Catalytic Dehydrogenation of Alkanes by PCP-Pincer Iridium Complexes Using Proton and Electron Acceptors

Article abstract of DOI:10.1021/acscatal.0c05160

Dehydrogenation to give olefins offers the most broadly applicable route to the chemical transformation of alkanes. Transition-metal-based catalysts can selectively dehydrogenate alkanes using either olefinic sacrificial acceptors or a purge mechanism to remove H2; both of these approaches have significant practical limitations. Here, we report the use of pincer-ligated iridium complexes to achieve alkane dehydrogenation by proton-coupled electron transfer, using pairs of oxidants and bases as proton and electron acceptors. Up to 97% yield was achieved with respect to oxidant and base, and up to 15 catalytic turnovers with respect to iridium, using t-butoxide as base coupled with various oxidants, including oxidants with very low reduction potentials. Mechanistic studies indicate that (pincer)IrH2 complexes react with oxidants and base to give the corresponding cationic (pincer)IrH+ complex, which is subsequently deprotonated by a second equivalent of base; this affords (pincer)Ir which is known to dehydrogenate alkanes and thereby regenerates (pincer)IrH2.

Full text of DOI:10.1021/acscatal.0c05160

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Research Article
Catalytic Dehydrogenation of Alkanes by PCP−Pincer Iridium
Complexes Using Proton and Electron Acceptors
Arun Dixith Reddy Shada, Alexander J. M. Miller, Thomas J. Emge, and Alan S. Goldman*
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ABSTRACT: Dehydrogenation to give olefins offers the most
broadly applicable route to the chemical transformation of alkanes.
Transition-metal-based catalysts can selectively dehydrogenate
alkanes using either olefinic sacrificial acceptors or a purge
mechanism to remove H2; both of these approaches have
significant practical limitations. Here, we report the use of
pincer-ligated iridium complexes to achieve alkane dehydrogen-
ation by proton-coupled electron transfer, using pairs of oxidants
and bases as proton and electron acceptors. Up to 97% yield was
achieved with respect to oxidant and base, and up to 15 catalytic
turnovers with respect to iridium, using t-butoxide as base coupled
with various oxidants, including oxidants with very low reduction
potentials. Mechanistic studies indicate that (pincer)IrH2 com-
plexes react with oxidants and base to give the corresponding cationic (pincer)IrH+ complex, which is subsequently deprotonated by
a second equivalent of base; this affords (pincer)Ir which is known to dehydrogenate alkanes and thereby regenerates (pincer)IrH2.
KEYWORDS: alkane dehydrogenation, C−H activation, pincer complexes, proton-coupled electron transfer, iridium catalysis
as olefin accrues. Such acceptorless dehydrogenation requires
significant energy input and the rate of H2 expulsion is
proportional to the low H2 concentrations. The long reaction
times allow for undesirable secondary reactions such as
isomerization, resulting in the loss of valuable regioselectiv-
ity.5−7 Moreover, the partial pressure of the hydrogen produced
is necessarily extremely low, rendering it very difficult to realize
its significant potential value as a byproduct.
An approach that is very distinct from the transfer of hydrogen
to an olefin or purging H2 from the solution would be the
separate removal of electrons and protons. In contrast with the
sacrifice of an olefin or the high energy input required for
acceptorless dehydrogenation, the transfer of electrons and
protons could proceed as in a fuel cell,36−40 yielding useful
energy. Alternatively, in the absence of O2 at the cathode, the
protons could be reduced to give hydrogen. Although this would
require the input of electrical energy, it could yield H2 at a very
low energetic cost compared with the electrolysis of waterand
the “byproduct” would be a valuable olefin instead of
dioxygen.41,42
INTRODUCTION
■
The dehydrogenation of alkanes to give olefins offers almost
unlimited potential as a route to their catalytic conversion to
commodity chemicals or to alkanes of higher fuel value, e.g.,
through olefin coupling after dehydrogenation.1,2 Over the past
decades, great progress has been made toward the use of
molecular transition-metal catalysts for alkane dehydrogen-
ation.3−7 They offer highly attractive regioselectivity for the
terminal position of n-alkanes in some cases6,8−10 and operating
temperatures much lower than required with conventional
heterogeneous catalysts.11,12 The most widely studied class of
such catalysts has been pincer-ligated iridium complexes.1,13−32
Alkane dehydrogenation to give olefins and H2 is highly
endothermic.33 Two general approaches have been developed to
address the challenge thus presented in the application of
transition-metal-based molecular catalysts. Most commonly, a
sacrificial olefinic hydrogen acceptor is used, in which case the
overall reaction is approximately thermoneutral. Not only does
this introduce the cost of the olefin, however, it also raises the
need for separation of hydrogenated and any unreacted
acceptor.
Received: November 25, 2020
Revised: February 8, 2021
Published: February 22, 2021
An alternative approach to the use of sacrificial acceptors is
simply to purge the hydrogen byproduct of dehydrogenation
from the solution, typically by refluxing.34,35 However, at the
relatively low temperatures compatible with molecular catalysts,
the steady-state concentration of H2 is extremely low even at low
concentration of olefin product and becomes increasingly lower
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Nonelectrochemical approaches to dehydrogenation based
on the transfer of electrons and protons are also potentially very
attractive. Dioxygen is the most obviously favorable acceptor
based on thermodynamic and economic considerations, while
the byproduct, water, could be easily separated from the mixture.
The use of dioxygen for alkane dehydrogenation presents
challenges, including oxidative decomposition of the catalyst,
unwanted oxidation reactions of the alkane and olefin products,
and safety issues in dealing with alkanes, oxygen, and
catalysts.43−45 These problems, however, can be circumvented
via a system for electron and proton transfer in which O2 does
not need to come into contact with either a catalyst or a reagent.
This approach is exemplified most famously in the case of the
Wacker oxidation of ethylene,46 in which case electrons are
ultimately transferred to O2 by Cu(I), which is returned to
solution as Cu(II).
Evidence for the viability of separated proton-coupled
electron-transfer (PCET) reactions47,48 for the activation of
C−H bonds in general41,48−50 and by pincer-iridium systems in
particular51,52 has been demonstrated recently. In this paper, we
report this approach, using one-electron oxidants and Brønsted
bases,47,49 for the catalytic dehydrogenation of alkanes.
Incorporation of such reactions into either electrochemical or
nonelectrochemical systems offers the possibility of a
fundamentally different and highly attractive class of alkane
dehydrogenation reactions.
relative to Cp2Fe+.49 For example, for KOtBu, the pKa of the
conjugate acid in acetonitrile is reportedly 40,47 while E° for
Cp2Fe+ is by definition zero. Thus, the Cp2Fe+/KOtBu couple
has a BDFEeff of 109.7 kcal/mol, which is much greater than is
thermodynamically required for COA dehydrogenation.
BDFEeff(Ox + BH+)
= 1.37pKa(BH+) + 23.1E°(A+/0) + 54.9 kcal/mol
(5)
Our initial choices of an oxidant and Brønsted base, respectively,
were Ag+ and the t-butoxide anion.57 Its very high basicity
notwithstanding, t-butoxide has been demonstrated to be
surprisingly compatible with various oxidants.47 To a p-xylene-
d10 solution of (tBu4POCOP)IrH2 (20 mM; Scheme 1) and COA
Scheme 1. (Pincer)Ir Units Used in This Work
RESULTS AND DISCUSSION
(300 mM), 2.0 equivalents (based on Ir) of Ag[BF4] and 2.0
equivalents of KOtBu were added. Heating to 120 °C rapidly
resulted in a color change from dark brown to orange-brown and
the formation of silver mirror on the wall of the NMR tube
(Scheme 2, first reaction). The 31P NMR spectrum showed a
single signal at δ 177.2. The 1H NMR spectrum revealed a triplet
at δ −42.96 (2JP‑H = 13.1 Hz), indicative of a hydride ligand trans
■
Dehydrogenation of cyclooctane (COA) is somewhat less
endothermic than dehydrogenation of typical alkanes,33 and
cycloalkane dehydrogenation does not introduce complications
due to the formation of various double-bond isomers.
Additionally, the resulting olefin, cyclooctene (COE), tends to
bind to metal centers less strongly than linear olefins53 and has
less tendency to form allyl complexes;54 for both reasons, it is
therefore less likely to inhibit catalysis. COA has therefore long
been the standard substrate of choice for studies of alkane
dehydrogenation catalysis, at least at the early stages of catalyst
development.3−7,55
The free energy of dehydrogenation of COA, to give COE and
H2, is ΔG° = 15.5 kcal/mol, while ΔG° = 97.2 kcal/mol for the
formation of 2H• from H2.33 Therefore, for the formation of
COE and two hydrogen atoms from COA, ΔG° = 112.7 kcal/
mol (eqs 1−3).
58
t
to a vacant coordination site, and inequivalent Bu groups.
These NMR spectroscopic features are very similar to those of
[(tBu4POCOP)Ir(H)(acetone)][B(C6F5)4] (31P NMR δ 174.4;
1H NMR δ −42.28, 2JP‑H = 12.4 Hz),59 synthesized by Brookhart
via the reaction of (tBu4POCOP)IrH2 with hydride abstraction
agent [Ph3C][B(C6F5)4]. In the absence of any evidence for a
ligand other than the hydride and pincer (i.e., a ligand analogous
to the acetone in Brookhart’s complex) or any obvious source of
such ligand, we expect that the BF4 anion would coordinate
weakly at the site trans to the tBu4POCOP ipso carbon. However,
the presence of either a weakly coordinating solvent molecule or
an adventitious species cannot be excluded.
COA → COE + H2 ΔG° = 15.5 kcal/mol
(1)
Continued heating of the reaction mixture for 24 h at 120 °C
H2 → 2H• ΔG° = 97.2 kcal/mol
(2)
1
results in the formation of COE (0.9 eq as determined by H
COA → COE + 2H• ΔG° = 112.7 kcal/mol
NMR and gas chromatography) and the reappearance of
(
(3)
tBu4POCOP)IrH2 (Scheme 2, second reaction).58 This is
The effective bond dissociation free energy (BDFEeff) of an
oxidant/base pair is the free energy of the reaction of a free H
atom with that pair, to give the respective reduced and
protonated species.47,56 For reaction 4 to be ergoneutral (ΔG°
= 0), therefore, the BDFEeff for the oxidant/base couple must be
(112.7 kcal/mol)/2 or 56.35 kcal/mol. It is perhaps worth
noting that this required BDFEeff is much less than the BDFE of
an alkane C−H bond.
readily explained in that deprotonation of the cationic hydride
(Scheme 3) by the remaining equivalent of KOtBu should afford
the key Ir(I) intermediate, (tBu4POCOP)Ir, that is presumed to
undergo C−H addition and then β−H elimination to yield
olefin in the cycle for catalytic alkane dehydrogenation by
(
tBu4POCOP)IrH2.53
The overall reaction shown in Scheme 2 was also conducted at
150 °C, for 16 h, giving the same yield of COE (90% relative to
Ag[BF4] or KOtBu), as determined by gas chromatography
(Table 1, entry 1). The reaction also gave a 90% yield when
conducted in neat COA (Table 1, entry 2).
The overall reaction of Scheme 2 comprises a catalytic cycle
for dehydrogenation of COA driven by proton-coupled electron
COA + 2Ox+ + 2B → COE + 2Ox + 2BH+
(4)
Equation 5 gives the value of BDFEeff for any oxidant/base
couple (for acetonitrile), where pKa(BH+) is the pKa of the
conjugate acid of B and E°(A+/0) is the reduction potential
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Scheme 2. Reaction of (tBu4POCOP)IrH2 with Ag[BF4] and KOtBu in Cyclooctane
Scheme 3. Proposed Pathway for the Reaction of [(tBu4POCOP)IrH][BF4] with KOtBu and Cyclooctane
Table 1. Dehydrogenation of Cyclooctane by Pincer Iridium Complexes with [Ag][BF4] and KOtBu (150 °C, 16 h)
concentration KOtBu
(mM)
concentration AgBF4
(mM)
concentration
COA
concentration product (COE)
(mM)
TON
(yield)
a
entry
catalyst (concentration)
1
2
3
4
5
6
7
8
(
(
(
(
tBu4POCOP)IrH2 (20 mM)
tBu4POCOP)IrH2 (20 mM)
tBu4POCOP)IrH2 (20 mM)
tBu4POCOP)IrH2 (4 mM)
40
40
80
120
80
80
none
120
40
40
80
120
80
none
80
120
300 mM
18
18
36
56
0.9 (90%)
0.9 (90%)
1.8 (90%)
14 (93%)
0
neat (7.4 M)
neat (7.4 M)
neat (7.4 M)
neat (7.4 M)
neat (7.4 M)
neat (7.4 M)
neat (7.4 M)
none
not detected
not detected
not detected
58
(
(
tBu4POCOP)IrH2 (20 mM)
tBu4POCOP)IrH2 (20 mM)
0
0
(MeO-tBu4POCOP)IrH2 (4
mM)
14.5 (97%)
9
(
tBu4PCP)IrH2 (4 mM)
120
120
neat (7.4 M)
58
14.5 (97%)
a
TON = number of turnovers based on the iridium catalyst; yield based on silver salt or KOtBu.
transfer. To our knowledge, this is the first reported example of
this approach to alkane dehydrogenation.
initially investigated. KOtBu and the Cp2Fe+ salts were added
(30 equiv each) to the COA solutions of (tBu4PCP)IrH2 (4
mM), and the mixture was heated at 150 °C for 16 h. Turnover
numbers were very limited (1.2, 0.8, and 2.0 TO, respectively,
out of a theoretical limit of 15 TO; Table 2, entries 1−3), but
this was considered likely attributable to the very low solubility
of these salts in alkane solvent.
Ferrocenium salts that were expected to be more soluble than
those of the parent Cp2Fe+ were then investigated.65 With BF4
and PF6 salts of bis(acetylcyclopentadienyl)iron(III), 2.0 and
While Scheme 2 represents a single catalytic turnover for
dehydrogenation, multiple turnovers can be achieved. The
addition of 4 equiv each of Ag[BF4] and KOtBu to a COA
solution of (tBu4POCOP)IrH2 (20 mM) resulted in the
formation of 36 mM COE, a 90% yield, and 1.8 TO (catalytic
turnovers; Table 1, entry 3). Much higher quantities of Ag[BF4]
and KOtBu were not feasible, but with a lower concentration of
(
tBu4POCOP)IrH2 (4 mM) and 30 equiv of Ag[BF4] and
KOtBu, 56 mM COE was produced (14 TO and 93% yield based
on oxidant and base; Table 1, entry 4).
Table 2. Catalytic Dehydrogenation of Cyclooctane by
(
tBu4PCP)IrH2 with Various Metallocenium Salts as
a
Control experiments were conducted in which the conditions
given in Table 1, entry 3, were reproduced but with the omission
of (tBu4POCOP)IrH2, Ag[BF4], or KOtBu. No COE formation
was observed in these experiments (Table 1, entries 5, 6, and 7).
Comparable results were obtained with the other (pincer)Ir
complexes of Scheme 1. With the same conditions as used for
the experiment of Table 1, entry 4, but with the use of (MeO-
tBu4POCOP)IrH2 or (tBu4PCP)IrH2 (Scheme 1), approximately
equal or greater yields of COE were obtained, 58 mM in both
cases (14.5 TO, 97% yield; Table 1, entries 8 and 9).
Oxidants
entry
metallocenium salt
E° (vs Fc/Fc+)
TON
1
2
3
4
5
6
7
8
[Fc][BF4]
[Fc][PF6]
0.00
0.00
0.00
1.2
0.8
2
2.2
2
7.4
8
10
9
[Fc][BArF4]
[Ac2Fc][PF6]
[Ac2Fc][BF4]
[Me2Fc][PF6]
[Me2Fc][BF4]
[iPr2Fc][BF4]
[iPr2Fc][PF6]
[Cp2Co][PF6]
[Cp*2Co][PF6]
b
,
c
c
0.51
0.51
b
,
d
e
e
−0.11 , −0.10
−0.11 , −0.10
−0.11
−0.11
−1.33
−1.94 , −1.91
d
While the use of Ag+ as oxidant serves as proof of principle, it
surely does not represent a practical advance over the use of
olefinic acceptors. A practical oxidant must be easily reoxidized,
ultimately either by oxygen or at an anode of an electrochemical
cell, whereas Ag+ forms metallic silver. Turning our attention to
such oxidants, we considered metallocenium cations, which also
offer the possibility of “tuning” oxidation potentials and thereby
the reaction thermodynamics. [Cp2Fe][BF4], [Cp2Fe][PF6],
d
d
9
10
11
d
f
,f
12
8
g
a
Conditions: (tBu4PCP)IrH2 (4 mM), metallocene (120 mM), KOtBu
b
(120 mM), COA solvent, 150 °C, and 16 h. CH2Cl2 solvent, ref 60.
c
d
e
Acetonitrile solvent, ref 61. CH2Cl2 solvent, ref 62. Acetonitrile
F
F
f
g
and [Cp2Fe][BAr4 ] (BAr4 = B[3,5-(CF3)2C6H3]4) were
solvent, ref 63. CH2Cl2 solvent, ref 64. Acetonitrile solvent, ref 64.
3011
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Scheme 4
2.2 TO COE were obtained, respectively (Table 2, entries 4 and
5). The methylcyclopentadienyl derivatives gave better results,
8.0 and 7.4 TO, respectively, and the isopropylcyclopentadienyl
derivatives gave 10 and 9 TO, respectively (Table 2, entries 6−
9). Note that the solubility of even these salts in alkane solvent is
still very limited.
the former dihydride complex reacting rapidly and the latter
reacting over the course of 16 h. The cationic complexes were
identified by the H NMR chemical shift characteristic of a
1
hydride ligand trans to a vacant coordination site at −44.85 and
−42.96 ppm, respectively, Scheme 4.58
Thus, 2 equiv of Ag[BF4] effectively acts to remove a hydride
anion.57,70 The implication that this “oxidative removal of
hydride” should be possible electrochemically has already been
demonstrated;52,71 electrochemical oxidation of (tBu4PCP)IrH4
(either preceding or following the loss of H2) in the presence of
lutidine gives [(tBu4PCP)IrH(lutidine)+].52
A much weaker oxidant than Cp2Fe+ is Cp2Co+ (E° = −1.33
V). For the Cp2Co+/KOtBu couple, according to eq 5, BDFEeff is
equal to 79.0 kcal/mol. Although Cp2Co+ is rarely viewed as an
oxidant (rather, cobaltocene is prized as a reductant36), it is still
capable of pairing with t-butoxide to provide about 23 kcal/mol
of thermodynamic driving force for the dehydrogenation of
COA. The use of cobaltocenium as an oxidant was therefore of
interest in this context to probe the driving force needed above
the theoretical value. Additionally, we were interested to see if an
oxidant/base couple with a BDFEeff much less than that of the
cyclooctane C−H bond (ca. 87 kcal/mol66−68) would be
effective.
Slow evaporation of a pentane/benzene (3:1) solution of the
cationic tBu4PCP complex at room temperature afforded orange-
brown crystals. Single-crystal X-ray diffraction revealed the
presence of a water molecule coordinated to iridium (Figure 1).
Surprisingly, the use of cobaltocenium actually gave a higher
TON (12) (Table 2 entry 10) than ferrocenium, or any of the
ferrocenium derivatives investigated, under the same reaction
conditions. The observation of the characteristic chemical shift
of cobaltocene at 55 ppm69 as well as the formation of t-butanol
confirmed the reaction stoichiometry of eq 4.
Remarkably, even the very weak oxidant decamethylcobalto-
cenium, Cp*2Co+, (E° = −1.94 V)64 was found to be an effective
oxidant. A total of 8 TO was obtained with 30 equiv
[Cp*2Co][PF6] (Table 2, entry 11; 53% yield), or somewhat
less than with [Cp2Co][PF6], but equal to the highest values
obtained with Cp2Fe+ derivatives. The BDFEeff of the Cp*2Co+/
KOtBu couple is 64.9 kcal/mol, much less than that of the
ferrocenium/KOtBu couples but still significantly greater than
the BDFEeff that is thermodynamically required for COA
dehydrogenation. Thus, consistent with a mechanism in which
the C−H bonds are cleaved by the Ir center, not directly by the
oxidant/base couple, dehydrogenation can proceed even though
the BDFEeff of the oxidant/base pair is much less than the BDFE
of the cyclooctane C−H bond.
We would exercise caution in drawing mechanistic con-
clusions based on the above experiments since the metal-
locenium salts are not fully soluble. Given that caveat, however,
we consider that the improved yields from cobaltocenium vs
ferrocenium may be attributable to overoxidation by the latter.
Notably, the oxidation/deprotonations are proposed to yield the
Ir(I) species, (pincer)Ir, and it seems plausible that this species
in particular may be susceptible to oxidation in a catalytically
unproductive degradation. The same explanation might also
apply to the observation that higher TONs were achieved with
methyl- and isopropyl-substituted ferrocenium than with the
more strongly oxidizing acetyl derivative (Table 2).
Figure 1. ORTEP structure of [(tBu4PCP)Ir(H)(H2O)][BF4] at the
50% probability level (C-bound hydrogen atoms omitted for clarity).
Selected bond lengths (Å) and angles (deg): Ir(1)−C(1) 2.008(3);
Ir(1)−O(1W) 2.207(2); Ir(1)−P(1) 2.3108(8); Ir(1)−P(2)
2.3137(8); Ir(1)−H(1A) 1.599(7); C(1)−Ir(1)−O(1W)
177.57(11); C(1)−Ir(1)−P(1) 83.41(9); O(1W)−Ir(1)−P(1)
98.86(7); O(1W)−Ir(1)−P(2) 94.29(7); P(1)−Ir(1)−P(2)
166.84(3); C(1)−Ir(1)−H(1A) 86.3(3); and O(1W)−Ir(1)−H(1A)
92.8(3).
It is not clear if the major solution phase species observed by 1H
NMR likewise has a bound molecule of water. Arguing against
that possibility, however, is the observation that addition of 1
equiv water to a C6D6 solution of [(tBu4PCP)IrH]+ results in the
formation of (tBu4PCP)Ir(OH)H72 at room temperature; the
fate of the lost proton (eq 6) is not clear. In a related manner,
under the same conditions where alkane dehydrogenation was
observed, i.e., in the presence of KOtBu, addition of water led to
conversion to (tBu4PCP)Ir(OH)H and catalysis was completely
inhibited.
Isolation of the Cationic Intermediate. The reaction of
either (tBu4PCP)IrH2 or (tBu4POCOP)IrH2 with Ag[BF4] at
room temperature, even in the absence of added base, results in
the formation of the corresponding (pincer)IrH+ species with
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Scheme 5. Proposed Pathway for the Formation of the Trans C−H Addition Product of Acetophenone via Loss of 2H from
(
tBu4PCP)IrH2 through Hydrogenation of Norbornene (ref 80) and through Oxidation and Deprotonation (Present Work)
[(tBu4PCP)IrH]+ + H2O → (tBu4PCP)Ir(H)(OH) + H+
The formation of the trans-C−H adduct 1-trans is
inconsistent with C−H addition occurring while the carbonyl
oxygen is coordinated to iridium, i.e., with the carbonyl acting as
a “directing group.”80 It strongly implicates the transition state
TS-1, the same transition state as is proposed for addition
following loss of hydrogen to olefin. By extension, it implies the
same prior intermediate, the three-coordinate (tBu4PCP)Ir(I)
fragment, obtained by deprotonation of (tBu4PCP)IrH+ in the
case of the present reaction.
Use of Other Bases. We attempted the use of several bases
as alternatives to KOtBu for catalytic COA dehydrogenation, in
all cases with 10 mM (tBu4PCP)IrH2 in p-xylene-d10 with 400
mM COA, 8 equiv of Ag[BF4] and 8 equiv of base (thus the
theoretical yield is 4 equiv or 40 mM COE). After 16 h at 150
°C, 3.8 equiv of COE was obtained with the use of KOtBu under
these conditions. When triethylamine or iPr2NH was used in its
place, no COE was observed (Table 3, entries 2 and 3), which is
(6)
Weak binding of water to the Ir center is indicated by the long
Ir−O bond distance, 2.207(2) Å, a value typical of cationic Ir
aquo complexes,73−78 which compares with 2.00(3) Å reported
for (tBu4PCP)Ir(OH)H.72 By contrast, the Ir−P and Ir−C bond
lengths, as well as the P−Ir−C angles, are identical for aquo and
hydroxy72 complexes within the standard error of the crystallo-
graphic determinations.58
Trapping the Ir(I) Intermediate. As noted above,
deprotonation of the cationic oxidation species, (pincer)IrH+,
would afford the Ir(I) intermediates, (pincer)Ir, which are
presumed to be the species that effect alkane dehydrogenation in
the catalyses with olefinic acceptors.53,79 This fragment, at least
in the case of the tBu4PCP pincer ligand, has been reported to
undergo an unusual and characteristic reaction with arenes with
coordinating functional groups (acetophenone, nitroben-
zene).80 Specifically, it undergoes addition of the C−H bond
ortho to the functional group to give a six-coordinate complex in
which the added C and H are positioned mutually trans, while
the coordinating functional group is trans to the tBu4PCP ipso
carbon (1-trans; Scheme 5). It was therefore concluded that the
coordinating group did not act as a “directing” group but rather
acted to trap the initial C−H addition product. Importantly, the
C−H-trans isomer 1-trans was found to be the kinetic product of
C−H addition and O-coordination, as evidenced by the fact that
it slowly isomerized to the thermodynamically more stable C−
H-cis isomer, 1-cis, upon heating (Scheme 5).80
Table 3. Use of Various Bases to Promote PCET-Driven
a
Dehydrogenation
entry
base
KOtBu
Et3N
iPr2NH
pKaH (MeCN)
pKaH (H2O) TON COE
b
c
1
2
3
4
5
6
7
40
20
3.8
d
d
18.8
10.7
e
none
none
none
none
2.8
e
19
11
b
KN(SiMe3)2
tBuPN(pyrr)3
LiNH2
34
d
d
28.4
17.5
f
f
41 (DMSO)
36 (THF)
38
g
LiiPr2N (LDA)
3.2
a
Conditions: (tBu4PCP)IrH2 (10 mM), [Ag][BF4] (80 mM), base (80
b
mM), COA (400 mM), 150 °C, and 16 h in p-xylene-d10. Ref 47.
To help determine if C−H addition induced by oxidation/
deprotonation proceeded through the same key intermediate
resulting from hydrogenation of olefin, 2 equiv of Ag[BF4] and
KOtBu were added to a p-xylene-d10 solution of (tBu4PCP)IrH2
and acetophenone. The immediate formation of (tBu4PCP)IrH+
c
d
e
f
g
Ref82. Ref 81. Extrapolated from values in ref 81. Ref 83. Ref 84.
presumably attributable to these being much weaker bases than
KOtBu.47 Potassium bis(trimethylsilyl)amide (KHMDS) and
tBuPN(pyrr)3 phosphazene81 were also found to be ineffective
(Table 3, entries 4 and 5), perhaps because they are sterically
very hindered or perhaps because they are insufficiently basic.
Lithium amide and lithium diisopropylamide appeared to
successfully deprotonate (tBu4PCP)IrH+, although the resulting
conjugate acids coordinated to iridium to give (tBu4PCP)Ir-
(amine). Continued heating, however, did result in catalysis,
1
was observed by H NMR spectroscopy. Heating at 100 °C
afforded C−H adduct 1-trans, i.e., the same product that was
previously obtained from the reaction of (tBu4PCP)IrH2 with
norbornene in the presence of acetophenone.80 Further heating
at 120 °C resulted in isomerization to the thermodynamically
more stable 1-cis, again as previously observed in the reaction
with norbornene (Scheme 5).80
3013
https://dx.doi.org/10.1021/acscatal.0c05160
ACS Catal. 2021, 11, 3009−3016

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
although yields were somewhat lower than obtained with KOtBu
(Table 2, entries 6 and 7).
Brunswick, New Jersey 08903, United States; orcid.org/
0000-0002-2774-710X; Email: alan.goldman@rutgers.edu
Authors
SUMMARY AND CONCLUSIONS
■
Arun Dixith Reddy Shada − Department of Chemistry and
Chemical Biology, Rutgers, The State University of New Jersey,
New Brunswick, New Jersey 08903, United States
Alexander J. M. Miller − Department of Chemistry, University
of North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290, United States; orcid.org/0000-0001-9390-
3951
Thomas J. Emge − Department of Chemistry and Chemical
Biology, Rutgers, The State University of New Jersey, New
Brunswick, New Jersey 08903, United States
We report the first example of alkane dehydrogenation by
proton-coupled electron transfer using molecular catalysts
(pincer-ligated iridium complexes) and pairs of oxidants and
bases. Previous approaches to alkane dehydrogenation with
molecular catalysts have relied on sacrificial alkene acceptors or
forcing conditions that facilitate the removal of H2. The PCET
approach introduced here operates at relatively mild conditions.
Mechanistic studies suggest a sequence of oxidation and
deprotonation steps. In the absence of added base, the reaction
of 2 equivalents of Ag[BF4] with (tBu4PCP)IrH2 was found to
rapidly yield (tBu4PCP)IrH+, i.e., a hydride ligand was in effect
removed by oxidation. Subsequent deprotonation leads to C−H
bond activation; evidence points to the formation of the same
three-coordinate d8 species that is produced in the reaction of
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c05160
Notes
(
tBu4PCP)IrH2 with olefin, i.e., (tBu4PCP)Ir, which is known to
be the key intermediate in alkane transfer dehydrogenation by
tBu4PCP)IrH2. The (tBu4POCOP)Ir fragment shows compara-
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
■
ble catalytic activity.
This work was supported by the U. S. Department of Energy
Office of Science (DE-SC0020139).
Remarkably, even extremely weak oxidants such as Cp*2Co+
were found to be effective. The deprotonation of (pincer)IrH+ is
apparently the energetically most demanding step under the
range of conditions we investigated; accordingly, the dehydro-
genation cycle with these catalysts requires a very strong base.
These results suggest that catalytically active fragments that
are much more easily deprotonated than (tBu4PCP)Ir or
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05160.
■
sı
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AUTHOR INFORMATION
Corresponding Author
Alan S. Goldman − Department of Chemistry and Chemical
Biology, Rutgers, The State University of New Jersey, New
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̈
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ACS Catal. 2021, 11, 3009−3016