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)].

ABSTRACT: Rhodium-alkene complexes of the pincer ligand κ³-C₅H₃N-2,6-(OPⁱPr₂)₂ (PONOP-ⁱPr) have been prepared and structurally characterized: [Rh(PONOP-ⁱPr)(η²-alkene)][BAr^F₄] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; Ar^F = 3,5-(CF₃)₂C₆H₃]. Only one of these, alkene = COD, undergoes a reaction with H₂ (1 bar), to form [Rh(PONOP-ⁱPr)(η²-COE)][BAr^F₄] (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 η²-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-ⁱPr)(η²-H₂)][BAr^F₄], which has been independently synthesized and undergoes exchange with free H₂ on the NMR time scale, as well as with D₂ to form free HD. When the H₂ addition to [Rh(PONOP-ⁱPr)(η²-ethene)][BAr^F₄] is interrogated using pH₂ at higher pressure (3 bar), this produces the dihydrogen complex as a transient product, for which enhancements in the ¹H NMR signal for the bound H₂ ligand, as well as that for free H₂, 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.

SUBMITTER: Johnson A 

PROVIDER: S-EPMC8456414 | biostudies-literature |

REPOSITORIES: biostudies-literature