?²-Alkene Complexes of [Rh(PONOP-ⁱPr)(L)]⁺ Cations (L = COD, NBD, Ethene). Intramolecular Alkene-Assisted Hydrogenation and Dihydrogen Complex [Rh(PONOP-ⁱPr)(?-H₂)]⁺

Rhodium-alkene complexes of the pincer ligand ?3-C5H3N-2,6-(OPiPr2)2 (PONOP-iPr) have been prepared and structurally characterized: [Rh(PONOP-iPr)(?2-alkene)][BArF4] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; ArF 3,5-(CF3)2C6H3]. Only one these, alkene COD, undergoes a reaction with H2 (1 bar), to form [Rh(PONOP-iPr)(?2-COE)][BArF4] (COE cyclooctene), while others show no significant reactivity. This COE complex does not undergo further hydrogenation. difference in reactivity between COD other alkenes is proposed be due intramolecular alkene-assisted reductive elimination complex, which ?2-bound diene can engage bonding its additional unit. H/D exchange experiments on ethene that from reversibly formed alkyl hydride intermediate likely rate-limiting high barrier. The final product hydrogenation would dihydrogen [Rh(PONOP-iPr)(?2-H2)][BArF4], has independently synthesized free NMR time scale, as well D2 HD. When addition [Rh(PONOP-iPr)(?2-ethene)][BArF4] interrogated using pH2 at higher pressure (3 this produces transient product, for enhancements 1H signal bound ligand, H2, are observed. unique example partially negative line-shape effect, enhanced signals observed being explained by processes already noted.