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Combined Experimental and Computational Study of the Mechanism of Acceptorless Alcohol Dehydrogenation by POCOP Iridium Pincer Complexes

Polukeev, Alexey V. LU ; Abdelaziz, Omar Y. LU and Wendt, Ola F. LU (2022) In Organometallics 41(7). p.859-873
Abstract

Iridium pincer complexes of the type (POCOP)Ir (POCOP = 2,6-(tBu2PO)2C6H3) are very productive catalysts for dehydrogenation of secondary alcohols. To our surprise, we found that turnover frequencies demonstrated by (POCOP)IrH2 (IrH2) are higher in more dilute solutions of the catalyst, which triggered a mechanistic study of alcohol dehydrogenation by IrH2. Here, we provide strong evidence that acceleration by dilution is related to the rate-limiting mass transfer of hydrogen, which, so far, has not received much attention in the literature. Using experimental and computational methods, we show that dehydrogenation has two high-barrier steps, namely the reaction of IrH2 with alcohol to give (POCOP)IrH(OR) (IrH(OR)) and subsequent... (More)

Iridium pincer complexes of the type (POCOP)Ir (POCOP = 2,6-(tBu2PO)2C6H3) are very productive catalysts for dehydrogenation of secondary alcohols. To our surprise, we found that turnover frequencies demonstrated by (POCOP)IrH2 (IrH2) are higher in more dilute solutions of the catalyst, which triggered a mechanistic study of alcohol dehydrogenation by IrH2. Here, we provide strong evidence that acceleration by dilution is related to the rate-limiting mass transfer of hydrogen, which, so far, has not received much attention in the literature. Using experimental and computational methods, we show that dehydrogenation has two high-barrier steps, namely the reaction of IrH2 with alcohol to give (POCOP)IrH(OR) (IrH(OR)) and subsequent β-elimination in the latter. Depending on the alcohol and reaction conditions, IrH(OR) can be formed via an associative pathway that includes proton transfer to the hydride or a dissociative mechanism that involves hydrogen elimination from IrH2 to give a 14e (POCOP)Ir species. Rapid re-hydrogenation of IrH(OR) or the 14e (POCOP)Ir by dissolved hydrogen is responsible for the rate retardation in more concentrated solutions of the catalyst. The suggested mechanism gives a satisfactory quantitative description of the catalytic cycle, such that kinetic curves and reaction orders in the catalyst can be reproduced.

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organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Organometallics
volume
41
issue
7
pages
15 pages
publisher
The American Chemical Society (ACS)
external identifiers
  • scopus:85127232504
ISSN
0276-7333
DOI
10.1021/acs.organomet.2c00012
language
English
LU publication?
yes
id
2ae84c20-ac47-400d-a7d8-f0ecb604d49f
date added to LUP
2022-06-29 11:37:39
date last changed
2023-12-19 19:19:29
@article{2ae84c20-ac47-400d-a7d8-f0ecb604d49f,
  abstract     = {{<p>Iridium pincer complexes of the type (POCOP)Ir (POCOP = 2,6-(tBu2PO)2C6H3) are very productive catalysts for dehydrogenation of secondary alcohols. To our surprise, we found that turnover frequencies demonstrated by (POCOP)IrH2 (IrH2) are higher in more dilute solutions of the catalyst, which triggered a mechanistic study of alcohol dehydrogenation by IrH2. Here, we provide strong evidence that acceleration by dilution is related to the rate-limiting mass transfer of hydrogen, which, so far, has not received much attention in the literature. Using experimental and computational methods, we show that dehydrogenation has two high-barrier steps, namely the reaction of IrH2 with alcohol to give (POCOP)IrH(OR) (IrH(OR)) and subsequent β-elimination in the latter. Depending on the alcohol and reaction conditions, IrH(OR) can be formed via an associative pathway that includes proton transfer to the hydride or a dissociative mechanism that involves hydrogen elimination from IrH2 to give a 14e (POCOP)Ir species. Rapid re-hydrogenation of IrH(OR) or the 14e (POCOP)Ir by dissolved hydrogen is responsible for the rate retardation in more concentrated solutions of the catalyst. The suggested mechanism gives a satisfactory quantitative description of the catalytic cycle, such that kinetic curves and reaction orders in the catalyst can be reproduced. </p>}},
  author       = {{Polukeev, Alexey V. and Abdelaziz, Omar Y. and Wendt, Ola F.}},
  issn         = {{0276-7333}},
  language     = {{eng}},
  number       = {{7}},
  pages        = {{859--873}},
  publisher    = {{The American Chemical Society (ACS)}},
  series       = {{Organometallics}},
  title        = {{Combined Experimental and Computational Study of the Mechanism of Acceptorless Alcohol Dehydrogenation by POCOP Iridium Pincer Complexes}},
  url          = {{http://dx.doi.org/10.1021/acs.organomet.2c00012}},
  doi          = {{10.1021/acs.organomet.2c00012}},
  volume       = {{41}},
  year         = {{2022}},
}