Reinvestigating catalytic alcohol dehydrogenation with an iridium dihydroxybipyridine catalyst

Article abstract of DOI:10.1021/acs.organomet.0c00398

The examined catalyst [Cp*Ir(H₂O)(6,6′-dhbp)]²⁺ (1; 6,6′-dhbp = 6,6′-dihydroxy-2,2′-bipyridine) was reported in 2012 as a highly efficient (92% conversion) and selective catalyst for the conversion of benzyl alcohol to benzaldehyde as the sole product via acceptorless dehydrogenation. We report herein that the observed conversion and selectivity data are not accurate but may have resulted, in part, from other products being produced that are not easily detected. Specifically, benzoic acid is formed as a byproduct via the disproportionation of benzaldehyde, but at high temperatures, most of the benzoic acid produced is converted in situ to benzene and carbon dioxide. While we can explain the observed selectivity, we cannot explain the observed conversion to products. In our hands, we observed 15% conversion to products under the original conditions. Other alcohol substrates were also examined and gave lower conversion to products and decreased selectivity in comparison with the original report. Acceptorless alcohol dehydrogenation to generate aldehydes is a potentially transformative technology which can allow chemists to replace stoichiometric oxidants that produce waste with efficient catalysts that only generate H₂ gas as a byproduct. Thus, clarification of the 2012 report to indicate what conditions can lead to high efficiency and selectivity is a worthy topic of discussion in the literature.

Full text of DOI:10.1021/acs.organomet.0c00398

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Article
Reinvestigating Catalytic Alcohol Dehydrogenation with an Iridium
Dihydroxybipyridine Catalyst
Wenzhi Yao, Alexa R. DeRegnaucourt, Emily D. Shrewsbury, Kylie H. Loadholt, Weerachai Silprakob,
Fengrui Qu, Timothy P. Brewster,* and Elizabeth T. Papish*
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ABSTRACT: The examined catalyst [Cp*Ir(H2O)(6,6′-dhbp)]2+ (1; 6,6′-
dhbp = 6,6′-dihydroxy-2,2′-bipyridine) was reported in 2012 as a highly
efficient (92% conversion) and selective catalyst for the conversion of benzyl
alcohol to benzaldehyde as the sole product via acceptorless dehydrogen-
ation. We report herein that the observed conversion and selectivity data are
not accurate but may have resulted, in part, from other products being
produced that are not easily detected. Specifically, benzoic acid is formed as
a byproduct via the disproportionation of benzaldehyde, but at high
temperatures, most of the benzoic acid produced is converted in situ to
benzene and carbon dioxide. While we can explain the observed selectivity,
we cannot explain the observed conversion to products. In our hands, we
observed 15% conversion to products under the original conditions. Other
alcohol substrates were also examined and gave lower conversion to
products and decreased selectivity in comparison with the original report. Acceptorless alcohol dehydrogenation to generate
aldehydes is a potentially transformative technology which can allow chemists to replace stoichiometric oxidants that produce waste
with efficient catalysts that only generate H2 gas as a byproduct. Thus, clarification of the 2012 report to indicate what conditions can
lead to high efficiency and selectivity is a worthy topic of discussion in the literature.
6,6′-dhbp = 6,6′-dihydroxy-2,2′-bipyridine), which at 1.5 mol
% was able to give a 92% yield of pure benzaldehyde (100%
selectivity) after 20 h of reflux in water.
INTRODUCTION
■
Catalytic acceptorless alcohol dehydrogenation is an environ-
mentally friendly method for converting alcohols into
aldehydes (Scheme 1).1−5 This method has the potential to
However, in 2014 a report by Brewster, Cundari, Heinekey,
et al. described the aldehyde−water shift reaction for
converting various aldehydes into the corresponding carboxylic
acids.7 This study included 1 as a catalyst with propionalde-
hyde as the substrate (the corresponding reaction with
benzaldehyde was not reported for this catalyst). In their
report, carboxylic acid could be obtained from aldehyde via
two unique routes. The aldehyde−water shift (AWS) reaction
is shown in Scheme 1 and converts water plus aldehyde to 1
equiv of carboxylic acid plus 1 equiv of H2. Alternatively, a
disproportionation reaction (Scheme 2) can convert 2 equiv of
benzaldehyde to benzoic acid (or benzoate) and benzyl
alcohol. Under basic conditions, the uncatalyzed Cannizzaro
reaction (Scheme 2a) can occur.8 With an iridium catalyst (e.g.
1) and neutral or acidic conditions, a metal-catalyzed
Scheme 1. Acceptorless Alcohol Dehydrogenation as Shown
with Benzyl Alcohol to Generate an Aldehyde and/or a
Carboxylic Acid as the Product
a
a
After aldehyde formation, the aldehyde−water shift (AWS) reaction
is shown.
replace stoichiometric methods that use an oxidizing agent
(e.g., PCC) and generate waste products. Stronger oxidants
(e.g., CrO3) are known to overoxidize alcohols and produce
the carboxylic acid product rather than the desired aldehyde.
Therefore, the report from Fujita et al. in 2012 describing
catalytic and selective alcohol dehydrogenation to form
exclusively aldehyde in high yields was an exciting develop-
ment.6 While various catalysts were tested, the best catalyst
employed in their report was [Cp*Ir(H2O)(6,6′-dhbp)]2+ (1;
Received: June 11, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00398
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Scheme 2. (a) The Cannizzaro Reaction Differs from AWS
in That It Does Not Generate Hydrogen and It Generates
Alcohol as a Byproduct and (b) Metal-Catalyzed
Disproportionation Reaction Converts 2 equiv of
Benzaldehyde to Benzoic Acid and Benzyl Alcohol
Scheme 3. Dehydrogenation of Benzyl Alcohol to Form
both Benzaldehyde and Benzoic Acid
observation of benzaldehyde formation, significant quantities
1
of benzoic acid were observed by H NMR. This was further
corroborated by the observation of a pH drop during the
reaction, which is attributable to the formation of benzoic acid.
Following the CHEM 413 experiments in the Fall of 2018,
we began efforts to reproduce Fujita’s results6 in the Papish
group research laboratory during 2019. We synthesized
[Cp*Ir(H2O)(6,6′-dhbp)][OTf]2 (1) in the prescribed
manner,6 characterized it fully, found that it matched literature
data as described in the Supporting Information, and used it in
experiments to rule out any influence of chloride with 1-Cl. In
disproportionation reaction is more likely to occur via [Ir]−H
formed in situ (Scheme 2b). With 0.8 mol % of 1, aqueous
propionaldehyde was converted to the carboxylic acid in 51%
yield with the reactivity being primarily due to disproportio-
nation (96%) with a small amount (4%) of AWS reactivity
observed.7 Since the focus of this paper was on obtaining
carboxylic acid in good yields without alcohol byproduct
formation, other catalysts were pursued that favored AWS
reactivity, including [Cp*Ir(OH2)bipy]2+ (2) and [(η6-p-
cymene)Ru(OH2)(bipy)]2+, and these catalysts showed an
ability to convert benzaldehyde to benzoic acid with a
selectivity for carboxylic acid as high as 95% (via 95% AWS,
though often with low conversion).7,9,10 Other groups have
pursued other catalysts (using mostly Ru) for AWS and closely
related reactivity.11−13 Though the reaction conditions differed
slightly, given that both the Heinekey and Fujita papers had
benzaldehyde in a flask (as either a starting material or a
product) with a catalyst 1 or similar in aqueous solution, it
seemed highly unlikely to us that both papers could be
simultaneously consistent. This means that either the
benzaldehyde is inert and can be generated without further
reactivity (the Fujita results) or the benzaldehyde can produce
benzoic acid (the Heinekey results). We report herein that the
observed selectivity for oxidative dehydrogenation of benzyl
alcohol with 1 is sensitive to temperature, and at higher
temperatures an apparent selectivity for benzaldehyde is
observed, yet at lower temperatures significant quantities of
benzoic acid are formed (Scheme 3).
our experience, catalyst 1 (synthesized in the Papish group6,15
)
at 1.5 mol % when it is heated at 110 °C with benzyl alcohol in
aqueous solution under nitrogen produces benzaldehyde
(∼97.6% of converted material) and benzoic acid (∼2.4% of
converted material) (entry 4, Table 1). These values in Table 1
represent an apparent selectivity, since we did not trap any
volatile products (vide infra). This reaction also shows a pH
drop from 3.2 at the start of the reaction to 3.0 at the end of
the reaction. However, our percent conversion is somewhat
variable, averaging around 18%, far less than the 92% reported
in the literature by Fujita.6 Anticipating that contamination of
the inert reaction atmosphere by room air may be causing the
oxidation of substrate, we attempted this same reaction in air
(entry 5) and observed no significant difference in the results.
In an effort to further check our results, we reached out to
Brewster, the lead author of the Heinekey study (now at the
University of Memphis), for independent verification. Qual-
itatively similar results were obtained with the catalyst
synthesized and tested by the Brewster group (the catalyst
synthesized in the Brewster laboratory has also been fully
characterized, as described in the Supporting Information).
Furthermore, the catalyst made by one group was shared with
the other for catalytic trials and similarly consistent results
were obtained.16
These results stand in contrast to Fujita’s observations
(entry 1) and prompted us to contact Profs. Fujita and
Yamaguchi for clarification on the experimental procedure. We
obtained a response which indicated that it was important for
the reaction to be run at 130 °C for a vigorous reflux reaction
under argon but that it was otherwise fairly insensitive to the
conditions. This prompted us to study the reaction at 130 °C
under nitrogen (entry 6) or argon (entry 7). Qualitatively,
these two trials gave similar results in terms of conversion (7−
15%) and selectivity (95−98% aldehyde formed). In all of
RESULTS AND DISCUSSION
■
This study began as part of a CHEM 413 Advanced Inorganic
Chemistry Lab experiment for senior undergraduate students
at the University of Alabama during 2017−2018. As part of a
laboratory assignment, students attempted to reproduce the
benzyl alcohol dehydrogenation with iridium catalysts as
reported by Fujita et al.6 The students were provided with
[Cp*IrCl(6,6′-dhbp)]Cl (1-Cl) that was synthesized in the
Papish research group and is known to form 1 in situ by
exchange of chloride for water.14 However, in addition to the
1
these trials in Table 1, both GC and H NMR analyses show
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a
Table 1. Catalytic Dehydrogenation of Benzyl Alcohol with 1 in Water under Various Conditions
sel (%)
entry
source
Fujita6
T of oil bath (°C)
atmosphere
1 (mol %)
conversn (%)
aldehyde
100
88(13)
acid
1
2
3
4
5
6
7
8
130
80
95
110
110
130
130
130
Ar
N2
N2
N2
air
N2
Ar
Ar
1.5
1.5
1.5
1.5
1.5
1.5
1.5
10
92
6(1)
0
c
d
d
this work
this work
this work
this work
this work
this work
this work
12(13)
5(5)
c
10(4)
17.7(3)
14(2)
7(3)
14.6(2)
37(1)
95(5)
97.6(3)
97.5(2)
b
b
c
2.4(3)
2.5(2)
d
d
95(6)
5(6)
b
b
98.2(4)
99.1(1)
1.8(4)
0.9(1)
a
All reactions were carried out with 0.25 mmol of benzyl alcohol and 1 (at the catalyst loading shown in the fifth column) in water for 20 h. All
yields were determined via GC analysis with biphenyl as an internal standard. Reactions were conducted under the atmosphere indicated in the
b
fourth column. We note that % conversion is defined here as 100 × ((moles of aldehyde) + (moles of acid))/(moles of benzyl alcohol). These
reactions were performed at the University of Alabama (UA) in triplicate, and a majority of these entries have been verified by either another
c
student at UA or co-workers at the University of Memphis. These reactions were performed at the University of Memphis (UM) in triplicate, and
d
a majority of these entries have been verified by co-workers at UA. These selectivity measurements had high estimated standard deviations.
a
Table 2. Catalytic Dehydrogenation of Various Substrates with 1 in Water
conversn (%)
sel (%)
b
Fujita,6
this work
aldehyde
acid
,
entry
catalyst (loading (mol %))
substrate
c
c
1
2
3
4
5
6
7
1 (2)
1 (2)
1 (1.5)
1 (1.5)
1 (2)
4-chlorobenzyl alcohol
4-bromobenzyl alcohol
4-methoxybenzyl alcohol
4-methylbenzyl alcohol
cyclohexanol
92
93
93
94
80
92
25
2.0(2)
3(1)
21(6)
27(3)
>99(1)
75(2)
<1(1)
c
c
25(2)
9(5)
3(1)
91(5)
97(1)
d
d
d
8(3)
NA
NA
de
,
d
d
1 (1)
2 (0.5)
1-phenylethanol
benzyl alcohol
15(4)
7(2)
NA
NA
>99(1)
<1(1)
a
All reactions were carried out with 0.25 mmol of substrate and catalyst in water for 20 h. All yields were determined via GC analysis with biphenyl
as an internal standard. Reactions were done with vigorous reflux conditions (oil bath at 130 °C) and under Ar as done by the Fujita group.
Catalyst loadings were selected to match those in the Fujita report for the selected substrate. We note that % conversion is defined here as 100 ×
b
((moles of aldehyde) + (moles of acid))/(moles of benzyl alcohol). Fujita et al. report 100% selectivity for the aldehyde and 0% selectivity for the
c
d
acid with all substrates.6 The selectivity is difficult to measure accurately for these entries with low percent conversion. Carboxylic acids and
aldehydes are not possible products for the oxidation of secondary alcohols. Here, the ketone is the sole product and the selectivity is not
e
applicable. This value was confirmed by 1H NMR analysis, showing a mixture of acetophenone (minor component) and 1-phenylethanol (major
1
component). H NMR analysis of the methyl peaks showed 7% conversion, in agreement with 10% conversion by GC analysis for this trial. GC
analysis is expected to give greater precision.
that the major isolated species is leftover unreacted benzyl
alcohol (82−93%). We also explored the influence of using
lower temperatures for the reaction (entries 2 and 3), and we
observed a similar percent conversion and worse selectivity at
80 °C. Furthermore, hypothesizing that increased catalyst
loading may allow us to achieve the Fujita results, we
performed the reaction at a higher catalyst loading (10 mol
%, entry 8). However, the improved conversion of 37% is still
less than the Fujita results, and the selectivity is similar (99.1%
aldehyde). In no instance were we able to reproduce the published
combination of conversion and selectivity.
We also tested other substrates to determine if our results
were unique to benzyl alcohol (Table 2). Fujita reported that
the oxidative dehydrogenation of 4-R-benzyl alcohols with
catalyst 1 results in complete selectivity for the corresponding
aldehyde with isolated yields of 92−94% (entries 1−4). Our
results (under conditions identical with those in Fujita’s paper
for each substrate) show low conversion (2−3%) with halide
substitution and more moderate conversion (21 and 27%) with
methyl and methoxy substitution, respectively (entries 1−4).
These trials showed apparent selectivity for the aldehyde in
some cases (entries 1 and 4) but also significant formation of
the corresponding carboxylic acid (9 and 25%) for some
substrates (entries 2 and 3). The remaining quantitatively
measured material was the starting benzyllic alcohol (73−97%
depending on substrate). Fujita also reported that catalyst 1
facilitated the oxidative dehydrogenation of secondary alcohols
to the corresponding ketones in high yields: for example,
cyclohexanol and 1-phenylethanol produced ketones in 80 and
92% yields, respectively. In contrast, we observed conversion
to products of 8 and 15%, respectively, when we used the same
conditions (entries 5 and 6; the remaining quantitated material
is recovered unreacted secondary alcohol).
Furthermore, we prepared another catalyst from Fujita’s
paper to determine if our results were unique to catalyst 1. The
catalyst [Cp*Ir(OH2)bipy](OTf)2 (2; 0.5 mol %) was
reported for the oxidative dehydrogenation of benzyl alcohol
with 25% conversion.6 In contrast, we obtained 7% conversion
with freshly prepared, analytically pure catalyst under the
reported conditions (entry 7). Overall, it appears that our
results are not unique to a particular substrate or catalyst. It is
unlikely that our results stem from an impurity in the catalyst
or the substrates. All substrates were purchased from
1
commercial sources, and H NMR analysis shows that the
substrates are >99% pure. The catalysts were synthesized as
previously reported and characterized by NMR spectroscopy,
single-crystal X-ray diffraction for 1 (which matches the prior
structure6), and elemental analysis to eliminate the possibility
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of trace impurities.6,14 Full synthetic details and character-
ization data are given in the Supporting Information.
inert gas (the standard Fujita setup) at 120 °C for 24 h.
Reaction mixtures were diluted with isopropyl alcohol and
injected into a GC/FID for analysis. Crucially, benzene was
observed in this open-system reaction and the product
distribution (alcohol:aldehyde:acid) was consistent with all
the reactions undertaken above. Thus, it is highly likely that
benzene was produced in earlier trials.
The results in Table 1 suggest that at higher temperatures
less benzoic acid is being isolated at the end of the reaction.
Initially, this seems counterintuitive because Scheme 1 shows
that forming benzaldehyde releases 1 equiv of H2 but forming
benzoic acid releases 2 equiv of H2. Thus, one would expect
acid production to be both enthalpically and entropically
favorable. Increased temperature should only increase amounts
of benzoic acid by providing a driving force for gas release, and
these steps should be irreversible with hydrogen escaping via
the bubbler. These results prompted us to look at the aldehyde
reactivity directly by using benzaldehyde as a substrate with 1.5
mol % of 1 in water at 130 °C. This reaction led to 23%
conversion, with the observed selectivity producing 97% benzyl
alcohol and 3% benzoic acid (Table S1). These results can
only be explained by a disproportionation reaction (Scheme
2), or a combination of disproportionation and AWS, which is
followed by decomposition of benzoic acid, most likely to form
benzene and CO2 (Scheme 4). Prior to this discovery we had
Other possible explanations for the discrepancy between our
work and that of Fujita were also probed but were found not to
be supported by experimental evidence. The Fujita paper does
not mention the pH of the reaction mixture, but
correspondence with the author confirms that the pH was
not regulated and distilled, unbuffered water was used for the
reaction. Furthermore, Fujita has informed us that the reaction
is not sensitive to trace contaminants, and we have observed
similar results upon using ultrapure unbuffered water for this
reaction. Thus, one can presume that the pH used for the
reaction by Fujita is 3.2 (which is the pH we observed in
entries 2−7 of Table 1), which results from the acidic
properties of catalyst 1. In fact, the pH observed comes close
to that predicted from the average pKa values of 4.1 for the
diprotic acid 1.17 Given that the properties of a catalyst can
change as it is deprotonated,14,15,18−21 we used catalyst 1 with
added base to achieve pH 7−10 at the start of the reaction to
see if we could obtain results closer to those observed by Fujita
et al. (entry 1, Table 1). As shown in Table S2 in the
Supporting Information, these experiments did not signifi-
cantly change the percent conversion (3−5%) or the selectivity
(∼97% selectivity for benzaldehyde with ∼3% benzoic acid).
We also observe that the pH drops significantly during these
reactions (ending at pH 4 and 5 when starting at pH 7 and 10,
respectively), presumably due to the formation of benzoic acid.
Catalyst instability or sensitivity to a trace contaminant22−24
is frequently blamed when catalytic results in the literature
cannot be reproduced. However, as noted above, an increased
loading of catalyst 1 (Table 1, entry 8) to compensate for
purported catalyst decomposition does not result in
quantitative conversion to the products. Notably, this catalyst
is stable and long-lived in aqueous solution, as demonstrated
by published studies of 1 (as the sulfate salt) and 1-Cl (which
produces 1 in situ) catalyzing CO2 hydrogenation with base
(pH 8.5) in aqueous solution over 18 h.17,20 The continued
catalytic output over the course of the reaction time frame17
and UV−vis analysis of the catalyst at the end of the reaction20
both demonstrate stability. Similarly, catalysts 1 and 1-Cl are
stable in acidic aqueous solution (pH 1.9) when they are used
for formic acid dehydrogenation.20,25 Thus, it appears that this
catalyst is stable but not particularly active for acceptorless
dehydrogenation of benzyl alcohol, which is a very different
reaction vs CO2 hydrogenation (and the reverse reaction) from
both a thermodynamic and kinetic perspective.
Scheme 4. Heating Benzaldehyde in Water with Catalyst 1
Leading to Benzyl Alcohol as the Major Product and
a
Benzoic Acid as the Minor Product
a
This can be explained by a disproportionation followed by
decomposition of benzoic acid.
not specifically looked for benzene in our GC analysis of the
product mixtures. Under vigorous reflux conditions at 130 °C
it is possible that benzene could have escaped the reaction
flask, and thus, our detection. Furthermore, the percent
recovery of organic material was never 100%, suggesting that
some mass loss due to product decomposition during the
dehydrogenation reaction was possible.
CONCLUSIONS
■
With this knowledge in hand after obtaining the results in
Scheme 4, the Papish laboratory performed the reaction of
benzyl alcohol with catalyst 1 (1.5 mol %) in water in a sealed
tube at 130 °C for 20 h. After the reaction mixture was cooled
to room temperature, an extraction was performed with
Fujita’s 2012 paper has been cited over 250 times as of August
2020. Over 100 of these citations use iridium, but only 31 (12
from Fujita et al.) use catalyst 1 or a close derivative. Many
papers use 1 (or attempt to use 1) for other reactions or use
significantly different substrates or solvents, which precludes a
comparison with Fujita’s results,26−28 but a few papers are
relevant here. Do et al. reported that 1 (2 mol %) in aqueous
solution was not able to dehydrogenate benzyl alcohol (and
several other substrates used by Fujita et al.) to form the
corresponding aldehyde, but this result may be attributed to
1
CDCl3. H NMR data suggested, and GC analysis confirmed,
the presence of benzene. As a second verification, the Brewster
laboratory also sought out the production of benzene (see the
Supporting Information). A set of reactions were run in
Schlenk flasks capped with septa under a positive pressure of
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the low temperature (40 °C) used.29 Bruijnincx et al. used
catalyst 1 in 1:1 water and dioxane with base present at T =
130 °C to first cleave the C−C bond in lignin models and then
disproportionate the resulting aldehyde to form both alcohol
and carboxylic acid.30 For this multistep reaction, yields of the
corresponding carboxylic acid were as high as 33% for the
product 3,4-dimethoxybenzoic acid.30 Various conditions were
used in this paper, but in no case was high conversion to
products accompanied by the exclusive formation of only the
aldehyde product. More frequently, high conversion to
products resulted in the formation of approximately equal
parts alcohol, aldehyde, and carboxylic acid. However,
significant differences in the solvent, the base present
(Na2CO3), and the substrate could play some role in the
different results observed by Bruijnincx vs Fujita. Similar to
Bruijnincx’s work, other catalysts using Ir, Ru, and other metals
also convert alcohols to carboxylic acids by a mechanism
similar to that shown in Schemes 1 and 2.31−33 In another
paper that cites Fujita et al., Li et al. report that o-aminobenzyl
alcohols underwent dehydrogenative oxidation with 1 to form
an aldehyde, which then underwent an intermolecular
cyclization reaction that precluded carboxylic acid formation.34
Though the authors assert that initial acceptorless dehydrogen-
ation of the benzylic alcohol is crucial to the reaction, little
mechanistic support was provided. Additionally, stoichiometric
KOH was included in this reaction, suggesting that the
Cannizzaro reaction should occur if the benzaldehyde
derivative is formed in appreciable amounts. Thus, this report
is largely distinct from the Fujita report. In summary, we were
unable to find any literature that directly reproduces or
confirms the Fujita conclusions, but several papers do
indirectly call into question the validity of the Fujita paper
and are more in line with our observations.
Now that we have established that carboxylic acid formation
is supported by literature precedent, we can discuss the
decarboxylation reaction. Complex 1 and closely related
iridium complexes have been previously demonstrated to
function as highly active catalysts for formic acid decom-
position to form CO2 and H2.20,25,35 Thus, it is not surprising
that this catalyst can perform a similar reaction with benzoic
acid. The release of CO2 and formation of benzene could
become more competitive as the reaction temperature is
increased due to the driving force required for this reaction.
Thus, it is possible that, under Fujita’s conditions, 100%
selectivity may have been observed for the aldehyde due to
decomposition of benzoic acid, as illustrated in Scheme 4. In
our hands, at lower temperatures (80 and 95 °C) we obtain
more benzoic acid and apparently worse selectivity.
rule out the possibility that 1 is sensitive to a trace
contaminant. Catalytic reactions in general are known to be
sensitive to conditions and trace contaminants that can
enhance or be detrimental to catalysis.22−24 Thus, it is of the
utmost importance that all experimental details and sources of
reagents be reported for new catalytic methods. Any
experimental details that can lead to higher percent conversion
would be useful for this important reaction, which allows a
stoichiometric oxidant to be replaced with a green and
potentially recyclable catalyst for the acceptorless dehydrogen-
ation of substrates.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00398.
■
sı
Experimental details on the synthesis of the catalyst 1
and its characterization data, further details on the
catalytic studies and characterization of the products
formed, and GC method details (PDF)
Accession Codes
CCDC 2025971 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Timothy P. Brewster − Department of Chemistry, University of
Memphis, Memphis, Tennessee 38152, United States;
orcid.org/0000-0002-7654-4106; Email: tbrwster@
memphis.edu
Elizabeth T. Papish − Department of Chemistry and
Biochemistry, University of Alabama, Tuscaloosa, Alabama
35487, United States; orcid.org/0000-0002-7937-8019;
Email: etpapish@ua.edu
Authors
Wenzhi Yao − Department of Chemistry and Biochemistry,
University of Alabama, Tuscaloosa, Alabama 35487, United
States; orcid.org/0000-0002-4874-0169
Alexa R. DeRegnaucourt − Department of Chemistry and
Biochemistry, University of Alabama, Tuscaloosa, Alabama
35487, United States
Emily D. Shrewsbury − Department of Chemistry and
Biochemistry, University of Alabama, Tuscaloosa, Alabama
35487, United States
Kylie H. Loadholt − Department of Chemistry, University of
Memphis, Memphis, Tennessee 38152, United States
Weerachai Silprakob − Department of Chemistry and
Biochemistry, University of Alabama, Tuscaloosa, Alabama
35487, United States
Our overall aim in this work has been to clarify the literature
record and find conditions that can lead to efficient catalytic
activity in the acceptorless dehydrogenation of substrates with
catalyst 1. Despite our explanation of the observed selectivity,
we do not have an explanation for the discrepancies we find in
our results vs Fujita’s results with respect to percent
conversion. While percent conversions can vary in our hands,
we observe ∼15% conversion with benzyl alcohol as the
substrate at 130 °C. Thus, the literature and our results herein
support that (1) aldehydes are generally not inert and can react
further to form carboxylic acids, (2) catalyst 1 does not
universally give quantitative conversion to the aldehyde (or
ketone) for dehydrogenation reactions and the results depend
on the temperature and the exact substrates used, and (3)
catalyst 1 appears to be stable,17,20,25 but we cannot definitively
Fengrui Qu − Department of Chemistry and Biochemistry,
University of Alabama, Tuscaloosa, Alabama 35487, United
States; orcid.org/0000-0002-9975-2573
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00398
Notes
The authors declare no competing financial interest.
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Iridium and Ruthenium Complexes of NHC and Pyridinol Derived
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ACKNOWLEDGMENTS
■
We thank the NSF (award CHE-1800214) for funding this
research, including the use of research experiments in the
teaching lab and support to E.D.S. as a summer REU student.
We thank NSF CHE MRI 1828078 and UA for purchase of the
SC XRD instrument. T.P.B. acknowledges support from a
grant from The University of Memphis College of Arts and
Sciences Research Grant Fund. This support does not
necessarily imply endorsement by the University of Memphis
of research conclusions. We thank the University of Alabama
for partial support of the experiments done in CHEM 413. We
also thank the students in CHEM 413 during the Fall of 2017
and 2018 for performing initial test runs on the reactions
herein, and we thank Dalton Burks and Chance Boudreaux for
being teaching assistants for these laboratory experiments. We
thank Sanjit Das for valuable suggestions.
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