Effects of Sequential Ligation of Molybdenum Cation by Chalcogenides on Electronic Structure and Gas-Phase Reactivity

Abstract

The molybdenum chalcogenide cations MoX n + (X = O, S; n = 1−3) are studied by a combined experimental and theoretical approach. The monoligated species MoO+ and MoS+ both have (4Σ-) ground states that formally arise from spin-pairing of Mo+ (6S) with O (3P) and S (3P), respectively. Similarly, the bent triatomic MoX2 + cations exhibit doublet ground states (2A1). The trichalcogenides MoO3 + and MoS3 + also have doublet ground states and exhibit similar C 3 v -symmetrical structures; however, distinct energetic differences are found in that MoO3 + is much less stable than MoS3 +, due to the necessity to ionize a strong Mo−O double bond in neutral MoO3. Sequential addition of chalcogenides to molybdenum goes hand in hand with an increase of the formal oxidation state of the metal. As a result, the ionization energies (IEs) increase with the electronegativity and the number of the chalcogenide atoms added: IE(MoO) = 7.9 ± 0.3 eV, IE(MoO2) = 8.7 ± 0.3 eV, IE(MoO3) = 11.7 ± 0.3 eV, IE(MoS) = 7.7 ± 0.3 eV, IE(MoS2) = 8.6 ± 0.3 eV, and IE(MoS3) = 8.9 ± 0.3 eV. Thermochemical considerations in conjunction with the ion/molecule reaction bracketing technique and theoretical results provide reevaluated values for the bond dissociation energies (in kcal/mol): Mo+−O 118 ± 2, OMo+−O 131 ± 5, O2Mo+−O 62 ± 17, Mo+−S 88 ± 14, SMo+−S 100 ± 14, and S2Mo+−S 88 ± 14. Notable differences are observed in the gas-phase reactivity of the MoX n + cations. In general, the molybdenum sulfides are less reactive than the corresponding oxides. The monoligated MoX+ cations promote C−H bond activation of hydrocarbons, while the MoX2 + cations and also MoS3 + are somewhat less reactive. The high-valent transition-metal oxide MoO3 + is the most reactive species and is even capable of activating methane.

Gas-phase activation of methane by ligated transition-metal cations

“…H-atom abstraction from methane by MnO ϩ (51) and MgO ϩ (62) is also promoted by the radical character of the oxygen atom, although theory implies a significant participation of the metal and the formation of an insertion intermediate (62,81), as is reflected by the significant KIEs. MoO 3 ϩ exhibits a KIE of the same magnitude, thus pointing to the operation of a similar mechanism (60). In the case of OFeOH ϩ (59) and FeO ϩ (52, 53), even larger KIEs are observed that, at least in the latter case, can be ascribed to the formation of an intermediate insertion species in conjunction with spin cross-over (81)(82)(83)(84)(85).…”

Aktivierung der C‐H‐Bindung von Methan bei Raumtemperatur durch nacktes [P4O10].+

“…[3] Dabei wurden massenselektierte, sich im Grundzustand befindende Metalloxide mit unterschiedlicher Stöchiometrie untersucht, unter anderem [MgO] + , [4] [FeO] + , [5] [MoO 3 ]C + , [6] [ReO 3 (OH)]C + , [7] [OsO 4 ]C + , [8] [V 4 O 10 ]C + [9] und oligomere [(Al 2 O 3 ) x ]C + -Cluster (x = 3-5). [10] Unter den Aluminiumoxidcluster-Kationen zeigten die mit einer ungeraden Zahl an Aluminiumatomen, die folglich über kein sauerstoffzentriertes Radikal verfügen, keinerlei Reaktivität gegenüber Methan.…”

“…6 + mit einem Argonpuls thermalisiert. [18] Nach sorgfältiger Re-Isolierung und einer variablen Reaktionszeit von 0-5 s wurden die Ionen über eine Standard-Anregungs-Detektions-Sequenz charakterisiert.…”

Thermische Aktivierung von Methan: Es geht auch ohne Übergangsmetalle

“…[9] Auch experimentell sind die Elementarschritte in der Chemie des Magnesiums und seiner Oxide vielfach untersucht worden, [12][13][14][15][16][17][18][19] [20] von wässrigen Magnesium(II)-nitrat-Lösungen bei härteren Ionisierungsbedingungen vor und berichten erstmals von der Gasphasenreaktivität dieses Kations. [21][22][23] + gefunden (KIE = 2.0), [29] während im [V 4 kann, der KIE nur 1.35 beträgt [30] und damit nahe dem KIE von ca. 1.3 für die H(D)-Abstraktion aus Methan durch freie OH-Radikale liegt.…”