Sulfinato Iron(III) complex for electrocatalytic proton reduction

Article abstract of DOI:10.1021/ic5030394

We report the first example of a sulfinato Fe(III) complex acting as a highly active electrocatalyst for proton reduction. The sulfinate binds to the metal through oxygen, resulting in a seven-membered chelate ring that is likely hemilabile during catalys

Full text of DOI:10.1021/ic5030394

Article
pubs.acs.org/IC
Sulfinato Iron(III) Complex for Electrocatalytic Proton Reduction
Andrew C. Cavell,† Carolyn L. Hartley,† Dan Liu,† Connor S. Tribble,† and William R. McNamara*,†
†Department of Chemistry, College of William and Mary, 540 Landrum Drive, Williamsburg, Virginia 23185, United States
S
* Supporting Information
ABSTRACT: We report the first example of a sulfinato
Fe(III) complex acting as a highly active electrocatalyst for
proton reduction. The sulfinate binds to the metal through
oxygen, resulting in a seven-membered chelate ring that is
likely hemilabile during catalysis. Proton reduction occurs at
−1.57 V versus Fc/Fc+ in CH3CN with an ic/ip = 13 in
CH3CN (kobs = 3300 s−1) and an overpotential of 800 mV.
The catalysis is first order with respect to [catalyst] and second
order with respect to [trifluoracetic acid]. An 11% increase in
catalytic activity is observed in the presence of water,
suggesting that sulfinate moieties are viable functional groups
for aqueous proton reduction catalysts.
catalyst. Herein we report a sulfinato Fe(III) polypyridyl
complex (2) that forms a seven-membered chelate ring with
Fe(III) (Figure 1). This complex is significantly more active
INTRODUCTION
■
With dwindling fossil fuel reserves and rising greenhouse gas
emissions, the development and use of renewable energy
sources is critical.1 Using photons to split water into O2 and H2
provides a promising method of harvesting solar energy. To
fully realize this goal, proton reduction catalysts must be
developed that are made from earth-abundant materials.1 Since
iron is the most abundant transition metal, it is of interest to
explore the use of iron in complexes that are highly active and
stable for hydrogen generation catalysis.
Other first row transition metals such as Co and Ni have
been studied extensively for use as proton reduction catalysts.2
In the pursuit of improved stability, pentadentate polypyridyl
ligands have been used to form complexes that generate
hydrogen from aqueous solutions.3 Although cobalt and nickel
complexes are widely studied, few iron catalysts have been
examined that both operate at modest potentials and are highly
active for proton reduction. To this end, there have been many
efforts to mimic the active site of hydrogenase enzymes.4 There
are other recent examples of iron catalysts that are only active
in nonaqueous media.5 Notably, there have also been recent
advances in using iron complexes as catalysts for electrocatalytic
H2 oxidation.6 However, examples of iron-containing proton
reduction catalysts that are stable in aqueous media are more
limited and do not yet approach the activity of Ni or Co
catalysts.7
Recently, we reported a mononuclear iron complex (1)
containing a polypyridyl monophenolate ligand that was stable
in aqueous solution and generated hydrogen in CH3CN with a
kobs = 1000 s−1.8 The mechanism involves protonation of the
phenolate prior to subsequent protonation and reduction
events (CECE or CEEC). We reasoned that if the dissociation
of the protonated phenolate was involved in the rate-limiting
step, replacing the phenolate with a functionality that formed a
less favorable chelate ring would improve the activity of the
Figure 1. (left) Iron polypyridyl monophenolate complex (1). (right)
Iron polypyridyl sulfinate complex (2).
than the phenolate catalyst for proton reduction with an ic/ip =
13 (kobs = 3300 s−1), where ic is the catalytic current and ip is
the peak current of the complex when no acid is added (see
Supporting Information).
Fe(II) and Fe(III) sulfinate complexes have been of great
interest in the development of structural mimics of cysteine
dioxygenase (CDO).9 CDO utilizes the S-oxygenation of
cysteine, which is crucial for the biosynthesis of pyruvate and
taurine.9 To date, very few CDO mimics have been reported
that make use of O2 as the oxidant.9,10 Fe sulfinate complexes
are typically synthesized by reacting the thiolate complex with
pure O2, resulting in S-oxygenation to give the sulfinate
product.9 However, in obtaining the desired complex 2, S-
oxygenation occurs rapidly in air. Owing to rapid S-oxygen-
ation, efforts to isolate the pure thiolate complex in our
Received: December 18, 2014
© XXXX American Chemical Society
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laboratory have been unsuccessful using standard Schlenk
techniques (Scheme 1).
bond to the unbound oxygen and a single bond to the oxygen
that is coordinated to iron.
Cyclic voltammograms (CVs) of the complex (2) in
acetonitrile reveal a reversible redox couple for Fe(II/III) at
−0.28 V versus Fc/Fc+. A peak separation of 70 mV is observed
under these conditions, which is consistent with the peak
separation observed for the Fc/Fc+ redox couple under the
same conditions. With the addition of known concentrations of
trifluoracetic acid (TFA) in acetonitrile, an irreversible
reduction event is observed at −1.57 V versus Fc/Fc+.
Calculations using the catalytic half-wave potential and the
Eref for TFA in acetonitrile indicate that the catalysis occurs
with an 800 mV overpotential.11 When more acid is added, a
larger current enhancement is observed (Figure 3).
Scheme 1. Synthesis of 2
RESULTS AND DISCUSSION
■
The ligand was synthesized using a modified literature
procedure to give 3 in moderate yield (45%). The free ligand
(3) is stable in air with no ligand oxidation observed after
several weeks. The sulfinate complex (2) was then obtained by
deprotonating the thiolate ligand (3) using triethylamine and
treating the solution with FeCl3 in a methanol solution under
argon. The resulting complex was recrystallized from dichloro-
methane and then ethanol to give the pure product as a brown
solid. X-ray quality crystals were obtained through slow
diffusion of diethyl ether through acetonitrile.
The structure confirms that the sulfinato complex (2) is
coordinated through the oxygen of the sulfinate resulting in a
seven-membered chelate ring (Figure 2). The structure shows a
Figure 3. CVs of 2 in CH3CN with 0.1 M TBAPF6 (black) upon
addition of 2.2 mM (green), 4.4 mM (blue), 6.6 mM (red), and 8.8
mM (orange) TFA at ν = 200 mV/s.
The linear relationship between ic and [TFA] suggests a
second-order dependence on proton concentration (see
Supporting Information). The catalyst is highly active for
proton reduction with an ic/ip = 13, corresponding to a kobs
=
3300 s−1 (see Supporting Information). Although the method
used to calculate kobs accurately represents simple pseudo-first-
order systems, it is also used to determine turnover frequencies
(TOFs) for more complicated systems to provide a point of
comparison for different catalysts.3a,c,12 This compares
favorably with other iron complexes in the literature.5,7
Complex 2 is also significantly more active than the phenolate
complex (1), which gave an ic/ip = 7.8 under identical
conditions, corresponding to a kobs = 1000 s−1.
Furthermore, when [TFA] is held constant and catalyst is
introduced, a catalytic wave is also observed at −1.57 V.
Addition of more catalyst then results in a larger current
enhancement (Figure 4). After several additions of 2 there is a
linear relationship between [catalyst] and ic/ip. This suggests
that there is a first-order dependence on [catalyst], resulting in
an overall rate expression of rate = k[2][H+]2.
When acid is added to the phenolate complex (1) the redox
couple for Fe(II)/Fe(III) shifts from −0.6 to −0.3 V versus Fc/
Fc+ (Figure 5, right).8 The shift in redox couple suggests that a
new complex is formed upon addition of acid. Owing to the fact
Figure 2. ORTEP diagram of 2. Ellipsoids are at the 50% probability
level with hydrogen omitted for clarity.
distorted octahedral complex with O−Fe−N and N−Fe−Cl
bond angles of 170.22° and 161.41°, respectively. The Fe−O
for the sulfinato group is 1.934 Å and is longer than what was
reported for the Fe−O of the phenolate complex (1). The S−O
bond lengths of 1.451 and 1.527 Å are consistent with a double
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Upon addition of TFA, the onset of the catalytic wave is
observed at the same potential for both complexes 1 and 2.
This suggests that both ligands are hemilabile in nature, leaving
the same Fe-NNN core intact during later stages of the catalytic
cycle. The phenolate in complex 1 is first protonated and
becomes labile upon reduction. The sulfinate group of complex
2 forms a less-favored seven-membered chelate ring, increasing
the lability of this site. The resulting difference in catalytic
activity of 1 and 2 indicates that the dissociation of the phenol
group in complex 1 is likely involved in the slow step of the
catalytic cycle. Therefore, by replacing it with a more labile
sulfinate group, a significant increase in rate is observed.
With many iron catalysts operating at more cathodic
potentials, it is crucial to choose a proton source that is not
reduced at these potentials. Figure 6 shows that the background
reduction of [TFA] does not contribute significantly to the
value of ic/ip.13 Although homoconjugation is observed for
TFA, the lack of background reduction at −1.57 V versus Fc/
Fc+ makes it viable for studying this complex.
Since it is of great interest to develop catalysts that can
reduce protons in aqueous solutions, it is also important to
determine if 2 is active in the presence of water. Figure 7 shows
the CVs of 2 in CH3CN with 11 mM TFA added (green).
Upon addition of 100 μL (1.1 M) of water, an increase in
catalytic activity is observed (blue). This increase corresponds
to an 11% improvement in catalytic performance for 2 with
water present. The increase in activity suggests that the
unbound sulfinate is stabilized by the presence of a polar, protic
solvent such as water. At high concentrations of water, the
complex does not decompose but is sparingly soluble. Owing to
solubility issues, we could not perform an experiment in 1:1
CH3CN/H2O to directly compare 1 and 2 in aqueous
environments. Solubility also limited our ability to test the
activity of 2 in purely aqueous buffered solutions. However, the
increase in ic/ip observed with water added suggests that water-
soluble catalysts with sulfinato moieties have the potential to be
highly active in aqueous solutions.
Figure 4. CVs in CH3CN with 0.1 M TBAPF6 containing 44 mM TFA
with 0.2 mM 2 (green), 0.4 mM 2 (blue), 0.6 mM 2 (red), and 0.8
mM 2 at ν = 200 mV/s.
that the most basic site of the complex is the phenolate, the first
step in the catalytic cycle involves protonation of the oxygen. In
contrast, upon addition of TFA, the redox couple for Fe(II)/
Fe(III) does not shift for complex 2 (Figure 5, left), indicating
that this catalyst operates under a different mechanism. With no
observed shift in the redox couple, the first step is proposed to
involve an electrochemical reduction, which is followed by
protonation and reduction events (ECEC or ECCE). It is also
possible that catalysis occurs through a more complicated
mechanism than what can be deduced from electrochemical
analysis alone.
Figure 5. (left) CVs of 2 without added TFA (black) and with 8.8 mM TFA (blue). (right) CVs of 1 without added TFA (black) and with 8.8 mM
TFA (blue).
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seven-membered chelate ring. The catalysis is first order with
respect to [catalyst] and second order with respect to [TFA]. It
is also active in the presence of water with an increase in activity
observed after the addition of 100 μL (1.1 M) H2O. The
incorporation of a sulfinate moiety has resulted in the
development of a more active iron catalyst and has helped to
elucidate the mechanism of the previously reported iron
phenolate catalyst.
EXPERIMENTAL SECTION
■
Materials. All experiments were performed using standard Schlenk
air-free techniques under an Ar atmosphere unless otherwise indicated.
All chemicals were purchased from Fischer Scientific and were used
without further purification unless noted otherwise.
Instrumentation. 1H and 13C spectra were recorded on an Agilent
400MR DD2 spectrometer operating in the pulse Fourier transform
mode. Chemical shifts are referenced to residual solvent. Elemental
analysis was carried out by the CENTC Elemental Analysis Facility at
the University of Rochester, funded by NSF CHE-0650456. UV−vis
spectra were recorded using an Agilent Cary 60 UV−vis Spectropho-
tometer using air-free solutions in sealed quartz cuvettes. All
electrochemical experiments were performed under an atmosphere
of Ar using a CH Instruments 620D potentiostat. CVs were acquired
using a standard three-electrode cell. Prior to each acquisition, the
working electrode (glassy carbon) and the auxiliary electrode
(platinum) were polished using 0.05 μm alumina powder paste on a
cloth-covered polishing pad, followed by rinsing with water and
acetonitrile. The reference used was a saturated calomel electrode
(SCE) electrode unless otherwise noted. Ferrocene was used as an
internal standard to correct for drifting of the reference electrode and
changes in concentration, and all potentials are reported relative to the
ferrocene/ferrocenium (Fc/Fc+) redox couple. Controlled potential
coulometry was carried out in a sealed 500 mL cell using vitreous
carbon working and counter electrodes and a Ag wire reference
electrode separated by porous VYCOR frits. A CH Instruments 620D
potentiostat combined with a CH Instruments 680 A booster was used
for the CPC. Gas analysis for H2 was performed using a Bruker Scion
436 gas chromatograph equipped with a TCD using Ar carrier gas and
calibrated with H2/CH4 gas mixtures of known composition.
X-ray Diffractometry. Single crystals were mounted on glass
fibers. All data for 2 were collected using graphite-monochromated Mo
Kα radiation on a Bruker SMART Apex II CCD platform
diffractometer. The structure was solved using SIR20114 and refined
using SHELXL-2014/7. The space group P43 was determined based
on CSD statistics and having solved the structure in space groups P1
and P21 and noting the higher symmetry visually and via the Addsym
function of program Platon. Systematic absences clearly chose P42;
however, no solution was obtainable in that space group. A solution in
space group P21 that included modeling for pseudomerohedral
twinning also did not produce a better result than that obtained in
P43. A direct-methods solution was calculated, which provided most
non-hydrogen atoms from the E-map. Full-matrix least-squares/
difference Fourier cycles were performed, which located the remaining
non-hydrogen atoms. All non-hydrogen atoms were refined with
anisotropic displacement parameters. All hydrogen atoms were placed
in ideal positions and refined as riding atoms with relative isotropic
displacement parameters.
Figure 6. CVs of 22 mM TFA in 0.1 M TBAPF6 in CH3CN prior to
(black) and after (blue) the addition of 0.5 mM 2 at ν = 200 mV/s. A
reduction peak corresponding to the catalytic reduction of hydrogen is
visible at −1.57 V only upon addition of 2.
Figure 7. CVs of 2 in a CH3CN solution with 0.1 M TBAPF6 and 11
mM TFA in dry conditions (green) and upon addition of 100 μL (1.1
M) of water (blue) at ν = 200 mV/s. The black trace represents the
CV of 2 in CH3CN with no acid added.
CONCLUSION
■
Reflection contributions from highly disordered solvent, located in
channels along the crystallographic 43 axes at the corners of the unit
cell, were fixed and added to the calculated structure factors using the
Squeeze routine of program Platon, which determined there to be 68
electrons in 212 Å3 removed per unit cell.
In conclusion, an iron complex with a sulfinato moiety has been
found to be highly active for electrocatalytic proton reduction.
It is the first example of a sulfinato Fe(III) complex that is an
active catalyst for this reaction. The sulfinate binds to the metal
through oxygen, resulting in a seven-membered chelate ring.
The catalyst reduces protons at −1.57 V versus Fc/Fc+ (800
mV overpotential) with an ic/ip = 13, making it significantly
more active (kobs = 3300 s−1) than the previously reported
phenolate complex (1). This higher activity is likely a result of
the hemilabile nature of the ligand due to the presence of the
Syntheses. The ligand (3) was synthesized from thiosalicylic acid
using a modified literature procedure.14
o-Mercaptobenzyl Alcohol. Thiosalicylic acid (3.0908 g, 0.02
mol) in 75 mL of dry diethyl ether was degassed with Ar. This was
combined with LiAlH4 (1.1385 g, 0.03 mol) under air-free conditions,
and the solution was stirred at room temperature for 1 h. The solution
was cooled in an ice bath, and 4 mL H2O was added dropwise,
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followed by 20 mL of 10% H2SO4. The solution was then allowed to
stir under argon for 48 h. The reaction mixture was washed with
diethyl ether (3 × 30 mL), and the organic layer was dried using
MgSO4 and evaporated to yield o-mercaptobenzyl alcohol, a yellow oil.
(82% yield) The 1H and 13C NMR spectra matched reported values.14
3-(2-Hydroxymethylphenylsulfanyl)propionitrile. A solution
of o-mercaptobenzyl alcohol (0.9974 g, 7.12 mmol) in 15 mL of
ethanol was prepared and degassed. This was combined with a
degassed solution of NaOH (0.4058 g, 0.01 mol) in 5 mL of H2O and
10 mL of ethanol. Degassed bromopropionitrile (0.6 mL, 7.1 mmol)
was added dropwise under air-free conditions, and the mixture was
allowed to stir at room temperature for 5 h. The resulting solution was
filtered and evaporated to yield an oily yellow solution. This was
dissolved in 25 mL of diethyl ether, washed with 10 mL of 5% NaOH
and 10 mL of H2O, then dried with MgSO4 and evaporated to yield a
suitable for X-ray diffraction were grown by diffusion of diethyl ether
into a concentrated solution of 2 in CH3CN. HR MS: m/z for
(C19H18Cl2FeN3O2S)Na+ expected = 500.973841 m/z found =
500.974354 (see Supporting Information, Figure S14). Anal. Calcd
for 2 FeC19H18Cl2N3O2S: C, 47.63; H, 3.79; N, 8.77%. Found: C,
47.77; H, 4.12; N, 8.44%. Structure was obtained using X-ray
diffraction of a single crystal.
Controlled-Potential Coulometry. Controlled-potential coulom-
etry experiments (CPC) were conducted in a closed 500 mL four-neck
round-bottom flask. Complex 2 (0.3 mg, 0.00063 mmol) was added to
50 mL of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)
in CH3CN. The flask was capped with two vitreous carbon electrodes
and a silver wire reference electrode, all submerged in the solution and
separated by VYCOR frits. The flask was degassed using Ar for 20 min,
while the solution was stirred. Using a Hamilton gas syringe, 10 mL of
Ar was removed from the flask and replaced with 10 mL of CH4 for
reference. A CV of the solution was then taken from −0.1 to −1.8 V
versus Fc/Fc+ to identify the potential at which proton reduction
occurs. A CPC was run at −1.6 V for 1800 s, while the solution
continued to stir. Upon completion of the experiment, a 0.10 mL
sample of vapor from the flask was removed using a Hamilton gas
syringe and injected into a GC. The ratio of H2 to CH4 in the sample
was compared to a calibration curve to determine the total volume of
H2 produced during the experiment. A faradaic yield of 98% was
observed for 2. No hydrogen was observed when the experiment was
run without catalyst.
Catalyst Concentration Dependence. A 5.2 mM stock solution
of 2 was prepared by dissolving 0.0125 g of 2 crystals with 0.1 M
TBAPF6 in CH3CN in a 5 mL volumetric flask. A 5 mL solution of 0.1
M TBAPF6 CH3CN was prepared in an electrochemical cell. 200 μL of
1.1 M TFA (44 mM) was added to the cell, which was degassed with
Ar. CVs were taken at 200 mV/s without any catalyst, then in the
presence of 0.2, 0.3, 0.4, and 0.5 mM catalyst from the 2 stock
solution. CVs were obtained using a glassy carbon working electrode, a
Pt auxiliary, and an SCE reference electrode. The working and
auxiliary electrodes were polished with 0.05 μm alumina powder paste
prior to each acquisition. All ic values were obtained using the method
described in the Supporting Information of this work.
Scan Rate Dependence. In an electrochemical cell, 1.0 mg of 2
(2.09 μmol) was dissolved in 5 mL of CH3CN with 0.1 M TBAPF6
and degassed with argon. CVs were taken upon addition of 11 mM
TFA at various scan rates ranging from 5 to 14 V/s. CVs were
obtained using a glassy carbon working electrode, a Pt auxiliary, and an
SCE reference electrode. The working and auxiliary electrodes were
polished with 0.05 μm alumina powder.
Determination of Overpotential. The tendency of TFA to
exhibit homoconjugation in acetonitrile solutions often leads to
inaccurate determinations of overpotential if this effect is not taken
into account.11 This renders standard calculations of overpotential
using pKa unreliable.11 Overpotentials were determined by calculating
the difference between the half-wave potential of the catalytic
reduction and Eref. Eref refers to a theoretical reduction potential that
takes into account the effects of homoconjugation at acid
concentrations used in electrochemical experiments.11b This method
gave overpotential values of ∼800 mV for both TFA and tosic acid.
1
white solid, 3-(2-hydroxymethylphenylsulfanyl)propionitrile. The H
and 13C NMR spectra matched reported values.14
3-(2-Bromomethylphenylsulfanyl)propionitrile. Solid 3-(2-
hydroxymethylphenylsulfanyl)propionitrile (0.2739 g, 1.473 mmol)
was dissolved in 22 mL of dichloromethane and degassed with argon.
This was added to a Schlenk flask under air-free conditions, and the
reaction was cooled in an ice bath. 1.0 M PBr3 (0.6 mL, 0.6 mmol) was
added dropwise to the Schlenk, and the solution was allowed to stir for
4 h. The clear yellow-orange solution was washed with 10 mL of 10%
NaOH and 10 mL of H2O, dried with MgSO4, filtered, and evaporated
to yield 3-(2-bromomethylphenylsulfanyl)propionitrile as a clear
yellow oil (80% yield). The 1H and 13C NMR spectra matched
reported values.14
N-(2-Propionitilemercaptobenzyl)-N,N-bis(2-pyridylmethyl)-
amine. 3-(2-bromomethylphenylsulfanyl)propionitrile (0.347 g, 1.355
mmol) was dissolved in 20 mL of ethyl acetate and degassed with
argon. This was added to an air-free Schlenk flask, followed by the
addition of a degassed solution of dipicolylamine (0.3 mL, 1.671
mmol) in 15 mL of ethyl acetate, and then a degassed solution of Et3N
(1 mL, 7.17 mmol) in 15 mL of ethyl acetate. This stirred under argon
for 72 h. The solution was then filtered and evaporated to yield N-(2-
propionitrilemercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine, which
was purified through a silica gel column run in 7:3 ethanol/ethyl
1
acetate and collected at 59% yield. The H and 13C NMR spectra
matched reported values.14
N-(2-Mercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine (3). N-
(2-propionitrilemercaptobenzyl)-N,N-bis(2-pyridylmethyl)amine
(40.7 mg, 0.112 mmol) was dissolved in 10 mL of methanol and
degassed. The solution was then combined with NaOMe (9.7 mg,
0.180 mmol) in a Schlenk flask under air-free conditions. The solution
was refluxed under argon for 72 h. The resulting clear amber colored
solution was filtered and evaporated. The resulting solid was dissolved
in 13 mL of dichloromethane (DCM) and washed with 13 mL of
deionized H2O to quench the remaining NaOMe. The lower orange-
brown organic layer was collected and evaporated to yield a brown oil.
The ligand was purified through use of a silica gel column in 9:1
DCM/MeOH. The purified N-(2-mercaptobenzyl)-N,N-bis(2-
pyridylmethyl)amine (3) was collected at 45% yield as a brown oil.
The 1H and 13C NMR spectra matched reported values.11 For a
representative 1H NMR spectrum, see Supporting Information, Figure
1
S12. H NMR (CDCl3): ∂ 8.51 (s, 2H), 7.50−7.67 (m, 4H), 7.10−
7.37 (m, 7H), 3.87 (s, 2H), 3.80 (s, 4H). High-resolution mass
spectrometry (HR MS): m/z for (C19H19N3S)H+ expected =
322.137 245 m/z found = 322.137 527.
ASSOCIATED CONTENT
■
[FeCl2(L-OSO)] (2). The sulfinato Fe(III) complex was synthesized
according to the following procedure: 1 (0.100 g, 0.312 mmol) and
Et3N (0.04 mL, 0.312 mmol) were dissolved in 10 mL of MeOH and
degassed with Ar. FeCl3·6H2O (0.843 g, 0.312 mmol) was dissolved in
10 mL of MeOH and degassed with Ar. The two solutions were
combined under air-free conditions to yield a brown solution with
some visible precipitate. The reaction was stirred at room temperature
for 12 h and filtered. The filtrate was evaporated, and the resulting
solid was dissolved in DCM to remove impurities through
recrystallization. Additional recrystallizations were performed in
EtOH. A dark solid of 2 was collected with a 71% yield. Crystals
S
* Supporting Information
Determination of kobs, sample calculations of kobs, CV data, GC
calibration curve, proton concentration study, peak current
density versus TFA, scan rate study, dip test study, controlled
potential coulometry, FeCl3 control study, UV−vis spectra,
tosic acid concentration study, NMR spectrum, HR-MS data,
X-ray crystallographic data (including CIF file), ORTEP
diagram, and selected bond lengths and angles. This material
is available free of charge via the Internet at http://pubs.acs.org.
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(10) Lee, Y.-M.; Hong, S.; Morimoto, Y.; Shin, W.; Fukuzumi, S.;
Nam, W. J. Am. Chem. Soc. 2010, 132, 10668.
AUTHOR INFORMATION
Corresponding Author
*E-mail: wrmcnamara@wm.edu.
Notes
■
(11) (a) Roberts, J. A. S.; Bullock, R. M. Inorg. Chem. 2013, 52,
3823−3835. (b) Fourmond, V.; Jacques, P. A.; Fontecave, M.; Artero,
V. Inorg. Chem. 2010, 49, 10338−10347.
(12) (a) Helm, M. L.; Stewart, M. P.; Bullock, R. M.; Rawkowski-
Dubois, M.; Dubois, D. L. Science 2011, 333, 863.
The authors declare no competing financial interest.
(13) McCarthy, B. D.; Martin, D. J.; Rountree, E. J.; Ullman, A. C.;
ACKNOWLEDGMENTS
■
Dempsey, J. L. Inorg. Chem. 2014, 53, 8350−8361.
W.R.M would like to thank R. D. Pike for helpful discussions.
W.R.M. would also like to thank W. W. Brennessel and the X-
ray crystallography facility at the Univ. of Rochester for solving
the crystal structure of 2. This work was funded by the Virginia
Space Grant Consortium New Investigator Award.
(14) Thapper, A.; Behrens, A.; Fryxelius, J.; Johansson, M. H.;
Prestopino, F.; Czaun, M.; Rehder, D.; Nordlander, E. Dalton Trans.
2005, 3566−3571.
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DOI: 10.1021/ic5030394
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