Oxygen atom transfer to a half-sandwich iridium complex: Clean oxidation yielding a molecular product

Article abstract of DOI:10.1021/ja413023f

The oxidation of [Ir(Cp)(phpy)(NCAr^F)][B(Ar^F) ₄] (1; Cp* = η⁵-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC^1′-pyridine-κN, NCAr ^F = 3,5-bis(trifluoromethyl)benzonitrile, B(Ar^F

Full text of DOI:10.1021/ja413023f

Article
pubs.acs.org/JACS
Oxygen Atom Transfer to a Half-Sandwich Iridium Complex: Clean
Oxidation Yielding a Molecular Product
Christopher R. Turlington, Peter S. White, Maurice Brookhart, and Joseph L. Templeton*
W. R. Kenan Laboratory, Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290, United States
S
* Supporting Information
ABSTRACT: The oxidation of [Ir(Cp*)(phpy)(NCArF)][B(ArF)4] (1; Cp* = η5-pentamethylcyclopentadienyl, phpy = 2-
phenylene-κC1′-pyridine-κN, NCArF = 3,5-bis(trifluoromethyl)benzonitrile, B(ArF)4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]-
borate) with the oxygen atom transfer (OAT) reagent 2-tert-butylsulfonyliodosobenzene (sPhIO) yielded a single, molecular
product at −40 °C. New Ir(Cp*) complexes with bidentate ligands derived by oxidation of phpy were synthesized to model
possible products resulting from oxygen atom insertion into the iridium−carbon and/or iridium−nitrogen bonds of phpy. These
new ligands were either cleaved from iridium by water or formed unreactive, phenoxide-bridged iridium dimers. The reactivity of
these molecules suggested possible decomposition pathways of Ir(Cp*)-based water oxidation catalysts with bidentate ligands
that are susceptible to oxidation. Monitoring the [Ir(Cp*)(phpy)(NCArF)]+ oxidation reaction by low-temperature NMR
techniques revealed that the reaction involved two separate OAT events. An intermediate was detected, synthesized
independently with trapping ligands, and characterized. The first oxidation step involves direct attack of the sPhIO oxidant on the
carbon of the coordinated nitrile ligand. Oxygen atom transfer to carbon, followed by insertion into the iridium−carbon bond of
phpy, formed a coordinated organic amide. A second oxygen atom transfer generated an unidentified iridium species (the
“oxidized complex”). In the presence of triphenylphosphine, the “oxidized complex” proved capable of transferring one oxygen
atom to phosphine, generating phosphine oxide and forming an Ir−PPh3 adduct in 92% yield. The final Ir−PPh3 product was
fully characterized.
INTRODUCTION
■
Isolated terminal oxo complexes of the late transition metals
(groups 9−11) are rare.1 Despite being invoked as key
intermediates in biological2 and industrial3 processes, only
two mononuclear examples exist: a PtIV−oxo complex
supported by a tridentate P−C−N pincer ligand4,5 and a
(mesityl)3IrV−oxo compound (Figure 1).6,7 A bridging oxo
ligand in a dinuclear iridium species also exhibits an unusually
short iridium−oxygen bond distance (1.858 Å) and may be
considered a third example of a late-transition-metal−oxo
complex.8 Notably, reports of oxo complexes of gold,9
platinum,10 and palladium,11 stabilized by polyoxometalate
Figure 1. (a) Late-transition-metal−oxo complexes. (b) An asym-
metric bridging oxo ligand with an unusually short iridium−oxygen
ligands, have recently been retracted due to misidentification of
bond length.
the metal in the metal−oxo bond.12
Iridium complexes that serve as precursors for water
harsh oxidizing conditions required for water oxidation.22−25
Reports implicate degradation to iridium-based nanoparticles
oxidation catalysts have been intensely studied,13−19 and IrV−
oxo complexes were originally invoked as reactive intermedi-
ates.20,21 However, these iridium catalysts, often possessing a
Cp* ligand and a κ2 chelating ligand in the coordination sphere,
Received: December 21, 2013
have been plagued by oxidative ligand degradation under the
Published: February 26, 2014
© 2014 American Chemical Society
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Scheme 1. Low-Temperature Oxidation of 1 with sPhIO
(IrOx·nH2O is a known water oxidation catalyst)26,27 when
using either ceric ammonium nitrate or electrochemical
methods for oxidation.28,29 Systems employing sodium period-
ate, a less harsh oxidant, have achieved molecular water
oxidation30 and C−H activation,31−33 yet even in this system
Cp* ligand oxidation and loss is observed.34,35 For molecular
catalysts using sodium periodate as oxidant, nucleophilic attack
on a terminal iridium−oxo moiety has been suggested as an
explanation for the observed catalyst activity.36−38 Despite
much effort, no experimental evidence for Cp*IrV−oxo
complexes has been reported.
1
Figure 2. Expansion of the aromatic region of the H NMR spectrum
of the OC at −40 °C. When the OC is formed from oxidation of 1
with excess sPhIO, 2 equiv of sPhIO is generated relative to the OC.
Recently we reported low-temperature oxidations of [Ir-
(Cp*)(phpy)(NCArF)][B(ArF)4] (1; Cp* = η5-pentamethyl-
cyclopentadienyl, phpy = 2-phenylene-κC1′-pyridine-κN,
NCArF = 3,5-bis(trifluoromethyl)benzonitrile, B(ArF)4
=
oxidation, the Cp* methyl resonance (singlet, 15H) shifted
dramatically upfield from 1.68 to 1.24 ppm, and one of the phpy
proton resonances shifted at least 0.5 ppm upfield to 6.48 ppm
(no phpy proton resonance appeared upfield of 7.11 ppm in 1).
Importantly, the NCArF moiety was still present in the
coordination sphere (the H chemical shifts observed for the
nitrile ligand did not correspond to those of the free ligand). At
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), a complex re-
lated to numerous water oxidation catalysts, with the oxygen
atom transfer (OAT) reagents iodosobenzene (PhIO) and
dimethyldioxirane (DMDO).39 Oxidation with PhIO and
DMDO resulted in decomposition of 1, rather than oxidation
of the metal to form an IrV−oxo moiety.
1
1
In an extension of this work, we discovered that the OAT
−70 °C, the H NMR spectrum of the OC decoalesced into
two isomers (see the Supporting Information).43 Isomers could
result from restricted rotation of the aryl−pyridine bond of a
new κ2 ligand, arising from structural modification of the phpy
ligand upon oxidation. Samples of the OC were unstable above
−20 °C, at which temperature the oxidized complex began
decomposing and reacting with excess sPhIO. Oxidation at
temperatures higher than −20 °C led to intractable materials;
the resulting NMR spectra were complicated and suggested
ligand oxidation and degradation.28 Despite utilizing low-
temperature column chromatography and low-temperature
crystallization techniques, our efforts to isolate the OC proved
unsuccessful.
reagent 2-tert-butylsulfonyliodosobenzene (sPhIO),40−42
a
more soluble analogue of PhIO, reacted cleanly with 1 at low
temperatures. A single product was generated, and Cp*
degradation was not implicated. Reaction with HCl led to an
Ir(III) product reflecting insertion into the Ir−C bond of the phpy
ligand. Insertion follows oxidation of an electron-deficient nitrile
ligand, and an intermediate has been identified and prepared
independently. Although the final low-temperature metal-based
oxidation product could not be isolated, introducing triphenyl-
phosphine yielded triphenylphosphine oxide and an Ir−PPh3
adduct in 92% yield. Experimental evidence implicating a high-
valent IrV−oxo monomer remains elusive.
When the oxidation of 1 was conducted in the presence of
exogenous NCArF, an intermediate could be detected (Scheme 2).
RESULTS AND DISCUSSION
■
Low-Temperature Oxidation with the Soluble
Iodosoaryl Reagent 2-tert-Butylsulfonyliodosobenzene
(sPhIO). When complex 1 was treated with 4 equiv of the OAT
reagent sPhIO at −40 °C in dichloromethane-d2, 91%
conversion (based on an internal standard) to a new,
temperature-sensitive iridium species was observed by 1H
NMR spectroscopy (Scheme 1). Two equivalents of 2-tert-
butylsulfonyliodobenzene (sPhI, the reduced oxidant) was also
generated, indicating two oxygen atom transfers per iridium.
The reaction did not reach full conversion unless excess oxidant
was used, presumably due to the limited solubility of iodonium
ylides40 and slow second-order kinetics as the reagent
concentrations dwindled.
Scheme 2. Observation of an Intermediate (Int) in the
Presence of Excess Nitrile during Formation of the OC
The intermediate (Int) had chemical shifts in the aromatic region
similar to those of the OC, suggesting that modification of the
phpy ligand had occurred in a previous step. The Int cleanly
converted to the OC over time at −40 °C in the presence of
sPhIO. Presumably, added NCArF trapped an iridium species with
a vacant coordination site, which allowed observation of the
transient Int. In the absence of trapping ligand, the unsaturated
iridium species reacted too quickly to be observed by low
-temperature NMR spectroscopy.
The reactive oxidized complex (OC) could only be
characterized by low-temperature NMR spectroscopy, and
this complex displayed 1H NMR resonances for Cp* and phpy
that were clearly distinct from those of 1 (Figure 2). Upon
Complexes Representing Oxygen Atom Insertion
Products. When attempts to isolate the OC failed, the
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Scheme 3. Synthesis of Proligands 2 and 3
synthesis of potential oxidation products was pursued. Inspired
by reports of olefin insertion into the iridium−carbon bond of
[Ir(Cp*)(phpy)(olefin)]+ complexes,44 we hypothesized that
oxygen atom insertion into either the iridium−carbon or the
iridium−nitrogen bond of phpy was possible. To investigate
this possibility, we independently prepared complexes exhibit-
ing such structural features.
A proligand that could serve as a model for oxygen atom insertion
into the iridium−carbon bond of phpy, 2-(2-hydroxyphenyl)-
pyridine (2), has been reported.45 A different synthetic route to 2
was employed to also allow access to a phpy-derived ligand
representing oxygen atom insertion into both the iridium−carbon
and iridium−nitrogen bonds. The proligand 2-(2-hydroxyphenyl)-
pyridine N-oxide (3) was synthesized in one step using a modified
coupling procedure (Scheme 3).46,47 Acceptable yields were achieved
when the reaction solution was heated above the boiling point of the
solvent (toluene) in a closed reaction vessel. Employing lower
temperatures resulted in lower yields and only partial conversion of
the 2-bromophenol substrate. The yield for this reaction was low in
comparison to other coupling reactions reported for pyridine
N-oxides and bromoarenes.46 This is likely due to the properties of
the 2-bromophenol reagent, which is sterically demanding and can
also bind to the metal as a phenoxide ligand.48 Reduction of 3 to the
known compound 2 required conditions more forcing than expected
for reduction of N-oxides.46 Full conversion could be achieved when
3 was heated at reflux for 48 h in the presence of ammonium
formate and Pd/C. It may be that the N-oxide of 3 is stabilized by
hydrogen bonding to the phenolic hydrogen, which lowers its
reactivity.
Figure 3. ORTEP drawing of 4, with partial numbering scheme (50%
probability thermal ellipsoids). Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å), bond angles (deg), and torsion angles
(deg): Ir1−O1 2.082(2), Ir1−N1 2.100(3), Ir1−Cl1 2.4070(8), C11−O1
1.325(4); O1−Ir1−Cl1 85.81(7), N1−Ir1−O1 82.74(10), Cl1−Ir1−N1
86.28(7), Ir1−O1−C11 114.34(19); C11−C6−C5−N1 27.9(5).
Metalation of 3 was more challenging because of the poor
solubility of the pyridine N-oxide.46 It was necessary to use at
least 1 mL of THF for every 2 mg of proligand to ensure
complete dissolution of the sodium salt of 3. When this ratio of
THF to 3 was used, deprotonation of the phenol by NaH and
cannula filtration onto [Ir(Cp*)Cl2]2 yielded Ir(Cp*)(2-(2-
pyridyl)phenoxide N-oxide)Cl (5; Scheme 5). The [Ir(Cp*)Cl2]2
Metalation of 2 was accomplished following deprotonation of
the proligand by an insoluble base (NaH) in THF. This
solution was filtered by cannula directly into a vessel with
[Ir(Cp*)Cl2]2 (Scheme 4). The sodium ion from the salt of
Scheme 4. Metalation of 2, Yielding Complex 4
Scheme 5. Metalation of 3, Yielding Complex 5
dimer had to be carefully dried by heating to 50 °C under high
vacuum overnight. This prevented protonation of the sodium salt
of 3 prior to reaction with the metal. In addition, the product
displayed extreme sensitivity to moisture. Product instability will
be discussed later.
The growth of crystals of 5 was accomplished by preparing a
saturated solution of the complex in pentane and storing in a
glovebox freezer at −25 °C. Structural characterization of 5
represents a rare example of an iridium complex with a bound
N-oxide (Figure 4).52,53 The iridium−oxygen bond length of
the coordinated N-oxide is 2.111 Å, which is comparable to the
rhodium−oxygen bond length of a coordinated pyridine N-oxide
(2.080 Å) in a [Rh(cycloocta-1,5-diene)(μ-pyridinyl N-oxide)]2
deprotonated 2 abstracted 1 equiv of chloride from the iridium,
thus allowing κ2 coordination of the ligand. The iridium
product, Ir(Cp*)(2-(2-pyridyl)phenoxide)Cl (4), was stable to
both moisture and air and could be conveniently purified by
chromatography on silica.
The X-ray structure of 4 was obtained and is shown in Figure 3.
The aromatic rings of the bound chelating ligand 2 are canted by
27.9°. The constraint imposed on the conjugated aryl system is the
consequence of forming a six-membered metallacycle. Both the
iridium−oxygen bond distance (2.082 Å) and the iridium−
nitrogen bond distance (2.100 Å) were within expected bonding
distances for iridium complexes.49−51
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Figure 5. ORTEP drawing of 6, with partial numbering scheme (50%
probability thermal ellipsoids). The anions and hydrogen atoms are
omitted for clarity. Selected bond lengths (Å), bond angles (deg), and
torsion angles (deg): Ir1−O1 2.1294(17), Ir1−O1′ 2.1373(17), Ir1−N1
2.099(2), C1−O1 1.361(3); O1−Ir1−O1′ 72.49(7), O1′−Ir1−N1
80.12(7), N1−Ir1−O1 80.96(7), Ir1−O1−Ir1′ 107.51(7); C1−C6−
C7−N1 31.4(4).
Figure 4. ORTEP drawing of 5, with partial numbering scheme (50%
probability thermal ellipsoids). Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å), bond angles (deg), and torsion angles
(deg): Ir1−O1 2.101(2), Ir1−O2 2.111(2), Ir1−Cl1 2.4130(7), C1−O1
1.338(4), O2−N1 1.350(3); O1−Ir1−Cl1 83.59(6), O2−Ir1−O1
82.44(8), Cl1−Ir1−O2 91.67(6), Ir1−O1−C1 119.64(19), Ir1−O2−N1
123.36(17); C1−C6−C7−N1 51.3(4).
insertion into the iridium−carbon bond of a bidentate ligand
is possible, formation of 6 could represent an example of a
deactivation pathway implicating a bidentate ligand in an
Ir(Cp*) water oxidation catalyst. Many studies citing catalyst
decomposition propose Cp* degradation pathways,22−25,34,35
but the role of the bidentate ligand is not clear. Complexes that
are known to be molecular catalysts when driven by the
sacrificial oxidant NaIO4 include bidentate ligands with pyridine
ligands or propanolate ligands, which are less likely to facilitate
oxygen atom insertion than an iridium−carbon bond.30,34
As mentioned earlier, 5 is sensitive to water. When 2.0 μL of
water (0.11 mmol) was added to a solution of 5 (6.5 mg,
0.012 mmol) in dry dichloromethane-d2, the 1H NMR
resonances of 5 disappeared after 1 min of vigorous shaking.
One product of decomposition was the protonated proligand 3,
which was identified in the 1H NMR spectrum (99%
conversion, on the basis of an internal standard). The sensitivity
of 5 to water may arise from the weak bonding interaction
between the hard pyridine N-oxide donor ligand and iridium.
Substitution of water for the pyridine N-oxide, therefore, would
result in κ1 coordination of the 2-(2-pyridyl)phenoxide N-oxide
ligand (Scheme 7). Protonation of the phenoxide-bound ligand
3 is attractive as the second step leading toward cleavage of 3
from iridium. Water molecules coordinated to metals are acidic
(pKa < 10) and could serve as the proton donor.56 Loss of the
oxidized ligand 3 demonstrates that a modified bidentate ligand
may be cleaved from Ir(Cp*) complexes. The final identity of
the iridium product formed from 5 and water has not been
determined, but it may be an [Ir(Cp*)(OH)Cl]n species.
Formation of a water-sensitive seven-membered metallacycle by
oxygen atom insertion into both the iridium−carbon and the
iridium−nitrogen bonds is an improbable decomposition
pathway for molecular iridium species involving bidentate
ligands, since oxygen atom insertion into an iridium−nitrogen
bond is unprecedented. None of the independently synthesized
iridium complexes prepared with oxidized phpy ligands were
dimer (the analogous distances in the two reported square-
planar Ir(I) complexes were 2.031 and 2.030 Å).54 The
octahedral geometry of 5 and the seven-membered metallacycle
with a coordinated N-oxide are unique for iridium. The seven-
membered metallacycle forces the two aryl rings to deviate
from planarity by a remarkable 51.3°, a phenomenon that has
also been reported in metal complexes with the isoelectronic κ2
chelating ligand 2,2′-bipyridine N,N′-dioxide.55 The Ir−OPh
bond distance is 2.101 Å, which is comparable to the analogous
distance in 4.
Reactivity of Complexes with Oxidized phpy Ligands
and Implications for Catalyst Degradation. The neutral
iridium chloride complexes 4 and 5 were distinct from the OC,
because the OC included a nitrile ligand. Replacement of the
chloride ligand by NCArF in complex 4 was attempted by
combining Na[B(ArF)4] and 4 in the presence of NCArF. A
new iridium complex was formed after 24 h, but surprisingly,
the nitrile ligand was not coordinated. X-ray diffraction analysis
on a suitable crystal revealed the dinuclear iridium product
[Ir(Cp*)(2-(2-pyridyl)-μ-phenoxide)]2[B(ArF)4]2 (6), with
bridging phenoxide ligands (Figure 5). The structure of 6
showed an elongated carbon−oxygen bond of 1.361 Å (for
comparison, the same bond in 4 is 1.325 Å). The Ir2O2 core
was roughly planar, and the iridium nuclei do not interact, as
they are separated by 3.441 Å. Remarkably, the Cp* methyl
1
protons resonated at 0.59 ppm in the H NMR spectrum,
which is an upfield shift of greater than 0.7 ppm from the
starting material 4. This shift is likely accounted for by the Cp*
methyl groups being positioned within the shielding region of
the arylpyridine ligands. In the absence of the benzonitrile
ligand, 6 was formed in several hours. Dimer 6 was not sensitive
to air or moisture, and a 21 h exposure to either carbon
monoxide or ethylene failed to cleave the dimer into
mononuclear components (Scheme 6). If oxygen atom
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Scheme 6. Formation of 6 and Reaction with Trapping Ligands
Scheme 7. Proposed Reaction of 5 with Water
1
identical with the OC, as assessed by H NMR spectroscopy.
However, their properties and behavior yielded insights into
potential decomposition pathways involving phpy ligands in
water oxidation catalysts.
Reaction of the Oxidized Complex with HCl. When
independent synthesis failed to illuminate the identity of the
OC, efforts were made to cleave the modified phpy ligand from
the OC and characterize it. Accordingly, HCl(g) was purged
through a freshly prepared sample of the OC in dichloro-
methane-d2 at −40 °C. This led to a new iridium product with
the Cp*, nitrile, and phenylpyridine ligand components within
the coordination sphere (Scheme 8). In addition, HCl reacted
Figure 6. ORTEP drawing of 7, with partial numbering scheme (50%
probability thermal ellipsoids). The anion and all hydrogen atoms
except the alcohol OH are omitted for clarity. Selected bond lengths
(Å), bond angles (deg), and torsion angles (deg): Ir1−Cl1 2.4035(7),
Ir1−N1 2.109(2), Ir1−N2 2.123(2), N2−C12 1.294(4), N2−C11
1.441(4), O1−C12 1.316(4); Ir1−N2−C12 128.22(19), C11−N2−C12
121.7(2), N2−C12−O1 121.7(3), N2−C12−C13 126.8(3); C11−C6−
C5−N1 37.3(4).
Scheme 8. Treatment of the OC with HCl, Forming 7
between the aryl groups in the other six-membered metalla-
cycles for complexes 4 (27.9°) and 6 (31.4°). The resonances
1
with excess sPhIO (presumably forming sPhICl2, a milder
oxidant),57 which slowed decomposition of the iridium
complex when the sample was warmed. After the samples of
the OC treated with HCl were warmed to room temperature,
the reaction mixture was washed with water, and the solvent
(dichloromethane) was removed in vacuo. Residual sPhI was
removed by washing the solids copiously with pentane, and
crystals suitable for X-ray analysis were grown by layering
pentane on top of a concentrated solution of the metal species
in dichloromethane. The molecular structure revealed the
iridium product as [Ir(Cp*)(3,5-bis(trifluoromethyl)-N-(2-(2-
pyridyl)phenyl)benziminol)Cl][B(ArF)4] (7; Figure 6). Nota-
bly, the nitrile had inserted into the iridium−carbon bond of
phpy and the original nitrile carbon now possessed a hydroxyl
group. The new iminol moiety was verified by the bond lengths
reported in the structure. The C−N bond length was 1.294 Å,
and the C−O bond length was 1.316 Å, verifying the identity of
the imine CN and C−O bonds of 7, respectively. In addition,
the torsion angle between the two aryl rings of the modified
phpy ligand (37.3°) was greater than the torsion angles
in the H NMR spectrum of 7 were similar to those observed
for the OC, but in addition there appeared a broad singlet
assigned to the alcohol OH at 12.1 ppm.
Synthesis of Complexes Relevant to Oxidation of 1.
The identities of the Int and OC were not apparent from the
structural characterization of the HCl addition product 7. Only
one oxygen atom was incorporated into the ligand framework
of 7, while integration of sPhI after formation of the OC
suggested that 2 equiv of oxygen atoms had been consumed.
The identity of the sixth ligand in the Int and the OC was
unknown, and chloride presumably displaced a ligand in the
OC on the way to form 7. The new phpy-derived ligand in 7,
however, appeared to be related to the phpy-derived ligands of
the OC and Int, since the aromatic resonances in the 1H NMR
spectra of all three complexes included a characteristic
resonance near 6.5 ppm. The resonance at 12.1 ppm for the
hydroxyl proton only appeared upon treating the OC with HCl,
which led us to believe that an amide was present in the OC,
and furthermore, this amide reflected net insertion of an
oxidized nitrile ligand under the oxidative conditions of the
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Scheme 9. Synthesis of Proligand 8 and Iridium Complexes Relevant to sPhIO Oxidation
reaction. Conversion of a coordinated amide to an iminol has
been observed at iridium and other metals.58
The reaction of 10 with sPhIO at −40 °C was also
conducted. The oxidation reaction starting from 10 cleanly
generated the OC in 90% yield (based on an internal standard)
after 4 min of stirring at −40 °C. Importantly, only 1 equiv of sPhI
was generated (Figure 7), further establishing the identity of 10 as
A proligand modeling the phpy-derived ligand corresponding
to an amide was independently prepared. Synthesis of bis-CF3-
substituted benzamide 8 was readily accomplished by an amide
coupling reaction between 2-(2-pyridyl)aniline59 and the
commercially available 3,5-bis(trifluoromethyl)benzoyl chloride
(Scheme 9). The conditions for the amide coupling followed a
closely related procedure for the coupling of aminoquinolines
and acid chlorides.60
Metalation of 8 was accomplished using the same strategy as
was used for metalation of proligands 2 and 3. Proligand 8 was
stirred over NaH in THF for 1.5 h and then filtered by cannula
directly onto solid [Ir(Cp*)Cl2]2 dimer (Scheme 9). The dimer
was carefully dried by heating at 50 °C overnight under high
vacuum. The product of metalation, Ir(Cp*)((3,5-bis-
(trifluoromethyl)benzoyl)(2-(2-pyridyl)phenyl)amide)Cl (9),
was stable to moisture and air, allowing for chromatography
and purification on silica gel. Importantly, protonation of 9 with
HBF4 and counterion exchange with Na[B(ArF)4] yielded 7,
which proved to be identical (as confirmed by NMR spectros-
copy) with the species obtained when the OC was reacted with
HCl.
1
Figure 7. Expansion of the aromatic region of the H NMR spectrum
of the OC at −40 °C. When the OC is formed by oxidation of the
isolated intermediate 10 with sPhIO, only 1 equiv of sPhI is generated.
the Int observed in the oxidation of 1 in the presence of excess
NCArF. Excess sPhIO was again required for full conversion of the
intermediate 10 to the OC. Note that intermediate 10 is not
observed in the oxidation reaction of 1 with sPhIO in the absence
of NCArF. In this case, the intermediate species is not stabilized by
added trapping ligands, although it may be stabilized by interaction
with the solvent (dichloromethane-d2).
Neither complex 9 nor 10 exhibited dynamic behavior at
−70 °C in their H NMR spectra (the OC decoalesced into
two isomers at this temperature). That single isomers were
observed by 1H NMR spectroscopy at −70 °C for complexes 9
and 10 possibly indicates that further modification of the κ2
chelating ligand occurred upon oxidation of the Int.
Identity of Intermediate. Substitution for the chloride
ligand in 9 was achieved using Na[B(ArF)4] in the presence of
NCArF, yielding [Ir(Cp*){(3,5-bis(trifluoromethyl)benzoyl)(2-
(2-pyridyl)phenyl)amide}(NCArF)][B(ArF)4] (10; Scheme 10).
1
Scheme 10. Synthesis and Oxidation of 10
Mechanisms of First and Second Oxidation Steps.
Following identification of the Int as 10, independent synthesis
of 10 allowed the individual oxidation steps to be studied in
greater detail. These individual steps in the overall reaction of 1
with sPhIO are described in Scheme 11. Initial OAT to 1 results
1
1H NMR spectral signals for 10 at −40 °C matched the H
NMR resonances observed for the Int formed in the reaction of
1, sPhIO, and excess NCArF at −40 °C. Specifically, the Cp*
methyl groups (15H) of 10 resonated at 1.42 ppm at low
temperature, while the same signal for the Int resonated at 1.45
Scheme 11. Individual Steps in the Oxidation of 1 with
sPhIO
1
ppm. The H NMR spectrum of 10 also revealed doublets at
8.66 ppm (J = 5.5 Hz) and 6.51 ppm (J = 7.5 Hz), which
matched well with doublets identified for the Int at 8.70 ppm
(J = 5.5 Hz) and 6.52 ppm (J = 8.0 Hz). Furthermore, two
1
triplets in the H NMR spectrum of 10 (7.41 ppm, J = 6.5 Hz;
7.04 ppm, J = 7.0 Hz) were equivalent to two triplets observed
for the Int (7.47 ppm, J = 6.5 Hz; 7.07 ppm, J = 7.0 Hz). Good
agreement between the two 1H NMR spectra at −40 °C
convinced us that independently prepared complex 10 was in fact
the Int observed under oxidative conditions in the presence of
exogenous NCArF.
in oxidation of NCArF and insertion into the iridium−carbon
bond of phpy, forming an amide. A second OAT step follows,
and this step converts the intermediate to the OC and is
accompanied by the generation of the second equivalent of sPhI.
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Mechanism of Nitrile Oxidation in the First Oxidation
Step. The kinetics of the first oxidation step were studied to
determine if dissociation of the nitrile ligand was a prerequisite
for nitrile oxidation. When a 6.3 mM solution of 1 with 6 equiv
of sPhIO was mixed at −70 °C, the first oxidation step was too
Scheme 13. Possible Mechanisms for Nitrile Oxidation and
Insertion
1
fast to monitor by H NMR spectroscopy, both in the absence
and in the presence of 44 equiv of NCArF (Scheme 12).
Scheme 12. Rate of OAT to 1 (Top) and Rate of NCArF
Dissociation from 1 (Bottom)
Since the rate of the first oxidation step was too fast to
monitor under our conditions, ligand substitution experiments
were conducted to measure the rate constant of nitrile
dissociation from iridium. When 6 equiv of triphenylphosphine
was added to 1, first-order kinetics for the conversion of 1 to
[Ir(Cp*)(phpy)(PPh3)]+ (1-PPh3) were observed at −40 °C.
The first-order rate constant for conversion of 1 to 1-PPh3 was
determined to be 6.19 × 10−4 s−1 (R2 = 0.998). In addition to
PPh3, acetonitrile was also selected for ligand substitution.
Although acetonitrile is not a strong ligand, full displacement
of NCArF was observed, forming [Ir(Cp*)(phpy)(NCCH3)]+
(1-NCCH3).39,61 When 30 equiv of acetonitrile was added to 1 at
−40 °C, first-order kinetics were also observed, and the rate of
conversion to 1-NCCH3 was determined to be 6.05 × 10−4 s−1
(R2 = 0.999). Good agreement between the observed rates for
conversion of 1 to the PPh3 and acetonitrile adducts indicated
that the values reflected the rate of NCArF dissociation from 1
(∼6 × 10−4 s−1). Since the rate of oxidation is much faster than
the rate of nitrile dissociation from 1, direct oxidation of the
coordinated nitrile must occur (sPhIO does not oxidize NCArF
in the absence of iridium). Oxidation of a coordinated nitrile to
an amide has been observed only rarely.62−64
Oxidation is likely to occur via nucleophilic attack of sPhIO
on the nitrile carbon of a σ-bound NCArF ligand,65 akin to well-
established hydrolysis mechanisms of coordinated nitriles.66
Importantly, complex 1 does not react with water in solution at
room temperature, which rules out product formation via nitrile
hydrolysis. The oxidations here were run under anhydrous
conditions. The reagent sPhIO is considered to be nucleophilic
(the reported bond distance between the iodine and oxygen is
characteristic of a single bond, which leaves extra electron
density on the oxygen).67 Initial attack of sPhIO at the central
carbon of NCArF on 1 would yield the intermediate “1a”
pictured at the top of Scheme 13. Migration of two electrons is
required to break the oxygen−iodine bond of sPhIO and
release sPhI. These electrons could come (route a) directly
from the iridium−carbon bond of phpy, which would
accomplish insertion in a one-step concerted process without
accessing the Ir(V) oxidation state. Alternatively, in a two-step
process, the metal could be oxidized (route b), generating a
transient Ir(V)nitrene ripe for insertion. A recently reported
catalytic amidation reaction of arenes with cyclometalating
directing groups using an Ir(Cp*)-based catalyst led to two
possible analogous mechanisms (direct insertion versus metal
oxidation and insertion).68 The catalytic reaction employed acyl
azides as the amide source, effectively a two-electron oxidant. In
addition, a reported oxidation of an [Ir(Cp*)(phpy)NCCH3]+
complex with the nitrene transfer reagent PhINTs led to
sulfonamide insertion into the iridium−carbon bond of phpy.61
An IrV−nitrene intermediate was postulated. Less than a dozen
stable Ir(III) complexes with multiply bound nitrogen ligands
are known, but stable IrV−nitrenes have not been reported.69
From our experimental evidence, an IrV−nitrene is neither
implicated nor excluded in the first oxidation step of 1 with
sPhIO.
Mechanism of the Second Oxidation Step. The kinetics of
oxidation from the intermediate 10 to the OC were studied in
the presence of excess nitrile ligand. Plots of ln[10] versus time
showed linear behavior when the reactions were run in the
presence of excess sPhIO. It was found that the rate of reaction
was inhibited by excess nitrile (Scheme 14a). For example, the
second-order rate constant for oxidation in the absence of
added nitrile ligand was [1.68( 0.13)] × 10−3 M−1 s−1, but
when 45 equiv of nitrile was added, the rate slowed to
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(Supporting Information). The rate constant for dissociation of
nitrile (k1) from 10 was estimated to be 1.7 × 10−3 s−1, and the
ratio k2/k−1 was calculated from the slope of the regression to
be ∼4.5. The rate of oxidation of Int*, therefore, is roughly 5
times faster than trapping with NCArF. The kinetic data are
consistent with reversible nitrile dissociation that competes
with irreversible oxidation. Notably, dissociation of nitrile in the
second oxidation step contrasted starkly with behavior observed
in the first oxidation step, where coordinated NCArF
underwent nucleophilic attack.
Scheme 14. (a) Kinetics of OAT to 10 in the Presence of
NCArF and (b) Proposed Reaction Pathway
To ascertain whether or not NCArF was perhaps oxidized
after OAT to the metal center, we prepared and oxidized
analogues of the Int using the different trapping ligands
p-tolunitrile (NCArMe) and pyridine N-oxide (OPy). Notably,
OPy is resistant to oxidation. If oxidation of the various
analogues yielded the OC in each case, it would suggest that
the sixth ligand of the intermediate was inconsequential to the
formation of the OC. Accordingly, the complexes [Ir(Cp*)-
{(3,5-bis(trifluoromethyl)benzoyl)(2-(2-pyridyl)phenyl)amide}-
(NCArMe)][B(ArF)4] (11, Int-NCArMe) and [Ir(Cp*){(3,5-
bis(trifluoromethyl)benzoyl)(2-(2-pyridyl)phenyl)amide}(OPy)]-
[B(ArF)4] (12, Int-OPy) were prepared following the same
strategy described for the synthesis of 10.
Oxidation of both Int-NCArMe and Int-OPy with excess
sPhIO yielded 1 equiv of sPhI and the same OC as 10 (Scheme 15).
[0.65( 0.06)] × 10−3 M−1 s−1. Adding 5 or 15 equivalents of
nitrile also slowed the rate of the reaction, but only marginally
in comparison to the reaction with no added NCArF. Inhibition
from added NCArF suggested that dissociation of NCArF was
necessary for the second oxidation step to occur. Direct
oxidation of the Cp* or the phpy ligand 8 was not implicated,
as sPhIO and the neutral iridium-chloride complex 9 did not
react at −40 °C.
Scheme 15. Formation of the OC using Different κ1 Ligands
Postulated elementary steps for the second oxidation are
depicted in Scheme 14b. The reaction was assumed to be first-
order in the sPhIO reagent, although no attempt was made to
determine the dependence of the rate constant on the sPhIO
concentration. When the steady-state approximation is applied
to Int*, the five-coordinate iridium species resulting from
dissociation of NCArF, the rate law shown in eq 1 can be
k1k2[10][sPhlO]
rate =
The oxidation of Int-OPy gave the OC in 98% yield (using an
internal standard) after 25 min at −30 °C. Apparently OPy binds
more tightly to iridium than NCArF and required increased
temperatures and more time for the reaction to reach
completion. Higher temperatures yet (−20 °C) were required
to observe a reaction with Int-NCArMe, which is consistent with
a dissociative mechanism and the use of NCArMe, a ligand more
strongly binding than NCArF or OPy. After 45 min, the yield of
the OC peaked at 53% and then began to decline (internal
standard), even though full conversion of Int-NCArMe had not
occurred. Presumably after 45 min, the rate of decomposition of
the OC surpassed the rate of formation of the OC via reaction of
Int-NCArMe and sPhIO. Decomposition products were observed
k−1[NCArF] + k2[sPhlO]
(1)
derived. The sPhIO concentration was treated as a constant in
the oxidation reactions, since complete dissolution of a 6-fold
molar excess of sPhIO (relative to 10) was achieved. The
observed rate constant (kobs), therefore, was described in terms
of the rate constants of the elementary steps and the
concentration of sPhIO (eq 2). When the reciprocals of the
k1k2[sPhlO]
rate = kobs[10] kobs
=
k−1[NCArF] + k2[sPhlO]
(2)
1
in the H NMR experiment, which was consistent with the
appearance of decomposition products in the oxidation of 1 with
sPhIO at −20 °C. Importantly, free nitrile ligand or free OPy was
observed in the 1H NMR spectra after oxidation of 10, 11, or 12.
Conversion of the cationic intermediate to the OC is thus
independent of the identity of the κ1 ligand. In addition,
oxidation of 1 in the absence of excess NCArF also rapidly
generated the OC, despite the fact that no nitrile was available
after coordinated NCArF was oxidized in the first OAT step.
four kobs values were plotted against their respective
concentrations of the nitrile ligand (eq 3), the data points
were fitted by linear regression and yielded an R2 value of 0.988
⎛
⎞
k−1
k k [sPhlO]
1
k1
1/kobs
=
[NCArF] +
⎜
⎝
⎟
⎠
(3)
1
2
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Scheme 16. Reaction of the OC with PPh3
That the yield of the OC is higher with OPy as the sixth ligand
(98%) of the intermediate than with NCArF as the sixth ligand
(90%) may reflect the fact that OPy is stable to oxidation.
These experiments, in addition to the observation of free nitrile
were grown by layering pentane on top of a concentrated
solution of 13 in dichloromethane. The structure of 13 revealed
the coordinated benzamide functionality with a C−N bond length
of 1.357 Å and a CO bond length of 1.228 Å (Figure 8). For
1
or OPy in the H NMR spectra, confirmed that the sixth
ligands in 10−12 were not oxidized after dissociation and
simply acted as placeholder ligands.
Oxidation of the intermediate 10, therefore, requires NCArF
dissociation from iridium and yields a species that does not
react with free NCArF. Nitrile dissociation from intermediate
10 exposes an electrophilic Ir(III) center, which the
nucleophilic sPhIO oxidant can attack.70 Importantly, the OC
is not simply a coordinated sPhIO molecule in the sixth
coordination site, as 1 equiv of sPhI is generated, indicating
complete transfer of the oxygen atom to iridium. Although the
exact identity of the OC is not clear from the spectroscopic
data, several possibilities are offered that reflect dissociation of
the sixth ligand and incorporation of a second oxygen atom.
These include, but are not limited to, an IrV−oxo complex, an
IrIII−iminopercarboxylate complex (with a fused 5,6-membered
iridacycle and a coordinated peroxide), Ir(V) dimers with bis-μ-
oxo ligands, and an IrIII−hydroxamate complex (oxygen atom
insertion into the iridium−nitrogen bond of the benzamide
donor). Importantly, a mononuclear IrV−oxo complex might be
expected to be paramagnetic like the known isoelectronic
RuIV−oxo complexes, which would complicate observation by
NMR spectroscopy.71 Terminal metal oxo ligands can also
interact to form bis-μ-oxo dimers.72 An IrIII−iminopercarbox-
ylate complex could form via nucleophilic attack of the amide
either on a terminal, electrophilic oxo ligand (reducing the
metal to Ir(III) and forming an oxygen−oxygen bond) or on
the oxygen atom of a coordinated sPhIO molecule. The amide
served as a base in the presence of HCl; therefore, it might also
serve as a nucleophile. Insertion into the iridium−nitrogen
bond of the benzamide is a plausible pathway for hydroxamate
formation.
Figure 8. ORTEP drawing of 13, with partial numbering scheme (50%
probability thermal ellipsoids). The anion and all hydrogen atoms are
omitted for clarity. Selected bond lengths (Å), bond angles (deg), and
torsion angles (deg): Ir1−P1 2.3594(6), Ir1−N1 2.151(2), Ir1−N2
2.105(2), N2−C12 1.357(4), N2−C11 1.420(3), O1−C12 1.228(3);
Ir1−N2−C12 122.93(17), C11−N2−C12 119.3(2), N2−C12−O1
123.4(2), N2−C12−C13 119.3(2); C11−C6−C5−N1 35.3(4).
comparison, the iminol functionality of 7 included a CN bond
distance of 1.294 Å and a C−O bond distance of 1.316 Å, which
are distinct from those of 13.
The reactivity of the OC was explored to see if it could act as
an oxidant. The OC did not react with styrene, trans-stilbene,
or 2,3-dimethyl-2-butene at −20 °C. In addition, the OC did
not react with water (delivered as a solution in acetone-d6)
between −40 and −20 °C. However, when 4 equiv of PPh3 was
added to a freshly generated solution of the OC and warmed to
−20 °C, phosphine oxidation to the oxide (OPPh3) was
observed, in addition to a new iridium species (92% conversion,
internal standard) after 75 min (Scheme 16). The iridium
complex was identified as [Ir(Cp*){(3,5-bis(trifluoromethyl)-
benzoyl)(2-(2-pyridyl)phenyl)amide}(PPh3)][B(ArF)4] (13)
The source of the oxygen atoms in the phosphine oxidation
reaction was closely examined. Importantly, the calculated O
PPh3/13 ratio was 3.0 at the end of the reaction, on the basis of
1
integration of the H NMR spectrum (Supporting Informa-
tion). The sPhIO oxidant is known to react with phosphine in
the absence of metal.40 Only 2 equiv of the sPhIO reagent (out
of 4 equiv) reacted with iridium starting material 1 to form the
OC. Two equivalents of OPPh3, therefore, was ascribed to
direct oxidation of PPh3 by residual sPhIO. The third oxidative
equivalent, however, could only be accounted for by OAT from
the OC. This conclusion was strengthened by the fact that the
third equivalent of OPPh3 grew in concomitantly with the
appearance of 13. This behavior is consistent with direct OAT
from the OC to PPh3. OAT from the OC leaves a vacant
coordination site, which is trapped by PPh3 to form 13.
Notably, the (mesityl)3IrV−oxo complex has been reported to
oxidize phosphines and arsines but does not react with olefins
1
by comparing the H NMR spectrum to the spectrum of an
independently prepared sample of 13. Notably, the Cp* methyl
protons of the new iridium species and complex 13 resonated
at 1.32 and 1.31 ppm, respectively. In addition, the aromatic
regions in the 1H NMR spectra were essentially identical
(Supporting Information). Complex 13 has been fully
characterized, and crystals suitable for X-ray diffraction studies
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or water.7 The oxidized complex, therefore, holds the capacity
to cleanly oxidize PPh3 and regenerate the same Cp*/κ2 ligand
framework of intermediate 10. This serves as a unique example
of an oxidized Ir(Cp*) complex that reacts to form a well-
characterized, molecular product.
in dichloromethane-d2. The total solution volume was 800 μL. The
ligand displacement reaction was monitored by 1H NMR spectroscopy
at −40 °C, and the concentration of 1 was calculated from the ratio of
the Cp* integrals for 1 and the respective adduct. Plotting ln [1] as a
function of time yielded graphs displaying good linear behavior
(Supporting Information).
Oxidation of 10 with sPhIO in the Presence of NCArF. A 10 mg
portion of sPhIO (29 μmol) was completely dissolved in dichloro-
methane-d2 at room temperature and then chilled to −78 °C. The
sPhIO remained in solution when cooled. A solution with 9.0 mg of
dissolved 10 (5 μmol) and NCArF (0−45 equiv) in dichloromethane-
d2 was slowly added to the sPhIO solution at −78 °C, bringing the
total sample volume to 800 μL. The oxidation reaction was monitored
by 1H NMR spectroscopy at −40 °C, and the concentration of 10 was
calculated from the ratio of the Cp* integrals for 10 and the OC.
Plotting ln [10] versus time yielded charts with good linear correlation
(Supporting Information).
CONCLUSIONS
■
Oxidation of 1 with the OAT reagent sPhIO yielded a single,
molecular compound at −40 °C. This is a rare example of an
Ir(Cp*) complex related to water oxidation catalysts that has
not suffered from Cp* ligand decomposition when oxidized.
When the oxidized complex could not be isolated, ligands
corresponding to potential oxidation of phpy by oxygen atom
insertion into the iridium−carbon and/or iridium−nitrogen
bonds were synthesized and metalated. Complex 4, with a κ2
ligand modeling insertion into the iridium carbon bond of
phpy, formed an unreactive phenoxide-bridged iridium dimer
(6) upon chloride abstraction. Another iridium complex (5),
with a κ2 ligand representing the product of oxygen atom
insertion into the iridium−carbon and iridium−nitrogen bonds
of phpy, was sensitive to moisture, and the bidentate ligand was
cleaved from iridium. Two sequential OAT reactions were then
uncovered in the low-temperature oxidation of 1, and an
intermediate was isolated and characterized. The intermediate
revealed that OAT to the carbon of the nitrile ligand coupled
with insertion into the iridium−carbon bond of phpy had
occurred in the first step. Nucleophilic attack of the oxidant on
the coordinated nitrile was implicated, and an IrV−nitrene may
have been generated transiently. The second OAT was found to
occur upon ligand dissociation from iridium, but the oxidized
complex could not be identified. OAT from the oxidized
complex to phosphine, however, was observed in high yield and
regenerated an iridium complex with the same Cp* and
benzamide ligands as the intermediate 10. This work offers
insights into possible deactivation pathways for iridium
complexes under harsh oxidizing conditions and describes the
reactivity and product identity of an oxidized, molecular iridium
species.
Synthesis of [Ir(Cp*)(phpy)NCArF][B(ArF)4] (1). The synthetic
procedure has been reported previously.39
Synthesis of [Ir(Cp*)(phpy)NCArF][SbF6] ([1][SbF6]). In a
100 mL Schlenk flask, 150 mg of Ir(Cp*)(phpy)Cl (0.29 mmol),
52 μL of NCArF (0.31 mmol), 105 mg of AgSbF6 (0.31 mmol), and
20 mL of dichloromethane were combined. The mixture was stirred
for 4 h in the dark, at which point the mixture was filtered by cannula.
The solvent was reduced to a minimal volume and added slowly to
rapidly stirred pentane chilled to −78 °C. The solid was isolated by
filtration and dried by heating to 40 °C overnight under high vacuum,
yielding 206 mg of the product (74%). 1H NMR (500 MHz, CD2Cl2):
δ 8.80 (d, J = 6.0, 1H), 8.12 (s, 1H), 7.95 (m, 2H), 7.92 (s, 2H), 7.80
(d, J = 7.5, 1H), 7.78 (d, J = 7.5, 1H), 7.37 (t, J = 6.5, 1H), 7.32 (t, J =
6.5, 1H), 7.23 (t, J = 7.5, 1H), 1.75 (s, 15H).
Synthesis of the “Oxidized Complex” (OC). A solution of
10 mg of 1 (6 μmol) in 0.4 mL of dichloromethane-d2 was slowly
added to an NMR tube charged with 8 mg of sPhIO (24 μmol,
4 equiv) and cooled to −40 °C. The mixture was thoroughly mixed for
1 min using a thin copper wire. The sample was transported cold
(−78 °C) to an NMR probe cooled to −40 °C. 1H NMR spectroscopy
revealed 91% conversion of 1 to the OC. Alternatively, a solution of
7 mg of 10 (4 μmol) in 0.4 mL of dichloromethane-d2 was slowly added
to an NMR tube charged with 7 mg of sPhIO (21 μmol, 5 equiv) and
cooled to −40 °C. The mixture was thoroughly mixed for 4 min using
a thin copper wire. Conversion to the OC (90%) was confirmed by 1H
NMR spectroscopy at −40 °C (using the 4-proton resonance of the
[B(ArF)4] anion at 7.54 ppm as an internal standard). The OC was
unstable above −20 °C. 1H NMR (500 MHz, CD2Cl2, −40 °C,
multiple resonances were obscured): δ 8.80 (d, J = 5.0, 1H), 7.89
(d, J = 7.5, 1H), 7.78 (s, 1−2H), 7.71 (s, 8H), 7.60 (m, 1−2H), 7.53
(s, 4H), 7.10 (m, 2H), 6.47 (m, 1H), 1.21 (s, 15H). 13C{1H} NMR
(125 MHz, CD2Cl2, −40 °C, one carbon was not observed): δ 171.1,
161.5 (1:1:1:1 quartet, JC−B = 49), 154.5, 152.4, 145.0, 140.2, 139.6,
136.1, 134.5, 133.6, 131.5, 130.5, 129.7, 128.5 (q, 2-bond JC−F = 32),
127.7, 125.7, 124.9, 124.3 (q, 1-bond JC−F = 270), 123.9, 122.8
(q, 1-bond JC−F = 271), 117.3, 86.2, 8.6.
EXPERIMENTAL SECTION
■
Materials and Methods. All reactions were performed under an
atmosphere of dry argon using standard Schlenk and drybox
techniques, unless noted otherwise. Argon was purified by passage
through columns of BASF R3-11 catalyst and 4 Å molecular sieves.
Methylene chloride, hexane, and pentane were purified under an argon
atmosphere and passed through a column packed with activated
alumina. All other chemicals were used as received without further
purification. 1H and 13C NMR spectra were recorded on Bruker
AVANCEIII400, AVANCEIII500, and AVANCEIII600 spectrometers.
1H NMR and 13C NMR chemical shifts were referenced to residual 1H
and 13C signals of the deuterated solvents. Unless stated otherwise,
yields of NMR reactions were determined using hexamethyldisiloxane
as an internal standard. IR absorption spectroscopy was carried out on
a Bruker Alpha-P ATR absorption spectrophotometer. X-ray
diffraction studies were conducted on a Bruker-AXS SMART APEX-
II diffractometer. Crystals were selected and mounted using Paratone
oil on a MiteGen Mylar tip. Elemental analyses were performed by
Robertson Microlit Laboratories of Madison, NJ. Caution! An
explosion hazard has been reported using the initially published
procedure for preparation of the OAT reagent 2-tert-butylsulfonyl-
iodosobenzene (sPhIO).40,41 In this work we have used a recent
procedure published by Lin.42
Synthesis of 2-(2-Hydroxyphenyl)pyridine (2). In a thick-
walled, 100 mL glass vessel, 333 mg of 2-(2-hydroxyphenyl)pyridine
N-oxide (1.8 mmol), 1.06 g of ammonium formate (16.8 mmol, 9.5
equiv), 80 mg of 5% Pd/C (2 mol %), and 16 mL of methanol were
added. The reaction vessel was sealed, covered with aluminum foil, and
heated to 95 °C overnight. After it was cooled to room temperature
the next morning, the mixture was filtered and the solvent was
removed under reduced pressure. The oil was diluted with dichloro-
methane and eluted through a silica gel column with dichloromethane/
hexane mixtures. After the solvent was removed under reduced pressure,
183 mg of a pale yellow oil was isolated (60%). The product matched the
spectral properties previously reported45 for this compound. H NMR
1
(500 MHz, CD2Cl2): δ 14.27 (s, O−H, 1H), 8.52 (d, J = 4.5, 1H), 7.95
(d, J = 8.5, 1H), 7.88 (t, J = 7.5, 1H), 7.83 (d, J = 8.0, 1H), 7.29 (m, 2H),
6.96 (d, J = 8.5, 1H), 6.91 (t, J = 7.0, 1H).
Synthesis of 2-(2-Hydroxyphenyl)pyridine N-Oxide (3). In a
glovebox, 118 mg of Pd(OAc)2 (0.53 mmol, 5 mol %), 460 mg of
Kinetics. Ligand Dissociation. Complex 1 (8.0 mg, 5 μmol) was
dissolved in dichloromethane-d2 and added slowly to a chilled solution
(−40 °C) of PPh3 (7.4 mg, 28 μmol) or acetonitrile (8 μL, 150 μmol)
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[HPtBu3]BF4 (1.58 mmol, 14 mol %), 2.9 g of K2CO3 (21 mmol,
1.9 equiv), and 4 g of pyridine N-oxide (42 mmol, 3.8 equiv) were
placed in a 100 mL pressure flask. Toluene (20 mL) was added,
followed by 1.3 mL of 2-bromophenol (11 mmol, 1 equiv). The
reaction vessel was closed securely, brought out of the glovebox, and
heated to 135 °C overnight with stirring. The next morning, the
mixture was cooled to room temperature and filtered through Celite.
The Celite was washed twice with dichloromethane and then twice
with acetone. The solvent was removed, and the crude material was
chromatographed on silica gel using acetone/hexane (1/4 v/v, Rf =
0.07). The product fractions were collected, the solvent was removed,
and the solid was heated to 50 °C under vacuum overnight. A 940 mg
amount of product was isolated (45%). Anal. Calcd (found) for
C11H9NO2: C, 70.58 (70.33); H, 4.85 (4.80); N, 7.48 (7.38). 1H NMR
(400 MHz, CD2Cl2): δ 11.16 (s, 1H), 8.40 (d, J = 6.5, 1H), 7.69
(d, J = 8.5, 1H), 7.59 (t, J = 7.5, 1H), 7.46 (t, J = 8.0, 1H), 7.42 (m,
2H), 7.03 (m, 2H). 13C{1H} NMR (150 MHz, CD2Cl2): δ 159.7,
151.4, 140.5, 132.5, 131.4, 130.0, 129.3, 124.7, 121.4, 120.4, 120.3.
Synthesis of Ir(Cp*)(2-(2-pyridyl)phenoxide)Cl (4). A Schlenk
flask was charged with 92 mg of NaH (3.8 mmol), 35 mL of THF, and
92 μL of 2-(2-hydroxyphenyl)pyridine (109 mg, 0.63 mmol). The
mixture was stirred at room temperature for 45 min and then filtered
by cannula onto a sample of 225 mg of [Ir(Cp*)(Cl)2]2 (0.28 mmol)
that had previously been heated to 50 °C under vacuum for 18 h. The
mixture was stirred for 7 h and then filtered. The solvent was reduced
to a minimal volume under reduced pressure and then was added
slowly to rapidly stirred pentane (20 mL) chilled to −78 °C. The solid
was collected by filtration and then eluted through a silica gel column
using acetone/dichloromethane (1/4 v/v). The solvent was removed
under reduced pressure, and the yellow solid was heated to 50 °C
under high vacuum overnight, yielding 230 mg of product (76%).
Yellow crystals suitable for X-ray diffraction analysis were grown by
vapor diffusion of pentane into a concentrated solution of the product
in dichloromethane. Using Olex2,73a the structure was solved with the
olex2.solve structure solution program using charge flipping and
refined with the XL refinement package using least-squares
minimization.73b Crystal data for C21H23NOClIr (Mr = 533.05):
monoclinic, space group P21/c (No. 14), a = 7.22210(10) Å, b =
14.9351(2) Å, c = 17.0443(2) Å, β = 93.7670(10)°, V = 1834.47(4) Å3,
Z = 4, T = 100 K, μ(Cu Kα) = 15.486 mm−1, Dcalcd = 1.930 g/mm3,
c = 14.3917(6) Å, α = 75.867(2)°, β = 87.839(2)°, γ = 74.111(2)°, V =
1279.14(9) Å3, Z = 2, T = 100 K, μ(Cu Kα) = 15.083 mm−1, Dcalcd
=
1.867 g/mm3, 23060 reflections measured (6.34 ≤ 2θ ≤ 140.34), 4711
unique (Rint = 0.0304), which were used in all calculations. The final R1
value was 0.0216 (>2σ(I)), and the wR2 value was 0.0529 (all data), with
a goodness of fit on F2 of 1.086. Anal. Calcd (found) for C21H23ClIrNO2:
1
C, 45.94 (47.14); H, 4.22 (4.50); N, 2.55 (2.27). H NMR (500 MHz,
CD2Cl2): δ 8.17 (d, J = 6.0, 1H), 7.73 (t, J = 7.5, 1H), 7.36 (d, J = 8.0,
1H), 7.33 (td, J = 7.0 and 1.5, 1H), 7.28 (td, J = 7.5 and 1.5, 1H), 7.07
(dd, J = 7.5 and 1.5, 1H), 6.93 (d, J = 7.5, 1H), 6.65 (t, J = 7.0, 1H), 1.55
(s, 15H). 13C{1H} NMR (150 MHz, CD2Cl2): δ 166.5, 155.9, 143.9,
134.4, 131.6, 131.1, 128.8, 127.4, 124.8, 121.7, 115.8, 81.7, 9.1.
Synthesis of [Ir(Cp*)(2-(2-pyridyl)-μ-phenoxide)]2[B(ArF)4]2
(6). A 100 mL Schlenk flask was charged with 100 mg of
Ir(Cp*)(2-(2-pyridyl)phenoxide)Cl (0.19 mmol), 207 mg of Na[B-
(ArF)4] (0.23 mmol), and 30 mL of dichloromethane. After it was
stirred for 7 h, the mixture was filtered and the solvent of the filtrate
was removed. The solid was redissolved in a minimal volume of
acetone and was slowly added to rapidly stirred pentane (20 mL)
chilled to −78 °C. The mixture was filtered and the solid was heated to
40 °C overnight under vacuum, yielding 176 mg of a yellow solid
(69%). Yellow crystals suitable for X-ray diffraction were grown by
vapor diffusion of pentane into a concentrated solution of the product
in dichloromethane. Using Olex2,73a the structure was solved with the
olex2.solve structure solution program using charge flipping and
refined with the XL refinement package using least-squares minimiza-
tion.73b Crystal data for C54H37BCl2F24IrNO (Mr = 1445.76): triclinic,
space group P1 (No. 2), a = 12.8140(2) Å, b = 13.7210(2) Å, c =
̅
18.1145(3) Å, α = 106.1770(10)°, β = 110.3230(10)°, γ = 98.0160(10)°,
V = 2767.97(7) Å3, Z = 2, T = 100.15 K, μ(Cu Kα) = 6.664 mm−1,
Dcalcd = 1.735 g/mm3, 39637 reflections measured (5.56 ≤ 2θ ≤ 140.22),
10164 unique (Rint = 0.0210), which were used in all calculations. The
final R1 value was 0.0237 (>2σ(I)) and the wR2 value was 0.0599 (all
data), with a goodness of fit on F2 of 1.087. Anal. Calcd (found) for
C106H70N2B2F48Ir2O2: C, 46.78 (46.91); H, 2.59 (2.54); N, 1.03 (1.08).
1H NMR (500 MHz, CD2Cl2): δ 8.97 (d, J = 6.0, 2H), 8.23 (m, 4H),
7.89 (d, J = 8.0, 2H), 7.82 (m, 2H), 7.72 (s, 16H), 7.61 (t, J = 7.5, 2H),
7.55 (s, 8H), 7.28 (t, J = 7.5, 2H), 7.17 (d, J = 8.0, 2H), 0.59 (s, 30H).
13C{1H} NMR (150 MHz, CD2Cl2): δ 162.1 (1:1:1:1 quartet, JC−B = 50),
160.8, 154.3, 152.1, 141.9, 135.1, 133.9, 130.2, 129.2 (q, 2-bond JC−F
=
33119 reflections measured (7.88 ≤ 2θ ≤ 140.5), 3490 unique (Rint
=
31), 127.7, 126.4, 125.9, 125.7, 124.9 (q, 1-bond JC−F = 271), 122.1,
0.0319), which were used in all calculations. The final R1 value was
0.0207 (>2σ(I)), and the wR2 value was 0.0472 (all data), with a
goodness of fit on F2 of 1.181. Anal. Calcd (found) for C21H23ClIrNO:
C, 47.31 (47.10); H, 4.35 (4.34); N, 2.63 (2.46). 1H NMR (500 MHz,
CD2Cl2): δ 8.72 (d, J = 5.5, 1H), 7.78 (t, J = 8.0, 1H), 7.72 (d, J = 8.0,
1H), 7.49 (d, J = 7.5, 1H), 7.21 (overlapping triplets, J = 7.0 and 7.5,
2H), 6.91 (d, J = 8.0, 1H), 6.63 (t, J = 7.5, 1H), 1.34 (s, 15H).
13C{1H} NMR (100 MHz, CD2Cl2): δ 169.2, 155.7, 153.8, 138.2,
131.4, 128.8, 128.2, 123.1, 122.9, 122.2, 116.5, 84.6, 8.6.
Synthesis of Ir(Cp*)(2-(2-pyridyl)phenoxide N-oxide)Cl (5).
In a 100 mL Schlenk flask, 93 mg of 2-(2-hydroxyphenyl)pyridine
N-oxide (0.5 mmol), 100 mg of NaH (4.2 mmol), and 70 mL of THF
were added. The mixture was stirred at room temperature overnight.
Simultaneously, 187 mg of [Ir(Cp*)(Cl)2]2 (0.24 mmol) was heated
at 50 °C under high vacuum overnight. The next day, the THF
solution was filtered with a cannula onto the metal dimer. The mixture
was stirred for 1.5 h at room temperature, after which time the mixture
was filtered by cannula. The solvent was removed from the filtrate, and
the resulting solid was redissolved in minimal dichloromethane. The
solution was slowly added to rapidly stirred pentane chilled to −78 °C.
The supernatant was filtered off by cannula, and 135 mg of a red solid
was isolated after drying under high vacuum overnight (52%). The
compound was stored in a glovebox freezer. Orange crystals suitable
for X-ray diffraction were grown by storing a saturated solution of the
product in pentane at −25 °C. Using Olex2,73a the structure was
solved with the olex2.solve structure solution program using charge
flipping and refined with the XL refinement package using least-squares
minimization.73b Crystal data for C23H27Cl5IrNO2 (Mr = 718.91):
117.8, 85.9, 7.9.
Synthesis of [Ir(Cp*)(3,5-bis(trifluoromethyl)-N-(2-(2-
pyridyl)phenyl)benziminol)Cl][B(ArF)4] (7). A solution of 50 mg
of 1 (0.032 mmol) in 1 mL of dichloromethane was added to an NMR
tube chilled to −40 °C and charged with 40 mg of sPhIO (0.12 mmol).
The reaction was mixed thoroughly for 1 min using a thin piece of
copper wire, forming the OC. HCl(g) was then gently bubbled through
the reaction for 1 min at −40 °C. The solids were allowed to settle, and
the solution was decanted into an acetylation vial at room temperature.
The organic layer was washed three times with water, and then the
solvent was removed. The solids were washed three times with pentane
and then recrystallized twice by layering pentane on top of a
concentrated solution of 7 in dichloromethane. The solvents were
decanted off, and the crystalline material was dried under high vacuum,
yielding 16 mg of 7 (31%). Alternatively, 9 (56 mg, 0.072 mmol) was
treated with HBF4·OEt2 (10 μL, 0.073 mmol) and stirred for 30 min in
dichloromethane. A 68 mg portion of Na[B(ArF)4] (0.077 mmol) was
added, and the mixture was stirred for an additional 30 min before being
filtered. The solvent was reduced to minimal volume, and the product
was recrystallized by slowly adding the solution to rapidly stirred
pentane chilled to −78 °C. The product was collected by filtration and
dried under vacuum (93 mg, 79%). Yellow crystals suitable for X-ray
diffraction were obtained by layering pentane on top of a concentrated
solution of 7 in dichloromethane. Using Olex2,73a the structure was
solved with the olex2.solve structure solution program using charge
flipping and refined with the XL refinement package using least-squares
minimization.73b Crystal data for C62H39BClF30IrN2O (Mr = 1636.41):
monoclinic, space group P21/n (No. 14), a = 13.1858(3) Å, b =
20.1500(5) Å, c = 24.1403(6) Å, β = 100.1810(10)°, V = 6312.9(3) Å3,
triclinic, space group P1 (No. 2), a = 9.1491(4) Å, b = 10.4209(4) Å,
̅
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Z = 4, T = 100 K, μ(Cu Kα) = 5.694 mm−1, Dcalcd = 1.722 g/mm3,
was filtered by cannula. The solvent was reduced to a minimal volume,
and the concentrated solution was added slowly to chilled, rapidly
stirred pentane (20 mL). The solid was isolated by cannula filtration
and then heated to 30 °C under high vacuum for 3 days, yielding 283
mg of a yellow powder (79%). Anal. Calcd (found) for
C71H41N3BF36IrO: C, 46.37 (46.53); H, 2.25 (2.22); N, 2.28 (2.20).
1H NMR (600 MHz, CD2Cl2, 5 °C): δ 8.70 (d, J = 6.0, 1H), 8.20 (s,
2H), 8.15 (s, 1H), 8.11 (d, J = 7.8, 1H), 8.05 (t, J = 7.8, 1H), 7.76 (s,
3H), 7.72 (s, 8H), 7.63 (d, J = 7.2, 1H), 7.54 (s, 4H), 7.45 (t, J = 6.6,
1H), 7.15 (t, J = 7.2, 1H), 7.09 (t, J = 7.2, 1H), 6.55 (d, J = 7.8, 1H),
1.48 (s, 15H). 13C{1H} NMR (150 MHz, CD2Cl2, 5 °C, one carbon
merged to others): δ 169.8, 162.9 (1:1:1:1 quartet, JC−B = 50), 156.1,
153.9, 147.1, 141.2, 139.7, 135.0, 134.2, 133.4 (q, 2-bond JC−F = 35),
131.5, 131.1, 131.0 (q, 2-bond JC−F = 33), 130.5, 130.4, 129.0 (q, 2-
bond JC−F = 29), 128.5, 126.3, 124.8 (q, 1-bond JC−F = 272), 125.2,
64683 reflections measured (3.72 ≤ 2θ ≤ 140.22), 11930 unique (Rint
=
0.0407), which were used in all calculations. The final R1 value was
0.0283 (I > 2σ(I)), the and wR2 value was 0.0695 (all data), with a
goodness of fit on F2 of 1.036. Anal. Calcd (found) for
C62H39N2BF30IrOCl: C, 45.51 (45.21); H, 2.40 (2.20); N, 1.71
1
(1.71). H NMR (600 MHz, CD2Cl2): δ 12.06 (s, 1H), 8.80 (d, J =
6.0, 1H), 8.12 (d, J = 4.2, 2H), 7.97 (s, 1H), 7.85 (dd, J = 7.8 and 1.8,
1H), 7.72 (s, 8H), 7.60 (overlapping triplets, J ≈ 5, 2H), 7.56 (s, 4H),
7.56 (overlapping d, 1H), 7.40 (td, J = 7.8 and 1.2, 1H), 7.33 (td, J = 7.8
and 1.2, 1H), 6.57 (dd, J = 7.8 and 0.6, 1H), 1.33 (s, 15H). 13C{1H}
NMR (150 MHz, CD2Cl2): δ 171.0, 162.1 (1:1:1:1 quartet, JC−B = 50),
154.9, 154.4, 142.9, 141.7, 135.1, 132.8, 132.6 (q, 2-bond JC−F = 35),
132.0, 131.8, 131.7, 130.0, 129.4, 129.2 (q, 2-bond JC−F = 32), 127.0,
126.3 (pseudo t, 3-bond JC−F = 3), 125.1, 125.0 (q, 1-bond JC−F = 272),
124.9, 122.6 (q, 1-bond JC−F = 272), 117.8 (pseudo t, 3-bond JC−F = 4),
90.2, 8.4.
124.7, 123.5 (pseudo t, 3-bond JC−F = 4), 123.2 (q, 1-bond JC−F
=
272), 122.0 (q, 1-bond JC−F = 272), 117.9, 117.7 (pseudo t, 3-bond
Synthesis of 3,5-Bis(trifluoromethyl)-N-(2-(2-pyridyl)-
phenyl)benzamide (8). A 2 dram acetylation vial was charged
with 200 mg of 2-(2-pyridyl)aniline (1.18 mmol), 124 mg of NEt3
(1.23 mmol), and 4 mL of dry dichloromethane and capped with a
septum. After it was stirred for 10 min, the reaction mixture was stirred
for an additional 10 min as the solution was cooled to 0 °C. A 321 mg
aliquot of 3,5-bis(trifluoromethyl)benzoyl chloride (1.16 mmol) was
injected via syringe, and the reaction mixture was stirred overnight as
the ice bath melted. The next day, 10 mL of dichloromethane was
added, and the solution was washed once with water and twice with
1 M aqueous NaOH. The organic layer was dried with MgSO4 and
filtered, and the solvent was removed under reduced pressure. The
crude product was chromatographed on silica gel using ethyl acetate/
hexane (15/85 v/v, Rf = 0.28), and 414 mg of a white solid was
isolated (87%). Anal. Calcd (found) for C20H12N2F6O: C, 58.54
(58.80); H, 2.95 (3.02); N, 6.83 (6.72). 1H NMR (500 MHz,
CD2Cl2): δ 14.01 (s, N−H, 1H), 8.80 (d, J = 8.0, 1H), 8.69 (d, J = 5.0,
1H), 8.56 (s, 2H), 8.09 (s, 1H), 7.93 (m, 2H), 7.86 (dd, J = 8.0 and
1.0, 1H), 7.51 (td, J = 7.5 and 1.0, 1H), 7.38 (td, J = 5.0 and 3.0, 1H),
7.28 (t, J = 7.5, 1H). 13C{1H} NMR (150 MHz, CD2Cl2): δ 162.3,
158.2, 147.4, 138.6, 138.2, 132.3 (q, 2-bond JC−F = 33), 130.7, 129.1,
128.2, 128.1, 125.3, 125.2, 124.5, 123.6 (q, 1-bond JC−F = 271), 123.3,
122.8, 121.8.
JC−F = 4), 112.1, 90.8, 8.6.
Synthesis of [Ir(Cp*){(3,5-bis(trifluoromethyl)benzoyl)(2-(2-
pyridyl)phenyl)amide}(NCArMe)][B(ArF)4] (Int-NCArMe, 11). Com-
plex 9 (100 mg, 0.13 mmol), 127 mg of Na[B(ArF)4] (0.14 mmol),
NCArMe (p-tolunitrile, 17 μL, 0.14 mmol), and 20 mL of
dichloromethane were stirred for 2 h, at which point the mixture
was filtered by cannula. The solvent was reduced to a minimal volume,
and the concentrated solution was added slowly to chilled, rapidly
stirred pentane (20 mL). The solid was isolated by cannula filtration
and then heated to 40 °C under high vacuum overnight, yielding
148 mg of the product (67%). When 10 mg of 11 (0.006 mmol) and
14 mg of sPhIO (0.041 mmol) were mixed at −20 °C in dichloro-
1
methane-d2, the OC was observed in 53% yield after 45 min by H
NMR spectroscopy. Anal. Calcd (found) for C70H45N3BF30IrO: C,
48.96 (48.69); H, 2.64 (2.63); N, 2.45 (2.28). H NMR (500 MHz,
1
CD2Cl2): δ 8.72 (dd, J = 5.5 and 0.5, 1H), 8.11 (d, J = 7.5, 1H), 8.06
(td, J = 7.5 and 1.5, 1H), 7.75 (s, 3H), 7.73 (s, 8H), 7.62 (dd, J = 6.0
and 1.5, 1H), 7.56 (s, 4H), 7.53 (d, J = 8.0, 2H), 7.46 (td, J = 7.0 and
1.5, 1H), 7.27 (d, J = 8.0, 2H), 7.12 (td, J = 8.0 and 1.5, 1H), 7.06 (td,
J = 7.5 and 1.5, 1H), 6.50 (dd, J = 7.5 and 1.0, 1H), 2.38 (s, 3H), 1.49
(s, 15H). 13C{1H} NMR (150 MHz, CD2Cl2): δ 170.2, 162.1 (1:1:1:1
quartet, JC−B = 50), 156.4, 153.9, 147.8, 147.6, 141.1, 140.7, 135.2, 133.6,
131.3, 131.2 (q, 2-bond JC−F = 33), 131.1, 130.9, 130.7, 130.3, 129.3 (q, 2-
bond JC−F = 31), 128.7, 126.1, 125.0, 124.9 (q, 1-bond JC−F = 270), 124.4,
123.4 (q, 1-bond JC−F = 272), 123.3 (pseudo t, 3-bond JC−F = 4), 121.6,
117.8 (pseudo t, 3-bond JC−F = 4), 106.0, 90.3, 22.2, 8.7.
Synthesis of [Ir(Cp*){(3,5-bis(trifluoromethyl)benzoyl)(2-(2-
pyridyl)phenyl)amide}(OPy)][B(ArF)4] (Int-OPy, 12). Complex 9
(90 mg, 0.12 mmol), 114 mg of Na[B(ArF)4] (0.13 mmol), OPy
(pyridine N-oxide, 0.13 mmol), and 20 mL of dichloromethane were
stirred for 2 h, at which point the mixture was filtered by cannula. The
solvent was reduced to a minimal volume and the concentrated
solution was added slowly to chilled, rapidly stirred pentane (20 mL).
The solid was isolated by cannula filtration and then dried under high
vacuum overnight, yielding 150 mg of the red product (76%). Complex
12 was unstable in solution at room temperature after several hours.
When 10 mg of 12 (0.006 mmol) and 18 mg of sPhIO (0.053 mmol)
were mixed at −30 °C in dichloromethane-d2, the OC was observed in
98% yield after 25 min by 1H NMR spectroscopy. Anal. Calcd (found) for
C67H43N3BF30IrO2: C, 47.47 (47.18); H, 2.56 (2.49); N, 2.48 (2.29). 1H
NMR (500 MHz, CD2Cl2, −40 °C): δ 8.90 (d, J = 5.0, 1H), 8.07 (d, J =
7.5, 1H), 8.03 (t, J = 7.5, 1H), 7.74−7.70 (m, 5H), 7.71 (s, 8H), 7.54−
7.52 (m, 2H), 7.53 (s, 4H), 7.34 (t, J = 7.5, 1H), 7.11 (t, J = 7.0, 2H),
6.99 (t, J = 7.0, 1H), 6.94 (t, J = 7.5, 1H), 6.32 (d, J = 7.5, 1H), 1.26 (s,
15H). 13C{1H} NMR (125 MHz, CD2Cl2, −40 °C, one carbon merged
to others): δ 169.4, 161.6 (1:1:1:1 quartet, JC−B = 50), 155.2, 153.5, 146.6,
141.6, 140.4, 139.6, 138.3, 135.2, 134.5, 130.8, 130.6, 129.6, 129.3, 128.6
(q, 2-bond JC−F = 31), 125.9, 125.5, 124.3 (q, 1-bond JC−F = 271), 123.9,
123.5, 122.9, 122.8 (q, 1-bond JC−F = 271), 117.4, 86.2, 8.3.
Synthesis of Ir(Cp*)((3,5-bis(trifluoromethyl)benzoyl)(2-(2-
pyridyl)phenyl)amide)Cl (9). A mixture of 150 mg of NaH (6.25
mmol), 450 mg of proligand 8 (1.10 mmol), and 30 mL of THF were
stirred for 1.5 h and then filtered by cannula onto 400 mg of
[Ir(Cp*)(Cl)2]2 (0.50 mmol). The iridium dimer had been dried by
heating at 50 °C under high vacuum the previous night. After the
mixture was stirred for 24 h, the mixture was filtered by cannula and
the solvent was removed under reduced pressure. The solid was
redissolved in minimal dichloromethane and loaded onto a short
(10 cm) silica gel column. Residual ligand was eluted first with
dichloromethane, and then the metal species was eluted with acetone/
dichloromethane mixtures (1/3 v/v, then 1/2 v/v). It was necessary to
include NEt3 (1%) in the eluent, since the metal species was basic and
spread extensively as it traveled down the silica column. The product
fractions were selected by their yellow solution color, and the solvent
was removed. The solids were placed under high vacuum for 1.5 h, and
515 mg of a yellow solid was isolated (66%). Anal. Calcd (found) for
1
C30H26N2F6IrOCl: C, 46.66 (46.80); H, 3.39 (3.38); N, 3.63 (3.69). H
NMR (400 MHz, CD2Cl2): δ 8.95 (d, J = 6.0, 1H), 7.92 (m, 2H), 7.81 (s,
2H), 7.67 (s, 1H), 7.56 (dd, J = 8.0 and 1.6, 1H), 7.34 (td,
J = 6.0 and 3.2, 1H), 7.05 (td, J = 7.6 and 1.2, 1H), 6.93 (td, J = 7.6
and 1.2, 1H), 6.51 (d, J = 8.0, 1H), 1.41 (s, 15H). 13C{1H} NMR (150
MHz, CD2Cl2): δ 170.8, 156.0, 155.6, 150.4, 142.7, 139.1, 131.7, 130.7,
130.6 (q, 2-bond JC−F = 33), 130.4, 130.3, 128.6, 124.4, 123.7 (q, 1-bond
JC−F = 271), 122.6, 122.5 (pseudo t, 3-bond JC−F = 4), 122.4, 86.5, 8.7.
Synthesis of [Ir(Cp*){(3,5-bis(trifluoromethyl)benzoyl)(2-(2-
pyridyl)phenyl)amide}(NCArF)][B(ArF)4] (Int, 10). Complex 9
(150 mg, 0.19 mmol), 184 mg of Na[B(ArF)4] (0.21 mmol), NCArF
(3,5-bis(trifluoromethyl)benzonitrile, 35 μL, 0.21 mmol), and 20 mL
of dichloromethane were stirred for 3 h, at which point the mixture
Synthesis of [Ir(Cp*){(3,5-bis(trifluoromethyl)benzoyl)(2-(2-
pyridyl)phenyl)amide}(PPh3)][B(ArF)4] (13). Complex 9 (57 mg,
0.074 mmol), 72 mg of Na[B(ArF)4] (0.081 mmol), 21 mg of
triphenylphosphine (PPh3, 0.080 mmol), and 20 mL of dichloromethane
3992
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were stirred for 2 h, at which point the mixture was filtered by cannula.
The solvent was reduced to a minimal volume, and the concentrated
solution was added slowly to chilled, rapidly stirred pentane (20 mL). The
solid was isolated by cannula filtration and then dried under high vacuum
overnight, yielding 98 mg of the yellow product (71%). Yellow crystals
suitable for X-ray diffraction were grown by layering pentane on top of a
concentrated solution of 13 in dichloromethane. Using Olex2,73a the
structure was solved with the XS structure solution program using the
Patterson method and refined with the ShelXL refinement package using
least-squares minimization.73b Crystal data for C80H53BF30IrN2OP (Mr =
(3) (a) Somorjai, G. A.; Li, Y. Introduction to Surface Chemistry and
Catalysis; Wiley: Hoboken, NJ, 2010. (b) Rogal, J.; Reuter, K.;
Scheffler, M. Phys. Rev. B: Condens. Matter 2007, 75 (20), 205433.
(c) Shelef, M. Chem. Rev. 1995, 95, 209−225. (d) Taylor, R. A.; Law,
D. J.; Sunley, G. J.; White, A. J. P.; Britovsek, G. J. P. Angew. Chem., Int.
Ed. 2009, 48, 5900−5903.
(4) Poverenov, E.; Efremenko, I.; Frenkel, A.; Ben-David, Y.; Shimon,
L.; Leitus, G.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Nature
2008, 455, 1093−1096.
(5) Efremenko, I.; Poverenov, E.; Martin, J. M. L.; Milstein, D. J. Am.
Chem. Soc. 2010, 132, 14886−14900.
(6) Hay-Motherwell, R. S.; Wilkinson, G.; Hussain-Bates, B.;
Hursthouse, M. B. Polyhedron 1993, 12, 2009−2012.
(7) Jacobi, B. G.; Laitar, D. S.; Pu, L.; Wargocki, M. F.; DiPasquale, A.
G.; Fortner, K. C.; Schuck, S. M.; Brown, S. N. Inorg. Chem. 2002, 41,
4815−4823.
(8) McGhee, W. D.; Foo, T.; Hollander, F. J.; Bergman, R. G. J. Am.
Chem. Soc. 1988, 110, 8543−8545.
(9) Cao, R.; Anderson, T. M.; Piccoli, P. M. B.; Schultz, A. J.; Koetzle,
T. F.; Geletii, Y. V.; Slonkina, E.; Hedman, B.; Hodgson, K. O.;
1862.51): triclinic, space group P1 (No. 2), a = 12.7575(2) Å, b =
̅
15.9555(2) Å, c = 37.5887(5) Å, α = 85.0951(7)°, β = 84.2762(7)°,
γ = 81.0887(7)°, V = 7502.10(18) Å3, Z = 4, T = 100 K, μ(Cu Kα) =
4.753 mm−1, Dcalcd = 1.649 g/mm3, 288052 reflections measured
(2.368 ≤ 2θ ≤ 140.36), 27692 unique (Rint = 0.0395, Rσ = 0.0191), which
were used in all calculations. The final R1 value was 0.0283 (I > 2σ(I)),
and the wR2 value was 0.0680 (all data), with a goodness of fit on F2 of
1.054. Anal. Calcd (found) for C80H53N2BF30IrOP: C, 51.60 (51.77); H,
2.87 (2.64); N, 1.50 (1.42). 1H NMR (600 MHz, CD2Cl2): δ 8.21 (d, J =
7.8, 1H), 8.16 (d, J = 8.4, 2H), 7.96 (t, J = 7.2, 1H), 7.86 (d, J = 5.4, 1H),
7.72 (overlapping singlets, 10H), 7.63 (m, 2H), 7.59 (s, 1H), 7.56 (s,
4H), 7.57−7.52 (m, 4H), 7.46 (d, J = 7.8, 1H), 7.15 (t, J = 8.4, 2H), 7.06
(t, J = 7.2, 1H), 6.97 (t, J = 7.2, 1H), 6.77−6.74 (m, 3H), 6.60 (t, J = 6.0,
1H), 6.35 (d, J = 7.8, 1H), 5.98 (t, J = 9.0, 2H), 1.31 (s, 15H). 13C{1H}
Hardcastle, K. I.; Fang, X.; Kirk, M. L.; Knottenbelt, S.; Kogerler, P.;
̈
Musaev, D. G.; Morokuma, K.; Takahashi, M.; Hill, C. L. J. Am. Chem.
Soc. 2007, 129, 11118−11133.
NMR (150 MHz, CD2Cl2, one carbon merged to others): δ 171.3 (JC−P
=
(10) Anderson, T. M.; Neiwert, W. A.; Kirk, M. L.; Piccoli, P. M. B.;
Schultz, A. J.; Koetzle, T. F.; Musaev, D. G.; Morokuma, K.; Cao, R.;
Hill, C. L. Science 2004, 306, 2074−2077.
2), 162.1 (1:1:1:1 quartet, JC−B = 50), 158.1, 156.2 (JC−P = 3), 146.9,
141.0, 138.3, 137.5 (JC−P = 11), 135.1, 134.0 (JC−P = 9), 132.9 (JC−P = 9),
132.7 (JC−P = 3), 132.4, 132.2 (JC−P = 2), 131.9, 130.7, 130.5 (JC−P = 2),
130.3, 129.8 (JC−P = 10), 129.8 (q, 2-bond JC−F = 35), 129.7, 129.2 (q,
2-bond JC−F = 34), 128.9 (JC−P = 11), 128.2 (JC−P = 10), 127.0, 126.6,
(11) Anderson, T. M.; Cao, R.; Slonkina, E.; Hedman, B.; Hodgson,
K. O.; Hardcastle, K. I.; Neiwert, W. A.; Wu, S.; Kirk, M. L.;
Knottenbelt, S.; Depperman, E. C.; Keita, B.; Nadjo, L.; Musaev, D.
G.; Morokuma, K.; Hill, C. L. J. Am. Chem. Soc. 2004, 127, 11948−
11949.
(12) O’Halloran, K. P.; Zhao, C.; Ando, N. S.; Schultz, A. J.; Koetzle,
T. F.; Piccoli, P. M. B.; Hedman, B.; Hodgson, K. O.; Bobyr, E.; Kirk,
M. L.; Knottenbelt, S.; Depperman, E. C.; Stein, B.; Anderson, T. M.;
Cao, R.; Geletii, Y. V.; Hardcastle, K. I.; Musaev, D. G.; Neiwert, W.
125.5, 124.9 (q, 1-bond JC−F = 271), 124.9, 124.3, 123.4 (q, 1-bond JC−F
=
271), 123.0 (septet, 3-bond JC−F = 4), 117.8 (septet, 3-bond JC−F = 4),
90.5 (JC−P = 2), 9.8.
ASSOCIATED CONTENT
■
S
* Supporting Information
A.; Fang, X.; Morokuma, K.; Wu, S.; Kogerler, P.; Hill, C. H. Inorg.
̈
Figures, text, and CIF files giving NMR spectra of the OC and
of new compounds, full structure reports, and detailed kinetic
analysis. This material is available free of charge via the Internet
at http://pubs.acs.org.
Chem. 2012, 51, 7025−7031.
(13) McDaniel, N. D.; Coughlin, F. J.; Tinker, L. L.; Bernhard, S. J.
Am. Chem. Soc. 2008, 130, 210−217.
(14) Dzik, W. I.; Calvo, S. E.; Reek, J. N. H.; Lutz, M.; Ciriano, M. A.;
Tejel, C.; Hetterscheid, D. G. H.; de Bruin, B. Organometallics 2011,
30, 372−374.
AUTHOR INFORMATION
■
Corresponding Author
E-mail for J.L.T.: joetemp@unc.edu.
(15) Hetterscheid, D. G. H.; Reek, J. N. H. Chem. Commun. 2011, 47,
2712−2714.
(16) Blakemore, J. D.; Schley, N. D.; Balcells, D.; Hull, J. F.; Olack,
G. W.; Incarvito, C. D.; Eisenstein, O.; Brudvig, G. W.; Crabtree, R. H.
J. Am. Chem. Soc. 2010, 132, 16017−16029.
Notes
The authors declare no competing financial interest.
(17) Savini, A.; Bellachioma, G.; Ciancaleoni, G.; Zuccaccia, C.;
Zuccaccia, D.; Macchioni, A. Chem. Commun. 2010, 46, 9218−9219.
ACKNOWLEDGMENTS
■
(18) Lalrempuia, R.; McDaniel, N. D.; Muller-Bunz, H.; Bernhard, S.;
̈
This research was supported by the UNC EFRC “Center for Solar
Fuels”, an EFRC funded by the U.S. Department of Energy, Office
of Science, Office of Basic Energy Sciences, under award DE-
SC0001011. C.R.T. acknowledges support from the National
Science Foundation Graduate Research Fellowship Program under
Grant No. DGE-1144081 and from the National Science
Foundation under Grant No. CHE-1058675.
Albrecht, M. Angew. Chem., Int. Ed. 2010, 49, 9765−9768.
(19) Miranda-Soto, V.; Parra-Hake, M.; Morales-Morales, D.;
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131, 8730−8731.
(21) Zhou, M.; Schley, N. D.; Crabtree, R. H. J. Am. Chem. Soc. 2010,
132, 12550−12551.
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