- Complex 1, an iridium(III) complex, was shown to catalytically dehydrogenate benzyl alcohol and isopropanol at 100°C using molecular oxygen as the hydrogen acceptor. - The addition of acetic acid increased the turnover number (TON) of both reactions by over 10-fold, and also increased the initial rate of benzyl alcohol dehydrogenation. - Lewis acids generally inhibited the dehydrogenation reactions when added, except for lithium triflate and sodium bis(trifluoromethanesulfonyl)imide which showed slightly higher TONs.

1. Previous work has shown successful stoichiometric
a l k a n e d e h y d r o g e n a t i o n u s i n g c o m p l e x
(dmPhebox)Ir(OAc)2(OH2) (1). Complex 1 activated the
C-H bonds of alkanes at 200 °C and after a rate limiting
β-hydride elimination, (dmPhebox)Ir(OAc)(H) (2) was
formed. The starting material 1 was regenerated using
O2 and HOAc at 25 °C completing a stoichiometric cycle
(Scheme 1).2
Scheme 1
Alcohol Dehydrogenation by an Iridium (III)
Complex using O2 as a Hydrogen Acceptor
Lisa Qiu†, Ashley M. Wright‡, Karen I. Goldberg‡
†Department of Chemistry, Carleton College, Northfield, MN, 55057
‡Department of Chemistry, University of Washington, Seattle, WA, 98195
Synthesis of Ir(III) Complex
Acknowledgements
References
This work was supported by NSF through the Center for
Enabling New Technologies through Catalysis (CENTC;
CHE-1205189). We thank Prof Tom R. Cundari, Mr. Dale
Pahls, Prof. Melanie S. Sanford, Prof. Elon A. Ison, Prof.
William D. Jones, and other members of CENTC for
helpful discussions.
Alcohol Dehydrogenation
We proposed alcohol dehydrogenation could be
achieved using complex 1. A similar pathway as alkane
dehydrogenation is envisioned (Scheme 2). Lower C-H
activation energy in alcohols should result in a lower
reaction temperature.
Scheme 2
1. Allen, K. E.; Heinekey, D. M.; Goldman, A. S.;
Goldberg, K. I. Organometallics, 2013, 32, 1579.
2. Allen, K. E.; Heinekey, D. M.; Goldman, A. S.;
Goldberg, K. I. Organometallics, 2014, 33, 1337.
3. Ito, J.; Nishiyama, H. Eur. J. Inorg. Chem. 2007,
1114.
4. Ito, J.; Shiomi, T.; Nishiyama, H. Adv. Synth. Catal.
2006, 348, 1235.
Introduction
This project focuses on the conversion of alcohols to
their respective ketones/aldehydes using molecular
oxygen as the terminal hydrogen acceptor (eq. 1). This
method has numerous large-scale industrial applications
in the production of fuels and chemicals
alcohol + 0.5 O2 aldehyde/ketone + H2O (eq. 1)
Contact Information
E-mail: qiul@carleton.edu
Complex 1 is an Iridium (III) complex supported by 2,6-
bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl),
dmPhebox, ligand. It has been shown to activate arene
and alkane C-H bonds.3, 4
Dehydrogenation of Benzyl Alcohol and
Isopropanol
Addition of Lewis Acids
Effects of Excess Acetic Acid
Conclusions
• Complex 1 was shown to catalytically dehydrogenate
benzyl alcohol and isopropanol at 100 °C
• Lewis acids did not appear to help facilitate reaction
• Presence of acetic acid increased TON of benzyl
alcohol and isopropanol dehydrogenation reactions
• Acetic acid also increased initial reaction rate in the
dehydrogenation of benzyl alcohol
Complex 1 successfully catalytically dehydrogenated
benzyl alcohol and isopropanol at 100 °C (eq. 2 & 3)
(Table 1). Benzyl alcohol appeared to be less easily
oxidized by complex 1 than isopropanol. C-H activation
of the benzyl ring could be attributing to low turnover
number (TON). Following this, we focused on optimizing
reaction conditions to maximize TON.
neat, air, 100 °C
OH O
0.1 - 1 mol % [Ir](OAc)2(OH2)
H2O (eq. 2)+0.5 O2+
neat, air, 100 °C
0.1 - 1 mol % [Ir](OAc)2(OH2)OH O
H2O (eq. 3)+0.5 O2+
O
N
Ir
N
O
[Ir] =
We investigated the catalytic activity of 1
towards the dehydrogenation of benzyl
alcohol and isopropanol (eq. 2 & 3).
When acetic acid was added to the cyclohexane
solution, TON for benzyl alcohol and isopropanol
reactions increased (Table 5), which suggests that acid
can increase the reactivity.
We proposed that the presence of Lewis acid in solution
would bind with acetate, opening up a coordination site
of the Ir center (Scheme 4).
Scheme 4
Different Lewis acids were added to dehydrogenation
reactions of benzyl alcohol (eq. 4) and isopropanol (eq.
5) (Table 3 & 4). In the presence of Lewis acid, TONs
decreased with the exception of LiOTf and NaBARf,
suggesting that Lewis acid inhibited the reaction. Solvent
system is very polar, which may lead to weak
interactions between Lewis acid and Ir catalyst.
Alcohol Substrate Ir Catalyst (mol %) TON
Benzyl Alcohol 0.10 12
Isopropanol 0.15 73
Table 1. TON of the dehydrogenation reactions of benzyl alcohol and
isopropanol. Reactions ran neat at 100 °C under the presence of air. After
18h, NMR spectral data were obtained and TONs were calculated.
Lewis Acid TON
None 12
LiOAc 3.9
NaOAc 8.6
KOAc 1.6
CsOAc 7.1
Table 2. TONs of the dehydrogenation reactions of
benzyl alcohol using complex 1 with the acetate salts. All
TONs with Lewis acids were lower than that without
Lewis acids.
neat, air, 100 °C, 18h
OH O
0.1 mol % [Ir](OAc)2(OH2)
1 mol % Lewis Acid
H2O
0.4mL
0.5 O2 (eq. 4)
Lewis Acid TON
None 73
LiOTf 85
NaOTf 52
KOTf 48
AgOTf 46
Zn(OTf)2 31
NaBARf 78
Table 3. TONs of the dehydrogenation reactions of
isopropanol using complex 1 with different Lewis acids.
LiOTf and NaBARf showed slightly higher TONs than the
run without any Lewis acids.
Non-polar solvent - Cyclohexane
Non-polar solvent may help facilitate better interaction
between Lewis acid and Ir catalyst. Isopropanol reaction
was done in cyclohexane with LiOTf (eq. 6) (Table 4).
Very little dehydrogenation occurred, which also
suggests that Lewis acids inhibit the reaction.
neat, air, 18h
0.15 mol % [Ir](OAc)2(OH2)
0.15 mol % Lewis AcidOH O
H2O
0.4mL
0.5 O2
(eq. 5)
Lewis Acid TON
LiOTf 1.6
LiOTf 1.9
LiOTf 8.9
Table 4. TONs of the dehydrogenation reactions of
isopropanol using complex 1 with LiOTf in cyclohexane.
C6H12, air, 18h
1 mol % [Ir](OAc)2(OH2)
5 mol % HOAc, 1 mol % LiOTfOH O
H2O0.5 O2
(eq. 6)
Alcohol
Substrate
Ir catalyst
(mol %)
Acetic Acid
(mol %)
TON
Benzyl Alcohol 1.4 0 1.6
Benzyl Alcohol 1.4 5.0 20
Isopropanol 1.0 0 6.3
Isopropanol 1.0 5.0 93
Table 5. TON of the dehydrogenation reactions of benzyl alcohol and
isopropanol in cyclohexane solvent with the addition of acetic acid
versus no addition. Reactions with 5 mol % acetic acid has over a ten-
fold increase in TON than its respective reaction without acetic acid.
Kinetic Studies of the Dehydrogenation of
Benzyl Alcohol
Dehydrogenation reactions of benzyl alcohol (eq. 7)
without acetic acid and with 5 mol % acetic acid (Figure
1) were monitored via NMR. With acetic acid, an
increase in the rate of the reaction was observed during
the first 4h.
Figure 1. TON vs time plot of the dehydrogenation reactions of benzyl alcohol
using complex 1. The green series represent the experiment without acetic acid
while red series represent the experiment with 5 mol % of acetic acid. An
increase in initial rate is observed during the first 4h of the reaction when there
is acetic acid present.
N
O
O
N
Ir OH2
OAc
AcO
- HOAc
N
O
O
N
Ir
O
H
O
R' R"
O
N
O
O
N
Ir
O
O
O
R' R"
OH
R'
R"
i) 5 KOH, N2 100°C
ii) 3 KMnO4, 0°C
iii) CH2O
iv) HCl
i) SOCl2, toluene 100°C
ii) 2-amino-2-
methyl-propanol,
Et3N
iii) MsCl, 0°C
OH
OH
O
O
N
O
O
N
N
O
O
N
Ir OH2
Cl
Cl
N
O
O
N
Ir OH2
OAc
AcO
Br
Br
IrCl6·H2O
NaHCO3, MeOH, H2O
60 °C, 1h
6 AgOAc
THF, 60 °C, 12h
1
N
O
O
N
Ir OH2
OAc
AcO
- HOAc
N
O
O
N
Ir
O
H
O
N
O
O
N
Ir
O O LA
O
R' R"
O
LA
+
R' R"
OH
R'
R"
However, catalysis does not occur:
• High C-H activation energy in alkanes resulted in
reactions running at high temperatures
• Complex 1 decomposed at 200 °C under O2
1
2
1
0
2
4
6
8
10
12
0 0.5 1 1.5 2 2.5 3 3.5 4
TON
Time (h)
C6D12, 1 atm O2, 100 °C
OH O
1 mol % [Ir](OAc)2(OH2)
0 or 5 mol % HOAc
H2O0.5 O2 (eq. 7)