Molecular wires using (Oligo)pyrroles as connecting units: An electron transfer study

Article abstract of DOI:10.1021/om4007533

A series of (oligo)pyrroles featuring redox-active terminal ferrocenyl groups (Fc₂-(^cC₄H₂NPh)_n (4, n = 1; 9, n = 2; 16, n = 3; 20, n = 4)) has been prepared using a Negishi C,C cross-coupling reaction

Full text of DOI:10.1021/om4007533

Article
pubs.acs.org/Organometallics
Molecular Wires using (Oligo)pyrroles as Connecting Units: An
Electron Transfer Study
Ulrike Pfaff,† Alexander Hildebrandt,† Dieter Schaarschmidt,† Tobias Ruffer,† Paul J. Low,‡
̈
and Heinrich Lang*,†
†Institute of Chemistry, Inorganic Chemistry, Faculty of Natural Sciences, Technische Universitat Chemnitz, D-09107 Chemnitz,
̈
Germany
‡School of Chemistry and Biochemistry, University of Western Australia, Crawley, Perth, Western Australia, Australia
S
* Supporting Information
ABSTRACT: A series of (oligo)pyrroles featuring redox-active terminal
ferrocenyl groups (Fc2-(cC4H2NPh)n (4, n = 1; 9, n = 2; 16, n = 3; 20, n
= 4)) has been prepared using a Negishi C,C cross-coupling reaction
protocol. The bi-, ter-, and quaterpyrrole wire moieties have been built
up by C,C cross-coupling reactions of trimethylsilyl-protected pyrrole
units in the presence of [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 as
precatalyst. The structural properties of the title compounds were
investigated by spectroscopic means and single-crystal X-ray diffraction
studies (9, 16, and 20). The influence of the increasing number of N-
phenylpyrrole units on the electronic interaction between the iron
centers was studied using electrochemistry (cyclic (CV) and square wave
voltammetry (SWV)) as well as spectroelectrochemistry (in situ UV/vis/
near-IR spectroscopy). With the exception of the diferrocenyl
quaterpyrrole 20, the application of [NnBu4][B(C6F5)4] as electrolyte allows the discrete oxidation of the ferrocenyl termini
(ΔE°′ = 450 mV (4), ΔE°′ = 320 mV (9), ΔE°′ = 165 mV (16)) in cyclic and square wave voltammograms. However, the iron
centers of 20 were oxidized simultaneously, generating dicationic 202+. Additionally, one (9) or two (16 and 20) pyrrole-related
well-defined reversible one-electron-redox processes were observed. The cyclic voltammetry data reveal that the splitting of the
ferrocene-based redox couples, ΔE°′, decreases with increasing oligopyrrole chain length and, hence, a greater metal−metal
distance. The trends in ΔE°′ with oligopyrrole structure also map to the electronic coupling between the ferrocene moieties, as
estimated by spectroelectrochemical UV/vis/near-IR measurements. Despite the fact that there is no direct metal−metal
interaction in diferrocenyl quaterpyrrole 20, a large absorption in the near-IR region is observed arising from photoinduced
charge transfer from the oligopyrrole backbone to the redox-active ferrocenyl termini. These charge transfer absorptions have
also been found in the dicationic oxidation state of the mono-(4), bi- (9), and terpyrroles (16). Within this series of
diferrocenyl(oligo)pyrroles this CT band is shifted bathochromically with increasing chain length of the backbone motif.
i.e. diferrocenyl polyenes,13 arenes,14−17 cumulenes,18 and
INTRODUCTION
■
oligo(phenylenevinylenes),19 and the redox properties and
During the last four decades significant interest has been
focused on π-conjugated organic materials, which in addition to
offering useful electronic and optical properties, for example,
electronic structures have been correlated with the wirelike
behavior of the bridge. Of particular relevance to the present
study are Sato’s ter- to sexithiophene bridged bis-ferrocene
complexes.20,21 These molecules were accessible by the Negishi
cross-coupling of FcZnCl with the appropriate iodo com-
pounds. However, despite the intuitively appealing chemical
structure, no direct interaction of the ferrocenyl units through
the oligo(thiophene) chain was observed.
In a recent study we were able to show that the electron-rich
pyrrole connecting unit is superior to the thiophene molecule
in terms of promoting electronic interactions between
conductivity and electrochromism, are also finding application
in the growing field of molecular electronics.1−6
A convenient method through which to assess the effective
delocalization of the π-electron density in these extended
organic materials is to assess the mixed-valence characteristics
of model complexes in which the target organic moiety serves
as a bridging unit between two (or more) redox-active
functionalities such as metallocenes7,8 and iron7,9 or ruthe-
nium9,10 half-sandwich compounds. Due to the excellent
thermal stability and electrochemical reversibility of the
ferrocene (Fe(II)/Fe(III)) redox couple, ferrocene is well-
suited to such investigations.11,12 Many such ferrocene-based
model systems with two redox-active termini have been
described featuring a variety of π-conjugated organic bridges,
Special Issue: Ferrocene - Beauty and Function
Received: July 29, 2013
Published: September 5, 2013
© 2013 American Chemical Society
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a
Scheme 1. Synthesis of Oligopyrroles 2, 8, 14, and 18
a
Reagents and conditions: (i) 60 °C, 12 h, tetrahydrofuran, 0.25 mol % [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2. NBS = N-bromosuccinimide.
ferrocenyl/ferrocenium termini.22 In that earlier work, a series
Scheme 2. Synthesis of Diferrocenyl (Oligo)pyrroles 4, 9, 16,
and 20
a
of 2,5-diferrocenyl-substituted furans, thiophenes, and pyrroles
were prepared and studied by electrochemical and spectroelec-
trochemical methods. The ferrocenyl moieties of these
compounds showed two well-separated reversible one-
electron-redox processes. The separation of the redox
potentials of 2,5-diferrocenyl-N-phenylpyrrole (ΔE°′ = 450
mV, supporting electrolyte [NnBu4][B(C6F5)4]) was found to
be much greater than those of 2,5-diferrocenylthiophene (ΔE°′
= 260 mV) under the same measurement conditions.16,22,23
Given the similar solvation, ion pairing, and inner- and outer-
sphere reorganization energies, the ΔE°′ values likely track
reasonably well with the magniture of “electronic coupling”
between the redox centers.24 This prompted us to investigate
model systems for molecular wires containing oligo(pyrrole)
units, which might offer the possibility to observe electron
transfer between the ferrocene-based redox centers through
relatively long oligo-heterocyclic backbones.
a
Reagents and conditions: (i): 60 °C, 12 h, tetrahydrofuran, 0.25 mol
% [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2. NBS = N-bromosuccini-
mide.
order to synthesize ter- (14) and quaterpyrroles (18), Me3Si-
protected bromo oligo(pyrroles) 10−12 were used in the cross-
coupling reactions (Scheme 1). Deprotection to give 14 and 18
was carried out with an excess of tetra-n-butylammonium
fluoride under moderate conditions (Experimental Section).
The introduction of the ferrocenyl substituents was realized
by the convenient synthetic methodology shown in Scheme 2.
Compound 1 and the fluorescent (oligo)pyrroles 7, 14, and 18
were treated with 2 equiv of N-bromosuccinimide, resulting in
the formation of dibrominated 2, 8, 15, and 19.25 Ferrocene
was monolithiated according to the procedures reported by
We herein describe the synthesis of a series of diferrocenes
bearing (oligo)pyrrole connecting units. (Spectro)-
electrochemical methods have been used to examine the effect
of an increasing distance between the ferrocenyl moieties on
the intermetallic communication of these complexes.
RESULTS AND DISCUSSION
Muller-Westerhoff27 followed by treatment with [ZnCl2·2thf]
■
̈
to give FcZnCl (3). Subsequent reaction of 3 with 1/3 equiv of
the dibromo compounds 2, 8, 15 and 19 under Negishi C,C
cross-coupling conditions and appropriate workup gave the
(oligo)pyrroles 4, 9, 16, and 20 as orange solids in moderate to
good yields (Experimental Section).
Synthesis and Characterization. Diferrocenyl (oligo)-
pyrroles Fc2-(cC4H2NPh)n (4, n = 1; 9, n = 2; 16, n = 3; 20, n =
4) have been prepared in two steps (Schemes 1 and 2),
involving assembly of the (oligo)pyrrole backbone (8, 14, and
18) followed by the attachment of the ferrocenyl termini.
Initially, N-phenylpyrrole was treated with 1 or 2 equiv of N-
bromosuccinimide, as described by Gilow et al.25 The
electrophilic bromination selectively forms mono- or dihalo-
genated pyrroles (2, 5). Bipyrrole 7 was synthesized by a
Negishi C,C cross-coupling reaction of 2-bromo-N-phenyl-
pyrrole (5) with the zinc species 2-ZnCl-cC4H3NPh (6), using
[Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 as precatalyst (Scheme
1).17,26 The workup and further reactions were carried out in
neutral or alkaline media to prevent protonation, which in turn
causes decomposition or polymerization of the molecules. In
The diferrocenyl (oligo)pyrroles 4, 9, 16, and 20 are stable to
air and moisture both in the solid state and in solution. They
are poorly soluble in nonpolar solvents such as n-hexane,
diethyl ether, and toluene but show good solubility in
dichloromethane and tetrahydrofuran. Each of the compounds
has been identified by NMR (1H, 13C{1H}) and IR spectros-
copy as well as elemental analysis. In addition, high-resolution
ESI-TOF mass spectrometric measurements and single-crystal
X-ray diffraction studies (9, 16, and 20) have been carried out.
The electrochemical behavior of 4, 9, 16, and 20 was
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Figure 1. ORTEP diagram (50% probability level) of the molecular structure of 9 with the atom-numbering scheme. Hydrogen atoms have been
omitted for clarity. Selected bond distances (Å) and angles (deg) and torsion angles (deg): average D−Fe = 1.647, N1−C14 = 1.383(3), C11−C12
= 1.368(3), C12−C13 = 1.396(3), C13−C14 = 1.370(3), C14−C21 = 1.462(3), C10−C11 = 1.464(3), C24−C31 = 1.467(3), N1−C15 = 1.436(3),
N2−C25 = 1.423(3); C11−C12−C13 = 108.6(2), C11−N1−C14 = 108.7(2), N1−C14−C21 = 122.5(2), N1−C11−C12 = 107.1(2), average D−
Fe−D = 178.3; N1−C14−C21−N2 = −155.8(2), C13−C14−C21−N2 = 33.9(4), C10−C11−N1−C15 = 1.0(4), C11−C12−C13−C14 = −0.3(3),
N1−C14−C21−C22 = 36.1(4) (D denotes the centroids of C5H4 and C5H5).
Figure 2. ORTEP diagram (50% probability level) of the molecular structure of 16 with the atom-numbering scheme. Hydrogen atoms and solvent
molecules have been omitted for clarity. Selected bond distances (Å) and angles (deg) and torsion angles (deg): average D−Fe = 1.666, N1−C33 =
1.425(7), N2−C39 = 1.453(6), N3−C45 = 1.443(6), N1−C11 = 1.394(6), N1−C14 = 1.407(6), C11−C12 = 1.373(7), C12−C13 = 1.405(7),
C13−C14 = 1.380(7), C10−C11 = 1.468(7), C14−C15 = 1.446(7), C18−C19 = 1.426(7), C22−C23 = 1.449(7); average D−Fe−D = 177.5, C11−
C12−C13 = 108.6(4), C11−N1−C14 = 108.8(4), N2−C18−C29 = 123.7(5), C18−N2−C39 = 121.4(4), C10−C11−N1 = 125.8(5); N1−C14−
C15−N2 = 156.0(5), N2−C18−C19−N3 = 149.9(5), C10−C11−N1−C33 = 5.6(8), C11−C12−C13−C14 = 0.3(6), N1−C14−C15−C16 =
−26.3(8) (D denotes the centroids of C5H4 and C5H5).
determined by cyclic voltammetry (CV) and square wave
voltammetry (SWV), and the redox products were studied
further by in situ UV/vis/near-IR spectroelectrochemical
methods.
more informative, with characteristic patterns of phenylpyrrole
resonances clearly indicating the different number of pyrrole
units. The terpyrroles 13−16 show a singlet for the protons of
the middle pyrrole core between 5.72 and 6.36 ppm
(Experimental Section). However, the bipyrroles 8−10 and
quaterpyrroles 17, 19, and 20 give two and four doublets
corresponding to the CH protons of the pyrrole ring.
Furthermore, these resonances are slightly shifted to higher
field with increasing chain length. As expected, the ferrocenyl
units with their C5H5 and C5H4 cyclopentadienyl rings give rise
The IR spectra are less meaningful for the determination of
the (oligo)pyrrolic chain length, since they only show typical
absorptions for the pyrrole moieties, the phenyl ring, and the
ferrocenyl units. The Me3Si-protected oligopyrroles 10, 11, 13,
and 17 show typical symmetric (νs‑CH3) and asymmetric
(νas‑CH3) stretching vibrations below 3000 cm−1 (Experimental
1
Section). In contrast to the IR spectra, H NMR spectra are
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Figure 3. ORTEP diagram (50% probability level) of the molecular structure of 20 with the atom-numbering scheme. Hydrogen atoms have been
omitted for clarity. Selected bond distances (Å) and angles (deg) and torsion angles (deg): average D−Fe = 1.653, N1−C15 = 1.426(4), N2−C25 =
1.428(3), N3−C35 = 1.417(4), N4−C45 = 1.432(3), C6−C11 = 1.453(4), C11−N1 = 1.391(4), C11−C12 = 1.374(4), C12−C13 = 1.411(4),
C13−C14 = 1.366(4), C14−N1 = 1.394(4), C14−C21 = 1.474(4), C24−C31 = 1.475(4), C34−C41 = 1.463(4), C44−C51 = 1.401(7); average
D−Fe−D = 177.6, C11−C12−C13 = 108.1(3), C11−N1−C14 = 109.1(2), N2−C24−C31 = 123.1(2), N3−C34−C41 = 122.6(3), N4−C44−C51
= 129.9(5), C6−C11−N1 = 125.1(3); C31−C32−C33−C34 = 0.0(3), N1−C14−C21−N2 = 98.9(3), N2−C24−C31−N3 = 69.3(4), N3−C34−
C41−N4 = 75.5(4), C7−C6−C11−N1 = −8.7(5), C35−N3−C34−C41 = −5.3(4), C43−C44−C51−C55 = 166.5(7) (D denotes the centroids of
C5H4 and C5H5).
to one singlet (C5H5) and two pseudotriplets (C5H4, AA′XX′
spin system, J = 1.80 Hz) in each case (Experimental Section).
The structures of the diferrocenyl-substituted oligopyrroles 9,
16, and 20 in the solid state have been determined by single-
crystal X-ray diffraction analysis. Suitable crystals were obtained
by diffusion of methanol (9 and 16) or toluene (20) into a
dichloromethane solution containing the respective compound
at ambient temperature. Important bond distances (Å), bond
angles (deg), and torsion angles (deg) are summarized in the
captions of Figures 1−3, while crystal and structure refinement
data are contained in the Experimental Section. The
delocalized systems such as benzene evince a τ value of 1,
while for localized systems τ approaches 0.22 For the
oligopyrroles average τ values of 0.799 (9), 0.878 (16), and
0.803 (20) were obtained, which are in the same range as for
unsubstituted pyrrole (0.830)29 and 2,3,4,5-tetraferrocenyl-1-
phenyl-1H-pyrrole (0.832).23
In the solid state the oligopyrroles adopt a conformation in
which the phenyl substituents point in different directions; in
addition, the pyrrole cores are not coplanar, as interplanar
angles of 36.2(2), 34.0(2)° (9), 27.2(3), 38.3(3)° (16) and
83.1(1), 70.0(1), 77.7(2)° (20) are observed. The CC bond
c
oligopyrroles crystallize in the triclinic space group P1 (9 and
̅
distances between the C4N heterocycles are not affected by
this torsion and are similar to those in related compounds.30,31
The ferrocenyl moieties in these compounds adopt an eclipsed
conformation (4.9(2)° (Fe1), −3.2(3)° (Fe2), 6.0(3)° (Fe3),
−9.3(3)° (Fe4), 9; 10.8(4)° (Fe1), 11.3(5)° (Fe2), 16; 4.2(3)°
(Fe1), −1.1(8)° (Fe2), 20) and are rotated by 21.1(1)° (Fe1)
and 35.8(2)° (Fe2) (9), 22.1(4)° (Fe1) and 27.9(3)° (Fe2)
(16), and 9.8(3)° (Fe1) and 12.5(5)° (Fe2) (20) out of the
plane of the adjacent pyrrole.
Electrochemistry. The redox properties of oligopyrroles 7,
13, and 17 and diferrocenyl-substituted (oligo)pyrroles 4, 9, 16,
and 20 have been determined by cyclic voltammetry and
square-wave voltammetry (Figures 4 and 5) and the redox
products studied further by UV/vis/near-IR spectroscopy using
spectroelectrochemical techniques (Figures 6 and 7 and Figures
SI1, SI2, SI4, and SI5 (Supporting Information)). Dichloro-
methane solutions containing the analyte (1.0 mmol L−1) and
[NnBu4][B(C6F5)4] (0.1 mol L−1)32 as supporting electrolyte
were used for all measurements. The cyclic voltammetry
20) and in the monoclinic space group P21/c (16). In the case
of bipyrrole 9, the asymmetric unit contains two independent
molecules. Each of the individual pyrrole heterocyclic moieties
are essentially planar, with rms deviations ranging from 0.0004
to 0.0066 Å (highest deviation observed for N1 (16) with
0.009(3) Å). Furthermore, electron delocalization within the
individual pyrrole moieties can be inferred from the CC bond
lengths, which fall between the distances of isolated single
(d0C−C = 1.54 Å)28 and double bonds (d0C−C = 1.34 Å).28 The
extent of delocalization can be expressed by calculating the
parameter τ as the normalized quotient of the single- and
double-bond lengths (eq 1, where d0C−C = 1.54 Å, d0CC = 1.34
(dC−C/dCC) − 1
τ = 1 +
0
1 − (dC−C0/dCC
)
(1)
Å; dC−C = distance of the appropriate single bond, dCC
=
distance of the appropriate double bond).22 Completely
Table 1. Cyclic Voltammetric Data (Potentials vs FcH/FcH+, Scan Rate 100 mV s−1) at a Glassy-Carbon Electrode of 1.0 mmol
L−1 Solutions of the Analytes in Dry Dichloromethane Containing 0.1 mol L−1 of [NnBu4][B(C6F5)4] as Supporting Electrolyte
at 25 °C
b
c
b
c
b
c
b
c
d
e
compd
E°′1 (mV) (ΔEp (mV))
E°′2 (mV) (ΔEp (mV))
E°′3 (mV) (ΔEp (mV))
E°′4 (mV) (ΔEp (mV))
ΔE°′ (mV)
Kc
a
4
−238 (68)
−250 (74)
−230 (68)
−175 (90)
212 (75)
70 (74)
450
320
165
4.08 × 107
2.58 × 105
6.17 × 102
9
810 (74)
480 (74)
725 (63)
16
20
−65 (74)
265 (66)
1080 (72)
a
b
c
d
See ref 23. E°′ = formal potential. ΔEp = difference between oxidation and reduction potentials. ΔE°′ = potential difference between the two
e
ferrocenyl-related redox processes. KC = comproportionation constant.
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Figure 4. Voltammograms of dichloromethane solutions containing 1.0 mmol L−1 of diferrocenyl (oligo)pyrroles 4, 9, 16, and 20 at 25 °C
(supporting electrolyte [N(nBu)4][B(C6F5)4] (0.1 mol L−1): (left) cyclic voltammograms (scan rate 100 mV s−1); (right) square-wave
voltammograms (scan rate 1 mV s−1).
measurements recorded at a scan rate of 100 mV s−1 are
summarized in Table 1. All redox potentials are referenced to
the FcH/FcH+ redox couple (E°′ = 0.00 mV, FcH = Fe(η5-
C5H5)2).33
separation of the ferrocene-based redox processes.20,21 It seems
likely that the larger redox splitting in the ferrocenyl-
functionalized pyrroles 4, 9, and 16 is caused by the smaller
energy gap between the ferrocenyl moieties and the
heterocyclic bridging moiety as well as the larger delocalization
of the pyrrole unit itself.23 Therefore, the oligopyrroles may
provide a more effective conduit for intermetallic charge
transfer between the ferrocene units through the π-conjugated
backbone.
In addition to the ferrocenyl-related redox events, at higher
potential the oxidation of the pyrrole units could be observed
for 9, 16, and 20. With increasing length of the pyrrolic π
system this oxidation process is shifted to cathodic potentials.
This behavior is not unexpected, as shown by the example of
extended π systems such as benzene, naphthalene, and
anthracene, for which a similar potential shift could be
observed.37,38 To verify the assignment of these additional
redox events, electrochemical measurements of the non-
ferrocenyl-substituted oligopyrroles 7, 13, and 17 have been
carried out (Figure 5, Table 2).
The first cycle in the voltammogram of bipyrrole 7 shows
four irreversible oxidation processes, while further cycles give
only an irreversible oxidation wave at 400 mV. This chemical
behavior is not unexpected, as upon oxidation a reactive radical
cation is formed and polymerization as well as other side
reactions can occur easily through the 5- and 5′-positions. In
contrast, the cyclic voltammogram of the bis(trimethylsilyl)-
substituted quaterpyrrole 17 shows two well-separated
reversible one-electron-redox processes with a potential
difference of ΔE°′ = 200 mV, which is stable over more than
10 cycles. Extension of the oligo(pyrrole) π system leads to the
stabilization of the radical cation and suppresses the
decomposition.
Figure 4 shows the cyclic and square wave voltammograms of
diferrocenyl (oligo)pyrroles 4, 9, 16, and 20. The ferrocenyl
units of 4, 9, and 16 are oxidized subsequently, while for 20 one
reversible redox process was observed. The relatively high ΔEp
value (90 mV) and the larger Ic,p and Ic,a values of this redox
event (Figure 4) indicate that two individual oxidation
processes take place in a close potential range and hence are
not resolved.34 Due to the increasing chain length and,
therefore, the larger metal−metal distance, the redox splitting
ΔE°′ between the ferrocenyl moieties decreases from 450 mV
(4) to 320 mV (9) to 165 mV (16) (Table 1), and the
corresponding comproportionation constants KC (RT ln Kc =
ΔE°′F) have been determined (Table 1).35 The larger Kc values
are indicative of the high thermodynamic stability of the mixed-
valent species with respect to disproportionation. Hence, these
molecules are suitable for spectroelectrochemical investigations
(vide infra), as the chemical equilibrium (eq 2) favors the
mixed-valent form of the mono-oxidized compounds.
n+1 2
[M1nM2
]
°′
Kc =
= eΔE F/RT
[M1nM2 ][M1n+1M2
]
n
n+1
(2)
In constrast to the oligo(pyrrole) bridged bis(ferrocene)
complexes 4, 9, and 16, the analogous thiophene systems
showed little or no redox splitting between the terminal
ferrocenyl units (ΔE°′ = 260 mV (2,5-diferrocenylthiophene,
dichloromethane, [N(nBu)4][B(C6F5)4] as supporting electro-
lyte);22 ΔE°′ = 130 mV (5,5′-diferrocenyl-2,2′-bithiophene,
dichloromethane, [N(nBu)4][B(C6H3(CF3)2)4] as supporting
electrolyte)16,36). The Sato group has synthesized related ter-,
quater-, quinque-, and sexithiophenes (substituted in position 3
with hexyl or methoxy groups to increase the solubility) with
terminal ferrocenyl moieties to create model systems for
molecular wires, but these complexes also exhibited little or no
A comparison with other unsubstituted oligopyrroles
reported in the literature is difficult because the cyclic
voltammetry data have been determined under different
measurement conditions (acetonitrile and [NEt4][BF4] as
supporting electrolyte). Hapiot et al. could detect one
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increase of the potential allows, starting from the neutral
(oligo)pyrroles 4, 9, 16, 17, and 20, the in situ generation of
cationic 4n+,23 9n+, 16n+, 17n+, and 20(n+1)+ (n = 1, 2) (Figure 6
and Figures SI1, SI2, SI4, and SI5 (Supporting Information)).
The near-IR data of 4 and its analysis have been published
elsewhere.23 Upon oxidation, diferrocenyl bipyrrole 9 shows an
increasing absorption in the near-IR region at 2000 nm, which
can be assigned to a CT (charge transfer) transition of the
mixed-valent species 9+. A further increase of the potential leads
to the collapse of this absorption envelope, and new bands at
1200 nm assigned to a LMCT (ligand-to-metal charge transfer)
transition can be observed. To determine the physical
parameters (wavenumber (νmax), extinction (εmax), full width
at half maximum (fwhm) (Δν1/2)) of the charge transfer
excitations in the near-IR region, deconvolution assuming
Gaussian-shaped bands has been applied (Table 3).
Surprisingly, upon enlargement of the pyrrolic chain the
characteristics of the charge transfer absorptions change.
Mixed-valent 9+ and 16+ showed much more intense
absorptions and a shift of the excitation maxima toward higher
energy. Deconvolution of the charge transfer bands in 9+ and
16+ revealed that three bands contribute to the absorption in
the near-IR region for both compounds (Table 3, and Figure 6
and Figure SI1 (Supporting Information)). None of these
transitions showed typical characteristics43,44 of an IVCT
(intervalence charge transfer) excitation and are much more
intense than the IVCT band found in 4+23. Therefore, it might
be concluded that the classical two-state model is not applicable
to the elongated pyrrole systems (9, 16), as a more complex
charge transfer behavior is observed. Especially for the
terpyrrole 16+ the characteristics of the CT absorption in the
near-IR resembles that for dicationic 162+, pointing to a strong
involvement of LMCT and interbridge charge transfer
interactions.
Figure 5. Cyclic voltammograms (scan rate 100 mV s−1) of
dichloromethane solutions containing 1.0 mmol L−1 of oligopyrroles
7, 13, and 17 at 25 °C (supporting electrolyte [N(nBu)4][B(C6F5)4]
(0.1 mol L−1)).
Table 2. Cyclic Voltammetric Data (Potentials vs FcH/FcH+,
Scan Rate 100 mV s−1) at a Glassy-Carbon Electrode of 1.0
mmol L−1 Solutions of the Analytes in Dry Dichloromethane
Containing 0.1 mol L−1 of [NnBu4][B(C6F5)4] as Supporting
Electrolyte at 25 °C
a
a
a
a
E°′1 (mV)
E°′2 (mV)
E°′3 (mV)
E°′4 (mV)
(ΔE
(ΔE
(ΔE
(ΔE
ΔE° ′
1c
pb
pb
pb
pb
(mV))
compd
(mV))
(mV))
(mV))
(mV)
7
280 (−)
120 (−)
5 (80)
400 (−)
605 (−)
205 (90)
930 (−)
905 (−)
1095 (−)
1190 (−)
13
17
Due to the large metal−metal distance, mixed-valent 20+
showed no coupling between its redox-active moieties and is
thermodynamically not stable under the applied conditions;
hence, the ferrocenyl termini in diferrocenyl quaterpyrrole are
oxidized simultaneously. Therefore, the electronic character of
the mixed-valent species cannot be determined by near-IR
spectroscopy. Nevertheless, 202+ and 203+ were generated in
situ within the near-IR measurement setup (Figure SI2
(Supporting Information)), showing remarkably intense CT
absorptions in the spectral range between 1250 and 1700 nm.
To prove the character of these absorptions, their solvatochro-
mic behavior was studied using solvents with different polarities
(dichloromethane (εo = 0.42), tetrahydrofuran (εo = 0.45),
acetonitrile (εo = 0.65)).45 The solvatochromic shifts thus
observed were negligible (<100 nm) and proved the solvent
independency of this charge transfer process (Figure SI3
(Supporting Information)). A deconvolution of the near-IR
spectrum of 202+ gives two large Gaussian-shaped functions at
5900 cm−1 (Δν1/2 = 2500 cm−1; εmax = 17600 L mol−1 cm−1)
and 7800 cm−1 (Δν1/2 = 3600 cm−1; εmax = 21350 L mol−1
cm−1). Comparison with the spectroelectrochemical measure-
ments of 42+, 92+, and 162+ showed that these compounds also
exhibit weak to strong CT bands in a range between 1000 and
1740 nm. Extension of the oligo(pyrrole) π system leads to a
bathochromic shift of these CT transitions, while simulta-
neously increasing their intensity (Figure 7). With a decrease of
the potential gap (electrochemical part Table 1, Figure 4)
between the second ferrocenyl-related redox process and the
first pyrrolic oxidation, the acceptor orbitals, within a ligand-to-
200
a
b
E°′ = formal potential. ΔEp = difference between oxidation and
c
reduction potentials. ΔE°′ = potential difference between two redox
processes.
irreversible redox event for the bipyrrole (E°′ = 600 mV) and
for the terpyrrole (E°′ = 280 mV), while for the quaterpyrrole
two reversible redox processes (ΔE°1′ = 160 mV, ΔE°2′ = 430
mV) with a ΔE°′ value compatible to our measurements (170
mV) was observed.39 Depending on the supporting electrolyte,
a different number of redox processes and varying potential
differences were found. It could also be shown that the use of
different electrolytes has a significant impact on the position
and reversibility of the redox potentials, which has already been
discussed in the literature.17,23,40,41 In comparison to the
ferrocenyl-substituted analogues 9, 16, and 20 the pyrrole-
related oxidation is shifted to cathodic potentials. The
ferrocenyl moieties of 9, 16, and 20 are oxidized at lower
potential than the appropriate oligopyrrole backbone; there-
fore, the ferrocenium units thus formed act as electron-
withdrawing substituents hindering the oxidation of the
heterocyle.
Spectroelectrochemistry. The spectroelectrochemical
studies were performed by stepwise increase of the potential
(step heights 25, 5, and 100 mV) from −200 to +1000 mV vs
Ag/AgCl in an OTTLE (optically transparent thin-layer
electrochemistry) cell42 in dichloromethane solutions of 4,23
9, 16, 17, and 20 (0.002 mol L−1) containing [NnBu4][B-
(C6F5)4] (0.1 mol L−1) as electrolyte at 25 °C. The stepwise
6111
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Figure 6. (left) UV/vis/near-IR spectra of 9 at rising potentials vs Ag/AgCl: (top) −200 to 350 mV; (bottom) 350 to 1000 mV. (right)
Deconvolution of the near-IR absorption at 350 mV (top) of in situ generated 9+ and at 900 mV (bottom) of in situ generated 92+ using three
Gaussian-shaped bands. Measurement conditions: 25 °C, dichloromethane, 0.1 mol L−1 [NnBu4][B(C6F5)4] as supporting electrolyte.
Table 3. Near-IR Data of the Diferrocenyl (Oligo)pyrroles
a
4n+, 9n+, 16n+, and 20(n+1)+ (n = 1, 2)
compd transition νmax (cm−1
)
εmax (L mol−1 cm−1
)
Δν1/2 (cm−1
)
b
4+
LMCT
IVCT
LMCT
LMCT
CT
4256
4820
8500
9900
4800
5800
7800
4700
8200
7400
5400
6650
9100
6200
8050
5900
7800
1805
4200
190
690
2369
900
42+
9+
1500
6900
8200
2700
1950
11400
4000
10700
7450
3600
11750
19900
17600
21350
3400
1700
2300
3850
1500
3900
2700
2100
2700
4050
2800
3800
2500
3600
CT
CT
c
92+
LF
CT
CT
CT
CT
CT
CT
CT
CT
CT
16+
Figure 7. Bathochromic shift of the near-IR spectra of 42+, 92+, 162+,
and 202+ (0.002 mol L−1 in dichloromethane) at 25 °C (supporting
electrolyte [NnBu4][B(C6F5)4] (0.1 mol L−1)).
162+
202+
behavior of 42+ in comparison to that of 92+, 162+, and 202+
again points to an involvement of interbridge charge transfer
excitations for the elongated pyrroles, while 42+ cannot possess
such absorptions.
a
In dry dichloromethane containing 0.1 mol L−1 of [NnBu4][B-
b
c
(C6F5)4] as supporting electrolyte at 25 °C. See ref 23. Ligand field
transition.
For comparison, the potential-depended UV/vis/near-IR
spectra of 5,5‴-bis(trimethylsilyl)quaterpyrrole (17) have been
measured (Figure SI5 (Supporting Information)). While no
absorption could be found in a spectral region similar to that of
the CT absorption, intense π−π* absorptions below 1250 nm
can be observed. Those sharp and intense bands could also be
metal charge transfer process, are raised, while the donor
orbitals are lowered in energy. This results in a lower optical
transition energy and thus a bathochromic shift. A comparable
phenomenon has been described by Zhu and Wolf on the
example of various ethynylferrocenyl (oligo)thiophenes.46 They
could show that a longer thiophene chain leads to a decreased
energy difference between the ferrocenyl-centered acceptor
orbital and the thiophenic donor orbitals.46 The different
found in 92+ (624 nm), 162+ (696 nm), and 202+ (830 nm). For
1
4, 9, and 16 (Figure SI2 (Supporting Information)) A1g
→
1E1g and A1g
→
1E2g d−d transitions of the ferrocenyl
1
substituents47 (465 and 650 nm (4), 450 and 670 nm (9), 520
6112
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carried out in an OTTLE cell42 from dichloromethane solutions
containing 0.1 mol L−1 of [NnBu4][B(C6F5)4] as supporting electrolyte
using a Varian Cary 5000 spectrometer. High-resolution mass spectra
were performed using a micrOTOF QII Bruker Daltonite workstation.
Electrochemistry. Measurements on 1.0 mmol L−1 solutions of
the analytes in dry, air-free dichloromethane containing 0.1 mol L−1 of
[NnBu4][B(C6F5)4] as supporting electrolyte were conducted under a
blanket of purified argon at 25 °C utilizing a Radiometer Voltalap PGZ
100 electrochemical workstation interfaced with a personal computer.
A three-electrode cell, which utilized a Pt auxiliary electrode, a glassy-
carbon working electrode (surface area 0.031 cm2), and an Ag/Ag+
(0.01 mol·L−1 AgNO3) reference electrode mounted on a Luggin
capillary, was used. The working electrode was pretreated by polishing
and 810 nm (16)) can be detected in the UV/vis region, which
are shifted bathochromically in comparison to ferrocene (322
and 437 nm).48 A bathochromic shift of the d−d transitions
with increasing chain length of the (oligo)pyrroles can be seen,
indicating that less energy is necessary for these excitations.49
CONCLUSION
■
The series of diferrocenyl-substituted (oligo)pyrroles 4, 9, 16,
and 20 could be synthesized using palladium-catalyzed Negishi
C,C cross-coupling reactions. Single-crystal X-ray diffraction
analysis of diferrocenyl bipyrrole (9), terpyrrole (16), and
quaterpyrrole (20) show a decrease in coplanarity in the
heterocyclic chain with a growing number of pyrrole units. The
redox behaviors of the title compounds have been investigated
by electrochemical methods such as cyclic voltammetry and
square wave voltammetry. While the ferrocenyl moieties in
quarerpyrrole 20 are oxidized simultaneously, the terminal units
of pyrroles 4, 9, and 16 show a redox splitting of 450 mV (4),
320 mV (9) and 165 mV (16) in the cyclic and square wave
voltammograms. Additionally, one (9) or two (16 and 20)
pyrrole-related well-defined reversible one-electron-redox pro-
cesses were observed. The metal−metal communication was
further proven by UV/vis/near-IR spectroelectrochemical
measurements, which showed absorptions in the near-IR
region. The characteristics of these absorptions have been
analyzed by deconvolution calculations and solvatochromic
experiments. While the mixed-valent compound 4+ possesses
intense IVCT absorptions, the classical two-state model seems
not to be applicable to the longer oligopyrroles (9+, 16+).
Diferrocenyl (oligo)pyrroles showed weak (4) to intense (9,
16, and 20) CT absorptions in the dicationic oxidation state. In
good agreement with other published examples,46 those bands
showed a bathochromic shift, when the chain length of the
backbone is increased.
1
on a Buehler microcloth first with 1 μm and then /4 μm diamond
paste. The reference electrode was constructed from a silver wire
inserted into a solution of 0.01 mol L−1 [AgNO3] and 0.1 mol L−1
[NnBu4][B(C6F5)4] in acetonitrile, in a Luggin capillary with a Vycor
tip. This Luggin capillary was inserted into a second Luggin capillary
with Vycor tip filled with a 0.1 mol L−1 [NnBu4][B(C6F5)4] solution in
dichloromethane.32 Successive experiments under the same exper-
imental conditions showed that all formal reduction and oxidation
potentials were reproducible within 5 mV. Experimental potentials
were referenced against an Ag/Ag+ reference electrode, but results are
presented referenced against ferrocene as an internal standard as
required by IUPAC.33 When decamethylferrocene was used as an
internal standard, the experimentally measured potential was
converted into E vs FcH/FcH+ by addition of −0.61 V.51 Data were
then manipulated on a Microsoft Excel worksheet to set the formal
redox potentials of the FcH/FcH+ couple to E°′ = 0.0 V. The cyclic
voltammograms, which are depicted, were taken after typically two
scans and are considered to be steady state cyclic voltammograms, in
which the signal pattern does not differ from the initial sweep.
General Procedure: Protection with Trimethylsilyl Group of
2 and 8. The dibromo (oligo)pyrroles 2 and 8 were dissolved in 50
mL of degassed tetrahydrofuran and treated with 1 equiv of n-
butyllithium at −80 °C. After 1 h of stirring at this temperature the
solution changed from colorless to pale green (10) or deep purple
(11). Then 1.1 equiv of trimethylsilyl chloride was added in a single
portion and the reaction mixture was slowly warmed to ambient
temperature and stirred overnight, whereby the color changed to
yellow. After evaporation of all volatiles the crude product was purified
by column chromatography (column size 3 × 25 cm, alumina
pretreated with triethylamine, n-hexane as eluent). The first fraction
contained the appropriate pure product, and all volatiles were removed
under reduced pressure.
EXPERIMENTAL SECTION
■
General Conditions. All reactions were carried out under an
atmosphere of nitrogen using standard Schlenk techniques. Tetrahy-
drofuran, n-hexane, and n-pentane were purified by distillation from
sodium/benzophenone ketyl, and toluene and diethyl ether were
purified by distillation from sodium. Dichloromethane and N,N-
dimethylformamide were purified by distillation from calcium hydride.
Data for 2-Bromo-5-trimethylsilyl-N-phenylpyrrole (10). 2,5-
Dibromo-N-phenylpyrrole (2; 8.29 g, 27.5 mmol), 1 equiv of n-
butyllithium (11.0 mL, 27.5 mmol) and 1.1 equiv of trimethylsilyl
chloride (3.83 mL, 30.0 mmol). Yield: 2.48 g (8.44 mmol, 31% based
on 2); colorless, air-sensitive solid, soluble in dichloromethane. Anal.
Calcd for C13H16BrNSi (294.26): C, 53.06; H, 5.48; N, 4.76. Found:
Reagents. nBuLi (2.5 M solution in n-hexane), BuLi (1.6 M
t
solution in n-pentane), ferrocene, KOtBu, N-phenylpyrrole, N-
bromosuccinimide, trimethylsilyl chloride, and tetra-n-butylammonium
fluoride were purchased from commercial suppliers and used without
further purification. 2-Bromo-N-phenylpyrrole (1), 2,5-dibromo-N-
phenylpyrrole (2),25 and [N(nBu)4][B(C6F5)4]50 were prepared
according to published procedures. [ZnCl2·2THF] was received by
drying ZnCl2 with thionyl dichloride and additional reaction with dry
tetrahydrofuran. The palladium catalyst [P(tC4H9)2C(CH3)2CH2Pd-
(μ-Cl)]2 was synthesized according to Clark et al.26
1
C, 53.00; H, 5.52; N, 4.65. Mp: 51 °C. H NMR (CDCl3, ppm): δ
−0.04 (s, 9 H, SiC3H9), 6.32 (d, 3JH4−H3 = 3.69 Hz, 1 H, H-4), 6.48 (d,
3JH3−H4 = 3.69 Hz, 1 H, H-3), 7.28 (m, 2 H, C6H5/o-H), 7.46 (m, 3 H,
C6H5). 13C{1H} NMR (CDCl3, ppm): δ −0.29 (SiC3H9), 107.43 (C-
2), 111.38 (C-4), 120.02 (C-3), 128.77 (C6H5), 128.91 (C6H5),
129.30 (C6H5), 136.85 (Ci-C6H5), 140.52 (Ci-C-SiC3H9). IR data
(KBr, cm−1): ν 750 (s, δoop‑C−H), 972 (s, δC−N), 1505 (s, νCC),
1597 (m, νCC), 2895 (w, νs‑CH3), 2954 (w, νas‑CH3), 3098 (w, νC−H).
HR-ESI-MS (m/z): calcd for C13H16BrNSi 295.0210, found 295.0216
[M].
1
Instruments. H NMR (500.3 MHz) and 13C{1H} NMR (125.8
MHz) spectra were recorded with a Bruker Avance III 500
spectrometer operating at 298 K in the Fourier transform mode.
Chemical shifts are reported in δ (parts per million) using
undeuterated solvent residues as internal standard (chloroform-d3,
Data for 5-Bromo-5′-trimethylsilyl-N,N′-diphenyl-2,2′-bipyrrole
(11). 5,5′-Dibromo-N,N′-bisphenyl-2,2′-bipyrrole (8; 4.98 g, 11.3
mmol), 1 equiv of n-butyllithium (4.53 mL, 11.3 mmol) and 1.1 equiv
of trimethylsilyl chloride (1.57 mL, 12.5 mmol). Yield: 4.12 g (9.46
mmol, 84% based on 8); colorless, air-sensitive solid, soluble in
dichloromethane. Anal. Calcd for C23H23BrN2Si (435.43): C, 63.44;
1
1H at 7.26 ppm and 13C{1H} at 77.16 ppm; acetone-d6, H at 2.05
1
ppm and 13C{1H} at 29.84, 206.26 ppm; dimethyl-d6 sulfoxide, H at
2.50 ppm and 13C{1H} at 39.52 ppm). Infrared spectra were recorded
using a FT-Nicolet IR 200 equipment. The melting points of analytical
pure samples (sealed off in nitrogen-purged capillaries) were
determined using a Gallenkamp MFB 595 010 M melting point
apparatus. Microanalyses were performed using a Thermo FLASHEA
1112 Series instrument. Spectroelectrochemical measurements were
1
H, 5.32; N, 6.43. Found: C, 63.68; H, 5.48; N, 6.39. Mp: 92 °C. H
NMR (CDCl3, ppm): δ −0.1 (s, 9 H, SiC3H9), 6.00 (d, 3JH4′−H3′ = 3.81
Hz, 1 H, H-4′), 6.03 (d, 3JH4−H3 = 3.51 Hz, 1 H, H-4), 6.18 (d, 3JH3′‑H4′
6113
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= 3.80 Hz, 1 H, H-3′), 6.36 (d, 3JH3−H4 = 3.62 Hz, 1 H, H-3), 6.84 (m,
2 H, C6H5′), 6.89 (m, 2 H, C6H5), 7.21−7.36 (m, 6 H, C6H5/C6H5′).
13C{1H} NMR (CDCl3, ppm): δ −0.05 (SiC3H9), 103.20 (C-5),
butyllithium (1.0 mL, 2.47 mmol), 1.0 equiv of [ZnCl2·2thf] (0.69 g,
2.47 mmol), 1 equiv of 11 (1.07 g, 2.47 mmol), and 0.25 mol % of
[Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 (4.23 mg). For column chro-
matography an n-hexane/diethyl ether mixture of ratio 10/1 (v/v) was
used as eluent. Yield: 0.99 g (1.4 mmol, 57% based on 11); colorless,
fluorescent solid, soluble in dichloromethane. Anal. Calcd for
C36H39N2Si2 (711.06): C, 77.70; H, 6.52; N, 7.88. Found: C, 77.37;
H, 6.70; N, 7.88. Mp: 211 °C. 1H NMR (CDCl3, ppm): δ −0.13 (s, 18
111.53 (C-3), 112.42 (C-3′), 112.70 (C-4′), 119.19 (C-4), 124.27 (Ci-
C-2), 127.64 (C6H5), 127.78 (C6H5′), 128.38 (C6H5), 128.49
(C6H5′), 128.51 (C6H5), 128.63 (C6H5′), 130.36 (Ci-C-2′), 135.60
(Ci-C6H5′), 138.35 (Ci-C6H5), 141.24 (Ci-C-SiC3H9). IR data (KBr,
cm−1): ν 751 (s, δoop‑C−H), 967 (m, δC−N), 1497 (s, νCC), 1595 (m,
νCC), 2889 (w, νs‑CH3), 2948 (w, νas‑CH3), 3100 (w, νC−H). HR-ESI-
MS (m/z): calcd for C23H23BrN2Si 437.0867, found 437.0845 [M]+.
General Procedure: Synthesis of Bipyrrole and Me3Si-
Protected Oligopyrroles. To a solution of the monobromo
(oligo)pyrroles 5 and 11 in 50 mL of degassed tetrahydrofuran was
added dropwise via a syringe 1 equiv of n-butyllithium (2.5 M in n-
hexane) at −80 °C. The reaction mixture was stirred for 1 h. Then
[ZnCl2·2thf] (1 equiv) was added in a single portion to generate in
situ 6 or 12. The reaction mixture was stirred for an additional 30 min
at 0 °C. Afterward, 0.25 mol % of [Pd(CH2C(CH3)2P(tC4H9)2)(μ-
Cl)]2 and 1 equiv of 5, 10, or 11 were added in a single portion and
the reaction solutions were stirred overnight at 60 °C. The crude
product was worked up by column chromatography (column size 3 ×
25 cm, alumina pretreated with triethyl amine). The first fraction
contained the reactants 5 and 11, while from the second fraction the
appropriate pure product could be obtained. All volatiles were
removed under reduced pressure.
3
H, SiC3H9), 5.93 (d, JH4−H3/H4‴−H3‴ = 3.64 Hz, 2 H, H-4/4‴), 6.02
(d, 3JH3−H4/H3‴−H4‴ = 3.64 Hz, 2 H, H-3/3‴), 6.04 (d, 3JH3′−H4′/H4″−H3″
= 3.50 Hz, 2 H, H-3′/4″), 6.07 (m, 4 H, C6H5/C6H5‴), 6.36 (d,
3JH4′−H3′/H3″−H4″ = 3.50 Hz, 2 H, H-4′/3″), 6.58 (m, 4 H, C6H5′/
C6H5″), 6.84 (m, 4 H, C6H5/C6H5‴), 6.98 (m, 2 H, C6H5/C6H5‴),
7.08 (m, 4 H, C6H5′/C6H5″), 7.18 (m, 2 H, C6H5′/C6H5″). 13C{1H}
NMR (CDCl3, ppm): δ −0.03 (SiC3H9), 111.98 (C-3′/4″), 112.13
(C-4′/3″), 112.26 (C-3/3‴), 119.16 (C-4/4‴), 125.53 (C6H5/
C6H5‴), 125.92 (Ci-C-2′/5″), 126.11 (Ci-C-5′/2″), 126.75 (C6H5/
C6H5‴), 127.01 (C6H5′/C6H5″), 127.68 (C6H5′/C6H5″), 127.99
(C6H5/C6H5‴), 128.05 (C6H5′/C6H5″), 130.94 (Ci-C-2/2‴), 135.11
(Ci-C6H5/Ci-C6H5‴), 138.52 (Ci-C6H5′/C6H5″), 141.20 (Ci-C-
SiC3H9/C-SiC3H9‴). IR data (KBr, cm−1): ν 756 (s, δoop‑C−H),
1497 (s, νCC), 1596 (m, νCC), 2893 (w, νs‑CH3), 2952 (w, νas‑CH3),
3097 (w, νC−H). HR-ESI-MS (m/z): calcd for C46H46N4Si2
710.3275, found 710.3256 [M].
General Procedure: Deprotection of the Oligopyrroles 13
and 17. The Me3Si-protected oligopyrroles 13 and 17, respectively,
were dissolved in 20 mL of degassed tetrahydrofuran, and 3 equiv of
tetra-n-butylammonium fluoride was added in a single portion at −40
°C. The reaction mixture was slowly warmed to ambient temperature
and stirred for 2 days, whereby the colorless solution changed to pale
blue. After evaporation of all volatiles the crude product was worked
up by filtration (column size 1.5 × 10 cm, alumina pretreated with
triethylamine). For column chromatography an n-hexane/diethyl ether
mixture of ratio 10/1 (v/v) was used as eluent. The first fraction
contained reactant 13 or 17, while from the second fraction the
appropriate pure product could be isolated. All volatiles were removed
under reduced pressure.
Data for N,N′-Diphenyl-2,2′-bipyrrole (7). 2-Bromo-N-phenyl-
pyrrole (5; 2.95 g, 13.28 mmol), 1 equiv of n-butyllithium (5.4 mL,
13.5 mmol), 1.0 equiv of [ZnCl2·2thf] (3.72 g, 13.5 mmol), 1 equiv of
2-bromo-N-phenylpyrrole (5; 2.95 g, 13.28 mmol), and 0.25 mol % of
[Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 (22.8 mg). For column chro-
matography an n-hexane/diethyl ether mixture of ratio 6/1 (v/v) as
eluent was used. Yield: 2.97 g (10.46 mmol, 77% based on 5);
colorless, fluorescent solid, soluble in dichloromethane. Anal. Calcd for
C20H16N2 (284.35): C, 84.48; H, 5.67; N, 9.85. Found: C, 84.08; H,
1
5.92; N, 9.90. Mp: 101 °C. H NMR (CDCl3, ppm): δ 6.32 (pt,
3
4
3JH4−H3 = 3.58 Hz, JH4−H5 = 2.96 Hz, 2 H, H-4), 6.44 (dd, JH3−H5
=
3
1.84 Hz, JH3−H4 = 3.54 Hz, 2 H, H-3), 6.57−6.59 (m, 4 H, C6H5/o-
H), 6.77 (dd, 4JH5−H3 = 1.80 Hz, 3JH5−H4 = 2.98 Hz, 2 H, H-5), 7.01 −
7.05 (m, 6 H, C6H5). 13C{1H} NMR (CDCl3, ppm): δ 109.13 (C-4),
112.59 (C-3), 122.41 (C-5), 123.90 (C6H5), 125.18 (Ci-C-2), 125.57
(C6H5), 128.56 (C6H5), 140.30 (Ci-C6H5). IR data (KBr, cm−1): ν 718
(s, δoop‑C−H), 1072 (m, νC−N), 1499 (s, νCC), 1599 (m, νCC), 3100
(w, νC−H). HR-ESI-MS (m/z): calcd for C20H16N2 566.2465, found
566.2485 [M]2.
Data for N,N′,N″-Triphenyl-2,2′:5′,2″-terpyrrole (14). 5,5″-Bis-
(trimethylsilyl)-N,N′,N″-triphenyl-2,2′:5′,2″-terpyrrole (13; 0.72 g,
1.26 mmol) and 3 equiv of tetra-n-butylammonium fluoride (1.19 g,
3.78 mmol). Yield: 0.51 g (1.19 mmol, 95% based on 13); colorless,
fluorescent solid, soluble in dichloromethane. Anal. Calcd for
C30H23N3 (425.52): C, 84.68; H, 5.45; N, 9.87. Found: C, 84.19; H,
5.77; N, 9.70. Mp: 132 °C. 1H NMR (CDCl3, ppm): δ 5.87 (m, 4Jmeta
3
= 1.23 Hz, 2 H, C6H5′), 6.19 (pt, JH4−H5/H4″−H5″ = 2.94 Hz,
Data for 5,5″-Bis(trimethylsilyl)-N,N′,N″-triphenyl-2,2′:5′,2″-ter-
pyrrole (13). 5-Bromo-5′-trimethylsilyl-N,N′-diphenyl-2,2′-bipyrrole
(11; 1.66 g, 3.8 mmol), 1 equiv of n-butyllithium (1.53 mL, 3.8
mmol), 1.0 equiv of [ZnCl2·2thf] (1.07 g, 3.8 mmol), 1 equiv of 2-
bromo-5-trimethylsilyl-N-phenylpyrrole (10; 1.12 g, 3.8 mmol), and
0.25 mol % of [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 (6.53 mg). For
column chromatography n-hexane was used as eluent. Yield: 0.91 g
(1.6 mmol, 42% based on 11); colorless, fluorescent solid, soluble in
dichloromethane. Anal. Calcd for C36H39N2Si2 (569.89): C, 75.87; H,
4
3JH4−H3/H4″−H3″ = 3.61 Hz, 2 H, H-4/4″), 6.23 (dd, JH3−H5/H3″−H5″
=
1.82 Hz, 3JH3−H4/H3″−H4″ = 3.60 Hz, 2 H, H-3/3″), 6.36 (s, 2 H, H-3′/
4′), 6.61 (m, 4 H, C6H5/C6H5″), 6.65 (m, 2 H, C6H5/C6H5″), 6.67
4
3
(dd, JH5−H3/H5″−H3″ = 1.82 Hz, JH5−H4/H5″−H4″ = 2.92 Hz, 2 H, H-5/
4
5″), 6.82 (tt, Jmeta = 1.21 Hz, 1 H, C6H5′), 7.04 (m, 6 H, C6H5/
C6H5′/C6H5″). 13C{1H} NMR (CDCl3, ppm): δ 108.82 (C-4/4″),
111.76 (C-3′/4′), 112.99 (C-3/3″), 122.27 (C-5/5″), 123.95 (C6H5/
C6H5″), 125.08 (C6H5′), 125.23 (C6H5′), 125.65 (C6H5/C6H5″),
126.05 (Ci-C-2/2″), 126.31 (C6H5′), 127.50 (Ci-C-2′/5′), 128.56
(C6H5/C6H5″), 138.29 (Ci-C6H5′), 140.33 (Ci-C6H5/Ci-C6H5″). IR
data (KBr, cm−1): ν 757 (s, δoop‑C−H), 1499 (s, νCC), 1599 (m,
νCC), 3055 (w, νC−H). HR-ESI-MS (m/z): calcd for C30H23N3
426.1965, found 426.1962 [M]+.
1
6.90; N, 7.37. Found: C, 75.37; H, 7.26; N, 7.31. Mp: 181 °C. H
NMR (CDCl3, ppm): δ −0.12 (s, 18 H, SiC3H9), 6.02 (s, 2 H, H-3′/
3
4′), 6.03 (d, JH4−H3/H4″−H3″ = 3.49 Hz, 2 H, H-4/4″), 6.28 (m, 2 H,
3
C6H5′), 6.36 (d, JH3−H4/H3″−H4″ = 3.54 Hz, 2 H, H-3/3″), 6.62 (m, 4
H, C6H5/C6H5″), 6.96 (m, 2 H, C6H5′), 7.09 (m, 5 H, C6H5/C6H5′/
C6H5″) 7.18 (m, 2 H, C6H5/C6H5″). 13C{1H} NMR (CDCl3, ppm): δ
0.01 (SiC3H9), 112.21 (C-3′/4′), 112.23 (C-3/3″), 119.19 (C-4/4″),
125.87 (C6H5′), 126.03 (C6H5/C6H5″), 127.03 (C6H5′), 127.15
(C6H5/C6H5″), 127.95 (Ci-C-2′/5′), 127.98 (C6H5′), 128.13 (C6H5/
C6H5″), 130.96 (Ci-C-2/2″), 135.07 (Ci-C6H5/Ci-C6H5″), 138.87 (Ci-
C6H5′), 141.24 (Ci-C-SiC3H9/C-SiC3H9″). IR data (KBr, cm−1): ν
756 (s, δoop‑C−H), 1496 (s, νCC), 1595 (m, νCC), 2899 (w, νs‑CH3),
2952 (w, νas‑CH3), 3091 (w, νC−H). HR-ESI-MS (m/z): calcd for
C36H39N3Si2 570.2755, found 570.2701 [M]+.
Data for N,N′,N″,N′′′′-Tetraphenyl-2,2′:5′,2″:5″,2‴-quaterpyrrole
(18). 5,5‴-Bis(trimethylsilyl)-N,N′,N″,N‴-tetraphenyl-
2,2′:5′,2″:5″,2‴-quaterpyrrole (17; 0.686 g, 0.96 mmol) and 3 equiv
of tetra-n-butylammonium fluoride (0.913 g, 2.88 mmol). Yield: 0.545
g (0.96 mmol, 100% based on 17); colorless fluorescent solid, soluble
in dichloromethane. Anal. Calcd for C40H30N4 (566.69): C, 84.78; H,
1
5.34; N, 9.80. Found: C, 84.60; H, 5.37; N, 9.80. Mp: 177 °C. H
3
NMR (CDCl3, ppm): δ 5.97 (m, Jortho = 7.83 Hz, 4 H, C6H5′/
3
C6H5″), 6.04 (d, JH3′−H4′/H4″−H3″ = 3.55 Hz, 2 H, H-3′/4″), 6.18 (m,
3
3JH4−H5/H4‴−H5‴ = 3.25 Hz, 2 H, H-4/4‴), 6.21 (m, JH5−H4/H5‴−H4‴
=
Data for 5,5‴-Bis(trimethylsilyl)-N,N′,N″,N‴-tetraphenyl-
2,2′:5′,2″:5″,2‴-quaterpyrrole (17). 5-Bromo-5′-trimethylsilyl-N,N′-
diphenyl-2,2′-bipyrrole (11; 1.07 g, 2.47 mmol), 1 equiv of n-
4
3.25 Hz, JH5−H3/H5‴−H3‴ = 1.80 Hz, 2 H, H-5/5‴), 6.23 (d,
3JH4′−H3′/H3″−H4″ = 3.59 Hz, 2 H, H-4′/3″), 6.56 (m, Jortho = 7.83
3
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Organometallics
Article
4
Hz, 4 H, C6H5/C6H5‴), 6.65 (m, JH3−H5/H3‴−H5‴ = 1.82 Hz, 2 H, H-
2‴), 126.64 (C6H5/C6H5‴), 127.32 (C6H5′/C6H5″), 127.67 (Ci-C-5′/
2″), 127.85 (C6H5′/C6H5″), 128.14 (C6H5/C6H5′/C6H5″/C6H5‴),
137.88 (Ci-C6H5/C6H5‴), 138.14 (Ci-C6H5′/C6H5″). IR data (KBr,
cm−1): ν 766 (s, δoop‑C−H), 1497 (s, νCC), 1596 (m, νCC), 3042
(w, νC−H). HR-ESI-MS (m/z): calcd for C40H28N4Br2 724.0694,
found 724.0659 [M].
General Procedure: Synthesis of Diferrocenyl Oligopyrroles
9, 16, and 20. Ferrocene and KOtBu (0.125 equiv) were dissolved in
20 mL of tetrahydrofuran, and the solution was cooled to −80 °C.
tButyllithium (2 equiv, 1.6 M in n-pentane) was added dropwise via a
3
3/3‴), 6.73 (m, Jortho = 7.83 Hz, 4 H, C6H5′/C6H5″), 6.88 (m, 2 H,
3
C6H5′/C6H5″), 7.03 (m, Jortho = 7.83 Hz, 4 H, C6H5/C6H5‴), 7.08
(m, 2 H, C6H5/C6H5‴). 13C{1H} NMR (CDCl3, ppm): δ 108.83 (C-
4/4‴), 111.51 (C-3/3‴), 112.14 (C-3′/4″), 112.92 (C-4/3″), 122.70
(C-5/5‴), 123.90 (C6H5/C6H5‴), 125.24 (Ci-2/2″), 125.28 (C6H5/
C6H5‴), 125.54 (C6H5′/C6H5″), 126.02 (Ci-C-5″/2‴), 126.31 (Ci-C-
5′/2″), 126.35 (C6H5′/C6H5″), 127.50 (C6H5′/C6H5″), 128.52
(C6H5/C6H5‴), 138.25 (Ci-C6H5′/ Ci-C6H5″), 140.34 (Ci-C6H5/Ci-
C6H5‴). IR data (KBr, cm−1): ν 769 (s, δoop‑C−H), 1500 (s, νCC),
1597 (m, νCC), 3100 (w, νC−H). HR-ESI-MS (m/z): calcd for
C40H30N4 566.2453, found 426.2465 [M].
syringe, and the solution was stirred for 1 h. Then [ZnCl2·2thf] (1
equiv) was added in a single portion. The reaction mixture was stirred
for an additional 30 min at 0 °C. Afterward, 0.25 mol % of
General Procedure: Synthesis of Dibromo Oligopyrroles 8,
15, and 19. According to a modified procedure from Gilow et al.25
unsubstituted N-phenyl(oligo)pyrroles 7, 14, and 18 were dissolved in
30 mL of degassed tetrahydrofuran, and 2 equiv of N-bromosuccini-
mide was added in a single portion at −78 °C. After 30 min of stirring
at this temperature the solution was slowly warmed to ambient
temperature and stirred for an additional 2 h. Then the pink (8) or
purple (15 and 19) reaction mixture was extracted three times with 20
mL of diethyl ether. The organic phase was washed three times with an
aqueous Na2CO3 solution and dried over MgSO4. The solvent was
evaporated to dryness under oil-pump vacuum, yielding a colorless
precipitate. The crude product was washed twice with cold n-pentane
(0 °C) and finally dried under oil-pump vacuum.
1
[Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 and /3 equiv of the bromoo-
ligopyrroles 8, 15, and 19 were added in a single portion, and the
reaction solutions were stirred overnight at 60 °C. The crude product
was worked up by column chromatography (column size 1.5 × 10 cm,
alumina pretreated with triethylamine). The first fraction contained
ferrocene, while from the second fraction 9, 16, or 20 could be
isolated. All volatiles were removed under reduced pressure.
Data for 5,5′-Diferrocenyl-N,N′-diphenyl-2,2′-bipyrrole (9). Fer-
rocene (0.35 g, 1.89 mmol), 0.125 equiv of KOtBu (27 mg, 0.24
mmol), 2 equiv of tert-butyllithium (2.35 mL, 3.78 mmol), 1.0 equiv of
1
[ZnCl2·2thf] (527 mg, 1.89 mmol), /3 equiv of 5,5′-dibromo-N,N′-
bisphenyl-2,2′-bipyrrole (8; 0.28 g, 0.63 mmol), and 0.25 mol % of
[Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2 (2.14 mg). As eluent for
column chromatography an n-hexane/diethyl ether mixture of ratio
10/1 (v/v) was used. Yield: 0.3 g (0.45 mmol, 73% based on 8); pale
orange solid, soluble in dichloromethane. Anal. Calcd for C40H32N2Fe2
(652.38): C, 73.64; H, 4.94; N, 4.29. Found: C, 73.61; H, 5.40; N,
4.10. Mp: 221 °C. 1H NMR (CDCl3, ppm): δ 3.84 (pt, JHH = 1.90 Hz,
4 H, C5H4), 3.97 (s, 10 H, C5H5), 3.98 (pt, JHH = 1.90 Hz, 4 H, C5H4),
6.03 (d, 3JH3−H4 = 3.67 Hz, 2 H, H-3), 6.30 (d, 3JH4−H3 = 3.67 Hz, 2 H,
H-4), 6.85 (m, 4 H, C6H5/o-H), 7.22−7.25 (m, 6 H, C6H5). 13C{1H}
NMR (CDCl3, ppm): δ 67.51 (C5H4), 67.56 (C5H4), 69.60 (C5H5),
79.26 (Ci-C5H4), 108.09 (C-4), 112.47 (C-3), 126.65 (C-5), 127.41
(C6H5), 128.27 (C6H5), 129.08 (C6H5), 131.85 (Ci-C-2), 139.41 (Ci-
C6H5). IR data (KBr, cm−1): ν 775 (s, δoop‑C−H), 1105 (m, νC−N),
1500 (s, νCC), 1590 (m, νCC), 3080 (w, νC−H). HR-ESI-MS (m/
z): calcd for C40H32N2Fe2 652.1260, found 652.1263 [M].
Data for 5,5′-Dibromo-N,N′-diphenyl-2,2′-bipyrrole (8). N,N′-
Diphenyl-2,2′-bipyrrole (3; 2.36 g, 8.31 mmol) and 2 equiv of N-
bromosuccinimide (2.96 g, 16.62 mmol). Yield: 2.88 g (6.54 mmol,
79% based on 3); colorless solid, soluble in dichloromethane. Anal.
Calcd for C20H14N2Br2 (442.15): C, 54.33; H, 3.19; N, 6.34. Found:
C, 54.39; H, 3.36; N, 6.28. Mp: 168.5 °C. 1H NMR (CDCl3, ppm): δ
6.05 (d, 3JH3−H4 = 3.80 Hz, 2 H, H-3), 6.19 (d, 3JH4−H3 = 3.80 Hz, 2 H,
H-4), 6.79 (m, 4 H, C6H5), 7.22−7.27 (m, 6 H, C6H5). 13C{1H} NMR
(CDCl3, ppm): δ 103.56 (C-5), 111.56 (C-3), 113.19 (C-4), 127.13
(Ci-C-2), 127.84 (C6H5), 128.38 (C6H5), 128.55 (C6H5), 137.90 (Ci-
C6H5). IR data (KBr, cm−1): ν 755, 764 (s, δoop‑C−H), 1070 (m,
νC−N), 1496 (s, νCC), 1595 (m, νCC), 3032 (w, νC−H). HR-ESI-
MS (m/z): calcd for C20H14N2Br2 441.9499, found 441.9499 [M].
Data for 5,5″-Dibromo-N,N′,N″-triphenyl-2,2′:5′,2″-terpyrrole
(15). N,N′,N″-Triphenyl-2,2′:5′,2″-terpyrrole (14; 0.43 g, 1.0 mmol)
and 2 equiv of N-bromosuccinimide (0.36 g, 2.0 mmol). Yield: 0.49 g
(0.84 mmol, 84% based on 14); colorless solid, soluble in
dichloromethane. Anal. Calcd for C30H21N3Br2 (583.32): C, 61.77;
Data for 5,5″-Diferrocenyl-N,N′,N″-triphenyl-2,2′:5′,2″-terpyrrole
(16). Ferrocene (383 mg, 2.06 mmol), 0.125 equiv of KOtBu (29 mg,
0.26 mmol), 2 equiv of tert-butyllithium (2.57 mL, 4.12 mmol), 1.0
1
1
equiv of [ZnCl2·2thf] (577 mg, 2.06 mmol),
/ equiv of 5,5″-
3
H, 3.63; N, 7.20. Found: C, 61.55; H, 3.74; N, 6.89. Mp: 181 °C. H
NMR ((CD3)2CO, ppm): δ 5.94 (d, JH3−H4/H3″−H4″ = 3.84 Hz, 2 H,
3
dibromo-N,N′,N″-triphenyl-2,2′:5′,2″-terpyrrole (15; 0.40 g, 0.69
mmol), and 0.25 mol % of [Pd(CH2C(CH3)2P(tC4H9)2)(μ-Cl)]2
(2.35 mg). As eluent for column chromatography an n-hexane/diethyl
ether mixture of ratio 2/1 (v/v) was used. Yield: 0.3 g (0.38 mmol,
55% based on 15); orange solid, soluble in dichloromethane. Anal.
Calcd for C50H39N3Fe2·0.85Et2O (856.55): C, 74.87; H, 5.59; N, 4.90.
Found: C, 74.56; H, 5.30; N, 4.90. Mp: 185 °C dec. 1H NMR
((CD3)2SO, ppm): δ 3.78 (pt, JHH = 1.88 Hz, 4 H, C5H4), 3.92 (s, 10
H-3/3″), 5.96 (s, 2 H, H-3′/4′), 6.17 (d, 3JH4−H3/H4″−H3″ = 3.84 Hz, 2
H, H-4/4″), 6.40 (m, 2 H, C6H5′), 6.72 (m, 4 H, C6H5/C6H5″), 7.09
(m, 2 H, C6H5′), 7.19 (m, 1 H, C6H5′), 7.27 (m, 4 H, C6H5/C6H5″),
7.34 (m, 2 H, C6H5/C6H5″). 13C{1H} NMR ((CD3)2CO, ppm): δ
103.24 (C-5/5″), 112.19 (C-3′/4′), 113.34 (C-3/3″), 113.53 (C-4/
4″), 126.63 (Ci-C-2/2″), 127.45 (C6H5′), 128.14 (C6H5/C6H5″),
128.44 (C6H5′), 128.60 (C6H5′), 129.01 (Ci-C-2′/5′), 129.03 (C6H5/
C6H5″), 129.25 (C6H5/C6H5″), 138.69 (Ci-C6H5/Ci-C6H5″), 139.02
(Ci-C6H5′). IR data (KBr, cm−1): ν 757 (s, δoop‑C−H), 1497 (s,
νCC), 1596 (m, νCC), 3059 (w, νC−H). HR-ESI-MS (m/z): calcd
for C30H21N3Br2 584.0157, found 584.0087 [M]+.
3
H, C5H5), 4.01 (pt, JHH = 1.88 Hz, 4 H, C5H4), 5.70 (d, JHH = 3.65
Hz, 2 H, H-3/3″), 5.72 (s, 2 H, H-3′/4′), 6.25 (d, 3JHH = 3.65 Hz, 2 H,
H-4/4″), 6.70 (m, 2 H, C6H5′/o−H), 6.82 (m, 4 H, C6H5/C6H5″/o-
H), 7.21 (m, 3 H, C6H5′), 7.32 (m, 4 H, C6H5/C6H5″/m-H), 7.39 (m,
2 H, C6H5/C6H5″/p-H). 13C{1H} NMR ((CD3)2SO, ppm): δ 66.28
(C5H4), 67.29 (C5H4), 69.20 (C5H5), 78.42 (Ci-C5H4), 107.67 (C4N),
111.34 (C4N), 111.64 (C4N), 125.48 (Ci-C-5/5″), 126.12 (Ci-C-2/
2″), 126.59 (C6H5′), 127.70 (C6H5/C6H5″), 127.86 (C6H5′), 127.98
(C6H5′), 128.34 (C6H5/C6H5″), 128.83 (C6H5/C6H5″), 131.25 (Ci-C-
2′/5′), 138.43 (Ci-C6H5′), 138.59 (Ci-C6H5/Ci-C6H5″). IR data (KBr,
cm−1): ν 768 (s, δoop‑C−H), 1497 (s, νCC), 1596 (m, νCC), 3055,
3098 (w, νC−H). HR-ESI-MS (m/z): calcd for C50H39N3Fe2
794.1918, found 794.1815 [M]+.
Data for 5,5‴-Dibromo-N,N′,N″,N‴-tetraphenyl-2,2′:5′,2″:5″,2‴-
quaterpyrrole (19). N,N′,N″,N‴-Tetraphenyl-2,2′:5′,2″:5″,2‴-quater-
pyrrole (18; 0.53 g, 0.94 mmol) and 2 equiv of N-bromosuccinimide
(0.33 g, 1.88 mmol). Yield: 0.62 g (0.86 mmol, 91% based on 18);
colorless solid, soluble in dichloromethane. Anal. Calcd for
C40H28N4Br2 (724.49): C, 66.31; H, 3.90; N, 7.73. Found: C, 65.31;
1
H, 3.98; N, 7.50. Mp: 178 °C. H NMR (CDCl3, ppm): δ 5.92 (d,
3JH4−H3/H4‴−H3‴ = 3.61 Hz, 2 H, H-4/4‴), 6.03−6.06 (m, 8 H, C6H5/
3
C6H5‴/H-3/3‴/H-3′/4″), 6.18 (d, JH4′−H3′/H3″−H4″ = 3.77 Hz, 2 H,
H-4′/3″), 6.58 (m, 4 H, C6H5′/C6H5″), 6.85 (m, 4 H, C6H5/C6H5‴),
7.00 (m, 2 H, C6H5/C6H5‴), 7.12 (m, 4 H, C6H5′/C6H5″), 7.21 (m, 2
H, C6H5′/C6H5″). 13C{1H} NMR (CDCl3, ppm): δ 102.88 (C-5/5‴),
111.52 (C-3/3‴), 112.02 (C-3′/4″), 112.50 (C-4′/3″), 112.78 (C-4/
4‴), 125.54 (Ci-C-2′/5″), 125.78 (C6H5/C6H5‴), 126.23 (Ci-C-2/
Data for 5,5‴-Diferrocenyl-N,N′,N″,N‴-tetraphenyl-
2,2′:5′,2″:5″,2‴-quaterpyrrole (20). Ferrocene (466 mg, 2.5 mmol),
0.125 equiv of KOtBu (35 mg, 0.31 mmol), 2 equiv of tert-butyllithium
(3.13 mL, 5.0 mmol), 1.0 equiv of [ZnCl2·2thf] (0.70 g, 2.5 mmol), 1/3
equiv of 5,5‴-dibromo-N,N′,N″,N‴-tetraphenyl-2,2′:5′,2″:5″,2‴-qua-
6115
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Organometallics
Article
terpyrrole (19; 0.60 g, 0.84 mmol), and 0.25 mol % of [Pd(CH2C-
(CH3)2P(tC4H9)2)(μ-Cl)]2 (4.30 mg). For column chromatography
an n-hexane/dichloromethane mixture of ratio 5/1 (v/v) was used as
eluent. Yield: 0.65 g (0.69 mmol, 83% based on 19); orange solid,
soluble in dichloromethane. Anal. Calcd for C40H46N4Fe2 (934.24): C,
77.10; H, 4.96; N, 5.99. Found: C, 77.03; H, 5.36; N, 5.83. Mp: 227
ACKNOWLEDGMENTS
■
We are grateful to the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for generous financial
support.
1
°C dec. H NMR (CDCl3, ppm): δ 3.80 (pt, JHH = 1.89 Hz, 4 H,
REFERENCES
■
C5H4), 3.95 (s, 10 H, C5H5), 3.97 (pt, JHH = 1.90 Hz, 4 H, C5H4), 5.95
(d, 3JHH = 3.58 Hz, 2 H, H-4/4‴), 6.03 (d, 3JHH = 3.67 Hz, 2 H, H-3′/
(1) Lapinte, C.; Paul, F. Coord. Chem. Rev. 1998, 180, 431−509.
(2) Low, P. J. Dalton Trans. 2005, 2821−2824.
3
4″), 6.06 (d, JHH = 3.56 Hz, 2 H, H-3/3‴), 6.14 (m, 4 H, C6H5/
C6H5‴), 6.30 (d, 3JHH = 3.67 Hz, 2 H, H-4′/3″), 6.55 (m, 4 H, C6H5′/
C6H5″), 6.86 (m, 4 H, C6H5/C6H5‴), 6.97 (m, 2 H, C6H5/C6H5‴),
7.10 (m, 4 H, C6H5′/C6H5″), 7.21 (m, 2 H, C6H5′/C6H5″). 13C{1H}
NMR (CDCl3, ppm): δ 67.47 (C5H4), 67.53 (C5H4), 69.60 (C5H5),
79.09 (Ci-C5H4), 108.09 (C4N), 111.77 (C4N), 112.32 (C4N), 112.41
(C4N), 125.47 (C6H5/C6H5‴), 126.07 (Ci-C-2/2″), 126.10 (Ci-C-2′/
5″), 126.53 (Ci-C-5/5‴), 126.77 (C6H5/C6H5‴), 127.04 (C6H5′/
C6H5″), 127.65 (C6H5′/C6H5″), 127.97 (C6H5/C6H5‴), 128.68
(C6H5′/C6H5″), 131.09 (Ci-C-5′/2″), 138.55 (Ci-C6H5/Ci-C6H5‴),
138.98 (Ci-C6H5′/Ci-C6H5″). IR data (KBr, cm−1): ν 769 (s,
δoop‑C−H), 1498 (s, νCC), 1596 (m, νCC), 3055, 3091 (w,
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̅
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ASSOCIATED CONTENT
* Supporting Information
■
S
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Reactions, Mechanisms, and Structure, 5th ed.; Wiley-VCH: New York,
2001; pp 20−36.
Figures and CIF files giving additional experimental and
crystallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
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Katritzky, A. R., Rees, C. W., Eds.; Pergamon Press: Oxford, U.K.,
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AUTHOR INFORMATION
Corresponding Author
*H.L.: e-mail, heinrich.lang@chemie.tu-chemnitz.de; tel, +49
(0)371-531-21210; fax, +49 (0)371-531-21219.
■
(30) Lotz, S.; Crause, C.; Olivier, A. J.; Liles, D. C.; Gorls, H.;
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Notes
(32) Gericke, H. J.; Barnard, N. I.; Erasmus, E.; Swarts, J. C.; Cook,
The authors declare no competing financial interest.
M. J.; Aquino, M. a. S. Inorg. Chim. Acta 2010, 363, 2222−2232.
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