Chapter 1
Mechanisms of Metal-Mediated C–N Coupling Processes: A Synergistic Relationship between Gas-Phase Experiments and Computational Chemistry

Robert Kretschmer, Maria Schlangen, and Helmut Schwarz

1.1 Introduction

As a consequence of the key positions that the elements carbon and nitrogen occupy in nature, C–N bond formation constitutes an important issue in the synthesis of various products ranging from chemical feedstocks to pharmaceuticals. Not surprisingly, over the last few decades, intensive research has been devoted to this timely topic [1], and the use of ammonia as a relatively inexpensive reagent for C–N coupling reactions has been found to be highly desirable [2]. However, despite the impressive progress reported on the development of new synthetic methodologies, there exists a lack of information on the precise, atomistic-level derived mechanisms in particular for the metal-mediated formation of nitrogen-containing organic molecules generated directly from ammonia. One way to gain such insight is to perform gas-phase experiments on “isolated” reactants. These studies provide an ideal arena for probing experimentally the energetics and kinetics of a chemical reaction in an unperturbed environment at a strictly molecular level without being obscured by difficult-to-control or poorly defined solvation, aggregation, counterion, and other effects. Thus, an opportunity is provided to reveal the intrinsic feature(s) of a catalyst, to explore directly the concept of single-site catalysts, or to probe in detail how mechanisms are affected by factors such as cluster size, different ligands, dimensionality, stoichiometry, oxidation state, degree of coordinative saturation, and charge state. In short, from these experiments, one may learn what determines the outcome of a chemical transformation [3]. In addition, thermochemical and kinetic data derived from these experiments provide a means to benchmark the quality of theoretical studies.

While the study of “naked” gas-phase species will, in principal, never account for the precise kinetic and mechanistic details that prevail at a surface, in an enzyme, or in solution, when complemented by appropriate, computationally derived information, these gas-phase experiments prove meaningful on the ground that they permit a systematic approach to address the above-mentioned questions; moreover, they provide a conceptual framework. The DEGUSSA process, which is the rather unique, platinum-mediated, large-scale coupling of CH4 and NH3 to generate HCN [4], serves as a good example. Mass spectrometry-based experiments [5] suggested both the key role of CH2NH as a crucial gas-phase transient and also pointed to the advantage of using a bimetallic system rather than a pure platinum-based catalyst for the C–N coupling step to diminish undesired, catalyst-poisoning “soot” formation [6, 7]. The existence of CH2NH was later confirmed by in situ photoionization studies [8] and catalysts that are currently employed contain silver-platinum alloys rather than pure platinum.

In this chapter, we focus on two types of gas-phase C–N coupling processes, Eqs. (1.2) and (1.2), using metal complexes bearing simple carbon- and nitrogen-based ligands and probing their thermal reactions with ammonia and hydrocarbons, respectively. While we will refrain from describing the various experimental techniques and computational methods or the way the reactive species [M(CHx)]+ and [M(NHx)]+ are generated [9], the emphasis will rather be on the elucidation of the often intriguing mechanisms of these metal-mediated coupling reactions.

1.2 From Metal-Carbon to Carbon–Nitrogen Bonds

1.2.1 Thermal Reactions of Metal Carbide and Metal Methylidene Complexes with Ammonia

The major ionic product in the reactions of [Ptn(C)]+ (n = 1, 2) with NH3 corresponds to dehydrogenation of the latter [10]. While there is no direct spectroscopic support for the structure assignment of the generated [Ptn(C,N,H)]+ ions, circumstantial evidence is provided by the ion/molecule reaction of the mass-selected product ions [Pt(C,N,H)]+ with NH3, Eq. (1.3).

Occurrence of reaction (1.3) suggests the presence of a preformed HCN (or HNC) ligand in [Pt(C,N,H)]+. Thus, in contrast to [Pt2(C,N,H)]+, generated from [Pt2C]+ and not being able to release HCN, Eq. (1.4), the mononuclear platinum carbide [Pt(C)]+ induces C–N bond formation upon reaction with NH3; apparently, this species serves as one of the late reactive intermediates to generate HCN from CH4 and NH3 [5].

In the thermal ion/molecule reactions of the singlet platinum methylidene clusters [Ptn(CH)]+ (n = 1, 2) with NH3, the dominant path corresponds to proton transfer to generate [NH4]+ [11]. In addition, for the mononuclear precursor, the couple [Pt(CH)]+/NH3 gives rise to the formation of [CH2NH2]+ concomitant with the loss of atomic platinum; clearly, transfer to and insertion of the electrophilic CH+ unit in a N–H bond of ammonia provides the methane iminium ion [CH2NH2]+.

This reaction, Eq. (1.1) with x = 1, has also been studied in quite some detail for the group 10 systems [M(CH)]+/NH3 (M = Ni, Pd, Pt), and remarkable metal-dependent differences have been noted [3]. For the couples [M(CH)]+/NH3 (M = Ni, Pt),1 the following branching ratios were obtained, Eq. (1.5); mechanisms of the various processes were uncovered by extensive density functional theory (DFT) calculations and deuterium-labeling experiments employing [M(CD)]+/NH3 and [M(CH)]+/ND3 [3].

Proton transfer to produce [NH4]+ and the neutral metal carbide MC is exothermic only for M = Pt as a consequence of the relatively small proton affinity (PA) of 780 kJ mol−1 for PtC as compared with PA(NH3) = 852 kJ mol−1; in contrast, PA(NiC) = 915 and PA(PdC) = 879 kJ mol−1 are too high to let [M(CH)]+ act as a Brønsted acid toward NH3.

Further, the elimination of a hydrogen atom, originating exclusively from the incoming ligand NH3, to generate eventually the amino-substituted metal carbene complex [M(CHNH2)]+ reflects thermochemical features. Specifically, the M–H bond strength of the central intermediate [H–M(CHNH2)]+ increases from nickel to platinum such that the reaction is exothermic for nickel but endothermic for the other two-metal complexes.

Clearly, C–N bond formation is also involved in the generation of [CH2NH2]+ as well as in the dehydrogenation paths to produce either [M(CHNH)]+ or isomeric [M(CNH2)]+. Depending on the metal, these two isomers are formed via different mechanisms; while for nickel and palladium, a σ-bond metathesis is operative, for platinum a sequence of oxidative addition/reductive elimination is involved [12]. In addition, in the formation of [CH2NH2]+, the actual mechanism of the intracomplex hydrogen rearrangement, that is, a direct [1.2] migration versus a metal-mediated hydrogen transfer is quite affected by the electronic structure of the intermediate [M(CH–NH3)]+ [3].

1.2.2 How Metals Control the C–N Bond-Making Step in the Coupling of CH4 and NH3

Under thermal conditions, the system [Pt]+/CH4/NH3 reacts with 76–80% efficiency [5] relative to the collision rate, to form [Pt(CH2)]+; dehydrogenation of ammonia by atomic Pt+ to produce [Pt(NH)]+ is endothermic [5, 13]. Further, if independently generated [Pt(NH)]+ is reacted with CH4, the products [Pt(CH2)]+ and NH3 are mainly formed (85%), presumably in a σ-metathesis process with [CH2NH2]+/PtH (10%), and [Pt(CNH)]+/2H2 (5%) generated as by-products. Thus, it is the metal carbene complex [Pt(CH2)]+ that serves as the key intermediate in the C–N coupling of CH4 and NH3. As shown in Eq. (1.6), in addition to minor proton transfer to generate [NH4]+, the two major products are associated with the formation of C–N bonds. On the basis of labeling experiments, both [CH2NH2]+ and [Pt(CHNH2)]+ are formed in clean reactions in which [Pt(CH2–NH3)]+ serves as the central precursor. As mentioned, extensive labeling experiments complemented by DFT calculations shed light on the mechanisms of the reactions [5].

As shown in Figure 1.1, in the dehydrogenation of intermediate 3, which is also accessible in a detour 1 + NH3 → [H3N–Pt–CH2]+ (2) → 3, the platinum center is exploited as a “catalyst.” According to DFT calculations, the sequence of metal-mediated N–H and C–H bond activations to generate 6 is energetically favored over the alternative path commencing with a C–H bond activation (3 → 5 → 6). The metal-free, symmetry-forbidden [1.2] hydrogen migration/elimination path (3 →...