Über die Rolle von [CuO]⁺ bei spinselektiven Wasserstoff‐ und Sauerstoff‐Übertragungen in der Aktivierung von Methan

Abstract

Das fehlende Stück in einem fesselnden Puzzle: Mehr als zehn Jahre nach der theoretischen Vorhersage, dass kationisches [CuO]+ ein wirkungsvoller Kandidat für die Umwandlung von Methan zu Methanol sein müsste, konnte dieses Oxid in der Gasphase erzeugt werden. Eine Kombination aus Massenspektrometrie und DFT‐Rechnungen offenbart die entscheidende Rolle der Zweizustandsreaktivität sowie die Bedeutung von sauerstoffzentrierten Radikalen hinsichtlich der Selektivität in der Oxidation von Methan.

Doping Effects in Cluster‐Mediated Bond Activation

“…[83] and [84].A st ot he general features of an efficient metal-oxide-mediated gas-phase HATf rom hydrocarbons, the consensus views can be summarized as follows: [68] 1) Thep resence of unpaired, high spin density at the abstracting,p referentially terminal oxygen atom of M À O t C is crucial. [86] While these general reactivity patterns pertain to both homo-and heteronuclear cluster oxides,t he latter systems permit to modify crucial properties,f or example,s pin densities or local charge distributions around the reaction centers by judicious choices of the dopants.A sag ood example on how the spin density at the H-accepting oxygen atom can be changed by doping,let us discuss the reactivity of [MgO] n C + (n = 1-7) clusters toward hydrocarbons.W hile diatomic [MgO]C + initiates HATe ven from CH 4 ,t he larger clusters with n ! [68] and am ore recent, rather intriguing one for the indirect process corresponds to the couple [CuO] + /CH 4 .…”

“…[68] and am ore recent, rather intriguing one for the indirect process corresponds to the couple [CuO] + /CH 4 . [86] While these general reactivity patterns pertain to both homo-and heteronuclear cluster oxides,t he latter systems permit to modify crucial properties,f or example,s pin densities or local charge distributions around the reaction centers by judicious choices of the dopants.A sag ood example on how the spin density at the H-accepting oxygen atom can be changed by doping,let us discuss the reactivity of [MgO] n C + (n = 1-7) clusters toward hydrocarbons.W hile diatomic [MgO]C + initiates HATe ven from CH 4 ,t he larger clusters with n ! 2are completely inert toward methane,even though the reactions are exothermic.W hy is this so?I n contrast to diatomic [MgO]C + ,in[Mg 2 O 2 ]C + the spin is equally distributed over the two bridging oxygen atoms of the cluster; intracluster spin-density transfer and the associated kinetic barrier for HATf rom CH 4 are too high in energy to be accessible under thermal conditions.T hus,H AT is only observed with those substrates that have weaker CÀHbonds, as for example,propane or butane.…”

Mechanistic Aspects of Gas‐Phase Hydrogen‐Atom Transfer from Methane to [CO].+ and [SiO].+: Why Do They Differ?

“…This process is barrier-free and proceeds without the formation of a longlived encounter complex. dination site at the metal atom, such as [MnO]C + , [15] [FeO]C + , [16] [MgO]C + , [17] [CuA C H T U N G T R E N N U N G O] + , [18] [Pb-O]C + , [19] [SnO]C + , or [GeO]C + . [14] On the other hand, the indirect (or metal-mediated) pathway includes an initial coordination of methane to the metal center, followed by its migration towards the reactive oxo site at which the actual C À H bond fission occurs.…”

“…A theoretical analysis of the respective HAT proA C H T U N G T R E N N U N G cesses reveals two distinctly different mechanistic pathways for [CO]C + and [SiO]C + , and a comparison to the higher metal oxides of Group 14 emphasizes the particular role of carbon as a second-row p element. dination site at the metal atom, such as [MnO]C + , [15] [FeO]C + , [16] [MgO]C + , [17] [CuA C H T U N G T R E N N U N G O] + , [18] [Pb-O]C + , [19] [SnO]C + , or [GeO]C + . [20] In the context of interstellar chemistry, also the reactivity of cationic carbon monoxide [CO]C + towards CH 4 was studied quite some time ago.…”

Linking Ion and Neutral Chemistry in CH Bond Electrophilic Activation: Generation and Detection of HO2. Reactive Radicals in the Gas Phase

“…The most striking example is the production of methyl radical CH 3 . ,2a–c,e–j, 3a,b which can dimerize to give ethane. Likewise, processes of the type shown in Eq.…”