Bond Activation by Metal–Carbene Complexes in the Gas Phase

Abstract

"Bare" metal-carbene complexes, when generated in the gas phase and exposed to thermal reactions under (near) single-collision conditions, exhibit rather unique reactivities in addition to the well-known metathesis and cyclopropanation processes. For example, at room temperature the unligated [AuCH2](+) complex brings about efficient C-C coupling with methane to produce C2Hx (x = 4, 6), and the couple [TaCH2](+)/CO2 gives rise to the generation of the acetic acid equivalent CH2═C═O. Entirely unprecedented is the thermal extrusion of a carbon atom from halobenzenes (X = F, Cl, Br, I) by [MCH2](+) (M = La, Hf, Ta, W, Re, Os) and its coupling with the methylene ligand to deliver C2H2 and [M(X)(C5H5)](+). Among the many noteworthy C-N bond-forming processes, the formation of CH3NH2 from [RhCH2](+)/NH3, the generation of CH2═NH2(+) from [MCH2](+)/NH3 (M = Pt, Au), and the production of [PtCH═NH2](+) from [PtCH2](+)/NH3 are of particular interest. The latter species are likely to be involved as intermediates in the platinum-mediated large-scale production of HCN from CH4/NH3 (the DEGUSSA process). In this context, a few examples are presented that point to the operation of co-operative effects even at a molecular level. For instance, in the coupling of CH4 with NH3 by the heteronuclear clusters [MPt](+) (M = coinage metal), platinum is crucial for the activation of methane, while the coinage metal M controls the branching ratio between the C-N bond-forming step and unwanted soot formation. For most of the gas-phase reactions described in this Account, detailed mechanistic insight has been derived from extensive computational work in conjunction with time-honored labeling and advanced mass-spectrometry-based experiments, and often a coherent description of the experimental findings has been achieved. As for some transition metals, in particular those from the third row, the metal-carbene complexes can be formed directly from methane, coupling of the so-generated [MCH2] species with an inert molecule such as CH4, CO2, or NH3 constitutes a route to activate and functionalize methane under ambient conditions. Clearly, while these gas-phase studies cannot be translated directly to formally related processes in solution or those that occur at a surface, they nevertheless provide a conceptual mechanistic understanding and permit researchers to probe directly the remarkable intrinsic features of these elusive molecules and, in a broader context, help to identify the active site of a catalyst, the so-called "aristocratic atoms".

Revealing Disparate Chemistries of Protactinium and Uranium. Synthesis of the Molecular Uranium Tetroxide Anion, UO4–

“…Given the reaction energetics were found to be similar, the difference in reactivity is driven by a higher kinetic barrier for PaO − with background gases in the ion trap (Figure 2b). The corresponding PaO 4 − product is not apparent in Figure 2a. To confirm that UO 4 − results from the reaction of UO 2 (C 2 O 4 ) − with O 2 , an excess of isotopically labeled 18 O 2 was introduced into the ion trap.…”

“…The corresponding PaO 4 − product is not apparent in Figure 2a. To confirm that UO 4 − results from the reaction of UO 2 (C 2 O 4 ) − with O 2 , an excess of isotopically labeled 18 O 2 was introduced into the ion trap. The result is shown in Figure 3, where the dominant product peak at 306 m/ z corresponds to reaction of UO 2 (C 2 O 4 ) − with 18 O 2 to yield UO 2 18 O 2 (unless specified otherwise, O refers to the 99.8% natural abundance 16 O isotope).…”

“…11 Bonding and reactivity of gas-phase actinide oxides, particularly uranium oxides, has received appreciable attention, both experimentally and computationally. 12−25 Of particular interest are molecules with an excess of oxygen atoms, such as UO 4 …”

“…17,27,32 A goal of gas-phase chemistry is to identify spontaneous exothermic reactions that can alternatively result in seemingly exotic high-energy species such as UO 4 − under thermal conditions. One approach to achieve such reactions is by selecting precursors that have ligands that decompose absent substantial kinetic barriers to yield stable neutral molecules, such as CO 2 .…”

“…An emphasis was on differences between these two neighboring actinides, which contrasts with the very similar chemistries of the homologous trivalent 4f lanthanides, Pr and Nd. A further aim was to identify a route to oxygen-rich molecular ions via ion−molecule reactions under thermal conditions; a new synthetic route to molecular UO 4 − was identified. The gas-phase pentavalent actinyl complexes PaO 2 (C 2 O 4 ) − and UO 2 (C 2 O 4 ) − were produced by very different approaches, a disparity that reflects very different chemistries of Pa V and U V .…”