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)][BArF4] [alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; ArF = 3,5-(CF3)2C6H3]. Only one of these, alkene = COD, undergoes a reaction with H2 (1 bar), to form [Rh(PONOP-iPr)(η2-COE)][BArF4] (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)][BArF4], 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)][BArF4] 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.
Highlights
Pincer complexes of the group 9 metals (Co, Rh, and Ir) are used widely in catalysis.[1]
When they are exposed to H2, a proposed intramolecular alkene-assisted reductive elimination leads to marked differences in both reactivity and selectivity in hydrogenation reactions, the outcome of which is dependent on the identity of the η2-bound alkene and, in particular, its ability to engage in bonding to the rhodium center with its additional alkene unit
We report the independent synthesis of the dihydrogen complex [Rh(PONOP-iPr)(η2H2)][BArF4] and some highly unusual observations during monitoring of the reaction of the corresponding ethene complex with paradihydrogen
Summary
Pincer complexes of the group 9 metals (Co, Rh, and Ir) are used widely in catalysis.[1]. Reductive elimination from an intermediate ethyl hydride is promoted by coordination of exogenous ethene (Scheme 1A), or even more strongly by H2 which returns Ir(carb− PNP)H2 instead. In this contribution, we report the synthesis of new cationic [Rh(PONOP-iPr)(η2-alkene)][BArF4] complexes [PONOP-iPr = κ3-C5H3N-2,6-(OPiPr2)[2]; alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; ArF = 3,5(CF3)2C6H3]. We report the synthesis of new cationic [Rh(PONOP-iPr)(η2-alkene)][BArF4] complexes [PONOP-iPr = κ3-C5H3N-2,6-(OPiPr2)[2]; alkene = cyclooctadiene (COD), norbornadiene (NBD), ethene; ArF = 3,5(CF3)2C6H3] When they are exposed to H2, a proposed intramolecular alkene-assisted reductive elimination leads to marked differences in both reactivity and selectivity in hydrogenation reactions, the outcome of which is dependent on the identity of the η2-bound alkene and, in particular, its ability to engage in bonding to the rhodium center with its additional alkene unit. H2 ligand, as well as for free H2, a consequence of the partially negative line-shape (PNL) effect that is in operation.[15]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
