Electronically intercommunicating iron centers in Di-and tetraferrocenyl pyrroles

Article abstract of DOI:10.1021/om100914m

Novel 2,5-diferrocenyl-1-phenyl-1H-pyrrole (4) and 2,3,4,5-tetraferrocenyl- 1-phenyl-1H-pyrrole (6) have been prepared by a 2-or 4-fold Negishi cross-coupling reaction of 2,5-dibromo-1-phenyl-1H-pyrrole (3) and 2,3,4,5-tetrabromo-1-phenyl-1H-pyrrole (5),

Full text of DOI:10.1021/om100914m

556 Organometallics 2011, 30, 556–563
DOI: 10.1021/om100914m
Electronically Intercommunicating Iron Centers in Di- and
Tetraferrocenyl Pyrroles§
Alexander Hildebrandt, Dieter Schaarschmidt, and Heinrich Lang*
€
€
Lehrstuhl fu€r Anorganische Chemie, Institut fu€r Chemie, Fakultat fur Naturwissenschaften, Technische
€
Universitat Chemnitz, Strasse der Nationen 62, 09111 Chemnitz, Germany
Received September 22, 2010
Novel 2,5-diferrocenyl-1-phenyl-1H-pyrrole (4) and 2,3,4,5-tetraferrocenyl-1-phenyl-1H-pyrrole
(6) have been prepared by a 2- or 4-fold Negishi cross-coupling reaction of 2,5-dibromo-1-phenyl-
1H-pyrrole (3) and 2,3,4,5-tetrabromo-1-phenyl-1H-pyrrole (5), respectively, with FcZnCl (2) (Fc=
Fe(η5-C5H4)(η5-C5H5)) in the presence of [(Ph3P)4Pd] as catalyst. The electronic and structural
properties of 4 and 6 were investigated with UV-vis spectroscopy and single-crystal X-ray
diffraction (6). Comparison of the appropriate bond distances in the pyrrole core system of 6
demonstrates considerable electron delocalization. Cyclic, square wave, and linear sweep voltam-
metry as well as in situ NIR spectro-electrochemistry highlight the electrochemical properties of both
compounds. Molecules 4 and 6 display two (4) or four (6) electrochemically reversible one-electron
0
0
0
0
transfer processes with remarkably high ΔE1/2 values and reduction potentials of E =-238 and E =
0
0
0
0
0
0
0
0
212 mV for 4 (ΔE1/2=450 mV) and E =-280, E =51, E =323, and E =550 mV for 6 (ΔE1/2
=
322, 264, and 233 mV) using [NBu4][B(C6F5)4] as the supporting electrolyte. The pyrroles could be
classified as class II systems according to Robin and Day. Additionally, 4[PF6]n (n = 1, 2) were
synthesized and studied, giving CV responses and NIR spectra identical to those obtained for 4 from
electrochemical oxidations.
Introduction
and heteroaromatics were given by the hexaferrocenylben-
zene of Volhardt,2 which can be electrochemically oxidized
in three redox waves with ΔE1/2 values of 130 and 250 mV,
respectively, several substituted ferrocenyl thiophenes (2,5-
diferocenylthiophene and 3,4-diferrocenylthiophene)3 with
less than 150 mV peak separation in the cylcovoltammetric
studies, and diferrocenylpyridine,3 which exhibits nearly no
observable metal-metal interaction, as the two ferrocenyl
moieties were oxidized simultaneously. The highest ΔE1/2
values among symmetric ferrocenyl heteroaromatics have
been reported in 1,3-di(ferrocenyl)benzo[b]thiophene (280 mV)
and 1,3-di(ferrocenyl)benzo[b]selenophene (305 mV) by
Sato and Ogawa.4 Winter and Kowalski synthesized azafer-
rocenes with heteroaromatics such as pyridine or thiophene
as bridging units and studied their electro- and spectro-
electrochemisty, resulting in higher ΔE1/2 values compared
to the appropriate ferrocenyl derivatives.5
Ferrocenyl-substituted aromatics and heteroaromatics
have attracted rising attention during recent years because
they can be regarded as model compounds for the investiga-
tion of metal-metal electronic communication. Moreover,
these π-conjugated organometallics may be suitable for the
design of novel electro-active materials.1 The tendency of the
iron centers in the mixed valence species to communicate
through the aromatic bridging system has mostly been
investigated by cyclovoltammetry (CV), whereas detailed
spectro-electrochemical studies of these compounds were
often neglected. However, the varying experimental condi-
tions in those studies hinder the comparability of their results
(e.g., the difference in the individual half-wave potentials
ΔE1/2). Examples of those ferrocenyl-substituted aromatics
§ Dedicated to Prof. Dr. Peter Kl€ufers on the occasion of his 60th birthday.
*Corresponding author. E-mail: heinrich.lang@chemie.tu-chemnitz.
de. Phone: þ49 (0)371-531-21210. Fax: þ49 (0)371-531-21219.
(1) (a) Jones, S. C.; Barlow, S.; O’Hare, D. Chem.;Eur. J. 2005, 11,
4473. (b) Barlow, S.; O'Hare, D. Chem. Rev. 1997, 97, 637. (c) Bruce, I. M.
Coord. Chem. Rev. 1997, 166, 91. (d) Ratner, M.; Jortner, J. Molecular
Electronics; Blackwell Science: Malden, MA, 1997. (e) Tour, M. J. Acc.
Chem. Res. 2000, 33, 791. (f) Collier, P. C.; Wong, W. E.; Belohradsky, M.;
Raymo, M. F.; Stoddart, F. J.; Kuekes, J. P.; Williams, S. R.; Heath, R. J.
Science 1999, 285, 391. (g) Carroll, R. L.; Gorman, Ch. B. Angew. Chem.,
Int. Ed. 2002, 41, 4378. (h) Robertson, N.; McGowan, C. A. Chem. Soc.
Rev. 2003, 32, 96. (i) Diallo, A. K.; Daran, J.-C.; Varret, F.; Ruiz, J.; Astruc,
D. Angew. Chem., Int. Ed. 2009, 48, 3141.
Recently, we have reported on the synthesis and electro-
chemistry of supercrowded 2,3,4,5-tetraferrocenyl thiophene
for the first time.6 Cyclic, square wave, and linear sweep
voltammetry together with in situ NIR spectro-electrochem-
istry highlighted that this compound displays four electroche-
mically reversible one-electron transfer processes indicating
(3) Iyoda, M.; Kondo, T.; Okabe, T.; Matsuyama, H.; Sasaki, S.;
Kuwatani, Y. Chem. Lett. 1997, 35.
(4) Ogawa, S.; Muraoka, H.; Kikuta, K.; Saito, F.; Sato, R. J.
Organomet. Chem. 2007, 692, 60.
(2) Yu, Y.; Bond, A. D.; Leonard, P. W.; Lorenz, U. J.; Timofeeva,
T. V.; Vollhardt, K. P. C.; Whitener, G. D.; Yakovenko, A. A. Chem.
Commun. 2006, 2572.
(5) Kowalski, K.; Winter, R. F. J. Organomet. Chem. 2009, 694, 1041.
(6) Hildebrandt, A.; Ruffer, T.; Erasmus, E.; Swarts, J. C.; Lang, H.
Organometallics 2010, 29, 4900.
€
r
pubs.acs.org/Organometallics
Published on Web 01/06/2011
2011 American Chemical Society

Article
Organometallics, Vol. 30, No. 3, 2011 557
Scheme 1. Syntheses of Ferrocenyl Pyrroles 4 from 2 and 3 and 6
from 2 and 5a
The electrochemical behavior of both compounds (CV, LSV,
SWV, NIR spectroscopy) was determined.
While the IR spectra of 4 and 6 are not very expressive, 1H
NMR spectroscopy shows some peculiarities. In 4 the two
CH pyrrole protons appear at 6.33 ppm as a singlet, while for
the cyclopentadienyl groups one singlet (C5H5) and two
pseudotriplets with JHH = 1.8 Hz (C5H4) centered at 3.88
and 4.02 ppm are found as expected for AA0XX0 spin
systems. In 6, the four ferrocenyl units show a distinct signal
pattern caused by the steric demand of the ferrocenyl groups
(Figure 1). The signals for the R protons of cyclopentadienyl
rings appear as very broad resonances, while those in β
position occur as poorly resolved pseudotriplets with the
expected coupling pattern (vide supra). The free rotation
around the pyrrole-ferrocenyl carbon-carbon bond could
be reduced by cooling NMR samples to -80 °C, resulting in
the observation of eight individual signals for the protons of
the C5H4 units (Figure 1). The exchange rates of those
protons can be determined by line-shape-fitting of the
NMR spectra at the appropriate temperatures. The activa-
tion parameters of the rotations around the pyrrole-ferrocenyl
C,C bond could be quantified from the exchange rates in
graphical analyses according to Eyring.10 The ferrocenyls in
the 2 and 5 position exhibit an activation enthalpy of ΔH‡=
a Conditions: (i) In tetrahydrofuran; (1) t-BuLi, -30 °C, 30 min; (2)
[ZnCl2 2thf]. (ii) In tetrahydrofuran, [Pd(PPh3)4] (1 mol %), 60 °C, 48 h.
3
significant electrostatic interaction among the four ferroce-
nyl units as oxidation progresses. Spectro-electrochemistry
confirmed that positive charges are localized on the Fcþ
groups in the mixed valent intermediates, as no charge
transfer bands of significant strength could be detected. In
continuation of this work we were interested in varying the
heteroatom of the cyclic core system to modify the electronic
properties and improve the electrochemical interaction be-
tween the terminal redox-active ferrocenyl functionalities. It
is well known that polypyrroles are electro-active materials
of high conductivity.7 For this reason we decided to use
pyrroles as organic bridging units between ferrocenyl groups
to decrease the energy gap between the redox-active groups
and the connecting system itself. This prompted us to synthe-
size the title compounds 2,5-diferrocenyl- and 2,3,4,5-tetra-
ferrocenyl-1-phenyl-1H-pyrrole, respectively, and to study
their electrochemical properties.
26.8 ((1.2) kJ mol-1 and an activation entropy of ΔS‡ =
3
-94.1 ((4.5) J mol-1 K-1; those in positions 3 and 4 have
3
3
values of ΔH‡ = 27.9 ((1.5) kJ mol-1 and ΔS‡ = -88.6
3
((5.6) J mol-1
Single crystals of 6 CHCl3 suitable for X-ray diffraction
K
-1, respectively.
3
3
3
analysis could be obtained by diffusion of n-hexane into a
chloroform solution containing 6 at 25 °C. The molecular
structure of 6 CHCl3 in the solid state is shown in Figure 2.
˚
Important bond distances (A), bond angles (deg), and tor-
3
sion angles (deg) are summarized in the caption of Figure 2.
Compound 6 CHCl3 crystallized in the triclinic space
3
group P1. The ferrocenyl substituents are rotated by
44.6(1)° (Fe1), 37.8(1)° (Fe2), 68.1(2)° (Fe3), and 18.3(1)°
(Fe4) out of the plane of the pyrrole core.11 The cyclopenta-
dienyl ligands at the iron centers exhibit an almost eclipsed
conformation (2-4°). As common for aromatic compounds
Results and Discussion
Synthesis and Characterization. Treatment of ferrocenyl
zinc chloride (2)8,9 with 2,5-dibromo-1-phenyl-1H-pyrrole
(3) in the molar ratio of 3:1 in the presence of catalytic
amounts of [Pd(PPh3)4] gave 2,5-diferrocenyl-1-phenyl-1H-
pyrrole (4) (Scheme 1). An excess of 2 is required to reach
complete conversion of the reactants. Under similar Negishi
C,C cross-coupling conditions 2,3,4,5-tetrabromo-1-phenyl-
1H-pyrrole (5) was reacted with FcZnCl (2) (Fc = Fe(η5-
C5H4)(η5-C5H5)), yielding 2,3,4,5-tetraferrocenyl-1-phenyl-
1H-pyrrole (6) (Scheme 1). After appropriate workup, this
molecule could be obtained as an orange solid material in
68% yield (Experimental Section).
˚
the C4N arrangement is planar (rms deviation 0.0244 A). It is
˚
noteworthy that the C1-C2 bond length (1.400(3) A), as a
formal double bond, is elongated, when compared with
˚
localized C,C double bonds (1.34 A). In addition, the
˚
C2-C3 bond (1.435(3) A) is shorter than those for localized
C,C single bonds (1.53 A).12 This behavior is already known
˚
for pyrroles,13 indicating strong electronic delocalization in
the core system, which is significantly more pronounced than
in the 2,3,4,5-tetraferrocenylthiophene.6
Pyrroles 4 and 6 are stable to air and moisture both in the
solid state and in solution. They have been identified by
elemental analysis, IR, UV-vis, and NMR (1H, 13C{1H})
spectroscopy. ESI TOF mass spectrometry and single-crystal
X-ray structure analysis (6) were additionally carried out.
€
(10) (a) Gunther, H. NMR Spectroscopy: Basic Principles, Concepts,
And Applications in Chemistry, 2nd ed.; John Wiley & Sons: New York,
1996. (b) Neuman, R. C., Jr.; Jonas, V. J. Org. Chem. 1974, 39, 925.
(11) The angles between the ferrocenyl substituents and the pyrrole
core have been determined by generating least-squares planes and
measuring the relative orientation.
(12) Smith, M. B.; March, J. March’s Advanced Organic Chemistry:
Reactions, Mechanisms, and Structure, 5th ed.; Wiley: New York, 2001; pp
20-36.
(7) (a) Curran, D.; Grimshaw, J.; Perera, S. D. Chem. Soc. Rev. 1991,
20, 391. (b) Li, S.; Macosko, C. W.; White, H. S. Adv. Mater. 1993, 5, 575.
(c) Fabre, B.; Simonet, J. Coord. Chem. Rev. 1998, 178-180, 1211. (d)
Reddinger, J. L.; Reynolds, J. R. Adv. Polym. Sci. 1999, 145, 57. (e)
MacDiarmid, A. G. Angew. Chem., Int. Ed. 2001, 40, 2581.
(8) Sanders, R.; Mueller-Westerhoff, U. T. J. Organomet. Chem.
1996, 512, 219.
ꢀ
(13) (a) Faigl, F.; Fogassy, K.; Szuics, E.; Kovaics, K.; Keseru,
€
+
G. M.; Harmat, V.; Bocskei, Z.; Toke, L. Tetrahedron 1999, 55, 7881.
(b) Takahashi, I.; Tsuzuki, M.; Keumi, T.; Kitajima, H.; Isa, K.; Hosoi, S.;
Tsuda, Y. Chem. Pharm. Bull. 1994, 42, 947. (c) Welch, J.; Tucker, C.;
Bereman, R.; Singh, P. Inorg. Chim. Acta 1986, 121, 77. (d) Cromer, D. T.;
Coburn, M. D.; Ryan, R. R.; Wasserman, H. J. Acta Crystallogr. 1986, C42,
1428.
(9) Negishi, E. I.; King, A. O.; Okukado, N. J. Org. Chem. 1977, 42,
1821.

558 Organometallics, Vol. 30, No. 3, 2011
Hildebrandt et al.
Figure 1. (Left) Experimental 1H NMR spectra of 6 in the range between 3.75 and 5.00 ppm; toluene-d8; various temperatures. (Right)
1
Simulated H NMR spectra of 6 with different exchange rates (protons of ferrocenyls in 2,5 position/protons of ferrocenyls in 3,4
position).
Table 1. Cyclovoltammetric Data (potentials vs FcH/FcHþ)a
compd
E0 (mV)
0
ΔEp (mV)b
ΔE1/2 (mV)c
4, [NBu4][PF6] wave 1
4, [NBu4][PF6] wave 2
4, [NBu4][B(C6F5)4] wave 1
4, [NBu4][B(C6F5)4] wave 2
-198
117
-238
212
80
85
68
75
315
450
a Scan rate 100 mV s-1 at a glassy-carbon electrode of 0.5 mmol dm-3
3
solutions of 4 in dry dichloromethane containing 0.1 mol dm-3 of
[N(nBu)4][PF6] or [N(nBu)4][B(C6F5)4] as supporting electrolyte at 25 °C.
=
b ΔEp = difference between oxidation and reduction potential. c ΔE1/2
potential difference between two redox processes.
(NIR spectroscopy) in dry dichloromethane utilizing 0.1
mol dm-3 [N(nBu)4][PF6] and [N(nBu)4][B(C6F5)4], respec-
tively, as supporting electrolyte. The data of the cyclovol-
tammetric experiments were carried out at scan rates of
100 mV s-1 and are summarized in Tables 1 (4) and 2 (6).
All potentials are referenced to the FcH/FcHþ redox couple
(Fc=Fe(η5-C5H4)(η5-C5H5)) as recommended by IUPAC.14
The cyclovoltamograms of 4 are shown in Figure 3, and
those of 6 in Figure 6. Pyrrole 4 exhibits two diffusion-
controlled ferrocenyl-related redox events independent of
the electrolyte used. Each of the two ferrocenyl substituents
shows a reversible electrochemical behavior with ΔEp values
of 80 and 85 mV for electrolyte [N(nBu)4][PF6] and 68 and 75
mV for [N(nBu)4][B(C6F5)4]. Electrochemical reversibility is
characterized by values of 59 mV.15 The use of dichloro-
methane and [N(nBu)4][B(C6F5)4] as solvent/supporting elec-
trolyte results in a better resolution as compared with the
system dichloromethane/[N(nBu)4][PF6], due to the lower
Figure 2. ORTEP diagram (50% probability level) of the mo-
lecular structure of 6 CHCl3 with the atom-numbering scheme.
All hydrogen atoms and one molecule of chloroform have been
3
˚
omitted for clarity. Selected bond distances (A), angles (deg),
and torsion angles (deg): average D-Fe = 1.649, C1-C2 =
1.400(3), C2-C3 = 1.435(3), C3-C4 = 1.386(3), N1-C1 =
1.393(3), N1-C4 = 1.394(3), N1-C5 = 1.430(3), C1-C11 =
1.471(3), C2-C21=1.476(3), C3-C31=1.492(3), C4-C41=
1.479(3); N1-C1-C2 = 107.3(2), C1-C2-C3 = 107.4(2),
C1-N1-C4 = 109.6(2), C1-N1-C5 = 125.8(2), average
D-Fe-D=177.18; N1-C1-C2-C3=4.7(3), C6-C5-N1-C1=
-71.0(3), N1-C1-C11-C12=52.9(4), N1-C4-C41-C42=
-0.4(4), C1-C2-C21-C22=-41.8(4), C4-C3-C31-C32=
60.2(3) (D=denotes the centroids of C5H4 or C5H5).
-
˚
ion-pairing capabilities (spherical diameter [PF6] , 3.3 A;
(14) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1984, 56, 461.
(15) (a) Evans, D. H.; O’Connell, K. M.; Peterson, R. A.; Kelly, M. J.
J. Chem. Educ. 1983, 60, 290. (b) Kissinger, P. T.; Heineman, W. R. J. Chem.
Educ. 1983, 60, 702. (c) Van Benshoten, J. J.; Lewis, J. Y.; Heineman, W. R.
J. Chem. Educ. 1983, 60, 772. (d) Mobbott, G. A. J. Chem. Educ. 1983, 60,
697.
(16) Gericke, H. J.; Barnard, N. I.; Erasmus, E.; Swarts, J. C.; Cook,
M. J.; Aquino, M. A. S. Inorg. Chim. Acta 2010, 363, 2222.
(17) (a) Barriere, F.; Geiger, W. E. J. Am. Chem. Soc. 2006, 128, 3980.
(b) LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76, 6395. (c)
Auger, A.; Muller, A. J.; Swarts, J. C. Dalton Trans. 2007, 3623. (d) Kemp,
K. C.; Fourie, E.; Conradie, J.; Swarts, J. C. Organometallics 2008, 27, 353.
(e) Trupia, S.; Nafady, A.; Geiger, W. E. Inorg. Chem. 2003, 42, 5480.
Electrochemistry and Spectro-electrochemistry
The redox properties of 4 and 6 were studied by cyclovol-
tammetry (CV), square wave voltammetry (SWV), linear
sweep voltammetry (LSV), and spectro-electrochemistry

Article
Organometallics, Vol. 30, No. 3, 2011 559
Figure 3. Voltammograms of dichloromethane solutions containing 0.5 mmol L-1 of 4 at 25 °C. (Left) Supporting electrolyte
3
[N(nBu)4][PF6]; CV (scan rate: 100 mV s-1) and LSV (scan rate 1 mV s-1). (Right) Supporting electrolyte [N(nBu)4][B(C6F5)4]; CV
3
3
(scan rate: 100 mV s-1), and LSV (scan rate 1 mV s-1).
3
3
[N(nBu)4][B(C6F5)4], ΔE1/2=260 mV), pointing to significant
electronic communication in the pyrrole derivative. This was
proven by in situ UV-vis-NIR spectroscopy (vide infra).
On the basis of the full reversibility of the CV waves, the
oxidized complexes 4[PF6]n (n = 1, 2) were considered as
accessible synthetic targets. According to a well-established
procedure,18 complex 4 was reacted with 1 equiv of
[FcH][PF6] in tetrahydrofuran at -20 °C. The initial dark
red solution immediately turned dark green, and after com-
pletion of the reaction, addition of n-hexane allowed the
precipitation of 4[PF6]. As the redox potential of the redox
couple 4þ/42þ is more positive than that of the ferrocene/
ferrocenium couple, the dication 4[PF6]2 was prepared by
reacting 4 with two stoichiometric amounts of [AgPF6] in
tetrahydrofuran at -20 °C.18 Excellent yields of spectro-
scopically pure materials were obtained by partial precipita-
tion and vacuum drying. Both cationic complexes gave CV
responses identical to that obtained for 4 (Experimental
Section).
The spectro-electrochemical studies were conducted by
stepwise increase of the potential from -500 to 1200 mV vs
Ag/AgCl in an OTTLE cell (OTTLE=optically transparent
thin-layer electrode) containing dichloromethane solutions
of 4 (1.0 mmol dm-3) and [N(nBu)4][PF6] (0.1 mol dm-3).
This procedure allows the generation of 4þ and 42þ from
neutral 4. Compound 4 does not contain any absorption in
the NIR range. In contrast, the spectra of the monooxidized
species 4[PF6] show an absorption characteristic for mixed
valence systems (=MV) Fe(II)/Fe(III) (Figure 4). The iso-
lated mono- and dioxidized compounds 4[PF6] and 4[PF6]2
Figure 4. NIR spectra of 4 at rising potentials (bottom: -200 to
225 mV; top: 250 to 600 mV vs Ag/AgCl) at 25 °C, in dichlor-
omethane, supporting electrolyte [N(nBu)4][PF6]. Arrows indi-
cate increasing or decreasing absorptions.
-
[B(C6F5)4] , 10 A),16 and, hence, in a superior quality of the
˚
electrochemical data for multiredox processes with posi-
tively charged analytes.17 LSV confirmed two individual
one-electron processes for the oxidation of the ferrocenyls.
The advantage of using [N(nBu)4][B(C6F5)4] as electrolyte
results in a significantly higher separation of the redox poten-
tials ([N(nBu)4][PF6], ΔE1/2 = 315 mV; [N(nBu)4][B(C6F5)4],
ΔE1/2=450 mV) (Table 1). Despite similar geometries and
therefore similar electrostatic interactions, the obtained ΔE1/2
values for pyrrole 4 are remarkably higher when compared
with analogous species measured under the same conditions,
i.e., 2,5-diferrocenylthiophene (([N(nBu)4][PF6], ΔE1/2=152 mV;
(18) (a) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877. (b)
Lohan, M.; Justaud, F.; Roisnel, T.; Ecorchard, P.; Lang, H.; Lapinte, C.
Organometallics 2010, 29, 4804.

560 Organometallics, Vol. 30, No. 3, 2011
Hildebrandt et al.
Table 2. Cyclovoltammetric Data (potentials vs FcH/FcHþ)a
compd
E0 (mV)
0
ΔEp (mV)b
ΔE1/2 (mV)c
6, [NBu4][PF6] wave 1
6, [NBu4][PF6] wave 2
-226
-26
72
72
200
160
6, [NBu4][PF6] wave 3
134
104
6, [NBu4][B(C6F5)4] wave 1
6, [NBu4][B(C6F5)4] wave 2
-280
51
62
63
322
264
233
6, [NBu4][B(C6F5)4] wave 3
6, [NBu4][B(C6F5)4] wave 4
323
62
550
61
a Scan rate 100 mV s-1 at a glassy-carbon electrode of 0.5 mmol dm-3
3
solutions of 6 in dry dichloromethane containing 0.1 mol dm-3 of
[N(nBu)4][PF6] or [N(nBu)4][B(C6F5)4] as supporting electrolyte at 25 °C.
b ΔEp =difference between oxidation and reduction potential. c ΔE1/2
potential difference between two redox processes.
=
Figure 5. Deconvolution of NIR absorptions of 4[PF6] using
three Gaussian-shaped bands determined by spectro-electro-
chemistry in an OTTLE cell: experimental data (solid line);
Gaussian-shaped absorptions (dashed line).
process at 134 mV, which is less reversible (ΔEp=104 mV).
This two-electron process could be resolved by using the
advanced supporting electrolyte [N(nBu)4][B(C6F5)4].16,17
The electrochemical data of 6 are summarized in Table 2.
The differences in the potentials of the first and second redox
couple of 200 mV ([N(nBu)4][PF6]) or 322 mV ([N(nBu)4]-
[B(C6F5)4]) are lower than those in pyrrole 4, due to its inc-
reased steric demand and therefore the decreased tendency of
the cyclopentadienyls to be coplanar with the pyrrole core
system. The ΔE1/2 values are higher than those for the
appropriate isostructural thiophene derivative,6 which indi-
cates stronger electronic interaction among the ferrocenyl
moieties in 6.
Scheme 2. Synthesis of the Monocation 4[PF6] from 4 and
[FcH][PF6] and the Dication 4[PF6]2 from 4 and [AgPF6] in
Tetrahydrofuran at -20 °C
In situ NIR measurements of 6 were performed in an
OTTLE cell containing a dichlormethane solution of 0.1
mol L-1 [N(nBu)4][B(C6F5)4] and 0.1 mmol L-1 of 6 in a
3
3
potential range between -500 and 1500 mV vs Ag/AgCl.
While the potential is increased stepwise (the step length
differs from 25 to 100 mV), 6 is oxidized to 6þ, 62þ, 63þ, and
finally 64þ. Upon raising the potential from -500 to 200 mV,
oxidation of pyrrole 6 occurs and the extinction of NIR
bands increases. In situ formed 6[B(C6F5)4] evinces a strong
broad band between 1500 and 2500 nm, similar to that
observed in 4[PF6] as well as a sharp absorption in the UV-
vis part of the spectra at 663 nm, which most probably can be
assigned to a ligand-to-metal charge transfer transition
(Figure 7).20 Further increase of the potential to 525 mV
leads to the in situ formation of 6[B(C6F5)4]2. As this oxida-
tion takes place, the extinction of the NIR absorptions
decreases. The LMCT band in the UV-vis region is shifted
hypsochromically to 607 nm (Figure 7). Compounds 6[B-
(C6F5)4]3 and 6[B(C6F5)4]4 could be in situ generated by
increasing the potentials to 800 and 1100 mV, respectively.
During these oxidation processes no NIR absorption could
be observed, and therefore, they are not depicted in Figure 7.
UV-vis absorptions could be found at 818 nm for 63þ and
possess the same characteristics as the in situ generated
species.
Deconvolution of the NIR absorption was achieved using
three distinct overlapping transitions with Gaussian shapes
(Figure 5). The fits are good enough to allow an almost exact
overlay of the sum of the spectral components with the
experimental spectra. Two bands were found at 4820 and
4250 cm-1, respectively. The most intense absortion (εmax
4200 L mol-1 cm-1) at 4820 cm-1 showed a characteristic
=
3
3
peak width at half-height for IVCT bands (2369 cm-1),19
while the band at 4250 cm-1 might be attributed to a ligand-
to-metal charge transfer absorption.20b A third broad
Gaussian-shaped band represents the baseline correction.
It is noteworthy that the NIR transitions of the MV complex
4[PF6] differ from 2,5-diferrocenylthiophene, for which no
IVCT absorptions are found between 800 and 2800 nm,
indicating the superior electron transfer properties of the
pyrrole molecule. As expected, the double-oxidized species
4[PF6]2 exhibits no IVCT band.
782 nm for 64þ
.
Depending on the electrolyte, tetraferrocenyl-pyrrole 6
exhibits three ([N(nBu)4][PF6]) or four ([N(nBu)4][B(C6F5)4])
separate reversible oxidation processes. Using the clas-
sical hexafluorophosphate anion as supporting electrolyte
leads to two one-electron processes at -226 mV (ΔEp =
72 mV) and -26 mV (ΔEp=72 mV) and one two-electron
Deconvolution of the NIR data of 6[B(C6F5)4] was per-
formed using three Gaussian-shaped overlapping absorp-
tions (Figure 8). The sum of the Gaussian functions closely
matches the measured spectrum. The IVCT band can be
(20) (a) Paul, F.; Ellis, B. G.; Bruce, M. I.; Toupet, L.; Roisnel, T.;
Costuas, K.; Halet, J.-F.; Lapinte, C. Organometallics 2006, 25, 649. (b)
Lohan, M.; Ecorchard, P.; R€uffer, T.; Justaud, F.; Lapinte, C.; Lang, H.
Organometallics 2009, 28, 1878.
(19) (a) D’Alessandro, D. M.; Keene, F. R. Chem. Soc. Rev. 2006, 35,
424. (b) Kaim, W.; Sarkar, B. Coord. Chem. Rev. 2007, 251, 584. (c) Paul, F.;
Lapinte, C. Coord. Chem. Rev. 1998, 178-180, 431.

Article
Organometallics, Vol. 30, No. 3, 2011 561
Figure 6. Voltamograms of dichloromethane solutions containing 0.5 mmol L-1 of 6 at 25 °C. (Left) Supporting electrolyte
3
[N(nBu)4][PF6]; CV (scan rate: 100 mV s-1). (Right) Supporting electrolyte [N(nBu)4][B(C6F5)4]; CV (scan rate: 100 mV s-1) and
3
3
LSV (scan rate 1 mV s-1).
3
Figure 8. Deconvolution of NIR absorptions of 6[B(C6F5)4]
using three Gaussian-shaped bands determined by spectro-
electrochemistry in an OTTLE cell.
Figure 7. NIR spectra of 6 at rising potentials (bottom: oxida-
tion of 6 to 6þ, -500 to 200 mV; top: oxidation of 6þ to 62þ, 250
to 525 mV vs Ag/AgCl) at 25 °C, in dichloromethane, support-
ing electrolyte [N(nBu)4][B(C6F5)4]. Arrows indicate increasing
or decreasing absorptions.
mixed valence species (eqs 1 and 2)21 and the classification
criterion of Brunschwig, Creutz and Sutin.22
1=2
ðΔν1=2Þtheo ¼ ð2310νmax
Γ ¼ 1 - ððΔν1=2Þ=ðΔν1=2
Þ
ð1Þ
ð2Þ
observed at 4752 cm-1 possessing an extinction of εmax=4900
L mol-1 cm-1 and a bandwidth at half-height of 2719 cm-1
Band shape analyses for both pyrrole compounds were
performed according to the Hush model for symmetric
.
3
3
Þ
theoÞ
Theoretical bandwidth at half-height could be calculated
as 3341 cm-1 for 4þ and 3313 cm-1 for 6þ. Therefore, Γ
values of 0.31 (4þ) and 0.18 (6þ) could be determined.
According to this classification 2,5-diferrocenyl-1-phenyl-
1H-pyrrole (4) and 2,3,4,5-tetraferrocenyl-1-phenyl-1H-pyr-
role (6) can be assigned as class II systems. The Hush theory
is a two-state model; however, in 6þ three iron(II) centers and
(21) (a) Lapinte, C. J. Organomet. Chem. 2008, 693, 793. (b) Hush,
N. S. Electrochim. Acta 1968, 13, 1005. (c) Astruc, D. Electron Transfer
and Radical Processes in Transition-Metal Chemistry; VCH: New York,
1995; p 630.
(22) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem. Soc. Rev. 2002,
31, 168.

562 Organometallics, Vol. 30, No. 3, 2011
Hildebrandt et al.
one iron(III) ion are present. This system is much more
complicated, and the use of Hush’s formula may be inade-
quate, as more than one IVCT process could appear in a
close range and, therefore, the bandwidth at half-height
could be enlarged.
that described previously24,6 from dichloromethane solutions
containing 0.1 mol L of [N(nBu)4][B(C6F5)4] or [N(nBu)4][PF6]
3
as supporting electrolyte using a Varian Cary spectrometer.
High-resolution mass spectra were recorded using a micrOTOF
QII Bruker Daltonite workstation.
Reagents. 2,5-Dibromo-1-phenyl-1H-pyrrole,25 2,3,4,5-tetra-
bromo-1-phenyl-1H-pyrrole,25 and [N(nBu)4][B(C6F5)4]26 were
prepared according to published procedures. All other chemi-
cals were purchased from commercial suppliers and were used as
received.
Conclusion
We have shown that novel 2,5-diferrocenyl-1-phenyl-1H-
pyrrole (4) and 2,3,4,5-tetraferrocenyl-1-phenyl-1H-pyrrole
(6) are accessible in a straightforward synthesis methodol-
ogy. As proven by dynamic NMR spectroscopy, the perfer-
rocenylated pyrrole (6) evinces rotation barriers in solution
even at room temperature, due to the sterical demand of the
four ferrocenyl groups. Comparison of the C,C double bond
and the single bond length in the pyrrole core system of 6,
determined by single-crystal X-ray diffraction, indicates
considerable electron delocalization. Organometallics 4 and
6 display two (4) or four (6) electrochemically reversible one-
Synthesis of 2,5-Diferrocenyl-1-phenyl-1H-pyrrole (4). To
920 mg (5 mmol) of ferrocene and 56 mg (0.5 mmol) of KOtBu
dissolved in 20 mL of tetrahydrofuran was added 4.6 mL
(7.5 mmol) of a 1.6 M solution of tert-butyllithium in n-pentane
at -30 °C. After 1 h of stirring, 2.2 g (8 mmol) of dry [ZnCl2 2thf]
3
was added. The solution was kept for 1 h at -30 °C and an
additional hour at 25 °C. Afterward, 35 mg (0.03 mmol) of
[Pd(PPh3)4] and 500 mg (1.66 mmol) of 2,5-dibromo-1-phenyl-
1H-pyrrole were added in a single portion, and the reaction
solution was stirred for 48 h at 60 °C. After evaporation of all
volatiles, the precipitate was dissolved in 200 mL of dichlor-
omethane and washed three times with 100 mL portions of
water. The organic phase was dried over MgSO4, and the solvent
was evaporated to dryness in an oil-pump vacuum. The remain-
ing solid was purified by column chromatography on alumina
using an n-hexane-toluene mixture of ratio 1:1 (v/v) as eluent.
All volatiles were removed under reduced pressure. Compound
4 was obtained as an orange solid. Yield: 526 mg (1.03 mmol,
62% based on 3.) Anal. Calcd for C30H25Fe2N (511.22): C,
70.48; H, 4.93; N, 2.74. Found; C, 70.52; H, 5.01; N, 2.69. Mp:
238 °C. IR data (KBr): 3089 m, 2923 w, 1597 w, 1498 s,1417 s,
1328 w, 1105 m, 1001 s, 819 s, 766 s. 1H NMR (CDCl3, δ): 3.88
(pt, JHH=1.8 Hz, 4H, C5H4), 4.02 (s, 10 H, C5H5), 4.07 (pt, JHH
=1.8 Hz, 4H, C5H4), 6.33 (s, 2H, C4H2N), 7.30-7.35 (m, 2H,
C6H5), 7.46-7.54 (m, 3H, C6H5). 13C{1H} NMR (CDCl3, δ):
66.81 (C5H4), 67.60 (C5H4), 69.56 (C5H5), 77.37 (Ci-C5H4),
79.28 (Ci-C4H2N), 108.16 (C4H2N), 128.78 (C6H5), 128.88
(C6H5), 130.08 (C6H5), 140.23 (Ci-C6H5). HR-ESI-MS [m/z]:
511.0680 [M]þ.
0
0
0
electron processes with formal reduction potentials of E =
0
0
0
0
0
0
0
-238 and E =212 mV for 4 and E =-280, E =51, E =
0
0
n
323, and E = 550 mV for 6 using [N( Bu)4][B(C6F5)4] as
supporting electrolyte. With ΔE1/2 values of 315 mV
([N(nBu)4][PF6]) and 450 mV ([N(nBu)4][B(C6F5)4]) the 2,5-
diferrocenyl-1-phenyl-1H-pyrrole (4) exhibits to the best of
our knowledge the largest separation of the oxidation po-
tentials among ferrocenyl aromatics or heteroaromatics ever
described in literature. Most probably this increased inter-
metallic communication, when compared to other ferrocenyl
heteroaromatics, can be explained by the decreasing of the
energy gap between the ferrocenyl moieties and the hetero-
cyclic core system and the increased delocalization in the
C4N unit itself. Spectro-electrochemical studies confirmed
electronic communication between the ferrocenyl moieties in
the monocations through the C4N core of both compounds.
This delimits the title compounds from the appropriate
thiophene systems,6 where the interaction between the fer-
rocenyl functionalities is mainly attributed to electrostatic
effects. The pyrroles could be classified as class II systems
according to Robin and Day23 by band shape analyses of the
IVCT absorptions.
Synthesis of 4[PF6]. A 300 mg (0.58 mmol) amount of 4
was dissolved in 10 mL of dry tetrahydrofuran, and 194 mg
(0.58 mmol) of FcH[PF6] was added at -20 °C in a single portion.
After 1 h of stirring the reaction mixture was allowed to warm to
25 °C, and stirring was continued for an additional 2 h. After
evaporation of all volatiles, the dark green solid was washed
three times with 50 mL portions of n-hexane. The remaining
solid was dried in the oil-pump vacuum. Complex 4[PF6] could
be isolated as a dark green solid. Yield: 364 mg (0.55 mmol, 95%
based on 4). Mp: 256 °C (dec). Anal. Calcd for C30H25F6Fe2NP
(656.18): C, 54.91; H, 3.81; N, 2.13. Found: C, 53.47; H, 3.91; N,
1.83. IR data (KBr): 3110 w, 2922 m, 2855 w, 1510 s, 1406 s, 1380
w, 1308 m, 1246 m, 1182 m, 844 s, 822 s. HR-ESI-MS [m/z]:
511.0680 [M - PF6]þ.
Synthesis of 4[PF6]2. A 200 mg (0.39 mmol) amount of 4 was
dissolved in 10 mL of tetrahydrofuran, and 197 mg (0.78 mmol)
of [AgPF6] was added at -20 °C in a single portion. After
stirring the reaction solution for 1 h at this temperature, it was
allowed to warm to 25 °C, and stirring was continued for an
additional 2 h. The dark solution was filtered from the in situ
formed silver particles through Celite. After evaporation of all
volatiles the residue was washed three times with 50 mL portions
of n-hexane. The remaining dark violet solid was dried in an oil-
pump vacuum. Yield: 302 mg (0.37 mmol, 95% based on 4). Mp:
232 °C (dec). Anal. Calcd for C30H25F12Fe2NP2 (801.14): C,
44.98; H, 3.15; N, 1.75. Found: C, 46.03; H, 3.46; N, 1.69. IR
data (KBr): 2924 s, 2853 m, 1655 m, 1637 m, 1527 w, 1495 m,
Experimental Section
General Conditions. All reactions were carried out under an
atmosphere of nitrogen using standard Schlenk techniques.
Tetrahydrofuran, toluene, n-hexane, and n-pentane were pur-
ified by distillation from sodium/benzophenone ketyl; dichlor-
omethane was purified by distillation from calcium hydride.
Instruments. Infrared spectra were recorded with a FT-Nico-
let IR 200. The 1H NMR spectra were recorded with a Bruker
Avance III 500 spectrometer operating at 500.303 MHz in
the Fourier transform mode; the 13C{1H} NMR spectra were
recorded at 125.800 MHz. Chemical shifts are reported in δ
(parts per million) downfield from tetramethylsilane with the
solvent as reference signal (1H NMR: CHCl3, δ 7.26; 13C{1H}
NMR: CDCl3, δ 77.00). The melting points of analytically pure
samples (sealed off in nitrogen-purged capillaries) were deter-
mined using a Gallenkamp MFB 595 010 M melting point
apparatus. Microanalyses were performed using a Thermo
FLASHEA 1112 Series instrument. Spectro-electrochemical
measurements were carried out in an OTTLE cell similar to
(23) Robin, M. B.; Day, P. Adv. Inorg. Chem. Radiochem. 1967, 10,
247.
(25) Gilow, H. M.; Burton, D. E. J. Org. Chem. 1981, 46, 2221.
(26) (a) Massey, A. G.; Park, A. J.; Stone, F. G. A. Proc. Chem. Soc.
1963, 212. (b) Massey, A. G.; Park, A. J. J. Organomet. Chem. 1964, 2, 245.
ꢁ
ꢁ
(24) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem. 1991,
317, 179.

Article
Organometallics, Vol. 30, No. 3, 2011 563
1461 w, 1261, w, 1109 s, 845 s. HR-ESI-MS [m/z]: 255.5345
.
0.01 mol L-1 [AgNO3] and 0.1 mol L-1 [N(nBu)4][B(C6F5)4]
3
3
[M - 2 PF6]2þ
in acetonitrile, in a Luggin capillary with a Vycor tip. This Luggin
capillary was inserted into a second Luggin capillary with Vicor
3
Synthesis of 2,3,4,5-Tetraferrocenyl-1-phenyl-1H-pyrrole (6).
To 1.22 g (6.54 mmol) of ferrocene and 112 mg (1 mmol) of
KOtBu dissolved in 30 mL of tetrahydrofuran was added 6.1 mL
(9.81 mmol) of a 1.6 M solution of tert-butyllithium in n-pentane
at -30 °C. After 1 h of stirring, 4.2 g (15 mmol) of dry
tip filled with a 0.1 mol L-1 [N(nBu)4][B(C6F5)4] solution in
3
acetonitrile. Successive experiments under the same experimen-
tal conditions showed that all formal reduction and oxidation
potentials were reproducible within 5 mV. Experimentally
potentials were referenced against an Ag/Agþ reference elec-
trode, but results are presented referenced against ferrocene as
an internal standard, as required by IUPAC.14 To achieve this,
since the ferrocene couple FcH/FcHþ interferes with the ferro-
cenyl signals of 4 and 6, each experiment was first performed in
the absence of any internal standard and then repeated in the
[ZnCl2 2thf] was added. The solution was kept for 1 h at -30 °C
3
and an additional hour at 25 °C. Afterward, 70 mg (0.06 mmol)
of [Pd(PPh3)4] and 500 mg (1.09 mmol) of 2,3,4,5-tetrabromo-
1-phenyl-1H-pyrrole were added in a single portion, and the
reaction solution was stirred for 48 h at 60 °C. After evapora-
tion of all volatiles, the precipitate was dissolved in 200 mL
of dichloromethane and washed three times with 100 mL
portions of water. The organic phase was dried over MgSO4,
and the solvent was evaporated to dryness in an oil-pump
vacuum. The remaining solid was purified by column chroma-
tography on alumina eluting with a n-hexane-toluene mixture
of ratio 1:1 (v/v). All volatiles were removed under reduced
pressure. Compound 6 was obtained as an orange crystalline
material. Yield: 634 mg (1.03 mmol, 68% based on 5). Anal.
Calcd for C50H41Fe4N (879.25): C, 68.30; H, 4.70; N, 1.59.
Found: C, 67.92; H, 4.81; N, 1.52. Mp: 182 °C. IR data (KBr):
3083 w, 2922 m, 2851 w, 1653 m, 1498 m, 1409 m, 1104 s, 1037 m,
1001 s, 817 vs, 736 m. 1H NMR (CDCl3, δ): 3.80 (s, 10H, C5H5),
3.95 (s, 10H, C5H5), 4.07 (bs, 4H C5H4), 4.10 (pt, JHH=1.8 Hz,
4H, C5H4), 4.19 (pt, JHH = 1.8 Hz, 4H, C5H4), 4.60 (bs, 4H,
C5H4). 13C{1H} NMR (CDCl3, δ): 66.43 (C5H4), 67.13 (C5H4),
69.00 (C5H4), 69.54 (C5H4), 70.24 (Ci-C5H4) 70.76 (C5H4), 72.43
(C5H4), 80.40 (Ci-C5H4), 84.63 (C4N), 122.59 (C4N), 128.31
(C6H5), 129.05 (C6H5), 132.09 (C6H5), 140.40 (Ci-C6H5). ESI-
MS [m/z]: 879.0651[M]þ.
presence of<1 mmol L-1 decamethylferrocene (Fc*). A sepa-
3
rate experiment containing only ferrocene and decamethylfer-
rocene was also performed. Data were then manipulated on a
Microsoft Excel worksheet to set the formal reduction poten-
tials of the FcH/FcHþ couple to 0.0 V. Under our conditions the
Fc*/Fc*þ couple was at -619 mV vs FcH/FcHþ, ΔEp=60 mV,
while the FcH/FcHþ couple itself was at 220 mV vs Ag/Agþ, ΔEp
=61 mV.27
Crystal data for 6 CHCl3: C51H42Cl3Fe4N, Mr = 998.61
˚
3
g mol-1, triclinic, P1, λ = 0.71073 A, a = 11.5111(6) A, b =
˚
3
˚
˚
11.9216(5) A, c=15.8448(11) A, R=84.922(4)°, β=76.734(5)°, γ
=70.897(4)°, V=1999.61(19) A , Z=2, Fcalcd=1.659 g cm-3, μ
3
˚
3
=1.663 mm-1, T=100 K, θ range=3.02-26.00°, reflections
collected 14 970, independent 7824 (Rint=0.0288), R1=0.0325,
wR2 = 0.0656 [I > 2σ(I)]. Single crystals of 6 CHCl3 were
3
obtained by diffusion of n-hexane into a chloroform solution
containing 6 at 25 °C. Data were collected with an Oxford
Gemini S diffractometer, with graphite-monochromatized Mo
˚
KR radiation (λ = 0.71073 A) using oil-coated shock-cooled
crystals. The structure was solved by direct methods and refined
Electrochemistry. Measurements on 1.0 mmol L-1 solutions
3
of 4 and 6 in dry, air-free dichloromethane containing
by full-matrix least-squares procedures on F2.28
0.1 mol L-1 of [N(nBu)4][B(C6F5)4] or [N(nBu)4][PF6] as supporting
3
Acknowledgment. We are grateful to the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for generous financial support.
electrolyte were conducted under a blanket of purified argon at
25 °C utilizing a Voltalap PGZ 100 radiometer 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/
Supporting Information Available: Crystallographic data of 6
as CIF files are available free of charge via the Internet at http://
pubs.acs.org. Crystallographic data of 6 are also available from
the Cambridge Crystallographic Database as file no. CCDC
792152.
Agþ (0.01 mol L-1 [AgNO3]) reference electrode, mounted on a
3
Luggin capillary was used. The working electrode was pre-
treated by polishing on a Buehler microcloth first with 1 μm
and then 1/4 μm diamond paste. The reference electrode was
constructed from a silver wire inserted into a solution of
(28) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46, 467. (b)
Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refine-
(27) (a) Ruiz, J.; Astruc, D. C. R. Acad. Sci. Paris, Seꢀr. II c 1998, 21.
(b) Ruiz Aranzaes, J.; Daniel, M.-C.; Astruc, D. Can. J. Chem. 2006, 84, 288.
€
ment; University of Gottingen, 1997.