile Addition.…”