Cp? versus Bis-carbonyl iridium precursors as CH oxidation precatalysts

Article abstract of DOI:10.1021/acs.organomet.6b00525

We previously reported a dimeric Ir^IV-oxo species as the active water oxidation catalyst formed from a Cp?Ir(pyalc)Cl {pyalc = 2-(2′-pyridyl)-2-propanoate} precursor, where the Cp? is lost to oxidative degradation during catalyst activation; this system can also oxidize unactivated CH bonds. We now show that the same Cp?Ir(pyalc)Cl precursor leads to two distinct active catalysts for CH oxidation. In the presence of external CH substrate, the Cp? remains ligated to the Ir center during catalysis; the active species-likely a highvalent Cp?Ir(pyalc) species-will oxidize the substrate instead of its own Cp?. If there is no external CH substrate in the reaction mixture, the Cp? will be oxidized and lost, and the active species is then an iridium-μ-oxo dimer. Additionally, the recently reported Ir(CO)₂(pyalc) water oxidation precatalyst is now found to be an efficient, stereoretentive CH oxidation precursor. We compare the reactivity of Ir(CO)₂(pyalc) and Cp?Ir(pyalc)Cl precursors and show that both can lose their placeholder ligands, CO or Cp?, to form substantially similar dimeric Ir^IV-oxo catalyst resting states. The more efficient activation of the bis-carbonyl precursor makes it less inhibited by obligatory byproducts formed from Cp? degradation, and therefore the dicarbonyl is our preferred precatalyst for oxidation catalysis.

Full text of DOI:10.1021/acs.organomet.6b00525

Article
pubs.acs.org/Organometallics
Cp* versus Bis-carbonyl Iridium Precursors as CH Oxidation
Precatalysts
Daria L. Huang,§ David J. Vinyard,§ James D. Blakemore,†,§ Sara M. Hashmi,‡ and Robert H. Crabtree*,§
†Department of Chemistry, University of Kansas, 1251 Wescoe Hall Drive, 2010 Malott Hall, Lawrence, Kansas 66045, United States
‡Department of Chemical and Environmental Engineering, Yale University, 9 Hillhouse Avenue, New Haven, Connecticut 06520,
United States
§Department of Chemistry, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States
S
* Supporting Information
ABSTRACT: We previously reported a dimeric IrIV-oxo
species as the active water oxidation catalyst formed from a
Cp*Ir(pyalc)Cl {pyalc = 2-(2′-pyridyl)-2-propanoate} pre-
cursor, where the Cp* is lost to oxidative degradation during
catalyst activation; this system can also oxidize unactivated CH
bonds. We now show that the same Cp*Ir(pyalc)Cl precursor
leads to two distinct active catalysts for CH oxidation. In the
presence of external CH substrate, the Cp* remains ligated to
the Ir center during catalysis; the active specieslikely a high-
valent Cp*Ir(pyalc) specieswill oxidize the substrate instead of its own Cp*. If there is no external CH substrate in the
reaction mixture, the Cp* will be oxidized and lost, and the active species is then an iridium-μ-oxo dimer. Additionally, the
recently reported Ir(CO)2(pyalc) water oxidation precatalyst is now found to be an efficient, stereoretentive CH oxidation
precursor. We compare the reactivity of Ir(CO)2(pyalc) and Cp*Ir(pyalc)Cl precursors and show that both can lose their
placeholder ligands, CO or Cp*, to form substantially similar dimeric IrIV-oxo catalyst resting states. The more efficient activation
of the bis-carbonyl precursor makes it less inhibited by obligatory byproducts formed from Cp* degradation, and therefore the
dicarbonyl is our preferred precatalyst for oxidation catalysis.
INTRODUCTION
■
The selective activation of CH bonds is a highly desirable
reaction that has attracted great interest for many years.1−7 CH
oxidation is an especially valuable version, as it is highly relevant
to selective chemical synthetic processes for late-stage
functionalization of organic molecules.8−14 Several metal-
loproteins are known to carry out oxidation reactions in nature
through metal-oxo species, most commonly using high-valent
first-row transition metals.15−20 These metalloproteins gen-
erally operate via intermediate radicals.21−25 In contrast, many
synthetic complexes, including those discussed here, have
metal-oxo intermediates, where an oxene-like insertion permits
a stereoretentive reaction.26,27
Figure 1. Cp*IrIII(pyalc)Cl (1), IrIV-oxo dimer (2), monomeric
Cp*IrIV(pyalc) reaction intermediate (3), and the IrI(CO)2(pyalc)
(4). Species 2 is thought to be a mixture of stereoisomers with X
possibly being H2O, acetate, or iodate under turnover conditions.
Species 3 is thought to be a resting state in the CH oxidation catalysis
with Y possibly being H2O, −OH, acetate, or iodate under turnover
conditions.
Although Cp* is oxidatively degraded under catalytic
conditions for WO,34,38−40 we were curious to see if rapid
degradation would still occur if more reactive CH substrates
were also present in the reaction mixture. We now report that
the Cp* complex 1 can retain its Cp* group under CH
oxidation conditions, and the catalyst progresses to 2 only
when no CH substrate is present. We have identified a
Cp*IrIV(pyalc) species (3) as a long-lived resting state in CH
In previous reports, we and others have demonstrated that
Cp*Ir compounds, such as Cp*Ir(pyalc)Cl (Figure 1, 1), are
competent CH oxidation precatalysts.28−33 Later reports
proposed an IrIV-oxo dimer as the active species for both
water oxidation (WO) and CH oxidation catalysis, following
oxidative degradation of the Cp* ligand to primarily give
AcOH.34 Despite extensive work on this system, the active form
of the catalyst is not yet fully defined, as it cannot be
crystallized for structural study. Recent spectroscopic and
HEXS data suggest a mono-μ-oxo di-iridium structure as the
preferred active species (2).35 A number of stereoisomers may
well be present, accounting for its reluctance to crystallize.36,37
Special Issue: Hydrocarbon Chemistry: Activation and Beyond
Received: June 28, 2016
© XXXX American Chemical Society
A
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a
Scheme 1. Different Reaction Protocols for CH Oxidation Catalysis
a
Protocol A has no preactivation phase, and all reagents are added at the same time, while protocol B involves a pre-activation phase that deliberately
removes the Cp* with oxidant prior to adding the CH substrate and more oxidant.
Figure 2. (a) UV−vis spectra over 1 h of a solution of 1 (1 mM) in the presence of 100 mM sodium 4-ethylbenzenesulfonate (EBS) as CH substrate
and 400 mM NaIO4 as oxidant. (b) UV−vis spectra of a solution of 1 (1 mM) after addition to a solution of NaIO4 and EBS (black line) versus a 1
mM solution of 1 in the presence of 400 mM NaIO4 alone (blue dashed line).
oxidation catalysis, suggesting that the active CH oxidation
species is a high-valent Ir(pyalc) species with the Cp* still
ligated to the metal center. The Cp*Ir(pyalc) active species will
oxidize the external CH substrate rather than its own Cp*
ligand. A Cp*IrIV species has previously been suggested as a
catalytic intermediate in CH activation catalysis,41,42 but to the
best of our knowledge this is the first report in which we
identify a Cp*IrIV resting state in solution-state reaction
conditions by means of a number of spectroscopic techniques.
Recently we reported a new Ir bis-carbonyl precursor (4) as
an improvement over 1 for WO catalysis.43 Here, we now show
that 4 is also a competent CH oxidation precatalyst. Both 1 and
4 likely form substantially similar species of general type 2 after
their placeholder ligands, CO and Cp*, respectively, undergo
oxidative degradation, but we see faster activation of the
dicarbonyl precatalyst. Moreover, 4, lacking the Cp* ligand,
does not form complicating byproducts, such as AcOH, iodate,
and methanol, during the Cp* degradation process. We show
that this change of the placeholder ligands makes the bis-
carbonyl precatalyst (4) more rapidly activating and efficient at
CH oxidation, making it the preferred precursor over 1.
a needed coligand for 2 to become catalytically active for CH
oxidation. This emphasizes the importance of acetic acid as an
in situ ligand for promoting catalysis for 2, something we were
able to discover and study only by using a precursor that lacks
Cp* ligands and thus lacks the AcOH degradation product.
RESULTS
■
Characterization of High-Valent Monomeric Cp*Ir
Species. In initial work by Zhou et al. with the Cp*Ir series
as CH oxidation precatalysts,28,30 the precatalyst was always
added simultaneously with the oxidant and substrate (Scheme
1, protocol A); no steps were taken to preactivate the Cp*Ir
precatalyst to remove the Cp* (protocol B). This Cp*
degradation process was reported only after the work by
Zhou et al. and therefore was not taken into account by them.
We could not then be sure if the active species in the CH
oxidation reaction was a mononuclear derivative of 1 with the
Cp* still present or whether the Cp* was degraded
preferentially over the CH substrate and that 2 oxidized the
CH bonds. The mononuclear catalytic species has been alluded
to before: Templeton and co-workers have isolated and
characterized a Cp*Ir compound that retains its placeholder
ligand and is still able to perform stoichiometric oxidation
reactions.41,44,45 They propose the active species as a high-
valent mononuclear Cp*Ir species. In the present paper, we
report several pieces of evidence that indicate that for CH
Importantly, the dicarbonyl precursor does not give AcOH as
a byproduct of activation, and thus the Ir-oxo species 2
produced in the activation process lacks this potential ligand.
We now find that 4 requires the addition of AcOH to give
significant yields in CH oxidation, thus indicating that AcOH is
B
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Figure 3. (a) Cyclic voltammogram of 1 mM 1 in MeCN with 0.1 M nBuN4PF6 as the supporting electrolyte. Scan rate: 200 mV/s with basal plane
graphite (BPG) working electrode. (b) UV−vis spectra of 1 before and after applying a 1 V potential for 30 min. More information can be found in
the Experimental Section.
Figure 4. X-band EPR spectra. (a) 1 in a representative CH oxidation reaction: 50 nmol (500 μM) of 1 is added to 250 mM NaIO4 and 50 mM
THF in water (black line). The reaction is allowed to proceed for 5 min prior to sparging and freezing. The simulation of the spectra is in red. (b)
Breakdown of the two simulation components.
oxidation reactions with precursor 1 the Cp* remains ligated to
the metal center, as suggested by our EPR and UV−vis
observations, which suggest we have a mononuclear Cp*IrIV
resting state of type 3. In contrast, dicarbonyl 4 rapidly loses
the placeholder ligand and forms a μ-oxo species as a catalytic
resting state, as shown later in this paper.
turns blue and absorbs at 610 nm (Figure 2b). This is the
procedure that Hintermair et al. use to form the IrIV-oxo dimer
(2), with the Cp* primarily degrading into acetic acid in the
preactivation process.34 Additionally, if a substrate, such as
sodium benzenesulfonate, is added that lacks very easily
oxidizable CH bonds, the Cp* placeholder ligand will also be
preferentially attacked and 2 will be formed (Figure S2).
Therefore, for protocol A to be effective, available and very
easily oxidizable CH bonds must be present in the substrate to
protect the Cp* from degradation.
This λmax = 485 nm species was further investigated by
electrochemistry. When a solution of 1 is scanned over a range
of potentials, we see a metal-centered redox process at 1 V vs
NHE with a peak-to-peak separation of 99 mV, which is
consistent with a quasireversible IrIII/IV couple (Figure 3a).
When a fixed potential of 1 V (vs NHE) is applied to a solution
of 1, a new spectral band grows in at 530 nm (Figure 3b),
similar to the λmax = 485 nm species produced from the
Under normal CH oxidation conditions, where the catalyst is
added to the substrate and oxidant without any preactivation of
the compound (protocol A), the reaction retains its orange
color and absorbs strongly at 485 nm (Figure 2a). This band
persists even after the CH oxidation reaction is completed and
1
remains stable in solution for more than 3 days. The H NMR
spectrum of the reaction mixture shows that the Cp* signal
remains, with no trace of acetic acid, a major byproduct of the
Cp* degradation as part of the activation process for WO
(Figure S1). In contrast, following protocol B with the
deliberate removal of the Cp* in a preactivation phase in
contact with oxidant alone to remove the Cp*, the solution
C
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chemical oxidation of 1 (Figure 2a). The shift from 485 to 530
nm may be due to the difference in solvent, electrolyte, and pH
conditions. More importantly, this species is clearly distinct
from 2, which absorbs at 610 nm, and supports the idea that,
though a high-valent Ir is being formed, the Cp* remains
bound to the metal center. Thomsen et al. could fully remove
the Cp* from 1 electrochemically only by applying a potential
of 1.4 V vs NHE over the course of 48 h.46 This is a promising
sign that Cp*Ir compounds may be efficient electrocatalysts for
CH oxidation without Cp* loss; work is ongoing in the lab to
further probe this point.
An EPR spectrum of 1 in a CH oxidation reaction using
protocol A gives a spectrum that appears broad and nearly axial
(Figure 4a). However, simulations show that the signal is best
fit when there are two unique species, both of which are
rhombic (Figure 4b). The dominant species (83%) was
modeled using the g-values 2.503, 2.293, and 1.426, and line
broadening was simulated using the g-strain tensor [0.149 0.160
0.333]. The minor feature (17%) was modeled using the g-
values 2.809, 2.091, and 1.877, and line broadening was
simulated using the g-strain tensor [0.307 0.118 0.220].
Although EPR examples of Cp*IrIV species are rare, these
values align with those reported by our group47,48 and
others.49−52 Importantly, the presence of an EPR signal
indicates that 2 is not the dominant CH oxidation species, as
it is EPR silent, at least in the usual IV,IV oxidation state.34 This
is also a unique example of a Cp*IrIV species observed in
reaction conditions, rather than oxidized by electrode or by
[Ru(bipy)3]3+. For the purposes of this paper, we will consider
the dominant feature as the main species in the reaction.
The fact that the observed EPR signal assigned to Ir(IV)
persists for over an hour of reaction time (Figure 5) is
this high-valent Cp*IrV(pyalc)-oxo or -oxene species, which
then oxidizes either the CH substrate, the Cp*, or the solvent
water depending on the reaction conditions.
Given that we have two potential catalytic species (2 or 3)
that can be derived from the precatalyst 1, we wondered if they
have the same reactivity for CH oxidation catalysis. If we follow
protocol A and form the active species 3, we find high oxidation
yields for a variety of substrates, as observed by Zhou (Table
1).28 However, if 1 is preactivated with 50 equiv of NaIO4 to
Table 1. CH Oxidation Yields for Select Substrates from 1,
a
Following Protocol A or B
a
Reaction conditions: Oxidant predissolved in H2O and added to the
substrate and precatalyst 1 and allowed to stir at 25 °C for 60 min in
air. Yields were determined by 1H NMR against TMSP-d4 as a
standard. More information can be found in the Experimental Section.
remove the Cp* and form 2 (protocol B), we find that the
addition of CH substrates and additional oxidant gives lower
oxidized CH product yields. We believe that this discrepancy is
due to inhibition of 2, the Cp* degradation products; the
preactivation step in protocol B requires that a significant
amount of periodate be used to remove the Cp* and
necessarily produces degradation byproducts AcOH and iodate,
but also formic acid, methanol, and ethanol, known powerful
reductants.38,40,46 The presence CH oxidation byproducts from
the Cp* degradation distinguishes protocols A and B; these
byproducts may well inhibit CH oxidation catalysis and cause
the difference in yields.
Ir(CO)2(pyalc), 4, as an Alternative CH Oxidation
Precatalyst. Considering that the obligatory degradation
products of the preactivation phase of 1 to 2, such as methanol,
ethanol, and formic acid, inhibit oxidation catalysis, we turned
to a different precatalyst, which produces the same or very
similar species to 2, but with much fewer activation byproducts.
The recently reported that the IrI(CO)2(pyalc) precursor (4)
was shown to be a robust WO precatalyst, in part because it
activates much more quickly and easily than the analogous 1
and with no observable byproduct.43 The CO ligands are lost
more easily and quickly than Cp*; more importantly, 4 requires
only 2 equiv of oxidant to become activated, compared with a
minimum of 50 equiv for 1. Given the success of precursor 4 as
a WO precatalyst and its facile preactivation, we decided to test
it for CH oxidation catalysis.
In optimizing the CH oxidation conditions for dicarbonyl 4,
we adapted what we learned from WO experiments, notably,
the requirement for the presence of AcOH to stabilize the
active species of type 2. For terminal oxidants, we tested several
different water-soluble and heterogeneous oxidants for the
oxidation of model substrate 4-ethylbenzenesulfonate (EBS) to
4-acetophenone sulfonate (APS) (Table 2). Of the 10 oxidants
tested, sodium periodate, periodic acid, and ceric ammonium
nitrate (CAN) gave the highest yields. However, given the well-
known difficulties53 of using CAN at pH higher than 2 and its
interference with UV−vis and NMR experiments, owing to its
Figure 5. EPR signals recorded in a representative CH oxidation
reaction over the course of 90 min.
consistent with the presence of an Ir(IV) resting state for the
system. This is also consistent with the UV−vis data (Figure
2a). This resting state is likely similar in structure to 3 and is a
mononuclear paramagnetic Cp*IrIV species. Additionally, the
signal does not change when screening other CH substrates
and solvents (Figures S3 and S4). This indicates that the
proposed monomeric [Cp*IrIV(pyalc)-Y]n+ resting state does
not have a CH substrate ligated to the metal center. The resting
state is likely a precursor to the active form, possibly a high-
valent Cp*IrV(pyalc)-oxo or -oxene, which we have not been
able to characterize yet but have previously described by
DFT.29 The rate-determining step is likely the oxidation of 3 to
D
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Cp*Ir(pyalc)Cl (1) for water-soluble substrates (Table 4). 4
gives high yields for heterocycles such as THF and pyrrolidine,
indicating it is not poisoned by substrate binding. As in the
prior work with the Cp* precursor, we checked whether these
reactions were stereoretentive. Oxidation of cis-decalin to cis-
decalol (entry 7) with retention of configuration indeed
indicates a concerted oxidation pathway as inferred by Zhou
et al.30 Cycloalkanes are also efficiently oxidized to the
corresponding diacid. We believe that the catalyst likely
performs the initial oxidation of the cycloalkane to the
corresponding ketone, and the ketone then goes through a
Baeyer−Villiger oxidation to form a lactone. Subsequent
lactone hydrolysis would be expected to give a hydroxy-
substituted carboxylic acid, which would then undergo further
oxidation to the diacid (entries 8−10), the observed final
product. However, it is unclear whether the catalyst or oxidant
gives the Baeyer−Villiger oxidation to form a lactone; many
peracids, periodic acid included, can perform Baeyer−Villiger O
atom insertion into ketones to form esters.54 However, periodic
acid cannot oxidize the cycloalkane without a catalyst, so the
initial oxidation is metal-driven. The product distribution also
indicates that the majority of the oxidant is being used in metal-
based oxidation of the cycloalkane to the cyclic ketone.
Table 2. Screened Oxidants Using 4 as a Precatalyst for EBS
Conversion to APS
a
oxidant
APS yield (%)
oxidant
PbO2
APS yield (%)
NaIO4
H5IO6
CAN
78
81
91
20
38
11
10
7
0
0
0
0
0
0
H2O2
tBuOOtBu
NaClO
tBuOOH
Oxone
KBrO3
LiO4
I2O5
NaIO3
air (control)
a
Reaction conditions: Oxidant predissolved/mixed in H2O and added
to the substrate, dicarbonyl precatalyst 4, and AcOH and allowed to
1
stir at 25 °C for 60 min. Yields were determined by H NMR against
TMSP-d4 as a standard. Mass balance was >95% for all reactions. More
information can be found in the Experimental Section.
color and paramagnetism, we used NaIO4 and H5IO6 for
subsequent experiments.
Interestingly, the amount of added acetic acid seems to
significantly affect the oxidation product yields (Table 3). In the
Table 3. EBS Oxidation Yields from 4 with Varying mol % of
a
Added AcOH
For organic substrates as a whole, the turnover frequencies
(TOFs) for 4 are generally better than those reported for 1. We
believe that the greater solubility of periodic acid over sodium
periodate in these organic systems may help increase yields.
However, 4 gave significant yields of oxidized CH products
only in a MeCN/H2O solvent system with H5IO6 as an oxidant.
mol % AcOH
APS yield (%)
0
0.5
1
14
21
39
48
47
5
t
Acetone/H2O and BuOH/H2O solvent systems gave much
50
lower yields, and hexafluoroisopropanol (HFIP)/H2O did not
work at all, as HFIP was immediately oxidized by H5IO6 even
without a catalyst. This solvent sensitivity distinguishes 4 from
1, as Zhou et al. reported that 1 was an efficient precatalyst in
all four solvent systems listed here. This may be because Zhou
et al. had been working under the conditions of protocol A, so
the active Ir species in this previous CH oxidation work still has
the Cp* ligated to the metal center to form a resting state of
type 3. The Cp*Ir active species may be more soluble and/or
less sensitive to solvent conditions than the active species for
precatalyst 4, likely an Ir-oxo dimer similar to 2, thus
maintaining many open sites. However, since 4 has higher
TOFs for organic substrates relative to 1, future CH oxidation
work would benefit from using 4 in terms of optimizing yields.
a
Reaction conditions: Oxidant predissolved in H2O and added to the
substrate, precatalyst 4, and AcOH and allowed to stir at 25 °C for 60
min in air. The amount of AcOH was varied between each reaction.
Yields were determined by H NMR against TMSP-d4 as a standard.
More information can be found in the Experimental Section.
1
absence of any acetic acid, very little EBS is converted to APS.
However, this does not seem to be the case because 1 degrades
into nanoparticles, which was shown previously to be
detrimental to oxidative catalysis.43 On the CH oxidation
time scale (30 min to 1 h), the Ir active species is shown to be
homogeneous by dynamic light scattering (DLS) with EBS
present in the reaction mixture, even if there is no acetic acid
present (Figure S6). The necessity for the presence of added
acetic acid for efficient CH oxidation indicates that acetic acid
not only stabilizes the homogeneous species43 but also acts as
an in situ ligand for the active species and thereby promotes CH
oxidation. A slight excess of AcOH is used in CH oxidation
reactions using dicarbonyl 4 as a precatalyst, as AcOH can be
oxidized as part of the reaction; small amounts of formate are
CONCLUSION
■
We show that Ir(CO)2(pyalc) (4) is a competent CH oxidation
precatalyst able to oxidize many substrates under a variety of
conditions, including the stereoretentive oxidation of cis-decalin
to cis-decalol. The precatalyst requires acetic acid to efficiently
oxidize CH bonds. This AcOH acts as both a stabilizing agent
against nanoparticle formation and a ligand that promotes CH
oxidation catalysis. In contrast, for WO, acetic acid acts only as
a stabilizing agent and not a promoter ligand. We also show
that the Cp*Ir(pyalc)Cl (1) precatalyst has two pathways to
oxidize CH bonds. Without preactivation to remove Cp*
before adding external CH substrate, the initially formed active
species is a high-valent mononuclear Ir-oxo species with the
Cp* still ligated to the metal center, suggested by the
mononuclear Cp*IrIV(pyalc) resting state of type 3, which we
have identified. If the Cp* is deliberately removed in a
preactivation step, the active species is now an IrIV-oxo dimer of
type 2, competent for WO and CH oxidation catalysis. A
1
observed in the H NMR product spectra. We do not see a
difference in yields if we use protocols A and B with dicarbonyl
precatalyst 4; that is, there is no difference in CH oxidation
product yields if we mix 4, oxidant, and CH substrate all at once
or if we preactivate 4 with a small amount of oxidant and then
add CH substrate and more oxidant. This is not unexpected, as
previous WO work has shown that 4 loses its CO placeholder
ligands and activates extremely quickly to a species similar to
2.43 Precatalyst 4 likely has no other active oxidation species
like the Cp*Ir precatalysts. For all CH oxidation experiments
with 4, we added all reagents at once (protocol A).
In testing a variety of different substrates for CH oxidation,
we find similar efficiencies between 4 and the analogous
E
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a
Table 4. CH Substrates Oxidized by 4
a
Reaction conditions: Oxidant predissolved in solvent and added to the substrate, precatalyst 4, and AcOH and allowed to stir at 25 °C for 60 min in
air. Yields were determined by 1H NMR against a standard. For entries 1−6, the solvent was H2O, the oxidant NaIO4, and the standard TMSP-d4.
b
For entries 7−10, the solvent was MeCN/H2O (4:1), the oxidant H5IO6, and the standard 1,3,5-trichlorobenzene. TOF is based on the oxidation of
the first CH bond. TOF calculated from Zhou et al.28 Yield was determined by GC/MS according to literature procedure.30 More information can
c
d
be found in the Experimental Section.
species substantially similar to 2 is also formed by the activation
of Ir(CO)2(pyalc). However, because 1 requires significantly
more periodate for activation to give 2, the increased amount of
obligatory coproduct, iodate, acts as an inhibitor. In the absence
of such byproducts though, 2 is an efficient CH oxidation
catalyst. This is confirmed through the more easily activated 4,
where minimal iodate is involved in preactivation. Given that 4
has been shown to more efficiently activate to 2 with fewer
F
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byproducts, and therefore is more efficient for WO43 and CH
oxidation reactions, future experiments will examine the
reactivity of IrIV-oxo dimer and will test Ir(CO)2(pyalc) as
the precursor rather than any Cp*Ir analogues. This is
especially true for electrochemical studies, where byproducts
must be minimized in order to prevent electrode side-reactions
and inhibition.
AUTHOR INFORMATION
Corresponding Author
*E-mail: robert.crabtree@yale.edu.
Notes
The authors declare the following competing financial
interest(s): D.L.H. and R.H.C. are currently submitting a
patent that contains intellectual property described in this
article.
■
EXPERIMENTAL SECTION
■
ACKNOWLEDGMENTS
General Procedures. All organic solvents were dried using a
Grubbs-type purification system. High-purity Milli-Q water was used
for all aqueous experiments. All chemicals were purchased from major
suppliers and used without further purification. Syntheses were
performed using standard Schlenk techniques under a dry atmosphere
of N2. The 2-(2′-pyridyl)-2-propanoate (pyalc) ligand,55
Cp*IrIII(pyalc)Cl,56 and IrI(CO)2(pyalc)43 were prepared according
■
This research was supported by the Center for Catalytic
Hydrocarbon Functionalization, No. DE-SC0001298 (D.L.H.
and R.H.C.), an Energy Frontier Research Center funded by
the U.S. Department of Energy. D.L.H. acknowledges support
by the National Science Foundation Graduate Research
Fellowship Program under Grant No. DGE-1122492 and the
Yale Dox Summer Research Fellowship. DLS measurements
were conducted in the Yale Facility for Light Scattering. We
thank Prof. Gary Brudvig for productive discussions about EPR
analysis.
1
to previous literature procedures. H NMR spectra were collected at
room temperature on a 400 or 600 MHz Varian spectrometer and
referenced to the residual solvent signal or an added standard (δ in
ppm, J in Hz). Dynamic light scattering experiments were conducted
based on a literature procedure.43
For CH oxidation reactions: 0.2 mmol of CH substrate was
dissolved with 2 μmol (1 mol %) of precatalyst and additive (only used
for precatalyst 4) in 2 mL of solvent. The oxidant (2 mmol) was
predissolved or mixed in 8 mL of solvent in a separate vial. The
oxidant was added to the substrate and catalyst solution to initiate the
reaction and allowed to stir at 25 °C for 60 min in air. With the
exception of cis-decalin, the reaction was stopped by taking a 1 mL
aliquot and removing the solvent in vacuo. The resulting solids were
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and an Oxford ESR-900 helium-flow cryostat. Samples were sparged
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ASSOCIATED CONTENT
■
S
* Supporting Information
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electrochemical and DLS data (PDF)
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DOI: 10.1021/acs.organomet.6b00525
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