Electron-rich CpIr(biphenyl-2,2′-diyl) complexes with π-accepting carbon donor ligands

Article abstract of DOI:10.1021/om300696t

Cp*Ir^III and CpIr^III complexes have attracted interest as catalysts for oxidative transformations, and highly oxidizing iridium species are postulated as key intermediates in both catalytic water and C-H bond oxidation. Strongly electron-donating ligand sets have been shown to stabilize Ir^IV complexes. We describe the synthesis and reactivity of complexes containing the CpIr(biphenyl-2,2′-diyl) moiety stabilized by a series of strong donor carbon-based ligands. The oxidation chemistry of these complexes has been characterized electrochemically, and a singly oxidized Ir^IV species has been observed by X-band EPR for the complex CpIr(biph)(p-CNCH₂SO₂C₆H₄CH ₃).

Full text of DOI:10.1021/om300696t

Article
pubs.acs.org/Organometallics
Electron-Rich CpIr(biphenyl-2,2′-diyl) Complexes with π‑Accepting
Carbon Donor Ligands
Jonathan Graeupner, Timothy P. Brewster, James D. Blakemore,† Nathan D. Schley,
Julianne M. Thomsen, Gary W. Brudvig,* Nilay Hazari,* and Robert H. Crabtree*
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107, United States
S
* Supporting Information
ABSTRACT: Cp*IrIII and CpIrIII complexes have attracted interest as
catalysts for oxidative transformations, and highly oxidizing iridium
species are postulated as key intermediates in both catalytic water and C−
H bond oxidation. Strongly electron-donating ligand sets have been
shown to stabilize IrIV complexes. We describe the synthesis and reactivity
of complexes containing the CpIr(biphenyl-2,2′-diyl) moiety stabilized by
a series of strong donor carbon-based ligands. The oxidation chemistry of
these complexes has been characterized electrochemically, and a singly
oxidized IrIV species has been observed by X-band EPR for the complex
CpIr(biph)(p-CNCH2SO2C6H4CH3).
INTRODUCTION
■
Cp*IrIII complexes are common in organometallic chemistry
but only recently have been found to be precatalysts for a
number of oxidative transformations.1−7 Several half-sandwich
IrIII species catalyze water oxidation,8−11 and our group has
recently developed a wide variety of complexes of this type that
are also active precatalysts for C−H bond oxidation.12−16 These
precatalysts give relatively high turnover frequencies for water
oxidation in comparison to ruthenium-based systems,17−25 but
Figure 1. T1, a water oxidation precatalyst with a highly donating
oxidative degradation of the molecular complexes is a
ligand set.12
concern.26−29 Recently, we investigated this degradation by
dynamic light scattering, identifying the conditions under which
the precatalysts remain molecular during turnover.30
single-electron-oxidation steps is plausible. We postulated that a
more strongly donating ligand set would further stabilize half-
Despite increased effort in the area, relatively little
mechanistic data are available regarding these iridium-catalyzed
oxidative transformations. A catalytic cycle involving a series of
sandwich IrIV species and allow us to study the reactivity in
more detail.
Specifically, we anticipated that moving to an X2 chelate from
the previously studied L-X chelate in T1 would increase σ
proton-coupled single-electron oxidations to generate an active
IrV oxo species has been proposed on the basis of kinetic,
electrochemical, and computational13 results, but experimental
data on the key high-valent intermediate(s) remain elusive. A
recent study from our group demonstrated that Cp*IrCl-
(κ2C2,C2′-1,3-diphenylimidazol-2-ylidene) (T1) (Figure 1), a
precatalyst for water oxidation that contains a highly donating
ligand set, undergoes one-electron oxidation with [Ru(bpy)3]3+
to generate an IrIV species that was observed by EPR
spectroscopy.12 The transient IrIV complex decomposes within
minutes at room temperature under an inert atmosphere.
Furthermore, this IrIV species contains an inner-sphere chloride
ligand and, thus, is unlikely to be involved directly in a catalytic
cycle for water or C−H bond oxidation where water is expected
to have replaced chloride. Nevertheless, the observation of an
IrIV species suggests that a mechanism involving sequential
donation to the metal center. In spite of its analogy with 2,2′-
dipyridyl and 2-phenylpyridine, biphenyl-2,2′-diyl (biph) has
rarely been used as a ligand for iridium systems.31−33
CpIr(biph)CO has been prepared by Rausch and co-workers
by the addition of fluorenone to CpIr(CO)I2, but this synthetic
route gives low yields.32 Subsequently, Wakatsuki and co-
workers prepared the Cp* analogue in a significantly higher
yield by addition of a sodium fluorenone complex to
[Cp*IrCl2]2.33 To date, these are the only reported half-
sandwich Ir(biph) complexes. A variety of biph-containing
transition-metal complexes have been prepared by oxidative
Received: July 23, 2012
Published: October 3, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of CpIr(biph)L Complexes31
Figure 2. ORTEP drawings of 1 (left) and 2 (right) shown at the 50% probability level. Solvent molecules and disorder modeling have been omitted
for clarity.
addition of the strained C−C bond in biphenylene,34−37 and in
1995, we reported that biphenylene can oxidatively add to
[Ir(cod)Cl]2 (where cod = 1,4-cyclooctadiene) to form
[(cod)Ir(biph)Cl]2 in high yields.31 This complex has
subsequently been used as a precursor for other Ir(biph)
species,38,39 as the cod ligand in [(cod)Ir(biph)Cl]2 is labile
and can be displaced by reaction with phosphine ligands.
Several IrIV derivatives were generated and examined by EPR
spectroscopy, including a unique IrIV boryl species that was
isolated and characterized by X-ray crystallography.
complexes have been investigated electrochemically, and for the
isonitrile complex a one-electron-oxidized IrIV species has been
observed and characterized by EPR spectroscopy.
RESULTS AND DISCUSSION
■
Synthesis of CpIr(biph)L Complexes. The addition of
sodium cyclopentadienide to a suspension of [Ir(cod)(biph)-
Cl]2 in dichloromethane generated CpIr(biph)(cod) (1) in
moderate yield. Interestingly, cod was found to bind the
resulting complex in the η2 mode rather than in the η4 chelate
or bridging modes more commonly observed (Scheme 1). Due
to the loss of the nonchelated olefin moiety, the resultant
complex CpIr(biph)(cod) (1) slowly dimerizes upon heating or
standing in solution. This dimerization, resulting in the
formation of complex 2, is slow at room temperature,
presumably due to the strength of the iridium−olefin bond.
Dimer 2 was never obtained in high purity, as it is not easily
Here we describe the synthesis of CpIr(biph)(η2-cod) and
find that the ligand set is so donating that only strong π
acceptor ligands can displace cod. The previously reported CO
complex has been obtained in moderate yield, and the new
monomeric species CpIr(biph)(CNCH2SO2PhMe) and a
dimeric species, [{CpIr(biph)}2(μ-η2:η2′-cod)], with a bridging
cod ligand, have been isolated and characterized. The new
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separated from 1 and the dimerization does not proceed to
completion. However, it was possible to characterize both the
monomeric and dimeric forms by X-ray crystallography (Figure
2). The structure of 1 represents only the second crystallo-
graphically characterized iridium complex in which cod binds in
an η2 fashion40 and the first example of an IrIII species.
Similarly, there exists only one previous example of an iridium
dimer containing a μ-η2:η2′-cod moiety,41 and there are no
prior examples of IrIII species. In all other crystallographically
characterized complexes, cod is found to chelate one iridium in
an η4 manner.
distribution was investigated as a function of iridium
concentration.
All reactions were carried out at 50 °C in 1,2-dichloroethane
under 1 atm of CO. In order to determine concentration
dependence, the reaction was run to completion in different
solvent volumes with a constant molar quantity of starting
material 1. Only carbonyl complex 3 and dimer 2 were
1
observed as products by H NMR spectroscopy. The product
distribution was determined as an average of two runs using the
relative integrations of the Cp signals (see the Supporting
Information). The results are shown in Figure 3.
1 crystallizes with minor disorder in the cyclooctadiene
moiety arising from the presence of two partially occupied
conformations. 2 crystallizes from dichloromethane/pentane as
a dichloromethane solvate in the high-symmetry P42bc space
group with two halves of two different dimers in the
asymmetric unit. The cod CC bond lengths in 1 of
1.399(15) and 1.314(14) Å for the coordinated and
uncoordinated olefins, respectively, are consistent with a
significant amount of π back-bonding from iridium, as would
be expected from an electron-rich metal.42 In 2, the olefinic
CC bond length is 1.42(2) Å, indicative of similar Dewar−
Chatt/metallocyclopropane character in the dimer.
At elevated temperatures cod can be exchanged, but only for
other π-accepting ligands (Scheme 1). Placing 1 under 1 atm of
CO in 5 mL of 1,2-dichloroethane at 50 °C affords a mixture of
the dimerized product 2 and the monomeric carbonyl complex
3. When the reaction is performed at a lower iridium
concentration (vide infra), the formation of 3 is favored. Full
conversion to 3 was obtained by carbonylation of 1 in a Parr
reactor under 68 atm of CO. Complex 3, which was previously
reported but not characterized,32 has now been isolated in 47%
yield and characterized by NMR and IR spectroscopy and
elemental analysis. The CO stretching band at 1994 cm−1 is at
the low end of the range seen with other previously reported
Cp*IrIII and CpIrIII carbonyl complexes and highlights the
strongly donating nature of the ligand set.43−48 Additionally,
treatment of complex 1 with a slight excess of C
NCH2SO2PhMe in dichloromethane at reflux yielded the
isonitrile complex 4 (Scheme 1).
Attempts to substitute the cod ligand in 1 with other types of
ligands were unsuccessful. Reactions with ligands such as
acetate, 2,4,6-trimethylaniline, and pyridine all returned
mixtures of starting material and dimerized product 2.
Surprisingly, even PPh3, PiPr3, and PMe3 were found to be
incapable of displacing cod from 1, suggesting that only strong
π-acceptor ligands can bind competitively. Furthermore, neither
treatment with iodine nor use of a high pressure of hydrogen
generated any new products. A full list of attempted ligand
substitution reactions is provided in the Supporting Informa-
tion.
Figure 3. Dependence of mole fraction of 2 and 3 on [1]. Reactions
were run at 50 °C in 1,2-dichloroethane under 1 atm of CO.
Diamonds correspond to the mole fraction of 3 after reaction. Squares
correspond to the mole fraction of 2.
Formation of the carbonyl species 3 is favored under more
dilute conditions, whereas dimer formation is favored when
using higher concentrations of the reacting complex 1. The
highest concentration tested, 3.87 mM 1, shows a product
distribution of about 3:1 of carbonyl complex 3 to dimer 2. A
distribution of 96:4 of carbonyl complex 3 to dimer 2 is
obtained when the reaction is run using 0.48 mM 1.
Unfortunately, further attempts to describe the kinetics of
this reaction were unsuccessful. Observing the formation of the
dimer 2 from monomer 1 was complicated by limited solubility
of 2 at the necessary concentrations for continuous observation
1
by H NMR spectroscopy.
Electrochemical Oxidation. With these complexes in
hand, we next turned our attention to the cyclic voltammetry
(CV) of 1, 3, and 4 to characterize their potentials for
oxidation. Since the ligand set is strongly σ donating, we
expected readily accessible metal-centered oxidations from IrIII
to IrIV, in analogy to previously reported complexes.12,13,27 The
cyclic voltammetry results are summarized in Figure 4.
Also unsuccessful were attempts to synthesize the Cp*
analogue of 1. Reaction of lithium pentamethylcyclopentadie-
nide with [Ir(cod)(biph)Cl]2 yielded only starting material and
decomposition products. This is a rare case in which the
Cp*IrIII complex is more difficult to access than the
corresponding CpIrIII complex.13
Product Distribution in Synthesis of CpIr(biph)CO. As
mentioned above, exposing the cod complex 1 to 1 atm of CO
in 1,2-dichloroethane yielded a mixture of both the carbonyl
species 3 and dimerized product 2. At reduced concentrations
of starting material 1, dimerized product 2 was observed in
smaller quantities. Therefore, the dependence of the product
Each of the compounds undergoes a quasi-reversible redox
event above 1.2 V vs NHE. These redox processes become
more reversible at higher scan rates, consistent with the limited
stability of the oxidized species formed at the electrode.
Specifically, at a scan rate of 100 mV/s, 1 has a midpoint
potential (E1/2) of 1320 mV with a peak separation of 157 mV.
The ratio of peak currents for the process (ip,c/ip,a) is 0.70,
suggesting that a majority of the species resulting from the
electrochemical oxidation survives to be reduced back to the
IrIII state. Isonitrile derivative 4 is slightly more reversible, with
a peak separation of only 100 mV and a ratio of peak currents
of 0.75. Interestingly, the net electronic contribution from the
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potentials for oxidation and more unstable oxidized iridium
species as a result.
The observed reduction potentials of 1, 3, and 4 are all
higher than those of other half-sandwich iridium complexes
studied in our laboratory.12,13,16,27 This is noteworthy, as the
donor power of biph is greater than that of the chelate ligand
present in any of our other complexes. However, the presence
of the strongly π-accepting ligand in the sixth coordination site
reduces the electron density around iridium enough to
overcome the additional donor power gained by using biph.
The lowest observed potential for oxidation of any of our
complexes is from T1 and centered at 0.9 V vs NHE.12 The L-
type ligand in this complex is the N-heterocyclic carbene, which
is an exceptionally poor π acceptor. T1 also contains Cp*,
which is a stronger electron donor than Cp. Thus, it appears
electrochemically that the net donor power of all ligands
present is optimal in complexes such as T1. If the ligand L in
the CpIr(biph)L framework could be replaced with a weaker π
acceptor, the reduction potential of the complexes could
perhaps be pushed below that of T1. Unfortunately, as
described above, this is not possible with our synthetic route.
EPR Spectroscopy. A relatively small number of stable IrIV
species have been previously reported,31,49−52 and few examples
of EPR spectra of Cp*IrIV species have been observed.12,53−55
Recently our laboratory designed a method for observing
unstable Cp*IrIV species by mixing solutions of Cp*IrIII
precursor and a stoichiometric oxidant, [Ru(bpy)3]3+, at cold
temperatures in a glovebox and then rapidly freezing the
mixture to trap oxidized iridium species. This procedure yielded
the EPR spectrum of the previously mentioned complex
[T1]+.12 In the case of this compound, quasi-reversible
electrochemical oxidation of the IrIII starting material gave a
strong indication that the oxidation product was stable, at least
transiently, and suitable for EPR work aimed at detecting
oxidized intermediate(s).
On the basis of these results, we applied the same method to
this family of CpIr(biph)L complexes. Analogous to previous
experiments,12 test complexes were first dissolved in a glassing
solvent mixture, 1/1 dichloromethane/toluene, in a glovebox.
These solutions were then mixed with 0.75 equiv of
[Ru(bpy)3]3+ in 1/1 dichloromethane/toluene and rapidly
frozen for EPR analysis.
Oxidation of the isocyanide complex 4 gives a weak rhombic
spectrum (Figure 5). This complex gave fairly reversible
electrochemistry (the ratio of peak currents was 0.75) (Figure
4), which is consistent with the ability to observe a metastable
oxidation product. The spectrum itself is unremarkable, with g
values at 2.44, ca. 2.08, and 1.91. Also present is a background
signal (in Figure 5, visible as the sloping baseline at lower
fields) attributable to unreduced [Ru(bpy)3]3+, as previously
reported.12,56 The midpoint potential obtained from the CV for
4 is at about 1.3 V vs NHE, while the standard reduction
potential for [Ru(bpy)3]3+/[Ru(bpy)3]2+ is 1.26 V vs NHE.
Thus, RuIII has enough oxidizing potential to oxidize complex 4.
The IrIV species generated from 4 shows limited thermal
stability. Warming the sample to room temperature for 20 min
results in the disappearance of the IrIV EPR signal. No EPR
signal was observed by mixing complex 1 or complex 3 with
[Ru(bpy)3]3+ under the same conditions used for 4. In the case
of 3, this is likely due to RuIII lacking sufficient driving force to
oxidize the CO-bound iridium center.
Figure 4. Cyclic voltammetry of 1 (top panel, a), 3 (middle panel, b),
and 4 (lower panel, c). Conditions: [Ir] = 10−3 M; dichloromethane
solution with 0.1 M NBu4PF6 as supporting electrolyte; basal-plane
graphite working electrode. Further details can be found in the
Experimental Section.
cod ligand must be similar to that of the isonitrile, since the E1/2
value for isonitrile 4 is 1330 mV, nearly identical with that of 1.
The midpoint potential for the CO complex 3 is 1550 mV
and the couple is less reversible, having a peak separation of 220
mV. Moreover, very little of the oxidized species survives to be
reduced, with a ratio of peak currents of only 0.35. Thus,
iridium is more electron deficient when bound to CO as
compared to the other ligands surveyed here, resulting in higher
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electrode, and Ag/0.1 M AgNO3 in acetonitrile was used as the
reference electrode. Ferrocene was used as an external standard to
calibrate the reference electrode versus NHE (Fc/Fc+: +0.690 V vs
NHE).59 Experiments were carried out in dry dichloromethane
solution containing 0.1 M tetrabutylammonium hexafluorophosphate
(Sigma-Aldrich) as the supporting electrolyte.
CpIr(biph)(cod) (1). A flame-dried 100 mL Schlenk flask
containing 638 mg of [Ir(cod)(biph)Cl]2 (1.31 mmol, 1 equiv) was
placed under nitrogen, and 10 mL of dry, degassed dichloromethane
was added via syringe, followed by 1.25 mL of sodium cyclo-
pentadienide (2.0 M in tetrahydrofuran) (2.50 mmol, 1.9 equiv). The
mixture was stirred under nitrogen for 3 h. The resulting solution was
filtered through Celite in air. Ten milliliters of diethyl ether was added,
and the solution was cooled overnight, forming a precipitate. The
precipitate was collected by vacuum filtration in air, washed with 10
mL of diethyl ether, and dried in vacuo to give the product as a dark
brown solid. Yield: 230 mg (33.9%). Crystals suitable for X-ray
diffraction were grown by layering diethyl ether on a saturated
dichloromethane solution. 1H NMR (400 MHz, CD2Cl2): δ 7.57 (dd,
J = 7.5, 0.8 Hz, 2H), 7.34 (dd, J = 7.6, 1.3 Hz, 2H), 6.88 (td, J = 7.5,
1.2 Hz, 2H), 6.73 (td, J = 7.4, 1.5 Hz, 2H), 5.55 (m, 2H), 5.34 (s, 5H),
2.68 (m, 2H), 2.40 (m, 2H), 2.30 (m, 2H), 1.98 (dd, J = 19.9, 10.4 Hz,
2H), 1.85 (m, 2H). 13C{1H} NMR (126 MHz, CD2Cl2): δ 32.01,
35.37, 64.27, 90.93, 121.00, 123.30, 125.54, 130.77, 140.50, 142.88,
154.09. Anal. Calcd for C25H25Ir: C, 58.00; H, 4.87. Found: C, 57.63;
H, 4.76.
Figure 5. X-band EPR spectrum of 4 upon mixing with 0.75 equiv of
[Ru(bpy)3]3+ in a 1/1 dichloromethane/toluene mixture. Conditions:
[Ir] = 10−3 M; microwave frequency 9.398 673 GHz; 7 K. For details,
see the Experimental Section.
CpIr(biph)(CO) (3). In a Parr reactor 48.2 mg of CpIr(biph)(cod)
(0.093 mmol) was dissolved in 21 mL of dry 1,2-dichloroethane. The
mixture was placed under 68 atm of CO gas and heated to 70 °C for
24 h with stirring. The resulting mixture was cooled to room
temperature, and the pressure was reduced to 1 atm. The solvent was
removed using a rotary evaporator, and the resulting light brown solid
was washed with hexane and dried in vacuo to yield pure product.
CONCLUSIONS
■
We have described a new synthetic route for CpIrIII complexes
of the strongly electron donating ligand biph and have explored
the reactivity of these complexes both synthetically and
electrochemically. This ligand framework is so electron
donating that the complex seems to require strong π acceptor
ligands in the sixth coordination site of iridium. Accordingly, we
find a low νCO in the infrared spectrum of CpIr(biph)CO of
1994 cm−1 in comparison with other literature CpIrCO
complexes. The isonitrile-bound compound CpIr(biph)(p-
CNCH2SO2C6H4CH3) shows a quasi-reversible iridium-cen-
tered oxidation by cyclic voltammetry, and the resulting
iridium(IV) species generated on oxidation has been observed
by EPR spectroscopy.
1
Yield: 22.5 mg (47.1%). H NMR (400 MHz, CD2Cl2): δ 7.55 (m,
4H), 7.08 (t, J = 7.4 Hz, 2H), 6.83 (t, J = 7.4 Hz, 2H), 5.70 (s, 5H).
13C{1H} NMR (126 MHz, CD2Cl2): δ 89.76, 121.76, 124.85, 126.34,
131.58, 141.77, 155.63, 166.85. Anal. Calcd for C18H13IrO: C, 49.41;
H, 2.99. Found: C, 49.17; H, 3.10.
CpIr(biph)(p-CNCH2SO2C6H4CH3) (4). A flame-dried 100 mL
Schlenk flask with an attached reflux condenser containing 42.0 mg of
CpIr(biph)(cod) (0.082 mmol, 1 equiv) and 25.5 mg of tosylmethyl
isocyanide (0.131 mmol, 1.6 equiv) was placed under nitrogen, and 10
mL of dry, degassed dichloromethane was added. The mixture was
heated to reflux for 24 h and then cooled to room temperature.
Solvent was removed by rotary evaporation, and the resulting solid was
dissolved in 5 mL of dichloromethane, filtered, and washed with excess
hexane. The product precipitated out in the filtrate and was filtered
and dried in vacuo to give a light brown solid. Yield: 22.5 mg (49.5%).
1H NMR (400 MHz, CD2Cl2): δ 7.49 (dd, J = 7.5, 1.1 Hz, 2H), 7.40
(dd, J = 7.6, 1.4 Hz, 2H), 7.31 (d, J = 8.3 Hz, 2H), 7.16 (d, J = 8.0 Hz,
2H), 6.94 (td, J = 7.4, 1.2 Hz, 2H), 6.69 (td, J = 7.4, 1.5 Hz, 2H), 5.48
(s, 5H), 4.55 (s, 2H), 2.35 (s, 3H). 13C{1H} NMR (126 MHz,
CD2Cl2): δ 21.92, 63.53, 87.92, 90.76, 121.12, 123.72, 125.74, 129.22,
130.57, 133.25, 136.37, 141.71, 146.62, 155.76. Anal. Calc. for
C26H22IrNO2S: C, 51.64; H, 3.67; N, 2.32. Found: C, 51.98; H,
3.83; N, 2.02.
Procedure for Product Distribution Experiment. A small
round-bottom flask containing 5 mg of CpIr(cod)(biph) (1) was
placed under nitrogen and 2.5, 5, 10, 15, or 20 mL of 1,2-
dichloroethane was added. The reaction was sparged with CO and
stirred at 50 °C for 24 h under 1 atm of CO. The solution was then
cooled rapidly to room temperature and the solvent removed using a
rotary evaporator. The resulting light brown solid was washed with
hexane and dried in vacuo. The product distribution was determined
by relative integration of the 1H NMR signals from the Cp protons of
each compound (2 or 3).
EXPERIMENTAL SECTION
■
[Ir(cod)Cl]2, [Ir(cod)(biph)Cl]2, and [Ru(bpy)3](PF6)3 were pre-
pared according to literature procedures.31,56,57 Dichloromethane was
dried on a Grubbs-type solvent purification system.58 Anhydrous 1,2-
dichloroethane was purchased from Aldrich in a Sure/Seal bottle to
ensure purity. All other reagents and materials were commercially
1
available and were used without further purification. H NMR spectra
were recorded at room temperature on a 400 MHz Bruker
spectrometer and referenced to the residual solvent peak (δ in ppm,
J in Hz). 13C{1H} NMR spectra were recorded on a 500 MHz Varian
spectrometer and referenced to the residual solvent peak. EPR spectra
were recorded on an X-band Bruker ELEXSYS E500 spectrometer
equipped with an Oxford ESR-900 cryostat. The nominal temperature
for experiments was 7 K. The modulation amplitude was set to 2 G
and the microwave frequency set to 9.398 673 GHz. Experiments were
recorded with a time constant of 41 ms. Elemental analysis was
performed by Robertson Microlit Laboratories (Ledgewood, NJ).
Electrochemical Studies. Electrochemical measurements were
made on a Princeton Applied Research Versastat 4-400 or Model 273
potentiostat/galvanostat using a standard three-electrode configura-
tion. A basal plane graphite electrode (surface area 0.09 cm2) was used
as the working electrode to minimize background oxidative current.
The electrode was constructed and prepared for experiments as
described previously.13 A platinum wire was used as the counter
Procedure for EPR Experiment. The method for preparation of
the EPR samples is a slight modification of our previously published
procedure.12 In a glovebox, 2 μmol of iridium complex was dissolved
in 1 mL of 1/1 dichloromethane/toluene and transferred to an EPR
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tube placed in a cold well. Separately, 0.75 equiv of [Ru(bpy)3](PF6)3
was dissolved in 1 mL of dichloromethane/toluene. The [Ru(bpy)3]-
(PF6)3 solution was then layered onto the frozen solution of iridium
complex and chilled. EPR samples were then removed from the
glovebox while cold and immediately stored in a liquid nitrogen bath.
Just prior to recording the spectra, the samples were briefly warmed
above the freezing point of the solvent mixture, rapidly mixed, and
refrozen, and spectra were recorded. Samples containing CpIr(biph)-
(p-CNCH2SO2C6H4CH3) (4) were then allowed to stand at room
temperature for 20 min prior to refreezing to examine the thermal
stability of the observed iridium(IV) species.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF files giving NMR spectra, the IR
spectrum of 3, a representative spectrum and raw data from the
synthesis of 3 at varied [1], and crystallographic details. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: robert.crabtree@yale.edu (R.H.C.); nilay.hazari@yale.
edu (N.H.); gary.brudvig@yale.edu (G.W.B.).
(23) Concepcion, J. J.; Jurss, J. W.; Templeton, J. L.; Meyer, T. J. J.
Am. Chem. Soc. 2008, 130, 16462.
(24) Tseng, H. W.; Zong, R.; Muckerman, J. T.; Thummel, R. Inorg.
Chem. 2008, 47, 11763.
(25) Yagi, M.; Tajima, S.; Komi, M.; Yamazaki, H. Dalton Trans.
Present Address
†Beckman Institute and Division of Chemistry and Chemical
Engineering, California Institute of Technology, Caltech MC
139-74, Pasadena, California 91125, United States.
2011, 40, 3802.
(26) Blakemore, J. D.; Schley, N. D.; Olack, G. W.; Incarvito, C. D.;
Brudvig, G. W.; Crabtree, R. H. Chem. Sci. 2011, 2, 94.
(27) Schley, N. D.; Blakemore, J. D.; Subbaiyan, N. K.; Incarvito, C.
D.; D’Souza, F.; Crabtree, R. H.; Brudvig, G. W. J. Am. Chem. Soc.
2011, 133, 10473.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(28) Zuccaccia, C.; Bellachioma, G.; Bolano, S.; Rocchigiani, L.;
̃
Savini, A.; Macchioni, A. Eur. J. Inorg. Chem. 2012, 2012, 1462.
(29) Grotjahn, D. B.; Brown, D. B.; Martin, J. K.; Marelius, D. C.;
Abadjian, M.-C.; Tran, H. N.; Kalyuzhny, G.; Vecchio, K. S.; Specht, Z.
G.; Cortes-Llamas, S. A.; Miranda-Soto, V.; van Niekerk, C.; Moore, C.
E.; Rheingold, A. L. J. Am. Chem. Soc. 2012, 133, 19024.
(30) Hintermair, U.; Hashmi, S. M.; Elimelech, M.; Crabtree, R. H. J.
Am. Chem. Soc. 2012, 134, 9785.
This material is based in part upon work supported as part of
the Argonne-Northwestern Solar Energy Research (ANSER)
Center, an Energy Frontier Research Center funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences, under Award Number DE-SC0001059 (R.H.C.,
G.W.B., J.D.B., and J.M.T., electrochemistry and EPR; T.P.B.,
synthesis) and a catalysis grant from the Division of Chemical
Sciences, Geosciences, and Biosciences, Office of Basic Energy
Sciences, of the U.S. Department of Energy through Grant No.
DE-FG02-84ER13297 (N.D.S., crystallography).
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