Ligand and electronic-structure effects in metal-mediated gas-phase activation of methane: A cold approach to a hot problem

Abstract

Gas-phase ion-molecule reactions, complemented by DFT and wave-function based electronic structure calculations, are presented, which permit rather detailed descriptions of two fundamental processes related to the activation of methane at room temperature. (i) Recent examples are discussed, which shed light on the role of oxygen-centered radicals in the first step of the oxidative dimerization of methane, i.e. the homolytic C-H bond cleavage of methane. (ii) Thermal ligand exchange processes of the type ML(+) + CH(4) --> M(CH(3))(+) + LH are analyzed in detail with an emphasis on L = H and F attached to various transition-metal cations M(+). It will be demonstrated inter alia that similar systems, e.g. MH(+) (M = Ni, Pd, Pt) in their room-temperature reactions with methane, actually exhibit fundamentally different mechanistic scenarios.

Chemistry with Methane: Concepts Rather than Recipes

“…Conceptually, the development of new mechanistic paradigms, such as two‐state reactivity17 or appreciating the role of relativistic effects12–14 in bond‐activation processes, has removed some of the mysteries and are now considered relevant in several areas far beyond the field of gas‐phase organometallic chemistry 62. 64 Further, the inclusion of ligated systems4j,m,n, 110 not only extends beyond the world of “idealized” naked‐atom chemistry; these systems also serve as more realistic models for reactive species or intermediates that are capable of activating methane in the condensed phase, as do the exciting cluster studies aimed at mimicking surface reactions 4a,g,k,l,n. 22, 24, 48b,c, 83, 111 A major breakthrough certainly was driven by computational chemistry.…”

Room-temperature methane activation by a bimetallic oxide clusterAlVO4+

“…More recently, C-H activation by nonmetal centered oxide clusters such as SO þ 2 [12] and P 4 O þ 10 [13] have been found by de Petris and Schwarz group, respectively. As has been suggested by Schwarz et al, spin-density effect [6,7,[14][15][16][17][18] may be the main reason for oxide radical cations to undergo the initial H-atom abstraction which shed light on the molecular view of so called active sites which are actually transition states or intermediates with a radical cationic character [19] on the surface of the real methane activation catalysts.…”

Detection of σ-alkane complexes of manganese by NMR and IR spectroscopy in solution: (η5-C5H5)Mn(CO)2(ethane) and (η5-C5H5)Mn(CO)2(isopentane)

“… 43 , 44 Simple alkanes are often very reactive towards metal ions in the gas phase, as shown by mass spectrometric methods. 45 Their reactivity may be understood via alkane complexation and, in some cases, the σ-CAM mechanism. 5 , 45 Alkane complexes are also postulated in Periana's most recent results on C–H functionalisation catalysed by heavy p-block metals.…”