Mechanistic Studies on the Gas‐Phase Dehydrogenation of Alkanes at Cyclometalated Platinum Complexes

Abstract

In the ion/molecule reactions of the cyclometalated platinum complexes [Pt(L-H)](+) (L=2,2'-bipyridine (bipy), 2-phenylpyridine (phpy), and 7,8-benzoquinoline (bq)) with linear and branched alkanes C(n)H(2n+2) (n=2-4), the main reaction channels correspond to the eliminations of dihydrogen and the respective alkenes in varying ratios. For all three couples [Pt(L-H)](+)/C(2)H(6), loss of C(2)H(4) dominates clearly over H(2) elimination; however, the mechanisms significantly differs for the reactions of the "rollover"-cyclometalated bipy complex and the classically cyclometalated phpy and bq complexes. While double hydrogen-atom transfer from C(2)H(6) to [Pt(bipy-H)](+), followed by ring rotation, gives rise to the formation of [Pt(H)(bipy)](+), for the phpy and bq complexes [Pt(L-H)](+), the cyclometalated motif is conserved; rather, according to DFT calculations, formation of [Pt(L-H)(H(2))](+) as the ionic product accounts for C(2)H(4) liberation. In the latter process, [Pt(L-H)(H(2))(C(2)H(4))](+) (that carries H(2) trans to the nitrogen atom of the heterocyclic ligand) serves, according to DFT calculation, as a precursor from which, due to the electronic peculiarities of the cyclometalated ligand, C(2)H(4) rather than H(2) is ejected. For both product-ion types, [Pt(H)(bipy)](+) and [Pt(L-H)(H(2))](+) (L=phpy, bq), H(2) loss to close a catalytic dehydrogenation cycle is feasible. In the reactions of [Pt(bipy-H)](+) with the higher alkanes C(n)H(2n+2) (n=3, 4), H(2) elimination dominates over alkene formation; most probably, this observation is a consequence of the generation of allyl complexes, such as [Pt(C(3)H(5))(bipy)](+). In the reactions of [Pt(L-H)](+) (L=phpy, bq) with propane and n-butane, the losses of the alkenes and dihydrogen are of comparable intensities. While in the reactions of "rollover"-cyclometalated [Pt(bipy-H)](+) with C(n)H(2n+2) (n=2-4) less than 15 % of the generated product ions are formed by C-C bond-cleavage processes, this value is about 60 % for the reaction with neo-pentane. The result that C-C bond cleavage gains in importance for this substrate is a consequence of the fact that 1,2-elimination of two hydrogen atoms is no option; this observation may suggest that in the reactions with the smaller alkanes, 1,1- and 1,3-elimination pathways are only of minor importance.

Modeling Metal-Catalyzed Polyethylene Depolymerization: [(Phen)Pd(X)]+ (X = H and CH3) Catalyze the Decomposition of Hexane into a Mixture of Alkenes via a Complex Reaction Network

“…Although the chemistry associated with this system was not explored any further, it is worth noting that these results suggest that the barriers for the extrusion of alkenes are higher than those associated with the dehydrogenation reactions. Indeed, related Pt complexes readily dehydrogenate alkanes in the gas phase …”

True and masked three-coordinate T-shaped platinum(II) intermediates

“…Further studies including labeling experiments support the reversibility of these “rollover” reactions. The highly unsaturated species 55 is still reactive and can coordinate and decompose XMe 2 molecules (X = S [128] and O [129]) and dehydrogenate alkanes [130]. Finally, other cyclometalation processes including “rollover” reactions have also been observed for the complex [Pt(Me)L(SMe 2 )] bearing a diimine ligand instead of the ubiquitous bipyridyl backbone [131].…”

A Two-Step Catalytic Cycle for the Acceptorless Dehydrogenation of Ethane by Group 10 Metal Complexes: Role of the Metal in Reactivity and Selectivity

“…While much of the focus of such gas-phase work has been focused on the crucial C–H bond activation step for methane, fewer studies have focused on higher chain alkanes and rarely have complete catalytic cycles been described . Given that ethylene is an important industrial commodity chemical with applications as a polymerization monomer and industrial reaction intermediate, it is surprising that there have been no gas-phase mechanistic studies exploring a catalytic cycle for the dehydrogenation of ethane involving C–H bond activation to form an organometallic intermediate. , Such studies would have direct relevance to the metal-catalyzed acceptorless dehydrogenation of an alkane (as shown for ethane in eq ). This endothermic reaction is highly desirable since it requires no harsh oxidants and only produces hydrogen as a byproduct .…”