An iridium(IV) species, [Cp*Ir(NHC)Cl]+, related to a water-oxidation catalyst

Article abstract of DOI:10.1021/om101016s

The Ir precatalyst (3) contains both a Cp* and a κ ²C²,C^2′-1,3-diphenylimidazol-2-ylidene ligand, a C-C chelate, where one C donor is the NHC and the other is a cyclometalated N-phenyl wingtip group. The structure of 3 was

Full text of DOI:10.1021/om101016s

Organometallics 2011, 30, 965–973 965
DOI: 10.1021/om101016s
An Iridium(IV) Species, [Cp*Ir(NHC)Cl]þ, Related to a
Water-Oxidation Catalyst
Timothy P. Brewster, James D. Blakemore, Nathan D. Schley, Christopher D. Incarvito,
Nilay Hazari,* Gary W. Brudvig,* and Robert H. Crabtree*
Department of Chemistry, Yale University, P.O. Box 208107, New Haven, Connecticut 06520-8107,
United States
Received October 26, 2010
0
The Ir precatalyst (3) contains both a Cp* and a κ2C2,C2 -1,3-diphenylimidazol-2-ylidene ligand,
a C-C chelate, where one C donor is the NHC and the other is a cyclometalated N-phenyl wingtip
group. The structure of 3 was confirmed by X-ray crystallography. Like our other recently described
Cp*Ir catalysts, this compound is a precursor to a catalyst that can oxidize water to dioxygen.
Electrochemical characterization of the new compound shows that it has a stable iridium(IV)
oxidation state, [Cp*IrIV(NHC)Cl]þ, in contrast with the unstable Ir(IV) state seen in our previous
cyclometalated [Cp*IrIII(2-pyridyl-20-phenyl)Cl] catalyst. The new iridium(IV) species has been
characterized by EPR spectroscopy and has a rhombic symmetry, a consequence of the ligand
environment. These results both support previous studies which suggest that Cp*Ir catalysts can be
advanced through the relevant catalytic cycle(s) in one-electron steps and help clarify the electro-
chemical behavior of this class of water-oxidation catalysts.
Introduction
protons to hydrogen or carbon-containing molecules to
other fuels.7-13 Homogeneous water oxidation catalysts,
which have well-defined structures and mechanisms of ac-
tion, are a key component of many of these proposed
systems.14 The so-called “blue dimer”, the first homogeneous
water-oxidation catalyst, was described in 1982 by Meyer
and co-workers.15 Since then, binuclear ruthenium-based water-
oxidation catalysts have been studied in great detail.16-20 More
recently, mononuclear ruthenium water oxidation catalysts
have emerged as attractive systems for catalytic water oxidation,
due to their efficacy and simplicity.21,22 Mechanistic investiga-
tions of both mononuclear and binuclear ruthenium catalysts
Rising demand for alternative energy due to geopolitical
concerns such as global climate change has led to increased
interest in carbon-neutral “green” fuels.1-3 In order to
produce these alternative fuels, an abundant and sustainable
source of reducing equivalents must be found. Green plants,
algae, and cyanobacteria utilize water as their source of
electrons and protons for sustaining life processes and redu-
cing carbon dioxide to carbohydrates. Specifically, genera-
tion of protons and electrons out of water is accomplished by
the enzyme-cofactor complex Photosystem II (PSII).4-6 In
PSII, a manganese-calcium cluster is sequentially oxidized
and ultimately releases four protons, four electrons, and
dioxygen from two water molecules (eq 1).
(8) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163.
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Acc. Chem. Res. 2009, 42, 1966.
2H2O f 4Hþ þ 4e- þ O2
ð1Þ
Many artificial photosynthesis and solar-fuel production
schemes, inspired by natural photosynthesis, propose using
water as a source of reducing equivalents for reduction of
(12) Brimblecombe, R.; Koo, A.; Dismukes, G. C.; Swiegers, G. F.;
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Inorg. Chem. 2008, 47, 1835.
*To whom correspondence should be addressed. E-mail: robert.
crabtree@yale.edu (R.H.C.); gary.brudvig@yale.edu (G.W.B.); nilay.
hazari@yale.edu (N.H.).
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2011 American Chemical Society
Published on Web 02/08/2011
pubs.acs.org/Organometallics

966 Organometallics, Vol. 30, No. 5, 2011
Brewster et al.
Since the isolation of the first stable free N-heterocyclic
carbene (NHC) by Arduengo in 1991, NHCs have seen
extensive use as ligands in organometallic complexes with
catalytic applications and can impart unique activity to
catalytic systems.30-33 NHC ligands are known for binding
tightly to metal centers and for their high donor capacity
relative to phosphines.31,34 Additionally, the relatively
strong σ-donor power of the ligand assists in stabilizing
high-valent metal complexes. Recently, Cp*Ir catalysts bear-
ing an N-heterocyclic carbene ligand have been shown to be
competent for water oxidation.35,36
Figure 1. Cp*Ir water oxidation catalysts and precatalysts.28,29
We now report the synthesis of the N-heterocyclic carbene
Cp*Ir complex 3 (Scheme 1), which is a competent catalyst
precursor for water oxidation. The stabilizing effect of the
NHC ligand on high-valent iridium is clearly seen by com-
paring the electrochemical behavior of 1 and 3. By electro-
chemistry, 1 lacks a stable iridium(IV) form, even in the
absence of water. However, 3 shows quasi-reversible forma-
tion of an iridium(IV) species. Furthermore, a singly oxi-
dized iridium(IV) species is now experimentally observable
by electron paramagnetic resonance (EPR) spectroscopy
using [RuIII(bpy)3]3þ (bpy = 2,20-bipyridine) as a primary
oxidant. This oxidant was chosen because it is sufficiently
strong to effect oxidation to Ir(IV) but not so strong as to
access the Ir(V) state.
have led to a consensus view of the requirement for high-valent
ruthenium intermediates which arise by oxidation via proton-
coupled electron transfer.20 In these mechanistic proposals,
oxidation from lower valent ruthenium occurs along with
deprotonation of water bound to the metal center. Eventually,
a sufficiently oxidizing intermediate is reached (usually a
ruthenium(V) oxo species) which is susceptible to either nu-
cleophilic attack from substrate water or accomplishes O-O
bond formation by oxo-oxo coupling.18 Such high-valent
intermediates have also found use in oxidative transformations
of organic molecules.23-26
Iridium catalysts also oxidize water. In 2008, Bernhard
and co-workers showed that, upon addition of cationic
iridium(III) bis(solvento) bis(2-pyridyl-20-phenyl) com-
plexes to solutions of cerium(IV) ammonium nitrate, oxygen
is evolved.27 More recently, we have investigated a series of
Cp*Ir oxidation catalysts which are capable of water oxida-
tion at high rates.28,29 Among these catalysts, 1 and 2,
which bear the cyclometalated phenylpyridine (ppy) ligand
(Figure 1), are among the most active, achieving turnover
numbers for oxygen evolution of 10 turnovers min-1 when
driven with cerium(IV) at pH 0.89. Catalyst 1 also has been
shown to mediate oxidative transformations of organic
molecules such as alkane hydroxylation.26 These oxidations
are anticipated to proceed via proton-coupled electron
transfer to form high-valent iridium-oxo intermediates.
However, no spectroscopic characterization of intermediates
has been possible for the Cp*Ir(ppy)L system, primarily due
to its fast rate of oxygen evolution and the consequent short
lifetimes of intermediates in solution. Reactions carried out
in water or in water/organic solvent mixtures rapidly lead to
product formation. DFT calculations have supported the
proposed mechanism of sequential one-electron oxidations
of the complex occurring via proton-coupled electron
transfer.29 The key intermediate for water oxidation was
proposed to be an iridium(V) oxo, which has favorable
energetics to undergo attack from nucleophilic water and
form the O-O bond.
Results and Discussion
0
Synthesis and Characterization of Cp*Ir(K2C2,C2 -NHC)-
(Cl). N,N0-Diphenylimidazolium chloride (4) was synthe-
sized by an adaptation of the standard preparation for N,
N0-dimesitylimidazolium chloride.37 Although 4 was origin-
ally synthesized by Wanzlick in the 1960s, its use in organo-
metallic chemistry has been limited, due to its relative
instability as a free carbene.38 For our purposes, the phenyl
wingtip groups were anticipated to provide necessary oxida-
tion resistance versus other alkyl or mixed aryl-alkyl NHCs.
Treatment of the imidazolium salt 4 with [Cp*IrCl2]2 in the
presence of potassium tert-butoxide generates 3 (Scheme 1).
Following chromatographic purification (see the Experimental
Section), 3 was isolated as an analytically pure yellow powder.
Crystals suitable for X-ray analysis were obtained from di-
chloromethane solution by vapor diffusion of pentane. The
solid-state structure is shown in Figure 2. The obtained struc-
˚
ture has an Ir-C(11) bond length of 1.998(5) A and an Ir-C-
(15) bond length of 2.047(5) A, consistent with those observed
˚
in similar Cp*Ir(NHC) complexes.35,39-41
(30) Arduengo, A. J.; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.
1991, 113, 361.
(31) de Fremont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev.
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Brudvig, G. W.; Crabtree, R. H. Inorg. Chem. 2009, 48, 488.
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J. Am. Chem. Soc. 2010, 132, 16017.
Weinheim, Germany, 2006.
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Incarvito, C. D.; Crabtree, R. H. Organometallics 2009, 28, 321.
(34) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239.
€
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S.; Albrecht, M. Angew. Chem., Int. Ed. 2010, 49, 9765.
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DOI: 10.1039/C0CC05108J.
(37) Nolan, S. P. U.S. Patent Application 2003-653668, 2006.
€
(38) Schonherr, H. J.; Wanzlick, H. W. Chem. Ber. 1970, 103, 1037.
(39) Xiao, X. Q.; Jin, G. X. J. Organomet. Chem. 2008, 693, 3363.
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(29) Hull, J. F.; Balcells, D.; Blakemore, J. D.; Incarvito, C. D.;
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131, 8730.
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Article
Organometallics, Vol. 30, No. 5, 2011 967
Scheme 1. Synthesis of Cp*Ir(K2C2,C2 -NHC)(Cl) (3)
0
shows that 3 gives faster rates of oxygen evolution at higher
loadings, suggesting multimolecular mechanisms of oxygen
evolution. A traditional log-log plot of the same data gives
an order of reaction in 3 of 1.85. By analogy with previously
studied Cp*Ir complexes,28,47 this suggests that the NHC
ligand may be lost under the harsh catalytic conditions,
though not under electrochemical conditions (vide infra),
to give a [Cp*Ir(H2O)3]2þ fragment which contributes to
catalysis (see the Supporting Information). Catalysts 1 and 2
exhibit much more unexceptional oxygen evolution traces
when exposed to water in the presence of cerium(IV)
(Figure 3).28,29 Thus, with cerium(IV), 3 exhibits activity
distinct from that of previously studied iridium half-sand-
wich compounds, suggesting a different mode of precatalyst
activation28 and the possibility of heterogeneous catalysis.
Overall, although 3 has been shown to be a competent
precatalyst, complex 1 exhibits superior rates of reaction.
As Ce(IV) oxidations require low pH which can contribute
to catalyst deactivation, we thus also screened sodium
periodate as the primary oxidant for oxygen evolution, as
it can function near neutral pH. Under our chosen condi-
tions (5 mM, pH ca. 5), periodate gives rates of 12-16
turnovers min-1. 3 shows deactivation at higher oxidant
loadings, suggesting a multimolecular deactivation pathway.
Interestingly, the oxygen evolution traces themselves appear
more as expected at low micromolar precatalyst loadings,
with a linear oxygen evolution trace, diminished lag phase,
and no initial oxygen consumption. This supports the hy-
pothesis that oxygen consumption is a product of the harsh
conditions required when using Ce(IV) as oxidant. However,
a log-log plot of the rate dependence gives an order of 0.67
in 3, suggesting complicated kinetics (see Supporting In-
formation). This complication may arise from the fact that
periodate can function as a one- or two-electron oxidant.48 A
more detailed study of the mechanism of periodate-driven
water oxidation is underway so as to allow direct comparison
of this result to a wide range of water oxidation catalysts.
Electrochemical Studies. Electrochemical studies allow us
to carefully examine the oxidation behavior of our com-
plexes without the complications that come from the pre-
sence of our primary oxidants, cerium(IV) and periodate.
Compounds 1-3 are soluble in acetonitrile and a 50/50
acetonitrile/water mixture and were investigated in these
solvents by cyclic voltammetry with a basal plane graphite
working electrode in a standard three-electrode configura-
tion (see the Experimental Section for details). As reported
previously, compound 1 displays three dominant irrevers-
ible waves in electrochemical studies in organic solvent
(Figure 4).29 The first oxidation, which occurs near 1.1 V
Figure 2. ORTEP drawing of the crystal structure of Cp*Ir-
(κ2C2,C2 -NHC)(Cl) (3). Thermal ellipsoids are shown at the
50% probability level. Hydrogen atoms are omitted for clarity.
0
Water Oxidation. Cp*Ir complexes have been shown to be
competent catalysts for a variety of oxidative transforma-
tions.42-46 Using cerium(IV) ammonium nitrate (CAN) as
the primary oxidant, 1 has been shown to be competent for
water oxidation,29 epoxidation of cyclooctene, and C-H
oxidation of cis-decalin to cis-decalol.26
In the case of the NHC precatalyst 3, two primary oxidants
were screened for driving water oxidation. By analogy with
previous work with 1 and 2,28,29 cerium(IV) ammonium
nitrate was chosen (78 mM, pH 0.89) and gives a rate of
oxygen evolution of 8 turnovers min-1 at 4.5 μM loading of
3. However, under these harsh conditions of very low pH and
high salt concentration, oxygen evolution data from a Clark-
type electrode show a distinct lag phase, slight consumption
of oxygen upon injection, and distinct upwardly concave
shape (Figure 3). The lag phase showing consumption of
oxygen is unique, not previously observed for other Cp*Ir
systems.28 It is, however, possible that this corresponds to a
slow dissociation of Cl- 36 followed by oxidation to OCl- or
higher oxyanions. Though kinetically slow under ambient
conditions, it is thermodynamically possible for oxygen to
oxidize Cl- to OCl-. In the presence of cerium(IV) and high-
valent iridium, this reaction may be kinetically accessible. It is
also possible, as noted below, that this oxygen consumption
corresponds to a ligand oxidation or ligand loss. The depen-
dence of the oxygen-evolution rate on precatalyst loading
(42) Jiang, B.; Feng, Y.; Ison, E. A. J. Am. Chem. Soc. 2008, 130,
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(43) Hanasaka, F.; Fujita, K. I.; Yamaguchi, R. Organometallics
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Leung, W. H. J. Organomet. Chem. 2010, 695, 1399.
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968 Organometallics, Vol. 30, No. 5, 2011
Brewster et al.
Figure 3. Oxygen evolution traces in 5 mL H2O using 78 mM cerium(IV) as the terminal oxidant: (left) complex 1 (10 μM loading);
(right) complex 3 (15 μM loading).
acetonitrile solutions containing tetrabutylammonium chlo-
ride show this oxidation corresponds to free chloride oxida-
tion (presumably to chlorine) at the basal plane graphite
electrode, as observed in similar cases by other groups.49
On the basis of the results in Figure 4, the 1.1 V oxidation
wave peak arises from oxidation of the chloride-bound
Cp*Ir(ppy)Cl (1). This oxidation is followed at higher po-
tentials by completely irreversible oxidation of the analogous
[Cp*Ir(ppy)MeCN]þ adduct. The cationic solvento complex
is thus significantly more difficult to oxidize than the neutral,
chloride-bound complex. This is consistent with the complete
loss of the 1.1 V irreversible oxidation when the chloride ligand
in 1 is substituted for triflate in 2. In the case of this solvento
complex, the 1.6 V oxidation wave does not appear to change
potential in comparison with 1. However, the 2 V oxidation
observed in the case of 1 does seem to change in appearance;
this could be due to the formation of different species in
solution. However, two factors complicate further analysis of
these oxidation waves. First, they are both completely irrever-
sible, which is likely a result of high instability in solution in the
oxidized form. Furthermore, the very high potential required
for observation of the ca. 2 V oxidation (and in the case of 1, the
presence of chloride) makes identification of even the exact
peak potential uncertain.
Supporting this model of the electrochemical data, upon
titration of chloride into solutions containing 2, the first
irreversible oxidation in 1 can be seen to reappear (see the
Supporting Information). This strongly supports the idea
that the first oxidation of 1 in Figure 4 is associated with the
chloride-bound complex. Additionally, the equilibrium be-
tween chloride-bound and solvent species can be shifted by
chloride addition so as to favor chloride-bound species; the
resulting 1.1 V oxidation wave plateaus in intensity after
addition of ca. 8-10 equiv of chloride per metal center.
Compound 3 has different redox behavior compared with
compounds 1 and 2. A wide scan from ca. 0.2 V to nearly 2.0
V vs NHE shows several oxidation waves (Figure 5, upper
Figure 4. Cyclic voltammograms of compounds 1 (bottom) and
2 (top) in acetonitrile with 0.1 M tetrabutylammonium perchlo-
panel). The first oxidation wave is quasi-reversible on a full-
width scan and occurs at E1/2 = 0.9 V. This appears to be
analogous to the irreversible oxidation wave which appears
at 1.1 V in the case of compound 1 (vide infra). The second
oxidation is fully irreversible and occurs at approximately
1.5 V; this is a slightly less oxidizing potential than that which
is required for the oxidation wave near 1.6 V in the case of
rate as supporting electrolyte. Conditions: ca. 3 mM catalyst,
scan rate 100 mV/s.
vs NHE, is totally irreversible. Upon removal of the chloride
ligand and substitution to form the triflate complex 2, this first
oxidation completely disappears. In both complexes 1 and 2,
there are two higher potential, irreversible oxidations which
occur near 1.6 and 2 V. In the case of complex 1, there is an
additional irreversible feature appearing as a shoulder on
the 1.6 V oxidation. Cyclic voltammograms collected on
(49) Kasuno, M.; Hayano, M.; Fujiwara, M.; Matsushita, T. Poly-
hedron 2009, 28, 425.

Article
Organometallics, Vol. 30, No. 5, 2011 969
Figure 6. Cyclic voltammograms: (top) 3 in acetonitrile (ca. 3
mM) containing electrolyte; (bottom) 3 in a 50/50 acetonitrile/
water mixture (ca. 2 mM, bold line) and background (dashed
line). The scan rate was 100 mV/s.
Figure 5. Cyclic voltammograms: (top) 3 (solid line) and 1
(dashed line) in acetonitrile solution with 0.1 M tetrabutylam-
monium perchlorate as supporting electrolyte; (bottom) com-
pound 3, centered on the lowest potential, quasi-reversible redox
process. Conditions: ca. 3 mM complex, scan rate 100 mV/s.
50 water/acetonitrile mix, the reversible oxidation peak
centered at 0.9 V disappears entirely, consistent with dis-
placement of all inner-sphere chloride for inner-sphere aqua
ligands. Furthermore, at ca. 1.4 V, there is onset of a
completely irreversible, catalytic oxidation process, indica-
tive of conversion of bound water into dioxygen. The
potential of this catalytic process is consistent with assign-
ment of the ca. 1.5 V process for 3 in MeCN as irreversible
oxidation of the acetonitrile-bound species. In the case of
water ligation, a redox-leveling effect due to proton-coupled
electron transfer processes is expected to result in nearly
isoenergetic oxidation couples leading to catalytic water
oxidation (i.e., IrIII-OH2/IrIV-OH, IrIV-OH/IrVdO).28,51
Similar changes in voltammetric response are seen in the case
of 2 in water/acetonitrile mixtures. In 50/50 acetonitrile/
water, 2 shows only a catalytic wave (Figure 7). In neither
case are deposition or decomposition processes evident from
the voltammetry. This suggests that ligand loss does not
occur under these conditions, since loss of ligand to form a
fragment such as [Cp*Ir(H2O)3]2þ would lead to alternative
catalytic processes that we have reported elsewhere.47
compounds 1 and 2. Computational analysis using density
functional theory (DFT) supports these data, confirming the
lower iridium(III)/iridium(IV) oxidation potential of com-
plex 3 in comparison to complex 1 (see the Supporting
Information for details).
Narrowing the scan window to contain only the quasi-
reversible feature at 0.9 V results in an improvement of the
electrochemical reversibility for the oxidation-reduction
process taking place (Figure 5, bottom panel). Examination
of the behavior of the peak confirms that the process is nearly
reversible, as judged by electrochemical theory.50 The peak
separation (ΔEp) is 76 mV, only slightly larger than the ideal
59 mV. Furthermore, the ratio of peak currents (ip,c/ip,a),
expected to be unity for an ideal reversible process, is found
experimentally to be 0.90. With this result, we expected that
the quasi-reversible process taking place was the one-elec-
tron oxidation-reduction of the Cp*IrIII(NHC)Cl complex
with chloride in the inner coordination sphere.
Addition of water to solutions containing 1-3 in acetoni-
trile with electrolyte results in a complete change in the
electrochemical response (Figure 6). For complex 3 in a 50/
EPR Observation of Iridium(IV) Species. The electroche-
mical results shown above imply that the iridium(IV) species
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D. A.; Gersten, S. W.; Hodgson, D. J.; Meyer, T. J. J. Am. Chem. Soc.
1985, 107, 3855.
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970 Organometallics, Vol. 30, No. 5, 2011
Brewster et al.
Figure 8. X-band EPR spectra: (solid line) 3 oxidized by 0.75
equiv of [RuIII(bpy)3]3þ; (dashed line) [RuIII(bpy)3]3þ control.
(PF6)3 were each dissolved in a 1/1 acetonitrile/toluene
solvent mixture (this mixture forms a glass and is suitable
for EPR studies). Solutions were cold when mixed, having
been chilled in the glovebox cold well by liquid nitrogen; the
final solution was removed at low temperature from the
glovebox and immediately frozen in liquid nitrogen for EPR
analysis. The results of the EPR experiments are summarized
in Figure 8.
The oxidized iridium complex (solid line in Figure 8) gives
a rhombic signal with a center crossing point at g = 2.17.
Initially, 3 is a low-spin iridium(III) complex with a d6
configuration and S = 0. The ruthenium(III) oxidant is low-
spin d5, S = 1/2, and gives the EPR signal shown as the dashed
line in Figure 8. This spectrum (axial with g omitted for clarity)
obtained for [RuIII(bpy)3]3þ is consistent with literature data.53
Upon treatment of 3with[RuIII(bpy)3]3þ, the resulting iridium-
Figure 7. Cyclic voltammograms: (top) 2 in acetonitrile (ca. 3
mM) containing electrolyte; (bottom) 2 in a 50/50 acetonitrile/
water mixture (ca. 3 mM). The scan rate was 100 mV/s.
obtained upon oxidation of 3 is stable at room temperature, but
on a limited time scale. Due to this limited stability, a bulk
electrolysis experiment to stoichiometrically prepare the iri-
dium(IV) species seemed unlikely to succeed. Thus, we turned
to the well-known chemical oxidant [RuIII(bpy)3]3þ for rapid
preparation of the oxidation product. [RuIII(bpy)3]3þ has a
standard redox potential, E°, of 1.25 V vs NHE in MeCN,
depending on the exact conditions.52 Thus, it is poised to afford
the product of the observed quasi-reversible electrochemical
wave which occurs at 0.9 V while not subsequently oxidizing the
complex to the product of the irreversible feature observed at
1.4 V. Also, oxidation with [RuIII(bpy)3]3þ should oxidize the
iridium complex quickly, affording time to rapidly freeze the
sample before disproportionation or decomposition of the
oxidized iridium species. The sample could then be analyzed
by EPR spectroscopy.
1
(IV) complex is apparently low-spin d5 with S = /2. Upon
reduction by the iridium, the ruthenium oxidant is EPR silent
with S = 0. Thus, the spectrum in Figure 8 is consistent with
generation of [Cp*IrIV(NHC)Cl]þ.
Few examples of Cp*IrIV complexes have been observed
by EPR, and most of those previously described bear at
least two identical strong donor ligands, such as methyl
groups.54-56 In this case, we have prepared a Cp*Ir complex
bearing a chelating, nonsymmetrical NHC ligand. In the
previously characterized Cp*IrIV species, the presence of
identical ligands induces higher symmetry, as is evident in
their axial EPR spectra. We know of no prior case in which a
Cp*IrIV species gives rise to a spectrum with rhombic
symmetry, which is consistent with the presence of three
different ligands in our Cp*IrIV species. Spin-orbit coupling
between the different energy levels leads to well-resolved gx,
[Ru(bpy)3](PF6)3 was prepared from [Ru(bpy)3](Cl)2 by
oxidation with PbO2 and precipitation from acid solution by
KPF6, according to Drago and co-workers,53 and stored
under vacuum in a freezer to minimize decomposition. In a
glovebox under a nitrogen atmosphere, 3 and [Ru(bpy)3]
(54) Diversi, P.; Fabrizi de Biani, F.; Ingrosso, G.; Laschi, F.;
Lucherini, A.; Pinzino, C.; Zanello, P. J. Organomet. Chem. 1999, 584,
73.
(55) Diversi, P.; Iacoponi, S.; Ingrosso, G.; Laschi, F.; Lucherini, A.;
Pinzino, C.; Uccello-Barretta, G.; Zanello, P. Organometallics 1995, 14,
3275.
(56) Kawabata, K.; Nakano, M.; Tamura, H.; Matsubayashi, G.-E.
Inorg. Chim. Acta 2004, 357, 4373.
(52) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
Vonzelewsky, A. Coord. Chem. Rev. 1988, 84, 85.
(53) Desimone, R. E.; Drago, R. S. J. Am. Chem. Soc. 1970, 92, 2343.

Article
Organometallics, Vol. 30, No. 5, 2011 971
Figure 9. Calculated SOMO for [Cp*IrIV(NHC)Cl]þ (left) and drawing rotated clockwise 90° about the vertical axis (right). Hydrogen
atoms are omitted for clarity.
gy, and gz components (2.53, 2.17, and 1.85, respectively). No
hyperfine coupling is observed, all iridium isotopes having
I = 3/2, consistent with little s character in the SOMO. Near
g = 2, a very weak signal corresponding to an organic radical
is visible, possibly arising from partial decomposition of the
ruthenium oxidant during preparation and storage. DFT
analysis of the iridium(IV) SOMO that would arise from
one-electron oxidation of complex 3 further supports the
assignment of the observed EPR signal to iridium(IV). The
SOMO (Figure 9) shows a largely metal-centered orbital
with little s character. This is consistent with the rhombic
spectrum observed experimentally.
After the initial EPR spectrum was obtained, the sample
was warmed to room temperature under argon. After 3 min,
the sample was refrozen and another EPR spectrum re-
corded. The newly acquired spectrum showed the same
features as previously observed with essentially no loss in
intensity, indicating that on a minute time scale, the iridium-
(IV) species is stable under an inert atmosphere. However,
repeating the same procedure and allowing the sample to
incubate at room temperature for 30 min caused the signal to
disappear; no other peaks are seen, and so any decomposi-
tion products are not EPR active. This result is consistent
with the electrochemistry results, which suggest limited
stability of the iridium(IV) species at room temperature.
1
A sample was similarly prepared for H NMR analysis
containing a 1/1 mixture of complex 3 and [Ru(bpy)3]3þ
.
Room-temperature spectra with a scan time of 3 s showed
diamagnetic peaks consistent with the presence of [RuII
(bpy)3]2þ, as verified by measurements on a pure sample.
Weak signals in the aromatic region indicate the presence of
an iridium(III) or a diamagnetic Ir(V) species. However,
these peaks are not well resolved and are overwhelmed by the
signal from [RuII(bpy)3]2þ (Figure 10). Given that there are
essentially no substrates present in the reaction mixture other
than the bound chloride that could undergo oxidization, it is
possible that the iridium(IV) species either directly oxidizes
chloride or disproportionates to iridium(III) and iridium(V)
before reaction or decomposition. This would give a wide
array of potential iridium-containing products and could
explain the poorly resolved spectrum.
In an attempt to observe iridium(IV) species directly by 1H
NMR, paramagnetic 1H NMR spectra were recorded at -35
°C. Unfortunately, even with a scan time of 30 ms, no new
signal was observed and the resulting NMR spectra only
contain the previously described diamagnetic signals. The
paramagnetic iridium(IV) species visible by EPR presum-
ably relaxes too rapidly to be observed by NMR. It is
noteworthy that no signal is observed that can be attributed
to [RuIII(bpy)3]3þ (Figure 10). The spectrum obtained of
Figure 10. 500 MHz 1H NMR spectra in 1/1 toluene/acetonitrile:
(A) diamagnetic spectrum of 1/1 3/[RuIII(bpy)3]3þ, δ 6.0-10.0
ppm; (B) paramagnetic spectrum of 1/1 3/[RuIII(bpy)3]3þ, δ -50
to þ50 ppm; (C) paramagnetic spectrum of [RuIII(bpy)3]3þ, δ -50
to þ50 ppm. Peaks are referenced to toluene solvent.
pure [RuIII(bpy)3]3þ under identical conditions matches the
literature data, thus validating the parameters used for

972 Organometallics, Vol. 30, No. 5, 2011
Brewster et al.
observing paramagnetic species.53 Furthermore, lack of a
ruthenium(III) signal in the mixed sample indicates total
reduction to ruthenium(II), confirming the data observed by
EPR spectroscopy and further affirming that the observed
EPR signal must arise from [Cp*IrIV(NHC)Cl]þ.
An analogous EPR experiment was run on catalyst 1. A
small signal, barely distinguishable from the noise, was
observed possibly due to an analogous iridium(IV) species.
As seen for complex 3, consumption of [RuIII(bpy)3]3þ was
complete, as observed by EPR. Consistent with the irrever-
sible oxidation observed in electrochemical studies (vide
supra), the iridium(IV) species generated upon one-electron
oxidation of 1 leads to rapid reaction, possibly via oxidation
of chloride. It is thus evident that the NHC ligand in complex
3 imparts greater stability to the high-valent intermediate
[Cp*IrIV(NHC)Cl]þ than the much weaker donor ppy ligand
present in complex 1.
This conclusion is also consistent with the observed higher
turnover frequency for 1 as compared to 3 for water oxida-
tion. Both results indicate that a comparatively less stable,
and more reactive, iridium(IV) species is obtained from
oxidation of the active form of 1 in comparison to the active
form of 3. While [Cp*IrIV(NHC)Cl]þ is likely not an active
participant in the catalytic cycle for water oxidation, a
reactive Ir(IV) species, as predicted by DFT,29 is shown to
be a plausible class of intermediate, as demonstrated by the
observation of the analogous example studied here.
ppm, J in Hz). Paramagnetic NMR spectra were obtained on a
500 MHz Bruker spectrometer at -35 °C with a scan time of 30
ms. 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 8.5
K. The modulation amplitude was set to 2 G and the microwave
frequency to 9.388 GHz. Experiments were recorded with a time
constant of 41 ms. Elemental analysis was performed by Atlan-
tic Microlabs Inc. (Norcross, GA).
0
Chloro(1,2,3,4,5-pentamethylcylopentadienyl)(K2C2,C2 -1,3-
diphenylimidazol-2-ylidene)iridium(III) (3). In a Schlenk flask,
343 mg of imidazolium salt 4 (1.34 mmol, 2 equiv), 535 mg of
[Cp*IrCl2]2 (0.67 mmol, 1 equiv), and 300 mg of KOtBu (2.67
mmol, 4 equiv) were combined. Dry dichloromethane (25 mL)
was then added, and the reaction mixture was stirred under
nitrogen at room temperature for 12 h. The resulting orange
solution was filtered through Celite and the solvent removed
under reduced pressure. Analytically pure product was obtained
using neutral alumina column chromatography with dichloro-
methane as the eluent. The product elutes as a yellow band with
an Rf value near zero and is isolated as a yellow powder. Crystals
for X-ray analysis were obtained by vapor diffusion of pentane
into dichloromethane. Isolated yield: 96 mg (10.9%). 1H NMR
(500 MHz, CD2Cl2): δ 1.42 (s, 15H), δ7.00 (m, 2H), 7.22 (m,
1H), 7.26 (d, J = 2.2, 1H), 7.45 (m, 1H), 7.54 (m, 3H), 7.69 (m,
1H), 7.97 (d, J = 7.6, 2H). 13C{1H} NMR (125 MHz, CD2Cl2): δ
9.18, 91.98, 110.75, 115.50, 122.30, 122.36, 126.08, 126.28,
128.44, 129.84, 137.82, 140.78, 143.93, 146.49, 164.60. Anal.
Calcd for C25H27ClIrN2: C, 51.49; H, 4.67; N, 4.80. Found, C,
51.22; H, 4.55; N, 4.79.
X-ray Diffraction Study of Compound 3. Crystal samples were
mounted in a polyimide MiTeGen loop with immersion oil. All
measurements were made on a Rigaku SCXMini diffractometer
with filtered Mo KR radiation at a temperature of 223 K. Two ω
scans consisting of 180 data frames each were collected. The data
frames were processed and scaled using the Rigaku CrystalClear
program.59 The data were corrected for Lorentz and polarization
effects. The structure was solved by direct methods and expanded
using Fourier techniques.60 The non-hydrogen atoms were refined
anisotropically, and hydrogen atoms were treated as idealized
contributions. The final cycles of full-matrix least-squares
refinement61 on F2 were applied until convergence of unweighted
Conclusions
A newly synthesized Cp*Ir compound, 3, bearing a cyclo-
20
metalated N,N0-diphenylimidazolyl ligand, Cp*Ir(κ2C2,C -
NHC)(Cl), has been synthesized and fully characterized by
spectroscopy and X-ray crystallography. This complex was
shown to be analogous to the Cp*Ir(ppy)Cl complex, 1,
previously described by us as a catalyst precursor for water
oxidation. The highly electron donating N-heterocyclic car-
bene ligand stabilizes a higher valent form of the compound,
[Cp*IrIV(NHC)Cl]þ, which can be prepared by electroche-
mical and chemical oxidation. This oxidized form of 3 can be
observed using EPR spectroscopy and has rhombic symme-
try. The existence of this transient species is consistent with a
mechanism for water-oxidation catalysis involving a series of
sequential, one-electron oxidations, as proposed previously
for the related compounds 1 and 2. This work suggests that
further tuning of the electronic properties of ligands on the
Cp*Ir scaffold may afford catalysts with useful reactivity
and spectroscopic signatures.
P
P
and weighted factors of R = ||Fo| - |Fc||/ |Fo| and Rw
=
P
P
{
[w(Fo2 - Fc2)2]/ [w(Fo ) ]}1/2. Crystal data and experimental
2 2
details are included in the Supporting Information.
Procedure for Sample Preparation for EPR and NMR Anal-
ysis. In a glovebox, 1.3 mg of 3 (0.0022 mmol) was dissolved in
500 μL of 1:1 d3-MeCN/d8-toluene. Separately, 1.6 mg of
[Ru(bpy)3](PF6)3 (0.0022 mmol) was dissolved in 500 μL of d3-
MeCN/d8-toluene. The solutions were combined in a cold well
and removed from the glovebox at low temperature. Samples
were frozen immediately upon removal from the glovebox.
Thermal stability was examined by warming the frozen sample
to room temperature under a constant flow of argon. Samples
were kept at room temperature for 3 min before refreezing and
analysis. Samples were then warmed to room temperature and
held under argon for 30 min before refreezing for analysis.
Electrochemical Studies. Electrochemical measurements were
made on a Princeton Applied Research Versastat 4-400 poten-
tiostat/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 reduce background oxida-
tion. The electrode consisted of a brass cylinder, sheathed in a
Teflon tube. At the tip of the brass, a two-part silver conducting
epoxy (Alfa Aesar) was used to firmly attach the basal plane
Experimental Section
[Cp*IrCl2]2 and [Ru(bpy)3](PF6)3 were prepared according to
literature procedures.53,57 N,N0-Diphenylimidazolium chloride
was synthesized from aniline, glyoxal, and paraformaldehyde
using an adaptation of a known method.37 CH2Cl2 and CH3CN
were dried on a Grubbs-type solvent purification system.58 All
other reagents and materials were commercially available and
used without further purification. Diamagnetic NMR spectra
were recorded at room temperature on a 400 or 500 MHz Bruker
spectrometer and referenced to the residual solvent peak (δ in
(57) Ball, R. G.; Graham, W. A. G.; Heinekey, D. M.; Hoyano, J. K.;
Mcmaster, A. D.; Mattson, B. M.; Michel, S. T. Inorg. Chem. 1990, 29,
2023.
(59) CrystalClear and CrystalStructure; Rigaku/MSC, The Woodlands,
TX, 2005.
(58) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518.
(60) Sheldrick, G. M. Acta Crystallogr., Sect. A 2008, 64, 112.
P
(61) Least-squares function minimized: w(Fo2 - Fc2)2.

Article
Organometallics, Vol. 30, No. 5, 2011 973
carbon electrode surface to the brass. Finally, the tip was sealed
with the organic solvent resistant, electrically insulating, two-
part epoxy Tra-bond 2151 (Emerson and Cuming, Canton,
MA). Immediately prior to experiments, the working electrode
was polished with 1 μm alumina paste, washed with copious
amounts of water, and allowed to dry completely. Then, the
surface of the working electrode was resurfaced with tape to
restore the gray, basal surface.
For nonaqueous electrochemical studies, a platinum wire was
used as the counter electrode, and a silver wire was used as the
pseudoreference electrode. Ferrocene was used as an external
standard to calibrate the reference electrode versus NHE (Fc/
Fcþ: þ0.690 V vs NHE in MeCN). Experiments were carried out
in a dry acetonitrile solution containing 0.1 M tetrabutylammo-
nium perchlorate (Fluka, electrochemical grade) as the support-
ing electrolyte. For mixed solvent system studies, water was
added to the acetonitrile electrolyte solutions in the indicated
proportions.
Kinetic Studies of Oxygen Evolution. Measurements of the
initial oxygen evolution rate were made with a YSI 5300A
Biological Oxygen Monitor. Prior to the start of each set of
experiments, the gas-permeable membrane was replaced to
ensure a high-quality response. The electrode, secured in a
Teflon tube, was inserted into a tight-fitting water-jacketed
glass vessel. The system was kept at a constant temperature of
25 °C. In a typical experiment, a freshly prepared cerium(IV)
solution in Milli-Q water (5 mL, 78 mM; pH 0.89) was allowed
toequilibratewithstirring for 7 min. Datacollectionover 5-7 min
showed equilibration of the system, and when a steady baseline
was achieved, 10 μL of catalyst solution was injected. Oxygen
evolution commenced immediately and typically exceeded the
maximum value for the system within 10 min of injection.
Computational Details. Density functional theory calcula-
tions were carried out using Gaussian 09 Revision A.02.62
Calculations were performed using the B3LYP functional and
the LANL2DZ basis set for iridium and the 6-31G** basis set
for all other atoms. The LANL2DZ pseudopotential was used
for iridium. Initial geometries were obtained using the coordi-
nates from X-ray structures, and all optimized structures were
verified using frequency calculations to check that they did not
contain any imaginary frequencies.
Acknowledgment. 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-PS02-08ER15944
(G.W.B., R.H.C., J.D.B., and T.P.B.) and the Center
for Catalytic Hydrocarbon Functionalization, CCHF
under Award Number DE-SC0001298 (R.H.C.) 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
DE-FG02-84ER13297 (N.D.S.). J.D.B. and T.P.B. thank
(62) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.;
Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.;
Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.;
Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima,
T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J., J. A.;
Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin,
K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.;
Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.;
Millam, N. J.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.;
Jaramillo, J.; Gomperts, R. E.; Stratmann, O.; Yazyev, A. J.; Austin, R.;
Cammi, C.; Pomelli, J. W.; Ochterski, R.; Martin, R. L.; Morokuma, K.;
Zakrzewski, V. G.;Voth, G. A.;Salvador, P.;Dannenberg, J. J.;Dapprich,
S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.;
Fox, D. J. Gaussian 09 revision A.02; Gaussian Inc., Wallingford, CT,
2004.
€
Gozde Ulas and Ravi Pokhrel for assistance in collecting
the EPR spectra. T.P.B. also thanks Meng Zhou and
Graham Dobereiner for insightful discussions.
Supporting Information Available: Text, tables, figures, and
a CIF file giving NMR spectra, supplemental electrochemis-
try, oxygen evolution data, xyz coordinates for calculated
structures, and crystallographic information for 3. This ma-
terial is available free of charge via the Internet at http://pubs.
acs.org.