期刊
CHEMICAL SCIENCE
卷 7, 期 4, 页码 2579-2586出版社
ROYAL SOC CHEMISTRY
DOI: 10.1039/c5sc04794c
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资金
- NSF under the CCI Center for Enabling New Technologies through Catalysis (CENTC) Phase II Renewal [CHE-1205189]
- Division Of Chemistry
- Direct For Mathematical & Physical Scien [1205189] Funding Source: National Science Foundation
A rhodium complex based on the bis-phosphine carbazolide pincer ligand was investigated in the context of alkane dehydrogenation and in comparison with its iridium analogue. (carb-PNP) RhH2 was found to catalyze cyclooctane/t-butylethylene (COA/TBE) transfer dehydrogenation with a turnover frequency up to 10 min(-1) and turnover numbers up to 340, in marked contrast with the inactive Ir analogue. TONs were limited by catalyst decomposition. Through a combination of mechanistic, experimental and computational (DFT) studies the difference between the Rh and Ir analogues was found to be attributable to the much greater accessibility of the 14-electron (carb-PNP) M(I) fragment in the case of Rh. In contrast, Ir is more strongly biased toward the M(III) oxidation state. Thus (carb-PNP)RhH2 but not (carb-PNP)IrH2 can be dehydrogenated by sacrificial hydrogen acceptors, particularly TBE. The rate-limiting segment of the (carb-PNP)Rh-catalyzed COA/TBE transfer dehydrogenation cycle is found to be the dehydrogenation of COA. Within this segment, the rate-determining step is calculated to be (carb-PNP) Rh(cyclooctyl)(H) undergoing formation of a beta-H agostic intermediate, while the reverse step (loss of a beta-H agostic interaction) is rate-limiting for hydrogenation of the acceptors TBE and ethylene. Such a step has not previously been proposed as rate-limiting in the context of alkane dehydrogenation, nor, to our knowledge, has the reverse step been proposed as rate-limiting for olefin hydrogenation.
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