Oxidation of Cymantrene Analogues of Ferrocifen: Electrochemical, Spectroscopic, and Computational Studies of the Parent Complex 1,1′-Diphenyl-2-cymantrenylbutene

Article abstract of DOI:10.1021/acs.organomet.8b00186

The oxidative electrochemical behavior of 1,1′-diphenyl-2-cymantrenylbutene (1), a cymantrene analogue of the breast cancer drug ferrocifen, was shown to involve the sequential electron-transfer series 1/1⁺/1²⁺ in dichloromethane/0.05 M [NBu₄][B(C₆F₅)₄] (E_1/2 values 0.78 and 1.18 V vs ferrocene). By a combination of spectroscopic and computational techniques, it was shown that the cymantrene functionality plays an important role in dissipating the positive charges in the oxidized compounds and is therefore an active participant in the redox events. The redox-active orbital goes from roughly equal degrees of organometallic and π-organic (diphenylolefin) makeup in 1 to increasingly organic based fractions in 1⁺ and 1²⁺. Structural changes mimicking those of oxidized tetrakis(aryl)ethylenes accompany the one-electron oxidations. There is sufficient unpaired electron density on the manganese center in 1⁺ to allow for oxidatively induced ligand exchange of one or more of the carbonyl ligands with donor ligands, including phosphites and pyridine. The complex Mn(CO)₂P(OPh)₃(η⁵-C₅H₄(Et)C=C(C₆H₅)₂) was prepared by the electrochemical switch method, wherein [Mn(CO)₂P(OPh)₃(η⁵-C₅H₄(Et)C=C(C₆H₅)₂)]⁺, produced by the oxidation of 1 in the presence of P(OPh)₃, was reduced back to the neutral CO-substituted complex.

Full text of DOI:10.1021/acs.organomet.8b00186

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Oxidation of Cymantrene Analogues of Ferrocifen: Electrochemical,
Spectroscopic, and Computational Studies of the Parent Complex
1,1′-Diphenyl-2-cymantrenylbutene
Kan Wu,† Ji Young Park,‡ Rachael Al-Saadon,§ Hyerim Nam,‡,∥,∇ Yujin Lee,‡,∥,∇ Siden Top,⊥
Gerard Jaouen,⊥,# Mu-Hyun Baik,*,‡,∥ and William E. Geiger*,†
́
†Department of Chemistry, University of Vermont, Burlington, Vermont 05405, United States
‡Center for Catalytic Hydrocarbon Functionalizations, Institute for Basic Science (IBS), Daejeon 34141, South Korea
§Department of Chemistry, Duke University, Durham, North Carolina 27708, United States
∥Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, South Korea
⊥Sorbonne Universite,
75005 Paris, France
́ ́
UPMC, CNRS, Institut Parisien de Chimie Moleculaire (IPCM), UMR 8232, 4 place Jussieu,
#PSL, Chimie ParisTech, 11 rue Pierre and Marie Curie, F-75005 Paris, France
S
* Supporting Information
ABSTRACT: The oxidative electrochemical behavior of 1,1′-diphenyl-2-cyman-
trenylbutene (1), a cymantrene analogue of the breast cancer drug ferrocifen, was
shown to involve the sequential electron-transfer series 1/1+/12+ in dichloro-
methane/0.05 M [NBu4][B(C6F5)4] (E1/2 values 0.78 and 1.18 V vs ferrocene).
By a combination of spectroscopic and computational techniques, it was shown
that the cymantrene functionality plays an important role in dissipating the
positive charges in the oxidized compounds and is therefore an active participant
in the redox events. The redox-active orbital goes from roughly equal degrees of
organometallic and π-organic (diphenylolefin) makeup in 1 to increasingly organic
based fractions in 1+ and 12+. Structural changes mimicking those of oxidized
tetrakis(aryl)ethylenes accompany the one-electron oxidations. There is sufficient
unpaired electron density on the manganese center in 1+ to allow for oxidatively
induced ligand exchange of one or more of the carbonyl ligands with donor
ligands, including phosphites and pyridine. The complex Mn(CO)2P(OPh)3(η5-C5H4(Et)CC(C6H5)2) was prepared by the
“electrochemical switch” method, wherein [Mn(CO)2P(OPh)3(η5-C5H4(Et)CC(C6H5)2)]+, produced by the oxidation of 1 in
the presence of P(OPh)3, was reduced back to the neutral CO-substituted complex.
activity.8,9 An unexplored aspect of the cymantrenyl-tagged
compounds is that dramatic changes in the redox potential and
other properties of 1 may be obtained with derivatives in which
INTRODUCTION
■
Modeled in part on the structure of the breast cancer drug tamox-
ifen (Scheme 1),1−3 a large number of ferrocenyl-derivatized
diaryl butenes have been tested for activity against cancer cells.4,5
one or more carbonyl group is replaced by an electron-donating
ligand.10 For example, the E1/2 value of the one-electron oxi-
Several of these “ferrocifen mimics” show promising antiprolifer-
dation of MnCp(CO)2(PPh3) is 650 mV negative of that of
ative activity against both estrogen receptor-positive and receptor-
MnCp(CO)3,11,12 and even greater shifts are possible with
negative cell lines.5 Replacing the phenyl group of tamoxifen
double substitution of the CO ligands.13 To better understand
by the ferrocenyl group increases the lipophilicity, and per-
these systems, we prepared several diaryl butenes substituted at
haps more importantly, facilitates its one-electron oxidation.
the 2-butenyl position with cyclopentadienylmanganese tricar-
Ruthenium and osmium analogues of ferrocifen are less cytotoxic
bonyl groups and investigated their oxidative redox properties.
to breast cancer cell lines, a factor attributed to the higher
M(II)/M(III) redox potentials of the heavier homologues.5,6
The communication describing our preliminary experimental
findings on 114 is supplemented in the present paper by cal-
Less widely studied are analogues in which the organometallic
unit has a half-sandwich structure. Here we describe the oxida-
tion of the cyclopentadienylmanganese tricarbonyl (cymantrene)
derivative 1, which may be seen as the parent of a class of group 5
culations giving information on the electronic and molecular
structures of 1 and its oxidation products, 1+ and 12+. We also
describe the oxidatively induced substitution of carbonyl groups
organometallic diarylbutenes. Although compound 1 shows only
weak antiproliferative action against breast cancer cell lines,7
Received: March 29, 2018
other cymantrenyl-labeled poly(aryl) butenes exhibit significant
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 1
EXPERIMENTAL DETAILS
■
All operations were carried out under nitrogen or argon using either
Schlenk or drybox techniques. Solvents were dried and purified either by
refluxing them in drying agents or by passing them down an alumina
column under argon. Infrared spectra were obtained on either an IRFT
BOMEM Michelson-100 spectrometer or an ATI-Mattson Infinity
Series FTIR instrument operating at a resolution of 4 cm−1. In situ IR
spectroelectrochemistry was carried out with a Remspec, Inc., fiber-optic
system using a methodology previously described.26 1H and 13C NMR
spectra were obtained with Bruker spectrometers, and mass spectra were
obtained with a Nermag R 10-10C unit using chemical ionization. EPR
spectra were obtained with a Bruker ESP 300E spectrometer.
[NBu4][B(C6F5)4] was recrystallized from dichloromethane/diethyl
ether after being prepared by metathesis of [NBu4]Br and
K[B(C6F5)4].27
Synthesis of Mn(CO)3(η5-C5H4)(Et)CC(C6H5)2 (1). TiCl4 (1.1 g,
6.0 mmol) was added dropwise to a suspension of zinc powder (0.78 g,
12.0 mmol) in 20 mL THF at 0 °C. A blue mixture was obtained, which
was refluxed for 2 h, whereupon the solution became black. The reaction
medium was allowed to come to room temperature, after which a
solution of benzophenone (0.73 g, 4.0 mmol) and propionylcyman-
trene28 (0.52 g, 2.0 mmol) in 10 mL of THF was added dropwise. The
resulting mixture was refluxed for 4 h, after which it was cooled to room
temperature and then poured into 100 mL of water and acidified to pH 1
with a diluted aqueous HCl solution. After the solution was extracted by
dichloromethane (3 × 50 mL) and washed with 50 mL of water, the
solvent was evaporated, leaving 1.17 g of crude product that was
chromatographed on a silica gel column with 1/50 diethyl ether/
petroleum ether as eluent, giving 0.38 g (46%) of the desired product.
It was recrystallized from diethyl ether/pentane, giving yellow crystals
with mp 102 °C. 1H NMR (300 MHz, d6-acetone): δ 7.39−7.17
(m, 10H, aromatic rings), 4.77 (t, J = 2.1 Hz, 2H, C5H4), 4.62 (t, J =
2.1 Hz, 2H, C5H4), 2.31 (q, 2H, J = 7.5 Hz, CH2CH3), 1.08 (t, 3H, J =
7.5 Hz, CH2CH3). 13C NMR (75.5 MHz, d6-acetone): δ 15.4 (CH3),
28.5 (CH2), 82.8 and 85.7 (CH, C5H4), 105.1 (Cip, C5H4), 127.7, 129.3,
129.4, 129.5, and 130.1 (CHarom), 132.9, 143.7, 144.2, and 144.4 (Cq of
two C6H4 and CC). IR (CH2Cl2): νCO 2017 and 1932 cm−1. MS (CI):
m/z 411 [MH]+; 326 [M − 3CO]+. Anal. Calcd for C24H19O3Mn: C,
70.25, H, 4.67. Found: C, 69.75, H, 4.55.
in 1 by donor ligands and report the E1/2 potentials of the
resulting carbonyl-substituted complexes.
The potential redox activity of organometallic-tagged tamoxifen-
type compounds is not restricted to the organometallic unit. In fact,
the 1,1′-diarylethylenyl position of these molecules is expected to
be redox-active, on the basis of the fact that most poly(aryl)-
ethylenes undergo two chemically reversible oxidations at acces-
sible potentials. For example, in dichloromethane/0.1 M [NBu4]-
[PF6], tetraanisylethylene is converted to its radical cation and
dication at 0.41 and 0.53 V vs ferrocene (FcH), respectively.15−18
Oxidation potentials of the diaryldiethyl analogue 3,4-bis-
(anisyl)-3-hexene are only slightly more positive.17 More recent
papers on tetraarylethylene redox chemistry are available.18−20
Consistent with the expectation that a ferrocenyl group should
be easier to oxidize than a diarylethylenyl group, the first oxi-
dations of 1,1′-diphenyl-2-ferrocenylbutene (2) and other mem-
bers of the ferrocifen family give monocations that have essen-
tially ferrocenium (Fe(III)) character.21−23 In contrast, oxidation
of the manganese center in 1 is expected to be more difficult than
oxidation of the diarylethylenyl group, on the basis of the fact that
E1/2 for the oxidation of cymantrene is 0.91 V vs FcH.24 Thus,
radical cations of the type 1+ might be expected to have spin and
charge that is highly localized in the diarylethylenyl (π-ligand)
part of the molecule. In fact, as we show by both experiment and
calculation, the HOMO of the parent complex 1, as well as
the SOMO of 1+, has similar amounts of organometallic and
π-organic character. It is of practical importance to obtain
a measure of the metal vs π-ligand makeup of these systems
owing to the fact that delocalization between the organometallic
group and the diarylethylenyl group is thought to facilitate
the follow-up reactions of positively charged ferrocifen deriv-
atives that exhibit strong biological activity.5,21−23 Further-
more, it has been shown that the degree of metal−ligand
charge delocalization in a cymantrene-type radical cation affects
the rates of carbonyl substitution reactions for the Mn(CO)3
group.25
Synthesis of Mn(CO)2P(OPh)3(η5-C5H4(Et)CC(C6H5)2). A 32 mL
portion of a CH2Cl2/0.05 M [NBu4][B(C6F5)4] solution that was
3.04 mM in 1 (40 mg) and 4.5 mM in P(OPh)3 (44 mg) was placed in
the working compartment of a three-compartment electrochemical cell.
With application of an applied potential of 1.0 V vs ferrocene to a
platinum-basket electrode, the solution was electrolyzed for 30 min, at
which time 1.0 F had been passed and the solution had changed color
from its original pale yellow to pink. After CV scans were obtained which
showed that 1 had been quantitatively converted to [Mn(CO)2P-
(OPh)3(η5-C5H4(Et)CC(C6H5)2)]+ (E1/2 = 0.46 V), the applied
potential at the platinum basket was changed to 0.1 V. This initiated the
reduction of the phosphite-substituted cation and resulted in passage of
0.85 F as the solution went from pink to green. The working com-
partment solution was evaporated while being protected from light.
A pentane extract of the crude product was evaporated to dryness, and
the resulting yellow solid was spotted on an alumina column with
dichloromethane and eluted with diethyl ether. The yellow band trailing
the unreacted triphenyl phosphite was evaporated, giving 27 mg (40%)
of a yellow powder that was identified as Mn(CO)2P(OPh)3(η5-
C5H4(Et)CC(C6H5)2) on the basis of its IR spectrum (νCO 1956,
1893 cm−1 in CH2Cl2) and the fact that the potential of its reversible
oxidation (E1/2 = 0.46 V) matched that obtained from cyclic
voltammetry scans when 1 was oxidized in the presence of triphenyl
phosphite (see Results and Discussion).
The present study was simplified by the fact that the anodic
electron-transfer reactions of 1 are not complicated by the types
of follow-up reactions that would be expected if the aryl moieties
in 1 contain hydroxyl or other groups that are subject to oxidative
activation. A separate study has been carried out on the
p-hydroxyl- and p-methoxy-substituted analogues of 1, and
those results will be reported in a separate paper.
Electrochemistry. Voltammetry was carried on with a three-
electrode format, using a Pt wire for the auxiliary electrode and a
homemade Ag/AgCl electrode for the experimental reference electrode.
The latter consisted of a AgCl-coated silver wire that was immersed in a
solution of 0.05 M [NBu4][B(C6F5)4] in dichloromethane which was
separated from the working electrode compartment by a fine-porosity
B
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
glass frit. The potentials in this paper are all referenced to the ferrocene/
ferrocenium couple,29 the potential of which was obtained by the in situ
method30 at an appropriate point in the experiment. Working electrodes
for voltammetry measurements were 1 or 2 mm glassy-carbon disks
obtained from Bioanalytical Systems and polished before use with
Buehler diamond paste. A platinum-basket working electrode was used
in a three-compartment H cell for bulk electrolysis. The potentiostat was
a PARC Model 273 instrument interfaced to a computer controlled by
homemade software. The temperature of the electrolyte solution was
controlled to within 1 °C by a TFS Flexi-Cool system. In general, the
electrochemical procedures followed those described in an earlier
paper.24 Well-known diagnostics31 were applied to cyclic voltammetry
measurements at different scan rates to check for diffusion control and
chemical and electrochemical reversibility of the redox couples. Digital
simulations were carried out using Digisim 3.0 (Bioanalytical Systems)
on background-subtracted cyclic voltammograms.
Computational Methods. All calculations were performed at the
B3LYP-D332 level of density functional theory implemented in the
Q-Chem 4.4 package.33 For geometry optimizations, the 6-31G** level
basis set34 was used for all main-group elements and Mn was repre-
sented using the Los Alamos LANL2DZ basis set,35 which is a double-ζ
quality basis set including effective core potentials. Convergence to local
minima was confirmed by vibrational frequency calculations that showed
no imaginary frequency. The energies of these species were re-evaluated
using the cc-pVTZ36 basis set for main-group elements and LANL2DZ
for Mn. This is a well-established and thoroughly tested protocol for
calculating redox potentials37 and modeling redox processes. Solvation
energy corrections were obtained using a continuum solvation model
(c-pcm) employing the double-ζ basis set, and zero-point vibrational
energy and entropy corrections were obtained also using the double-ζ
basis set. The dielectric constant of dichloromethane was set to 9.08 at
room temperature.38 Vibrational frequencies computed using the har-
monic oscillator model at the B3LYP/double-ζ level of theory were
scaled by a factor of 0.9613.39 Cartesian coordinates of all computed
structures are given in the Supporting Information. The percent orbital
contributions for fragments were obtained using the Chemissian40
program.
Figure 1. Experimental (solid line) and simulated (circles) CVs of
1.25 mM 1 in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at 298 K and 0.2 V s−1
on a 1 mm glassy-carbon electrode.
consumed an additional 1.0 F and produced the dication 12+ in
high yield. Cathodic re-electrolysis at Eappl = 0 V regenerated over
90% of the neutral starting material. The favorable stabilities of
the oxidation products facilitated their spectral characterization.
Carbonyl-region IR spectra were obtained either by the in situ
fiber-optic method27 or by simple transfer of an electrolysis
sample to a solution IR cell. Examples of in situ spectra are shown
in Figures 2 and 3 for the stepwise conversion of 1 to 1+ and of 1+
to 12+, respectively.
RESULTS AND DISCUSSION
■
Oxidation of 1. Electrochemical studies of the anodic reac-
tions of 1 were carried out in dichloromethane, using either
[NBu4][PF6] or [NBu4][B(C6F5)4] as the supporting electro-
lyte. Owing to the fact that a single cyclic voltammetry (CV) scan
through the first oxidation wave resulted in severe electrode fouling
in [PF6]−-based electrolyte, the electrochemical work reported in
this paper was carried out in [B(C6F5)4]−-based electrolyte, the
use of which circumvented adsorption problems and gave repro-
ducible voltammograms at glassy-carbon electrodes. Positive-
going CV scans of the parent compound 1 showed two reversible
one-electron oxidations at E1/2(1) = 0.78 V and E1/2(2) = 1.18 V
vs FeCp20/+. That the two reactions are diffusion-controlled,
quasi-Nernstian processes (see electron-transfer series in eq 1)
was confirmed by evaluation of the standard CV diagnostic cri-
teria31 and by digital simulations (Figure 1). The latter was used
to determine the diffusion coefficient of 1 as 1.5 × 10−5 cm2 s−1 at
298 K. Both 1+ and 12+ are stable on the electrolysis time scale
and were characterized by spectroscopic methods.
Figure 2. Fiber-optic IR spectra of solutions of 1.5 mM 1 in CH2Cl2/
0.05 M [NBu4][B(C6F5)4] as the bulk oxidation proceeded at 273 K to
1 F passed, with Eappl = 0.9 V.
−e−
−e−
2+
1 HoooooI 1+ HoooooI 1
ΔE1/2 = 400 mV
(1)
0.78 V
1.18 V
Spectral Characterization of Oxidation Products. To more
broadly characterize the oxidation products, the monocation and
dication of 1 were generated by bulk anodic electrolysis at 273 K.
Electrolysis at Eappl = 0.9 V consumed 1.0 F, giving complete
conversion to the monocation 1+, as shown by linear scan
voltammetry. Continuation of the electrolysis at Eappl = 1.3 V
Figure 3. Fiber-optic IR spectra of 1.5 mM 1 in CH2Cl2/0.05 M
[NBu4][B(C6F5)4] as the bulk oxidation proceeded at 273 K from 1 F
passed to 2 F passed, with Eappl = 1.3 V.
It is well-known that metal−carbonyl stretching frequencies
increase with increasing positive charge at the metal center as
C
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
a
Table 1. Summary of Metal−CO IR Absorptions for 1, 1+, and 12+ in CH2Cl2/0.05 M [NBu4][TFAB] at 273 K
exptl
DFT calcd
b
c
b
c
compound
νsym
νasym
1934
⟨νCO
⟩
Δ⟨νCO
⟩
νsym
νasym
1979
⟨νCO
⟩
Δ⟨νCO⟩
MnCp(CO)3
[MnCp(CO)3]+
2022
2118
2017
2042
2097
1963
2078
1960
2024
2072
2035
2197
2028
2056
2104
1998
2080
1991
2032
2080
2058
+115
1959, 2085
1969, 1978
2015, 2024
+82
1
1931
1+
12+
2015
+64
+41
+89
2049, 2069
+112
1989, 2075
a
b
c
0/+
Data on MnCp(CO)3 (298 K) were taken from reference 24. ⟨νCO⟩ (cm−1) = (νsym + 2νasym)/3. Average value referenced to neutral
compound in cm−1.
Figure 4. EPR spectrum of 1+ at 77 K. The sample was prepared by exhaustive electrolysis of 1 in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at 253 K, with
Eappl = 0.9 V.
back-donation of electron density from the metal to the carbonyl
π* orbital is reduced, resulting in a stronger C−O bond.41−45
In the present case, it is informative to compare the increases in the
average carbonyl stretching frequencies for 1+ and 12+ with those
observed for the oxidation of cymantrene itself24 (see Table 1).
Note that we use a weighted average of the three carbonyl absorp-
tion frequencies, counting the asymmetric stretch twice for com-
pounds in which the two asymmetric νCO bands are not resolved.
This analysis shows that removal of two electrons from 1 is
required to produce the same degree of spectral shift (+112 cm−1)
as that seen with removal of a single electron from cymantrene
(+115 cm−1). In concert, the average increase in νCO (+64 cm−1)
for the monocation 1+ is only slightly more than half of that
observed for the cymantrene radical cation. We have also com-
puted the vibrational frequencies using DFT calculations, and the
results are convincingly similar, although in that case the com-
puted energy shifts are slightly smaller. Calculations confirm that,
to reproduce the frequency shift calculated for the oxidation of
cymantrene (+82 cm−1), two electrons must be removed from 1
to give a calculated shift of +89 cm−1 in 12+. Significant fractions
of the positive charges are thus seen to reside on the dipenylethylenyl
segment as electrons are removed from the molecule.
characteristic of a highly metal centered Mn(II) system. The
frozen-solution spectrum of 1+, shown in Figure 4, is nearly
isotropic, with ⟨g⟩ ≈ 2.01 and A(55Mn) (I = 5/2) hyperfine
splitting (hfs) of approximately 21 G. These features are in stark
contrast to those reported for [MnCp(CO)3]+, which has a
highly anisotropic g value and an average Mn hfs of 70 G.24
Calculations: Structures and Redox-Active Orbitals.
The DFT-optimized structures of 1, 1+, and 12+ (Figure 5) reveal
how electron removal affects the molecular structures of this
series of redox-active molecules. In the neutral species 1, the
double bond between C2 and C3 is intact at a bond length of
1.358 Å, which is slightly longer than the value (∼1.33 Å)
typically seen in simple olefins.46 The dihedral angle (C1−C2−
C3−C4) of 167° is a bit distorted from that of an ideal planar
geometry. Both deviations from an idealized olefin structure are
due to the steric demands of the substituents in this tetrasubstituted
olefin. The fact that the C2−C3 bond lengthened by ∼0.06 Å to
1.422 Å in 1+ is a consequence of removal of olefin π character, an
effect that is enhanced with removal of the second electron. The
resulting C2−C3 bond length of 1.489 Å in 12+ begins to
approach that of a standard C−C single bond. Decreases in the
dihedral angle from the 167° of 1 to 146° in 1+ and then to 127°
in 12+ are also consistent with a decreasing degree of double-
bond character for C2−C3 as electrons are removed.
The rich contribution of the diphenylethynyl moiety to the
SOMO is also indicated by the EPR spectrum of 1+, which is not
D
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Figure 5. Optimized structures of 1, 1+, and 12+. Bond lengths are given in Å.
Figure 6. Representative structures of 1, 1+, and 12+.
Figure 7. Redox-active orbitals of 1, 1+, and 12+. The isodensity value for the surface plots was set to 0.05 au.
The calculations also show that there is a significant change in
the nature of the metal−ligand bonding as electrons are removed
from 1. There is a notable shortening of the C2−C5 distance
from 1.478 to 1.438 and 1.403 Å in 1, 1+, and 12+, respectively,
consistent with the progress toward a fulvene-type tautomer as
the oxidation proceeds. The newly formed quasi-olefinic C2−C5
fragment can then engage the Mn center. The Mn−C1 distance
in 1 is 3.231 Å, indicating that there is no electronic interaction
between these atoms in the neutral complex. As oxidation
introduces fulvene character to the ligand and a partial double
bond is formed between C2 and C5, the Mn−C2 distance
shortens to 2.981 Å and finally to 2.758 Å in 1+ and 12+,
respectively. The structural features of the dication are highly
reminiscent of the [Mn(CO)3(η4:η2-diphenylfulvene)]+ struc-
ture observed previously by Gleiter.47,48 A schematic illustration
of the interplay between the electronic and molecular structures
of the electron-transfer series is shown in Figure 6.
1-ethyl-2-diphenylethene (olefin) moiety. By inspecting the atomic
orbital contribution of these fragments to the redox-active orbital,
we can obtain the percent contribution of each fragment. The
HOMO of 1, found at an energy of −5.668 eV, has about 2:1
ratio of π-ligand and organometallic character (33% Mn(C5H4)-
(CO)3). Specifically, the HOMO of 1 consists of 13% Mn, 13%
Cp, 6% CO, and 67% olefin. The SOMO of 1+ shows a nearly
identical distribution of 15, 12, 5, and 68%, respectively. This
composition of the redox-active orbitals stands in stark contrast
to the SOMO of the [MnCp(CO)3]+ complex, which is formed
by mixing 73% Mn, 9% Cp, and 18% CO based fragment orbitals.
The redox-active orbital in 1 is derived from the interaction of the
alkenyl π-MO with the nonbonding d(xy) orbital of the low-spin
Mn(I) d6 center that is aligned parallel to the cyclopentadienyl
plane. Removal of an electron from the HOMO of 1 leads to a
substantial lowering of all orbital energies, with the singly occupied
1+-SOMO being found at an electronic energy of −9.031 eV.
Because the contributions of the organometallic fragment in
1-HOMO and 1+-SOMO are essentially the same, as illustrated
in Figure 7, the orbital shapes remain nearly identical, despite the
somewhat severe changes in geometrical structure described
above. Removal of the unpaired electron associated with 1+-SOMO
More detailed information about the frontier orbitals of the
three members of the electron-transfer series are obtained from
examination of Figure 7. To understand the origin of the electron
removed during oxidation, we divided 1 into four different
fragments: Mn, cyclopentadienyl ring (Cp), carbonyls, and the
E
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
completes the oxidation and the resulting 94%-ligand based
LUMO of 12+is found at an energy of −10.898 eV.
P(OPh)3 for CO shifts in the cymantrene series Mn(η5-
C5H4R(CO)3) (R = H, Me, I, CHO),25 most likely reflecting
the increased charge delocalization in 1+. A triphenyl phosphite
substitution rate of 4.7 × 102 M−1 s−1 was determined for 1+ by
digital simulation, about an order of magnitude slower than that
reported for the cymantrene radical cation.25
Bulk anodic electrolysis (Eappl = 0.9 V) of 1 in the presence of
P(OPh)3 released 1 F of charge and gave a single product
showing a reversible wave at E1/2 = 0.46 V for the reduction of the
CO-substituted cation [Mn(CO)2P(OPh)3(η5-C5H4(Et)C
C(C6H5)2)]+. Changing Eappl to 0.2 V required 1 F of charge
and resulted in efficient conversion to neutral Mn(CO)2P-
(OPh)3(η5-C5H4(Et)CC(C6H5)2). This sequence can be
followed in Figure 9. The dotted line was recorded before
The spectroscopic and computational data are in agreement
that the stepwise oxidation of 1 involves orbitals which are
delocalized between the cymantrenyl and diphenylethylenyl parts
of the molecule and that the redox orbital becomes more π-ligand
based as the oxidation proceeds from the neutral complex to the
corresponding dication.
Redox-Switched Metal−Carbonyl Substitution. The fact
that the radical cation 1+ contains significant cymantrenyl radical
character suggests that the manganese carbonyl center should be
susceptible to associative-driven CO substitution reactions.11,25,49−53
It is possible to take advantage of redox-dependent substitution
rates in order to prepare CO-substituted complexes having
metals in either the 17-electron or 18-electron electronic con-
figurations under mild conditions. The “electrochemical switch”
method outlined in eqs 2−4 shows how this might be carried out
for a hypothetical metal complex MLn(CO). In this sequence, the
neutral metal carbonyl complex is anodically oxidized to its
radical cation (eq 2), which reacts with L′, producing the radical
cation of the substitution product (eq 3). Owing to the fact
that L′ is invariably a stronger donor than CO, the potential
E
1/2(ML′) is less positive than E1/2(MCO), assuring that the
substitution product [MLnL′]+ remains in the positively charged
state as the electrolysis proceeds. [MLnL′]+ may then be cath-
odically reduced to give the desired neutral substitution product
(eq 4), which is then extracted from the electrolyte solution.
A literature example of this process is the preparation of Cr(η6-
C6H6)(CO)2PPh3 from Cr(η6-C6H6)(CO)3.54
Figure 9. CVs of 1.0 mM 1 in the presence of 1.5 mM P(OPh)3 in
CH2Cl2/0.05 M [NBu4][B(C6F5)4] before (dotted line) and after
(dashed line) bulk oxidation at Eappl = 0.9 V and after bulk reduction
(solid line) at Eappl = 0.2 V at 253 K and 0.2 V s−1, with a 1 mm GCE.
MLn(CO) − e− ⇌ [MLn(CO)]+
[MLn(CO)]+ + L′ → [MLnL′]+ + CO
[MLnL′]+ + e− ⇌ MLnL′
1/2(ML′)
E1/2(MCO)
(2)
(3)
(4)
beginning the anodic electrolysis and is analogous to the solid
line in Figure 8. After completion of the electrolysis, the only
electroactive compound in solution is the radical cation of the
substitution product, responsible for the reversible cathodic wave
at 0.46 V (dashed line). The solid line was recorded after
completion of the cathodic “back-electrolysis” and arises from
the reversible oxidation of the neutral substitution product
Mn(CO)2P(OPh)3(η5-C5H4(Et)CC(C6H5)2). Comparison
of the currents before and after the double electrolysis suggested
that the in situ yield of Mn(CO)2P(OPh)3(η5-C5H4(Et)C
C(C6H5)2) was greater than 80%.55 As described in Experimental
Details, workup of the electrochemically switched solution allowed
isolation of the product, identified by its IR spectrum (Figure S1
in the Supporting Information) in 40% (unoptimized) yield.
E
Application of the electrochemical switch method to the
preparation of the triphenyl phosphite complex Mn(CO)2P-
(OPh)3(η5-C5H4(Et)CC(C6H5)2) followed the expected
protocol. Figure 8 shows a CV scan of 1 when a slight excess
Figure 8. CVs of 1.0 mM 1 in the absence (dashed line) and the
presence (solid line) of 1.5 mM P(OPh)3 in CH2Cl2/0.05 M
[NBu4][B(C6F5)4] at 298 K and 0.2 V s−1, with a 1 mm GCE.
of P(OPh)3 is added to the solution. A reduction in the chemical
reversibility of the 10/+ couple is noted by comparing the scans
taken before (dashed line) and after (solid line) addition of tri-
phenyl phosphite. The follow-up product is seen to have a revers-
ible reduction at E1/2 = 0.46 V, consistent with the replacement of
a single CO by P(OPh)3 in 1. The E1/2 shift of 0.32 V from that of
10/+ is somewhat less than the average of 0.40 V observed for the
Figure 10. CVs of 0.90 mM 1 in the absence (dotted line) and the
presence of 0.54 mM (long dashed line) and 2.70 mM (solid line)
P(OiPr)3 in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at 253 K and 0.2 V s−1,
with a 3 mm GCE.
F
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 2. Potentials vs Ferrocene in CH2Cl2/0.05 M [NBu4][B(C6F5)4] for the First Oxidation of Mn(CO)3−nLn(η5-C5H4(Et)C
C(C6H5)2)
ligand (L)
CO
n
E1/2 (V)
total shift from E1/2 of 1 (V)
EL(cym)
EL(lit)
comment
a
0
1
1
1
1
1
2
2
2
0.78
0.46
n.a.
0.99
0.67
0.46
0.45
0.45
0
0.99
b
P(OPh)3
P(OiPr)3
−0.32
−0.53
−0.54
−0.54
−0.99
−1.09
−1.30
−1.31
0.62
0.25
literature n.a.
literature n.a.
a
P(OMe)3
P(nBu)(OEt)2
pyridine
0.24
0.42
0.24
a
−0.21
−0.31
−0.52
−0.53
0.25
P(OMe)3
P(nBu)(OEt)2
P(OiPr)3
additional shift of −0.55 V from n = 1
additional shift of −0.76 V from n = 1
additional shift of −0.78 V from n = 1
a
b
Reference 57. Reference 25.
In the case of three phosphites having cone angles smaller than
that of P(OPh)3, examples of multiple CO substitutions were
observed. An example is shown in Figure 10, which gives the CVs
of 1 in the presence of two different concentrations of P(OiPr)3.
At a substoichiometric concentration of the phosphite (long dashed
line), product waves are seen at E1/2 values of 0.25 and −0.49 V,
which we attribute to the monosubstituted and disubstituted,
[Mn(CO)3−n[P(OiPr)3]n(η5-C5H4(Et)CC(C6H5)2)]+, n = 1
and 2, respectively. With excess P(OiPr)3 (solid line), only the
more negative wave for the disubstituted product is observed.
Assignments of the new waves to n = 1 or n = 2 substitution
products were made on the basis of the expected shifts of the E1/2
values from that (0.78 V) of the first oxidation of the tricarbonyl
parent compound 1. Predictions of the shifts are possible for two
phosphites, for which the so-called “ligand electronic parameter”
has been reported.57 This parameter was originally given as EL on
the basis of the oxidation of Cr(CO)5L complexes.57 In concert
with the slightly different shifts in E1/2 values that have been
observed for Mn(π-C5H4R)(CO)2L complexes,11,25,58 we use
the symbol EL(cym) for the ligand electronic parameter of the
cymantrene family of complexes, including 1. As shown in Table 2,
the EL(cym) values determined for P(OPh)3 and P(OMe)3 match
well with those reported in previous literature, confirming the
assignments of the mono-CO-substituted products. The sim-
ilarity of the EL(cym) values for P(OiPr)3 and P(nBu)(OEt)2 to
that of P(OMe)3 suggests that these three ligands have essentially
the same donor strengths.
The CV curves assigned to the second CO substitution
products for L = P(OMe)3, P(OiPr)3, and P(nBu)(OEt)2 are
again shifted to potentials more negative than those observed for
the singly substituted products. The sequential shifts are essen-
tially identical for the L = P(OMe)3 system, for which the oxidation
goes smoothly from 0.78 to 0.24 and −0.31 V as n increases from
0 to 2 in Mn(CO)3−n(P(OMe)3)n(η5-C5H4(Et)CC(C6H5)2).
Somewhat larger shifts (ca. −0.77 V) for the apparent n = 2
products are seen for the other small phosphites, in comparison
to values of only −0.45 V for the first substitution products. We
cannot rule out the possibility that the more highly substituted
products for L = P(OMe)3 or P(OiPr)3 are trisubstituted, but we
think it more likely that the somewhat irregular shifts arise from
electronic effects.
Figure 11. CVs (0.2 V s−1, 1 mm GCE) of 0.9 mM 1 in the presence of
2.7 mM pyridine before (dashed line) and after (solid line) bulk
oxidation (Eappl = 0.9 V) in CH2Cl2/0.05 M [NBu4][B(C6F5)4] at 253 K.
the Mn(CO)2(pyridine) group (1933 and 1865 cm−1 reported
for MnCp(CO)2(py))59 but also showed bands for considerable
unsubstituted material (Figure S2 in the Supporting Informa-
tion). The EL(cym) value of zero determined for L = pyridine
(vide infra) is somewhat larger than its reported EL value
(0.25).57
The above data show that, by substitution of one or more
carbonyl groups by donor ligands, the oxidation potential of
Mn(CO)3−nLn(η5-C5H4(Et)CC(C6H5)2) can be altered by
1 V or more, allowing the possibility of generating a family of
compounds that is either easier or more difficult to oxidize than
their ferrocenyl analogues.
CONCLUSIONS
■
On the basis of the known differences in oxidative redox
potentials among cymantrene, ferrocene, and the diphenyleth-
ylene moieties, the observation that the first oxidation of 1 occurs
at a potential much higher (0.78 V) than that of the ferrocenyl
analogue 2 (∼0 V)21,56 was expected. Interestingly, however, this
process involves loss of an electron from a HOMO orbital of 1
that is highly delocalized over the entire organometallic/π-organic
framework, leading to a well-distributed sharing of positive
charge in the monocation 1+. There is sufficient positive charge at
the Mn(CO)3 group of 1+ to facilitate the rapid substitution of
one or more CO ligands by donor ligands, including several
phosphites and pyridine. The resulting CO-substituted com-
pounds have much milder oxidation potentials, in some cases
being easier to oxidize than 2. The fact that the SOMO of the
radical cation 1+ has mixed organometallic/π-organic character
and the finding that the fraction of positive charge on the organic
segment increases as electrons are removed will be helpful in
Redox-switched pyridine for CO substitution on the CV time
scale was also observed for 1, as indicated in Figure 11, although
the reaction was not completed in that case. The E1/2 value of the
pyridine substitution product (−0.21 V) is shifted positively by
0.99 V in comparison to the 0/+ potential of the tricarbonyl
complex. IR spectra of a solution taken after a “redox switched”
electrolysis had bands at ∼1930 and 1845 cm−1 attributable to
G
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(16) Aalstad, B.; Parker, V. D. J. Electroanal. Chem. Interfacial
Electrochem. 1982, 136, 251−258.
understanding both the follow-up reactions and the thermody-
namic potential shifts that accompany the oxidations of ana-
logues having more reactive groups in the phenyl rings.14 Elec-
trochemical and computational studies of the p-hydroxy and
p-methoxy derivatives of 1 will be detailed in a separate paper.
(17) Rathore, R.; Lindeman, S. V.; Kumar, A. S.; Kochi, J. K. J. Am.
Chem. Soc. 1998, 120, 6931−6939.
(18) Schreivogel, A.; Maurer, J.; Winter, R.; Baro, A.; Laschat, S. Eur. J.
Org. Chem. 2006, 2006, 3395−3404. The potentials in this paper are
reported vs ferrocene/ferrocenium. Values reported vs other reference
electrodes (see refs 15−18) would give similar potentials on conversion
to the ferrocene reference.
ASSOCIATED CONTENT
* Supporting Information
■
S
(19) Talipov, M. R.; Boddeda, A.; Hossain, M. M.; Rathore, R. J. Phys.
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.8b00186.
Org. Chem. 2016, 29, 227−233.
(20) Mori, T.; Inoue, Y. J. Phys. Chem. A 2005, 109, 2728−2740.
IR spectra and xyz coordinates (PDF)
Cartesian coordinates for calculated structures (XYZ)
̀
(21) (a) Hillard, E.; Vessieres, A.; Thouin, L.; Jaouen, G.; Amatore, C.
Angew. Chem., Int. Ed. 2006, 45, 285−290. (b) Messina, P.; Labbe,
́
E.;
Buriez, O.; Hillard, E. A.; Vessieres, A.; Hamels, D.; Top, S.; Jaouen, G.;
́
Frapart, Y. M.; Mansuy, D.; Amatore, C. Chem. - Eur. J. 2012, 18, 6581−
6587.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for M.-H.B.: mbaik2805@kaist.ac.kr.
*E-mail for W.E.G.: William.Geiger@uvm.edu.
■
(22) Tan, Y. L. K.; Pigeon, P.; Hillard, E. A.; Top, S.; Plamont, M.-A.;
Vessier
Trans. 2009, 10871−10881.
(23) Hillard, E.; Vessieres, A.; Le Bideau, F.; Plaz, D.; Spera, D.; Huche,
̀
es, A.; McGlinchey, M. J.; Muller-Bunz, H.; Jaouen, G. Dalton
̈
̀
́
ORCID
M.; Jaouen, G. ChemMedChem 2006, 1, 551−559.
William E. Geiger: 0000-0002-9152-8550
(24) Laws, D. R.; Chong, D.; Nash, K.; Rheingold, A. L.; Geiger, W. E.
J. Am. Chem. Soc. 2008, 130, 9859−9870.
(25) Wu, K.; Laws, D. R.; Nafady, A.; Geiger, W. E. J. Inorg. Organomet.
Polym. Mater. 2014, 24, 137−144.
Present Address
∇Samsung Electronics, Suwon 16677, South Korea.
Notes
(26) Shaw, M. J.; Geiger, W. Organometallics 1996, 15, 13−15.
(27) LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76,
6395−6401.
(28) Fischer, E. O.; Fellman, W. J. Organomet. Chem. 1963, 1, 191−
199.
(29) (a) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461−466.
(b) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877−910.
(30) Gagne, R. E.; Koval, C. A.; Lisensky, G. C. Inorg. Chem. 1980, 19,
2854−2855.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
M.-H.B. acknowledges support from the Institute for Basic
Science (IBS-R10-D1) in Korea. W.E.G. acknowledges support
from the National Science Foundation (CHE-1212339 and
CHE-1565441).
(31) (a) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.;
Wiley: New York, 2001; Chapter 6. (b) Geiger, W. E. In Laboratory
Techniques in Electroanalytical Chemistry, 2nd ed.; Kissinger, P. T.,
Heineman, W. R., Eds.; Marcel Dekker: New York, 1996; Chapter 23.
(32) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372. (b) Grimme, S.;
Antony, J.; Ehrlich, S.; Krieg, H. J. Chem. Phys. 2010, 132, 154104.
(33) Shao, Y.; Gan, Z.; Epifanovsky, E.; Gilbert, A. T. B.; Wormit, M.;
Kussmann, J.; Lange, A. W.; Behn, A.; Deng, J.; Feng, X.; Ghosh, D.;
Goldey, M.; Horn, P. R.; Jacobson, L. D.; Kaliman, I.; Khaliullin, R. Z.;
REFERENCES
■
(1) (a) Jordan, V. C. J. Med. Chem. 2003, 46, 883−908. (b) Jordan, V.
C. J. Med. Chem. 2003, 46, 1081−1111. (c) Jordan, V. C. J. Natl. Cancer
Inst. 2014, 106 (11), dju296.
(2) Wu, X.; Hawse, J. R.; Subramaniam, M.; Goetz, M. P.; Ingle, J. N.;
Spelsberg, T. C. Cancer Res. 2009, 69, 1722−1727.
(3) Relevance of Breast Cancer Hormone Receptors and other Factors
to the Efficacy of Adjuvant Tomoxifen: Patient-level Meta-analysis of
Randomised Trials Lancet 2011, 378, 771−784.
́
Kus, T.; Landau, A.; Liu, J.; Proynov, E. I.; Rhee, Y. M.; Richard, R. M.;
Rohrdanz, M. A.; Steele, R. P.; Sundstrom, E. J.; Woodcock, H. L.;
Zimmerman, P. M.; Zuev, D.; Albrecht, B.; Alguire, E.; Austin, B.; Beran,
G. J. O.; Bernard, Y. A.; Berquist, E.; Brandhorst, K.; Bravaya, K. B.;
Brown, S. T.; Casanova, D.; Chang, C.-M.; Chen, Y.; Chien, S. H.;
Closser, K. D.; Crittenden, D. L.; Diedenhofen, M.; DiStasio, R. A.; Do,
H.; Dutoi, A. D.; Edgar, R. G.; Fatehi, S.; Fusti-Molnar, L.; Ghysels, A.;
Golubeva-Zadorozhnaya, A.; Gomes, J.; Hanson-Heine, M. W. D.;
Harbach, P. H. P.; Hauser, A. W.; Hohenstein, E. G.; Holden, Z. C.;
Jagau, T.-C.; Ji, H.; Kaduk, B.; Khistyaev, K.; Kim, J.; Kim, J.; King, R. A.;
Klunzinger, P.; Kosenkov, D.; Kowalczyk, T.; Krauter, C. M.; Lao, K. U.;
Laurent, A. D.; Lawler, K. V.; Levchenko, S. V.; Lin, C. Y.; Liu, F.;
Livshits, E.; Lochan, R. C.; Luenser, A.; Manohar, P.; Manzer, S. F.; Mao,
S.-P.; Mardirossian, N.; Marenich, A. V.; Maurer, S. A.; Mayhall, N. J.;
Neuscamman, E.; Oana, C. M.; Olivares-Amaya, R.; O’Neill, D. P.;
Parkhill, J. A.; Perrine, T. M.; Peverati, R.; Prociuk, A.; Rehn, D. R.;
Rosta, E.; Russ, N. J.; Sharada, S. M.; Sharma, S.; Small, D. W.; Sodt, A.;
(4) Vessier
Trans. 2006, 529−541.
(5) Jaouen, G.; Vessier
8817 and references therein.
(6) Lee, H. Z. S.; Buriez, O.; Chau, F.; Labbe,
C.; Jaouen, G.; Vessieres, A.; Leong, W. K.; Top, S. Eur. J. Inorg. Chem.
2015, 2015, 4217−4226.
(7) Hillard, E. A.; Vessier
Huche;
̀
es, A.; Top, S.; Beck, W.; Hillard, E. A.; Jaouen, G. Dalton
̀
es, A.; Top, S. Chem. Soc. Rev. 2015, 44, 8802−
E.; Ganguly, R.; Amatore,
́
̀
̀
es, A.; Top, S.; Pigeon, P.; Kowalski, K.;
́
Jaouen, G. J. Organomet. Chem. 2007, 692, 1315−1326.
(8) Dallagi, T.; Saidi, M.; Jaouen, G.; Top, S. Appl. Organomet. Chem.
2013, 27, 28−35.
(9) Dallagi, T.; Saidi, M.; Vessier
J. Organomet. Chem. 2013, 734, 69−77.
(10) Connelly, N. G.; Geiger, W. E. Adv. Organomet. Chem. 1984, 23,
1−93.
̀ ́
es, A.; Huche, M.; Jaouen, G.; Top, S.
(11) Kochi, J. K. J. Organomet. Chem. 1986, 300, 139−166.
(12) Atwood, C. G.; Bitterwolf, T. E.; Geiger, W. E. J. Electroanal.
Chem. 1995, 397, 279−285.
Stein, T.; Stuck, D.; Su, Y.-C.; Thom, A. J. W.; Tsuchimochi, T.;
̈
Vanovschi, V.; Vogt, L.; Vydrov, O.; Wang, T.; Watson, M. A.; Wenzel,
J.; White, A.; Williams, C. F.; Yang, J.; Yeganeh, S.; Yost, S. R.; You, Z.-
Q.; Zhang, I. Y.; Zhang, X.; Zhao, Y.; Brooks, B. R.; Chan, G. K. L.;
Chipman, D. M.; Cramer, C. J.; Goddard, W. A.; Gordon, M. S.; Hehre,
W. J.; Klamt, A.; Schaefer, H. F.; Schmidt, M. W.; Sherrill, C. D.; Truhlar,
D. G.; Warshel, A.; Xu, X.; Aspuru-Guzik, A.; Baer, R.; Bell, A. T.; Besley,
N. A.; Chai, J.-D.; Dreuw, A.; Dunietz, B. D.; Furlani, T. R.; Gwaltney, S.
(13) Pike, R. D.; Rieger, A. L.; Rieger, P. H. J. Chem. Soc., Faraday
Trans. 1 1989, 85, 3913−3925.
(14) Wu, K.; Top, S.; Hillard, E. A.; Jaouen, G.; Geiger, W. E. Chem.
Commun. 2011, 47, 10109−10111.
(15) Phelps, J.; Bard, A. J. J. Electroanal. Chem. Interfacial Electrochem.
1976, 68, 313−335.
H
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
R.; Hsu, C.-P.; Jung, Y.; Kong, J.; Lambrecht, D. S.; Liang, W.;
Ochsenfeld, C.; Rassolov, V. A.; Slipchenko, L. V.; Subotnik, J. E.; Van
Voorhis, T.; Herbert, J. M.; Krylov, A. I.; Gill, P. M. W.; Head-Gordon,
M. Mol. Phys. 2015, 113, 184−215.
(34) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257.
(35) Hay, P. J.; Wadt, W. J. Chem. Phys. 1985, 82, 270−283.
(36) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007−1023.
(37) Baik, M.-H.; Friesner, R. A. J. Phys. Chem. A 2002, 106, 7407−
7412.
(38) Wohlfarth, C. Static Dielectric Constants of Pure Liquids and
Binary Liquid Mixtures. In Supplement to IV/6; Lechner, M. D., Ed.;
Springer Berlin Heidelberg: Berlin, Heidelberg, 2008; pp 67−68.
(39) CCCBDB listing of precalculated vibrational scaling factors; https://
cccbdb.nist.gov/vibscalejust.asp (Accessed 2 Mar. 2018).
(40) Chemissian v4.52; www.chemissian.com (accessed Nov 02, 2017).
(41) Goldman, A. S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1996, 118,
12159−12166.
(42) Ehlers, A. W.; Ruiz-Morales, Y.; Baerends, E. J.; Ziegler, T. Inorg.
Chem. 1997, 36, 5031−5036.
(43) Willner, H.; Aubke, F. Angew. Chem., Int. Ed. Engl. 1997, 36,
2402−2425.
(44) Niu, S.; Thomson, L. M.; Hall, M. B. J. Am. Chem. Soc. 1999, 121,
4000−4007.
(45) Xu, Q. Coord. Chem. Rev. 2002, 231, 83−108.
(46) Borden, W. T. Chem. Rev. 1989, 89 (5), 1095−1109.
(47) (a) Gleiter, R.; Bleiholder, C.; Rominger, F. Organometallics 2007,
26, 4850−4859. (b) Bleiholder, C.; Rominger, F.; Gleiter, R.
Organometallics 2009, 28, 1014−1017.
(48) Volland, M. A. O.; Kudis, S.; Helmchen, G.; Hyla-Kryspin, I.;
Rominger, F.; Gleiter, R. Organometallics 2001, 20, 227−230.
(49) Chong, D.; Laws, D. R.; Nafady, A.; Costa, P. J.; Rheingold, A. L.;
Calhorda, M. J.; Geiger, W. E. J. Am. Chem. Soc. 2008, 130, 2692−2073.
(50) Orpen, A. G.; Connelly, N. G. Organometallics 1990, 9, 1206−
1210.
(51) Therien, M. J.; Ni, C.-L.; Anson, F. C.; Osteryoung, J. G.; Trogler,
W. C. J. Am. Chem. Soc. 1986, 108, 4037−4042.
(52) Basolo, F. New J. Chem. 1994, 18, 19−24.
(53) Geiger, W. E. Organometallics 2007, 26, 5738−5765.
(54) Camire Ohrenberg, N.; Paradee, L. M.; Dewitte, R. J., III; Chong,
D.; Geiger, W. E. Organometallics 2010, 29, 3179−3186.
(55) Currents in the cyclic and linear scan voltammograms are
proportional to the square root of the diffusion coefficient of the analyte.
The diffusion coefficient of the triphenyl phosphite derivative is
expected to be considerably lower than that of the tricarbonyl complex,
owing to its higher molecular weight. For a discussion of how variations
in molecular weight affect the diffusion coefficients in a family of
molecules, see: Flanagan, J. B.; Margel, S.; Bard, A. J.; Anson, F. A. J. Am.
Chem. Soc. 1978, 100, 4248−4253.
(56) In ref 21, the E1/2 potential of 4 is given as 0.02 V vs SCE in
methanol/[NBu4][BF4]. Conversion to the ferrocene potential is an
estimate.
(57) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271−1285.
(58) Atwood, C. G.; Geiger, W. E.; Bitterwolf, T. E. J. Electroanal.
Chem. 1995, 397, 279−285.
(59) Giordano, P. J.; Wrighton, M. S. Inorg. Chem. 1977, 16, 160−166.
I
DOI: 10.1021/acs.organomet.8b00186
Organometallics XXXX, XXX, XXX−XXX