Mechanistic studies on the gas-phase dehydrogenation of alkanes at cyclometalated platinum complexes.

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.