1 Introduction
Catalysts that can convert methane directly into higher-value-added commodities have long been sought, but breaking the thermodynamically strong, kinetically inert C-H bonds in a controlled way under mild conditions remains a central challenge (Geng et al., 2017). Reactivity studies of transition-metal ions in the gas phase, and, in particular, aspects related to the ongoing challenge of selective activation of inert C-H and C-C bonds, have been studied intensely over the past decades (Howell and Burkinshaw, 1983; Dubois, 1989; Eller and Schwarz, 1991; Balcells et al., 2010; Dobereine and Crabtree, 2010; Roithova and Schröder, 2010; Jana et al., 2011). In recent years, how ligation affects the electronic structure at the transition-metal center has been systematically investigated (Howell and Burkinshaw, 1983; Dubois, 1989; Schlangen et al., 2007; Schlangen et al., 2007; Schlangen and Schwarz, 2008; Dede et al., 2009; Li et al., 2016a; Sun et al., 2016; Zhou et al., 2016; Zhou et al., 2017a; Zhou et al., 2017b; Zhou et al., 2017c; Schwarz et al., 2017; Schwarz et al., 2017; Yue et al., 2017). The ligand can change the electronic structure of the metal center through a shift in the electronic state, or provide a more efficient reaction center, so the addition of a single ligand to a metal center has been widely used to prepare reactants for C-H bonds activation (Chen et al., 1997; Rodgers et al., 2000; Li et al., 2009).
Irikura and Beauchamp (Irikura and Beauchamp, 1989; Irikura and Beauchamp, 1991a; Irikura and Beauchamp, 1991b) discovered that Pt+ as a 5d transition metal dehydrogenates methane to yield the corresponding carbene complexes Pt (CH2)+. Bare Pt+ also has been found to catalyze the reaction of methane with molecular oxygen in the gas phase to produce methanol, formaldehyde and other oxidation products (Wesendrup et al., 1994). Subsequently, a series of activation studies around transition metal Pt+ were carried out (Achatz et al., 2000; Wheeler et al., 2016). Recently, it has been reported that Pt− is able to selectively activate one C-H bond in methane, which represents the first example of methane activation by atomic anions (Liu et al., 2019).
Open-shell ligands X form a covalent bond with the metal cation and thereby increase the formal oxidation state, for example, X = F, Cl, Br, I, OH, NH, O (Schlangen et al., 2007; Dede et al., 2009), which often increases reactivity. For example, bare Cr+ is one of the least reactive transition metal cations, whereas CrCl+ is significantly more reactive (Mandich et al., 1986). Clearly, this example demonstrates that an appropriately chosen ligand can enhance the selectivity of a reagent at the expense of reactivity (Schlangen et al., 2007). Similarly, the naked cations M+ (M = Fe, Co, Ni, Ru, Rh, Pd) do not bring about thermal C-H bond activation of methane (Halle et al., 1982; Tolbert and Beauchamp, 1986; Tolbert et al., 1986; Schultz et al., 1988; Musaev et al., 1993; Musaev and Morokuma, 1994; Westerberg and Blomberg, 1998), but the corresponding MH+ cations (Schilling et al., 1986; Elkind and Armentrout, 1987; Schilling et al., 1987; Schilling et al., 1987; Ohanessian et al., 1990; Zhang and Bowers, 2004; Li et al., 2009; Wang and Andrews, 2009) give rise to efficient H/CH3 ligand switches.
It is not surprising that the nature of the ligand X controls the outcome of a given ion-molecule reaction, as, for example, demonstrated in a systematic investigation of FeX+ cations with acetone (Schröder et al., 1993). The number of ligands also affects the reaction activity. With respect to the activation of methane, CrF+ is not sufficient, and CrF2+ does not react with CH4, whereas CrF3+ and CrF4+ are able to activate the C-H bonds of methane (Mazurek et al., 1998).
Schlangen et al. have reported the studies on ligand and substrate effects in gas-phase reactions of NiX+/RH couples (X = F, Cl, Br, I; R = CH3, C2H5, n-C3H7, n-C4H9) (Schlangen et al., 2007). The results indicate that NiF+ is the only NiⅡ halide complex that brings about thermal activation of methane to eliminate HF, whereas the nickel-halide cations NiCl+, NiBr+, and NiI+ react only with large alkanes. In the elimination of HX (X = F, Cl, Br, I), the formal oxidation state of the metal ion appears to be conserved, and the importance of this reaction channel decreased in going from NiF+ to NiI+. A reversed trend is observed in the losses of H2, which dominate the gas-phase ion chemistry of NiI+/RH couples. Schröder and Schwarz (2005) reported the reactions of methane with PtX+ (X = H, Cl, Br and CHO) using mass spectrometry and found that these species are able to activate methane.
Here, we report our calculated results for the PtX+/CH4 (X = F, Cl, Br, I) systems. The key issues for our study are the mechanistic details of methane catalyzed by ligated transition metal PtX+/CH4 (X = F, Cl, Br, I).
