Activation of X–H and X–D bonds (X = O, N, C) by alkaline-earth metal monoxide cations: experiment and theory

Abstract

Experimental investigations are reported for reactions of MO (+) (M = Ca, Sr, and Ba) with elemental hydrides water, ammonia and methane proceeding in the gas phase at 295 +/- 3 K in helium buffer gas at a pressure of 0.35 +/- 0.01 Torr. Measurements were taken with an inductively-coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer and a novel electrospray ion source/ion selection quadrupole/selected-ion flow tube/triple quadrupole (ESI/qQ/SIFT/QqQ) mass spectrometer. All three alkaline-earth metal oxide ions exclusively abstract a H-atom from the three hydrides with rate coefficients > 1 x 10(-11) cm(3) molecule(-1) s(-1). Formation of metal hydroxide ion was followed by sequential addition of water or ammonia, but not methane. Density functional calculations have provided potential energy surfaces for the X-H bond activations leading to H-atom abstraction as well as those for O-atom transfer and H(2)O elimination (with ammonia and methane). A comparison of experimental and theoretical isotope effects points toward a mechanism involving the direct atom transfer from XH and XD to O in MO (+)via a three-centered transition structure.

Thermal hydrogen-atom transfer from methane: A mechanistic exercise

“…The [MgO] •+ /CH 4 couple serves to illustrate the key features [49], and the simplified potential-energy surface (PES) is shown in Figure 3. Analogous findings hold true for the heavier alkaline-earth oxides [MO] •+ (M = Ca, Sr, Ba) [50]. The initially generated adduct complex has gained enough internal energy for the hydrocarbon to migrate around the metal center toward the reactive oxo site at which the C H bond fission occurs.…”

“…While the alternative rebound mechanism to generate methanol is thermochemically even more favorable than HAT, it is kinetically and entropically much less attractive. Finally, in contrast to the direct hydrogenatom abstraction from CH 4 , which features rather small kinetic isotope effects, the metal-mediated, indirect homolytic C H bond cleavages exhibit larger KIEs, typically >2.0 [46,50]. While HAT, when mediated by diatomic [MO] •+ ions, has many features in common, for some reagents a fundamental difference seems to exist with regard to the regioselectivities when reacted with higher alkanes: in contrast to oxides of transition metals which prefer to attack internal C H bonds, in the reactions of n-alkanes with [MgO] •+ a preferential activation of a terminal, primary C H bond has been noted [51].…”

Chemie mit Methan: Studieren geht über Probieren!

“…B. MgO + C, [40] MO + C (M = Ca, Sr, Ba), [41] PbO + C, [42] sie umfassen außerdem das nichtmetallische SO 2 + C, [43] die gemischten Metall/Nichtmetall-Cluster P n V 4Àn O 10 + C (n = 1, 2, 4), [44] Dimetalloxide AlVO 4 + C, [45] sowie Schema 2. Vereinfachtes Schema des Katalysezyklus der C-C-Kupplung von Methan (aus Lit.…”

“…[41] Für den Mechanismus der Wasserstoffabstraktion aus CH 4 In der Gasphasenreaktion des mehrkernigen Oxidclusters V 4 O 10 + C mit Methan [48] bildet sich offenbar kein langlebiger Begegnungskomplex V 4 + ···CH 3 C wird das nur schwach gebundene Methylradikal freigesetzt. [37] Der Verlauf dieses ziemlich effizienten Prozesses, der mit 60 % der Stoßrate erfolgt und mit einem intramolekularen kinetischen Isotopeneffekt von 1.35 behaftet ist, [37,49] [40] Abschließend sei bemerkt, dass in dem hier diskutierten Kontext kinetische Isotopeneffekte als experimentelle Sonde zur mechanistischen Klassifizierung von homolytischen C-HBindungsbrüchen brauchbar zu sein scheinen.…”

High reactivity of nanosized niobium oxide cluster cations in methane activation: A comparison with vanadium oxides

“…14, 15 The active species include many metal [16][17][18][19][20][21][22][23][24][25] (such as vanadium, V) or heteronuclear (such as X-V, in which X = Al, 26 Si, 27 P, 28,29 Y, 30 or Nb 31 ) oxide cluster cations and a few anionic ones, such as La 6 O 10 −32 and PtAl 2 O 4 − . 33 Most of these oxide clusters are with particular stoichiometry to satisfy ∆ = 1, in which ∆ is defined as ∆ = 2 y − nx + q for an oxide cluster M x O y q to indicate its oxygen-richness or poorness (q is the charge number and n counts the highest oxidation state of element M).…”