Methane Activation by 5 d Transition Metals: Energetics, Mechanisms, and Periodic Trends

Abstract

Although it has been known for almost three decades that several 5d transition-metal cations will activate methane at room temperature, a more detailed examination of these reactions across the periodic table has only recently been completed. In this Minireview, we compare and contrast studies of the kinetic energy dependence of these reactions as studied using guided-ion-beam tandem mass spectrometry. Thermochemistry for the various products observed (MH , MH , MC , MCH , MCH , and MCH ) are collected and periodic trends evaluated and discussed. The mechanisms for the reactions as elucidated by synergistic quantum chemical calculations are also reviewed. Recent spectroscopic evidence for the structures of the MCH dehydrogenation products are discussed as well.

Ta2+-mediated ammonia synthesis from N2and H2at ambient temperature

“…As has been shown time and again, gas-phase experiments provide an ideal arena for tackling many challenging mechanistic issues at a strictly molecular level, such as investigating the detailed course of chemical reactions, including those that are industrially relevant (74)(75)(76)(77)(78)(79)(80)(81)(82)(83)(84)(85)(86)(87)(88). Structurally properly characterized gas-phase clusters have been chosen as prototypical models to probe the active sites in (including but not limited to) heterogeneous catalysis aimed at a better understanding of the intrinsic factors that govern reactivity patterns in the condensed phase (89,90).…”

Phenyl-Group Exchange in Triphenylphosphine Mediated by Cationic Gold–Platinum Complexes—A Gas-Phase Mimetic Approach

“…NMR spectra were recorded using a Bruker Avance II + 400 MHz WB NMR spectrometer with a 5 mm ATM-BBFO probe head and a variable-temperature device. 1 H and 13 C chemical shifts are reported relative to TMS. The (residual) solvent peaks were used for calibration.…”

“…Mass-Spectrometric Reactivity Experiments. The cation-mode ESI mass spectrum of [(Ph 3 P)AuPt(PPh 3 ) 3 ]-[BAr 4 F ] (1[BAr 4 F ]) dissolved in diethyl ether is dominated by three main signals at m/z 1439, 1177, and 721 (see Figure 4; the m/z assignments are based on the isotopologues containing 1 H, 13 While the signals for 2 and 3 are unequivocally identified by their isotope pattern, the isotope pattern of complex 1 appears poorly resolved and is slightly shifted toward a lower m/z ratio in our ion-trap mass spectrometer (see Figure 3 and Figure 5a). However, an Ultra-Zoom Scan in the Selected-Ion Monitoring-mode (SIM mode) clearly identifies the signal as [Au,Pt,(PPh 3 ) 4 ] + (1; Figure 5b).…”

Selective C−H Bond Cleavage in Methane by Small Gold Clusters

“…Consequently, the encounter complex Au x CH 4 + represents the energetically most‐stable structure and only one C−H bond can be activated and cleaved upon collision with a relatively energetic He atom, thus resulting in a mixture of the encounter complexes and the gold cluster–hydrido–methyl complexes observed in the present investigation. With respect to such a “gentle” activation gold clearly differs from that of the other 5d metals . The hydrido methyl complexes are, thermodynamically, considerably less stable than metal–carbene complexes and thus they are ideally suited for further selective functionalization under thermal conditions, opening new vistas for catalytic transformation of methane into desired valuable products with possible chemical and energy‐related applications.…”