Electron-transfer processes in 3,4-diferrocenylpyrroles: Insight into a missing piece of the polyferrocenyl-containing pyrroles family

Article abstract of DOI:10.1021/om400901w

3,4-Diiodo-1-(triisopropylsilyl)-1H-pyrrole (1), 3,4-diferrocenyl-1- (triisopropylsilyl)-1H-pyrrole (2), and 3,4-diferrocenyl-1H-pyrrole (3) were prepared and characterized using spectroscopic methods and X-ray crystallography. UV-vis spectra of 2 and 3 were correlated with their density functional theory (DFT)-calculated electronic structures as well as theoretically predicted by the time-dependent (TD) DFT-calculations vertical excitation energies. Redox properties of 2 and 3 were investigated using cyclic voltammetry, differential pulse voltammetry, and spectroelectrochemical approaches. Ferrocene-centered oxidation processes in 2 and 3 were found to be separated by ～180 and ～300 mV in DCM/TBAP and DCM/(NBu ₄)[B(C₆F₅)₄] systems, respectively. Stepwise spectroelectrochemical oxidation of 2 and 3 allowed us to obtain spectroscopic signatures of the mixed-valence [2]⁺ and [3] ⁺ cations. Hush analysis of the intervalence charge-transfer band in [2]⁺ and [3]⁺ is suggestive of class II (in Robin and Day classification) mixed-valence behavior. Electronic structures of neutral and spin-localized/delocalized single-electron oxidized mixed-valence cations of 2,5-di-, 3,4-di-, and 2,3,4,5-tetraferrocenylpyrroles were investigated by DFT calculations to resolve current uncertainties regarding the first oxidation process of tetraferrocenylpyrrole.

Full text of DOI:10.1021/om400901w

Article
pubs.acs.org/Organometallics
Electron-Transfer Processes in 3,4-Diferrocenylpyrroles: Insight into a
Missing Piece of the Polyferrocenyl-Containing Pyrroles Family
Wil R. Goetsch, Pavlo V. Solntsev, Casey Van Stappen, Anatolii A. Purchel, Semen V. Dudkin,
and Victor N. Nemykin*
Department of Chemistry and Biochemistry, University of Minnesota Duluth, 1039 University Drive, Duluth, Minnesota 55812,
United States
S
* Supporting Information
ABSTRACT: 3,4-Diiodo-1-(triisopropylsilyl)-1H-pyrrole (1), 3,4-diferrocenyl-1-
(triisopropylsilyl)-1H-pyrrole (2), and 3,4-diferrocenyl-1H-pyrrole (3) were prepared
and characterized using spectroscopic methods and X-ray crystallography. UV−vis
spectra of 2 and 3 were correlated with their density functional theory (DFT)-
calculated electronic structures as well as theoretically predicted by the time-
dependent (TD) DFT-calculations vertical excitation energies. Redox properties of 2
and 3 were investigated using cyclic voltammetry, differential pulse voltammetry, and
spectroelectrochemical approaches. Ferrocene-centered oxidation processes in 2 and
3 were found to be separated by ∼180 and ∼300 mV in DCM/TBAP and DCM/
(NBu4)[B(C6F5)4] systems, respectively. Stepwise spectroelectrochemical oxidation of 2 and 3 allowed us to obtain
spectroscopic signatures of the mixed-valence [2]+ and [3]+ cations. Hush analysis of the intervalence charge-transfer band in
[2]+ and [3]+ is suggestive of class II (in Robin and Day classification) mixed-valence behavior. Electronic structures of neutral
and spin-localized/delocalized single-electron oxidized mixed-valence cations of 2,5-di-, 3,4-di-, and 2,3,4,5-tetraferrocenylpyr-
roles were investigated by DFT calculations to resolve current uncertainties regarding the first oxidation process of
tetraferrocenylpyrrole.
range metal−metal coupling; in the majority of these cases π-
conjugated ligands are used. Among the poly(ferrocene)-
containing systems, the most studied bridging ligands are
(poly)ethynes,11 (poly)ethenes,9,10 porphyrines,14 aromatic
heterocycles,15,16 and carbocycles.17 While the (poly)ethyne
and (poly)ethene bridges have been known for decades, the
porphyrins and the heterocyclic platforms have attracted
attention only in recent years. For instance, in a series of
excellent publications, Lang and co-workers reported the
synthesis, electrochemical and spectroelectrochemical studies
of a series of 2,5-diferrocenyl- and supercrowded 2,3,4,5-
tetraferrocenyl-containing pyrroles, furanes, and thiophenes.15
They demonstrated that the (poly)ferrocenyl-containing
heterocycles can be sequentially oxidized, forming a set of
mixed-valence spin-localized cations. On the basis of similarities
in electrochemical data between 2,5-diferrocene and 2,3,4,5-
tetraferrocene derivatives, these authors suggested that the first
oxidation process observed in 2,3,4,5-tetraferrocene-substituted
pyrroles, furanes, and thiophenes is localized at the ferrocene
group connected to the pyrrolic α-position.15 Such assignment,
however, could potentially suffer from the lack of redox
property data on 3,4-diferrocenyl pyrroles, furanes, and
thiophenes, which, to the best of our knowledge, have never
been reported. In order to improve the current understanding
of redox properties and electronic communications in
INTRODUCTION
■
Preparation of nanometer-scale molecular systems with
controllable redox or electronic conductivity properties is of
great interest for modern technology.1 Because of their well-
defined redox properties and robust structure, mono- and
poly(ferrocene)-containing compounds were proposed as
candidates for potential application in molecular photonics,
(opto)electronics, redox-driven fluorescence markers, and
sensors for toxic ions.2 Among these organometallic platforms,
compounds with strong metal−metal coupling are particularly
interesting from fundamental and practical points of view.3
Such systems have been intensely studied in recent decades
because of their fundamentally interesting multiredox and
electronic communication properties.4 From a practical point of
view, such molecular modules could have potential applications
in molecular electronics, quantum cellular automata, and
optoelectronic materials for use in high-speed photonic or
redox-switchable devices.5 Often, formation of the mixed-
valence states in metallocenyl-containing molecules is respon-
sible for the aforementioned properties. Mixed-valence
polyferrocene derivatives have been known for several decades,
and the factors affecting their redox and electronic
communication properties have been thoroughly investi-
gated.3−13 In the majority of cases, the metal−metal coupling
is facilitated by a bridging ligand between the ferrocene groups
as well as by the intermetal distance in these compounds. In
cases involving relatively large metal−metal distances (10−15
Å), the nature of the bridging ligand become crucial for long-
Received: September 9, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 1. Synthesis of Target Ferrocene-Containing Pyrroles
polyferrocene pyrroles, furanes, and thiophenes, we have
prepared and characterized the redox properties of the missing
members of the group −3,4-diferrocenyl-1-(triisopropylsilyl)-
1H-pyrrole (2) and 3,4-diferrocenyl-1H-pyrrole (3). We also
conducted extensive density functional theory (DFT) and time-
dependent (TD) DFT calculations on neutral and mixed-
valence cations of 2,5-diferrocenyl-, 3,4-diferrocenyl-, and
2,3,4,5-tetraferrocenylpyrroles in order to clarify their redox
behavior and spin localization properties.
the reaction between ferrocenyl-zinc chloride and 1, following a
procedure reported by Lang and co-workers for the preparation
of 2,3,4,5-tetraferrocenylpyrrole.15 Using this approach, 2 was
successfully prepared in 7% to 52% yields in several trials. An
optimal yield of 2 could be achieved by heating the reactants for
72 h at 60 °C (Scheme 1) and allows production of target
compound 2 in gram quantity. In addition to target compound
2, small amounts of ferrocene, bis(ferrocene), and other
byproducts were detected in the reaction mixture. The new
diferrocenyl compound 2 is air stable in the solid state and
soluble in a variety of organic solvents. Deprotection of 2 can
be easily achieved using tetrabutylammonium fluoride23 in
organic solvent to give pure 3,4-diferrocenyl-1H-pyrrole (3)
after standard purification (Scheme 1). Unlike protected
pyrrole derivative 2, compound 3 is not stable in solution
and degrades within several days in the majority of organic
solvents. This compound, however, could be stored in a solid
state for several weeks if refrigerated.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of 3,4-Diferrocenyl-1-
(triisopropylsilyl)-1H-pyrrole (2) and 3,4-Diferrocenyl-
1H-pyrrole (3). It is well known that pyrrole undergoes
predominant electrophilic substitution at the α-position, while
preparation of β-substituted pyrroles is more challenging.18 The
most common synthetic pathways for preparation of β-
substituted pyrroles include, but are not limited to, (i)
utilization of a removable deactivating group at α-substituted
pyrroles to direct the entry of an electrophile to the β-
position,19 (ii) acid-mediated isomerization of the easily
available α-isomers,20 and (iii) direct substitution of an N-
(phenylsulfonyl)pyrrole with certain electrophiles, described,
for instance, by Anderson and co-workers21 and Rokach and
co-workers.22 Another approach to the synthesis of β-
substituted pyrroles is based on the use of a bulky protecting
group at the nitrogen atom, such as triisopropylsilyl, to obstruct
electrophilic attack at the α-position.23 Considerable selectivity
for the β-pyrrolic position is indeed observed, and the process
of removing the protecting group is easily attainable with the
use of tetraalkylammonium fluoride.24
The 1H and 13C NMR spectra of 2 and 3 consist of
characteristic signals and correlate well with their structures
1
(Figures S1−S4). In particular, H NMR spectra of 2 and 3
consist of one signal of pyrrolic α-protons and three signals of
ferrocene substituents. In addition, two signals of the protecting
−Si(i-Pr)3 group and a pyrrolic NH signal were observed in the
1H NMR spectra of 2 and 3, respectively. Similarly, 13C NMR
spectra of pyrroles 2 and 3 consist of two pyrrolic signals and
four signals of monosubstituted ferrocene groups. In addition,
two signals of the protecting −Si(i-Pr)3 group were observed in
the 13C NMR spectrum of 2. UV−vis spectra of 2 and 3 are
shown in Figure 1 and consist of a weak band at 22 320 cm−1
(448 nm) or 22 270 cm−1 (449 nm) and an intense band
observed at 36 230 cm−1 (276 nm) or 36 760 cm−1 (272 nm)
for pyrrole derivatives 2 and 3, respectively.
Following a previous report by Bray and co-workers,23 we
used a bulky triisopropylsilyl protecting group at the pyrrolic
nitrogen atom to obstruct halogenation in the 2,5-pyrrolic
positions. Indeed, reaction of N-(triisopropylsilyl)pyrrole with
either bromine or iodine at low temperature results in
formation of 3-mono- as well as 3,4-dihalopyrrole. In our
hands, the bromination reaction resulted in formation of the
mixture of 3-bromo- and 3,4-dibromopyrrole, which was
challenging to separate in required large quantities using
conventional chromatography methods. Iodination of N-
(triisopropylsilyl)pyrrole in the presence of mercuric acetate,
however, yielded almost pure 3,4-diiodo-1-(triisopropylsilyl)-
1H-pyrrole (1), which was further purified using standard
techniques (Scheme 1).
Two different strategies were explored for conversion of 1 to
the 3,4-diferrocenyl-1-(triisopropylsilyl)-1H-pyrrole (2). First,
we explored palladium-catalyzed coupling reaction between
bis(ferrocenyl)mercury and 1, following methodology de-
scribed earlier by Beletskaya25 and co-workers, which was
successfully used for the preparation of several ferrocene-
containing derivatives in our laboratory.26 This reaction,
unfortunately, reproducibly resulted in formation of only
trace quantities of 2. In the second approach, we explored
Figure 1. Experimental (top and middle) and TDDFT-predicted
(bottom) UV−vis spectra of compounds 2 and 3. Vertical bars
represent excitation energies and oscillator strengths calculated by the
TDDFT approach.
B
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Crystal Structure Description. Information regarding the
chemical structure of 1 and 2 was obtained on the basis of X-ray
crystallography (Figure 2). The results of the X-ray structure
refinement of 1 and 2 are presented in Table S1, while selected
bond distances and angles are shown in Table 1. The pyrrole
rings of 1 and 2 are essentially planar. The root-mean-square
(RMS) deviations for the planes are 0.002 Å (1) and 0.004 Å
(2), respectively, while the largest deviation of the atoms from
the corresponding planes is 0.003(2) Å for C(1) and 0.002(4)
Å for C(3) for 1 and 2, respectively. The torsion angles N(1)−
C(1)−C(2)−C(3) and C(2)−C(3)−C(4)−N(1) are 0.6(2)°,
0.1(2)° and −0.2(4)°, −0.5(4)°, for 1 and 2, respectively. At
the same time, out-of-plane deviations of the silicon atoms are
different for 1 and 2. Thus, the torsion angles Si(1)−N(1)−
C(1)−C(2) and C(2)−C(1)−N(1)−Si(1) are equal to
−173.4(1)° and 163.1(2)° for 1 and 2, respectively. In
addition, the deviation of the silicon atoms and from a least-
squares plane of the pyrrole ring (N(1), C(1), C(2), C(3), and
C(4)) is 0.170(1) and 0.421(1) Å for 1 and 2, respectively.
Iodine atoms in 1 are coplanar with the pyrrole plane. Thus, the
deviation of the iodine atoms from the pyrrole plane is
−0.098(1) Å for I(1) and −0.066(1) Å for I(2), while torsion
angle I(1)−C(2)−C(3)−I(2) is −0.8(3)°. It is worth noting
that the torsion angle C(5)−C(2)−C(3)−C(15) in 2 is almost
the same and equals −1.6(6)°, which could not be expected
taking into account the bulk character of the ferrocene groups.
Indeed, values for the torsion angle in related systems reported
in the literature are significantly larger (7.24° and 4.46° for
related compounds).27 Orientation of the ferrocene groups in 2
deviates significantly from the effective C2 symmetry of this
ferrocene-containing pyrrole as observed by NMR spectroscopy
in solution. Thus, corresponding torsion angles C(1)−C(2)−
C(5)−C(9) and C(4)−C(3)−C(15)−C(19) are 141.0(4)°
and 67.4(5)°, respectively. It is interesting to note that the
2,3,4,5-tetraferrocenyl-pyrrole adopts a similar conformation of
the ferrocene groups in the 3- and 4-positions with
corresponding torsion angles of 131.74/131.71° and 134.70/
−105.64°.15 A similar conformation of the ferrocenyl
substituents with local C2 symmetry was reported earlier for
3,4-diferrocenemaleoimides, and corresponding torsion angles
were found to be −154.84/−141.69°26 and −152.76/−
146.02°.15 Fe−C distances in 2 are in the regular range
2.036(4)−2.071(3) Å, with an average Fe−C distance of
2.049(4) Å.
Because of the low stability of 3 in solution for extended
periods of time required for a slow growth of its monocrystals
suitable for X-ray crystallography, we were not able to collect
single-crystal data on this compound. In the majority of cases,
the solution for crystallization became dark and precipitated
black micropowders in several days. In one such attempt,
however, along with the regular black precipitate, we were able
to manually separate several poor-quality dark orange crystals,
which were analyzed by X-ray diffraction (Figure 2, Tables S1
and 1) and found to be 5-hydroxy-3,4-diferrocenyl-1H-pyrrol-
2(5H)-one (3Ox, Scheme 1). Formation of the 3Ox complex
can be viewed as a partial oxidation/hydrolysis of 3 in wet
toluene in a regular atmosphere.
The crystal structure consists of two molecules of 3Ox and
two molecules of toluene. Carbon atoms in the 2- and 5-
positions are not chemically equivalent: one type of carbon
atom is sp3-hybridized and utilizes an alcohol function (C(4)
and C(28)), while the second one is sp2-hybridized and may be
viewed as a carbonyl component of an amide (C(1) and
C(25)). Bond distances for carbonyl component C(1)−O(1)
and O(3)−C(25) are 1.21(2) and 1.25(2) Å, respectively, and
are very close to another cyclic amide (1.214(4) and 1.204(4)
Å) reported earlier.26 The configuration of the C(4) and C(28)
Figure 2. ORTEP drawings of X-ray structures of compounds 1, 2,
and 3Ox. Thermal ellipsoids are at the 50% probability level.
Hydrogen atoms are omitted for clarity.
C
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 1. Selected Bond Distances (Å) and Angles (deg) for Complexes 1, 2, and 3Ox·PhMe Determined by X-ray
Crystallography
Compound 1
I(1)−C(2)
I(2)−C(3)
Si(1)−N(1)
Si(1)−C(8)
Si(1)−C(5)
Si(1)−C(11)
2.066(2)
2.061(2)
1.791(2)
1.881(2)
1.882(2)
1.882(2)
C(3)−C(2)−I(1)
C(4)−C(3)−I(2)
C(1)−N(1)−C(4)
C(1)−N(1)−Si(1)
C(4)−N(1)−Si(1)
I(1)−C(2)−C(3)−I(2)
128.00(15)
125.38(16)
107.13(17)
125.75(14)
126.69(14)
−0.8(3)
Compound 2
C(25)−Si(1)
C(28)−Si(1)
C(31)−Si(1)
N(1)−Si(1)
C(2)−C(5)
1.891(4)
1.883(4)
1.879(3)
1.782(3)
1.465(5)
1.480(4)
C(1)−C(2)−C(5)
124.2(3)
125.4(3)
−33.7(5)
67.4(5)
C(4)−C(3)−C(15)
C(1)−C(2)−C(5)−C(6)
C(4)−C(3)−C(15)−C(19)
C(2)−C(1)−N(1)−Si(1)
C(3)−C(4)−N(1)−Si(1)
Fe···π
163.1(2)
−163.0(3)
C(3)−C(15)
Fe−C(average)
Compound 3Ox·PhMe
O(1)−C(1)
1.21(2)
C(1)−C(2)
1.50(2)
O(2)−C(4)
1.43(1)
C(2)−C(3)
1.37(2)
N(1)−C(1)
1.38(2)
C(3)−C(4)
1.56(2)
N(1)−C(4)
1.41(2)
O(3)−C(25)
1.25(2)
O(4)−C(28)
1.38(1)
N(2)−C(25)
1.33(2)
N(2)−C(28)
1.47(2)
C(25)−C(26)
1.48(2)
C(26)−C(27)
1.33(2)
C(27)−C(28)
1.53(2)
C(1)−N(1)−C(4)
O(1)−C(1)−C(2)
C(3)−C(2)−C(1)
N(1)−C(4)−O(2)
O(2)−C(4)−C(3)
O(1)−C(1)−C(2)−C(3)
C(2)−C(3)−C(15)−C(19)
C(25)−C(26)−C(29)−C(30)
Fe−C(average)
112.4(11)
128.8(12)
108.4(10)
114.2(11)
114.3(9)
−169.8(13)
166.2(12)
−152.3(12)
2.03(1)
O(1)−C(1)−N(1)
N(1)−C(1)−C(2)
C(2)−C(3)−C(4)
N(1)−C(4)−C(3)
C(4)−N(1)−C(1)−O(1)
N(1)−C(1)−C(2)−C(3)
C(1)−C(2)−C(5)−C(6)
C(28)−C(27)−C(39)−C(40)
Fe···π
124.7(12)
106.5(12)
107.6(10)
104.1(11)
170.1(13)
10.2(13)
155.1(11)
16.1(18)
1.638(6), 1.651(7),
1.651(7), 1.651(6),
1.640(7), 1.658(6),
1.626(6), 1.633(6)
Chart 1. Structures of Complexes 4Me, 4Ph, 5Me, and 5Ph
atoms differs and corresponds to S and R type, thus eliminating
carbon−carbon double bond. The orientation of the ferrocenyl
groups also differs from that observed in 2. They adopt local C2
symmetry, as was previously observed for similar amide
systems.26 In addition, a strong hydrogen bonding between
the molecules in a solid state was observed in the crystal
structure of 3Ox. Two molecules of 3Ox of the same type form
a centrosymmetric dimer via hydrogen bonding between amide
groups and oxygen atoms of the carbonyl groups (N(1)···
O(1)[−x,1−y,−z] = 3.02(1) Å, N−H···O = 153.5° and N(2)···
O(3)[1−x,1−y,−z] = 2.98(1) Å, N−H···O = 154.2°). These
dimers are additionally bonded by a hydrogen bond between
the hydroxo group and the oxygen atom of the carbonyl group
(O(1)···O(4)[1−x,y,z]) = 2.74(2) Å and O(3)···O(2) =
overall chirality of the crystal (space group P1). The oxygen
̅
atoms of the hydroxo groups significantly deviate from pyrrole
planes (1.02(1) and 0.98(1) Å) built on N(1)−C(1)−C(2)−
C(3)−C(4) and N(2)−C(25)−C(26)−C(27)−C(28) atoms,
respectively. O−C−C angles for sp3-hybridized carbon atoms
are 107.6(10)° (O(2)−C(4)−C(3)) and 114.6(9)° (O(4)−
C(28)−C(27)), while C−O distances are 1.43(1) Å (O(2)−
C(4)) and 1.38(2) Å (O(4)−C(28)). All these geometric
parameters ultimately suggest the sp3-hybridized state for
carbons C(4) and C(28). The distances between carbons in 3
and 4 positions are 1.37(2) and 1.33(2) Å, which are shorter
than the same distance in 2 (1.443(5) Å) and correspond to a
D
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2.74(1) Å), forming one-dimensional chains along the
crystallographic a-axis.
tetrasubstituted pyrroles.15a−c Similar to the 2,5-diferrocenyl-
pyrrole systems,15a two oxidations were assigned to single-
electron ferrocene-centered processes. Because of the well-
documented suppression of the ion-paring for the TBAF
electrolyte,15 the separation between these oxidation waves is
smaller (ΔE1/2 = 170 and 180 mV for 2 and 3, respectively) in
TBAP electrolyte compared to that observed in TBAF
electrolyte (ΔE1/2 = 300 and 290 mV for 2 and 3, respectively).
Interestingly, the separation between the first two oxidation
waves in 2 and 3 in both electrolytes is significantly smaller
than similar values reported for 2,5-diferrocenyl-1-phenyl-
pyrrole (315 mV in TBAP and 450 mV in TBAF) or 2,3,4,5-
tetraferrocenyl-1-phenylpyrrole (330 mV in TFAB).15a,b It was
unusual to see a decrease in ΔE1/2 in 2 and 3, as we would
expect electronic communication to be improved between two
3,4-disubstituted ferrocenyl ligands, as the distance between the
two substituents is smaller than the ferrocene−ferrocene
distance in 2,5-disubstituted pyrroles. A large separation
between redox waves in 2 and 3 in both electrochemical
systems is indicative of the large comproportionation constants
Kc for the 2[2 or 3]+ ⇄ [2 or 3]0 + [2 or 3]2+ process.28
Because of the large values of Kc, one could expect that the
spectroscopic signatures of the mixed-valence [2]+ and [3]+
might be obtained using a spectroelectrochemical approach.
The UV−vis−NIR spectroscopic signatures of the different
oxidized forms of pyrrole derivatives 2 and 3 were obtained
using a spectroelectrochemical approach. Stepwise oxidation
was performed in DCM/TBAP and DCM/TBAF systems with
close results (Figures 4 and 5). Removal of the first electron
Electrochemical and Spectroelectrochemical Studies.
In order to investigate redox properties and evaluate the degree
of metal−metal coupling in the 3,4-diferrocene-substituted
pyrroles, compounds 2 and 3 were studied by cyclic
voltammetry (CV) and differential pulse voltammetry (DPV)
methods in a low-polarity solvent (DCM) using a common
electrochemical electrolyte, [N(C4H9)4]ClO4 (TBAP), and a
noncoordinating electrolyte, [N(C4H9)4][B(C6F5)4] (TBAF).
The latter electrolyte was shown to provide superior resolution
for redox waves in numerous coupled systems and in particular
for poly(ferrocenyl)pyrroles, furans, thiophenes, and maleoi-
mides.15,26 Experimental CV and DPV data for pyrrole
derivatives 2 and 3 are presented in Figure 3 and summarized
Figure 3. CV (blue) and DPV (red) data for compounds 2 and 3 in
DCM/0.05 M TBAF (top) and DCM/0.1 M TBAP (bottom) systems
at room temperature. CV scan rates are 100 mV/s.
in Table 2. In both electrolyte systems, two clearly defined
reversible oxidation processes have been observed. The first
oxidation potential in 2 and 3 in both electrolyte systems is
significantly lower compared to the parent ferrocene, similar to
what has been previously observed for the 2,5-di- and 2,3,4,5-
Table 2. Redox Properties of Pyrroles 2−5 (see Chart1 for
a
Structures 4 and 5)
compound
electrolyte
Ox1, mV Ox2, mV ΔE, mV
Kc
2
3
TBAP
−225
−150
−198
−226
−180
−170
−238
−206
−280
−280
−50
+30
175
180
315
200
300
290
450
410
331
265
908
TBAP
1103
b
d
Figure 4. Spectroelectrochemical oxidation of compound 2 in DCM/
0.3 M TBAP (top and middle) and DCM/0.15 M TBAF (bottom)
systems.
4Ph
(NBu4)[PF6]
(NBu4)[PF6]
TBAF
+117
−26
+120
+120
+212
+204
+51
211 039
b
d
5Ph
2102
2
3
117 712
79762
TBAF
b
d
4Ph
TBAF
40 386 252
from pyrrole derivative 2 or 3 in both systems results in the
appearance of two distinct bands located at ∼1000 and ∼2200
nm in the NIR region connected by a broad plateau. For both
ferrocenyl pyrroles, the former band is about an order of
magnitude more intense than the latter band. Upon electrolysis
of the mixed-valence [2]+ or [3]+ at the second oxidation
potential (DCM/TBAP system), the intensity of the weak band
observed at ∼2200 nm only slightly decreases, the initial band
c
d
4Me
TBAF
8 513 809
b
d
5Ph
TBAF
393 375
c
d
5Me
TBAF
−15
30 146
a
Potentials vs FcH/FcH+ ( 10 mV), scan rate 100 mV/s at platinum
working electrode in dry DCM, 0.1 M TBAP or 0.05 M TBAF
b
electrolyte at room temperature. 0.1 M TBAF or (NBu4)[PF6] was
used, ref 15a. 0.1 M TBAF was used, ref 15b. Kc values: this work.
c
d
E
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
electronic coupling matrix element (Hab) and the degree of
delocalization (α2).30 These parameters can be estimated using
eqs 1 and 2, and the Fe−Fe distances extracted from the
corresponding crystal structures or DFT-predicted geometries
of neutral compound 2.
Hab = 2.05 × 10−2[(νmaxεmaxΔν1/2
)
1/2/rab]
(1)
α2 = 4.24 × 10−4[(εmaxΔn1/2)/(rab2ηmax)]
(2)
Both ∼1000 and ∼2200 nm NIR bands observed in [2]+ and
[3]+ cannot be assigned as IVCT bands because neither of
these would disappear during the second oxidation process.
Thus, the broad plateau at ∼1800 nm, which increases under
[2] → [2]+ or [3] → [3]+ transformation and decays under the
[2]+ → [2]2+ or [3]+ → [3]2+ process, was assigned as an IVCT
band. Fitting of the IVCT band in [2]+ with the Gaussian-shape
bands (Figure S8) gave the necessary information for
calculation of Hab and α2 via eqs 1 and 2. The maximum
molar absorption coefficient of the IVCT band (143 M−1
cm−1), the energy of νmax of 5568 cm−1 and Δν1/2 of 3383
cm−1, and the crystallographic Fe−Fe distance of 5.934 Å
yielded Hab = 180 cm−1 and α2 = 1.04 × 10−3. These data along
with the Γ-test30 value (0.046) clearly suggest that 2+ belongs
to a weakly coupled class II mixed-valence system in Robin−
Day classification.31 It should be noted, however, that these
values should be treated with caution because the broad and
nonprominent nature of the IVCT band could cause some
error in the Δν1/2 value during the fitting procedure. In spite of
such possible error, IVCT band deconvolution analysis, which
predicts small intramolecular interactions in the mixed-valence
[2]+ and [3]+, is in excellent agreement with the electro-
chemical data on these systems.
DFT and TDDFT Calculations on Neutral Poly-
(ferrocenyl)pyrroles. DFT and TDDFT calculations were
performed to acquire insight into the nature of the electronic
structure, spectroscopy, and redox properties of ferrocene-
containing pyrroles 2 and 3. First, we have studied the influence
of the triisopropylsilyl group on the electronic structure and
MO composition of 2 and 3. In order to investigate such an
influence, we have calculated the electronic structure of 2 at X-
ray geometry and compared it to those obtained for (i) the
geometry-optimized structure of 2 (structure 2a) and (ii) the
geometry optimized (C2 symmetry) structure of 3. The
molecular orbital energy diagram, molecular orbital composi-
tions, and representative shapes of important molecular orbitals
predicted using the B3LYP exchange−correlation functional
with Wachter’s full-electron basis set for Fe, the 6-311G(d)
basis for C and N, and the 6-31G basis set for H are shown in
Figures 6 and 7. MO contribution analysis suggests that in all
cases the HOMO is dominated by the contributions from the
two ferrocene substituents with a minor contribution coming
from the β-pyrrolic and nitrogen atoms. Such MO shape creates
a potential orbital electron-transfer pathway for electron
transfer from one ferrocene substituent to another. Similar
HOMO shapes were observed in the previously reported
dimethyl (Z)-2,3-bis(ferrocenyl)-2-butenedioate and (Z)-2,3-
bis(ferrocenyl)maleimide systems.26 In addition, the composi-
tion of the HOMO in pyrrole derivatives 2 and 3 also suggests
that the first oxidation should be ferrocene-centered, in
agreement with experimental data. In all cases, the LUMO is
again dominated by contributions from the two ferrocene
substituents with minor contributions coming from the α- and
Figure 5. Spectroelectrochemical oxidation of compound 3 in DCM/
0.3 M TBAP (left) and DCM/0.15 M TBAF (right) systems.
at ∼1000 nm undergoes a blue shift to ∼900, and the NIR
plateau that connects these two bands disappears (Figures 4
and 5). Reduction of [2]2+ is completely reversible in both
electrochemical systems studied (Figure S5), and a similar
reversibility was observed for [3]2+ in the DCM/TBAP system
(Figure S6). Overall, these electrochemical and spectroelec-
trochemical results for pyrrole derivatives 2 and 3 correlate well
with reported data for both the 2,5-diferrocenylpyrroles and
2,3,4,5-tetraferrocenylpyrroles published earlier.15 The reversi-
bility of the oxidation processes for 3 in the DCM/TBAF
system under spectroelectrochemical conditions, however, is
quite different. Although, upon the oxidation of [3]+, the low-
energy band at ∼2200 nm does not lose intensity and the NIR
band at ∼1000 nm undergoes a blue shift to ∼900 nm, the
resulting [3]2+ cation cannot be reduced back to the neutral 3
(Figure S6). Such reproducible behavior can be attributed to
polymerization of [3]+ or [3]2+ on the working electrode,
which is a typical behavior for numerous α-unsubstituted NH
pyrrole derivatives.29 In order to test this hypothesis, we
conducted several electropolymerization experiments on
complex 3 in the DCM/TBAF system (Figure S7). Indeed,
when complex 3 was carefully cycled close to the first two
ferrocene-based oxidation potentials, no electropolymerization
was observed (Figure S7a). When the cycling potential was
increased by several hundredths of a millivolt (to mimic the
overpotential required by our custom-made spectroelectro-
chemical cell), electropolymerization was clearly observed in
CV experiments (Figure S7b). Such electropolymerization can
be initiated only by traces of unidentified impurity present in
the electrochemical system because the oxidation potential of
the pyrrole fragment in complex 3 is located outside the
electrochemical window (Figure S7c). Similar electropolyme-
rization of complex 3 in the DCM/TBAP system was not
observed because of the much lower second oxidation potential
of ferrocene substituents.
Although signals observed in the inter-valence charge transfer
(IVCT) region in the mixed-valence complexes [2]+ and [3]+
are rather weak, we were able to conduct a deconvolution
analysis for a better resolved and more intense IVCT band
region in [2]+. The two key parameters for the mixed-valence
[2]+ that could be estimated using the Hush model are the
F
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
and 3. Indeed, it has been shown that the TDDFT approach
can accurately predict both energies and intensities in small
ferrocene-containing compounds.33 The TDDFT-predicted
UV−vis spectrum of 3 along with its corresponding vertical
excitations is provided in Figure 1 in comparison with the
experimental spectra of pyrrole derivatives 2 and 3. TDDFT-
calculated vertical excitation energies and expansion coefficients
are provided in Table S2. In general, the TDDFT-predicted
spectrum of 3 is in very good agreement with the experimental
UV−vis spectra of both ferrocene derivatives 2 and 3 (Figure
1). The experimental spectra of 2 and 3 are composed of two
primary bands, the first (abbreviated as band 1 below) being
weak and broad, centered around 22 320 or 22 270 cm−1 (for 2
and 3, respectively) and spanning from ∼16 600 to ∼26 600
cm−1. TDDFT predicts eight excited states in this spectral
envelope. All of these excited states can be viewed as
predominantly d−d transitions, but only the first and the fifth
excited states contribute significantly to the band 1 profile
because excited states 2−4 and 6−8 have negligible intensities
(Table S2). In agreement with their predominant d−d
character and experimental data, overall TDDFT-predicted
intensities for the band 1 spectral envelope are quite low. The
second and more prominent band 2 of the experimental spectra
of pyrrole derivatives 2 and 3 appears at 36 230 and 36 760
cm−1, respectively. TDDFT calculations predict that excited
states 9−30 contribute to this band’s profile. Out of these
contributors, however, only excited states 21, 22, and 25 (Table
S2) have significant TDDFT-predicted intensities to form the
absorption band profile of the band 2 region. All of these
excited states have significant contributions from LMCT and/
or MLCT single-electron transitions in addition to their d−d
character. Overall, TDDFT calculations are in good agreement
with experimental data and suggest that the band 1 spectral
region is dominated by d−d transitions, while the higher energy
band 2’s spectral region has significant charge-transfer
character.
Figure 6. DFT-predicted frontier orbital energies of pyrroles 2 and 3.
Similar to the 3,4-ferrocene-disubstituted compounds 2 and
3, DFT predicts that the HOMO and the LUMO in 2,5-
di(ferrocenyl)pyrrole (4) are dominated by the ferrocene
substituents, although the contribution from the pyrrole
heterocycle to these MOs is significantly higher (∼40% for
the HOMO and ∼20% for the LUMO) than those in 3,4-
diferrocenyl analogues 2 and 3 (Figure 8). The HOMO and the
LUMO in 2,3,4,5-tetraferrocene-1H-pyrrole (5) are also
ferrocene-centered, although the HOMO is dominated by
contributions from 3,4- while the LUMO is dominated by
contributions from the 2,5-ferrocenyl substituents (Figure 8).
Thus, DFT calculations predict that the first oxidation in
compounds 2−5 should be ferrocene-centered, in agreement
with the experimental data. More interestingly, the ground-state
DFT calculations predict that the first oxidation in the
tetraferrocenylpyrrolic system 5 should be centered at the
3,4-ferrocenyl substituents, which contradicts the earlier
electrochemical data-based hypothesis of Lang and co-work-
ers.15
DFT Calculations on Mixed-Valence Poly(ferrocenyl)-
pyrrole Cations. The DFT calculations on the mixed-valence
[3]+ and [4]+ complexes in localized C1 and delocalized C2
symmetries (Table 3) suggest that the localized structures are
5.3 and 34.9 kcal/mol more stable than the corresponding
delocalized structures of pyrrole derivatives [3]+ and [4]+,
respectively. These findings are in agreement with the IVCT
band deconvolution analysis for pyrrole derivatives 2−4,
Figure 7. Molecular orbital compositions for pyrroles 2 (top) and 3
(bottom) calculated at the DFT level.
β-pyrrolic carbon atoms. In general, our DFT calculations on
2a and 3 are suggestive of a marginal influence of the
triisopropylsilyl protecting group on the MO energies and
compositions, and thus in all other calculations the smaller
ferrocene-containing pyrrole 3 variant was used. Such results
are not surprising because it is expected that the electron-
donating properties of proton and triisopropylsilyl substituents
should be close to each other.32
Accurate prediction of the electronic structure of 3, which is
a truncated version of 2, allows for the assignment of the
experimentally observed bands in the UV−vis spectra of both 2
G
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
localized C1 symmetry structures, manual localization of the
initial spin density on one out of four ferrocene groups was
implemented prior to and following geometry optimization.
Similarly, in the case of delocalized C2 symmetry structures of
[5]+, we performed three different optimizations: (i) no
restrictions on spin density; (ii) spin density at the 2,5-
ferrocene groups; (iii) spin density at the 3,4-ferrocene groups.
Finally, we also ran a set of DFT calculations on the N-Ph
derivate of [5]+ (compound 5Ph) using the experimentally
determined geometry for this compound15 with and without
initial spin localization.
The analysis of our data (Table 4 and Figures 9 and 10) is
suggestive of the following trends: (i) the global energy
Table 4. DFT-Predicted Relative Energies and Spin
Densities for Local and Global Minima in C1 (Localized)
and C2 (Delocalized) Structures of the Mixed-Valence [5]+
Complex
spin density
symmetry
ΔE (kcal/mol) Fe-2
Fe-5
Fe-3
Fe-4
C2
delocalized
Fc-2,5
Fc-3,4
Fc-2,5
Fc-3,4
Fc-2
7.30
7.37
9.70
3.86
7.78
43.14
6.50
0.00
1.20
0.33
0.30
0.13
0.33
0.04
1.02
0.01
0.00
0.00
0.33
0.30
0.13
0.33
0.04
0.00
1.15
0.00
0.00
0.10
0.12
0.20
0.10
0.52
0.00
0.01
1.27
0.00
0.10
0.12
0.20
0.10
0.52
0.00
0.02
0.00
1.26
C1
Fc-5
Fc-3
Fc-4
Figure 8. Molecular orbital compositions for pyrroles 4 (top) and 5
(bottom) calculated at the DFT level.
Table 3. DFT-Predicted Relative Energies and Spin-
Densities in C2 (Delocalized) and C1 (Localized) Geometries
of the Mixed-Valence [3]+ and [4]+
spin density
Figure 9. DFT-predicted energy profiles for [5]+ in C2 and C1
symmetries.
symmetry
ΔE, kcal/mol
Fe2
Fe5
Fe3
Fe4
Complex [3]+
C2
C1
5.28
0.00
0.64
1.28
0.64
0.00
Complex [4]+
C2
C1
34.89
0.00
0.49
1.15
0.49
0.03
according to which mixed-valence diferrocenes [2]+−[4]+
should have localized class II (in Robin−Day classification)31
character.
In order to resolve the above-mentioned controversy
regarding the first oxidation process in 2,3,4,5-tetraferrocenyl-
pyrroles, an extensive set of DFT calculations on relative
energies and spin density localization/delocalization in [5]+
mixed-valence cation were performed. Because the mixed-
valence [5]+ cation, at least theoretically, can adopt localized C1
(class II in Robin−Day classification)31 or delocalized C2 (class
III in Robin−Day classification) geometries, both of these
possibilities were taken into account. In addition, in the case of
Figure 10. DFT-predicted energy profiles for [5Ph]+ in C1 symmetry.
minimum for all optimized localized and delocalized mixed-
valence structures is represented by the complex [5]+ with
electron density localized at one out of two β-pyrrolic ferrocene
H
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
substituents; (ii) the lowest energy localized C1 structure in
which electron density is localized at α-pyrrolic ferrocene is 6.5
kcal/mol higher in energy than the global minimum for the C1
symmetry localized structures; (iii) the minimum energy for the
delocalized C2 symmetry structure belongs to the structure with
spin density delocalized over ferrocenes at the 2,5-positions;
this structure is 3.89 kcal/mol higher in energy than the global
minimum for the localized C1 structures; (iv) the lowest energy
delocalized C2 symmetry structure in which spin density is
delocalized over β-pyrrolic ferrocene groups is 3.92 kcal/mol
higher in energy than the C2 symmetry structure in which spin
density is delocalized over α-pyrrolic ferrocene groups; (v) in
the case of the X-ray crystal structure of [5Ph]+, DFT
calculations again predict that the global minimum should be
described by the localized structure in which spin density is
localized over one β-pyrrolic ferrocene group, while the lowest
energy structure in which spin density is localized over an α-
pyrrolic ferrocene group is 3.61 kcal/mol higher in energy.
Thus, DFT calculations suggest that the β-ferrocene localized
C1 structure should be more energetically stable than any other
mixed-valence singly oxidized species, which is in agreement
with the ground-state calculations presented above on neutral
5. Despite this agreement, however, one should be careful
about ultimate assignments of the nature of electrochemically
or chemically generated [5]+ because the DFT-predicted
energies of the 2,5-delocalized C2 symmetric and localized α-
pyrrolic C1 symmetric [5]+ cations are only 3.89 and 6.5 kcal/
mol higher in energy, and this energy order could be easily
affected by the electronic and steric properties of the
substituent at the nitrogen atom as well as by the polarity of
the solvent used for spectroelectrochemical or chemical
oxidation experiments. In addition, while the B3LYP
exchange−correlation functional could be viewed as a good
starting point for DFT calculations on energies of the mixed-
valence poly(ferrocenyl)-containing systems,34 it might be
expected that variation of the amount of Hartree−Fock
exchange in a given exchange−correlation functional can alter
the relative energies and degree of spin density localization/
delocalization in possible spin isomers of [5]+.35
assigned with the help of their electronic structures, and
excited-state energies calculated using DFT and TDDFT
methods. It was found that the low-energy, low-intensity
band observed in the UV−vis spectra of 2 and 3 can be
assigned as ferrocene-centered d−d transitions, while the more
intense UV band could be associated with the charge-transfer
excitations. Redox properties of 2 and 3 were investigated using
cyclic voltammetry, differential pulse voltammetry, and
spectroelectrochemical approaches. Ferrocene-centered oxida-
tion processes in 2 and 3 were found to be separated by ∼180
and ∼300 mV in DCM/TBAP and DCM/(NBu4)[B(C6F5)4]
systems, respectively. Stepwise spectroelectrochemical oxida-
tion of 2 and 3 allowed us to obtain spectroscopic signatures of
the mixed-valence [2]+ and [3]+ cations, which are charac-
terized by two NIR bands observed at ∼1000 and ∼2200 nm.
Hush analysis of the intervalence charge-transfer band in [2]+
and [3]+ is suggestive of class II (in Robin and Day
classification) mixed-valence behavior. Electronic structures of
neutral and spin-localized/delocalized single-electron-oxidized
mixed-valence cations 2,5-di- (compound 4), 3,4-di- (com-
pound 3), and 2,3,4,5-tetraferrocenopyrroles (compound 5)
were investigated by DFT calculations. DFT predicts that the
first electron in 5 should be removed from one of ferrocene
groups located at a β-pyrrolic position, although the structure of
[5]+ with spin density localized at one of ferrocene groups at
the α-pyrrolic position is only 6.5 kcal/mol higher in energy.
EXPERIMENTAL SECTION
■
Materials. Caution! All organomercurials are highly toxic. Extreme
care is necessary when handling all products and their solutions. All
commercial reagents were ACS grade and were used without further
purification. Silica gel (60 Å, 63−100 μm) and aluminum oxide
(activity I, 50−200 μm) for column chromatography were purchased
from Dynamic Adsorbents, Inc. All reactions were performed under a
dry argon atmosphere with flame-dried glassware. Dry toluene and
ether were obtained by distillation over sodium, dry DCM was
obtained by distillation over calcium hydride prior to experiments, and
dry THF was obtained by distillation over Na/K alloy with diphenyl
ketone. Tetrabutylammonium tetrakis(pentafluorophenyl)borate
((NBu4)[B(C6F5)4]) was prepared according to the literature
procedure.36
The overall problem with providing an accurate assignment
of the redox properties in tetraferrocenyl-containing pyrroles,
thiophenes, and furans might be even more complex. Indeed,
experimental redox data on 2, 4Ph, and 5Ph in TBAP or
(NBu4)[PF6] electrolytes (Table 2) cannot resolve such
uncertainties as the first oxidation potential of 2, which appears
virtually identical to that of the tetraferrocenyl compound 5Ph,
while the first oxidation potential of 4Ph is also close to that in
5Ph (ΔE1/2 ∼30 mV). Similarly, in the TBAF/DCM system,
the first oxidation potential of 2 is 26 mV higher than the first
oxidation potential in 4Me and 58 mV higher than in 4Ph,
while all of these are significantly higher than in the
tetrasubstituted derivatives 5Me and 5Ph. Moreover, recently
Lang and coauthors reported that the oxidation potentials for
2,5-diferrocenyl- and 3,4-diferrocenylthiophenes are exactly the
same and differ significantly from the first oxidation potential in
2,3,4,5-tetraferrocenylthiophene.15
Instrumentation. A Varian Unity INOVA NMR instrument was
used to evaluate spectra taken at 500 MHz frequency for protons and
125 MHz for carbon atoms. Each were referenced to TMS as an
internal standard, and chemical shifts were recorded in parts per
million. All UV−vis data were obtained on a JASCO-720
spectrophotometer at room temperature. Electrochemical measure-
ments were conducted using a CHI-620C electrochemical analyzer
utilizing the three-electrode scheme. Either carbon or platinum
working, auxiliary, and reference electrodes were used in a 0.05 M
solution of TBAF (or 0.1 M TBAP) in DCM with redox potentials
corrected using an internal standard (decamethylferrocene) in all
cases. Spectroelectrochemical data were collected using a custom-made
1 mm cell, a working electrode made of platinum mesh, and a 0.30 M
solution of TBAP (or 0.15 M TBAF) in DCM. Elemental analysis was
performed by Atlantic Microlab, Inc. in Atlanta, GA.
Synthesis. Preparation of N-(Triisopropylsilyl)pyrrole. The syn-
thesis of the N-(triisopropylsilyl)pyrrole was performed by a modified
method reported by Muchowski.37 Pyrrole (6.04g, 84.4 mmol) was
added dropwise to a solution of potassium-bis-trimethylsilylamide
(16.72 g, 83.8 mmol) in anhydrous THF under an argon atmosphere.
The reaction mixture was stirred for 18 h. Solvent was evaporated
under vacuum, triisopropylsilyl chloride (16.49g, 85.5 mmol) was
added dropwise to dry potassium pyrrole salt (83.8 mmol) in
anhydrous THF, and the mixture was stirred for 1 h. The solvent was
evaporated under vacuum; ether was added to the reaction mixture,
which was washed with water. The ether layer was dried over sodium
CONCLUSIONS
■
3,4-Diiodo-1-(triisopropylsilyl)-1H-pyrrole (1), 3,4-diferrocen-
yl-1-(triisopropylsilyl)-1H-pyrrole (2), and 3,4-diferrocenyl-1H-
pyrrole (3) were prepared and characterized using NMR and
UV−vis spectroscopic methods as well as X-ray crystallography
and elemental analysis. UV−vis spectra of 2 and 3 were
I
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
sulfate, and pure N-triisopropylpyrrole (12.73 g, 68% yield) was
obtained as a clear oil after vacuum distillation. 1H NMR of this
compound is in agreement with the reported data37 (δ, 500 MHz,
CDCl3, TMS): 1.09 (d, 18 H, J = 7.4 Hz, CH3), 1.45 (m, 3 H, J = 7.4
Hz, CH), 6.32 (t, 2 H, α-pyrrolic), 6.80 (t, 2 H, β-pyrrolic).
as the hybrid PBE1PBE functional. The first 30 states were calculated
for all TDDFT runs. In all calculations, Wachter’s full-electron basis
set42 was used for Fe, the 6-311G(d)43 for C and N, and the 6-31G44
basis for H. Equilibrium geometries were confirmed by frequency
calculations and specifically by the absence of the image frequencies.
All calculations were performed using Gaussian 03 or Gaussian 09
software.45 Molecular orbital analysis was conducted using the
VMOdes 8.1 program.46
3,4-Diiodo-1-(triisopropylsilyl)pyrrole, 1. This compound was
prepared following a slightly modified published procedure.37 Iodine
(0.95 g, 4 mmol) in 200 mL of DCM was added dropwise to a mixture
of mercuric acetate (1.28 g, 4 mmol) and N-(triisopropylsilyl)pyrrole
(0.51 g, 2 mmol) in DCM at 0 °C over a period of 1 h. The reaction
mixture was stirred for 3.5 h at 0 °C, and the solvent removed under
vacuum. The product was washed with hexane, and the solvent was
evaporated under reduced pressure to obtain the crude product as an
oil (0.494 g, 51% yield). The NMR spectra were identical to those
reported by Muchowski,37 and additional purification has not been
applied. 1H NMR (δ, 500 MHz, CDCl3, TMS): 1.08 (d, 18H, J = 7.49
Hz, CH3), 1.41 (sept, 3H, J = 7.49 Hz, CH(i-Pr)), 6.79 (s, 2H, α-
pyrrolic).
Geometries of the mixed-valence [2]+−[4]+ were investigated using
delocalized C2 symmetries or localized C1 symmetries. In the later case,
initial guesses for DFT calculations were generated using the spin
localization fragmentation procedure available in Gaussian 09. To
investigate the spin localization/delocalization properties in the mixed-
valence [5]+ pyrrole, the spin localization fragmentation procedure
available in Gaussian 09 was also used. In the case of C2 symmetries of
pyrrole derivative [5]+ initial guesses were generated by delocalization
of the spin density over the 3,4- or 2,5-positions. In the case of the
localized C1 symmetries of the mixed-valence pyrrole derivative [5]+
initial guesses were generated by localization of spin densities at the 2-
or 3-position with consequent geometry optimizations.
X-ray Crystallography. Single crystals suitable for X-ray crystallo-
graphic analysis of 1 and 2 were obtained by a slow evaporation of a
concentrated hexane solution, while crystals of 3Ox were obtained by a
slow evaporation of a toluene/hexane (1:1 v/v) mixture. X-ray
diffraction data were collected on a Rigaku Rapid II image plate
diffractometer using graphite-monochromated Mo Kα radiation (λ =
0.71073 Å) at 123 K. Multiscan absorption corrections were applied to
the data using the CrystalClear 2.0 program. The structures were
solved by direct methods implemented in SIR-9247 and refined by a
full-matrix least-squares method based on F2 using SHELXL-97 or
SHELXL-2013 and SHELXLE software.48 All non-hydrogen atoms
were refined in anisotropic approximation, while hydrogen atoms were
refined in a “riding” mode. The analyses of the structures and
visualization of the results were done using PLATON software.49 The
crystal structure of 3Ox was found to be affected by a nonmerohedral
twinning, where two components of the twin were related by the
transformation matrix −1 0 0 1.25 1 0.826 0 0 −1. This matrix was
used to generate a new hkl file in HKLF5 form. The final fraction of
twin components after the refinement was 0.22. Additional restraints
(SAME, DELU) were used for one toluene solvent molecule.
Bis-3,4-ferrocenyl-1-(triisopropylsilyl)pyrrole, 2. To a solution of
2.79 g (15 mmol) of ferrocene and 0.17 g (1.5 mmol) of KOt-Bu in 50
mL of THF was added dropwise 13.8 mL (23.5 mmol) of a 1.6 M
solution of tert-butyllithium in n-pentane at −42 °C (dry ice/
acetonitrile) to the reaction mixture. After stirring for 1 h, 6.76 g (24.1
mmol) of ZnCl2(THF)2 was added as a single portion, and the
reaction mixture was left to stir for 1 h at −42 °C and an additional 1 h
at room temperature. After this period of time, 0.104 g (0.09 mmol) of
[Pd(PPh3)4] along with 2.39 g (5 mmol) of 1,2-diiodo-N-
(triisopropylsilyl)pyrrole (1) were added to the reaction mixture.
The reaction mixture was refluxed for 72 h. After evaporation of the
solvent to dryness, the product was extracted with 500 mL of DCM.
The organic layer was washed with water (3 × 150 mL) and dried over
MgSO4. The solvent was removed under reduced pressure, and the
crude product was purified by column chromatography (Al2O3) using
a hexane/toluene (9:1, v/v) mixture as eluent. Additional purification
of the product was achieved by fractional crystallization from hexanes,
giving 1.54 g (52%) of orange crystals of 2, mp 120−121 °C. Anal.
Calcd for C33H41Fe2NSi·0.4C6H14 (traces of hexanes were observed in
1
the H NMR spectrum of 2): C, 67.93; H, 7.50; N, 2.24. Found: C,
1
67.85; H, 7.51; N, 2.16. H NMR (δ, 500 MHz, CDCl3, TMS): 6.81
(s, 2H, α-pyrrolic), 4.33 (s, 4H, α-Cp), 4.14 (s, 4H, β-Cp), 4.03 (s,
10H, Cp), 1.52 (sept, 3H, CH, iPr), 1.19 (d, 18H, CH3). 13C NMR, δ:
123.5 (α-pyrrole), 122.1 (β-pyrrole), 77.37 (Cipso-Cp), 69.4 (α-Cp),
69.1 (β-Cp), 67.3 (CpH), 18.1 (CH), 12.0 (CH3).
ASSOCIATED CONTENT
■
S
* Supporting Information
Bis-3,4-ferrocenylpyrrole, 3. A solution of tetra-n-butylammonium
fluoride (0.85 mL of a 1 M solution, 0.85 mmol) in THF was added to
a stirred solutions of N-(triisopropylsilyl)-3,4-differocenylpyrrole (0.50
g, 0.85 mmol) in THF (3 mL). After 10 min at room temperature, the
solution was diluted with ether, and the organic phase was washed
with water and dried over MgSO4. Then, the organic phase was
purified by flash chromatography on neutral alumina with a mixture of
DCM−triethylamine (100:1, v/v) as eluent. The first yellow fraction
(Rf 0.85) afforded compound 3, which was recrystallized from DCM−
ether. Yield: 0.15 g (41.0%). Mp: 142−143 °C (dec). Anal. Found: C,
65.87; H, 5.37; N, 2.76. Calcd for C24H21Fe2N·0.5C4H10O: C, 66.14;
1
Experimental H and 13C NMR spectra and crystallographic
and DFT data for compounds 2 and 3. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vnemykin@d.umn.edu. Phone: +1 (218) 7266729.
■
Notes
The authors declare no competing financial interest.
1
H, 5.55; N, 2.97. H NMR (ppm, CDCl3, TMS, δ): 4.09 (s, 20H,
ACKNOWLEDGMENTS
CpH), 4.17 (m, 4H, β-Cp), 4.31 (m, 4H, α-Cp), 6.94 (s, 2H, α-
pyrrolic), 8.08 (br, H, NH). 13C NMR (CDCl3, TMS, δ): 120.3 (β-
pyrrole), 117.6 (α-pyrrole), 82.17 (CipsoCp), 69.05 (α-Cp), 69.04
(CpH), 67.12. (β-Cp).
■
Generous support by the NSF CHE-1110455 and NSF MRI
CHE-0922366 grants, Minnesota Supercomputing Institute,
and University of Minnesota Grant-in-Aid to V.N.N. is greatly
appreciated. We wish to acknowledge Yuriy Zatsikha for the
help with electropolymerization studies.
DFT Calculations. The initial geometry of complex 2 was taken
from the X-ray analysis and optimized at the DFT level of theory,
using hybrid PBE1PBE38 or B3LYP39 exchange−correlation func-
tionals or pure GGA BP86.40 It was found that the hybrid B3LYP
exchange−correlation functional provides the best agreement between
theory and experiment for structures of complexes 2 and 3 when
tested against pure GGA BP86 and hybrid PBE1PBE exchange−
correlation functionals. TDDFT calculations for vertical excitation
energies in complexes 2 and 3 were performed using a hybrid B3LYP
exchange−correlation functional, which in the series of test
calculations outperformed pure BP86 and BPW9141 GGAs as well
REFERENCES
■
(1) (a) Heath, J. R.; Ratner, M. A. Phys. Today 2003, 56, 43−49.
(b) Chen, J.; Lee, T.; Su, J.; Wang, W.; Reed, M. A. Encycl. Nanosci.
Nanotechnol. 2004, 5, 633−662. (c) Weiss, J. Coord. Chem. Rev. 2010,
254, 2247−2248. (d) Nano and Molecular Electronics Handbook;
Lyshevski, S. E., Ed.; CRC Press: New York, 2007. (e) Bayley, H.
Nature (London, U. K.) 2010, 467, 164−165. (f) Giacalone, F.; Martin,
J
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
N. Adv. Mater. (Weinheim, Ger.) 2010, 22, 4220−4248. (g) Jurow, M.;
Schuckman, A. E.; Batteas, J. D.; Drain, C. M. Coord. Chem. Rev. 2010,
254, 2297−2310. (h) Chung, A.; Deen, J.; Lee, J.-S.; Meyyappan, M.
Nanotechnology 2010, 21, 412001/1−412001/22. (i) Belosludov, R.
V.; Farajian, A. A.; Baba, H.; Mizuseki, H.; Kawazoe, Y. Jpn. J. Appl.
Phys., Part 1 2005, 44, 2823−2825. (j) Belosludov, R. V.; Farajian, A.
A.; Kikuchi, Y.; Mizuseki, H.; Kawazoe, Y. Comput. Mater. Sci. 2006,
36, 130−134. (k) Lee, S. U.; Belosludov, R. V.; Mizuseki, H.; Kawazoe,
Y. J. Phys. Chem. C 2007, 111, 15397−15403. (l) Lee, S. U.;
Belosludov, R. V.; Mizuseki, H.; Kawazoe, Y. Small 2008, 4, 962−969.
(2) (a) Fabre, B. Acc. Chem. Res. 2010, 43, 1509−1518. (b) Mizuseki,
H.; Belosludov, R. V.; Uehara, T.; Lee, S. U.; Kawazoe, Y. J. Korean
Phys. Soc. 2008, 52, 1197−1201. (c) Uehara, T.; Belosludov, R. V.;
Farajian, A. A.; Mizuseki, H.; Kawazoe, Y. Jpn. J. Appl. Phys., Part 1
2006, 45, 3768−3771. (d) Kaim, W.; Lahiri, G. K. Angew. Chem., Int.
Ed. 2007, 46, 1778−1796. (e) Kaim, W.; Sarkar, B. Coord. Chem. Rev.
2007, 251, 584−594. (f) Chisholm, M. H.; Patmore, N. J. Acc. Chem.
Res. 2007, 40, 19−27. (g) Arumainayagam, C. R.; Lee, H.-L.; Nelson,
R. B.; Haines, D. R.; Gunawardane, R. P. Surf. Sci. Rep. 2010, 65, 1−44.
(3) (a) Astruc, D.; Ornelas, C.; Ruiz Aranzaes, J. J. Inorg. Organomet.
Polym. Mater. 2008, 18, 4−17. (b) Santi, S.; Orian, L.; Donoli, A.;
Bisello, A.; Scapinello, M.; Benetollo, F.; Ganis, P.; Ceccon, A. Angew.
Chem., Int. Ed. 2008, 47, 5331−5334. (c) Cowan, D. O.; LeVanda, C.;
Park, J.; Kaufman, F. Acc. Chem. Res. 1973, 6, 1−7.
(4) (a) Kaifer, A. E. Eur. J. Inorg. Chem. 2007, 5015−5027.
(b) Wagner, M. Angew. Chem., Int. Ed. 2006, 45, 5916−5918.
(c) Barlow, S.; O’Hare, D. Chem. Rev. 1997, 97, 637−670. (d) Miller,
J. S.; Epstein, A. J. Angew. Chem., Int. Ed. Engl. 1994, 33, 385−415.
(e) Epstein, A. J.; Miller, J. S. Synth. Met. 1996, 80, 231−237.
(f) Barlow, S. Inorg. Chem. 2001, 40, 7047−7053. (g) Giuffrida, G.;
Campagna, S. Coord. Chem. Rev. 1994, 135−136, 517−531. (h) Ernst,
S.; Kasack, V.; Kaim, W. Inorg. Chem. 1988, 27, 1146−1148.
(5) (a) Ferrocenes: Ligands, Materials and Biomolecules; Stepnicka, P.,
Ed.; John Wiley & Sons, Ltd.: Chichester, England, 2008. (b) Vlcek,
A., Jr. Chemtracts 2001, 14, 1−6. (c) Kowalski, K.; Linseis, M.; Winter,
R. F.; Zabel, M.; Zalis, S.; Kelm, H.; Kruger, H.-J.; Sarkar, B.; Kaim, W.
Organometallics 2009, 28, 4196−4209. (d) Nakaya, T.; Namiki, K.;
Murata, M.; Kanaizuka, K.; Kurashina, M.; Fujita, T.; Nishihara, H. J.
Inorg. Organomet. Polym. Mater 2008, 18, 124−130. (e) Nakashima, S.
Recent Res. Dev. Pure Appl. Chem 1998, 2, 247−273.
(6) (a) Ding, F.; Wang, H.; Wu, Q.; Van Voorhis, T.; Chen, S.;
Konopelski, J. P. J. Phys. Chem. A 2010, 114, 6039−6046. (b) Hadt, R.
G.; Nemykin, V. N. Inorg. Chem. 2009, 48, 3982−3992. (c) Santi, S.;
Durante, C.; Donoli, A.; Bisello, A.; Orian, L.; Ceccon, A.; Crociani, L.;
Benetollo, F. Organometallics 2009, 28, 3319−3326. (d) Masuda, Y.;
Shimizu, C. J. Phys. Chem. A 2006, 110, 7019−7027. (e) Mochida, T.;
Takazawa, K.; Matsui, H.; Takahashi, M.; Takeda, M.; Sato, M.;
Nishio, Y.; Kajita, K.; Mori, H. Inorg. Chem. 2005, 44, 8628−8641.
(f) Oda, T.; Nakashima, S.; Okuda, T. Bull. Chem. Soc. Jpn. 2003, 76,
2129−2134. (g) Dong, T.-Y.; Chang, L.-S.; Lee, G.-H.; Peng, S.-M.
Organometallics 2002, 21, 4192−4200. (h) Nakashima, S.; Isobe, H.;
Okuda, T.; Kai, M. Inorg. Chim. Acta 2000, 306, 215−221. (i) Dong,
T.-Y.; Ho, P.-H.; Lai, X.-Q.; Lin, Z.-W.; Lin, K.-J. Organometallics
2000, 19, 1096−1106. (j) Cowan, D. O.; Kaufman, F. J. Am. Chem.
Soc. 1970, 92, 219−220. (k) Cowan, D. O.; Levanda, C. J. Am. Chem.
Soc. 1972, 94, 9271−9272. (l) Iijima, S.; Saida, R.; Motoyama, I.; Sano,
H. Bull. Chem. Soc. Jpn. 1981, 54, 1375−1379. (m) Boehm, M. C.;
Gleiter, R.; Delgado-Pena, F.; Cowan, D. O. J. Chem. Phys. 1983, 79,
1154−1165. (n) Dong, T. Y.; Hendrickson, D. N.; Iwai, K.; Cohn, M.
J.; Geib, S. J.; Rheingold, A. L.; Sano, H.; Motoyama, I.; Nakashima, S.
J. Am. Chem. Soc. 1985, 107, 7996−8008.
Nowik, I.; Pichlmaier, M.; Bolte, M.; Lerner, H.-W.; Winter, R. F.;
Herber, R. H.; Wagner, M. Dalton Trans. 2009, 2940−2950.
(8) (a) Siemeling, U.; Jutzi, P.; Bill, E.; Trautwein, A. X. J. Organomet.
Chem. 1993, 463, 151−154. (b) Venkatasubbaiah, K.; Doshi, A.;
Nowik, I.; Herber, R. H.; Rheingold, A. L.; Jakle, F. Chem.Eur. J.
2008, 14, 444−458. (c) Venkatasubbaiah, K.; Nowik, I.; Herber, R. H.;
Jaekle, F. Chem. Commun. 2007, 2154−2156. (d) Venkatasubbaiah, K.;
Pakkirisamy, T.; Lalancette, R. A.; Jakle, F. Dalton Trans. 2008, 4507−
4513. (e) McAdam, C. J.; Robinson, B. H.; Simpson, J.; Tagg, T.
Organometallics 2010, 29, 2474−2483.
(9) (a) Chen, Y. J.; Pan, D.-S.; Chiu, C.-F.; Su, J.-X.; Lin, S. J.; Kwan,
K. S. Inorg. Chem. 2000, 39, 953−958. (b) Patoux, C.; Coudret, C.;
Launay, J.-P.; Joachim, C.; Gourdon, A. Inorg. Chem. 1997, 36, 5037−
5049. (c) Ribou, A.-C.; Launay, J.-P.; Sachtleben, M. L.; Li, H.;
Spangler, C. W. Inorg. Chem. 1996, 35, 3735−3740. (d) Dong, T. Y.;
Ke, T. J.; Peng, S. M.; Yeh, S. K. Inorg. Chem. 1989, 28, 2103−2106.
(e) Fry, A. J.; Jain, P. S.; Krieger, R. L. J. Organomet. Chem. 1981, 214,
381−390.
(10) (a) Oton, F.; Ratera, I.; Espinosa, A.; Tarraga, A.; Veciana, J.;
Molina, P. Inorg. Chem. 2010, 49, 3183−3191. (b) Kurosawa, M.;
Nankawa, T.; Matsuda, T.; Kubo, K.; Kurihara, M.; Nishihara, H. Inorg.
Chem. 1999, 38, 5113−5123.
(11) (a) Levanda, C.; Bechgaard, K.; Cowan, D. O. J. Org. Chem.
1976, 41, 2700−2704. (b) Powers, M. J.; Meyer, T. J. J. Am. Chem. Soc.
1978, 100, 4393−4398. (c) Plenio, H.; Hermann, J.; Sehring, A.
Chem.Eur. J. 2000, 6, 1820−1829. (d) Blackbourn, R. L.; Hupp, J. T.
Chem. Phys. Lett. 1988, 150, 399−405. (e) Cohn, M. J.; Timken, M.
D.; Hendrickson, D. N. J. Am. Chem. Soc. 1984, 106, 6683−6689.
(12) (a) Kadish, K. M.; Xu, Q. Y.; Barbe, J. M. Inorg. Chem. 1987, 26,
2565−2566. (b) Xu, Q. Y.; Barbe, J. M.; Kadish, K. M. Inorg. Chem.
1988, 27, 2373−2378. (c) Maiya, G. B.; Barbe, J. M.; Kadish, K. M.
Inorg. Chem. 1989, 28, 2524−2527. (d) Solntsev, P. V.; Sabin, J. R.;
Dammer, S. J.; Gerasimchuk, N. N.; Nemykin, V. N. Chem. Commun.
2010, 6581−6583.
(13) (a) Figueira-Duarte, T. M.; Lloveras, V.; Vidal-Gancedo, J.;
Gegout, A.; Delavaux-Nicot, B.; Welter, R.; Veciana, J.; Rovira, C.;
Nierengarten, J.-F. Chem. Commun. 2007, 4345−4347. (b) Santi, S.;
Orian, L.; Durante, C.; Bencze, E. Z.; Bisello, A.; Donoli, A.; Ceccon,
A.; Benetollo, F.; Crociani, L. Chem.Eur. J. 2007, 13, 7933−7947.
(c) Horikoshi, T.; Kubo, K.; Nishihara, H. J. Chem. Soc., Dalton Trans.
1999, 3355−3360. (d) Mueller-Westerhoff, U. T.; Eilbracht, P. J. Am.
Chem. Soc. 1972, 94, 9272−92744. (e) Morrison, W. H., Jr.;
Hendrickson, D. N. J. Chem. Phys. 1973, 59, 380−386. (f) Delgado-
Pena, F.; Talham, D. R.; Cowan, D. O. J. Organomet. Chem. 1983, 253,
C43−C46. (g) Cohn, M. J.; Timken, M. D.; Hendrickson, D. N. J. Am.
Chem. Soc. 1984, 106, 6683−6689. (h) Jiao, J.; Long, G. J.; Rebbouh,
L.; Grandjean, F.; Beatty, A. M.; Fehlner, T. P. J. Am. Chem. Soc. 2005,
127, 17819−17831. (i) McAdam, C. J.; Brunton, J. J.; Robinson, B. H.;
Simpson, J. J. Chem. Soc., Dalton Trans. 1999, 2487−2496. (j) Diallo,
A. K.; Absalon, C.; Ruiz, J.; Astruc, D. J. Am. Chem. Soc. 2011, 133,
629−641. (k) Ochi, Y.; Suzuki, M.; Imaoka, T.; Murata, M.; Nishihara,
H.; Einaga, Y.; Yamamoto, K. J. Am. Chem. Soc. 2010, 132, 5061−
5069. (l) Sakamoto, R.; Murata, M.; Nishihara, H. Angew. Chim., Int.
Ed 2006, 45, 4793−4795. (m) Boyd, D. A.; Cao, Z.; Song, Y.; Wang,
T.-W.; Fanwick, P. E.; Crutchley, R. J.; Ren, T. Inorg. Chem. 2010, 49,
11525−11531. (n) Xu, G.-L.; Crutchley, R. J.; DeRosa, M. C.; Pan, Q.-
J.; Zhang, H.-X.; Wang, X.; Ren, T. J. Am. Chem. Soc. 2005, 127,
13354−13363.
(14) (a) Jin, Z.; Nolan, K.; McArthur, C. R.; Lever, A. B. P.; Leznoff,
C. C. J. Organomet. Chem. 1994, 468, 205−212. (b) Poon, K.-W.; Yan,
Y.; Li, X. Y.; Ng, D. K. P. Organometallics 1999, 18, 3528−3533.
(c) Nemykin, V. N.; Lukyanets, E. A. ARKIVOC 2010, i, 136−208.
(d) Nemykin, V. N.; Kobayashi, N. Chem. Commun. 2001, 165−166.
(e) Lukyanets, E. A.; Nemykin, V. N. J. Porphyrins Phthalocyanines
2010, 14, 1−40. (f) Gonzalez-Cabello, A.; Claessens, C. G.; Martin-
Fuch, G.; Ledoux-Rack, I.; Vazquez, P.; Zyss, J.; Agullo-Lopez, F.;
Torres, T. Synth. Met. 2003, 137, 1487−1488. (g) Loim, N. M.;
Abramova, N. V.; Sokolov, V. I. Mendeleev Commun. 1996, 46−47.
(h) Burrell, A. K.; Campbell, W. M.; Jameson, G. B.; Officer, D. L.;
(7) (a) Kramer, J. A.; Herbstein, F. H.; Hendrickson, D. N. J. Am.
Chem. Soc. 1980, 102, 2293−2301. (b) Jones, S. C.; Barlow, S.;
O’Hare, D. Chem.Eur. J. 2005, 11, 4473−4481. (c) Rulkens, R.;
Lough, A. J.; Manners, I.; Lovelace, S. R.; Grant, C.; Geiger, W. E. J.
Am. Chem. Soc. 1996, 118, 12683−12695. (d) Yuan, Y.-F.; Zhang, L.-
Y.; Hu, A.-G.; Wang, J.-T.; Liu, W.-Y.; Ding, T.-Z. Polyhedron 1999, 18,
1247−1251. (e) Kaufmann, L.; Breunig, J.-M.; Vitze, H.; Schoedel, F.;
K
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Boyd, P. D. W.; Zhao, Z.; Cocks, P. A.; Gordon, K. C. Chem. Commun.
1999, 637−638. (i) Shoji, O.; Okada, S.; Satake, A.; Kobuke, Y. J. Am.
Chem. Soc. 2005, 127, 2201−2210. (j) Kubo, M.; Mori, Y.; Otani, M.;
Murakami, M.; Ishibashi, Y.; Yasuda, M.; Hosomizu, K.; Miyasaka, H.;
Imahori, H.; Nakashima, S. J. Phys. Chem. A 2007, 111, 5136−5143.
(k) Nemykin, V. N.; Barrett, C. D.; Hadt, R. G.; Subbotin, R. I.;
Maximov, A. Y.; Polshin, E. V.; Koposov, A. Y. Dalton Trans. 2007,
3378−3389. (l) Nemykin, V. N.; Galloni, P.; Floris, B.; Barrett, C. D.;
Hadt, R. G.; Subbotin, R. I.; Marrani, A. G.; Zanoni, R.; Loim, N. M.
Dalton Trans. 2008, 4233−4246. (m) Nemykin, V. N.; Rohde, G. T.;
Barrett, C. D.; Hadt, R. G.; Bizzarri, C.; Galloni, P.; Floris, B.; Nowik,
I.; Herber, R. H.; Marrani, A. G.; Zanoni, R.; Loim, N. M. J. Am. Chem.
Soc. 2009, 131, 14969−14978. (n) Galloni, P.; Floris, B.; de Cola, L.;
Cecchetto, E.; Williams, R. M. J. Phys. Chem. C 2007, 111, 1517−1523.
(o) Nemykin, V. N.; Rohde, G. T.; Barrett, C. D.; Hadt, R. G.; Sabin, J.
R.; Reina, G.; Galloni, P.; Floris, B. Inorg. Chem. 2010, 49, 7497−7509.
(p) Rohde, G. T.; Sabin, J. R.; Barrett, C. D.; Nemykin, V. N. New J.
Chem. 2011, 35, 1440−1448. (q) Vecchi, A.; Gatto, E.; Floris, B.;
Conte, V.; Venanzi, M.; Nemykin, V. N.; Galloni, P. Chem. Commun.
2012, 48, 5145−5147. (r) Sharma, R.; Gautam, P.; Mobin, S. M.;
Misra, R. Dalton Trans. 2013, 42, 5539−5545. (s) Samanta, S.; Mittra,
K.; Sengupta, K.; Chatterjee, S.; Dey, A. Inorg. Chem. 2013, 52, 1443−
1453. (t) Bakar, M. A.; Sergeeva, N. N.; Juillard, T.; Senge, M. O.
Organometallics 2011, 30, 3225−3228. (u) Subbaiyan, N. K.;
Wijesinghe, C. A.; D’Souza, F. J. Am. Chem. Soc. 2009, 131, 14646−
14647. (v) Burrell, A. K.; Campbell, W.; Officer, D. L. Tetrahedron
Lett. 1997, 38, 1249−1252. (w) Wang, H. J.H.; Jaquinod, L.;
Olmstead, M. M.; Vicente, M. G. H.; Kadish, K. M.; Ou, Z.; Smith,
K. M. Inorg. Chem. 2007, 46, 2898−2913. (x) Vecchi, A.; Galloni, P.;
Floris, B.; Nemykin, V. N. J. Porphyrins Phthalocyanines 2013, 17, 165−
196.
(22) (a) Rokach, J.; Hamel, P.; Kakushima, M.; Smith, G. M.
Tetrahedron Lett. 1981, 22, 4901−4904. (b) Kakushima, M.; Hamel,
P.; Frenette, R.; Rokach, J. J. Org. Chem. 1983, 48, 3214−3219.
(23) Bray, B. L.; Mathies, P. H.; Naef, R.; Solas, D. R.; Tidwell, T. T.;
Artis, D. R.; Muchowski, J. M. J. Org. Chem. 1990, 55, 6317−6328.
(24) Corey, E. J.; Snider, B. B. J. Am. Chem. Soc. 1972, 94, 6190−
6192.
(25) Beletskaya, I. P.; Tsvetkov, A. V.; Latyshev, G. V.; Tafeenko, V.
A; Lukashev, N. V. J. Organomet. Chem. 2001, 637−639, 653−663.
(26) (a) Solntsev, P. V.; Dudkin, S. V.; Sabin, J. R.; Nemykin, V. N.
Organometallics 2011, 30, 3037−3046. (b) Solntsev, P. V.; Goetsch, W.
R.; Nemykin, V. N. Organometallics 2011, 30, 6636−6640.
(c) Hildebrandt, A.; Lehrich, S. W.; Schaarschmidt, D.; Jaeschke, R.;
Schreiter, K.; Spange, S.; Lang, H. Eur. J. Inorg. Chem. 2012, 1114−
1121.
(27) (a) Klimova, E. I.; Klimova, T.; Backinowsky, L. V.; Flores-
Alamo, M.; Ortiz-Frade, L. A.; Garcia, M. M. Mendeleev Commun.
2010, 20, 312−313. (b) Klimova, E. I.; Beretsneva, T. K.; Toscano, R.
A.; Stivalet, J. M. M.; Garcia, M. M. Synth. Commun. 2007, 37, 889−
900.
(28) (a) D’Alessandro, D. M.; Keene, F. R. Chem. Rev. 2006, 106,
2270−2298. (b) D’Alessandro, D. M.; Keene, F. R. Dalton Trans.
2004, 3950−3954.
(29) (a) Sabouraud, G.; Sadki, S.; Brodie, N. Chem. Soc. Rev. 2000,
29, 283−293. (b) Clark, J. C.; Fabre, B.; Fronczek, F. R.; Vicente, M.
G. H. J. Porphyrins Phthalocyanines 2005, 9, 803−810. (c) Graczyk-
Zajac, M.; Vassiliev, S. Yu.; Vorotyntsev, M. A.; Tsirlina, G. A. J. Solid
State Electrochem. 2010, 14, 2039−2048. (d) Martinez, F. J.; Gonzalez,
B.; Alonso, B.; Losada, J.; Garcia-Armada, M. P.; Casado, C. M. J.
Inorg. Organomet. Polym. Mater. 2008, 18, 51−58. (e) Ouerghi, O.;
Senillou, A.; Jaffrezic-Renault, N.; Martelet, C.; Ben Ouada, H.;
Cosnier, S. J. Electroanal. Chem. 2001, 501, 62−69.
(15) (a) Hildebrandt, A.; Schaarschmidt, D.; Lang, H. Organo-
metallics 2011, 30, 556−563. (b) Hildebrandt, A.; Schaarschmidt, D.;
Claus, R.; Lang, H. Inorg. Chem. 2011, 50, 10623−10632.
(c) Hildebrandt, A.; Lang, H. Dalton Trans. 2011, 40, 11831−
11837. (d) Speck, J. M.; Claus, R.; Hildebrandt, A.; Rueffer, T.;
Erasmus, E.; van As, L.; Swarts, J. C.; Lang, H. Organometallics 2012,
31, 6373−6380. (e) Hildebrandt, A.; Ruffer, T.; Erasmus, E.; Swarts, J.
C.; Lang, H. Organometallics 2010, 29, 4900−4905. (f) Miesel, D.;
Hildebrandt, A.; Korb, M.; Low, P. J.; Lang, H. Organometallics 2013,
32, 2993−3002. (g) Pfaff, U.; Hildebrandt, A.; Schaarschmidt, D.;
(30) (a) Hush, N. S. Prog. Inorg. Chem. 1967, 8, 391−444. (b) Creutz,
C. Prog. Inorg. Chem. 1983, 30, 1−73. (c) Hush, N. S. Coord. Chem.
Rev. 1985, 64, 135−157.
(31) Robin, M. B.; Day, P. Adv. Radiochem. 1967, 10, 247−422.
́
(32) (a) Domingo, L. R.; Perez, P.; Contreras, R. J. Org. Chem. 2003,
68, 6060−6062. (b) Leffler, J. E.; Grunwald, E.Rates and Equilibria of
Organic Reactions; Wiley: New York, 1963.
(33) (a) Green, A. G.; Kiesz, M. D.; Oria, J. V.; Elliott, A. G.;
Buechler, A. K.; Hohenberger, J.; Meyer, K.; Zink, J. I.; Diaconescu, P.
L. Inorg. Chem. 2013, 52, 5603−5610. (b) Plazuk, D.; Zakrzewski, J.;
Nakatani, K.; Makal, A.; Wozniak, K.; Domagala, S. RSC Adv. 2012, 2,
3512−3524. (c) Huesmann, H.; Foerster, C.; Siebler, D.; Gasi, T.;
Heinze, K. Organometallics 2012, 31, 413−427. (d) Li, Y. L.; Han, L.;
Mei, Y.; Zhang, J. Z. H. Chem. Phys. Lett. 2009, 482, 217−222.
(e) Fabrizi de Biani, F.; Manca, G.; Marchetti, L.; Leoni, P.; Bruzzone,
S.; Guidotti, C.; Atrei, A.; Albinati, A.; Rizzato, S. Inorg. Chem. 2009,
48, 10126−10137. (f) Solntsev, P. V.; Spurgin, K. L.; Sabin, J. R.;
Heikal, A. A.; Nemykin, V. N. Inorg. Chem. 2012, 51, 6537−6547.
(g) Nemykin, V. N.; Hadt, R. G. J. Phys. Chem. A 2010, 114, 12062−
12066. (h) Nemykin, V. N.; Makarova, E. A.; Grosland, J. O.; Hadt, R.
G.; Koposov, A. Y. Inorg. Chem. 2007, 46, 9591−9601. (i) Nemykin, V.
N.; Maximov, A. Y.; Koposov, A. Y. Organometallics 2007, 26, 3138−
3148.
(34) (a) Mohammadi, N.; Ganesan, A.; Chantler, C. T.; Wang, F. J.
Organomet. Chem. 2012, 713, 51−59. (b) Zaater, S.; Brahimi, M.;
Rahmouni, A. Polyhedron 2012, 31, 29−36. (c) Zapata, F.; Caballero,
A.; Espinosa, A.; Tarraga, A.; Molina, P. J. Org. Chem. 2008, 73, 4034−
4044. (d) Zapata, F.; Caballero, A.; Espinosa, A.; Tarraga, A.; Molina,
P. Org. Lett. 2008, 10, 41−44. (e) D’Souza, F.; Chitta, R.; Gadde, S.;
Islam, D.-M. S.; Schumacher, A. L.; Zandler, M. E.; Araki, Y.; Ito, O. J.
Phys. Chem. B 2006, 110, 25240−25250. (f) Vrcek, V.; Buehl, M.
Organometallics 2006, 25, 358−367. (g) Siebler, D.; Foerster, C.;
Heinze, K. Dalton Trans. 2011, 40, 3558−3575. (h) Hadt, R. G.;
Nemykin, V. N. Inorg. Chem. 2009, 48, 3982−3992. (i) Herber, R. H.;
Nowik, I.; Grosland, J. O.; Hadt, R. G.; Nemykin, V. N. J. Organomet.
Chem. 2008, 693, 1850−1856. (j) Dammer, S. J.; Solntsev, P. V.;
Sabin, J. R.; Nemykin, V. N. Inorg. Chem. 2013, 52, 9496−9510.
Ruffer, T.; Low, P. J.; Lang, H. Organometallics 2013, 32, 6106−6117.
̈
(h) Hildebrandt, A.; Lang, H. Organometallics 2013, 32, 5640−5653.
(16) (a) Yang, H.-B.; Ghosh, K.; Zhao, Y.; Northrop, B. H.; Lyndon,
M. M.; Muddiman, D. C.; White, H. S.; Stang, P. J. J. Am. Chem. Soc.
2008, 130, 839−841. (b) Hildebrandt, A.; Schaarschmidt, D.; van As,
L.; Swarts, J. C.; Lang, H. Inorg. Chim. Acta 2011, 374, 112−118.
(c) Pfaff, U.; Hildebrandt, A.; Schaarschmidt, D.; Hahn, T.; Liebing, S.;
Kortus, J.; Lang, H. Organometallics 2012, 31, 6761−6771. (d) Lin, J.
T.; Yang, M.-F.; Tsai, C.; Wen, Y. S. J. Organomet. Chem. 1998, 564,
257−266. (e) Hildebrandt, A.; Wetzold, N.; Ecorchard, P.; Walfort, B.;
Rueffer, T.; Lang, H. Eur. J. Inorg. Chem. 2010, 3615−3627. (f) Inkpen,
M. S.; Albrecht, T.; Long, N. J. Organometallics 2013, 32, 6053−6060.
(17) (a) Chen, Y. J.; Pan, D. S.; Chiu, C. F.; Su, J. X.; Lin, S. J.; Kwan,
K. S. Inorg. Chem. 2000, 39, 953−958. (b) Huang, P.; Jin, B.; Liu, P.;
Cheng, L.; Cheng, W.; Zhang, S. J. Organomet. Chem. 2012, 697, 57−
64. (c) Steffens, S.; Prosenc, M. H.; Heck, J.; Asselberghs, I.; Clays, K.
Eur. J. Inorg. Chem. 2008, 1999−2006. (d) Kaleta, K.; Strehler, F.;
Hildebrandt, A.; Beweries, T.; Arndt, P.; Rueffer, T.; Spannenberg, A.;
Lang, H.; Rosenthal, U. Chem.Eur. J. 2012, 18, 12672−12680.
(e) Breuer, R.; Schmittel, M. Organometallics 2013, 32, 5980−5987.
(18) Anderson, H. J.; Loader, C. E. Synthesis 1985, 353−364.
(19) Loader, C. E.; Anderson, H. J. Can. J. Chem. 1981, 59, 2673−
2676.
(20) Carmona, O.; Greenhouse, R.; Landeros, R.; Muchowski, J. M. J.
Org. Chem. 1980, 45, 5336−5339.
(21) Anderson, H. J.; Loader, C. E.; Xu, R.; Le, N.; Gogan, N. J.;
McDonald, R.; Edwards, L. G. Can. J. Chem. 1985, 63, 896−902.
L
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(35) (a) Salzner, U. J. Chem. Theory Comput. 2013, 9, 4064−4073.
(b) Rinkevicius, Z.; Telyatnyk, L.; Salek, P.; Vahtras, O.; Agren, H. J.
Chem. Phys. 2003, 119, 10489−10496. (c) Stampfl, C.; Mannstadt, W.;
Asahi, R.; Freeman, A. J. Phys. Rev. B 2001, 63, 155106/1−155106/11.
(d) Barone, V.; Bencini, A.; Ciofini, I.; Daul, C. A.; Totti, F. J. Am.
Chem. Soc. 1998, 120, 8357−8365. (e) Chou, J. P.; Chen, H. Y. T.;
Hsing, C. R.; Chang, C. M.; Cheng, C.; Wei, C. M. Phys. Rev. B 2009,
80, 165412/1−165412/10. (f) Fromager, E.; Knecht, S.; Jensen, H. J.
A. J. Chem. Phys. 2013, 138, 084101/1−084101/14. (g) Namazian, M.;
Lin, C. Y.; Coote, M. L. J. Chem. Theory Comput. 2010, 6, 2721−2725.
(h) Martin, J.; Baker, J.; Pulay, P. J. Comput. Chem. 2009, 30, 881−883.
(i) Nemykin, V. N.; Hadt, R. G. Inorg. Chem. 2006, 45, 8297−8307.
(j) Nemykin, V. N.; Hadt, R. G.; Belosludov, R. V.; Mizuseki, H.;
Kawazoe, Y. J. Phys. Chem. A 2007, 111, 12901−12913. (k) Hadt, R.
G.; Nemykin, V. N.; Olsen, J. G.; Basu, P. Phys. Chem. Chem. Phys.
2009, 11, 10377−10384.
(36) Barriere, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980−
3989.
(37) Desales, J.; Greenhouse, R.; Muchowski, J. M. J. Org. Chem.
1982, 47, 3668−3672.
(38) Adamo, C.; Barone, V. J. Chem. Phys. 1999, 110, 6158−6169.
(39) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee,
C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789.
(40) (a) Becke, A. D. Phys. Rev. A 1988, 38, 3098−3100. (b) Perdew,
J. P. Phys. Rev. B 1986, 33, 8822−8824.
(41) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244−13249.
(42) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033.
(43) (a) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72,
5639−5648. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299−
310.
(44) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257−2261.
(45) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; et al. Gaussian 09,
Revision A.01; Gaussian, Inc.: Wallingford, CT, 2009. For full citation,
please see Supporting Information.
(46) Nemykin,V. N.; Basu, P. VMOdes Program, Revision A 7.2;
University of Minnesota Duluth and Duquesne University, 2001;
2003; 2005.
(47) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.;
Burla, M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27,
435.
(48) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.
(49) Burnett, M. N.; Johnson, C. K. ORTEP-III: Oak Ridge Thermal
Ellipsoid Plot Program for Crystal Structure Illustrations; Oak Ridge
National Laboratory Report ORNL-6895; 1996.
M
dx.doi.org/10.1021/om400901w | Organometallics XXXX, XXX, XXX−XXX