Iridium(iii) catalyzed trifluoroacetoxylation of aromatic hydrocarbons

Article abstract of DOI:10.1039/c4ra06251e

A tridentate, NNC-tb (where NNC-tb = 2-(pyridin-2-yl)benzo[h]quinoline) ligated Ir^III complex (NNC-tb)Ir(Ph)(4-MePy)(TFA), 11 along with analogues are very active for CH activation as evidenced by rapid catalytic H/D exchange between benzene and trifluoroacetic acid-d₁ (DTFA). The complexes were examined with a variety of oxidants for the catalytic conversion of benzene to phenyltrifluoroacetate. Herein, the synthesis and characterization of (NNC-tb)Ir complexes is described along with the reactivity of these complexes towards arenes and alkanes.

Full text of DOI:10.1039/c4ra06251e

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Iridium(III) Catalyzed Trifluoroacetoxylation of
Aromatic Hydrocarbons
Steven M. Bischof,¥ Brian G. Hashiguchi,¥,* Kapil S. Lokare,¥ Niles Gunsalus,¥
Mohammed Yousufuddin,┼ Roy A. Periana,¥*
¥The Scripps Energy & Materials Center, Chemistry Department, The Scripps Research Institute,
Jupiter, FL 33458.
┼Center for Nanostructured Materials, Chemistry Department, University of Texas at Arlington,
Arlington, TX 76019.
Email: bhashigu@scripps.edu
RECEIVED DATE (to be automatically inserted after your manuscript is accepted)
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Abstract
A tridentate, NNCꢀtb (where NNCꢀtb = 2ꢀ(pyridinꢀ2ꢀyl)benzo[h]quinoline) ligated IrIII
complex (NNCꢀtb)Ir(Ph)(4ꢀMePy)(TFA), 11 along with analogues are very active for CH
activation as evidenced by rapid catalytic H/D exchange between benzene and trifluoroacetic
acid – d1 (DTFA). The complexes were examined with a variety of oxidants for the catalytic
conversion of benzene to phenyltrifluoroacetate. Herein, the synthesis and characterization of
(NNCꢀtb)Ir complexes is described along with the reactivity of these complexes towards arenes
and alkanes.
Keywords:
Catalysis, Iridium, Benzene Oxidation, CꢀH Activation, CꢀH Functionalization
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Introduction
Catalytic CH activation followed by oxyꢀfunctionalization of hydrocarbons to generate
alcohols using strong electrophiles has proven a viable strategy for hydrocarbon oxidation
(Scheme 1).26ꢀ32 Systems based on soft, oxidizing cations such as HgII,33ꢀ35 PtII,36ꢀ47 PdII,19, 48ꢀ51
and AuI/AuIII
generate oxyꢀfunctionalized products from methane at product concentrations
52, 53
>1 M. However, the most active systems utilize concentrated, oxidizing acids and suffer from
catalyst poisoning due to the generation of product and H2O that bind to the strongly
electrophilic center.54 This inhibition has rendered these systems impractical and provided the
motivation for the development of new systems that do not suffer from such inhibition.55
Scheme 1. General catalytic cycle for CH activation/MꢀR functionalization of hydrocarbons.
The use of less electrophilic, less oxidizing cations such as those based on IrIII or RhIII has
been under exploration, as these species do not show sensitivities to water or product, and
reactions can be carried out in nonꢀoxidizing, relatively inert solvents such as CF3CO2H
(HTFA).56ꢀ59 Indeed, several reports have now shown that these weakly electrophilic cations can
undergo CH activation with hydrocarbons in carboxylic acid solvents to generate MꢀR
intermediates.60ꢀ62 However, efficient catalysts have not been developed with these systems
likely due to high energy barriers for oxyꢀfunctionalization of the MꢀR intermediate that is
required for a complete catalytic cycle, Scheme 1.63ꢀ66
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Recently, several groups have published examples of homogeneous catalyzed oxidations of
aromatics with transition metal catalysts proposed to proceed via a CH activation mechanism
(Eq. 1). For example, Sanford reported a Pd acetate complex that catalyzes H/D exchange of
benzene and leads to the phenylꢀester when the system is treated with PhI(OAc)2 as the
oxidant.67 Yu and coworkers reported several examples of the direct functionalization of
aromatics with O2 also using Pd catalysts.68ꢀ70 An important thrust of our work has been the
design of new homogeneous catalysts for the CH activation of alkanes and arenes. We recently
t
reported IrIII complexes bearing the BuꢀNNC ligand (tBuꢀNNC = 6ꢀphenylꢀ4,4’ꢀdiꢀtertꢀbutylꢀ
2,2’ꢀbipyridine) capable of undergoing CH activation with strong CH bonds such as methane and
benzene.71ꢀ74 It was shown that (tBuꢀNNC)Ir(Et)(C2H4)(TFA) , 1, in the presence of an oxidant,
such as NaIO4, gave ~6 turnovers (TON) of methyltrifluoroacetate after 3 h at 180 °C (Scheme
2)..However, after 3 h, further H/D exchange or oxidation products were not observed. This is
likely due to deactivation of the catalyst via pooling at higher oxidation states (e.g. Ir(IV or V)
that are likely not capable of undergoing CH activation.37
(1)
Scheme 2. Oxidation of methane to MeTFA by 1 in HTFA.
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Another key issue with the (tBuꢀNNC)Ir system was the apparent lack of thermal stability of
the (tBuꢀNNC)Ir systems above 100 °C that likely limited the efficiency of the system. Studies
suggested that cleavage of the cyclometalated Irꢀcarbon bond, leading to loss/oxidation of the
ligand or metal center, may have contributed to the complexes’ low yield relative to the observed
rates of H/D exchange and the observed loss of activity in extended run times. To address this,
we investigated the use of new ligand designs to improve the stability of the catalytic system to
the oxidizing and acidic reaction conditions. We also explored coupling the CH activation
reaction to a functionalization reaction with potentially practical oxidants to generate phenol
from benzene.
Results and Discussion
In an effort to design a more robust catalytic system, similar to the originally reported NNC
system, we explored related iridium complexes with a geometrically restricted or “tied back”
NNC motif. In the original report of the Ir complexes and subsequent work, it was discovered
that the cyclometalled ring underwent decyclometallation as observed by incorporation of
deuterium selectively in the ortho position of the phenyl ring.74 We believe this loss of
cyclometalation may be a root cause of the lack of thermal stability observed in these complexes.
We postulated that fusing the phenyl ring to one of the pyridines should prevent the
decyclometalation of the phenyl ring (Figure 1) as it would require loss of the chelating ligand.
This led to the synthesis and study of an (NNCꢀtb) (NNCꢀtb = 2ꢀ(pyridinꢀ2ꢀ
yl)benzo[h]quinoline) ligated Ir system. The expectation was that a fused benzoquinoline motif
should restrict the degrees of freedom of the cyclometalated phenyl group and lead to higher
barriers for decyclometallation and higher thermal stability.
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Figure 1. Parent NNC ligand system (tBu-NNC) compared to the “tied back” motif (NNC-tb).
The (NNCꢀtb)Ir motif is readily accessible by following a similar synthetic methodology to
those previously reported (Scheme 3).71 Warming a methylene chloride solution of [Ir(COE)2Cl]2
with 1 eq of NNCꢀtb ligand75 from ꢀ50 °C under an ethylene atmosphere, followed by stirring
overnight at RT provided the six coordinate, IrIII complex, (NNCꢀtb)Ir(Et)(C2H4)(Cl) (3) as an
orange solid in 60% yield over two steps. The freely rotating ethylene ligand is observed as a
singlet at δ = 4.1 ppm in the 1HꢀNMR. Replacement of the chloride is achieved by treatment with
AgTFA and stirring at RT for 48 h to give (NNCꢀtb)Ir(Et)(C2H4)(TFA) (2). Finally, the ethyl
group can be removed as ethane by stirring for 24 h at RT in neat HTFA to give (NNCꢀ
tb)Ir(C2H4)(TFA)2 (4). All compounds were characterized by multinuclear NMR (1H, 13C, and
19F), HRꢀMS, elemental analysis, and Xꢀray crystallography (Figure 2, for 3, Figure 3, for 2, and
Figure 4 for 4, respectively). Based on previous work with 1 in our group, heating 2 or 4 in hot
HTFA should lead to loss of ethane and ethylene and generate a mixture of 5 and 6 under
catalytic conditions (Scheme 3).
Scheme 3. Synthesis of (NNCꢀtb)Ir(R)(L)(X) compounds 3-6.
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Figure 2. ORTEP structure of (NNCꢀtb)Ir(Et)(C2H4)(Cl) (3) with ellipsoids at 50%.
Figure 3. ORTEP structure of (NNCꢀtb)Ir(Et)(C2H4)(TFA) (2) with ellipsoids at 50%.
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Figure 4. ORTEP structure of (NNCꢀtb)Ir(C2H4)(TFA)2 (4) with ellipsoids at 50%.
Analysis of the crystal structures for complexes 2-4 revealed increasing electrophilic
character leading to a tightly bound ethylene as the electronꢀwithdrawing TFA groups are added.
Substitution of the Cl for TFA leads to an increase in the electrophilicity at the metal center as
evidenced by the decreased C=C bond length (Table 1). Back donation from filled Ir πꢀorbitals
would be expected to increase the C=C bond length. The observed decrease in C=C bond length
in moving from complex 3 to the more electron deficient complex, 2, is consistent with less
backꢀbonding and more characteristic of a bound ethylene rather than a metalocyclopropane. In
all cases, the bound ethylene is in plane with the NNC ligand in the crystal structures. The C=C
bond length of coordinated ethylene in complex 3 is 1.386(16) Å, slightly longer than the C=C
distance of 1.335 Å of free ethylene76 and shorter than a typical CꢀC single bond (1.54 Å). The
bond distance is more closely resemblant of a delocalized CꢀC bond, such as that of benzene
(1.394 Å). The C=C bond distances in complexes 2 and 4 are 1.377(13) and 1.341(14) Å,
respectively. Successive substitution of the TFA ligand on the iridium center results in a decrease
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of the C=C bond length across a range of ~0.045 Å. The structural data is consistent with an
appreciable electrophilic iridium center and a coordinated ethylene, which is consistent with
solution NMR data for complexes 2-4. The IrꢀC distances are not particularly sensitive to
changes in ligands on Ir and vary from 2.163(9) to 2.188(8) Å (Table 1).
Table 1. Comparison of the C=C and IrꢀC bond lengths (Å).
Entry Complex
C=C
Ir-C
1
2
3
4
Ethylene
1.335
–
(NNCꢀtb)Ir(Et)(C2H4)(Cl) (3)
1.386(16)
2.179(10), 2.180(10)
2.163(9), 2.172(8)
2.182(8), 2.188(8)
(NNCꢀtb)Ir(Et)(C2H4)(TFA) (2) 1.377(13)
(NNCꢀtb)Ir(C2H4)(TFA)2 (4) 1.341(14)
Complex 2 was then examined for the CH activation reaction with a variety of hydrocarbons.
H/D exchange reactions between hydrocarbons and deuterated DTFA revealed the system is
active for aromatic sp2 CꢀH bonds (Figure 5). Heating a 0.5 M solution of benzene in DTFA
containing 5 mM of 2 at 150 °C for 3 h followed by GCꢀMS analysis showed 25% of the CH
bonds had been converted to CD bonds (Table 2). With these positive results, reactions with
benzylic and aliphatic CH bonds were examined (Figure 5). For example, heating a stirred
solution of DTFA containing 5 mM 2 for 2 h at 180 °C under 500 psi of CH4 led to H/D
exchange between methane and the DTFA solvent. Analysis of the gas phase of the reaction
mixture indicated that ~5% of the CH4 was converted to 5.1% CH3D, 0.1%, CH2D2, 0.0% CHD3,
and 0.0% CD4. In comparison, 1 was shown to give ~30% conversion of CH4 to 21.2% CH3D,
6.6% CH2D2, 1.3% CHD3, and 0.6% CD4, under comparable conditions. It is important to note
that the reaction mixtures remained completely homogeneous post reaction, in all cases,
suggesting that catalyst decomposition is unlikely. Additionally, control reactions show that no
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background H/D exchange is observed between solvent and methane in the absence of either
catalyst. This would suggest that the H/D exchange is the result of reversible CH activation.
Thus, the generation of (NNCꢀr)Ir(CH3)X2 intermediates is plausible, where X is TFA or HTFA.
Table 2. H/D exchange of benzene and DTFA at 150 °C for 3 h catalyzed by 2.
TOF
Run
C6H6 C6H5D C6H4D2 C6H3D3 C6H2D4 C6H1D5 C6D6
TON
(x10-2 s-1)
Background
No Catalyst
95.2
40.5
43.2
4.6
0.1
6.4
5.9
0.0
6.7
6.0
0.0
9.1
8.1
0.0
0.0
―
7
1
2
11.0
10.5
13.7
13.1
12.6
13.2
2.80
2.72
302
293
Figure 5. H/D exchange results using 2 in DTFA.
In addition to methane, complex 2 also catalyzes H/D exchange with other alkanes and
arenes such as nꢀoctane, cyclohexane, mesitylene, and nꢀpentane in trifluoroacetic acid based on
NMR and GCꢀMS analysis. With nꢀoctane and nꢀpentane, exchange is observed at all positions
with a small preference for the primary position. This is not surprising, as activation of stronger,
primary CH bonds is consistent with other reports on CH activation systems.77ꢀ79 This selectivity
can be attributed to the stronger MꢀC bonds formed from scission of primary, sp3 CH bonds as a
main driving force for the observed regioselectivity. Secondary CH bonds lead to a weaker MꢀC
bond, which disfavors formation of the internal MꢀC intermediate. Furthermore, these results are
also persuasive evidence against an exchange mechanism involving radicals. The lack of
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observed H/D exchange on the sp2 CH bond of mesitylene is the result of an extraordinarily fast
background reaction between DTFA solvent and mesitylene (likely the result of electrophilic
aromatic substitution reactions with the methyl groups stabilizing the carbocation intermediates).
This extraordinarily fast background reaction makes further H/D exchange at this position
difficult to detect. In comparison to 1, the observed rates of H/D exchange catalyzed by 2 are
typically equivalent within error for benzylic CH bonds and slower for aliphatic hydrocarbons. A
possible reason for this disparity will be dicussed later in this manuscript.
Recently, we utilized a method that allows for distinguishing between rate limiting steps of
CH bond coordination or cleavage.80 If the system was following typical SchultzꢀFlory behavior,
C6D1H5 should build up followed by gradual increase in C6D2H4. Given that higher isotopologues
of benzene are observed, it would suggest that multiple CH activation events are occuring. H/D
exchange reactions between C6H6 and DTFA were monitored by sampling the liquid phase by
GCꢀMS analysis after neutralization of the acid and extraction with diethyl ether. Heating 5 mM
solutions of 2 in a 0.5 M solution of benzene in DTFA and sampling the reaction over 3 h led to
parallel incorporation of deuterium (Figure 6). The parallel incorporation of deuterium is
suggestive of CH bond coordination as the rate determining step for the CH activation reaction
of benzene and can be modeled by the hypothetical energy diagram shown in Figure 7.
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Figure 6. Benzene H/D exchange catalyzed by 2 monitored over 3 h at 100 °C.
Figure 7. Representative energy diagram for CH activation that leads to parallel incorporation.
R
H
LnM
X
R
CH Coordination
LnM
H
X
CH Cleavage
M-R + H-X
[M(RH)]+[X]-
M-X + R-H
In moving to the “tied back”, NNCꢀtb ligand, we expected the system to become more stable
to the reaction conditions. We found it perplexing, therefore, that our new complex, 2, exhibited
less activity for H/D exchange with aliphatic hydrocarbons than did the parent complex, 1. This
led us to question the lability of the bound ethylene in 2. However, to avoid any complications
by the inclusion of the ethyl ligand, we choose to use complex 4 instead of 2 to examine the
lability of only ethylene. Previously, it was reported that NNC ligated Ir complexes underwent
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facile loss of ethylene under similar conditions.74, 81, 82 The loss of ethylene from 4 (Eq. 2) was
monitored by preparing a 15 mM solution in HTFA in a sealed JꢀYoung NMR tube under argon.
The NMR tube contained a coaxial insert containing 1,3,5ꢀtrimethoxybenzene in DMSOꢀD6. The
NMR tube was heated at 100 °C and monitored over 5 days by 1H NMR (See SI). Monitoring of
the ethylene signal at 4.9 ppm revealed that loss of the ethylene from 4 was in fact much slower
than in previously developed (tBuꢀNNC)Ir complexes. A T1/2 = ~1000 min was observed for the
loss of the Irꢀethylene signal.
(2)
In an effort to improve catalytic CH activation in this system, we designed Ir complexes that
were based on the NNCꢀtb ligand, but did not have a bound ethylene. Previous work in our group
has shown that heating NNC ligated Ir complexes (7) in neat hydrocarbon such as benzene led to
a bridging Ir dinuclear species (8).83 Treatment of the chloride dinuclear species, 8, with a
coordinating amine base such as pyridine gave rise to the formation of a mononuclear complex
(tBuꢀNNC)Ir(Ph)(Py)(Cl) (9, Scheme 4). Following a similar methodology, 3 was heated in neat
benzene at 160 °C for 6 h in a sealed glass vessel under vacuum. Removal of the solvent and
treatment of the residue with neat 4ꢀmethylpyridine (4ꢀMePy) for 2 days, followed by flash
column chromatography gave (NNCꢀtb)Ir(Ph)(4ꢀMePy)(Cl) (10) in 37% yield over two steps as
a brown powder. The use of unsubtitued pyridine led to a mixture of products that proved
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1
difficult to resolve by chromatography or crystalization. Specifically, H NMR interpretations
were challenging due to the nature of 21 aryl protons with 17 of them being chemically distinct
and overlapping. We found that addition of the methyl group on the pyridine in the 4ꢀposition
provided a means of identifying pyridine ligated Ir complexes where the methyl group is used as
a reference point. Furthermore, the increased solubility of the 4ꢀmethylpyridine complex in
organic solvents makes further manipulations simpler. Following isolation of 10, stirring in
DCM with 4 eq of AgTFA gave (NNCꢀtb)Ir(Ph)(4ꢀMePy)(TFA) (11) in quantitative yield
(Scheme 5).
Scheme 4. Synthesis of (tBuꢀNNC)Ir(Ph)(Py)(Cl) (9).83
Scheme 5. Synthesis of (NNCꢀtb)Ir(Ph)(4ꢀMePy)(TFA) (11).
We then set out to determine if 11 was an effective catalyst for CH activation of aromatic
substrates. Our group’s previous work suggested that the IrꢀPh intermediates of the parent Ir(tBuꢀ
NNC) complex 1, 8 or 9, show similar catalytic rates to the parent compounds within
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experimental error. It was our hope, in moving to the NNCꢀtb ligated systems, that 11 would be
much more active than 2, due to the slower loss of ethylene in the NNCꢀtb ligated iridium
t
systems than those previously reported for BuꢀNNC ligated iridium systems. Therefore, we
heated solutions of 11 under identical conditions to those reported for 2 (150 °C, 3 h) and found
that the IrꢀPh complex led to significantly increased rates of H/D exchange between C6H6 and
DTFA. The rate of H/D exchange was so fast that reactions did not remain pseudoꢀfirstꢀorder
with respect to benzene, preventing extraction of suitable rate data. The extraorinarly high rates
of H/D exchange using 11 prompted us to compare these new complexes to some of our
previously reported active CH activation catalysts.71, 74 Our goal in development of the “tiedꢀ
back” NNC was to improve stability (and hopefully enhance catalytic rates) of the parent NNC
complex. Pseudoꢀfirstꢀorder conditions for H/D exchange between benzene and DTFA catalyzed
by 11 were realized by running the reaction at 75 °C for 1 h. In order to ensure that the phenyl
substituent on the catalyst did not significantly contribute to the observed H/D exchange, the
concentration of benzene in solution was increased to 5 M and the catalyst concentration
remained at 5 mM. Control studies run with 11 in DTFA indicate that deuteration of the phenyl
substituent occurs rapidly, even at room temperature, and that deuterium is incorporated into the
resultant benzene (see SI). Catalysts 1, 2, and 11 were screened under identical conditions in
order to make a direct comparison of their activity. In all three cases, the reaction mixtures
remained homogeneous post reaction, suggesting that catalyst decomposition is not occurring.
The results of this study indicate that 11 is approximately an order of magnitude faster than 1 and
2 (Table 3). Interestingly, there are no observed differences in rate between the “tiedꢀback” and
parent NNC complexes within experimental error. This would suggest a common mechanism for
CH activation between 1 and 2. The higher observed rate for 11 correlates well with a more
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tightly bound ethylene in 2 than had been expected and it is likely that our increase in catalytic
activity, when using 11, results from the generation of a highly active Ir cation possessing open
coordination sites that are not readily accessible when ethylene is present in the complex,
especially at lower temperatures. Thus, slow loss of ethylene from 2 results in moderate catalyst
inhibition at the start of reactions.
Table 3. Comparison of various IrꢀNNC catalyst CH activation systems.
Catalyst
TON
7.5 ± 0.7
8 ± 2
TOF (x 10-3)
2.0 ± 0.1
2.5 ± 0.7
25 ± 4
1
2
90 ± 16
11
Conditions: 5 mM catlayst, 5 M C6H6 in DTFA, at 75 °C under Argon and analyzed by GCꢀMS
after 1 h.
Given that both 2 and 11 were competent for CH activation of benzene, we examined them for
catalytic the oxyꢀfunctionalization of benzene. Catalytic functionalization was attempted using
either 2 or 11 with an assortment of one and two – electron oxidants. We had particularly been
interested in the use of air – regenerable oxidants, such as CuII, with the idea of using Wacker –
like conditions for hydrocarbon oxidation. Unfortunately, 2 did not exhibit increased activity for
benzene functionalization when compared to 1 and a maximum TON of 6.9 was achieved (see
SI). Interestingly, 11 did not show any activity for benzene functionalization although it was
highly active for CH activation. Attempts to stoichiometrically functionalize the phenyl
substituent on 11 resulted in the formation of benzene resulting from protonolysis of the phenyl
substituent. Catalytic runs were also unsuccessful. Although no TON was observed with 11 for
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the oxyꢀfunctionalization of benzene, it is possible that the 4ꢀMePy substituent is interfering with
the functionalization. Further study is needed to elucidate the details of this system.
Conclusion
In our efforts to design catalysts for hydrocarbon oxidation in weakly acidic media several
new Ir complexes based on the use of a cyclometalated NNC ligand have been synthesized and
examined for reactions with hydrocarbons. Contrary to our expectations, ligating Ir with a “tied
back” version of the NNC ligand (see complex 2) relative to the original “nonꢀtied back”complex
did not result in increased rates of H/D exchange. Our studies have indicated that the lower rate
of exchange with aliphatic substrates, such as methane, may be due to a more tightly bound
ethylene in the precursor to the active catalyst and not from catalyst decomposition. The slow
loss of ethylene from 2 may inhibit entrance into the catalytic cycle from the iridium preꢀcatalyst
as two cisꢀcoordination sites are required for CH activation. Replacement of the ethylene ligand
with 4ꢀMePy (see complex 11) resulted in substantially increased rates of H/D exchange with
benzene. Importantly, the use of oxidants, such as CuII, gave oxyꢀfunctionalized aryl products of
benzene as either phenol or phenyltrifluoroester using complex 2. However, the yields are low
and there is a background reaction facilitated by Cu in HTFA under the reactions conditions.
Unfortunately, complex 11 gave only trace amounts of oxyꢀfunctionalized products and further
investigation is needed to elucidate the reason for this. Further investigations of this reaction
with iridium and other metals are ongoing in our lab.
Experimental Section
General Considerations: All manipulations were carried out using an argon filled MBraun
glovebox and standard Schlenk techniques using oven dried glassware (>1 h at 110 °C under
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vacuum, ꢀ30 mm Hg). Methylenechloride was HPLC grade from Fisher and distilled from P2O5
under argon prior to use. Benzene was HPLC grade from EMD and distilled from
Na/benzophenone under argon prior to use. Pentane was HPLC grade from EMD and distilled
over Na/benzophenone under argon prior to use. Reagentꢀgrade chemicals and solvents were
purchased from Sigma Aldrich or Alfa Aesar and used as is unless otherwise specified.
Deuterated solvents were purchased from Cambridge Isotope Inc. and degassed and stored under
argon before use. Deuterated methylenechloride was further stored over 4Å molecular sieves.
Deuterated trifluoroacetic acid was degassed before use by several freezeꢀpumpꢀthaw cycles and
stored under argon. Trifluoroacetic acid was HPLC grade from Fisher and degassed before use
by several freezeꢀpumpꢀthaw cycles and stored under argon. IrCl3•(H2O)x was obtained from
Pressure Chemical. Elemental Analyses were performed by Columbia Analytical Services;
Tucson, Arizona or Atlantic Microlab, Inc. of Norcross, Georgia. Electrospray Ionization (ESI)
mass spectroscopy was performed at the University of Illinois Mass Spec Facility; Urbana,
Illinois. Liquid phase organic products were analyzed with a Shimadzu GCꢀMS QP2010S
equipped with crossꢀlinked methyl silicone gum capillary column, RTXꢀ5. Gas measurements
were performed using an Agilent GasPro column. The retention times of the products were
confirmed by known standards. NMR spectra were obtained on a Bruker Digital Avance III 400
1
19
(400.132 MHz for H, 100.623 MHz for 13C, and 376.461 MHz for F) spectrometer. Chemical
shifts are given in ppm relative to residual solvent proton resonances or to a stated internal or
external standard. Liquid phase flash chromatography was performed on a Teledyne Isco
Combiflash Rf and all solvents were of HPLC grade and obtained from Fisher or EMD. Xꢀray
crystallography was performed at the Center for Nanostructured Materials located at the
University of Texas at Arlington.
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benzo[h]quinoline n-oxide: A solution of benzo[h]quinoline (1.8 g, 10.0 mmol) dissolved in
40 mL of CHCl3 was cooled in an ice bath. To this cooled solution, MCPBA (3.0 g, 15.0 mmol)
in 60 mL of CHCl3 was added and the reaction stirred for 4 d at RT. The solution was washed
with K2CO3, dried with MgSO4, and concentrated in vacuo. The residue was washed with diethyl
ether to give a crystalline solid in 65ꢀ70% yield. Characterization of the compound corresponded
to that previously reported.87
2-chlorobenzo[h]quinoline: A mixture of benzo[h]quinoline n-oxide (13.8 g, 71 mmol) in
60 mL of POCl3 was refluxed for 30 min. The cooled reaction mixture was poured into crushed
ice, neutralized with concentrated NH4OH solution, and extracted with CH2Cl2 (3 x 50 mL). The
combined extracts were washed with water, dried over MgSO4, and evaporated under reduced
pressure. The residue was flashꢀchromatographed on silica with a gradient from hexanes to
CH2Cl2 as the eluent to give 2-chlorobenzo[h]quinoline (44ꢀ48%) as the first major fraction.
Characterization of the compound corresponded to that previously reported.87
2-bromobenzo[h]quinoline: The reaction was performed in a well dried round bottom flask
equipped with a reflux condenser under an argon atmosphere. To a solution of 2-
chlorobenzo[h]quinoline in acetic acid (60 ml) kept under reflux, ~10mL of 48% HBr was
added slowly. The reaction was then refluxed for 6 h. The excess of solvent was removed in
vacuo and the resulting product washed with a NaHCO3 saturated aqueous solution, extracted
into CH2Cl2, and dried over MgSO4. The residue after removing the solvent was flashꢀ
chromatographed on silica with a gradient from hexanes to CH2Cl2 as the eluent to yield 55ꢀ68%
of 2-bromobenzo[h]quinoline. Characterization of the compound corresponded to that
previously reported.88
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2-(2’-pyridyl)-benzo[h]quinoline: A mixture of 2-bromobenzo[h]quinolone (1.9g 7.36
mmol), 2ꢀ(triꢀnbutylstannyl) pyridine (1.2eq, 3.76g, 8.83 mmol), and
tetrakis(triphenylphosphine)palladium(0) (0.1eq, 850 mg, 0.74 mmol) was loaded into an oven
dry Schlenk flask with a reflux condenser.89 The solids were dissolved in dry toluene (60 mL)
and refluxed under argon for 2 d. After cooling to room temperature, water was added and the
mixture was concentrated under reduced pressure. Flash chromatography of the residue on silica
gel, eluting with hexanes, followed by EtOAc/hexanes (1:3) gave the product (NNC-tb) as a
white/yellow solid (1.7 g, 90%). Characterization of the compound corresponded to that
previously reported.90
(NNC-tb)Ir(Et)(C2H4)(Cl) (3): A Schlenk bomb was loaded with 3 g (3.35 mmol) of
[Ir(COE)2Cl]2 and then dissolved in dry/degassed pentane in the glovebox. The flask was cooled
to ꢀ50 °C under argon and ethylene was bubbled through for 30 min followed by slow warming
to RT. The reaction was then cooled to ꢀ50 °C and transferred by cannula to a precooled (ꢀ50 °C)
Schlenk frit and washed with cold pentane (3 x 50 mL) to remove COE while under continuous
ethylene pressure. The receiving flask was then changed and the system washed at RT with dry
CH2Cl2 (3 x 30 mL) all while under ethylene pressure. To the collected filtrate solution at ꢀ50 °C,
1.72 g (2.23 mmol) of NNC-tb dissolved in 45 mL of dry CH2Cl2 was cannula transferred in.
The flask was then warmed to RT and stirred for 16 h to yield a red/orange solution with a
precipitate, which was evacuated to dryness on a high vac line. The reaction was then filtered
over a 1 x 3” plug of silica gel first washing with ether, then EtOAc, followed by EtOAc with
10% methanol to elute the product as an orange band. There is a red/brown band that elutes with
EtOAc as well as ligand/COE with diethyl ether that may be discarded. The orange residue was
dissolved in DCM and crashed out in stirring pentane followed by filtration on a sintered glass
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frit to give a 65% yield of (NNC-tb)Ir(Et)(C2H4)(Cl). 1H NMR (CD2Cl2, 400 MHz): δ 9.35 (d, J
= 5.1 Hz, 1H), 8.24 (d, J = 7.8 Hz, 1H), 8.19 (d, J = 8.3 Hz, 1H), 8.12 (d, J = 8.3 Hz, 1H), 8.01
(t, J = 7.8 Hz, 1H), 7.91 (d, J = 8.9 Hz, 1H), 7.85 (d, J = 6.8 Hz, 1H), 7.62 (m, 4H), 4.1 (s, 4H,
C2H4), 0.41 (m, 1H, IrꢀCH2ꢀCH3), 0.25 (m, 1H, IrꢀCH2ꢀCH3), ꢀ0.44 (t, J = 7.6 Hz, 3H, IrꢀCH2ꢀ
13
CH3). C NMR {1H} (CD2Cl2, 100 MHz): δ 163.9, 163.0, 162.3, 158.4, 153.2, 151.2, 144.8,
144.4, 135.0, 131.7, 124.9, 124.6, 122.7, 119.6, 116.2, 115.1, 65.9. HRꢀMS QꢀTof calc. for
C20H17N2Ir [M+H–Cl–C2H4]+: 478.1021; found: 478.1020. Anal. Calc. for C28H36ClIrN2: C
53.53; H 5.78; N 4.46, Cl 5.64; Found: C 53.44, H 5.67, N 4.39, Cl 5.59. Xꢀray: crystals were
obtained by crystallization from a CH2Cl2 solution at ꢀ20 °C.
(NNC-tb)Ir(Et)(C2H4)(TFA) (2): In a 30 mL amberized vial with a magnetic stir bar, 250
mg of (NNC-tb)Ir-(Et)(C2H4)(Cl) and 102 mg (1.1 eq) of AgTFA was dissolved in 25 mL of
dry DCM. The reaction was stirred in the dark for 2 d and then filtered over celite and evacuated
to dryness to afford an orangeꢀred powder. The compound was recrystallized from DCM/Pentane
1
at ꢀ20 °C to yield 76% of (NNC-tb)Ir(Et)(C2H4)(TFA). H NMR (CD2Cl2, 400 MHz): δ 9.42
(d, J = 5.6 Hz, 1H), 8.32 (d, J = 8.3 Hz, 1H), 8.26 (d, J = 7.8 Hz, 1H), 8.12 (m, 2H), 7.96 (d, J =
8.5 Hz, 1H), 7.87 (d, J = 7.6 Hz, 1H), 7.67 (m, 4H), 4.1 (m, J = 19 Hz, 4H, C2H4), 0.51 (m, 1H,
IrꢀCH2ꢀCH3), 0.36 (m, 1H, IrꢀCH2ꢀCH3), ꢀ0.48 (t, J = 7.6 Hz, 3H, IrꢀCH2ꢀCH3). 13C NMR {1H}
(CD2Cl2, 100 MHz): δ 160.00, 153.32, 152.88, 143.39, 140.38, 139.76, 137.08, 135.47, 132.18,
131.80, 131.68, 127.82, 126.92, 123.55, 123.50, 121.35, 117.28, 111.84, 66.68 (s, IrꢀC2H4),
14.83 (s, IrꢀCH2CH3), and ꢀ14.17 (s, IrꢀCH2CH3). The carbons from the –OC(O)CF3 group were
19
not observed. F NMR {1H} (CD2Cl2, ref. to CFCl3, 376 MHz): δ ꢀ75.77. HRꢀMS QꢀTof calc.
for C22H16N2O2F3Ir [M+H+–Et]+: 590.0793; found: 590.0786. Elemental Analysis Calc. for
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C30H36F3IrN2O2: C 51.05; H 5.14; F 8.07; N 3.97; Found C 50.75; H 4.98; F 8.14; N 3.96. Xꢀray:
crystals were obtained by crystallization from a CH2Cl2/ether solution at ꢀ20 °C.
(NNC-tb)Ir(C2H4)(TFA)2 (4): In a 100 mL RBF, 232 mg of (NNC-tb)Ir-(Et)(C2H4)(TFA)
was dissolved in 25 mL of HTFA. The reaction was stirred for 1 d and evacuated to dryness to
give a yellow powder. The compound was purified by flash column chromatography on silica gel
1
using a 1:1:1 mixture of hexanes:DCM:EtOAc to give 77% of (NNC-tb)Ir(C2H4)(TFA)2. H
NMR (CD2Cl2, 400 MHz): δ 9.53 (d, J = 5.3 Hz, 1H), 8.42 (d, J = 8.3 Hz, 1H), 8.37 (d, J = 7.8
Hz, 1H), 8.27 (d, J = 8.4 Hz, 1H), 8.20 (dt, J = 7.9, 1.5 Hz, 1H), 8.01 (d, J = 8.9 Hz, 1H), 7.91
13
(dd, J = 6.1, 2.3 Hz, 1H), 7.76 (m, 4H), 5.02 (s, 4H, C2H4). C NMR (CD2Cl2, 100 MHz): δ
163.8 (q, J = 36.1 Hz), 160.8, 154.8, 154.1, 153.9, 143.9, 141.0, 138.9, 135.4, 133.4, 131.9,
131.3, 128.9, 128.1, 127.2, 123.8 (t, J = 6.3 Hz), 117.7, 112.0 (q, J = 290.3 Hz), 73.1. 19F NMR
{1H} (CD2Cl2, ref. to CFCl3, 376 MHz): δ ꢀ75.60. HRꢀMS QꢀTof calc. for C24H16N2O4F6Ir
[M+H]+: 703.0644; found: 703.0638. Anal. Calc. for C30H31F6IrN2O4·H2O: C 44.61; H 4.12; N
3.47, found C 44.73, H 4.06, N 3.46. Xꢀray: crystals were obtained by crystallization from a
CH2Cl2/nꢀhexane (1:1) solution at ꢀ20 °C.
(NNC-tb)Ir(Ph)(4-methylpyridine)(Cl) (10): In a thickꢀwalled Schlenk bomb containing a
magnetic stir bar and a resealable PTFE valve, 300 mg (0.55 mmol) of (NNC-tb)Ir-
(Et)(C2H4)(Cl) was dissolved in 100 mL of dry benzene under argon. The red solution was then
frozen in liq. N2 and the head space was evacuated. The flask was thawed and then heated at 160
oC for 6 h in a well stirred oil bath. During the course of the reaction and upon cooling, a dark
brown solid precipitated from solution. The solvent was then removed under vacuum to give a
dark colored residue. In a 250 mL RBF, the residue was dissolved in 30 mL of 4ꢀmethylpyridine.
The reaction was stirred for 2 d under air at RT. The solvent was then removed by rotary
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evaporation to give a dark brown residue. Flash chromatography on silica eluting with Hexanes
1
to neat THF yielded the product as the second dark brown band in 37% yield over two steps. H
NMR (CD2Cl2, 400 MHz): δ 9.29 (d, J = 5.0 Hz, 1H), 8.12 (t, J = 3.9 Hz, 1H), 8.08 (d, J = 8.9
Hz, 1H), 8.04 (d, J = 6.3 Hz, 2H), 7.98 (dt, J = 7.8, 1.5 Hz, 1H), 7.91 (q, J = 21.6, 8.6 Hz, 2H),
7.84 (d, J = 8.8 Hz, 1H), 7.68 (dt, J = 5.3, 1.0 Hz, 1H), 7.55 (m, 3H), 6.72 (d, J = 5.6 Hz, 4H),
6.42 (m, 3H), 2.09 (s, IrꢀC5H4NꢀCH3, 1H). 13C NMR (CD2Cl2, 100 MHz): δ 158.6, 158.5, 155.2,
150.9, 150.4, 150.1, 149.3, 146.6, 138.9, 136.7, 136.7, 135.8, 134.8, 132.5, 132.3, 131.9, 128.7,
128.1, 126.2, 125.8, 125.6, 123.3, 123.2, 121.9, 118.2, 117.0, 21.1 (IrꢀC5H4NꢀCH3). HRꢀMS Qꢀ
Tof calc. for C30H24N3IrCl [M+H]+: 654.1288; found: 654.1296. Anal. Calc. for C30H23ClIrN3: C
55.16, H 3.55, N 6.43; found, C 54.98, H 3.42, N 6.45.
(NNC-tb)Ir(Ph)(4-methylpyridine)(TFA) (11): In a 100 mL RBF, 150 mg of (NNCꢀtb)Irꢀ
Ph.4MePy.Cl and 202 mg (4 eq) of AgTFA were dissolved in 80 mL of dry DCM. The reaction
was stirred in the dark for 3 d. The sample was then filtered over a plug of alumina with a celite
bottom and washed with DCM and then DCM/10%MeOH to elute a yellow/red band. The
solution was then evacuated to dryness. The residue was then recrystallized from DCM/pentane
at ꢀ20 °C to give >90% yield of the title product. Alternatively, the residue can be sonicated in
diethyl ether (3 x 30 mL) followed by centrifugation and decanting of the liquid to reisolate the
1
residue. This procedure affords purity levels of that achieved by recrystallization. H NMR
(CD2Cl2, 400 MHz): δ 10.16 (d, J = 5.4 Hz, 1H), 8.17 (d, J = 7.9 Hz, 1H), 8.01 (m, 3H), 7.93 (d,
J = 8.6 Hz, 1H), 7.88 (d, J = 6.4 Hz, 2H), 7.83 (d, J = 8.5 Hz, 1H), 7.65 (dt, J = 7.2, 1.3 Hz, 1H),
7.59 (m, 3H), 6.78 (d, J = 6.1 Hz, 2H), 6.56 (m, 2H), 6.48 (m, 3H), 2.11 (s, IrꢀC5H4NꢀCH3, 3H).
13C NMR (CD2Cl2, 100 MHz): δ 158.58, 157.31, 156.09, 155.71, 149.82, 149.75, 148.29,
145.28, 138.99, 135.50, 135.30, 134.69, 132.08, 131.62, 129.94, 129.36, 129.11, 126.61, 126.02,
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125.36, 123.42, 123.18, 122.32, 118.64, 116.90, and 21.15 (IrꢀNC5H4ꢀCH3). The carbons from
the –OC(O)CF3 group were not observed. 19F NMR {1H} (CD2Cl2, 376 MHz): δ ꢀ75.45. HRꢀMS
QꢀTof calc. for C26H18F3IrN3O2 [MꢀPh]+: 654.0981; found: 654.654.0987. Anal. Calc. for
C32H23F3IrN3O2: C 52.60, H 3.17, N 5.75; found, C 52.31, H 3.26, N 5.83.
General Procedure for H/D Exchange (gases): A 4 mL high pressure stainless steel bomb
containing a glass liner with a magnetic stir bar was charged with 1.0 mL (1.31 x 10ꢀ2 mol) of a
CF3COOD solution containing 5 mM catalyst. The flask was heated for 120 min on a
temperature controlled aluminum block maintained at 180 °C (unless otherwise mentioned). The
gas phase was sampled and analyzed by GCꢀMS. The percent deuterium incorporation was
determined by deconvoluting the mass fragmentation pattern for methane using a Microsoft
Excel program.81 An important assumption is built into the program is that there are no isotope
effects on the fragmentation pattern of methane. The parent ion peak of methane is relatively
stable towards fragmentation and can be used to quantify the exchange reactions. The mass
fragmentation pattern from m/z of 12 to 21 was analyzed for each reaction and compared to
control reactions not containing catalyst. The results obtained by this method are accurate within
± 5% of deuterium incorporation or loss.
General Procedure for H/D Exchange (liquids): A 4 mL Schlenk flask with a resealable
PTFE valve and a magnetic stir bar was charged with 1.0 mL (1.31 x 10ꢀ2 mol) of a CF3COOD
solution containing 0.5 M substrate and 5 mM of catalyst (unless otherwise mentioned). The
flask was heated for 180 min in a well stirred oil bath maintained at 150 °C for arenes and 180
1
°C for all other hydrocarbons. The liquid phase was sampled and analyzed by GCꢀMS and H
NMR. Liquid hydrocarbon substrates were compared to control reactions not containing catalyst
and any background reactions were subtracted accordingly.
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The percent deuterium incorporation into benzene when determined by GCꢀMS was
performed by deconvoluting the mass fragmentation pattern of benzene using a Microsoft Excel
program.81 An important assumption is built into the program is that there are no isotope effects
on the fragmentation pattern of benzene. The parent ion peak of benzene is relatively stable
towards fragmentation and can be used to quantify the exchange reactions. The mass
fragmentation pattern from m/z of 78 to 84 was analyzed for each reaction and compared to
control reactions not containing catalyst. The results obtained by this method are accurate within
± 5% of deuterium incorporation or loss. All other hydrocarbons, besides benzene, were
determined by 1H NMR comparison of control and catalytic reactions.
H/D Exchange Over Time Leads to Parallel Incorporation of D: Following our general
procedures, H/D exchange reactions were sampled at 30, 60, 120, and 180 mins and monitored
by GCꢀMS.
Associated Content
Notes: The authors declare no competing financial interest.
Supporting Information: Synthetic data, calibration curves for GC data interpretation,
labeling studies, oxyꢀfunctionalization studies, crystallographic data, and CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org.
Acknowledgements
We thank Dr. Kenneth Young for helpful discussions regarding the synthesis and
characterization of the iridium complexes. This work was supported as part of the Center for
Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center Funded by the
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U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award
Number DEꢀSC0001298 (to R.A.P.) and the Scripps Research Institute, Scripps Florida.
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