Redox chemistry of noninnocent quinones annulated to 2Fe2S cores

Article abstract of DOI:10.1021/om400913p

Noninnocent ligands that are electronically coupled to active catalytic sites can influence the redox behavior of the catalysts. A series of (μ-dithiolato)Fe₂(CO)₆ complexes, in which the sulfur atoms of the μ-dithiolato ligand are bridged by 5-substituted (Me, OMe, Cl, t-Bu)-1,4-benzoquinones, 1,4-naphthoquinone, or 1,4-anthraquinone, have been synthesized and characterized. In addition, the bis-phosphine complex derived from the 1,4-naphthoquinone-bridged complex has also been prepared and characterized. Cyclic voltammetry of these complexes shows two reversible one-electron reductions at potentials 0.2 to 0.5 V less negative than their corresponding parent quinones. In the presence of acetic acid two-electron reductions of the complexes result in conversion of the quinones to hydroquinone moieties. EPR spectroscopic and computational studies of the one-electron-reduced complexes show electron delocalization from the semiquinones to the 2Fe2S moieties, thereby revealing the noninnocent behavior of these ligands with these catalysts.

Full text of DOI:10.1021/om400913p

Article
pubs.acs.org/Organometallics
Redox Chemistry of Noninnocent Quinones Annulated to 2Fe2S
Cores
Gabriel B. Hall,† Jinzhu Chen,†,§ Charles A. Mebi,†,⊥ Noriko Okumura,†,∥ Matthew T. Swenson,†
Stephanie E. Ossowski,† Uzma I. Zakai,†,# Gary S. Nichol,†,△ Dennis L. Lichtenberger,† Dennis H. Evans,‡
and Richard S. Glass*,†
†Department of Chemistry and Biochemistry, The University of Arizona, Tucson, Arizona 85721, United States
‡Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States
S
* Supporting Information
ABSTRACT: Noninnocent ligands that are electronically
coupled to active catalytic sites can influence the redox
behavior of the catalysts. A series of (μ-dithiolato)Fe2(CO)6
complexes, in which the sulfur atoms of the μ-dithiolato ligand
are bridged by 5-substituted (Me, OMe, Cl, t-Bu)-1,4-
benzoquinones, 1,4-naphthoquinone, or 1,4-anthraquinone,
have been synthesized and characterized. In addition, the bis-
phosphine complex derived from the 1,4-naphthoquinone-
bridged complex has also been prepared and characterized.
Cyclic voltammetry of these complexes shows two reversible
one-electron reductions at potentials 0.2 to 0.5 V less negative than their corresponding parent quinones. In the presence of
acetic acid two-electron reductions of the complexes result in conversion of the quinones to hydroquinone moieties. EPR
spectroscopic and computational studies of the one-electron-reduced complexes show electron delocalization from the
semiquinones to the 2Fe2S moieties, thereby revealing the “noninnocent” behavior of these ligands with these catalysts.
core. According to a recent search of the CSD6,7 more than 600
catalysts of this class have been synthesized and characterized,
INTRODUCTION
The splitting of water consists of two half-reactions: oxidation
■
and all catalyze the reduction of protons to hydrogen under
appropriate conditions.8 In most cases, H2 production occurs at
potentials substantially more negative than the thermodynamic
potential.9,10
For example, we have studied H2 production using benzene
and hydroquinone moieties annulated to 2Fe2S cores.11−13
These complexes are excellent catalysts for H2 production from
weak acids. The mechanism for catalysis is shown in Scheme 1.
Notably it features protonation most favored at iron after 2e−
reduction to generate a μ-bridged hydride. A further 1e−
reduction is needed to render the complex sufficiently hydridic
to react with acetic acid. Because of this extra 1e− reduction,
these catalysts suffer from a > 0.5 V overpotential.
to form molecular oxygen and the reduction of protons to
molecular hydrogen. One broad class of catalysts for the latter
half-reaction is inspired by the active sites of [FeFe]-
hydrogenases,1−3 which are extremely efficient enzymes for
reversible reduction of protons to hydrogen with exceptionally
high turnover rates at very modest overpotential.4,5 The
common feature of this class of catalysts is a butterfly 2Fe2S
core as depicted schematically in Chart 1. In addition to the
Chart 1. Schematic Diagram of a General Class of 2Fe2S
Catalysts for Hydrogen Production
The H-cluster of [FeFe]-hydrogenase features a 4Fe4S
cluster, which is part of the redox chain connecting the active
site to the surface of the enzyme, and addition of an electron to
a model of the H-cluster shows that the electron is delocalized
over the entire H-cluster.14,15 Consequently, a number of
examples have been reported in the literature16−18 in which an
easily reduced moiety is incorporated into active site mimics. If
the easily reduced moiety is electronically coupled with the
2Fe2S catalytic site,19 and protonation occurs at the 2Fe2S
advantage of earth-abundant elements iron and sulfur
comprising the core, a wide range of ligands can be bound to
the iron centers and a variety of functional moieties can be
attached to the sulfur atoms to tune the reduction potentials,
facilitate geometry reorganizations, shuttle protons, accelerate
electron transfer, and electronically couple ligands to the 2Fe2S
Received: September 20, 2013
Published: October 7, 2013
© 2013 American Chemical Society
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Scheme 1. Mechanism of Electrocatalytic Production of
Hydrogen from a Weak Acid by Hydroquinone Complexes
Scheme 2. Methods A and B for Preparation of the Quinone
Metal Complexes in this Study
a
a
Potentials are for the molecule with R = OMe.
Chart 2. Quinone Metal Complexes Examined in this Study
Table 1. DDQ Oxidation of Hydroquinone 2Fe2S
Complexes to the Corresponding Quinone Metal Complexes
2b−e and 3a as Shown in Scheme 2
quinone metal complex
reaction time (min)
isolated yield (%)
2b
2c
2d
2e
3a
90
90
81
95
77
90
83
60
60
120
with DDQ provided the corresponding quinones 2b−e and 3a
in 77−90% isolated yields (Table 1).
Naphthoquinone adduct 3a proved especially stable, easily
obtainable, and easy to work with, so subsequent reactions are
reported for this complex. Quinone metal complex 3a could be
reduced back to the hydroquinone adduct with sodium
cyanoborohydride in 86% yield, but due to its facile air
oxidation, pure samples of the hydroquinone were difficult to
prepare.
Treatment of naphthoquinone adduct 3a with excess Ph3P
and Me3NO, similar to the previous studies of benzoquinone
complexes,35 afforded the corresponding bis-phosphine 3b in
19% isolated yield. An ORTEP drawing of the molecular
structure of 3b is shown in Figure 1. An interesting feature of
the structure of 3b is that the quinone moiety is face-to-face
with a phenyl ring from each of the triphenylphosphine ligands
with 3.5884(10) Å separating the centroid of each ring plane.
Anthraquinone q4 did not afford the corresponding
hydroquinone metal complex, but instead yielded the
anthraquinone metal complex 4 directly in 57% pure isolated
yield.
Electrochemistry. Figure 2 shows the voltammograms of
the quinone-containing catalysts 2b, 3a, and 4 along with the
voltammograms of the corresponding free quinones q2b, q3,
and q4 (Chart 3). In metal complexes with redox-active ligands
in which the electronic interaction between the metal and the
ligands is weak, initial reduction can occur either at the metal or
at the ligand depending on the inherent reduction potentials of
the separated sites36,37 and giving rise to redox behavior
core, a complex with the desired activity might be obtained. We
now report active site mimics 2−4 (Chart 2) in which readily
reducible quinone moieties are electronically coupled to the
2Fe2S core in an effort to design active H2 catalysts with low
overpotential. Such complexes appear promising considering
electronic couplings with metals have been reported for related
ligands: 1,2-dithiolenes,20−28 1,2-benzenedithiolates,29,30 1,2-
benzoquinone (catecholates),31−33 and 1,2-dithiolatonaphtho-
quinone.34 Furthermore 2a was reported before35 and is more
easily reduced than the corresponding benzene or hydro-
quinone complexes.13,35
RESULTS AND DISCUSSION
■
Synthesis and Structure. A synthesis of the dithiobenzo-
quinone metal complex 2a has been reported previously.35 In
this previous synthesis, shown schematically in Scheme 2 as
method A for a general quinone, the photocycloaddition of the
quinone to the disulfide complex S2Fe2(CO)6 (5) yields the
corresponding hydroquinone complex of type 7. Oxidation of
the hydroquinone complex 7 with 2,3-dichloro-5,6-dicyano-1,4-
benzoquinone, DDQ, provided the benzoquinone complex 2a.
We have since reported13 an alternative synthesis of hydro-
quinones of type 7, shown as method B in Scheme 2. In
method B the quinone is reacted with the dithiol complex
(HS)2Fe2(CO)6 (6) catalyzed by piperidine to yield the
hydroquinone complex 7. Oxidation of these hydroquinones
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reflects the electronic interaction between the component parts.
Ligands that alter the redox chemistry of metal complexes are
said to behave “noninnocently” and can have important
consequences for the chemistry of such systems.20,31,38
The voltammograms of the free quinones and the quinone-
containing metal complexes each show two reversible one-
electron processes with similar separations between the peaks.
For the free quinones the first peak is reduction of the neutral
quinone to the anion-radical semiquinone, and the second peak
corresponds to reduction to the dianion. Similarly, the peaks of
the quinone-containing metal complexes are assigned to
reduction predominantly of the quinone moiety in the complex
to the anion radical and dianion. The primary electrochemical
effect of appending the quinone to the metal complex is a
substantial positive shift of the potentials in the metal
complexes, +0.402−0.468 V for E°1 and +0.223−0.416 V for
E°2 (results summarized in Table 2). Thus the S2Fe2(CO)6
Figure 1. Structure of 3b with displacement ellipsoids at the 50%
probability level. The structure has 2-fold rotational symmetry; the
numbered half of the molecule indicates the asymmetric unit.
Table 2. Standard Potentials of Quinone Metal Complexes
in Comparison to the Corresponding Quinones
b
b
molecule
E°1/V
E°2/V
ΔE°1 /V
ΔE°2 /V
a
q2b
−1.028
−0.626
−1.041
−0.552
−1.083
−0.661
−0.794
−0.424
−1.174
−0.718
−1.082
−1.207
−0.739
−1.457
−1.234
−1.587
−1.202
−1.640
−1.315
−1.469
−1.240
−1.675
−1.293
−1.714
−1.667
−1.251
a
2b
−0.402
−0.489
−0.422
−0.370
−0.223
−0.385
−0.325
−0.224
c
q2c
c
2c
c
q2d
c
2d
c
q2e
c
2e
a
q3
a
3a
−0.456
−0.092
−0.382
0.039
c
3b
a
q4
a
4
−0.468
−0.416
a
Determined by simulations of cyclic voltammograms obtained in 0.10
M n-Bu4NPF6/CH2Cl2 with a 0.3 cm diameter glassy carbon working
electrode. Potentials referred to the Fc+/Fc potential also determined
in dichloromethane. The quinone-containing metal complexes were
not stable in acetonitrile. Diffusion coefficients used in simulations are
not reported, as there was uncertainty in the concentrations due to
b
evaporation of solvent. Potential of quinone minus potential of
c
quinone metal complex. Determined by averaging the potential of
maximum current for the anodic and cathodic peaks.
Figure 2. Voltammograms of ca. 0.5 mM quinone metal complexes 2b,
3a, and 4 as well as the corresponding organic quinones: q2b, q3, and
q4. The currents have been divided by the concentrations of the
compounds to normalize for slight variations in concentration.
Dichloromethane with 0.10 M n-Bu4NPF6, glassy carbon working
electrode, 0.10 V/s.
portion acts as a very strong electron-withdrawing substituent
on the quinone. This substituent effect indicates that the
S2Fe2(CO)6 “group” is roughly equivalent to three chlorine
atoms, as judged by the observation that the potential for one-
electron reduction in acetonitrile of trichloro-1,4-benzoquinone
is 0.43 V less negative than that of 1,4-benzoquinone itself.39
The dianions produced by electrochemical reduction of 2b,
3a, and 4 render reduction localized primarily on the
S2Fe2(CO)6 portion of the molecule more difficult. For
example, the overall two-electron reduction12 potential for
the benzenedithiolato complex 8 (Chart 4) is −1.47 V vs Fc+/
Fc (Fc = ferrocene) in dichloromethane (DCM), but as Figure
2 shows, further reduction of the quinonedithiolato metal
complexes 2b2−, 3a2−, and 42− is not observed in the potential
range of these scans to around −2 V.
Chart 3. Quinones in This Study
Thus the quinone-containing complexes are easily reduced,
even more so than the free quinones, with E°1 values falling in
the range −0.424 to −0.739 V vs Fc+/Fc. The reduced
complexes potentially could catalyze the reduction of protons
analogous to the separated species. The extent that the
voltammograms differ from those of the separated species
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Chart 4. (μ-Benzenedithiolato)Fe2(CO)6
Scheme 3. Proposed Reduction Processes of the
Methoxybenzoquinone Metal Complex 2b in the Presence of
a
Acetic Acid
to form hydrogen through a series of electron transfers and
protonations (perhaps coupled) at iron. However, we find that
protonation of the reduced complexes is favored at the quinone
moiety, probably at the stage of the anion radical depending on
the strength of the acid. To illustrate this, Figure 3 shows
a
The hydroquinone complex at bottom right undergoes reversible
two-electron reduction at −1.315 V and catalyzes the reduction of
protons from acetic acid to hydrogen at around −2 V, as shown in
Scheme 1.13
concentration to that of the complex exceeds 2, for it is only
then that there is excess acetic acid in the diffusion−kinetic
layer. For ratios less than or equal to 2, the acetic acid is
consumed to form the hydroquinone.41
When a stronger acid was used (benzoic acid; pKa = 20.4),
the same effects as those in Figure 3 were seen. Conversely,
higher concentrations of the weaker acid (2-bromophenol; pKa
= 23.9) were required to see reduction to the hydroquinone.
Quinone-containing catalysts 2b and 4 behaved in an analogous
way when studied with acetic acid.
Figure 3. Voltammograms of 0.48 mM 3a with additions of 0, 1, and 5
mM acetic acid. Mercury-film working electrode. Other conditions are
as in Figure 2
The reduction of the quinone moiety rather than H2
production as pointed out above shows that protonation at
oxygen of the reduced species occurs rather than at the 2Fe2S
moiety. This could be due to the lack of electronic
communication between moieties in the reduced species as
reported before for a bma (bma = 2,3-bis(diphenylphosphino)-
maleic anhydride) complexed through a bridging apical S to a
2Fe2S moiety.18,42 To determine whether there is electronic
communication between reduced semiquinone and 2Fe2S
moieties, electron paramagnetic resonance (EPR) spectroscopic
and computational studies of these species were undertaken.
EPR of the One-Electron-Reduced Complexes. The
cyclic voltammograms indicate that the first two reductions of
the quinone metal complexes are predominantly quinone in
character, although shifted substantially less negative from the
reduction potentials of the uncomplexed quinones. To probe
the delocalization of charge and spin density to the 2Fe2S core
in the reduced complexes, EPR spectroscopic experiments were
performed on the electrochemically produced anion-radicals
from 2b,c,d,e and 3a,3b (Figure 4) and compared with the
spectra obtained from the parent organic semiquinones of
q2b,c,d,e and q3 (the EPR spectra obtained for the organic
semiquinone anion-radicals agree well with those reported in
the literature).43 The gav values and hyperfine splitting
constants were obtained for both the organic semiquinones
voltammograms of 0.49 mM 3a obtained with 0, 1, and 5 mM
of added acetic acid (pKa = 22.4 in acetonitrile).40 With no acid,
the voltammogram is the same as seen in Figure 2. Addition of
1 mM acid causes a small increase in the first reduction peak,
and the second reduction peak is replaced by a much larger
quasireversible reduction peak near −1.5 V. Scheme 3
illustrates an explanation for the increases in reduction current
with the addition of acid. First, the addition of 1 mM acid is
stoichiometrically sufficient to convert the quinone-ligand to its
hydroquinone form, as shown in Scheme 3. Second, from our
previous studies13 of the reduction of the related hydroquinone
complexes, it is known that the hydroquinone complexes
undergo a quasireversible two-electron reduction near −1.5 V.
The reduction near −1.5 V in Figure 3 then includes the one-
electron reduction of the semiquinone complex and the two-
electron reduction of the hydroquinone complex. In the
presence of 5 mM acid, the first reduction peak is still larger,
as determined by the protonation equilibrium, the next
reduction corresponding to the hydroquinone becomes higher
and sharper, and a new peak appears near −1.9 V. This new
catalytic peak corresponds to the reduction of acetic acid to
dihydrogen and acetate as catalyzed by the in situ generated
hydroquinone-containing complex, as previously reported.13
The catalytic peak is not present until the ratio of acetic acid
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uncomplexed organic semiquinone as the spin density is
delocalized to the metals in the complexes.
The hyperfine constant due to the phosphorus atoms in 3b•−
is low in comparison to most other hydrogenase active site
mimics with bound phosphine ligands.46−48 These previous
cases differ from the present case in that the previous EPR
studies are performed on the radical cations of the complexes in
two instances and for the radical anion protonated at the metal−
metal bond in the third instance, and the SOMOs of these
species are calculated to be primarily on the iron atoms. In the
radical anion of the present case of 3b•− the SOMO is primarily
on the semiquinone portion of the molecule and delocalized to
the iron atoms, so there is a smaller percentage of metal
character. Examination of the SOMO of 3b•−, shown in Figure
5, identifies an additional reason for the modest hyperfine
Figure 4. EPR Spectra for the semiquinone obtained on electro-
chemical reduction of 2b, 2c, 2d, 2e, 3a, and 3b.
Table 3. EPR Spectroscopic Parameters of Metal Complex
Radical Anions (Parameters for Uncomplexed Organic
Semiquinones in Parentheses)
a
semiquinone complex
gav
aH (MHz)
2b•−
2.0177 (2.009)
2.017 (2.008)
2.017 (2.008)
2.016 (2.008)
gav
−1.1 (−2.5)
−2.80 (−5.32)
−3.1 (−5.5)
−4.4 (−6.9)
aP (MHz)
2c•−
2d•−
2e•−
naphthosemiquinone complex
3a•−
3b•−
2.017 (2.008)
2.017 (2.008)
−4.1
a
All aH values reported are for proton connected to the C6 position of
the quinone.
Figure 5. Calculated SOMO of 3b•
.
splitting by 31P. That is, just as the protons are situated on the
node of the semiquinone π orbital, the phosphines are situated
on the node of the SOMO at the metal d orbitals. Furthermore,
the metal orbital portion of the SOMO has δ symmetry with
respect to the Fe−P bond, so there is no π back-bonding from
this orbital to the phosphines. Thus the phosphine splitting in
the EPR is solely due to spin polarization on the iron atoms and
not due to spin delocalization onto the phosphorus atoms.
Nevertheless, delocalization of the unpaired electron from
the quinone to the 2Fe2S moiety is established. Agreement of
the calculated aH values shown in Table 4 with the experimental
and semiquinone metal complexes under the same conditions
(aH values are known to change with solvent)43−45 and are
reported in Table 3.
Comparison of the parent organic semiquinone g values with
those of the corresponding semiquinone metal complexes
shows an increase in gav values for the complexed semiquinone
from 2.008 to 2.016−2.017. This suggests greater spin−orbit
coupling, attributable to delocalization of the unpaired electron
from the semiquinone moiety to the 2Fe2S moiety. In addition,
the aH values for the uncomplexed organic semiquinones are
reduced to approximately half their values in the corresponding
2Fe2S complexed species. This provides further evidence for
delocalization of the unpaired electron from the quinone to the
2Fe2S moiety. Comparison of the EPR spectra of the
semiquinone obtained from 3a with that from 3b, in which
there are now phosphine ligands coordinated to the metals,
shows a dramatic difference. The former shows a broad singlet,
but the latter a distinct triplet. The additional hyperfine is due
to splitting by the two equivalent 31P nuclei of the Ph3P ligands,
demonstrating spin density on the P2Fe2S2 moiety. The
observed phosphorus hyperfine is 4.1 MHz.
Table 4. Calculated EPR Spectroscopic Hyperfine Splittings
and Spin Densities (Uncomplexed Organic Semiquinones in
Parentheses)
spin density (%)
a
b
semiquinone complex
aH (MHz)
Fe
C
2b•−
2c•−
2d•−
2e•−
2.4 (5.0)
3.3 (6.0)
3.3 (6.3)
4.3 (7.2)
18.32
18.8
18.7
15.6
1.4 (4.7)
2.8 (6.2)
3.0 (6.3)
4.3 (8.9)
Density functional theory (DFT) computations of the
structures and hyperfine constants compare favorably with
spin density (%)
1
the experimental values. As observed experimentally, the H
a
b
naphthosemiquinone complex
aP (MHz)
−7.3
Fe
C
hyperfine constants of the complexes are calculated to fall to
about half the values of the uncomplexed organic semi-
quinones. Since the hydrogen atoms are situated on the node of
the SOMO (singly occupied molecular orbital), the hyperfine
splitting is proportional to the spin density on the carbon atom,
which is calculated to fall to about half the value of the
3a•−
3b•−
22.7
17.4
0.8 (2.0)
1.3 (2.0)
a
b
The sum of both iron centers. Mulliken spin density for the carbon
atom in the C6 position of the parent quinone and the corresponding
carbon in the naphthoquinone analogues.
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167.1 (s, Cq, C−SFe), 134.3 and 127.1 (s, CH), 131.5 (s, Cq); IR
(KBr) cm−1 2086 (100), 2039 (100), 2023 (100), 1980 (100) [ν(C
O)], 1663 (90) [ν(CO)]; MS (EI) m/z (relative intensity) 500
(M+, 1), 472 (M+ − CO, 14), 444 (M+ − 2 CO, 20), 416 (M+ − 3
CO, 2), 388 (M+ − 4 CO, 12), 360 (M+ − 5 CO, 22), 332 (M+ − 6
values shown in Table 3 supports the calculation of the spin
density distribution. As shown in Table 4, the calculated spin
density on the carbon α to the remaining proton in 2b,c,d,e
shows reduction of spin density in the metal-complexed
semiquinones as compared to the uncomplexed organic
semiquinones. This Fe spin density can be varied based on
the functionalization of the quinone moiety, as shown in Table
4.
+
CO, 36), 288 (3), 276 (18), 244 (5), 232 (7), 176 (Fe2S2 , 24), 166
(7), 56 (Fe+, 1). Anal. Calcd for C16H4Fe2O8S2: C, 38.43; H, 0.81.
Found: C, 38.37; H, 0.99.
Syntheses of 2b−e follow an analogous method to that for 3a, and
experimental details can be found in the Supporting Information.
Reduction of 3a to the Corresponding Hydroquinone. To a
solution of 3a (100 mg, 0.20 mmol) in EtOH (40 mL) was added
sodium cyanoborohydride (14 mg 0.22 mmol). The solution was then
stirred at room temperature for 6 h. The solvent was removed under
vacuum, and the residue was separated by flash silica gel column
chromatography by using CH2Cl2 as eluent to give the corresponding
hydroquinone (83 mg, 83% yield): dark crystals, which were
recrystallized from dichloromethane and hexane.
CONCLUSIONS
■
Oxidation of hydroquinones annulated to 2Fe2S moieties
provided the corresponding quinone complexes in good yield.
Communication between the quinone and 2Fe2S moieties was
shown by the considerable positive shift in the reduction
potentials. The quinones were demonstrated to electronically
communicate with the 2Fe2S moieties through cyclic
voltammetry, EPR spectroscopy, and DFT computations.
This interaction in the semiquinones could be tuned by
changing functionalization of the quinone moiety. Thus the
spin density on the metal centers changes from 16% to 23% by
changing from chloroquinone to naphthoquinone, with
intermediate values for methyl, methoxy, and tert-butyl-1,4-
benzoquinone. Despite this electronic communication, initial
2e− reduction in the presence of acid gave the corresponding
hydroquinone product, and further reduction was required for
the production of molecular hydrogen.13 These systems
demonstrate that quinone π-systems linked in this way with
the sulfur atoms of the 2Fe2S core behave noninnocently in
their electronic interactions, reduce the reduction potentials of
the complexes, but protonate on oxygen rather than at the
2Fe2S, resulting in initial ligand not proton reduction.
Synthesis of Fe2(CO)4(PPh3)2(μ-S2C10H4O2), 3b. Me3NO·2H2O
(133 mg, 1.20 mmol) was added to a solution of 3a (100 mg, 0.20
mmol) and PPh3 (315 mg, 1.20 mmol) in CH3CN solvent (40 mL).
The solution was stirred at room temperature for 12 h. The solvent
was then removed under vacuum, and the residue was separated by
flash silica gel column chromatography (hexanes/CH2Cl2, v/v = 2:3)
to afford complex 3b as dark red crystals (37 mg, 19%), which were
1
recrystallized dichloromethane and hexane: H NMR (CDCl3, 23 °C,
500 MHz) δ 7.49 and 7.18 (s each, 1:1 H); 13C NMR (CDCl3, 23 °C,
125 MHz) δ 214.7 (t, Cq, Fe(CO)2), 177.0 (CO), 165.4 (s, Cq, C−
SFe), 135.6 and 135.3 (d, CP of PPh3), 133.3 and 128.5 (t each, CH of
PPh3), 132.5 and 125.6 (s, CH), 131.8 (s, Cq), 129.7 (s, CH of PPh3);
IR (KBr) cm−1 2004 (100), 1960 (80), 1940 (90) [ν(CO)], 1662
(10) [ν(CO)]. Anal. Calcd for C50H34Fe2O6P2S2: C, 62.00; H, 3.54.
Found: C, 61.65; H, 3.76.
Fe2(CO)6(μ-S2C14H6O2), 4. Dark crystals. Single crystals were
1
obtained by recrystallization from dichloromethane and hexane: H
NMR (CDCl3, 23 °C, 500 MHz) δ 8.50, 8.02, and 7.70 (s each, 1:1:1
H); 13C NMR (CDCl3, 23 °C, 125 MHz) δ 206.3 (s Cq, Fe(CO)3,
177.3 (CO), 168.6 (s, Cq, C−SFe), 134.7 and 127.8 (s, Cq), 130.3,
130.2, and 130.0 (s each, CH); IR (KBr) cm−1 2085 (100), 2044
(100), 2015 (100), 2002 (100), 1990 (100) [ν(CO)], 1664 (70)
[ν(CO)]. Anal. Calcd for C20H6Fe2O8S2: C, 43.67; H, 1.10. Found:
C, 43.52; H, 1.35.
EXPERIMENTAL SECTION
■
General Procedures. All reactions were carried out under an
atmosphere of prepurified nitrogen by using standard Schlenk and
vacuum-line techniques. Tetrahydrofuran (THF) and hexane were
purified by distillation under nitrogen from sodium/benzophenone
ketyl. Dichloromethane was distilled from calcium hydride under
nitrogen. Acetonitrile was distilled once from P2O5 and then freshly
distilled from CaH2 under N2 before use. Ethanol was distilled from
magnesium under nitrogen. LiBEt3H (1 M in THF), trifluoroacetic
acid, 1,4-anthraquinone, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone,
sodium cyanoborohydride, Me3NO·2H2O, PPh3, and PMe3 (1.0 M
in THF) were purchased from Aldrich Chemical Co. and used as
received. Hydroquinones of type 7 were synthesized as previously
reported.13
Electrochemistry. For electrochemical experiments, the source
and treatment of the solvent and supporting electrolyte have been
described earlier.52 Experiments were conducted in acetonitrile or
dichloromethane with 0.10 M tetrabutylammonium hexafluorophos-
phate (Bu4NPF6) as supporting electrolyte. Electrodes, cells,
instrumentation, and electrochemical procedures have been de-
scribed.52 In the present study the potentiostat was an EG&G PAR
model 2273. The working electrode was a glassy carbon electrode
(0.071 cm2) or a mercury-film on a gold-disk electrode (0.080 cm2
prepared as described earlier).10 Its area was ascertained by studies of
the oxidation of ferrocene, whose diffusion coefficient is known.52
Evaluation of solution resistance was carried out as described earlier,52
and the resistance was partially compensated by electronic resistance
compensation with the remainder of the resistance applied when
simulating the data. The laboratory reference electrode was a silver
wire in contact with 0.010 M AgNO3, 0.10 M NBu4PF6, and
acetonitrile (AgRE). Its potential was frequently determined with
respect to the reversible ferrocenium/ferrocene couple, in dichloro-
methane, and all potentials are referred to ferrocene. Voltammetric
experiments were carried out at room temperature except for those in
dichloromethane with added acid, for which the temperature was 298
K.
1H and 13C NMR spectra were obtained with a Bruker DRX-500
spectrometer (chemical shifts refer to TMS). IR spectra were obtained
on a Nicolet Impact 410 spectrometer. UV−vis data were collected on
an Agilent 8453. All photochemical reactions were carried out utilizing
a Rayonet Srinivasan-Griffith apparatus.
Single-crystal X-ray intensity data for 3b were obtained using an
Agilent Technologies XCalibur diffractometer using graphite-mono-
chromated Mo Kα radiation and a crystal temperature of 150 K. The
structure of 3b was solved by direct methods with SIR-2004 and
refined by full-matrix least-squares on F2 using SHELXTL.49−51
Oxidation of 12 to Fe2(CO)6(μ-S2C10H4O2), 3a. To a solution of 12
(100 mg, 0.20 mmol) in CH2Cl2 (40 mL) was added 2,3-dichloro-5,6-
dicyano-1,4-benzoquinone (55 mg, 0.22 mmol). The solution was
then stirred at room temperature for 2 h. The solvent was removed
under vacuum, and the residue was separated by flash silica gel column
chromatography by using a hexanes/CH2Cl2 (2:1, v/v) solvent
mixture as eluant to give 3a (83 mg, 83%): dark crystals, which
were recrystallized from dichloromethane and hexane; 1H NMR
(CDCl3, 23 °C, 500 MHz) δ 8.00 (s, 1H), 7.72 (s, 1H); 13C NMR
(CDCl3, 23 °C, 125 MHz) δ 206.2 (s, Cq, Fe(CO)3), 177.6 (CO),
Digital simulations were conducted with DigiElch, version 3.0, a
software package for the digital simulation of common electrochemical
experiments (http://www.digielch.de).53 The fitting routine in that
program was used to establish the final best-fit parameter values for
many of the variables.
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Organometallics
Article
EPR Spectroscopy. All EPR spectra were collected on species that
were generated electrochemically in situ via bulk electrolysis in DCM
with 0.5 M tetrabutylammonium hexafluorophosphate as a supporting
electrolyte. The spectroelectrochemical cell was made of quartz with a
platinum counter and working electrode and a Ag/AgNO3 reference
electrode. An X-band Bruker Elexsys E500 spectrometer was used for
all spectra. Spectra were modeled with EasySpin software to extract
hyperfine constants and g values.54
the University of Newcastle upon Tyne by Dr. Ross Harrington
and Professor William Clegg. The EPR spectra were collected
by Andrei Astashkin of the EPR facility at the University of
Arizona.
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All computations were performed with ADF2009.01.55,56 Geometry
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ASSOCIATED CONTENT
* Supporting Information
■
S
Characterization details of 2b−e, cyclic voltammograms of
2c,d,e and 3b, simulations of EPR spectra of metal-quinone
anions, plot of catalytic current vs [HA] for 3a, tables of
crystallographic information for 3b, optimized geometries of
radical anions in a separate text file, instructions for viewing
geometries, and example ADF input file. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Author
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§J. Chen: Key Laboratory of Renewable Energy and Natural
Gas Hydrate, Guangzhou Institute of Energy Conversion,
Chinese Academy of Sciences, Guangzhou 510640, P. R. China.
⊥C. A. Mebi: Department of Physical Sciences, Arkansas Tech
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∥N. Okumura: Kinjo Gakuin University, Pharmacy, 2-1723
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#U. I. Zakai: Chemistry, The Organosilicon Research Center,
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△G. S. Nichol: School of Chemistry, The University of
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The support of the National Science Foundation through the
Collaborative Research in Chemistry Program (Grant No.
0527003) and Grant Nos. 1111718 and 1111570 are gratefully
acknowledged. The X-ray crystal data for 3b were collected at
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