Identifying and Evading Olefin Isomerization Catalyst Deactivation Pathways Resulting from Ion-Tunable Hemilability

Article abstract of DOI:10.1021/acscatal.0c03784

Hemilabile ligands are found in many leading organometallic catalysts, but it can be challenging to tune the degree of hemilability in a particular catalyst. This work explores the impact of cation-tunable hemilability on the speciation of iridium(III) pincer-crown ether catalysts during high-activity olefin isomerization. Under conditions where strong cation-macrocycle interactions are fostered and terminal olefin has been consumed, labilization of the aza-crown ether group leads to an η6-arene complex, wherein the pincer ligand is metallated at a different position. Arene complexes of styrene, naphthalene, and mesitylene were independently synthesized and found to exhibit diminished catalytic activity for allylbenzene isomerization. In response to these findings, a previously unreported catalyst bearing a synthetically modified pincer ligand was designed, resulting in a refined system that maintains high activity even when arene complexes are formed.

Full text of DOI:10.1021/acscatal.0c03784

pubs.acs.org/acscatalysis
Research Article
Identifying and Evading Olefin Isomerization Catalyst Deactivation
Pathways Resulting from Ion-Tunable Hemilability
Henry M. Dodge, Matthew R. Kita, Chun-Hsing Chen, and Alexander J. M. Miller*
Cite This: ACS Catal. 2020, 10, 13019−13030
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Hemilabile ligands are found in many leading
organometallic catalysts, but it can be challenging to tune the
degree of hemilability in a particular catalyst. This work explores
the impact of cation-tunable hemilability on the speciation of
iridium(III) pincer-crown ether catalysts during high-activity olefin
isomerization. Under conditions where strong cation−macrocycle
interactions are fostered and terminal olefin has been consumed,
labilization of the aza-crown ether group leads to an η6-arene
complex, wherein the pincer ligand is metallated at a different
position. Arene complexes of styrene, naphthalene, and mesitylene
were independently synthesized and found to exhibit diminished
catalytic activity for allylbenzene isomerization. In response to
these findings, a previously unreported catalyst bearing a synthetically modified pincer ligand was designed, resulting in a refined
system that maintains high activity even when arene complexes are formed.
KEYWORDS: iridium, pincer, olefin isomerization, hemilability, crown ether, pince−crown ether, switchable catalysis
upon dissociation of the substrate. Because the dynamic
behavior of hemilabile ligands involves a delicate energetic
balance, it can be difficult to tune or control hemilability.26
To introduce an element of tunability to hemilabile ligands,
we have developed “pincer-crown ether” ligands that
incorporate an aza-crown ether capable of cation binding
into a pincer framework.1,27 Substitution of a crown ether
oxygen donor by a substrate is often unfavorable when no
cations are present in a solution, but when cations are present,
the dissociated macrocycle can be stabilized by cation−crown
interactions and substrate binding becomes favorable.28
A remarkable degree of control over the activity of olefin
isomerization is achieved when iridium(III) pincer-crown ether
complexes are used in conjunction with simple alkali metal
salts.29 The cationic iridium hydride complex
[(κ5-15c5NCOPiPr)IrH][BArF4] (ArF = 3,5-bis-(trifluoromethyl)-
phenyl) (1, Scheme 1) isomerizes allylbenzene to (E)-β-
methylstyrene in dichloromethane at room temperature slowly
when no salts are added (TOF = 1.8 h−1). The addition of
NaBArF4 leads to modest rate acceleration (TOF = 5.4 h−1),
while the addition of LiBArF4·3Et2O leads to more than 1000-
INTRODUCTION
■
Binding of the substrate to a transition metal center is the entry
point of many catalytic cycles. While substrate binding is often
a rate-limiting process, there are few methods to simply and
systematically tune the rate of substrate association to the
metal center.1 Control over substrate binding therefore
remains a fundamental challenge in the field of homogeneous
catalysis.
One method to control substrate binding utilizes stimuli-
responsive supramolecular encapsulation. External stimuli can
open or close the “gate” to modulate access of the substrate to
the active site.2 Such a situation is frequently encountered in
biological catalysis with allosterically modulated enzymes.3−7
Synthetic catalysts have also been designed with artificial
allosteric cofactors through elegant control of the secondary
coordination sphere.2,8−14 Other methods to control substrate
binding focus on the primary coordination sphere.1 Photo-
chemical ligand dissociation can open a substrate binding
site,15,16 or a change in catalyst oxidation state can alter the
kinetics and thermodynamics of ligand substitution.17−20
Supporting ligands with Lewis basic sites can interact with
externally added Lewis acids to modulate catalyst properties as
well.21−23
Received: August 30, 2020
Revised: October 9, 2020
Hemilabile ligands, which are multidentate ligands featuring
both strong and weak donors, represent one opportunity to
gate substrate access to a metal center via the primary
coordination sphere.1,24,25 The weak donors can be displaced
by the substrate, allowing catalysis to occur, then reassociate
https://dx.doi.org/10.1021/acscatal.0c03784
© XXXX American Chemical Society
ACS Catal. 2020, 10, 13019−13030
13019

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
Scheme 1. Cation-Enhanced Olefin Isomerization by an Iridium Pincer-Crown Ether Catalyst, Highlighting Proposed Key
Intermediate A
fold rate enhancements (TOF = 2750 h−1).29 The dramatic
responsiveness of the catalyst to Li+ is attributed to a
significantly higher binding affinity for Li+ based on the size
of the macrocycle.28 The tunable ether hemilability in this
pincer-crown ether catalyst also renders rate enhancements
reversible: catalysis can be accelerated by Li+ ions or
decelerated by the addition of chloride ions. The proposed
mechanism of cation-promoted olefin isomerization is shown
in Scheme 1. The iridium hydrido olefin complex A was
proposed to be a key intermediate, motivating a search for its
existence.
Here, we report the development of a pincer-crown ether
olefin isomerization catalyst, guided by mechanistic studies
that revealed roles of both ether and amine hemilability. In situ
spectroscopic monitoring of olefin isomerization by 1 revealed
the formation of an unknown iridium species as the reaction
proceeded. The unknown species was identified as an
unexpected η6-arene complex with an altered site of metal-
lation. Authentic synthesis of model complexes allowed full
structural characterization and catalytic assessment showed
poor performance. In response to this mechanistic insight, the
pincer-crown ether supporting ligand backbone was syntheti-
cally modified to prevent a change in metallation. The catalyst
reported here maintains high activity and exhibits improved
durability under certain conditions. Additional mechanistic
studies show that ether hemilability is essential for rapid
isomerization, and amine hemilability (leading to η6-arene
complexes) can be toleratedbut that a change in metallation
is detrimental. By directly relating catalytic activity to catalyst
speciation, generally applicable insight into different modes of
hemilability emerges.
effects of Li+ concentration, reaction temperature, and Lewis
acidity of the cationic additive on the formation of B were
examined. In all cases, unless otherwise noted, >99% yield of β-
methylstyrene was observed at the end of the reaction
according to NMR spectroscopy.
To examine the effect of salt concentration on catalyst
speciation, the formation of B was monitored by NMR
spectroscopy during isomerization of allylbenzene (0.75 M in
CH2Cl2) at various LiBAr4F·3Et2O concentrations. Figure 1
shows that the yield of B after 16 h increased markedly with
increasing concentration of LiBArF4·3Et2O. The trend of Figure
1 is consistent with cation binding playing a role in the
formation of the unknown species.
RESULTS AND DISCUSSION
■
Spectroscopic Detection of Iridium-Containing Spe-
cies During Catalysis. To search for catalytic intermediates
during Li+ -promoted olefin isomerization by
[(κ5-15c5NCOPiPr)IrH]+ (1), we examined allylbenzene isomer-
ization under a range of conditions. Trace amounts of an
unknown iridium-containing species, denoted B and identified
1
by a characteristic hydride H nuclear magnetic resonance
(NMR) signal at −15.7 ppm, were observed in Li+-containing
reactions as isomerization reached completion. Monitoring
reactions beyond 1 h revealed that the unknown species
continued forming after isomerization was complete. The
Figure 1. Plot showing Li+ dependence of the formation of B during
allylbenzene isomerization at room temperature (7.5 mM 1, 0.75 M
allylbenzene, 16 h).
13020
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
To assess the influence of cation Lewis acidity on catalyst
speciation, the lithium salt LiAl(OC(CF3)3)4 was examined.
Excess Et2O has been shown to attenuate Li+-promoted rate
enhancements in olefin isomerization by pincer-crown ether
catalysts,29 leading to the hypothesis that the ether-free salt
LiAl(OC(CF3)3)4 might lead to stronger cation−crown
interactions at lower salt concentrations.30 Only 20 min after
mixing catalyst 1 (7.5 mM) and 2 equiv LiAl(OC(CF3)3)4 with
allylbenzene (0.75 M) in CH2Cl2, complete isomerization to β-
methylstyrene and complete conversion of 1 to B were
observed by NMR spectroscopy. The formation of B thus
depends on both the concentration of Li+ salts and the Lewis
acidity of the specific salt employed.
NMR δ −15.7). There is a single resonance in the 31P NMR
spectrum of the product (δ 161.4).
The spectroscopic data was vetted as a possible cationic
hydrido η2-alkene complex analogous to the postulated
catalytic intermediate A (Scheme 1, above). While the vinylic
resonances of η2-alkene complexes are typically found upfield
31,32
1
of the free olefin in the H NMR spectrum,
the opposite
trend was observed in the present case: the terminal vinylic
resonances were shifted 0.43 ppm downfield of free styrene
(Figure 2). On the other hand, the aromatic proton signals of
To probe the role of temperature, a reaction vessel
containing 5 mM 1, 25 mM LiBArF4·3Et2O, and 0.5 M
allylbenzene in CD2Cl2 was heated at 35 °C for 16 h. The
reaction produced B quantitatively according to NMR
spectroscopy. For comparison, allylbenzene isomerization at
room temperature (20 °C) in CD2Cl2 under these conditions
gave 61% yield of B. Under analogous conditions, but utilizing
the higher boiling solvent 1,2-dichloroethane-d4 and heating at
80 °C, 1 was fully converted to B within 1 h. These studies
show that elevated temperatures increase catalyst speciation.
The catalytic runs under variable conditions provide insight
into the factors that lead to a change in a catalyst state. At
room temperature with LiBArF4·3Et2O and short reaction times,
only trace amounts of B are observed. Formation of B
increases at high LiBArF4·3Et2O concentrations and with longer
reaction times. At elevated reaction temperatures or when
strongly Lewis acidic Li+ sources are used, formation of B is
quantitative even at short reaction times. Catalyst speciation
was therefore predicted to become problematic during high-
temperature isomerization reactions and when strongly Lewis
acidic cations are employed. In this context, it was important to
fully characterize B and determine the extent to which it is
detrimental to catalysis.
Synthesis and Characterization of Analogues of
Iridium-Containing Species B. Styrene was employed as a
nonisomerizable model olefin to aid in the characterization of a
complex analogous to B. Complex 1 and 20 equiv styrene were
stirred in CD2Cl2 for 24 h, but no reaction was observed.
When 1.5 equiv LiBArF4·3Et2O was included with complex 1
and 20 equiv styrene, however, full conversion of 1 to a single
new hydride-containing product was observed within 30 min
(Scheme 2). The chemical shift of the hydride resonance in the
product (1H NMR δ −15.5) is very similar to that of B (1H
1
Figure 2. H NMR spectra (downfield region) showing (a) styrene
alone, and (b) an isolated sample of styrene complex 2. Changes in
the chemical shifts of the aryl and vinylic resonances of styrene aided
in identification of 2 as an η6-styrene complex.
the bound styrene were shifted 0.54 ppm upfield, again
opposite to expectations for a styrene complex bound η2
through the vinyl group. The styrene chemical shifts are
more in line with an η6-arene complex of iridium.33
Scheme 2. Synthesis of an η6-Styrene Iridium Complex
Aided in Identification of the Unknown Species Observed in
Allylbenzene Isomerization
A three-legged piano stool complex capped by η6-styrene
could form upon dissociation of the amine arm of the pincer-
crown ether ligand. Significant changes in the 1H NMR
resonances for the aryl backbone of the pincer ligand indicated
a more complex rearrangement, however. The aryl resonances
for complex 1 appear as a triplet and two doublets. The aryl
resonances for complex 2 appear as a doublet (J = 7.8 Hz), a
doublet (J = 2.0 Hz), and a doublet of doublets (J = 7.8, 2.0
Hz). We therefore assign the species as an η6-styrene complex
in which the aryl backbone has reductively eliminated, rotated,
and oxidatively added at the C−H bond ortho to the
phosphinite to form [Li@(κ2-15c5NC′OPiPr)Ir(η6-styrene)H]2+
1
(2, Scheme 2). Consistent with this assignment, the H NMR
resonances corresponding to the crown ether protons appear
in a narrow chemical shift range, as expected for an aza-crown
13021
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
ether lacking any amine or ether interactions with iridium.34
We have previously observed “remetallation” from the 2-
position to the 6-position of the arene backbone in
stoichiometric reactivity studies of iridium(I) carbonyl
pincer-crown ether complexes in the presence of Ca2+.35
Similar examples of metallation at the “wrong” position can be
found in pincer complexes with phenyl backbones and
nitrogen donors.36,37 Our observations show that, under
certain conditions, both ethers and amines can act as
hemilabile ligands in the pincer-crown ether system, with
amine dissociation sometimes leading to undesired reaction
pathways.
(33%) had a hydride resonance at −15.9 ppm. Over the course
of 48 h, the major hydride signal (δ −16.7) disappears
completely and the minor hydride signal (δ −15.9) becomes
the sole product. The aryl resonances for the ligand backbone
of the intermediate species appear as a triplet and two
doublets, leading us to assign this intermediate as a bidentate
phenylphosphinite arene complex that has not undergone
remetallation (4′, Scheme 3). This species is proposed to then
undergo reductive elimination, rotation, and oxidative addition
of the other C−H bond ortho to the phosphinite to reach the
thermodynamically preferred remetallated product
[Li@(κ2-15c5NC′OPiPr)Ir(η6-mesitylene)H]2+ (4). The reac-
tion pathway may be the same for each arene ligand, but we
postulate that the electron-rich mesitylene ligand stabilizes the
intermediate arene adduct more than styrene and naphthalene
do,38 slowing down remetallation relative to the other cases
where no intermediate is observed.
To probe the generality of η6-arene complex formation,
cationic complex 1 was allowed to react with an electron-poor
arene (naphthalene) and an electron-rich arene (mesitylene).
When complex 1 was treated with 20 equiv naphthalene and
2.0 equiv LiBArF4·3Et2O in dichloromethane, full conversion to
a new species was observed over 16 h. The product is
spectroscopically almost identical to styrene complex 2 and is
thus assigned as [Li@(κ2-15c5NC′OPiPr)Ir(η6-naphthalene)-
H]2+ (3, Scheme 3). The reaction with naphthalene proceeds
The structural assignments of the arene complexes were
confirmed by an X-ray diffraction (XRD) study on single
crystals of 3, which were grown by layering pentane onto a
concentrated dichloromethane solution and allowing the
solvents to mix at −30 °C. The structure unambiguously
shows η6-naphthalene binding and the pincer ligand
remetallation that positions the crown ether away from the
metal center (Figure 3). Crystals of 3 exhibited whole-
molecule disorder, with the occupancy of the minor
component ∼20% (see the Supporting Information for
details).
Scheme 3. Synthesis of Three η6-Arene Iridium Complexes
in Which the Pincer Ligand Has Remetallated, Giving
Products with a Bidentate Ligand Binding Mode
Role of η6-Arene Complexes in Catalysis. Based on the
iridium complexes obtained using styrene, naphthalene, and
mesitylene, and their spectroscopic similarity to species B
observed in allylbenzene isomerization, B was assigned as the
remetallated arene complex of β-methylstyrene (the product of
markedly more slowly than the analogous reaction with styrene
(16 h vs 0.5 h). The reactivity difference is attributed to the
ability of styrene to support η2-coordination via the vinyl group
before intramolecular rearrangement to an η6-arene structure.
When mesitylene was used as the capping arene ligand, the
reaction rate fell between styrene and naphthalene, and a
previously unobserved intermediate appeared. The major
species (77%) 3 h after treating complex 1 with 30 equiv
mesitylene and 2.8 equiv LiBArF4·3Et2O in CH2Cl2 had a
hydride resonance at −16.7 ppm, while the minor species
Figure 3. Structural representation of the cation of 3 with ellipsoids
drawn at the 50% probability level. All hydrogen atoms except the
hydride are omitted for clarity. Selected distances (Å) and angles (°):
Ir1−P1 2.221(2), Ir1−C11 2.031(5), Ir1−H1 1.498, P1−Ir1−C11
78.68(14), P1−Ir1−H1 79.1, and C11−Ir1−H1 81.3.
13022
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
allylbenzene isomerization). Formation of B is significantly
slower than the formation of 2, consistent with β-
methylstyrene being released from the coordination sphere
of the catalyst upon its initial formation from allylbenzene,
before later coordinating in an η6 fashion.
catalytic cycle with slower individual steps. To distinguish
between these possibilities, evidence for the reversibility of the
ligand remetallation was sought. Styrene-d8 (10 equiv) was
added to an isolated sample of 2 in CD2Cl2, and the reaction
was monitored by NMR spectroscopy (Scheme 6). Arene
Observing a new complex formed during catalysis, especially
one formed in increasing amounts toward the end of a catalytic
run, does not necessarily implicate that species as a relevant
intermediate.39 Catalytic species B could be a relevant
intermediate that does not build up until isomerization reaches
completion, or it could be an unwanted off-cycle species. To
probe this, we tested the viability of 2 as a precatalyst in
allylbenzene isomerization (Scheme 4).
Scheme 6. Labeling Experiment Using Styrene-d8 to Probe
Reversibility of η6-Arene Complex Formation
Scheme 4. Catalytic Allylbenzene Isomerization by the η6-
Styrene Iridium Complex 2
exchange occurs rapidly, as confirmed by the disappearance of
resonances for bound protio styrene and the appearance of
resonances assigned to free protio styrene in the H NMR
1
Catalytic isomerization of allylbenzene (0.5 M) in CD2Cl2
by the Li+-containing η6-styrene complex 2 (5 mM) proceeded
very slowly (TOF = 3.2 h−1, first-order half-life t1/2 = 20.7 h)
relative to isomerization catalyzed by 1 in the presence of Li+
(TOF = 1870 h−1, zero-order half-life 0t1/2 = 1.6 min). During
isomerization by 2, there was no hydride resonance discernible
spectrum. There is also H/D scrambling of the olefinic protons
of styrene, and the hydride resonance of starting complex 2
diminishes (22% hydride remains by integration) due to the
formation of the corresponding iridium deuteride. There is,
however, no D incorporation into the aryl backbone of the
NCOP ligand. If the reductive elimination, rotation, oxidative
addition pathway that leads to the formation of 2 is reversible,
D incorporation would be expected at the aryl C−H bond
ortho to the phosphinite in 2 (Scheme 6). We conclude from
this experiment that the remetallation involved in the
formation of 2 is not reversible under the reaction conditions,
and therefore 2 operates via an independent catalytic cycle
(Scheme 5).
1
in the H NMR spectrum, indicating that the catalyst resting
state is not an iridium hydride. Upon completion of the
reaction, a hydride resonance matching that of B, assigned as
the η6-β-methylstyrene adduct, was observed. On the basis of
the slow rate of allylbenzene isomerization by 2, relative to 1 in
the presence of LiBAr4F·3Et2O, we conclude that the
remetallated η6-arene complex is not an intermediate in the
fast catalytic cycle of Scheme 5.
Catalysis involving η6-arene complexes could be slow
because of unfavorable re-entry into the original catalytic
cycle (requiring arene dissociation and ligand remetallation
back to the original binding mode); or it could be slow because
the η6-arene complexes proceed through an independent
The remetallated backbone of the arene complexes raised
the possibility that the role of Li+ in isomerization catalysis
might be to promote remetallation to generate a highly active
catalyst derived from 1. If this were the case, however, the
remetallated arene complex 2 should resemble an active
Scheme 5. Proposed Mechanism of Cation-Promoted Allylbenzene Isomerization, Including the Newly Identified η6-Arene
a
Complexes
a
A new catalyst with a methoxy substituent ortho to the phosphinite prevents remetallation and sustains catalytic activity.
13023
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
intermediate, so the low catalytic activity of 2 helps rule out
this mechanistic possibility. The η6-arene ligand itself is also
clearly not required for productive catalysis, given that 1-
hexene isomerization by 1 in the presence of Li+ was previously
reported to proceed at a comparable rate to allylbenzene
isomerization.29 To further support that the role of Li+ is not
to trigger remetallation of the pincer ligand and generate a
highly active, low-coordinate iridium center, a version of 1 with
the site of remetallation occupied by a blocking group was
prepared. If remetallation is a key step in highly active Li+-
promoted catalysis, then the new catalyst should not exhibit
dramatic Li+ enhancement since the remetallation pathway is
blocked.
Synthesis and Reactivity of a “Remetallation-Proof”
Catalyst. To prevent the process of remetallation observed
with 1, we turned to a pincer ligand with the site of
remetallation blocked by a methoxy group, (MeO‑15c5NCOPiPr)-
H.40 With the methoxy group ortho to the phosphinite, the
ligand presents only one possible site of metallation. We
previously reported iridium carbonyl complexes with this
particular pincer-crown ether ligand,40 but the carbonyl-free
analogue of 1 has not been reported.
Figure 4. Structural representation of 5 with ellipsoids drawn at the
50% probability level. All hydrogen atoms except the hydride are
omitted for clarity. Selected distances (Å) and angles (°): Ir1−P1
2.1861(7), Ir1−O3 2.352(2), Ir1−C1 1.974(3), Ir1−N1 2.235(2),
and Ir1−Cl1 2.4496(8); P1−Ir1−O3 100.13(6), O3−Ir1−Cl1
94.96(6), C1−Ir1−O3 87.81(10), C1−Ir1−Cl1 173.90(9), and
N1−Ir1−O3 78.24(8).
Allowing the ligand (MeO‑15c5NCOPiPr)H to react with
[Ir(COD)Cl]2 (COD = 1,5-cyclooctadiene) yielded
(κ4-MeO‑15c5NCOPiPr)Ir(H)(Cl) (5, Scheme 7). Single crystals
formation, with peaks corresponding to 6 (δ 142.4) and an η6-
arene complex (δ 151.2), which matched the ratio of products
based on the integration of the hydride resonances.
Interestingly, when the same reaction was run using LiAl(OC-
(CF3)3)4 instead of LiBAr4F·3Et2O, the η6-mesitylene adduct
Scheme 7. Synthesis of an Iridium Pincer-Crown Ether
Catalyst with a Modified Ligand Backbone
1
was generated in 73% yield in just 30 min, according to H
NMR spectroscopy. This result is attributed to the higher
Lewis acidity of Li+ in the ether-free salt LiAl(OC(CF3)3)4
compared to LiBArF4·3Et2O. While attempts to isolate this
normally metallated arene complex were unsuccessful, the
spectroscopic data are consistent with arene complexation
w i t h o u t r e m e t a l l a t i o n i n t h e f o r m a t i o n o f
[Li@(κ2-MeO‑15c5NCOPiPr)Ir(η6-mesitylene)H]2+.
The need for electron-rich arenes to induce arene binding by
complex 6 suggests that these arene complexes are less stable
than the remetallated arene complexes produced from 1. The
thermodynamic preference for remetallation could avoid steric
interactions between the arene ligand and the pendant aza-
crown ether moiety, or it could minimize electrostatic
repulsion between the cationic iridium center and Li+
encapsulated in the crown and their respective counteranions.
The reactivity of complex 6 enabled us to pose the question of
whether the low activity of complex 2 was due to the presence
of an η6-arene ligand or due to the remetallation of the pincer
backbone. Complex 6 was therefore examined for allybenzene
isomerization.
Catalytic Activity of the “Methoxy-Blocked” Complex
6. The new cationic iridium hydride complex 6 was compared
with original catalyst 1 to understand the impact of
remetallation on isomerization catalysis. Under standard
conditions of 1 mol % catalyst (0.5 M substrate, 5 mM
catalyst) in the presence of 1 equiv LiBArF4·3Et2O, both 1 and
6 isomerize allylbenzene to β-methylstyrene in less than 10
min at 25 °C with high stereoselectivity (98:2 E:Z). The Ir/Li+
systems are among the most highly active isomerization
catalysts known.41−45 For comparison, other iridium catalysts
require high temperatures (70−125 °C) to efficiently isomer-
ize terminal olefins to internal positions.43,46−48
of 5 suitable for an X-ray diffraction study (XRD) were grown
from a dichloromethane solution layered with pentane (Figure
4). Comparing the crystallographic parameters of 5 with the
previously reported complex (κ4-15c5NCOPiPr)Ir(H)(Cl),
which has no methoxy group ortho to the phosphinite, reveals
no significant differences between the two in the primary
coordination sphere of iridium. Chloride abstraction with
NaBArF4 provided the cationic complex [(κ5-MeO‑15c5NCOPiPr)-
IrH]+ (6, Scheme 7).
The ability of methoxy-containing complex 6 to bind arenes
in an η6 fashion was examined using styrene and mesitylene.
Treating complex 6 with styrene in the presence of 1.5 equiv
LiBArF4·3Et2O did not lead to any observed reaction after 30
min. This draws a sharp contrast to complex 1, which
undergoes quantitative conversion to η6-arene complex 2
under the same conditions, as described above. When 6 was
treated with the more electron-rich arene mesitylene under
otherwise identical conditions, the “normally metallated” arene
complex was generated in 22% yield in 30 min according to 1H
NMR spectroscopy. After 72 h, the yield of arene complex was
63%. The 31P NMR spectrum also reflects arene complex
13024
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
Even when the catalyst loading of 1 or 6 was lowered to 0.2
mol % (0.5 M substrate, 1 mM catalyst, 5 mM LiAl(OC-
(CF3)3)4) and the reaction was heated at 70 °C in 1,2-DCE-d4,
the isomerization activity was similar for the two catalysts
(Scheme 8). In situ monitoring of the catalyst speciation by
methylstyrene. At the conclusion of the reaction, the speciation
in reaction A shifted from catalyst 1 to an η6-arene complex
with a remetallated phenyl backbone (species B in Scheme 8),
while the speciation in reaction B shifted from 6 to an η6-arene
complex with the original metallation position (species C in
Scheme 8). The distinct speciation in each reaction was
reflected in distinct reactivity when a second aliquot of
allylbenzene was added and the reaction was allowed to
proceed for 40 min, with reaction A (with the remetallated
catalyst) giving 10% yield and reaction B (original metallation
position) giving 100% yield. After allowing both reactions to
proceed to completion, a third aliquot was added and the
reaction stirred for 80 min, resulting in 75% conversion for
reaction B and only 7% conversion for reaction A. The
methoxy-blocked catalyst 6 gives higher yields than the original
catalyst under these recycle conditions. This comparison
reveals that the formation of an arene complex does not
inherently deactivate the catalyst, but that remetallation is the
key process leading to lower activity.
Scheme 8. Comparisons of Olefin Isomerization by 1 and
the Methoxy-Blocked Catalyst 6
A more challenging substrate, 4-phenyl-1-butene, was
examined next. Access to the thermodynamically favored
isomer, 1-phenyl-1-butene, requires two positional isomer-
izations. At a low 0.2 mol % catalyst loading (0.5 M substrate,
1 mM catalyst, 5 mM LiAl(OC(CF3)3)4) at 40 °C in CD2Cl2,
catalyst 6 produced 1-phenyl-1-butene in 86% yield (TON =
430) after 5 h. The original catalyst 1 only gave 38% yield
(TON = 190) under the same conditions. In contrast to the
isomerization of allylbenzene, the resting states during both 1-
and 6-catalyzed isomerization of 4-phenyl-1-butene are arene
complexes under these conditions (Scheme 8). However, only
catalyst 1 undergoes remetallation during the isomerization,
leading to lower activity for this more challenging substrate.
The comparative catalysis studies show that catalyst 6
exhibits markedly higher activity than 1 for a challenging
substrate requiring multiple isomerizations, or when the
catalyst is recycled in multiple runs. In addition to
demonstrating the superior performance of 6 under certain
conditions, the comparisons provide important mechanistic
insights. Scheme 5 (above) shows the catalytic cycle proposed
for allylbenzene isomerization. The collection of studies is
consistent with binding of the terminal olefin allylbenzene
occurring faster than amine labilization and arene complex
formation, while binding of an internal olefin is slower than
amine labilization and arene complex formation. Thus, no
change in resting state is observed during allylbenzene
isomerization, but once all of the allylbenzene is consumed
only internal olefin remains; the catalyst converts to arene
complexes. This leads to lower activity for catalyst 1 in
recycling experiments. When multiple isomerizations are
required, as in 4-phenyl-1-butene isomerization, all of the
terminal olefin is consumed before the reaction is complete.
The mechanism proposed in Scheme 10 illustrates how the
arene complexes become the resting states during the reaction
because the rate of internal olefin substitution is slower than
the rate of arene complex formation, ka (internal) < karene. The
arene complexes derived from catalysts 1 and 6 have very
different activity. Catalyst 1 loses almost all activity upon arene
complex formation because irreversible remetallation to a
different position of the pincer backbone occurs. Catalyst 6
does not remetallate and retains high activity, presumably
because it can readily re-enter the original rapid catalytic cycle
from the η6-arene complex C (Scheme 8 and Scheme 10). A
NMR spectroscopy revealed that, under these conditions, the
resting state during catalysis was the starting catalyst (1 or 6).
These studies show that during allylbenzene isomerization, the
resting state is the starting catalysts; conversion to arene
complexes B or C only occurs after all allylbenzene is
consumed. The mechanistic implications of this are discussed
below.
Because catalyst 1 is converted to a new species at the
conclusion of the isomerization, the performance differs
markedly when attempting to recycle the catalyst. As shown
in Scheme 9, the two iridium catalysts (5 mM) were stirred in
CD2Cl2 containing allylbenzene (0.5 M) and LiAl(OC-
1
(CF3)3)4 (10 mM) for 20 min at 25 °C before H NMR
spectroscopic analysis revealed complete conversion to β-
Scheme 9. Comparison of the Recyclability of Catalysts 1
and 6 in Multiple Allylbenzene Isomerization Reactions
13025
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
Scheme 10. Proposed Mechanism of 4-Phenyl-1-Butene Isomerization Highlighting Change in Resting State to Arene
Complexes Because ka (internal) Is Smaller than karene
simple modification of the ligand backbone thus prevents
significant losses in catalyst activity under certain conditions.
chemical shifts are reported via absolute referencing to the
1
corresponding H spectra. The compounds (15c5NCOPiPr)Ir-
(H)(Cl),4 9 [(1 5 c 5 NCOPi P r )IrH][BAr4
(
]
(1),4 9
F
MeO‑15c5NCOPiPr),40 LiBAr4F·3Et2O,50 and LiAl(OC(CF3)3)4
51
CONCLUSIONS
■
were prepared according to literature procedures. Some
NaBArF4 was purchased from Matrix Scientific and purified
by crystallization and rigorous drying prior to use,52 and some
NaBArF4 was synthesized according to literature protocol.53
Allylbenzene, 4-phenyl-1-butene, styrene, and mesitylene were
freeze−pump−thaw degassed three times before drying via
passage over a small column of activated alumina and storage
over 3 Å molecular sieves. All other reagents were
commercially available and used without further purification.
Elemental analyses were performed by Robertson Microlit
Labs (Ledgewood, NJ). High-resolution mass spectrometry
was carried out with a Q Exactive HF-X (ThermoFisher,
Bremen, Germany) mass spectrometer. Samples in dichloro-
methane were introduced via a heated electrospray source
(HESI) at a flow rate of 10 μL/min. Xcaliber (ThermoFisher)
was used to analyze the data. Single-crystal X-ray diffraction
data was collected on a Bruker APEX-II CCD diffractometer at
either 150 or 273 K with Cu Kα radiation.
Mechanistic studies of cation-promoted olefin isomerization by
iridium pincer-crown ether complexes reveal unexpected
deactivation pathways. In the original catalyst structures,
amine dissociation leads to metallation at a different position
on the aryl backbone of the pincer and formation of an η6-
arene complex with very low activity. Access to the off-cycle
species is promoted by high concentrations of Li+, high
temperatures, and long reaction times. To block remetallation,
a methoxy group was installed in the pincer backbone. Under
forcing conditions, arene complexes are still observed for the
new catalyst 6, yet ≥75% yields of internal olefin are still
obtained.
The present study provides insight into the opportunities
and challenges associated with tunable hemilability. The
cationic iridium hydride catalyst 1 features a macrocycle with
the amine and two ethers bound. Labilizing the ethers through
cation−crown interactions is critical to achieving high activity
in this system. Labilizing the amine, however, can lead to
reductive elimination and metallation at a different position on
the aryl backbone. Amine hemilability is not inherently
problematic, however, as long as remetallation is prevented.
Synthesis of [Li@(κ2-15c5NC′OPiPr)Ir(η6-styrene)H]-
[BArF4]2 (2). In a glovebox, a 20 mL scintillation vial was
charged with 0.2137 g (0.1428 mmol) of 1 and 0.2330 g
(0.2134 mmol) LiBArF4·3Et2O. The solids were dissolved in 8
mL of CH2Cl2 and 0.328 mL of styrene (2.853 mmol, 20
equiv) was added, resulting in a color change of the solution
from yellow-orange to bright red. The solution was stirred for
15 min, then the solvent was removed under vacuum, and the
resulting orange solid was washed thoroughly with pentane.
The solid was dissolved in dichloromethane (8 mL), pentane
was layered onto the concentrated solution, and the solvents
were allowed to mix at −30 °C, yielding 0.3229 g of yellow
crystalline solid (2, 84%). The material was 99% pure by NMR
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were per-
formed using standard Schlenk and glovebox techniques under
a N2 atmosphere. NMR-scale reactions were prepared under
N2 and monitored in Teflon-sealed tubes. Pentane, diethyl
ether, toluene, and benzene were used in a glovebox without
purging, such that traces of those solvents were present in the
atmosphere and in the solvent bottles. Tetrahydrofuran was
actively avoided due to its ability to replace the Et2O donors in
LiBArF4·3Et2O, making the Li+ cation less Lewis acidic. H,
spectroscopy. H NMR (600 MHz, CD2Cl2): δ 7.75 (s, 16H,
1
1
31P{1H}, 19F{1H}, and 13C{1H} spectra were recorded on 500
or 600 MHz spectrometers at 298 K. NMR solvents were
purchased from Cambridge Isotope Laboratories, Inc., and
were freeze−pump−thaw degassed three times before drying
via passage over activated alumina and storage over 3 Å
BArH), 7.59 (s, 8H, BArH), 7.14 (d, J = 7.9 Hz, 1H, ArH),
6.87 (t, J = 6.3 Hz, 1H, styrene-ArH), 6.81 (d, J = 7.0 Hz, 2H,
styrene-ArH), 6.75 (t, J = 6.3 Hz, 1H, styrene-ArH), 6.68 (d, J
= 2.0 Hz, 1H, ArH), 6.66 (t, J = 6.2 Hz, 1H, styrene-ArH),
6.55 (dd, J = 7.8, 2.0 Hz, 1H, ArH), 6.46 (dd, J = 17.3, 10.9
Hz, 1H, styrene-CHCH2), 6.05 (d, J = 17.3 Hz, 1H, styrene-
CHCH2), 5.84 (d, J = 11.0 Hz, 1H, styrene-CHCH2), 3.60 (m,
1
molecular sieves. H and 13C chemical shifts are reported in
ppm relative to residual protio solvent resonances. 31P{1H}
13026
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
18H, crown-CH2 and Ar(CH2)N), 2.64 (bs, 4H, crown-
(CH2)N(CH2)), 2.53 (sep, J = 6.8 Hz, 1H, CH(CH3)2), 2.26
(sd, J = 7.3, 2.5 Hz, 1H, CH(CH3)2), 1.23 (dd, J = 15.9, 7.0
Hz, 3H, CH(CH3)2), 1.03 (dd, J = 17.6, 7.2 Hz, 3H,
CH(CH3)2), 0.95 (dd, J = 15.4, 7.1 Hz, 3H, CH(CH3)2), 0.91
(dd, J = 12.0, 6.7 Hz, 3H, CH(CH3)2), and −15.50 (d, J = 29.6
Hz, 1H, IrH). 13C NMR (151 MHz, CD2Cl2): δ 162.17 (q,
1JBC = 49.6 Hz, B-CAr), 141.05 (CAr), 135.23 (CBAr), 129.30 (q,
2JFC = 31.4 Hz, CBAr-CF3), 128.64 (styrene-CHCH2), 127.22
Anal. calcd for C97H72B2F48IrLiNO5P: C, 46.69; H, 2.91; N,
0.56. Found: C, 46.43; H, 2.08; N, 0.63.
Synthesis of [Li@(κ2-15c5NC′OPiPr)Ir(η6-mesitylene)H]-
[BArF4]2 (4). In the glovebox, a 150 mL Teflon-sealed reaction
vessel was charged with 0.2425 g (0.1620 mmol) of 1, 0.3537 g
(0.3239 mmol) LiBArF4·3Et2O, and 0.451 mL (3.242 mmol, 20
equiv) of mesitylene in 15 mL of CH2Cl2. The flask was
brought out of the glovebox and heated at 50 °C for 48 h. No
significant color change was observed. The solvent was
removed under vacuum, and the resulting orange solid was
washed thoroughly with pentane in the glovebox. Multiple
crystallization attempts were unsuccessful so 4 was isolated and
characterized as a crude orange solid (0.3188 g, 71% yield)
1
(styrene-CHCH2), 125.77 (CAr), 125.02 (q, JFC = 272.6 Hz,
BAr-CF3), 120.55 (CAr), 117.93 (sept, J = 4.1 Hz, CBAr), 112.47
(d, J = 12.0 Hz, CAr), 111.14 (d, J = 6.2 Hz, CAr), 103.36 (d, J =
3.4 Hz, styrene-CAr), 101.79, (styrene-CAr), 98.88 (d, J = 3.4
Hz, styrene-CAr), 98.70 (styrene-CAr), 97.90 (styrene-CAr),
68.03 (crown-CH2), 67.82 (crown-CH2), 67.46 (crown-CH2),
66.74 (crown-CH2), 58.62 (Ar(CH2)N), 51.25 (crown-(CH2)-
N(CH2)), 33.64 (d, 1JPC = 36.6 Hz, CH(CH3)2), 28.80 (d, 1JPC
= 46.9 Hz, CH(CH3)2), 17.42 (CH(CH3)2), 17.16 (d, J = 2.6
1
The material was 89% pure by NMR spectroscopy. H NMR
(600 MHz, CD2Cl2): δ 7.75 (s, 16H, BArH), 7.59 (s, 8H,
BArH), 7.09 (d, J = 7.4 Hz, 1H, ArH), 6.66 (d, J = 1.2 Hz, 1H,
ArH), 6.62 (dd, J = 7.8, 1.8 Hz, 1H, ArH), 6.30 (s, 3H,
mesitylene-ArH), 3.69 (s, 4H, crown-CH2), 3.60 (m, 8H,
crown-CH2), 3.54 (s, 4H, crown-CH2), 2.65 (m, 4H, crown-
(CH2)N(CH2)) 2.57 (s, 9H, mesitylene-CH3), 2.52 (m, 1H,
CH(CH3)2), 2.08 (m, 1H, CH(CH3)2), 1.29 (dd, J = 15.5, 7.3
Hz, 3H, CH(CH3)2), 1.00 (dd, J = 17.3, 6.8 Hz, 3H,
CH(CH3)2), 0.94 (dd, J = 17.3, 7.5 Hz, 3H, CH(CH3)2), 0.87
(dd, J = 18.3, 7.0 Hz, 3H, CH(CH3)2), and −15.87 (d, J = 30.2
Hz, 1H, IrH). 13C NMR (151 MHz, CD2Cl2): δ 162.15 (q,
1JBC = 49.9 Hz, B-CAr), 140.41 (CAr), 135.21 (CBAr), 129.28 (q,
2JFC = 31.4 Hz), 125.95 (CAr), 125.01 (q, 1JFC = 272.2 Hz, BAr-
CF3), 123.48 (d, J = 2.1 Hz, CAr), 117.91 (sept, J = 4.0 Hz,
2
Hz, CH(CH3)2), 16.23 (d, JPC = 5.8 Hz, CH(CH3)2), and
2
16.13 (d, JPC = 4.8 Hz, CH(CH3)2). 31P{1H} NMR (243
MHz, CD2Cl2): δ 161.43. HRMS: m/z calcd for (22+) 372.655,
found m/z 372.65183. Anal. calcd for C95H72B2F48IrLiNO5P:
C, 46.17; H, 2.94; N, 0.57. Found: C, 45.86; H, 2.31; N, 0.62.
Synthesis of [Li@(κ2-15c5NC′OPiPr)Ir(η6-naphthalene)-
H][BArF4]2 (3). In the glovebox, a 20 mL scintillation vial was
charged with 0.2304 g (0.1539 mmol) of 1, 0.3455 g (0.3164
mmol) of LiBArF4·3Et2O, and 0.4545 g (3.545 mmol, 20 equiv)
of naphthalene. The solids were dissolved in CH2Cl2 (8 mL)
and allowed to stir for 24 h, during which time the solution
changed from yellow-orange to bright yellow. The solvent was
removed under vacuum, and the resulting yellow solid was
washed thoroughly with pentane. The crude product was then
crystallized by layering pentane onto a concentrated dichloro-
methane solution and allowing the solvents to mix at −30 °C,
yielding 0.2802 g of yellow crystalline solid (3, 67%). The
C
BAr), 112.02 (CAr), 98.31 (d, J = 2.7 Hz, mesitylene-CAr),
68.02 (crown-CH2), 67.82 (crown-CH2), 67.44 (crown-CH2),
66.78 (crown-CH2), 58.72 (Ar(CH2)N), 51.29 (crown-(CH2)-
N(CH2)), 31.71 (d, 1JPC = 36.1 Hz, CH(CH3)2), 28.73 (d, 1JPC
= 48.1 Hz, CH(CH3)2), 20.00 (mesitylene-CH3), 17.69
(CH(CH3)2), 17.41 (d, J = 2.2 Hz, CH(CH3)2), 15.72 (d,
2
2JPC = 6.8 Hz, CH(CH3)2), and 15.59 (d, JPC = 6.0 Hz,
1
CH(CH3)2). 31P{1H} NMR (243 MHz, CD2Cl2): δ 159.02.
HRMS: m/z calcd for (42+ − Li+ + H+) 377.6640 found m/z
377.66448. Anal. calcd for C96H76B2F48IrLiNO5P: C, 46.36; H,
3.08; N, 0.56. Found: C, 45.74; H, 2.55; N, 0.73.
material was 85% pure by NMR spectroscopy. H NMR (600
MHz, CD2Cl2): δ 7.94 (ddd, J = 8.2, 6.2, 2.2 Hz, 1H, C10H8),
7.87 (m, 2H, C10H8), 7.78 (d, J = 8.0 Hz, 1H, C10H8), 7.76 (s,
16H, BArH), 7.59 (s, 8H, BArH), 7.51 (d, J = 7.0 Hz, 1H,
C10H8), 7.43 (d, J = 8.0 Hz, 1H, ArH) 7.02 (dd, J = 11.7, 5.8
Hz, 2H, C10H8), 6.56 (m, 3H, ArH and C10H8), 3.66 (s, 4H,
crown-CH2), 3.57 (m, 12H, crown-CH2), 2.62 (s, 4H, crown-
(CH2)N(CH2)) 2.50 (sept, J = 7.1 Hz, 1H, CH(CH3)2), 1.95
(m, 1H, CH(CH3)2), 1.10 (dd, J = 15.6, 6.9 Hz, 3H,
CH(CH3)2), 1.03 (dd, J = 21.3, 6.9 Hz, 3H, CH(CH3)2), 0.91
(dd, J = 18.9, 6.6 Hz, 3H, CH(CH3)2), −0.03 (dd, J = 17.6, 7.2
Hz, 3H, CH(CH3)2), and −20.00 (d, J = 25.1 Hz, 1H, IrH).
13C NMR (151 MHz, CD2Cl2): δ 162.17 (q, 1JBC = 49.6 Hz, B-
CAr), 142.70 (CAr), 135.41 (d, J = 9.6, C10H8), 135.22 (CBAr),
129.29 (2JFC = 31.4 Hz, CBAr-CF3), 128.45 (C10H8), 126.31
(C10H8) 125.79 (CAr), 125.01 (q, 1JFC = 272.1 Hz, BAr-CF3), δ
117.93 (sept, J = 4.0 Hz, CBAr), 112.26 (d, J = 11.7 Hz, CAr),
104.28 (d, J = 7.5 Hz, C10H8), 99.13 (d, J = 6.2 Hz, CAr), 94.53
(C10H8), 91.66 (C10H8), 67.99 (crown-CH2), 67.79 (crown-
CH2), 67.44 (crown-CH2), 66.72 (crown-CH2), 66.70 (crown-
CH2), 58.64 (Ar(CH2)N), 51.26 (d, J = 5.7 Hz, crown-
Synthesis of (κ4-MeO‑15c5NCOPiPr)Ir(H)(Cl) (5). A 250 mL
Teflon-sealed pressure vessel was charged with 0.4140 g
(0.6163 mmol) of [Ir(COD)(Cl)]2, 0.5960 g (1.264 mmol) of
(
MeO‑15c5NCOPiPr)H and 50 mL of toluene. The flask was
heated with stirring for 16 h at 90 °C, during which time the
color of the solution changed from bright red to bright yellow.
The reaction volume was reduced to ca. 20 mL under vacuum,
and the concentrated solution was layered with an excess of
pentane, giving bright yellow crystals of 5 (0.6230 g, 72%
1
yield). The material was found to be analytically pure. H
NMR (600 MHz, CD2Cl2): δ 6.51 (d, J = 8.1 Hz, 1H, ArH),
6.34 (d, J = 8.1 Hz, 1H, ArH), 4.92 (t, J = 11.0 Hz, 1H, crown-
CH2), 4.84 (m, 1H, crown-CH2), 4.39 (m, 2H, Ar(CH2)N),
4.24 (dd, J = 12.9, 11.6 Hz, 1H, crown-CH2), 4.07 (m, 2H,
crown-CH2), 3.91 (dd, J = 12.6, 10.2 Hz, 1H, crown-CH2),
3.77 (m, 3H, crown-CH2), 3.76 (s, 3H, Ar(OCH3)), 3.69 (d, J
= 8.0 Hz, 2H, crown-CH2), 3.63 (m, 4H, crown-CH2), 3.49 (d,
J = 11.2 Hz, 1H, crown-CH2), 3.45 (d, J = 10.0 Hz, 1H, crown-
CH2), 3.37 (dd, J = 11.1, 1.7 Hz, 1H, crown-CH2), 3.22 (dd, J
= 15.0, 3.1 Hz, 1H, crown-CH2), 2.91 (d, J = 14.2 Hz, 1H,
crown-CH2), 2.54 (m, 1H, CH(CH3)2), 2.37 (sept, J = 6.8 Hz,
1H, CH(CH3)2), 1.42 (dd, J = 17.0, 7.6 Hz, 3H, CH(CH3)2),
1.37 (dd, J = 13.7, 7.2 Hz, 3H, CH(CH3)2), 1.13 (dd, J = 18.8,
1
(CH2)N(CH2)), 34.87 (d, JPC = 35.3 Hz, CH(CH3)2), 28.05
1
(d, JPC = 47.6 Hz, CH(CH3)2), 17.50 (d, J = 2.3 Hz,
2
CH(CH3)2), 16.31 (d, JPC = 6.2 Hz, CH(CH3)2), 16.13
2
(CH(CH3)2), and 15.88 (d, JPC = 5.7 Hz, CH(CH3)2).
31P{1H} NMR (243 MHz, CD2Cl2): δ 162.05. HRMS: m/z
calcd for (32+ − Li+ − C10H8) 634.23, found m/z 634.22777.
13027
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
7.0 Hz, 3H, CH(CH3)2), 0.82 (dd, J = 15.4, 6.8 Hz, 3H,
CH(CH3)2), and −31.33 (d, J = 25.7 Hz, 1H, IrH). 13C{1H}
NMR (151 MHz, CD2Cl2): δ 151.35 (d, J = 4.0 Hz, CAr),
142.84 (d, J = 12.2 Hz, CAr), 141.00 (d, J = 3.6 Hz, CAr),
136.26 (d, J = 3.8 Hz, CAr), 113.59 (s, CAr), 107.49 (s, CAr),
76.42 (s, crown-CH2), 73.82 (s, crown-CH2), 73.06 (s, crown-
CH2), 72.54 (s, crown-CH2), 70.82 (d, J = 23.1 Hz, crown-
CH2), 69.79 (s, Ar(CH2)N), 69.35 (s, crown-CH2), 67.50 (d, J
= 1.8 Hz, crown-CH2), 64.86 (s, Ar-(OCH3)), 63.22 (d, J = 2.3
Hz, crown-CH2), 56.34 (s, crown-CH2), 31.89 (d, J = 34.1 Hz,
CH(CH3)2), 29.71 (d, J = 40.2 Hz, CH(CH3)2), 17.62 (s,
CH(CH3)2), 17.46 (d, J = 8.0 Hz, CH(CH3)2), 17.07 (d, J =
4.7 Hz, CH(CH3)2), and 16.50 (d, J = 3.6 Hz, CH(CH3)2).
31P{1H} NMR (202 MHz, CD2Cl2): δ 144.21. HRMS: m/z
calcd for (5 − Cl−) 664.24, found m/z 664.23566. Anal. calcd
for C24H42ClIrNO6P: C, 41.23; H, 6.05; N, 2.00. Found: C,
41.42; H, 5.63; N, 1.99.
solutions of 1 were prepared in 20 mL scintillation vials, and
increasing amounts of LiBArF4·3Et2O were added to each to
create solutions that were 7.8, 16.9, 38.3, and 78.8 mM LiBArF4·
3Et2O. Allylbenzene (100 equiv) was added to each vial via
syringe; then, the vials were sealed and allowed to stir for 16 h.
After 16 h, the reaction mixtures were analyzed by NMR
spectroscopy.
Procedure for Temperature Dependence of Con-
version to Arene Complex. In a glovebox, a 20 mL vial was
charged with 3.3 mg of 1, 11.7 mg of LiBArF4·3Et2O, and 441
μL of CD2Cl2, and the solution was transferred to a Teflon-
sealed NMR tube. Allylbenzene (29.2 μL, 100 equiv) was
added, and then the tube was sealed and heated at 35 °C for 16
h. The reaction was analyzed by NMR spectroscopy.
A separate 20 mL vial was charged with 4.4 mg of 1, 15.4 mg
of LiBArF4·3Et2O, and 587 μL of 1,2-dichloroethane-d4, and the
solution was transferred to a Teflon-sealed NMR tube.
Allylbenzene (38 μL, 100 equiv) was added; then, the tube
was sealed and heated at 80 °C for 1 h. The reaction was
analyzed by NMR spectroscopy.
Procedure for Allylbenzene Isomerization by
[Li@(κ2-15c5NC′OPiPr)Ir(η6-styrene)H][BAr4F]2 (2). In a glove-
box, a 20 mL scintillation vial was charged with 7.0 mg of 2,
520 μL of CD2Cl2, and 3.6 μL of mesitylene internal standard.
The solution was transferred to a Teflon-sealed NMR tube, 34
μL of allylbenzene was added, and the tube was sealed.
Reaction progress was monitored via NMR spectroscopy.
Procedure for Arene Exchange Experiment Using
Styrene-d8. In a glovebox, a 20 mL scintillation vial was
charged with 24.5 mg of 2 and 500 μL of CD2Cl2. The solution
was transferred to a Teflon-sealed NMR tube, 10.4 μL styrene-
d8 (10 equiv) was added, and the tube was sealed. Reaction
progress was monitored by NMR spectroscopy.
Procedure for Reaction of (6) with Styrene and
LiBArF4·3Et2O. In a glovebox, a 20 mL scintillation vial was
charged with 10.4 of 6, 11.3 mg of LiBArF4·3Et2O, and 2 mL of
CH2Cl2. Styrene (15.7 μL, 20 equiv) was added and the
solution was allowed to stir for 30 min, at which time the
solvent was removed under vacuum. The solids were washed
with pentane and then dissolved in CD2Cl2 and analyzed by 1H
NMR spectroscopy.
Synthesis of [(κ5-MeO‑15c5NCOPiPr)IrH][BArF4] (6). In a
glovebox, a 20 mL scintillation vial was charged with 0.6230 g
(0.8910 mmol) of (κ4-MeO‑15c5NCOPiPr)Ir(H)(Cl) (5) and
0.8650 g (0.9761 mmol) of NaBArF4. The solids were dissolved
in 15 mL of CH2Cl2 and allowed to stir for 3 h, during which
time the solution changed from bright yellow to orange. The
mixture was filtered over a short column of activated alumina;
then, the solvent was removed from the filtrate under vacuum,
giving 6 as a pale yellow powder (0.7610 g, 56% yield). The
1
material was 98% pure by NMR spectroscopy. H NMR (600
MHz, CD2Cl2): δ 7.77 (s, 8H, BArH), 7.62 (s, 4H, BArH),
6.56 (d, J = 8.6 Hz, 1H, ArH), 6.50 (d, J = 8.2 Hz, 1H, ArH),
4.50 (d, J = 15.2 Hz, 1H, crown-CH2), 4.34 (m, 1H, crown-
CH2), 4.25 (m, 4H, Ar(CH2)N, and crown-CH2), 4.09 (m,
2H, crown-CH2), 4.04 (m, 1H, crown-CH2), 4.02 (m, 1H,
crown-CH2), 4.00 (t, J = 2.3 Hz 1H, crown-CH2), 3.95 (dd, J =
4.1, 2.8 Hz 1H, crown-CH2), 3.92 (m, 1H, crown-CH2), 3.90
(t, J = 4.0 Hz, 1H, crown-CH2), 3.83 (m, 1H, crown-CH2),
3.81 (s, 3H, Ar(OCH3)), 3.73 (m, 2H, crown-CH2), 3.67 (t, J
= 2.5 Hz, 1H, crown-CH2), 3.64 (m, 2H, crown-CH2), 3.57
(dt, J = 11.0, 4.1 Hz, 1H, crown-CH2), 3.52 (m, 1H, crown-
CH2), 3.41 (dt, J = 13.5, 3.0 Hz, 1H, crown-CH2), 3.13 (dq, J
= 14.3, 2.8 Hz, 1H, crown-CH2), 2.49 (n, J = 7.0 Hz, 1H,
CH(CH3)), 2.42 (p, J = 7.0 Hz, 1H, CH(CH3)), 1.39 (dd, J =
14.9, 6.9 Hz, 3H, CH(CH3)), 1.28 (dd, J = 15.3, 6.9 Hz, 3H,
CH(CH3)), 1.12 (dd, J = 20.3, 7.0 Hz, 3H, CH(CH3)), 0.98
(dd, J = 16.4, 6.5 Hz, 3H, CH(CH3)), and −29.86 (d, J = 27.4
Hz, 1H, IrH). 13C{1H} NMR (151 MHz, CD2Cl2): δ 162.12
(d, 1JBC = 49.9 Hz, B-CAr), 143.24 (d, J = 11.7 Hz, CAr), 137.34
Procedure for Reaction of (6) with Mesitylene and
LiBArF4·3Et2O. In a glovebox, a 20 mL scintillation vial was
charged with 10.6 mg of 6, 11.5 mg of LiBArF4·3Et2O (1.5
equiv), and 600 μL of CD2Cl2. Mesitylene (19.3 μL, 20 equiv)
was added, and the solution was transferred to a Teflon-sealed
1
NMR tube. The reaction was analyzed by H and 31P NMR
2
(d, J = 4.3 Hz, CAr), 135.18 (CAr), 129.24 (q, JFC = 32.3 Hz,
spectroscopy after 30 min and 72 h, respectively.
C
BAr-CF3), 124.98 (q, 1JFC = 271.6 Hz, BAr-CF3), 117.88 (sept,
Procedure for Reaction of (6) with Mesitylene and
LiAl(OC(CF3)3)4. In a glovebox, a 20 mL scintillation vial was
charged with 11.8 mg of 6, 12.3 mg of LiAl(OC(CF3)3)4 (1.5
equiv), and 600 μL of CD2Cl2. Mesitylene (21.5 μL, 20 equiv)
was added, and the mixture was transferred to a Teflon-sealed
NMR tube. The reaction was analyzed by 1H NMR
spectroscopy after 30 min and 24 h, respectively.
Procedure for Catalyst Recycling Experiment. In a
glovebox, two scintillation vials were each charged with iridium
catalyst, LiAl(OC(CF3)3)4, CD2Cl2, and mesitylene internal
standard (in one vial: 4.9 mg of 1, 7.2 mg of LiAl(OC(CF3)3)4,
655 μL of CD2Cl2, 5 μL of mesitylene; in second vial: 4.8 mg
of 5, 7.0 mg of LiAl(OC(CF3)3)4, 629 μL of CD2Cl2, 5 μL of
mesitylene). Allylbenzene (100 equiv relative to iridium) was
added to each vial via syringe. The vials were sealed and
J = 3.8 Hz, CBAr), 115.77 (CAr), 109.55 (CAr), 78.53 (crown-
CH2), 75.44 (crown-CH2), 74.37 (crown-CH2), 74.05 (crown-
CH2), 71.22 (crown-CH2), 70.80 (crown-CH2), 68.97 (Ar-
(CH2)N), 67.99 (crown-CH2), 67.83 (crown-CH2), 63.16 (Ar-
(OCH3)), 60.82 (crown-CH2), 56.47 (crown-CH2), 31.18 (d,
1
1JPC = 33.1 Hz, CH(CH3)2), 29.23 (d, JPC = 41.8 Hz,
2
CH(CH3)2), 17.91 (CH(CH3)2), 16.82 (d, JPC = 8.2 Hz,
CH(CH3)2), 16.68 (CH(CH3)2), and 16.56 (CH(CH3)2).
31P{1H} NMR (243 MHz, CD2Cl2): δ 142.42. HRMS: m/z
calcd for (6+) 664.24, found m/z 664.23783. Anal. calcd for
C56H54BF24IrNO6P: C, 44.05; H, 3.56; N, 0.92. Found: C,
43.20; H, 3.21; N, 0.86.
Procedure for Li+ Dependence of Conversion to
Arene Complex. In a glovebox, four 7.5 mM CD2Cl2
13028
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
allowed to stir for 20 min, and then the solutions were
transferred to Teflon-sealed NMR tubes and 1H NMR spectra
were collected. The solutions were transferred back to
scintillation vials, a second aliquot of allylbenzene was added
(100 equiv), and the reactions were allowed to stir for 40 min.
The solutions were then transferred to Teflon-sealed NMR
tubes and 1H NMR spectra were collected. The reactions were
left in Teflon-sealed NMR tubes until both had reached
completion, and then they were transferred back to
scintillation vials and charged with a third aliquot of
allylbenzene (100 equiv). After stirring for 80 min, the
solutions were transferred to Teflon-sealed NMR tubes and 1H
NMR spectra were collected.
27599-3290, United States; orcid.org/0000-0003-0150-
9557
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c03784
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon the work supported by the
National Science Foundation under CAREER Award CHE-
1553802. Mass spectrometry studies were supported by the
National Science Foundation under Grant no. CHE-1726291.
Procedure for Allylbenzene Comparison at Low
Catalyst Loadings. In a glovebox, two 5 mM catalyst stock
solutions were prepared in scintillation vials (catalyst 1: 5.4 mg
in 360 μL of 1,2-DCE-d4; catalyst 6: 5.2 mg in 340 μL of 1,2-
DCE-d4). Two one dram (4 mL) vials were then charged with
2.4 mg of LiAl(OC(CF3)3)4 (5 equiv relative to iridium) each.
To each one dram vial, 100 μL of catalyst stock solution and
400 μL of CD2Cl2 were added. The mixtures were transferred
to Teflon-sealed NMR tubes and 33 μL of allylbenzene (500
equiv relative to iridium) was added to each. The tubes were
REFERENCES
■
(1) Miller, A. J. M. Controlling Ligand Binding for Tunable and
Switchable Catalysis: Cation-Modulated Hemilability in Pincer-
Crown Ether Ligands. Dalton Trans. 2017, 46, 11987−12000.
(2) Blanco, V.; Leigh, D. A.; Marcos, V. Artificial Switchable
Catalysts. Chem. Soc. Rev. 2015, 44, 5341−5370.
(3) Traut, T. Allosteric Regulatory Enzymes; Springer US: Boston,
MA, 2008.
̈
(4) Kovbasyuk, L.; Kramer, R. Allosteric Supramolecular Receptors
1
sealed and heated at 70 °C for 9 h before H NMR spectra
and Catalysts. Chem. Rev. 2004, 104, 3161−3188.
(5) Wiester, M. J.; Ulmann, P. A.; Mirkin, C. A. Enzyme Mimics
Based Upon Supramolecular Coordination Chemistry. Angew. Chem.,
Int. Ed. 2011, 50, 114−137.
were collected.
Procedure for 4-Phenyl-1-Butene Isomerization at
Low Catalyst Loadings. In a glovebox, two 5 mM catalyst
stock solutions were prepared in scintillation vials (catalyst 1:
6.1 mg in 815 μL of CD2Cl2; catalyst 6: 6.2 mg in 815 μL of
CD2Cl2). Two one dram (4 mL) vials were then charged with
2.4 mg of LiAl(OC(CF3)3)4 (5 equiv relative to iridium) each.
To each one dram vial, 100 μL of catalyst stock solution, 400
μL of CD2Cl2, and 10 μL of mesitylene internal standard were
added. The mixtures were transferred to Teflon-sealed NMR
tubes and 37.5 μL of 4-phenyl-1-butene (500 equiv relative to
iridium) was added to each. The tubes were sealed and heated
(6) Kremer, C.; Lutzen, A. Artificial Allosteric Receptors. Chem. −
̈
Eur. J. 2013, 19, 6162−6196.
(7) Ma, T.; Peng, Y.; Huang, W.; Ding, J. Molecular Mechanism of
the Allosteric Regulation of the Aγ Heterodimer of Human NAD-
Dependent Isocitrate Dehydrogenase. Sci. Rep. 2017, 7, No. 40921.
(8) Oliveri, C. G.; Ulmann, P. A.; Wiester, M. J.; Mirkin, C. A.
Heteroligated Supramolecular Coordination Complexes Formed via
the Halide-Induced Ligand Rearrangement Reaction. Acc. Chem. Res.
2008, 41, 1618−1629.
(9) Raynal, M.; Ballester, P.; Vidal-Ferran, A.; van Leeuwen, P. W.
N. M. Supramolecular Catalysis. Part 1: Non-Covalent Interactions as
a Tool for Building and Modifying Homogeneous Catalysts. Chem.
Soc. Rev. 2014, 43, 1660−1733.
1
at 40 °C for 5 h before H NMR spectra were collected.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c03784.
■
(10) Raynal, M.; Ballester, P.; Vidal-Ferran, A.; van Leeuwen, P. W.
N. M. Supramolecular Catalysis. Part 2: Artificial Enzyme Mimics.
Chem. Soc. Rev. 2014, 43, 1734−1787.
sı
(11) McConnell, A. J.; Wood, C. S.; Neelakandan, P. P.; Nitschke, J.
R. Stimuli-Responsive Metal−Ligand Assemblies. Chem. Rev. 2015,
115, 7729−7793.
NMR spectra and crystallographic details (PDF)
Crystallographic data (CIF)
Crystallographic data (CIF)
́
(12) Vlatkovic, M.; Collins, B. S. L.; Feringa, B. L. Dynamic
Responsive Systems for Catalytic Function. Chem. − Eur. J. 2016, 22,
17080−17111.
(13) Lifschitz, A. M.; Rosen, M. S.; McGuirk, C. M.; Mirkin, C. A.
Allosteric Supramolecular Coordination Constructs. J. Am. Chem. Soc.
2015, 137, 7252−7261.
AUTHOR INFORMATION
Corresponding Author
■
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; Email: ajmm@email.unc.edu
(14) Yoon, H. J.; Kuwabara, J.; Kim, J.-H.; Mirkin, C. A. Allosteric
Supramolecular Triple-Layer Catalysts. Science 2010, 330, 66−69.
(15) Julliard, M.; Chanon, M. Photoelectron-Transfer Catalysis: Its
Connections with Thermal and Electrochemical Analogs. Chem. Rev.
1983, 83, 425−506.
(16) Neilson, B. M.; Bielawski, C. W. Photoswitchable Metal-
Mediated Catalysis: Remotely Tuned Alkene and Alkyne Hydro-
borations. Organometallics 2013, 32, 3121−3128.
Authors
Henry M. Dodge − Department of Chemistry, University of
North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290, United States
Matthew R. Kita − Department of Chemistry, University of
North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290, United States
(17) Broderick, E. M.; Guo, N.; Vogel, C. S.; Xu, C.; Sutter, J.;
Miller, J. T.; Meyer, K.; Mehrkhodavandi, P.; Diaconescu, P. L. Redox
Control of a Ring-Opening Polymerization Catalyst. J. Am. Chem. Soc.
2011, 133, 9278−9281.
(18) Allgeier, A. M.; Mirkin, C. A. Ligand Design for Electrochemi-
cally Controlling Stoichiometric and Catalytic Reactivity of Transition
Metals. Angew. Chem., Int. Ed. 1998, 37, 894−908.
Chun-Hsing Chen − Department of Chemistry, University of
North Carolina at Chapel Hill, Chapel Hill, North Carolina
13029
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030

ACS Catalysis
pubs.acs.org/acscatalysis
Research Article
(19) Lorkovic, I. M.; Duff, R. R.; Wrighton, M. S. Use of the Redox-
Active Ligand 1,1′-Bis(Diphenylphosphino)Cobaltocene To Rever-
sibly Alter the Rate of the Rhodium(I)-Catalyzed Reduction and
Isomerization of Ketones and Alkenes. J. Am. Chem. Soc. 1995, 117,
3617−3618.
(20) Gregson, C. K. A.; Gibson, V. C.; Long, N. J.; Marshall, E. L.;
Oxford, P. J.; White, A. J. P. Redox Control within Single-Site
Polymerization Catalysts. J. Am. Chem. Soc. 2006, 128, 7410−7411.
(21) Azoulay, J. D.; Koretz, Z. A.; Wu, G.; Bazan, G. C. Well-
Defined Cationic Methallyl α-Keto-β-Diimine Complexes of Nickel.
Angew. Chem., Int. Ed. 2010, 49, 7890−7894.
Iridium(III) Complexes Containing a 4,6-Dimethyl-1,3-Benzenedi-
phenylimine Pincer Ligand. Inorg. Chem. 2013, 52, 515−526.
(38) Maitlis, P. M. (Pentamethylcyclopentadienyl)Rhodium and
-Iridium Complexes: Approaches to New Types of Homogeneous
Catalysts. Acc. Chem. Res. 1978, 11, 301−307.
(39) Goldman, A. S.; Landis, C. R.; Sen, A. Jack Halpern (1925-
2018). Angew. Chem., Int. Ed. 2018, 57, 4460.
(40) Gregor, L. C.; Grajeda, J.; Kita, M. R.; White, P. S.; Vetter, A. J.;
Miller, A. J. M. Modulating the Elementary Steps of Methanol
Carbonylation by Bridging the Primary and Secondary Coordination
Spheres. Organometallics 2016, 35, 3074−3086.
(41) Grotjahn, D. B.; Larsen, C. R.; Gustafson, J. L.; Nair, R.;
Sharma, A. Extensive Isomerization of Alkenes Using a Bifunctional
Catalyst: An Alkene Zipper. J. Am. Chem. Soc. 2007, 129, 9592−9593.
(42) Larsen, C. R.; Erdogan, G.; Grotjahn, D. B. General Catalyst
Control of the Monoisomerization of 1-Alkenes to Trans -2-Alkenes.
J. Am. Chem. Soc. 2014, 136, 1226−1229.
(43) Hassam, M.; Taher, A.; Arnott, G. E.; Green, I. R.; van Otterlo,
W. A. L. Isomerization of Allylbenzenes. Chem. Rev. 2015, 115, 5462−
5569.
(22) Liberman-Martin, A. L.; Bergman, R. G.; Tilley, T. D. A
Remote Lewis Acid Trigger Dramatically Accelerates Biaryl Reductive
Elimination from a Platinum Complex. J. Am. Chem. Soc. 2013, 135,
9612−9615.
(23) Liberman-Martin, A. L.; Levine, D. S.; Liu, W.; Bergman, R. G.;
Tilley, T. D. Biaryl Reductive Elimination Is Dramatically Accelerated
by Remote Lewis Acid Binding to a 2,2′-Bipyrimidyl−Platinum
Complex: Evidence for a Bidentate Ligand Dissociation Mechanism.
Organometallics 2016, 35, 1064−1069.
(44) Larsen, C.; Paulson, E.; Erdogan, G.; Grotjahn, D. A Facile,
Convenient, and Green Route to (E)-Propenylbenzene Flavors and
Fragrances by Alkene Isomerization. Synlett 2015, 26, 2462−2466.
(24) Bader, A.; Lindner, E. Coordination Chemistry and Catalysis
with Hemilabile Oxygen-Phosphorus Ligands. Coord. Chem. Rev.
1991, 108, 27−110.
́
́
́
(45) Albeniz, A. C.; Espinet, P.; Lopez-Fernandez, R.; Sen, A. A
Warning on the Use of Radical Traps as a Test for Radical
Mechanisms: They React with Palladium Hydrido Complexes. J. Am.
Chem. Soc. 2002, 124, 11278−11279.
(25) Angell, S. E.; Rogers, C. W.; Zhang, Y.; Wolf, M. O.; Jones, W.
E. Hemilabile Coordination Complexes for Sensing Applications.
Coord. Chem. Rev. 2006, 250, 1829−1841.
(26) Braunstein, P.; Naud, F. Hemilability of Hybrid Ligands and
the Coordination Chemistry of Oxazoline-Based Systems. Angew.
Chem., Int. Ed. 2001, 40, 680−699.
(46) Biswas, S.; Huang, Z.; Choliy, Y.; Wang, D. Y.; Brookhart, M.;
Krogh-Jespersen, K.; Goldman, A. S. Olefin Isomerization by Iridium
Pincer Catalysts. Experimental Evidence for an η 3 -Allyl Pathway and
an Unconventional Mechanism Predicted by DFT Calculations. J. Am.
Chem. Soc. 2012, 134, 13276−13295.
(47) Chianese, A. R.; Shaner, S. E.; Tendler, J. A.; Pudalov, D. M.;
Shopov, D. Y.; Kim, D.; Rogers, S. L.; Mo, A. Iridium Complexes of
Bulky CCC-Pincer N-Heterocyclic Carbene Ligands: Steric Control
of Coordination Number and Catalytic Alkene Isomerization.
Organometallics 2012, 31, 7359−7367.
(48) Cooper, A. C.; Caulton, K. G. Identification of an Elusive
Catalyst: IrH(η2-C6H4PtBu2)(Cl)(PtBu2Ph) as a Precursor for C=C
Bond Migration. Inorg. Chim. Acta 1996, 251, 41−51.
(49) Kita, M. R.; Miller, A. J. M. Cation-Modulated Reactivity of
Iridium Hydride Pincer-Crown Ether Complexes. J. Am. Chem. Soc.
2014, 136, 14519−14529.
(50) Carreras, L.; Rovira, L.; Vaquero, M.; Mon, I.; Martin, E.;
Benet-Buchholz, J.; Vidal-Ferran, A. Syntheses, Characterisation and
Solid-State Study of Alkali and Ammonium BArF Salts. RSC Adv.
2017, 7, 32833−32841.
(27) Yoo, C.; Dodge, H. M.; Miller, A. J. M. Cation-Controlled
Catalysis with Crown Ether-Containing Transition Metal Complexes.
Chem. Commun. 2019, 55, 5047−5059.
(28) Smith, J. B.; Kerr, S. H.; White, P. S.; Miller, A. J. M.
Thermodynamic Studies of Cation−Macrocycle Interactions in
Nickel Pincer-Crown Ether Complexes Enable Switchable Ligation.
Organometallics 2017, 36, 3094−3103.
(29) Kita, M. R.; Miller, A. J. M. An Ion-Responsive Pincer-Crown
Ether Catalyst System for Rapid and Switchable Olefin Isomerization.
Angew. Chem., Int. Ed. 2017, 56, 5498−5502.
(30) Ivanova, S. M.; Nolan, B. G.; Kobayashi, Y.; Miller, S. M.;
Anderson, O. P.; Strauss, S. H. Relative Lewis Basicities of Six
−
Al(ORF)4 Superweak Anions and the Structures of LiAl{OCH-
(CF3)2}4 and [1-Et-3-Me-1,3-C3H3N2][Li{Al{OCH(CF3)2}4}2].
Chem. − Eur. J. 2001, 7, 503−510.
̈
(31) Olgemoller, B.; Beck, W. A Cationic Ethylene(Hydrio)Iridium-
(III) Complex. Angew. Chem., Int. Ed. 1980, 19, 834.
(51) Krossing, I.; Reisinger, A. Chemistry with Weakly-Coordinating
Fluorinated Alkoxyaluminate Anions: Gas Phase Cations in
Condensed Phases? Coord. Chem. Rev. 2006, 250, 2721−2744.
(52) Martínez-Martínez, A. J.; Weller, A. S. Solvent-Free Anhydrous
(32) Turlington, C. R.; Harrison, D. P.; White, P. S.; Brookhart, M.;
Templeton, J. L. Probing the Oxidation Chemistry of Half-Sandwich
Iridium Complexes with Oxygen Atom Transfer Reagents. Inorg.
Chem. 2013, 52, 11351−11360.
(33) Jeong, H.; Joo, K. S.; Chin, C. S. Iridium(III) Complexes of
H6-Arenes with Olefinic and Cyclopropyl Substituents: Facile
Conversion to η3-Phenylallyl Complexes. Bull. Korean Chem. Soc.
1997, 18, 402−405.
(34) Camp, A. M.; Kita, M. R.; Grajeda, J.; White, P. S.; Dickie, D.
A.; Miller, A. J. M. Mapping the Binding Modes of Hemilabile Pincer-
Crown Ether Ligands in Solution Using Diamagnetic Anisotropic
Effects on NMR Chemical Shift. Inorg. Chem. 2017, 56, 11141−
11150.
F
Li+, Na+ and K+ Salts of [B(3,5-(CF3)2C6H3)4]−, [BAr4 ]−. Improved
Synthesis and Solid-State Structures. Dalton Trans. 2019, 48, 3551−
3554.
(53) Yakelis, N. A.; Bergman, R. G. Safe Preparation and Purification
of Sodium Tetrakis[(3,5-Trifluoromethyl)Phenyl]Borate (NaBAr24F):
Reliable and Sensitive Analysis of Water in Solutions of Fluorinated
Tetraarylborates. Organometallics 2005, 24, 3579−3581.
(35) Grajeda, J.; Kita, M. R.; Gregor, L. C.; White, P. S.; Miller, A. J.
M. Diverse Cation-Promoted Reactivity of Iridium Carbonyl Pincer-
Crown Ether Complexes. Organometallics 2016, 35, 306−316.
(36) Van der Zeijden, A. A. H.; Van Koten, G.; Luijk, R.; Grove, D.
M. Formation of Octahedral Iridium(III) Dihydrides from the
Reaction of Ortho-Chelated Aryliridium(I) Compounds with
Dihydrogen. Organometallics 1988, 7, 1556−1561.
(37) Wingard, L. A.; Finniss, M. C.; Norris, M.; White, P. S.;
Brookhart, M.; Templeton, J. L. Synthesis, Structure, and Reactivity of
13030
https://dx.doi.org/10.1021/acscatal.0c03784
ACS Catal. 2020, 10, 13019−13030