Synthesis and electrochemical reactivity of molybdenum dicarbonyl supported by a redox-active α-diimine ligand

Article abstract of DOI:10.1021/ic400348j

Low-valent molybdenum dicarbonyl complexes with a diazabutadiene [ ^mesDAB^R; [ArN=C(R)C(R)=NAr]; Ar = 2,4,6-trimethylphenyl (mes), R = H or CH₃] ligand have been synthesized and fully characterized. The title complexes exhibit elongated DAB C-N and shortened C-C bond lengths over the free ligand and other zerovalent molybdenum complexes of DAB. Compared to known examples theoretically described as iminato π-radicals (L^?-), the oxidation state assignment fits a molybdenum(II) description. However, Mo K-edge X-ray absorption spectroscopy indicates that the complexes are best described as molybdenum(0). This example demonstrates that caution should be exercised in assigning the oxidation state based on structural parameters alone. Cyclic voltammetry studies reveal an electrochemical-chemical process that has been identified by in situ Fourier transform infrared spectroelectrochemistry as cis-to-trans isomerization.

Full text of DOI:10.1021/ic400348j

Article
pubs.acs.org/IC
Synthesis and Electrochemical Reactivity of Molybdenum Dicarbonyl
Supported by a Redox-Active α‑Diimine Ligand
Isaac R. Corn,† Pablo D. Astudillo-Sanchez,† Michael J. Zdilla,†,‡ Phillip E. Fanwick,† Michael J. Shaw,§
́
Jeffrey T. Miller,⊥ Dennis H. Evans,*,† and Mahdi M. Abu-Omar*,†
†Brown Laboratory, Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907, United States
§Department of Chemistry, Southern Illinois University at Edwardsville, Edwardsville, Illinois 62026, United States
⊥Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439,
United States
S
* Supporting Information
ABSTRACT: Low-valent molybdenum dicarbonyl complexes
with a diazabutadiene [mesDABR; [ArNC(R)C(R)NAr]; Ar
= 2,4,6-trimethylphenyl (mes), R = H or CH3] ligand have been
synthesized and fully characterized. The title complexes exhibit
elongated DAB C−N and shortened C−C bond lengths over the
free ligand and other zerovalent molybdenum complexes of DAB.
Compared to known examples theoretically described as iminato π-
radicals (L•−), the oxidation state assignment fits a molybdenum-
(II) description. However, Mo K-edge X-ray absorption spectros-
copy indicates that the complexes are best described as
molybdenum(0). This example demonstrates that caution should
be exercised in assigning the oxidation state based on structural
parameters alone. Cyclic voltammetry studies reveal an electro-
chemical−chemical process that has been identified by in situ Fourier transform infrared spectroelectrochemistry as cis-to-trans
isomerization.
Scheme 1. Structural Features of the DAB Ligand in Its
Three Recognized States as α-Diimine (0), Iminate π
Radical (1−), or α-Diamide (2−)
INTRODUCTION
■
Noninnocent redox-active ligands have gained renewed
attention in the literature because their coordination complexes
are rich in bonding and electronic structure.1−4 New reactivities
of their complexes have been exploited based on the ligand’s
ability to act as a reservoir for electrons,5−8 allowing, for
example, reductive elimination and small-molecule activation
on d0 metal centers.9−11 1,4-Diaza-1,3-butadiene (DAB) ligands
have been widely studied in olefin polymerization,12−14 in
atom-transfer-radical polymerization,15−17 and for their elec-
tronic properties.18−21 These ligands can store up to two
electrons and are thought to be present as α-diimine (0),
iminate π radical (1−), or α-diamide (2−).22−25 The form of
the ligand is characterized by the C−N and C−C bond lengths
(see Scheme 1).26,27
While there are many examples of α-diimine-supported
molybdenum(0) carbonyls, the current literature focuses
mainly on tetracarbonyl, tricarbonylsolvato, and bipyridyl
complexes.28,29 In this paper, we describe the synthesis and
characterization of molybdenum dicarbonylbis(1,4-diaza-1,3-
butadiene) complexes. Much of the literature places an
emphasis on the ability to establish a correlation between the
DAB bond lengths and the oxidation state of the metal, and
indeed for most examples, such a correlation holds true.
Structural data for the reported complexes herein are in
agreement with a formal molybdenum(II) metal center and
monoanionic iminate π radical (1−) DAB ligands. However,
Mo K-edge X-ray absorption near-edge structure (XANES)
indicates that the oxidation state is molybdenum(0). Spectro-
scopic and computational data suggest significant covalency
with the electron density shared in orbitals spanning both
ligands and the metal center. Electrochemical investigations
reveal a one-electron oxidation followed by a chemical process.
The latter has been identified as an isomerization based on
difference Fourier transform infrared (FTIR) spectroelectro-
chemistry.
Received: February 13, 2013
Published: April 11, 2013
© 2013 American Chemical Society
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Table 1. X-ray Crystallographic Parameters
1
3
4
5
formula
fw
C42H48MoN4O2
736.82
C46H56Mo1N4O2
792.92
C26H28MoN2O4
528.46
C25H27MoN3O3
513.45
space group
a (Å)
P1 (No. 2)
P2/n (No. 13)
15.4335(11)
8.9648(7)
16.7602(13)
90
Pbca (No. 61)
8.4968(5)
20.4430(12)
28.4906(13)
90
P21/c (No. 14)
15.2745(10)
7.6639(4)
21.9287(14)
90
̅
11.0982(12)
11.1809(14)
17.421(4)
101.38(2)
95.65(3)
117.05(2)
1842.6(5)
150
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
T (K)
λ (Å)
92.030(6)
90
90
110.075(2)
90
90
2317.5(3)
150
4948.8(5)
150
2411.1(3)
150
0.71073
1.54184
1.136
1.54184
1.418
0.71073
1.414
ρ (g cm−3
μ (mm−1
)
1.328
)
0.385
2.595
4.675
0.559
transm coeff
0.716−0.981
0.071
0.925−0.925
0.069
0.509−0.869
0.057
0.819−0.956
0.071
a
R (Fo)
b
2
Rw (Fo )
0.159
0.186
0.158
0.181
a
b
2
2
2
2 2
R = ∑||Fo| − |Fc||/∑ |Fo| for Fo > 2σ(Fo ). Rw = [∑w(|Fo | − |Fc2|)2/∑w|Fo | ]1/2
.
valve, welded to each ultratorr fitting, served to isolate the samples
from air.
EXPERIMENTAL SECTION
■
All solvents were dried by distillation over sodium/benzophenone or
by a solvent purification system (Anhydrous Engineering Inc.),
degassed, and stored over molecular sieves prior to use. Molybdenum
hexacarbonyl, dimolybdenum(II) tetraacetate, bis(cyclopentadienyl)-
molybdenum(IV) dichloride, ferrocenium hexafluorophosphate, and
NBu4PF6 were used as received from commercial sources. The ligands
and complexes mesDABH, mesDABMe, Mo(CO)3(NCMe)3, Mo-
(CO)2(dppe)2, Mo(O)2(hoz)2, 4, and 5 were prepared following
literature procedures.30−36
NMR spectra were recorded on Bruker ARX 400 MHz
spectrometers using solvent proton resonance peaks as references.
IR spectra were recorded using a Thermo-Nicolet FTIR with
attenuated total reflectance (ATR). UV/vis spectra were collected
on a Shimazdu 2501 series spectrophotometer. Electron paramagnetic
resonance (EPR) spectra were collected on a Bruker EMX
spectrophotometer equipped with an Oxford Instruments liquid-
helium continuous-flow cryostat. Spectra were obtained using 9.4 GHz
frequency with 100 kHz field modulation.
X-ray Absorption Spectroscopy (XAS) Data Collection and
Analysis. Mo K-edge (20.000 keV) XAS measurements were
conducted on the insertion devise beamline of the Materials Research
Collaborative Access Team (MRCAT, 10-ID) at the Advanced Photon
Source (APS), Argonne National Laboratory. Ionization chambers
were optimized for the maximum current with linear response (ca. 1010
photons detected per second) using 65% dinitrogen and 35% argon
(10% absorption) in the incident X-ray detector and a mixture of
argon (25% absorption) in the transmission X-ray detector. A third
detector in the series simultaneously collected a Mo foil reference
spectrum with each measurement for energy calibration. The X-ray
beam was 0.8 × 0.8 mm, and data were collected in transmission
geometry from 250 eV before the edge to 1000 eV beyond the edge in
fast scan mode in about 5 min.
The energy of the preedge peak was determined from the maximum
in the first derivative of the first peak in the leading edge of the XAS
spectrum after calibration of the Mo foil for that spectrum. Standard
procedures based on WINXAS 3.1 software were used to normalize the
XAS data using linear and cubic fits in the preedge and postedge
regions of the spectra.
Electrochemistry. FTIR spectroelectrochemistry was conducted
on a Bruker tensor-27 with a Remspec fiber-optic probe equipped with
a low-temperature accessory. A platinum disk electrode was used as
the working electrode, and a silver wire in CH2Cl2 was used as the
reference electrode.37
A PARSTAT 2273 potentiostat−galvanostat was used in all of the
voltammetric experiments in dichloromethane. A silver wire immersed
in 0.010 M AgNO3/0.10 M n-Bu4NPF6/acetonitrile and served as a
reference electrode. The working electrode was glassy carbon with an
area of 0.0706 cm2. The potential of the silver working electrode was
frequently measured with respect to the ferrocenium/ferrocene couple
(Fc/Fc+) in the solvent used, and all potentials are reported versus
Fc+. For scan rate studies, the specific scan rates that were chosen
allowed for an approximately linear variation in log(ν): ν = 0.1, 0.2,
0.3, 0.5, 1.0, and 2.0 V s−1 [log(ν) = −1, −0.699, −0.523, −0.301, 0,
and 0.301].
Simulations were carried out using DigiElch 4.M, a software package
for the digital simulation of common electrochemical experiments
(http://www.elchsoft.com). When matching simulations with the
experimental voltammograms, the objective was to find the single set
of parameter values that provided the best average agreement between
simulation and experiment for the entire range of scan rates that were
employed. This means that a simulation at a given scan rate might be
improved by some variation of one or more simulation parameter
values, but this would lower the average goodness of fit for the entire
set of voltammograms.
The energy of the preedge in the Mo K-edge XANES was used to
determine the oxidation state of the samples and was calibrated from
the position of standards. Because several of the samples were air-
sensitive, all samples were prepared in a glovebox under air- and water-
free conditions. The samples were diluted with dry SiO2, ground to a
uniform composition to about 5% Mo, and pressed into a self-
supporting wafer. Six samples were gently pressed into a cylindrical
sample holder consisting of six wells. The sample thickness was
adjusted to give an absorbance (μx) of approximately 2 in the Mo edge
region and an edge step (Δμx) of approximately 1. The sample holder
with six samples was placed in a quartz tube (1 in. o.d. and 10 in.
length) sealed with Kapton windows by two ultratorr fittings. A ball
X-ray Crystallography. Detailed information concerning data
collection and software used in analysis of the data can be found in the
Supporting Information. Table 1 summarizes the crystallographic
parameters for complexes 1 and 3−5.
Synthesis of cis-Mo(CO)2(mesDABH)2 (1) and cis-Mo-
(CO)2(mesDABMe
) (3). To a solution of fac,fac-Mo(CO)3(MeCN)3
2
was added 2 equiv of the corresponding DAB ligand, and the resulting
solution was allowed to reflux for 6 h under a nitrogen blanket. The
solvent was removed under vacuum, dissolved in a minimal volume of
toluene, and recrystallized by the slow vapor diffusion of hexane.
1
1. Yield (0.1752 g): 75%. H NMR (C6D6, 20 °C): δ 7.31 (s, 2H),
7.05 (s, 2H), 6.79 (s, 2H), 6.70 (s, 6H), 2.24 (s, 6H), 2.07 (s, 6H),
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Figure 1. ORTEP-style X-ray molecular structures of 1 and 3 displayed at 50% probability. The molybdenum atom is purple and oxygen atoms are
red, nitrogen atoms blue, and carbon atoms gray, with hydrogen atoms and two of the mesityl groups from 3 omitted for clarity. Crystallographically
important bond distances (Å) [calculated by DFT]: (1) N11−C12 = 1.338(6) [1.352]; N14−C13 = 1.333(6) [1.347]; C12−C13 = 1.389(6)
[1.408]; N21−C22 = 1.336(6) [1.347]; N24−C23 = 1.339(6) [1.354]; C23−C24 = 1.394(6) [1.407]; (3) N1−C2 = 1.307(6); N4−C3 = 1.343(6);
C2−C3 = 1.377(7).
1.92 (s, 12H), 1.84 (s, 6H), 1.58(s, 6H). IR (solid ATR): ν(CO) 1928,
Table 2. K-Edge Energies for Molybdenum Coordination
Complexes
1853 cm−1. ESI-MS: m/z 739. Anal. Calcd for C42H48MoN4O2: C,
68.46; H, 6.57; N, 7.60. Found C, 66.07; H, 5.86, N, 6.97.
3. Yield (0.2698 g): 85%. 1H NMR (CDCl3, 20 °C): δ 6.99 (s, 4H),
6.89 (s, 4H), 2.33 (s, 6H), 2.29 (6H), 2.15 (s, 12H), 2.07 (6H), 2.03
(6H), 2.00 (12H). IR (solid ATR): ν(CO) 1928, 1853 cm−1. ESI-MS:
m/z 794. Anal. Calcd for C46H56MoN4O2: C, 69.68; H, 7.12; N, 7.07.
Found C, 72.24; H, 7.50; N, 7.03.
Mo K-edge energy/
keV
oxidation
state
complex
cis-Mo(CO)2(dppe)2
Mo2(CH3CO2)4
(Cp)2MoCl2
20.0001
20.0056
20.0092
20.0140
20.0007
20.0002
20.0001
20.0003
0
2+
4+
6+
0
a
Mo(O)2(hoz)2
cis-Mo(CO)2(mesDABH)2 (1)
cis-Mo(CO)2(mesDABMe)2 (3)
Mo(CO)4(mesDABMe) (4)
RESULTS AND DISCUSSION
■
0
Structural Characterization. Complex 1 crystallizes into
0
space group P1 and complex 3 into P2/n. Both complexes
Mo(CO)3(CH3CN)(mesDABH) (5)
0
̅
exhibit slightly distorted octahedral coordination environments.
The interesting crystallographic features of the complexes lie
within the DAB ligand C−N and C−C bonds. They exhibit
elongated C−N bonds from the free ligand, with bond lengths
averaging 1.33 Å and shortened C−C backbone bond lengths
averaging 1.39 Å. For comparison, the same C−C and C−N
bonds in complexes 4 and 5 average 1.47 and 1.3 Å in length,
respectively. On the basis of the crystal structure alone, it would
appear that each ligand would fit the partially reduced
a
Hoz = 2-(2′-hydroxyphenyl)oxazoline.
XANES spectra for 1 and 3 are displayed in Figure 2 alongside
the standard Mo(CO)2(dppe)2.
Density Functional Theory (DFT) Calculations. In an
attempt to better understand the observed changes in the DAB
C−N and C−C bond lengths, a molecular orbital (MO)
analysis was performed. Using the single-crystal X-ray structure
for the starting geometry of complex 1, geometry optimization
was carried out employing unrestricted wave functions at the
B3LYP/LanL2DZ38,39 level of theory, utilizing the Gaussian 09
suite of programs.40 Calculated bond lengths are compared to
experimental values from the X-ray structure in Figure 1, and
they agree within experimental error. Analysis of the highest
occupied molecular orbital (HOMO) and HOMO−1, which
are nearly isoenergetic, demonstrates extensive delocalization of
the four highest-energy electrons, as depicted in Figure 3. The
orbitals comprising these span both DAB ligands and the metal
center. The calculations predict that the most likely reasons for
the observed changes in the DAB bond lengths are due to the
C−C orbitals being π bonding in character and the N−C
orbitals having antibonding character. The high covalent nature
and delocalization of these frontier orbitals are responsible for
the observed bond changes in the solid state rather than a
formal oxidation at the molybdenum center and reduction of
the DAB ligand.
1
monoanionic motif, iminate π radical (1−). H NMR spectra
of 1 and 3 yield sharp, well-defined peaks in the diamagnetic
region and integrate to the appropriate number of correspond-
ing protons, indicating an S = 0 ground state. Furthermore,
both complexes show no signal in the X-band EPR spectrum.
Molecular structures of 1 and 3 along selected bond lengths are
shown in Figure 1.
Mo K-Edge XAS. In order to assign an oxidation state to the
title complexes, Mo K-edge energies were compared to a variety
of molybdenum coordination complexes with varying oxidation
states (Table 2). Plotting the formal oxidation states versus the
edge energies gave a linear correlation. Having established the
relationship between the oxidation state and edge energy for
molybdenum complexes of unambiguous oxidation state, it was
straightforward to make the assignment for the complexes cis-
Mo(CO)2(DAB)2 reported herein. There is a consistent 5 eV
per two oxidation states increase in the K-edge energy. With
the K-edge energies for 1 and 3 being fairly close to the
molybdenum(0) standards (Table 2), the oxidation state
assignment for 1 and 3 is decisively molybdenum(0). The
Electronic Spectrum. Complexes 1 and 3 are intensely
colored. The UV/vis spectrum of 1 is shown in Figure 4. The
absorption in the visible at λmax = 560 nm exhibits a large
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Figure 4. UV/vis spectrum of complex 1 in toluene.
band at 560 nm (Figure 5). The HOMO, HOMO−1, and
HOMO−2 are significantly metal in character (approximately
Figure 2. Molybdenum XANES spectra showing K-edge energies.
Blue: Mo(CO)2(mesDABH)2. Red: Mo(CO)2(mesDABme)2. Green:
Mo(CO)2(dppe)2.
Figure 5. Relative energies of HOMO−2 through LUMO +1 and
transitions comprising the MLCT band at 560 nm, as determined by
TD-DFT. Orbital contributions: HOMO, Mo = 0.44, DAB = 0.42,
COs = 0.14; HOMO−1, Mo = 0.46, DAB = 0.45, COs = 0.17;
HOMO−2, Mo = 0.46, DAB = 0.37, COs = 0.17; LUMO, Mo = 0.16,
DAB = 0.75, COs = 0.09; LUMO+1, Mo = 0.22, DAB = 0.61, COs =
0.17.
Figure 3. HOMO (top) and HOMO−1 (bottom) of complex 1 from
DFT calculations. MO isovalue = 0.0200; density = 0.0040 for both
cases.
80%), and they have the same symmetry as the LUMO and
LUMO+1, which are significantly mesDABH π* in character.
Therefore, the transition at 560 nm can be assigned to a metal-
to-ligand charge transfer (MLCT).
Electrochemical Studies. Slow-scan cyclic voltammetry
indicated an almost irreversible oxidation process with a peak
potential of −0.084 V vs Fc/Fc+ and an irreversible reduction
process at −0.721 V vs Fc/Fc+, which are directly related to
extinction coefficient (ε = 18000 L mol−1 cm−1), indicative of a
charge-transfer (CT) transition. Time-dependent DFT (TD-
DFT) revealed that transitions from the HOMO, HOMO−1,
and HOMO−2 to the LUMO and LUMO+1 compose the CT
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each other, as determined by varying scan rates. Figure 6
exemplifies a cyclic voltammogram at lower scan rates. At 0.1 V
Figure 7. Difference FTIR spectroelectrochemistry of complex 1:
(solid line, a) background taken with no applied potential and the
electrode potential held at peak Ia during spectral scans; (dashed line,
b) electrode potential held at peak Ia during background scans and at
peak IIc during spectral scans; (dotted line, c) background taken with
no applied potential and the electrode potential held at peak Ia during
spectral scans at −78 °C. Negative peaks indicate the starting material;
positive peaks indicate the products. The electrode potential at peak Ia
was 0.40 V and that at peak IIc −0.40 V vs Ag/AgCl. Data obtained in
CH2Cl2 containing 1.3 mM 1 and 0.17 M n-Bu4NPF6 and purged with
argon.
Figure 6. Background-corrected cyclic voltammograms of 1.0 mM
complex 1 in dichloromethane with 0.30 M n-Bu4NPF6 (solid black
line) at 0.1 V s−1 (resistance compensation = 150 Ω): (a) recorded
without CO; (b) recorded in a CO saturated solution. Simulations
(circles) are based on the reactions and parameters described in Table
SI-1 in the Supporting Information. All of the voltammograms were
recorded at 25 °C and 1 atm of pressure.
change that occurs upon the reduction of 2+, the disappearance
of the single peak at 1900 cm−1, and the reappearance of the
two peaks of 1 at 1928 and 1853 cm−1. Without added CO, the
diffusion of free CO from the electrode surface to the bulk
solution would be fast enough to prevent re-formation of the
cis-dicarbonyl complex 1. Thus, the absence of any new peaks
demonstrates that isomerization is the dominant process, and it
is indeed reversible. It is apparent that, upon oxidation, 1+
forms the trans-dicarbonyl isomer 2+. The latter exhibits only
one stretching frequency in the IR region, as would be expected
for an octahedral trans-dicarbonyl species. The X-band EPR
spectrum of the oxidation product of 1 (Figure 8) is consistent
with a metal-centered oxidation, with the oxidation state being
s−1, peak Ia is almost irreversible but becomes fully reversible at
scan rates greater than 3 V s−1. The height of peak Ia
corresponds to a one-electron oxidation. As the scan rate is
increased, peak IIc disappears, suggesting that the species
reacting at IIc is formed by some reaction of the cation formed
at Ia. For peak Ia, the simulation showed good agreement with
the experimental data considering a first-order coupled reaction
of 1+ in the forward direction, but it was not clear about the
reaction order in the backward direction and perhaps the
forward reaction involves the loss of carbon monoxide (CO).
Figure 6b, conducted under a CO atmosphere, indicates no
additional changes for peak Ia, but the reduction wave (IIc) has
some dependence on CO, yielding more reduction current.
This observation suggests that the oxidized species undergoes
some CO loss and that this is suppressed when the solution is
saturated with CO.
In differential FTIR spectroelectrochemistry experiments,
two new species were observed. Figure 7 (solid line, a)
indicates the disappearance of the two peaks at 1928 and 1853
cm−1 due to 1 when the spectrum was recorded with the
potential held at peak Ia (Figure 6). The product that is
formed, 2+, gives a single peak at 1900 cm−1, which indicates
the formation of a trans-dicarbonyl species. To rule out
dissociation of CO, the reduction of 2+ was followed by
difference IR. Figure 7 (dashed line, b) demonstrates the
14,41−44
1
the molybdenum(I), S = /2, complex.
The spectrum is
in agreement with the notion that the observed oxidation
occurs at the metal center.
In an attempt to capture 1+ (oxidized molybdenum with cis-
dicarbonyl), we conducted the spectroelectrochemistry experi-
ment at low temperature in a dry ice/acetone bath (−78 °C).
Figure 7 (dotted line, c) shows the IR spectrum of [cis-
Mo(mesDABH)2(CO)2]+ (1+). At −78 °C, isomerization to the
trans-dicarbonyl 2+ isomer is slow enough to allow observation
of 1+. The carbonyl stretching frequencies in 1+ shift to higher
frequencies by ca. 50 cm−1 compared to 1. This shift in the
stretching frequency along with isomerization, which has been
rationalized theoretically for other molybdenum dicarbonyl
complexes, is consistent with a metal-centered oxidation.45
All of the data in hand was used to construct a square scheme
for the observed reactivity (Scheme 2). Complex 1 undergoes a
reversible oxidation at E°1 of −0.112 V vs Fc/Fc+ to form 1+,
[cis-Mo(CO)2(mesDABH)2]+. The rate at which 1+ undergoes
cis-to-trans isomerization to give 2+ has been determined by
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CONCLUSION
■
In summary, new molybdenum complexes with two coordi-
nated DAB ligands have been synthesized and fully
characterized. Cyclic voltammetry studies show oxidation
followed by a chemical change, affording a new species that
reverts to the starting complex upon reduction. FTIR
spectroelectrochemistry has identified a reversible cis-to-trans
isomerization of the oxidized complex [Mo-
(CO)2(mesDABH/Me)]+ as the chemical process. An excellent
fit of the cyclic voltammetry data was obtained based on the
square scheme shown in Scheme 2, allowing determination of
the corresponding potentials and rate constants. Mo K-edge
XAS data established the oxidation state of the metal center as
molybdenum(0). While there are many examples of DAB
complexes where the bond lengths can be useful in oxidation
state assignment, the title complexes do not follow that
particular trend. Serendipitously, the bond lengths as
established by X-ray crystallography coincide exactly with the
established π-radical ligand.
Figure 8. X-band EPR spectrum of the oxidation product of 1 (2+)
recorded at 125 K as CH2Cl2 glass. gavg = 2.0027, gx = 2.0945, gy =
1.9965, gz = 1.9199. A(95 Mo/97 Mo) = 28 G.
ASSOCIATED CONTENT
* Supporting Information
■
S
X-ray crystallographic data in CIF format, experimental
prodcedures, ORTEP diagrams, bond lengths and angles,
additional spectra, cyclic voltammograms, electrochemical
simulation parameters, calculated HOMO and LUMO,
supporting references, and XYZ coordinates. This material is
available free of charge via the Internet at http://pubs.acs.org.
Scheme 2. Square Scheme Used To Fit the Cyclic
Voltammetry Data for Complex 1
a
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: mabuomar@purdue.edu (M.M.A.-O.), evansd@
purdue.edu (D.H.E.).
Present Address
‡Department of Chemistry, Temple University, Philadelphia,
PA 19122.
Notes
The authors declare no competing financial interest.
a
E°1 = −0.112 V vs Fc/Fc+; E°2 = −0.683 V vs Fc/Fc+; k1 = 1.2 s−1; k2
ACKNOWLEDGMENTS
■
≥ 10 s−1; k3 = 0.12 s−1. See the Supporting Information for more
details regarding the simulation parameters.
Funding for this research was provided by U.S. Department of
Energy, Office of Basic Energy Sciences (DOE-BES; Grant DE-
FG-02-06ER15794 to M.M.A.-O.), and the NSF (Grant CHE-
0715375 to D.H.E.). Use of the APS was supported by the
DOE-BES under Contract DE-AC02-06CH11357. MRCAT
operations are supported by the Department of Energy and the
MRCAT member institutions. Partial funding for J.T.M. was
provided by the Chemical Sciences, Geosciences and
Biosciences Division, U.S. Department of Energy, under
Contract DE-AC0-06CH11357.
simulations, k1 = 1.2 s−1 at 25 °C. Complex 2+ can be reversibly
reduced to form the trans neutral complex 2 at E°2 of −0.683 V
versus Fc/Fc+. Isomerization of 2 back to 1 occurs rapidly with
k2 > 10 s−1. The rate constant of carbonyl dissociation from 2+
has been determined to be k3 = 0.12 s−1. In addition, the cyclic
voltammetry experiments also revealed the presence of two
consecutive oxidation peaks at higher anodic potentials, IIIa (a
reversible peak) and IVa (an irreversible peak) in Figure SI-9 in
the Supporting Information. Peak IIIa is prevalent at low scan
rates, but its height, relative to peak IVa, decreases as the scan
rate increases. This observation suggests that peak IIIa is due to
oxidation of the ultimate product at peak Ia, 2+. Peak IVa (peak
potential 0.914 V vs Fc/Fc+ at 2.0 V s−1), which is prevalent at
rapid scan rates, is assumed to be due to oxidation of the initial
product formed at peak Ia, 1+. The fact that peak IVa is
chemically irreversible at all scan rates is consistent with a rapid
decomposition of the reputed 12+.
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