Long-range intramolecular electronic communication in bis(ferrocenylethynyl) complexes incorporating conjugated heterocyclic spacers: Synthesis, crystallography, and electrochemistry

Article abstract of DOI:10.1021/ic3024887

A new series of bis(ferrocenylethynyl) complexes, 3-7, and a mono(ferrocenylethynyl) complex, 8, have been synthesized incorporating conjugated heterocyclic spacer groups, with the ethynyl group facilitating an effective long-range intramolecular interaction. The complexes were characterized by NMR, IR, and UV-vis spectroscopy as well as X-ray crystallography. The redox properties of these complexes were investigated using cyclic voltammetry and spectroelectrochemistry. Although there is a large separation of ～14 A between the two redox centers, ΔE _1/2 values in this series of complexes ranged from 50 to 110 mV. The appearance of intervalance charge-transfer bands in the UV-vis-near-IR region for the monocationic complexes further confirmed effective intramolecular electronic communication. Computational studies are presented that show the degree of delocalization across the Fc-C≡C-C≡C-Fc (Fc = C ₅H₅FeC₅H₄) highest occupied molecular orbital.

Full text of DOI:10.1021/ic3024887

Article
pubs.acs.org/IC
Long-Range Intramolecular Electronic Communication in
Bis(ferrocenylethynyl) Complexes Incorporating Conjugated
Heterocyclic Spacers: Synthesis, Crystallography, and
Electrochemistry
Hakikulla H. Shah,† Rayya A. Al-Balushi,† Mohammed K. Al-Suti,† Muhammad S. Khan,*,†
Christopher H. Woodall,‡ Kieran C. Molloy,‡ Paul R. Raithby,*,‡ Thomas P. Robinson,‡ Sara E. C. Dale,‡
and Frank Marken*,‡
†Department of Chemistry, Sultan Qaboos University, P.O. Box 36, Al-Khodh 123, Sultanate of Oman
‡Department of Chemistry, University of Bath, Bath BA2 7AY, U.K.
S
* Supporting Information
ABSTRACT: A new series of bis(ferrocenylethynyl) com-
plexes, 3−7, and a mono(ferrocenylethynyl) complex, 8, have
been synthesized incorporating conjugated heterocyclic spacer
groups, with the ethynyl group facilitating an effective long-
range intramolecular interaction. The complexes were
characterized by NMR, IR, and UV−vis spectroscopy as well
as X-ray crystallography. The redox properties of these
complexes were investigated using cyclic voltammetry and
spectroelectrochemistry. Although there is a large separation of ∼14 Å between the two redox centers, ΔE1/2 values in this series
of complexes ranged from 50 to 110 mV. The appearance of intervalance charge-transfer bands in the UV−vis−near-IR region
for the monocationic complexes further confirmed effective intramolecular electronic communication. Computational studies are
presented that show the degree of delocalization across the Fc−CC−CC−Fc (Fc = C5H5FeC5H4) highest occupied
molecular orbital.
We have synthesized a new series of bis(ferrocenylethynyl)
complexes, 3−7, and a mono(ferrocenylethynyl) complex, 8,
incorporating novel heterocyclic spacer groups. Heterocyclic
spacer groups, quinoline and benzothiadiazole, have been
utilized for complexes 3 and 4, respectively (Scheme 1). These
spacers have been fruitfully used as such and in substituted
forms for the development of sensors, taking advantage of the
conjugated framework.54−60 Heterocyclic spacers such as
phenyl-substituted thieno[3,4-b]pyrazine have shown to be
excellent precursors for the production of low-band-gap
conjugated polymers61−63 and have been utilized for the
synthesis of complexes 5 and 6. Similarly, fused thiophenes
show better conjugation than their non-fused analogues64 and
thus are used for the synthesis of complexes 7 and 8.
Combining the conjugation properties of the spacers and
connecting them to a terminal ferrocene via a rigid rod such as
an alkyne should enhance the electronic communication
between the two metal centers.
INTRODUCTION
■
There is considerable contemporary interest in metal-
containing polymers in which metal centers are linked by
conjugated moieties because these have the potential for facile
electron transfer between metals; such species are potential
molecular wires, with application in the downscaling of diverse
electronic devices.1−15 Ferrocene-based materials have been
central to this research because the complexes are often
synthetically robust, show well-defined redox chemistry, and
readily support mixed-valence systems.16−22 In particular, a
wide range of ferrocene moieties linked by conjugated spacers
such as alkenes,16,23−27 alkynes,28 and/or aromatic rings29−32
have been synthesized and their properties reported. We have a
long-standing interest in this general area, and in particular the
synthesis and characterization of oligomeric platinum33−36 and
gold37−40 species linked by alkynes, because these serve as
model systems for long-chain polymers. In this paper, we turn
our attention to related systems in which ferrocene groups are
linked by alkyne/heterocyclic spacers, combining synthesis,
crystallography, and spectroelectrochemical studies. Several
previous studies have reported alkyne-bridged mixed-valence
bis-ferrocene complexes16,41−50 as well as structural studies of
ferrocene groups linked by alkynes and/or oligothiophenes,
fluorenes, or similar heterocycles.31,51−53
Herein we report the synthesis, characterization, and
electrochemical studies of complexes 2−8 along with X-ray
crystallographic studies of complexes 2, 4−6, and 8. The effect
of the different spacer groups on the redox properties of the
Received: November 14, 2012
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ic3024887 | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Synthesis of Bis(ferrocenylethynyl) Complexes 2−7 and Mono(ferrocenylethynyl) Complex 8
FABMS: m/z 211 (M+). Anal. Calcd for C12H10Fe: C, 68.62; H, 4.80.
Found: C, 68.53; H, 4.86.
ferrocene is investigated by cyclic voltammetry (CV). The
signatures of the monocationic species formed during the long-
range intramolecular interaction of the ferrocene units are
studied by spectroelectrochemistry. Pure spectra for the
monocations are established with the help of spectral
deconvolution. Interaction parameters such as the ferrocene-
to-ferrocene distance, the separation of the ferrocene reversible
potentials, and the features of the intervalence charge-transfer
(IVCT) band are discussed.
Fc−CC−CC−Fc (2). Ethynylferrocene (0.105 g, 0.50 mmol)
and diisopropylamine (5 mL) were mixed with catalytic amounts of
Pd(OAc)2 (2 mg), CuI (2 mg), and PPh3 (5 mg). The mixture was
allowed to reflux for 15 h under aerobic conditions, after which all
volatile components were removed under reduced pressure. The
residue was dissolved in CH2Cl2 and chromatographed through a silica
column using hexane/CH2Cl2 (1:1, v/v) as the eluent. The title
compound was obtained as a dark-red powder in 95% yield (0.20 g).
IR (CH2Cl2): 2148 cm−1 [ν(CC)]. 1H NMR (300 MHz, CDCl3): δ
3.94 (t, 4H, J = 1.7 Hz, Cp), 4.13 (s, 10H, Cp), 4.46 (t, 4H, J = 1.9 Hz,
Cp). FABMS: m/z 419 (M+). Anal. Calcd for C24H18Fe2: C, 68.95; H,
4.34. Found: C, 68.99; H, 4.29.
EXPERIMENTAL SECTION
■
All reactions were carried out under an inert atmosphere using Schlenk
techniques. Solvents were predried and distilled from appropriate
drying agents. All chemicals, unless otherwise stated, were obtained
from commercial sources and used as received. Preparative thin-layer
chromatography was performed on 0.7 mm silica plates. The key
starting material, ethynylferrocene, was synthesized by adaptation of a
literature method.25,65,66 The NMR spectra were recorded on a Bruker
AM-400 spectrometer in CDCl3. The 1H NMR spectra were
referenced to solvent resonances. IR spectra were recorded as
CH2Cl2 solutions, in a NaCl cell, on a Nicolet-Impact 400D FT-IR
spectrometer amd mass spectrometry (MS) spectra on a Kratos MS
890 spectrometer by electron impact and fast atom bombardment
(FAB) techniques. Microanalyses were performed in the Department
of Chemistry, University of Bath, Bath, U.K. Computations were
performed on the University of Bath’s High Performance Computing
Facility. Column chromatography was performed on either Kieselgel
60 (230−400 mesh) silica gel or alumina (Brockman grade II−III).
Synthesis. Ethynylferrocene65 (1). Acetylferrocene (2.14 g, 10
mmol) and triphenylphosphine (10.48 g, 40 mmol) in anhydrous
acetonitrile (20 mL) at 0 °C under an argon atmosphere were added
to 3.08 g (20 mmol) of tetrachlormethane in one portion. The mixture
was warmed to room temperature. Stirring was continued for 45 min,
and then 5 mL of distilled water was added to the solution. The
mixture was extracted with ether (50 mL × 3), washed with brine, and
then dried over anhydrous magnesium sulfate (MgSO4). Solvent was
evaporated, and the residue was dissolved in dichloromethane and
filtered through a plug of alumina. After removal of the solvent under
reduced pressure, a red crystalline intermediate compound was
obtained in 85% yield (2.4 g). A total of 1.68 g (6 mmol) of this
intermediate compound in 10 mL of dry THF at 0 °C was added to 8
mL (12 mmol) of n-BuLi (1.5 M in THF) under rigorous stirring for
10 min. The reaction mixture was warmed to room temperature, and
stirring was continued for 15 min followed by hydrolysis with 10 mL
of distilled water and stirring for another 10 min. The mixture was
extracted with ether (50 mL × 3) and the combined organic layer was
dried over MgSO4. After filtration through a plug of alumina and
removal of solvent under reduced pressure, the title compound 1 was
obtained as a red crystalline solid in 93% yield (1.1 g). IR (CH2Cl2):
2110 [ν(CC)], 3301 cm−1 [ν(CCH)]. 1H NMR (300 MHz,
CDCl3): δ 4.21 (s, 5H), 4.19 (t, 2H), 4.47 (t, 2H), 2.71 (s, 1H).
Fc−CC−R−CC−Fc (3; R = Quinoline-5,8-diyl). Under an
argon atmosphere, a solution of ethynylferrocene (0.23 g, 1.1 mmol)
and 5,8-diiodoquinoline67 (0.14 g, 0.5 mmol) in diisopropylamine (15
mL) was mixed with catalytic amounts of Pd(OAc)2 (3 mg), CuI (3
mg), and PPh3 (10 mg). The mixture was allowed to reflux for 24 h,
after which all volatile components were removed under reduced
pressure. The residue was dissolved in CH2Cl2 and chromatographed
through a silica column using hexane/CH2Cl2 (2:1, v/v) as the eluent.
The title compound was obtained as a dark-red power in 53% yield
1
(0.29 g). IR (CH2Cl2): 2188 cm−1 [ν(CC)]. H NMR (300 MHz,
CDCl3): δ 4.11 (pseudo-t, 4H, Cp), 4.31 (s, 10H, Cp), 4.62 (pseudo-t,
4H, Cp), 6.94 (dd, 1H, J = 12.1 Hz, spacer), 7.58 (d, 1H, J = 7.5 Hz,
spacer), 7.84 (d, 1H, J = 7.9 Hz, spacer), 8.77 (d, 1H, J = 8.3 Hz,
spacer), 8.92 (dd, 1H, J = 6.0 Hz, spacer). FABMS: m/z 546 (M+).
Anal. Calc for C33H23Fe2N: C, 72.89; H, 4.25. Found: C, 72.98; H,
4.29.
Fc−CC−R−CC−Fc (4; R = Benzothiadiazole-4,7-diyl). The
title bis(ferrocenylethynyl) compound was prepared by following a
procedure similar to that described above for 3 using ethynylferrocene
(0.23 g, 1.1 mmol) and 4,7-dibromobenzothiadiazole67 (0.14 g, 0.5
mmol), giving a brown solid in 82% yield (0.45 g). IR (CH2Cl2): 2184
cm−1 [ν(CC)]. 1H NMR (300 MHz, CDCl3): δ 4.04 (t, 4H, J = 3.4
Hz, Cp), 4.26 (s, 10H, Cp), 4.59 (pseudo-t, 4H, J = 3.8 Hz, Cp), 7.30
(d, 2H, J = 7.5 Hz, spacer). FABMS: m/z 553 (M+). Anal. Calcd for
C30H20Fe2N2S: C, 65.25; H, 3.65. Found: C, 65.31; H, 3.71.
Fc−CC−R−CC−Fc (5; R = Diphenylthienopyrazine-5,7-diyl).
The title bis(ferrocenylethynyl) compound was prepared by a
procedure similar to that described for 3 using ethynylferrocene
(0.23 g, 1.1 mmol) and 5,7-dibromodiphenylthienopyrazine68 (0.22 g,
0.5 mmol) to obtain a dark-violet powder in 72% yield (0.51 g). IR
1
(CH2Cl2): 2199 cm−1 [ν(CC)]. H NMR (300 MHz, CDCl3): δ
4.04 (pseudo-t, 4H, J = 3.4 Hz, Cp), 4.26 (s, 10H, Cp), 4.59 (pseudo-t,
4H, J = 3.8 Hz, Cp), 7.40 (m, 2H, J = 4.9 Hz, spacer), 7.60−7.63 (m,
8H, spacer). FABMS: m/z 705 (M+). Anal. Calcd for C42H28Fe2N2S:
C, 71.61; H, 4.01. Found: C, 71.68; H, 4.06.
Fc−CC−R−CC−Fc (6; R = Difluorodiphenylthienopyrazine-
5,7-diyl). The title bis(ferrocenylethynyl) compound was prepared by
a procedure similar to that described for 3 using ethynylferrocene
(0.23 g, 1.1 mmol) and 5,7-dibromo(difluorodiphenyl)-
thienopyrazine68 (0.24 g, 0.5 mmol) to obtain a dark-violet powder
B
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Inorganic Chemistry
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Table 1. Crystallographic Data for 4−6 and 8
4
5
6
8
empirical formula
fw
C30H20Fe2N2S
552.24
C42H28Fe2N2S
704.42
C42H26F2Fe2N2S
740.41
C20H11BrFeS3
483.23
cryst syst
space group
a (Å)
monoclinic
C2/c
triclinic
triclinic
triclinic
P1
̅
P1
̅
P1
̅
30.6586(3)
9.8566(1)
21.6227(3)
7.6536(5)
12.7415(7)
16.5787(11)
100.757(5)
97.360(6)
92.718(5).
1571.00(17)
2
7.4305(5)
13.2951(6)
16.4962(10)
82.729(4)
85.045(5)
80.538(5)
1590.97(16)
2
7.867(5)
10.000(5)
11.548(5)
77.054(5)
86.042(5)
87.945(5)
883.1(8)
2
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
volume (Å3)
Z
ρcalc (Mg m−3
μ(Mo Kα) (mm−1
134.389(1)
4669.35(9)
8
)
1.571
1.489
1.546
1.817
)
1.355
1.025
1.025
3.470
F(000)
2256
724
756
480
cryst size (mm)
θ range (deg)
reflns collected
0.20 × 0.16 × 0.16
4.14−25.35
38948
0.4 × 0.2 × 0.05
2.81−26.37
13087
0.30 × 0.30 × 0.10
2.79−24.71
14712
0.3 × 0.2 × 0.1
3.06−29.61
15957
indep reflns [R(int)]
max, min transmn
GOF on F2
4258 [0.0431]
0.879, 0.805
1.155
6388 [0.0603]
1.000, 0.873
0.981
5432 [0.0599]
1.000, 0.882
1.102
4434 [0.0450]
1.000, 0.758
0.955
final R1, wR2 [I > 2σ(I)]
final R1, wR2 (all data)
largest diff peak, hole (e Å−3
0.0368, 0.1040
0.0452, 0.1092
0.515, −0.914
0.0537, 0.1021
0.0888, 0.1178
0.639, −0.422
0.0615, 0.1640
0.0777, 0.1780
1.119, −0.870
0.0422, 0.1020
0.0684, 0.1165
0.788, −0.763
)
in 74% yield (0.55 g). IR (CH2Cl2): 2199 cm−1 [ν(CC)]. 1H NMR
(300 MHz, CDCl3): δ 4.04 (pseudo-t, 4H, J = 3.4 Hz, Cp), 4.26 (s,
10H, Cp), 4.59 (pseudo-t, 4H, J = 3.8 Hz, Cp), 6.74 (dd, 4H, J = 8.7
Hz, spacer), 7.33 (dd, 4H, J = 7.3 Hz, spacer). FABMS: m/z 741 (M+).
Anal. Calcd for C42H26F2Fe2N2S: C, 68.13; H, 3.54. Found: C, 68.78;
H, 3.49.
Fc−CC−R−CC−Fc (7; R = Dithienothiophene-2,5-diyl). The
title bis(ferrocenylethynyl) compound was prepared by a procedure
similar to that described for 3 using ethynylferrocene (0.23 g, 1.1
mmol) and 2,5-dibromodithienothiophene40 (0.16 g, 0.50 mmol) to
obtain an orange powder in 68% yield (0.21 g). IR (CH2Cl2): 2199
cm−1 [ν(CC)]. 1H NMR (300 MHz, CDCl3): δ 4.28 (s, 10H, Cp),
4.30 (pseudo-t, 4H, J = 3.4 Hz, Cp), 4.55 (pseudo-t, 4H, J = 3.8 Hz,
Cp), 7.39 (s, 2H, spacer). FABMS: m/z 612 (M+). Anal. Calcd for
C32H20Fe2S3: C, 62.76; H, 3.29. Found: C, 62.78; H, 3.19.
disk electrode was used as the working electrode and a platinum wire
served as the counter electrode, while a KCl-saturated calomel
electrode (Radiometer ref 401) served as the reference electrode.
Under these conditions, the reversible potential for ferrocene is E1/2
=
0.527 V. Electrolyte solutions were prepared from dichloroethane
(DCE) and [n-Bu4N][PF6] (Fluka, dried in oil-pump vacuum) as the
supporting electrolyte. The respective organometallic complexes were
added at ca. 1 mM concentration. Cyclic voltammograms were
recorded using a micro-Autolab III (Ecochemie, The Netherlands).
Digisim, version 2.0, was employed to simulate CV data.
Spectroelectrochemistry. Spectroelectrochemistry was per-
formed in a home-built optically transparent thin-layer electrolysis
(OTTLE) cell by laminating a silver wire (reference), platinum mesh
(10 mm × 7.5 mm, working), and platinum wire (auxiliary).71 The
electrode was used in a 0.1-cm-path-length quartz UV−vis cell. Spectra
were recorded with reference to the spectrum of the pure solvent by
carrying out an initial baseline correction without any potential applied
to the solvent-filled cell. UV−vis spectra were then recorded with
compounds dissolved in DCE with [n-Bu4N][PF6] at different applied
potentials. UV−vis data were obtained at a rate of 600 nm min−1. For
each measurement, the potential of the OTTLE cell was kept at a
constant value and the absorbance spectrum of the solution was
recorded between 200 and 1100 nm. Data analysis was based on
principle-component analysis programmed in MATLAB (version
Fc−CC−R (8; R = 5-Bromodithienothiophene-2-yl). The title
mono(ferrocenylethynyl) compound was prepared by reacting
ethynylferrocene (0.23 g, 1.1 mmol) and 2,5-dibromodithienothio-
phene (0.33 g, 1.0 mmol) at 60 °C for 12 h to obtain orange crystals in
1
54% yield (0.27 g). IR (CH2Cl2): 2199 cm−1 [ν(CC)]. H NMR
(300 MHz, CDCl3): δ 3.95 (pseudo-t, 2H, J = 3.8 Hz, Cp), 4.11 (s,
5H, Cp), 4.46 (pseudo-t, 2H, J = 3.8 Hz, Cp), 6.62 (s, 1H, spacer),
7.13 (s, 1H, spacer). FABMS: m/z 483 (M+). Anal. Calcd for
C20H11BrFeS3: C, 49.71; H, 2.29. Found: C, 49.78; H, 2.19.
Crystallography. Single-crystal X-ray diffraction experiments were
performed at 150(2) K on either an Oxford Diffraction Gemini A
Ultra CCD diffractometer (5, 6, and 8) or an Nonius Kappa CCD
diffractometer (2 and 4) using monochromatic Mo Kα radiation (λ =
0.71073 Å). For 5, 6, and 8, the sample temperature was controlled
using an Oxford Diffraction Cryojet apparatus; CrysAlis Pro was used
for collecting frames of data, indexing reflections, and determining
lattice parameters. For 2 and 4, temperature control using an Oxford
Cryostream device. A multiscan absorption correction was applied in
all cases. Structures were solved by direct methods using SHELXS-8669
and refined by full-matric least squares on F2 using SHELX-97.70
Crystallographic data for all complexes studied can be found in Table
1.
̂
̂
̂
̂
2010b, Mathworks, Inc.). The equation X = C·S + E was used
̂
̂
withX, the experimental spectral matrix at three selected potentials, C,
̂
̂
the concentration coefficients, S, the pure-component spectra, and E,
the error matrix to be minimized for deconvolution into the pure
spectra.72−74
RESULTS AND DISCUSSION
■
Synthesis and Spectroscopic Characterization. The
key starting material for the bis(ferrocenylethynyl) compounds,
ethynylferrocene (1), was prepared in good yield by adaptation
of a literature procedure by Luo et al.65 in preference to other
reported synthetic methods for this compound.25,66 The
dibromo/diiodo heteroaromatic precursors for 3−7 were
prepared as reported previously.40,67,68,75 The syntheses of 2−
Electrochemistry. Cyclic voltammograms were recorded in a
dried glass cell purged with purified argon. A 3-mm-diameter platinum
C
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Inorganic Chemistry
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8 are shown in Scheme 1. The cross-coupling reactions
between ethynylferrocene and dibromo/diiodo heteroaromatic
precursors in a 2:1 stoichiometry, in i-Pr2NH/CH2Cl2, in the
presence of a PdII/CuI catalyst readily gave the bis-
(ferrocenylethynyl) compounds 3−7, while the oxidative
homocoupling of 1 under aerobic conditions yielded complex
2. Complex 8 was synthesized by a 1:1 reaction between 1 and
2,5-dibromodithienothiophene using a lower reaction temper-
ature (60 °C) and a shorter reaction time (12 h). The products
of cross-coupling and homocoupling reactions were purified by
silica gel column chromatography, giving orange-red crystals (2,
4, and 8) and dark-blue/black crystals (5 and 6) in respectable
yields (50−90%). All bis(ferrocenylethynyl) complexes (2−7)
and the monoferrocenylethynyl complex 8 are indefinitely
stable to light and air at ambient temperature and were fully
characterized by IR and NMR spectroscopy, FAB-MS, as well
as by satisfactory elemental analysis.
The IR spectra of the bis(ferrocenylethynyl) complexes show
a single, sharp ν(CC) in the 2200−2010 cm−1 range,
characteristic of other ethynylferrocenyls42,43,76 containing
1
aromatic and heteroaromatic spacer groups. The H NMR
spectra showed a characteristic pattern of singlet and triplet
absorption at ∼4 ppm for the unsubstituted and substituted
cyclopentadienyl protons, respectively. The aromatic and
heteroaromatic spacers gave signals in the 7−8 ppm regions
as singlet, doublets, doublet of doublets, and complex
multiplets as expected. MS spectra (positive-mode FAB)
displayed the presence of molecular ions with characteristic
fragmentation patterns for the complexes. The structures of
complexes 2, 4−6, and 8 were confirmed by X-ray
crystallography.
The electronic absorption spectra of complexes 2−8 were
recorded in CH2Cl2 (Table 2). Each compound displays three
Table 2. UV−Vis Spectral and Spectroelectrochemical
Results for Neutral and Monocationic Forms of 1−8 in DCE
λmax/nm
[complex]
[complex]+
Figure 1. Structure of 4 showing (a, top) the asymmetric unit and
labeling scheme used in the text, (b, middle) dimerization via short
N···S contacts, and (c, bottom) the offset in heterocycle stacking.
Thermal ellipsoids are at the 30% level. Selected geometric data: Fe1−
C(1,5) ring centroid 1.6558(13), Fe1−C(6,10) ring centroid
1.6440(11), Fe2−C(21,25) ring centroid 1.6470(15), Fe1−C(26,30)
ring centroid 1.6570(15), C6−C11 1.432(3), C11−C12 1.186(3),
C12−C13 1.438(3), C16−C19 1.432(3), C17−C18 1.414(4), C19−
C20 1.187(3), C20−C21 1.430(4), N1−S1 1.616(2), N2−S1
1.613(2), C14−N1 1.343(3), C14−C15 1.430(3), C15−N2
1.346(3), S1−N1′ 3.098(2) Å; C12−C11−C6 177.4(3), C11−C12−
C13 178.1(3), C20−C19−C16 171.1(3), C19−C20−C21 179.5(4)°.
1
2
3
4
5
6
7
8
267, 399, 514
283, 323, 466
246, 282, 350, 468
292, 366, 457
281, 338
a
309, 395, 551, 766
a
268, 294, 340, 433, 568, 867
a
299, 391, 564, 786
308, 491, 515, 677
268, 297, 407, 501
316, 382
a
322, 370, 396, 478
261, 363, 466
265, 302, 342, 510, 1020
b
284, 308, 417, 563, 760
a
b
Spectroscopic data for the IVCT band. Spectroscopic data for the
MLCT band.
sets of absorption bands. Bands with λmax below 400 nm can be
attributed to a π−π* transition associated with the organic
spacer group. A weak absorption band at ∼450 nm is assigned
to a FeII d−d transition15 but is overlapped by the strong, broad
higher-energy peak at ∼400 nm arising from a π−π* transition
associated with the organic spacer group.
Structural Studies. The structures of 4−6 and 8 are shown
in Figures 1−4, respectively, along with selected geometric data.
Room temperature data for 2 have been reported previ-
ously,45,42 but our low-temperature data are included in Table 3
for a direct comparison across the range of structures at a
constant temperature. In 2, the two ferrocenyl units are linked
by a −CC−CC− spacer leading to a Fe···Fe separation of
9.5965(5) Å; the two ferrocenyl groups are disposed in an anti
manner with respect to each other at the termini of the
conjugated alkynes. The CC bond is the longest [1.201(3)
Å] seen in this study (Table 3) and is accompanied by a C−C
single bond between alkynes [1.374(4) Å], which is the
shortest observed, although the other structures reported
herein have the Fc−CC bonded to a heterocycle, not
another alkyne [Fc = (C5H5)Fe(C5H4)]. These bond lengths
suggest some delocalization along the CC−CC unit. The
−CC−CC− unit is essentially linear, with only minor
deviations from ideal bond angles of 180° at the sp C atoms
(Table 3); the −CC−CC− torsion angle is −8.7°. The
D
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S1···N1 = 3.098(2) Å], although the planes of the two
heterocycles are offset (Figure 2c).
The structures of 5 (Figure 2) and 6 (Figure 3a) are related
to that of 4 but now incorporate a thienopyrazine spacer
substituted at the 5 and 6 positions of the pyrazine ring; in 4,
the alkyne is bonded to the six-membered ring, while in 5 and
6, it is bonded to the smaller ring. The fused five- and six-
membered rings common to 4−6 have similar dimensions, and
although the alkynes are linked differently between the two
systems, this has little impact on the Fe···Fe separation [4,
13.4942(5) Å; 5, 13.0742(11) Å; 6, 13.2918(12) Å]. It should,
however, be noted that, in solution, free rotation about the Fc−
C(C) bond will lead to a variety of Fe···Fe distances. The
key bond lengths within the Fc−CC−C unit (figure captions
and Table 3) are the same within experimental error as those
for 4, but there are differences in the spatial orientation of the
Fc units with respect to each other. Thus, in 5, the two Fc units
are anti across the extended −CC−(Het)−CC moiety (as
seen in 2), while in 6, they approach orthogonality [torsion
angle between C5H4 planes C6−C10 and C19−C23 =
80.1(2)°] similar to 4; this has the effect of making the
Fe···Fe separation marginally shorter than that in 6. The lattice
structure of 6 (Figure 3b) shows short F···S contacts [F1···S1 =
3.315(3) Å] and π stacking of heterocycles in a head-to-tail
manner with an interplane separation of ca. 4 Å [plane centroid
to plane = 3.681(4) Å]. In contrast, there are no close
intermolecular contacts of any significance in 5 [shortest, C9−
H1···N2 = 2.650 Å], and although pairs of molecules stack with
a separation of ca. 4 Å [plane centroid to plane = 4.087(2) Å],
the two heterocycles are significantly offset with respect to each
other.
The structure of 8 (Figure 4a) has the Fc unit linked via the
alkyne to a 5-bromodithienothiophene-2-yl-fused tricycle. The
key bond distances and angles (figure captions) are similar to
the structures already described (Table 3). The Fc ring is
orthogonal to the heterocycle [torsion angle between the C6−
C10 ring and the best plane through the heterocycle =
85.21(11)°] so that the organometallic and heterocycle units
conjugate with different π components of the alkyne, as seen in
4 and 6. The lattice of 8 reveals short S···S contacts [S1···S2′
3.322(4) Å], generating dimers that stack with heterocycles
head-to-tail with each other [Figure 4b; plane centroid to plane
= 3.469(2) Å]. There is also a visible curvature of the CC−
Fc moiety away from the plane of the heterocycle, as seen also
in 4 and 6.
Figure 2. Structure of 5 showing the asymmetric unit and labeling
scheme used in the text. Thermal ellipsoids are at the 30% level.
Selected geometric data: Fe1−C(1,5) ring centroid 1.6477(19), Fe1−
C(6,10) ring centroid 1.6422(18), Fe2−C(19,23) ring centroid
1.6455(18), Fe1−C(24,28) ring centroid 1.6475(18), C8−C11
1.439(5), C11−C12 1.191(5), C12−C13 1.414(5), C16−C17
1.426(5), C17−C18 1.189(4), C18−C19 1.439(5), C16−S1
1.724(3), C13−S1 1.727(4), C14−N1 1.370(4), C14−C15
1.431(4), C15−N2 1.371(4), C15−C16 1.394(5) Å; C12−C11−C8
175.9(4), C11−C12−C13 174.3(4), C18−C17−C16 179.9(4), C17−
C18−C19 175.5(4)°.
C5H4 unit connected to this latter fragment is close to being
coplanar with it [torsion angle between C5H4 and C−CC =
178.84(17)°], and the exocyclic Fc−C(C) bond length
[1.428(3) Å], comparable with the C−C bond lengths within
the Cp ring, also suggests some degree of multiple-bond
character.
In 4, the two alkynes in 2 are further separated by a
benzothiadiazole-4,7-diyl spacer, increasing the Fe···Fe separa-
tion to 13.4942(5) Å (Figure 1a). While the exocyclic bond
between C5H4 and CC remains the same as those in 2 [C6−
C11 and C21−C20 = 1.432(3) and 1.430(4) Å, respectively],
CC appears to shorten [1.186(3) and 1.187(3) Å] but just
remains within 3σ of the analogous bond length in 2.
However, the bonds at the other end of the alkyne [C12−C13
and C16−C19 = 1.438(3) and 1.432(8) Å, respectively] are
lengthened with respect to 2, and collectively the data suggest a
more localized CC, which retains some possible conjugation
with the organometallic fragment but less so with the
heterocycle. This asymmetry is also manifested in the angular
distortion at C19 [171.1(3)°]. Moreover, the orientation of the
two Fc units with respect to each other also differs markedly
from that of 2. While each Fc remains coplanar with CC
[torsion ∠C8−C7−C6−C11 = −179.9°; torsion ∠C23−C22−
C21−C20 = −178.9°], the two Fc units are close to orthogonal
to each other [torsion angle between C5H4 planes C6−C10 and
C21−C25 = 108.74(12)°], implying that any conjugation is
with differing components of the π manifold at either end of
the molecule. The benzothiadiazole heterocycle is planar and
coplanar with the C6−C10 ring of Fc1 but twisted out of
conjugation with Fc2. In addition, there is a distinct curvature
of the CC−Fc2 fragment upward from the plane of the
heterocycle (a feature also seen in 6 and 8 but not 5; see
below). This difference may arise from the fact that there are
close S···N contacts between pairs of molecules (Figure 1b;
Overall, there is little variation in the geometric parameters
associated with any putative conjugation across these molecules
(Table 3), and, indeed, with similar systems previously
reported,43,44,46,47 save for the fact that data for 2 shows
potentially the greatest delocalization of the π-electron density
between metal centers. This is supported by the IR data, which
show a much lower ν(CC) for 2 (2148 cm−1) than the other
complexes structurally characterized (2184−2199 cm−1). There
are differences in the relative orientations (in the solid state) of
the Fc moieties at either end of the molecule, between those
that are anti (2 and 5) and those that are orthogonal (4, 6, and
8), although in all of these latter cases, there are significant
intermolecular contacts that conceivably cause reorientation of
the Fc units to accommodate packing.
Electrochemistry and Spectroelectrochemistry. Cyclic
voltammograms for the oxidation of bis(ferrocenylethynyl)
complexes 2−7 and mono(ferrocenylethynyl) complex 8 in
DCE were recorded as a function of the scan rate (20−1000
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Figure 3. Structure of 6 showing (a, top) the asymmetric unit and labeling scheme used in the text and (b, bottom) dimerization via F···S contacts.
Thermal ellipsoids are at the 30% level. Selected geometric data: Fe1−C(1,5) ring centroid 1.646(2), Fe1−C(6,10) ring centroid 1.647(2), Fe2−
C(19,23) ring centroid 1.639(2), Fe1−C(24,28) ring centroid 1.645(3), C8−C11 1.431(7), C11−C12 1.192(7), C12−C13 1.406(7), C16−C17
1.425(7), C16−S1 1.717(5), C17−C18 1.190(7), C18−C19 1.429(7), C13−S1 1.721(4), C16−S1 1.717(5), C14−N1 1.368(6), C15−N2 1.356(6),
F1···S1′ 3.315(3) Å; C12−C11−C8 176.4(5), C11−C12−C13 175.5(5), C18−C17−C16 176.3(5), C17−C18−C19 176.2(6)°. Symmetry
operation: x, y − 1, z.
mV s−1) and over a 0−1 V potential range. All complexes were
reversibly oxidized as expected for mono- or bis-
(ferrocenylethynyl) derivatives connected via conjugated
spacers29,30 (see Figure 5). E1/2 values ranging from 610 to
750 mV for the current series (compared to that for ferrocene,
E1/2 = 0.527 V) indicate the electron-withdrawing nature of the
spacers (see Table 4). Two clearly resolved overlapping
oxidation waves were observed in the case of complex 2
containing conjugated polymers resulted in enhanced con-
ductivities despite the absence of peak separation in the metal
redox waves.9,77−79
Some other reports have also shown substantial electronic
interactions even in the absence of any observable peak
separation. For example, despite small RuII/III peak separations
in the voltammetry, the hybrid metallopolymers bearing
bis(2,2′-bipyridyl)ruthenium moieties on a conjugated back-
bone80,81 showed electron diffusion coefficients greater than
those for comparable non-conjugated materials by an order of
magnitude. Broad CV peaks similar to those of complexes 3−7
have been reported for ferrocenylethynyl polyynes and
oligoynes, and the broadening is attributed to the presence of
closely spaced redox processes.82−84
(Figure 5A) with a separation of ca. ΔE1/2 = 105 mV [here
II
ΔE1/2 = E1/2 − E1/2I) with the midpoint potential E1/2
=
0.5(Ep,ox + Ep,red)]. This indicates a moderately strong
electronic interaction between the two Fe atoms, which is
not surprising given the shorter Fe−Fe distance (9.597 Å) and
good conjugation in this case.
For complexes 3−7, broadened CV peaks without significant
splitting of the midpoint potentials were observed under similar
conditions (see Figure 5). This could result from a considerably
longer Fe−Fe distance, i.e., ca. ∼14 Å. Several studies have
shown that substantial electronic interaction may still occur in
cases where conjugated organic spacers are used to link the
metal centers. Swager reported that redox matching between
the metal and organic components in several transition metal-
To investigate the extent of broadening of the CV peaks in
complexes 3−7, the CV features were reproduced by digital
simulation with Digisim (see Figure 5). Successful simulations
of the main features in experimental CVs showed that the
broadening in the CV peaks can be reconciled with the
presence of two individual, closely spaced, one-electron
processes. Figure 5 documents the agreement between the
experimental and simulated CVs for the bis(ferrocenylethynyl)
F
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Figure 4. Structure of 8 showing (a, top) the asymmetric unit and labeling scheme used in the text and (b, bottom) dimerization via short S···S
contacts. Thermal ellipsoids are at the 30% level. Selected geometric data: Fe1−C(1,5) ring centroid 1.6497(19), Fe1−C(6,10) ring centroid
1.6428(18), C8−C11 1.435(5), C12−C11 1.182(5), C13−C12 1.426(5), S1−C16 1.719(4), S1−C13 1.758(4), S3−C15 1.741(4), S3−C18
1.743(4), S2−C20 1.723(4), S2−C17 1.722(3), Br1−C20 1.867(4), S1···S2′ 3.322(4) Å; C12−C11−C8 179.7(4), C11−C12−C13 176.6(4)°.
Symmetry operation: 1 − x, 1 − y, 1 − z.
Table 3. Comparison of Key Structural Data for the Fc−CC−C Unit of 2, 4−6, and 8
CC (Å)
Fc−C(C) (Å)
(C)C−C (Å)
∠Fc−CC−C (deg)
Fe···Fe (Å)
a
2
4
1.201(3)
1.186(3)
1.187(3)
1.191(5)
1.189(4)
1.192(7)
1.190(7)
1.182(5)
1.428(3)
1.432(3)
1.430(4)
1.439(5)
1.439(5)
1.431(7)
1.429(7)
1.435(5)
1.374(4)
1.438(3)
1.432(8)
1.414(5)
1.426(5)
1.406(7)
1.425(7)
1.426(5)
178.7(2), 179.8(3)
177.4(3), 178.1(3)
171.1(3), 179.4(4)
175.9(4), 174.3(4)
179.9(4), 175.5(4)
176.4(5), 175.5(5)
176.3(5), 176.2(6)
179.7(4), 176.6(4)
9.5965(5)
13.4942(5)
5
6
8
13.0742(11)
13.2918(12)
a
Data collected at 150 K as part of this work. Room temperature data are given in refs 45 and 51.
Table 4. Electrochemical Data in Millivolts versus SCE for
Complexes 2−8 Obtained from Voltammograms in DCE
Containing 0.1 M [n-Bu4N]PF6 at ca. 20 °C (Errors Are
Estimated)
I
II
E1/2
E1/2
ΔE1/2 IVCT bandwidth
5
5
5
at half-height
εIVCT
a
complex (mV) (mV)
(mV)
(cm−1
)
(L mol−1 cm−1
)
2
3
4
5
6
7
8
645
612
630
650
650
620
610
750
667
740
720
730
672
105
55
2364
2512
1988
212
478
b
110
70
542
c
c
b
e
c
c
80
d
e
d
52
2843
441
e
e
a
Values obtained from digital simulation of the cyclic voltammograms.
b
Estimated values from digital simulation in the presence of an
Figure 5. Simulation curves (red circles) matched with cyclic
voltammograms (black line) for bis(ferrocenylethynyl) complexes 2
(A), 3 (B), 5 (C), and 7 (D) in DCE at 25 °C with 0.1 M [n-
Bu4N]PF6 as the supporting electrolyte and at scan rate 100 mV s−1.
c
unknown impurity. Due to the limited spectral window, the IVCT
band could not be located. IVCT band, in part, outside the analysis
range; thus,
d
e
a
bandwidth estimate is reported. Mono-
(ferrocenylethynyl) complex.
complexes 2, 3, 5, and 7. The simulation CVs for complexes 4
and 6 (not shown here) had minor additional, and so far
unidentified, impurity oxidation peaks.
A recent report demonstrated significant electronic commu-
nication in bis(ferrocenyl) complexes separated by electron-
withdrawing spacers and having Fe−Fe distances of <8 Å.16
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Figure 6. UV−vis spectra of complexes 2, 3, 7, and 8 in a DCE solution at different potentials applied in OTTLE cells with [n-Bu4N]PF6 as the
supporting electrolyte [data for the neutral spectra (i), the monocation (ii), and difference spectra (iii−v) are shown]: (A) 2 with (i) neutral, (ii) 650
mV, and (iii−v) 540, 650, and 710 mV; (B) 3 with (i) neutral, (ii) 620 mV, and (iii−v) 580, 620, and 660 mV ;(C) 7 with (i) neutral, (ii) 630
mV,and (iii−v) 600, 630, and 660 mV; (D) 8 with (i) neutral, (ii) 650 mV, and (iii−v) 620, 650, and 690 mV, respectively.
Interestingly, 2, which has a Fe−Fe distance of 9.597 Å, and 4,
which has a Fe−Fe distance of 13.494 Å, still exhibit similar
ΔE1/2, i.e., 105 and 110 mV, respectively, which is not too
dissimilar compared to values for p-diferrocenylbenzene.28 The
electron-withdrawing nature of the spacer could have an impact
on the net conjugation effect, overriding the Fe−Fe distance
effect in this particular comparison. Other bimetallic complexes
supported by bis(NHC) (NHC = N-heterocyclic carbene)
ligands exhibit weaker interactions (ΔE1/2 = 42−80 mV)
despite having direct metal−NHC connections and metal−
metal distances of less than 11 Å.82 ΔE1/2 values of 80 and 70
mV were found for 6 and 5, respectively, although they have
Fe−Fe distances similar to that of complex 4. This can be
explained by the fact that the connecting unit in 5 and 6 is
substituted thiophene, which is less conjugated than the
substituted benzene unit in complex 4. Recent reports suggest
that electron-withdrawing spacers play an important role in the
communication of the terminal ferrocene units.16,85,86 This is
reflected in the ΔE1/2 values for complexes 2−7, which range
from 50 to 110 mV. While there is no clear relation between
the half-wave potential splitting and the strength of the
electronic interaction between coupled redox sites,87,88 the
values of ΔE1/2 (Table 4) suggest that they belong to class II
according to the Robin and Day classification scheme89 with
modest coupling.
UV−vis spectra were recorded at different applied potentials
for complexes 2−8. An initial spectrum was collected in an
OTTLE cell without applying any potential, and a series of
spectra were then collected by gradually changing the applied
I
potential. The spectra collected in the proximity of the E1/2
value were used for deconvolution to obtain a pure spectrum of
the monocationic species (see the Experimental Section).
Figure 6 summarizes neutral and monocation spectra and
shows difference spectra where weak bands are more clearly
resolved. The oxidation of complexes 2−8 resulted in strong
absorption bands with λmax in the range 260−310 nm assigned
to a π−π* transition in the organic spacer groups. The shoulder
at ∼440−570 nm in these spectra is due to a Cp → FeIII ligand-
to-metal charge-transfer (LMCT) band and has been reported
for related compounds.90,91 The broad absorption bands close
to the near-IR (NIR) region can be assigned as IVCT bands.
Upon oxidation of 2 to 2+, the intensity of the low-energy
MLCT bands at 395 nm decreases, while the intensity of the
higher-energy, predominantly π−π* band increases. In
addition, new broad bands at 551 and ∼766 nm appear in
the spectrum. The NIR band was assigned as an IVCT
transition. The IVCT nature was confirmed because this band
disappears upon further oxidation by increasing the potential.92
Similar observations were found for 3, where during the
spectroelectrochemical oxidation of 3 to 3+ the intensity of the
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low-energy MLCT bands at 484 nm decreases, while the
intensity of the higher-energy π−π* band increases; in addition,
new bands at 568 and 867 nm appear in the spectrum. The
former band might consist of overlapping MLCT and LMCT
transitions. The NIR was assigned as an IVCT transition
(Figure 6B). The spectroelectrochemical oxidation of com-
plexes 4−7 is consistent with the data collected for 2 and 3
(Table 1); for 5 and 6, the IVCT bands could not be observed
in the spectral window. In Figure 6C(i),(ii), the spectra of
complexes 7 and 7+ show subtle shifts in the visible absorption,
where there is an increase in the intensity of the π−π* band at
higher energy and a red shift of the initial MLCT band at 466
nm to a new band at 510 nm. The IVCT band here appears at
much longer wavelength, ∼1020 nm, and continues outside the
analysis range. The LMCT band at 510 nm for complex 7 can
be compared with the LMCT band at 563 nm of complex 8,
i.e., a mono(ferrocenylethynyl) complex with similar spacer
group (Figure 6D). This band in 8 is further shifted to lower
energy compared to 7 because of the inductive effect of the
terminal bromine. Further, 8 shows a MLCT band at ∼760 nm.
It is interesting to compare optical and electrochemical data.
A stronger intramolecular interaction should correspond to an
increased ΔE1/2, but also the IVCT band oscillator strength
given by 4.6 × 10−9εmaxΔv1/2 (εmax is the extinction coefficient
maximum, and Δv1/2 is the half-width of the IVCT band93)
should increase. However, this predicted trend cannot be
confirmed here. As the ΔE1/2 value increases, the IVCT
bandwidth appears to decrease, which suggests a lower
oscillator strength at assumed similar extinction maxima. For
example, in the series of complexes 3, 2, and 4, the ΔE1/2 values
increase 50, 105, and 110 mV, but the IVCT bandwidth at half-
height decreases as 2512, 2364, and 1988 cm−1, respectively.
Clearly structural effects introduced by the spacer system and
additional configurational changes in solution could add
complexity and limit the applicability of the ΔE1/2 oscillator
strength correlation. Further experimental work, in particular
taking into account solvent polarity effects, will be desirable.
Computational Studies. IR and structural analysis
indicated that complex 2 may potentially have the greatest
delocalization of the π-electron density between metal centers.
The electrochemistry results (ΔE1/2 = 105 mV) for complex 2
comparable to complex 4 (ΔE1/2 = 110 mV) motivated us to
select complexes 2 and 4 to conduct computational studies to
gain better insight into the intramolecular interaction processes.
We have therefore attempted to model the delocalization
computationally using the B3LY94 hybrid density functional
under the Gaussian09 package95 for complexes 2 and 4. The
SDD pseudopotential and associated basis set96 was used for Fe
atoms, and the 6-31G(d)97 basis set was used for all other
atoms. Geometry optimizations were performed and frequency
calculations were used to confirm that the stationary points
were true minima; pictures of the highest occupied molecular
orbitals (HOMOs) for 2 and 4 are given in Figure 7. For 2, the
HOMO shows extensive delocalization across the whole
molecule, with contributions of 26 (Fe), 11 (C5H4), and 23%
(CC−CC) from the contributing fragments. Similarly, the
HOMO for 4 (Figure 7, right), although less symmetrical than
that for 2, has contributions of 33, 20 (Fe, Fe), 11, 8 (C5H4,
C5H4). 7, 7 (CC, CC), and 14% (C6NSN).
CONCLUSION
■
We have successfully established a synthetic protocol for
bis(ferrocenylethynyl) complexes 3−7 and mono-
(ferrocenylethynyl) complex 8 and characterized these
complexes using NMR, IR, mass, and UV−vis spectroscopies.
Complexes 4−6 and 8 were characterized by X-ray
crystallography. The redox properties of these complexes
were investigated using a CV approach and digital simulation
revealing two one-electron oxidation processes with differences
ranging from 50 to 110 mV. Spectroelectrochemistry
performed in an OTTLE cell gave a clear indication of the
formation of monocationic species. The appearance of IVCT
bands for complexes 2−4 and 7 further confirms the
monocationic species. This work is an example of longer
range electronic interaction where the Fe−Fe distance is ∼14 Å
and the spacer is electron-withdrawing. Computational studies
show that there is significant electron delocalization between
iron centers in the HOMOs of both 2 and 4. It is demonstrated
that the conjugated spacer is important in tuning the optical
and redox properties of the bis(ferrocenylethynyl) complexes.
The results obtained have important implications for the design
and synthesis of metal-containing conjugated polyynes and
oligoynes.
ASSOCIATED CONTENT
■
S
* Supporting Information
Crystallographic data for structural analysis (in CIF format).
This material is available free of charge via the Internet at
http://pubs.acs.org. These have also been deposited with the
Cambridge Crystallographic Data Center as CCDC 610675−
610679 for 2, 4−6, and 8, respectively. Copies of this
information may be obtained from the Director, CCDC, 12
Union Road, Cambridge CB21EZ, U.K. (fax +44-1233-336033;
e-mail deposit@ccdc.cam.ac.uk, or weblink www.ccdc.cam.ac.
uk).
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: msk@squ.edu.om (M.S.K.), p.r.raithby@bath.ac.uk
(P.R.R.), f.marken@bath.ac.uk (F.M.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge the Sultan Qaboos University, Oman, for a
research grant and for a research leave to M.S.K. H.H.S. and
R.A.A. acknowledge Sultan Qaboos University, Oman, for
Ph.D. scholarships. We are grateful to the British Council for a
PMI-2 grant (GS 216) that has supported M.S.K., M.K.A.,
H.H.S., P.R.R. and K.C.M. P.R.R. gratefully acknowledges
support from the EPSRC through the award of a Senior
Fellowship. We also thank Dr. Dariusz Matoga of Jagiellonian
University, Poland, for helpful discussions on some CV data.
Figure 7. HOMOs of 2 (left) and 4 (right).
I
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