η2-Alkene Complexes of [Rh(PONOP-iPr)(L)]+ Cations (L = COD, NBD, Ethene). Intramolecular Alkene-Assisted Hydrogenation and Dihydrogen Complex [Rh(PONOP-iPr)(η-H2)]+

Rhodium-alkene complexes of the pincer ligand kappa(3)-C5H3N-2,6-((OPPr2)-Pr-i)(2) (PONOP-Pr-i) have been prepared and structurally characterized: [Rh(PONOP-Pr-i)(eta(2)-alkene)][ BAr4F] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; Ar-F = 3,5-(CF3)(2)C6H3]. Only one of these, alkene = COD, undergoes a reaction with H-2 (1 bar), to form [Rh(PONOP-Pr-i)(eta(2)-COE)][BAr4F] (COE = cyclooctene), while the others show no significant reactivity. This COE complex does not undergo further hydrogenation. This difference in reactivity between COD and the other alkenes is proposed to be due to intramolecular alkene-assisted reductive elimination in the COD complex, in which the eta(2)-bound diene can engage in bonding with its additional alkene unit. H/D exchange experiments on the ethene complex show that reductive elimination from a reversibly formed alkyl hydride intermediate is likely rate-limiting and with a high barrier. The proposed final product of alkene hydrogenation would be the dihydrogen complex [Rh(PONOP-Pr-i)(eta(2)-H-2)][BAr4F], which has been independently synthesized and undergoes exchange with free H-2 on the NMR time scale, as well as with D-2 to form free HD. When the H-2 addition to [Rh(PONOP-Pr-i)(eta(2)-ethene)][BAr4F] is interrogated using pH(2) at higher pressure (3 bar), this produces the dihydrogen complex as a transient product, for which enhancements in the H-1 NMR signal for the bound H-2 ligand, as well as that for free H-2, are observed. This is a unique example of the partially negative line-shape effect, with the enhanced signals that are observed for the dihydrogen complex being explained by the exchange processes already noted.

Automated summary

Rhodium-alkene complexes with different alkenes have been studied for their reactivity towards H-2, with only COD showing significant reaction to form a COE complex. The differences in reactivity are attributed to intramolecular alkene-assisted reductive elimination in the COD complex. Further experiments on ethene complex suggest that reductive elimination from a reversibly formed alkyl hydride intermediate is likely rate-limiting and with a high barrier.