The influence of an ethynyl spacer on the electronic properties in 2,5-ferrocenyl-substituted heterocycles Dedicated to Prof. Dr. Claude Lapinte for his outstanding contribution to organometallic chemistry especially to electron transfer studies.

Article abstract of DOI:10.1016/j.poly.2014.02.008

A series of binuclear complexes based on five-membered heterocycles featuring alkynyl ferrocenyl groups, 2,5-(FcCC)₂-^cC₄H₂E (E = NPh, 3a; S, 3b; O, 3c), has been synthesized using the Sonogashira C,C cross-coupling protocol. The molecular structures of 3a and 3c in the solid state are discussed. The influence of the ethynyl unit on the electronic and (spectro)electrochemical properties of these compounds is shown by cyclic voltammetry (CV), square wave voltammetry (SWV) and in situ UV-Vis-NIR and IR spectroscopy. The high ΔE_p values (140-204 mV) of the single reversible redox events indicate that two individual oxidation processes take place in a close potential range. Nevertheless, the NIR measurements indicate weak and broad intervalence charge transfer (IVCT) absorptions for the mixed-valent monocationic species 3a-c⁺. Compared to 2,5-Fc₂-^cC₄H₂E (E = NPh, S, O) the absorptions for the IVCT transitions are shifted hypsochromically, showing that the charge transfer process is promoted at higher excitation energies, due to larger metal-metal distances in 3a-c. The occurrence of IVCT absorptions demonstrates that the 2,5-diethynylferrocenyl heterocycles are class II systems according to Robin and Day.

Full text of DOI:10.1016/j.poly.2014.02.008

Polyhedron xxx (2014) xxx–xxx
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
The influence of an ethynyl spacer on the electronic properties
in 2,5-ferrocenyl-substituted heterocycles
⇑
Ulrike Pfaff, Alexander Hildebrandt, Marcus Korb, Heinrich Lang
Technische Universität Chemnitz, Faculty of Natural Sciences, Institute of Chemistry, Inorganic Chemistry, D-09107 Chemnitz, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of binuclear complexes based on five-membered heterocycles featuring alkynyl ferrocenyl
groups, 2,5-(FcC„C)2-cC4H2E (E = NPh, 3a; S, 3b; O, 3c), has been synthesized using the Sonogashira
C,C cross-coupling protocol. The molecular structures of 3a and 3c in the solid state are discussed. The
influence of the ethynyl unit on the electronic and (spectro)electrochemical properties of these com-
pounds is shown by cyclic voltammetry (CV), square wave voltammetry (SWV) and in situ UV–Vis–NIR
Received 7 November 2013
Accepted 5 February 2014
Available online xxxx
Dedicated to Prof. Dr. Claude Lapinte for his
outstanding contribution to organometallic
chemistry especially to electron transfer
studies.
and IR spectroscopy. The high
DEp values (140–204 mV) of the single reversible redox events indicate
that two individual oxidation processes take place in a close potential range. Nevertheless, the NIR mea-
surements indicate weak and broad intervalence charge transfer (IVCT) absorptions for the mixed-valent
monocationic species 3a–c+. Compared to 2,5-Fc2-cC4H2E (E = NPh, S, O) the absorptions for the IVCT
transitions are shifted hypsochromically, showing that the charge transfer process is promoted at higher
excitation energies, due to larger metal–metal distances in 3a–c. The occurrence of IVCT absorptions
demonstrates that the 2,5-diethynylferrocenyl heterocycles are class II systems according to Robin and
Day.
Keywords:
Ferrocene
Alkyne
Heterocycles
(Spectro)electrochemistry
Electron transfer
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
n = 1, D D
E°0 = 130 mV; n = 2, E°0 = 100 mV; [NnBu4][BF4] as sup-
porting electrolyte). Furthermore, the research groups of Lapinte
[28–30], Low [31] and Bruce [8] investigated homo-binuclear
half-sandwich compounds of type [CpꢀdppeM(–C„C–)nMCpꢀdppe]
(M = Fe, Ru, Os; dppe = 1,2-bis(diphenylphosphino)ethane;
n = 1, 2, . . ., 8). With an increasing number of ethynyl spacer units,
and therefore a larger metal–metal distance, a smaller redox sepa-
ration and a shift of the redox processes for the terminal metal
centers to anodic potentials was observed. This shows that the
oxidation energy correlates with the length of the carbon chain.
Besides the construction of long carbon chains, the –C„C–
moiety has been applied to connect different aromatic backbone
units with redox-active metal-containing groups in order to build
up extended structures [32–35]. In this respect, Justin and Lin
have shown on the example of 2,3,4,5-tetra-ethynylferrocenyl
thiophene that the ethynyl functionality suppresses the electronic
coupling of the ferrocenyls through the heteroaromatic ring in the
mixed-valent species [36]. They found that all four ferrocenyls are
oxidized simultaneously (E°01 = 148 mV, [NnBu4][ClO4] as electro-
lyte). In contrast, 2,3,4,5-tetraferrocenyl thiophene possesses redox
events corresponding to the individual metal center with relatively
In the last decades, the research on ‘‘molecular wire molecules’’
within the field of organometallic chemistry has become more and
more intense, due to the increasing demand for miniaturization in
microelectronics. It could be shown that compounds with p-conju-
gated systems, such as sp2- and sp-hybridized carbon chains like
polyenes (–CH@CH–)n (n = 1–6) [1–4] and polyynes (–C„C–)n
(n = 1–8) [5–8], which act as connecting units between two termi-
nal redox-active groups, have great potential as components in
molecular electronics [1,9–12]. The electronic interaction between
the respective termini is highly dependent on the number [8,13,14]
and variety [15–20] of the linking groups. The influence of
the –C„C– moiety as connecting unit within the mixed-valent
systems on the electronic interactions has extensively been studied
[21–24], with ferrocenyl as redox-active terminal group, due to its
excellent electrochemical properties [25,26].
In 1976 Cowan et al. [27] described a series of diferrocenes
containing (–C„C–)n connectivities (n = 0, 1, 2) showing a decreas-
ing redox separation in the mixed-valent species, which depends
on the number of -C„C– units present (n = 0,
D
E°0 = 350 mV;
high
E°03 = 186 mV; [NnBu4][B(C6F5)4] as electrolyte) in the mixed-
valent state [18] .
redox
separations
(
D
E°01 = 219 mV,
D
E°02 = 360 mV,
D
⇑
Corresponding author. Tel.: +49 (0)371 531 21210; fax: +49 (0)371 531 21219.
E-mail address: heinrich.lang@chemie.tu-chemnitz.de (H. Lang).
http://dx.doi.org/10.1016/j.poly.2014.02.008
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
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2
U. Pfaff et al. / Polyhedron xxx (2014) xxx–xxx
On a series of diferrocenyl heterocycles, including diferrocenyl
0.1 mol Lꢁ1 [NnBu4][B(C6F5)4] solution in dichloromethane [42–
49]. Successive experiments under the same experimental condi-
tions showed that all formal reduction and oxidation potentials
were reproducible within 5 mV. Experimentally potentials were
referenced against an Ag/Ag+ reference electrode but results are
presented referenced against ferrocene as an internal standard
as required by IUPAC [50]. When decamethylferrocene was used
as an internal standard, the experimentally measured potential
was converted into E versus FcH/FcH+ by addition of ꢁ0.61 V
[51]. Data were then manipulated on a Microsoft Excel worksheet
to set the formal reduction potentials of the FcH/FcH+ couple to
E°0 = 0.0 V. Under our conditions the FcH/FcH+ couple itself was at
thiophene, -furan, and -pyrroles [18] it was observed that the elec-
tronic nature of the heterocycle has a great effect on the electronic
coupling of the ferrocenyl/ferrocenium termini. Within the here
presented work, we aim to show how the influence of the different
heterocyclic unit is changed, when the distance between the het-
erocyclic backbone and the redox-active ferrocenyl groups is in-
creased by introducing ethynyl spacer units. Therefore, a series of
2,5-di(ethynylferrocenyl) heterocycles including thiophene
[36,37], furan, and N-phenyl pyrrole have been synthesized and
characterized (spectro)electrochemically.
220 mV versus Ag/Ag+,
DEp = 61 mV [52–54]. The cyclic voltammo-
2. Experimental
grams were taken after typical two scans and are considered to be
steady state cyclic voltammograms in which the signal pattern
differs not from the initial sweep.
2.1. General data
All reactions were carried out under an atmosphere of nitrogen
using standard Schlenk techniques. Tetrahydrofuran and n-hexane
were purified by distillation from sodium/benzophenone ketyl;
diethyl ether was purified by distillation from sodium; dichloro-
methane, N,N-diisopropylamine and N,N-dimethylformamide were
purified by distillation from calcium hydride.
2.5. Spectroelectrochemistry
Spectroelectrochemical UV–Vis/NIR and IR measurements of
5.0 mmol Lꢁ1 (UV–Vis/NIR) or 10 mmol Lꢁ1 (IR) solutions of 3a–c
in dry dichloromethane containing 0.1 mol Lꢁ1 of [NnBu4][B(C6F5)4]
as the supporting electrolyte were performed in an OTTLE
(=Optically Transparent Thin-Layer Electrochemistry, quartz
windows for UV–Vis–NIR; CaF2 windows for IR) [55] cell with a
Varian Cary 5000 spectrophotometer (UV–Vis/NIR) or a Thermo
Nicolet IR 100 spectrometer (IR) at 25 °C. Between the spectro-
scopic measurements the applied potentials have been increased
step-wisely using step heights of 100, 50, 25 or 15 mV. At the
end of the measurements the analyte was reduced at ꢁ500 mV
for 15 min and an additional spectrum was recorded to prove the
reversibility of the oxidations.
2.2. Reagents
N-Phenylpyrrole, furan, N-bromosuccinimide, N-iodosuccini-
mide, 2,5-dibromo thiophene (1b), triphenylphosphane and cop-
per(I)iodide were purchased from commercial suppliers and
used without further purification. Ethynylferrocene (2) [38]
and [N(nBu)4][B(C6F5)4] [39,40] were prepared according to
published procedures. The palladium catalyst [PdCl2(PPh3)2] was
synthesized according to Miyaura et al. [41].
2.6. General procedure for the synthesis of 2,5-diethynylferrocenyl
heterocycles 3a-c
2.3. Instruments
1H 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 re-
ported in d (parts per million) using deuterated solvent residues
as internal standard (chloroform-d3: 1H at 7.26 ppm and 13C{1H}
at 77.16 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
an Gallenkamp MFB 595 010 M melting point apparatus.
Microanalyses were performed using a Thermo FLASHEA 1112
Series instrument. High-resolution mass spectra were performed
with a micrOTOF QII Bruker Daltonite workstation.
For the Sonogashira C,C cross-coupling reactions 6 mol% of [CuI]
and 0.5 mol% of [PdCl2(PPh3)2] were dissolved in 60 mL of Pr2NH
i
and the reaction mixture was stirred for 10 min. The orange solu-
tion was treated with 1 eq of 2,5-X2-cC4H2E (E = NPh (X = I), 3a; S
(X = Br), 3b; O (X = Br), 3c), [37] 2.1 eq of ethynylferrocene and
6 mol% of triphenylphosphane and was heated to reflux overnight.
The reaction mixture was cooled to ambient temperature and all
volatiles were removed in an oil pump vacuum. The residue was
washed with diethyl ether and filtered through a pad of Celite to
remove the ammonium salt until the eluent was colorless. The
orange filtrate was concentrated and purified by column chroma-
tography (column size: 1.5 ꢂ 10 cm, alumina). For column chroma-
tography a n-hexane/diethyl ether mixture of ratio 10:1 (v/v) was
2.4. Electrochemistry
used as eluent. The 1st fraction contained ethynylferrocene, while
from the 2nd fraction compounds 3a–c could be isolated. All
volatiles were removed under reduced pressure.
Measurements on 1.0 mmol Lꢁ1 solutions of the analytes in
dry air free dichloromethane containing 0.1 mol Lꢁ1 of [NnBu4]
[B(C6F5)4] as supporting electrolyte were conducted under a blan-
ket of purified argon at 25 °C utilizing a Radiometer Voltalap PGZ
100 electrochemical workstation interfaced with a personal com-
puter. 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 Lꢁ1 AgNO3) reference electrode mounted on a
Luggin capillary was used. The working electrode was pretreated
2.6.1. Data for 2,5-diethynylferrocenyl-N-phenyl pyrrole (3a)
2,5-Diiodo-N-phenyl pyrrole (1a) (0.50 g, 1.26 mmol), [CuI]
(14.5 mg, 0.08 mmol), [PdCl2(PPh3)2] (4.4 mg, 6.33ꢃ10ꢁ6 mol),
PPh3 (19.9 mg, 0.08 mmol) and 2.1 eq of FcC„CH (558 mg,
2.66 mmol) were reacted. Yield: 210 mg (0.38 mmol, 30% based
on 1a); orange solid, soluble in diethyl ether and dichloromethane.
Anal. Calc. for C34H25Fe2N (559.25 g/mol) [%]: C, 73.02; H, 4.51; N,
2.50. Found: C, 72.76; H, 4.57; N, 2.58%. Mp.: 188 °C. 1H NMR
[CDCl3, ppm] d: 4.02 (s, 10 H, C5H5), 4.16 (pt, JHH = 1.9 Hz, 4 H,
C5H4), 4.33 (pt, JHH = 1.9 Hz, 4 H, C5H4), 6.51 (s, 2 H, H-3/4), 7.45
(m, 1 H, C6H5/p-H), 7.56 (m, 2 H, C6H5), 7.61 (m, 2 H, C6H5).
13C{1H} NMR [CDCl3, ppm] d: 64.9 (FcC„C), 68.9 (C5H4), 70.0
(C5H5), 71.4 (C5H4), 77.7 (Ci-C5H4), 92.0 (FcC„C), 115.2 (C-3/4),
by polishing on a Buehler microcloth first with a 1 lm and then
with a 1/4 lm diamond paste. The reference electrode was con-
structed from a silver wire inserted into a solution of 0.01 mol Lꢁ1
[AgNO3] and 0.1 mol Lꢁ1 [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 a vycor tip filled with a
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U. Pfaff et al. / Polyhedron xxx (2014) xxx–xxx
3
118.0 (Ci-2/5), 127.9 (C6H5), 128.1 (C6H5/p-C), 128.5 (C6H5), 138.7
(Ci-C6H5). IR data [KBr, cmꢁ1
: 772 (s, do.o.p. C–H), 999 (s, dC–N),
1496 (s, C@C), 1592 (m, C@C), 2208 (m, C„C), 3091 (w, C–H).
34H25Fe2N: 559.0681, found:
3. Results and discussion
3.1. Synthesis and characterization
]
m
@
m
m
m
m@
HR-ESI-MS [m/z]: calcd for
C
559.0653 [M]+.
The synthetic approach to obtain the 2,5-diethynylferrocenyl
heterocyclic complexes 2,5-(FcC„C)2-cC4H2E (Fc = Fe( 5-C5H5)
5-C5H4); E = NPh, 3a; S, 3b; O, 3c) is outlined in Scheme 1 [37].
g
(g
2.6.2. Data for 2,5-diethynylferrocenyl furan (3c)
2,5-Diiodo-N-phenyl pyrrole (1a) and 2,5-dibromo thiophene
(1b) and furan (1c) have been prepared by an electrophilic substi-
tution of the protons in 2 and 5 position with N-iodo- or N-bromo-
succinimide according to Gilow et al. [58] and were used in the
synthesis of 3a–c without further purification. The ethynylferroce-
nyl substituents were introduced by the Sonogashira C,C cross-
coupling reaction of ethynylferrocene (2) with 1a–c. After appro-
priate work-up, compounds 3a-c were obtained as orange solids
in moderate yields (Section 2).
2,5-Dibromofuran (1c) (0.50 g, 2.21 mmol), [CuI] (25.3 mg,
0.13 mmol), [PdCl2(PPh3)2] (7.8 mg, 0.01 mmol), PPh3 (34.8 mg,
0.13 mmol) and 2.1 eq of ethynylferrocene (976 mg, 4.65 mmol)
were reacted. Yield: 234 mg (0.48 mmol, 22% based on 1c); orange
solid, soluble in diethyl ether and dichloromethane. Anal. Calc. for
C
28H20Fe2O (484.15 g/mol) [%]: C, 69.46; H, 4.16. Found: C, 69.67;
H, 4.30%. Mp.: 150 °C (decomp.). 1H NMR [CDCl3, ppm] d: 4.26 (s,
10 H, C5H5), 4.27 (pt, JHH = 1.9 Hz, 4 H, C5H4), 4.52 (pt, JHH = 1.9 Hz,
4 H, C5H4), 6.55 (s, 2 H, H-3/4). 13C{1H} NMR [CDCl3, ppm] d: 63.8
(FcC„C), 69.4 (C5H4), 70.3 (C5H5), 71.6 (C5H4), 75.8 (Ci-C5H4), 93.6
The title compounds 3a–c are stable to air and moisture both in
the solid state and in solution. They are poorly soluble in non-polar
solvents such as n-hexane, toluene, and diethyl ether but dissolve
rather well in dichloromethane and tetrahydrofuran. Characteriza-
tion details for 3a and 3c by NMR (1H, 13C{1H}) and IR spectros-
copy, ESI-TOF mass spectrometry as well as elemental analysis
are given in the Experimental Part. The electrochemical behavior
of 3a–c was determined by cyclic voltammetry (=CV), square wave
voltammetry (=SWV) and in situ UV–Vis/NIR and IR spectroscopy.
The 1H NMR spectra of 3a–c display one sharp singlet for the
C5H5 groups and two pseudo triplets (JHH = 1.9 Hz) for the protons
of the C5H4 ligands as expected for a AA ‘XX’ spin system [59]. For
the heteroaromatic moiety a singlet (6.51 ppm, 3a; 6.55 ppm, 3c)
could be detected. The signals are shifted to higher field due to
the mesomeric (+M) effect of the phenyl group and the better over-
lap of the nitrogen (pyrrole), or oxygen (furan) orbitals with the
carbon ones [60], when compared with 3b (7.04 ppm) [37]. Besides
the absorptions characteristic for the heterocyclic, phenyl and
ferrocenyl units, most distinctive in the IR spectra of 3a-c is the
(FcC„C), 115.5 (C-3/4), 137.8 (Ci-2/5). IR data [KBr, cmꢁ1
]
m
: 791 (s,
do.o.p.
@C–H), 964 (s, dC-C), 1429 (m, mC@C), 1578 (m, mC@C), 2204
(m,
m
C„C), 3097, 3130 (w, m@C–H). HR-ESI-MS [m/z]: calcd for
C28H20Fe2O: 484.0208, found: 484.0184 [M]+.
2.7. Single crystal X-ray diffraction analysis
Data for 3a and 3c were collected on an Oxford Gemini S diffrac-
tometer using graphite-monochromatized Mo
Ka radiation
(k = 0.71073 Å). The molecular structures were solved by direct
methods using SHELXS-97 [56] and refined by full-matrix least-
squares procedures on F2 using SHELXL-97 [57]. All non-hydrogen
atoms were refined anisotropically and a riding model was em-
ployed in the treatment of the hydrogen atom positions.
m
(C„C) vibration for the ethynyl unit at 2008 cmꢁ1
.
2.7.1. Crystal data for 3a
Single crystals of 3a were obtained by diffusion of toluene into a
dichloromethane solution containing 3a at 25 °C. C34H25Fe2N,
Mr = 559.25 g molꢁ1, crystal dimension 0.45 ꢂ 0.38 ꢂ 0.05 mm, tri-
3.2. Crystallography
ꢀ
clinic, P1, k = 0.71073 Å, a = 7.4190(4) Å, b = 10.6092(5) Å,
The molecular structures of 3a and 3c in the solid state have
been determined by single crystal X-ray diffraction analysis. Suit-
able crystals were obtained by diffusion of toluene into a saturated
dichloromethane solution containing the respective compound at
ambient temperature. Important bond distances (Å), bond angles
(°) and torsion angles (°) are summarized in the captions of Figs. 1
and 2. For crystal and structure refinement data see Experimental
Part. The crystallographic investigations confirm a molecular archi-
c = 32.0745(12) Å,
V = 2519.1(2) Å3, Z = 4,
T = 100 K, range = 3.10–25.25°, reflections collected 18342,
a
= 90.246(4)°, b = 93.613(4)°,
c
l
= 91.044 (4)°,
q
calcd = 1.475 g cmꢁ3
,
= 1.176 mmꢁ1
,
H
independent 9074, R1 = 0.0372, wR2 = 0.0821 [I P 2
r(I)].
2.7.2. Crystal data for 3c
ꢀ
Single crystals of 3c were obtained by diffusion of toluene into a
dichloromethane solution containing 3c at 25 °C. C28H20Fe2O,
tecture in which 3a crystallizes in the triclinic space group P1 with
two molecules in the asymmetric unit (Fig. SI5, Supporting Infor-
mation), while 3c crystallizes in the monoclinic space group P21/c.
The ferrocenyl groups of 3c are oriented parallel and are almost
coplanar with the heterocyclic core (torsion angle of the C5H4 unit
to the furan core: 0.95(28)° for Fe1 and 3.32(26)° for Fe2), which is
equal to the 2,5-diferrocenyl furan [19]. In contrast, the ferrocenyl
units at 3a are orthogonal oriented (87.33(7)°, 86.09(7)°). As a
Mr = 484.14 g molꢁ1
,
crystal dimension 0.40 ꢂ 0.07 ꢂ 0.07 mm,
monoclinic, P21/c, k = 0.71073 Å, a = 14.467(5) Å, b = 7.432(5) Å,
c = 19.091(5) Å, = 90°, b = 98.699(5)°,
= 90°, V = 2029.0(16) Å3,
Z = 4, T = 293 K,
calcd = 1.585 g cmꢁ3 = 1.448 mmꢁ1
range = 2.95–26.00°, reflections collected 9644, independent 3951,
R1 = 0.0507, wR2 = 0.1126 [I P 2 (I)].
a
c
q
,
l
,
H
r
Scheme 1. Synthesis of 2,5-diethynylferrocenyl-substituted heterocycles 3a-c.
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4
U. Pfaff et al. / Polyhedron xxx (2014) xxx–xxx
Fig. 1. ORTEP diagram (50% probability level) of the molecular structure of 3a with the atom numbering scheme. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å), angles (°) and torsion angles (°): average D–Fe = 1.644, average Fe1–Fe2 = 11.398, N1–C4 = 1.390(3), N1–C1 = 1.302(3), C1–C2 = 1.381(3),
C2–C3 = 1.402(3), C3–C4 = 1.381(3), C4–C17 = 1.424(3), C17–C18 = 1.196(3), C1–C5 = 1.416(3), C5–C6 = 1.201(3); N1–C1–C5 = 119.94(19); N1–C4–C17 = 122.61(19);
C4–C17–C18 = 175.7(2); C17–C18–C19 = 176.4(2), C1–C5–C6 = 174.2(2), C5–C6–C7 = 174.5(2), C4–N1–C1 = 108.26(18), N1–C1–C2 = 107.8(2), average D–Fe–D = 179.5;
C1–C5–C6–C7 = –34(4), C5–C6–C7–C11 = ꢁ154(2), C2–C3–C4–C17 = 176.6(2), C1–C2–C3–C4 = 0.5(3), C4–C17–C18–C19 = 25(7), C29–N1–C1–C5 = ꢁ1.8(3) (D = denotes the
centroids of C5H4 and C5H5).
Fig. 2. ORTEP diagram (50% probability level) of the molecular structure of 3c with the atom numbering scheme. Hydrogen atoms have been omitted for clarity. Selected
bond distances (Å), angles (°) and torsion angles (°): average D–Fe = 1.651, Fe1–Fe2 = 11.059, O1–C4 = 1.382(4), O1–C1 = 1.384(4), C1–C2 = 1.344(5), C2–C3 = 1.413(5),
C3–C4 = 1.351(5), C4–C17 = 1.411(5), C17–C18 = 1.201(5), C1–C5 = 1.401(5), C5–C6 = 1.199(5); O1–C1–C5 = 115.1(3); O1–C4–C17 = 114.2(3); C18–C17–C4 = 174.4(4);
C17–C18–C19 = 176.2(4), C6–C5–C1 = 176.2(4), C5–C6–C7 = 177.2(4), C4–O1–C1 = 106.7(3), C2–C1–O1 = 109.0(3), average D–Fe–D = 179.33; C1–C5–C6–C7 = 138(7),
C5–C6–C7–C11 = 46(7), C2–C3–C4–C17 = ꢁ179.3(4), C1–C2–C3–C4 = 0.1(4), C4–C17–C18–C19 = ꢁ34(9), (D = denotes the centroids of C5H4 and C5H5).
reason for this orientation steric interactions with the N-phenyl
substituent can be excluded. It is more likely that a large T-shaped
p–p stacking network within the packing is the decisive factor
Fig. SI1 and SI2). Dichloromethane solutions containing the analyte
(1.0 mmol Lꢁ1) and [NnBu4][B(C6F5)4] (0.1 mol Lꢁ1) [14,18,19,39,
40,42–49] as supporting electrolyte were used. The data of the
CV measurements have been recorded at a scan rate of 100 mVꢃsꢁ1
and are summarized in Table 1. All redox potentials are referenced
to the FcH/FcH+ redox couple (E°0 = 0.00 mV, FcH = Fe(d5-C5H5)2)
[50].
(Supporting Information Fig. SI5 and SI6) [61,62]. The conforma-
tion of the cyclopentadienyl ligands is rather eclipsed (3a,
6.66(18), ꢁ1.15(16), 2.58(21); 3c, 8.77(27)°) or between eclipsed
and staggered (3a, 11.22(16); 3c, 14.65(26)°). The geometrical dis-
tance between the two iron atoms is 11.0588(27) Å for 3c and
11.3503(6) and 11.4458(6) Å for the two molecules of 3a. The dif-
ference in the Fe-Fe distances between 3a and 3c is caused by the
different angle of the ethynylferrocenyl substituents with the het-
erocycle (3a, N1–C1–C5 = 119.94(19), N1–C4–C17 = 122.61(19)°;
3c, O1–C1–C5 = 115.1(3), O1–C4–C17 = 114.2(3)°) (Fig. 1 and 2).
Fig. 3 shows the cyclic and square wave voltammograms of
binuclear 3a–c. For 2,5-diethynylferrocenyl-N-phenyl pyrrole (3a)
a reversible redox event corresponding to the two ferrocenyl units
could be observed, giving a redox separation of
D
E°0 = 100 mV re-
solved by square wave voltammetry. Additionally, the application
of the method according to Richardson and Taube allows the
estimation of the
D
E°0 value (106 mV) by determining the signal
width at half of the maximum current within the square wave
voltammogram (193 mV) [63]. Both methods give a similar redox
splitting between the individual redox events of the redox active
termini (
the Richardson and Taube method). Thiophene 3b and furan 3c
3.3. Electrochemistry
The redox properties of 3a-c have been determined by cyclic
voltammetry (=CV), square-wave voltammetry (=SWV) (Fig. 3)
and UV–Vis/NIR spectroscopy (Fig. 4, Supporting information
D D
E°0 = 100 mV with deconvolution, E°0 = 106 mV with
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U. Pfaff et al. / Polyhedron xxx (2014) xxx–xxx
5
Fig. 3. Voltammograms of dichloromethane solutions containing 1.0 mmol Lꢁ1 of 3a–c at 25 °C. Supporting electrolyte [N(nBu)4][B(C6F5)4] (0.1 mol Lꢁ1). Left: Cyclic
voltammograms (scan rate: 100 mV sꢁ1). Right: Square-wave voltammograms (scan rate: 1 mV sꢁ1).
8000
6000
4000
2000
0
3000
2000
1000
0
3a
3a+
9000
7000
5000
3000
300
800
1300
1800
2300
2800
(cm-1)
Wavelength (nm)
Wavenumber
16000
3a+
3a2+
12000
8000
4000
0
300
800
1300
1800
2300
2800
Wavelength (nm)
Fig. 4. UV–Vis/NIR spectra of 3a at rising potentials vs Ag/AgCl: ꢁ200–450 mV (left top), 450 to 1000 mV (bottom). Deconvolution of the NIR absorptions at 450 mV (right
top) of in situ generated 3a+ using three Gaussian-shaped bands. Measurement conditions: 25 °C, dichloromethane, 0.1 mol Lꢁ1 [NnBu4][B(C6F5)4] as supporting electrolyte.
analogues show one redox event. However, the high
D
Ep value
intensities are likely to be underestimated. Hence, in situ spectro-
electrochemical UV–Vis/NIR and IR measurements have been car-
ried out, to get a deeper insight into the electronic properties of
the individual oxidation states.
(140 mV, 3b; 144 mV, 3c) of the appropriate single wave suggests
that two individual reversible one-electron processes take place in
a close potential range, indicating a certain degree of intermetallic
interaction in the molecules [64]. Therefore, the Richardson and
D
E°0 values (3b:
½Mn1Mn2þ1ꢅ2
Taube method has been applied, giving low
ꢄ
RT
0
E
ꢃF
KC ¼ eD
¼
ð1Þ
75 mV, 3c: 72 mV). The calculation of the comproportionation
constants (Eq. 1) leads to a procentual ratio of neutral, monocat-
ionic and dicationic species (Table 2). It is noteworthy that the
maximal enrichment of the monocationic species 3a–c+ is not
determinable within an OTTLE cell, therefore the obtained
½Mn1M2nꢅ½M1nþ1M2nþ1
ꢅ
In comparison to 2,5-Fc2-cC4H2E (E = NPh (ꢁ238 mV),
S
(ꢁ94 mV), O (ꢁ152 mV)) [18,49] the redox events of 3a–c are
shifted to more anodic potentials indicating that these compounds
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6
U. Pfaff et al. / Polyhedron xxx (2014) xxx–xxx
Table 1
the C„C units’ carbon atoms cause an electron withdrawing effect
[65] in addition to the longer metal–metal distance and therefore,
are responsible for the low redox separation. Compound 3b has
been studied before [36,37] showing a similar reversible redox
behavior. The two ferrocenyl moieties are oxidized simultaneously
with redox potentials of E°0 = 141 [36] and 560 mV [37], depending
on the different reference systems (FcH/FcH+, SCE) used in the
electrochemical measurements [25].
Cyclic voltammetry data (potentials vs FcH/FcH+), scan rate 100 mVꢃsꢁ1 at a glassy-
carbon electrode of 1.0 mmol Lꢁ1 solutions of the analytes in dry dichloromethane
containing 0.1 mol Lꢁ1 of [NnBu4][B(C6F5)4] as supporting electrolyte at 25 °C.
Compound
E°01 (mV)a
(D
Ep (mV))b
D
E°0 (mV)c
3a
3b
3c
121 (204)
50 (140)
70 (144)
106d
75
72
a
b
c
E°0 = formal potential.
D
Ep = difference between the oxidation and the reduction potential.
E°0 = potential difference between the two ferrocenyl-related redox processes
3.4. Spectroelectrochemistry
D
determined by the application of the Richardson and Taube method [63].
d
Spectroelectrochemical studies were performed by a stepwise
increase of the potential (step heights: 15, 25, 50 or 100 mV) from
ꢁ200 to 1000 mV versus Ag/AgCl in an OTTLE cell [55] using dichlo-
romethane solutions of 3a–c (0.005 mol Lꢁ1) containing [NnBu4]
When using the deconvolution of the redox separation of the oxidation
potentials in SWV (Fig. 3)
D
E°0 = 100 mV.
[B(C6F5)4] (0.1 mol Lꢁ1
) as electrolyte at 25 °C. During this
Table 2
Procentual ratios for 3a–cn+ (n = 0, 1, 2) determined using KC.
procedure 3a–c were oxidized to the mixed-valent 2,5-diethynylf-
errocenyl species 3a–c+ and finally to dicationic 3a–c2+ (Fig. 4 and
6, Supporting information Figs. SI–SI4).
Compd.
KC
n = 0 (%)
n = 1 (%)
n = 2 (%)
3an+
3bn+
3cn+
62
18.5
16.5
10
16
16.5
80
68
67
10
16
16.5
For 3a an increasing absorption in the NIR region at 1510 nm
could be observed, attributed to an IVCT transition (=Inter-Valence
Charge Transfer) of mixed-valent 3a+ (Fig. 4). The maximum of the
absorption is observed at 450 mV versus Ag/AgCl and with further
oxidation a simultaneous collapse and the formation of a new band
at 1090 nm occurred, which can be assigned to a LMCT (=Ligand-
to-Metal Charge Transfer) transition. Compounds 3b and 3c show
similar UV–Vis/NIR spectra during the successive in situ oxidation,
whereby the IVCT excitations are approximately half as intense as
in the pyrrole derivative 3a (Fig. 5 and Table 3, Supporting
information Figs. SI1 and SI2). The absorptions in the UV–Vis
region (250–750 nm) are caused by d–d transitions of the ferroce-
nyl substituents [14,66].
8000
+
Py3rarol
6000
4000
2000
0
+
Thiophen
3b
+
Furan
3c
Using the method of deconvolution, the bands for monocationic
mixed-valent 3a–c+ could be described using three overlapping
Gaussian-shaped functions, which sum fits well with the
experimental graph. This approach allows the determination of
300
800
1300
1800
2300
2800
Wavelength (nm)
the physical parameters (wavenumber (
Full-Width-at-Half-Maximum (=FWHM) (
m
max), extinction (
emax),
Fig. 5. UV–Vis/NIR spectra of in situ generated 3a–c+ (0.005 mol Lꢁ1 in dichloro-
methane, 25 °C, supporting electrolyte [NnBu4][B(C6F5)4] (0.1 mol Lꢁ1)).
D
m
1/2)) of the charge
transfer excitations in the NIR region (Table 3). Band-shape analy-
sis based on the two-state Hush model [67–69] for symmetric
mixed-valent species enables a classification of the diethynylferr-
ocenyl-substituted heterocycles according to Robin and Day [70].
Therefore, the electronic coupling parameter Hab has been
calculated (Eq. 2) and thus, the strength of coupling between the
two metal atoms can be estimated within the class II regime. The
geometrical iron-iron distances rab are derived from X-ray crystal-
lographic data (Fig. 1 and 2). In pyrrole 3a a barrier-free rotation in
solution will maximize the Fe–Fe distance in an antiparallel
are more difficult to oxidize and hence are more electron-poor. For
the series of diferrocenyl heterocycles a trend with rising oxidation
potentials (pyrrole < furan < thiophene) could be observed
allowing to categorize the cC4H2NPh cycle as the most electron rich
and the cC4H2S as the most electron poor system within the series,
while for 3a–c a reversed trend is found. The 2,5-(C„C)2-cC4H2NPh
unit of 3a shows to be more electron withdrawing than the appro-
priate 2,5-(C„C)2-cC4H2S entity. Furthermore, the sp-character of
100
90
80
70
60
50
40
100
90
80
70
60
50
40
2300
2200
2100
2000
1900
1800
2300
2200
2100
2000
1900
1800
Wavenumber (cm-1
)
Wavenumber (cm-1)
Fig. 6. IR spectra of 3a at rising potentials vs Ag/AgCl collected from a spectroelectrochemical experiment. Left: -200 mV to 510 mV (3a ? 3a+). Right: 510 mV to 1000 mV
(3a+ ? 3a2+). The blue line belongs to the mixed-valent species 3a+. Measurement conditions: 25 °C, dichloromethane solution containing 10.0 mmol Lꢁ1 of 3a, 0.1 mol Lꢁ1
[NnBu4][B(C6F5)4] as supporting electrolyte.
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U. Pfaff et al. / Polyhedron xxx (2014) xxx–xxx
7
Table 3
NIR data of 3a–c+.a
c
d
Compd.
Transition
mmax (cmꢁ1) [nm]
emax (L molꢁ1 cmꢁ1
)
emax (L molꢁ1 cmꢁ1
)
D
m1/2 (cmꢁ1
)
Hab (cmꢁ1
)
3a+
LFb
IVCT
LMCT
LF
IVCT
LMCT
LF
IVCT
LMCT
4075 [2454]
6625 [1510]
9170 [1090]
4065 [2460]
7865 [1271]
10670 [937]
3975 [2515]
7450 [1342]
10560 [947]
395
1500
640
140
620
1180
130
380
495
1875
800
205
910
1735
195
565
600
359
3980
4000
1010
4370
2730
750
3b+
3c+
268
207
4
4370
2850
1095
1634
a
In dry dichloromethane containing 0.1 mol Lꢁ1 of [NnBu4][B(C6F5)4] as supporting electrolyte at 25 °C.
b
c
Ligand field transition.
Extinction values considering the maximum of the enrichment of the monocationic species 3a–c+ in the chemical equilibrium (vida supra).
d
As rab geometrical iron–iron distances derived from X-ray crystallographic data were used (3a, average 11.398 Å; 3c, 11.059 Å) (Figs. 1 and 2).
conformation. For 3b, the iron-iron distances of the pyrrole and fur-
an molecule were used to calculate Hab and the average value is gi-
ven within the limits of error. Due to the synperiplanar orientation
of the ferrocenyl substituents in the solid state, the iron-iron dis-
tance in 3c (11.059 Å) is somewhat shorter than in 3a (11.398 Å).
Table 4
Spectroelectrochemical IR data for 3a–cn+ (n = 0, 1, 2).
Compd.
mC„C (cmꢁ1) (n = 0)a
mC„C (cmꢁ1) (n = 1)
mC„C (cmꢁ1) (n = 2)
3an+
2208
2167
2184
2187
2212
2191
2194
2242
2197
2214
2207
3bn+
3cn+
2205
2204
The lower extinction emax leads to a smaller Hab value. Please, note
that the geometrical distances of the structures in the solid state are
usually longer than the actual electron transfer distances and
therefore, Hab is systematically underestimated. Nevertheless, a
trend can be found considering these values. The degree of interme-
tallic communication decreases in the series 3a, 3b, 3c.
a
Data collected from IR measurements in KBr at ambient temperature.
4. Conclusions
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
vmax
ꢃ
emax
rab
ꢃ
D
v1=2
Hab ¼ 2:06 ꢂ 10ꢁ2
ð2Þ
The 2,5-diethynylferrocenyl heterocycles 2,5-(FcC„C)2-cC4H2E
(E = NPh, 3a; S, 3b; O, 3c) were available from a Sonogashira C,C
cross-coupling of the corresponding halogenated heterocycles
and ethynylferrocene. The structures of these compounds in the
solid state were determined by single crystal X-ray diffraction
analysis showing a difference in the Fe–Fe distances between 3a
(11.398 Å) and 3c (11.059 Å), which is caused by the different an-
gle of the ethynylferrocenyl substituents with the heterocyclic
The broader IVCT bands of 2,5-diethynylferrocenyl thiophene
3b and furan 3c (
D
m
1/2 = 4370 cmꢁ1), in comparison to pyrrole
3a, are shifted to higher wavenumbers indicating larger CT excita-
tion energies (Table 3, Fig. 5). In contrast to 3a–c, the IVCT excita-
tions of 2,5-Fc2-cC4H2E (E = NPh, S, O) [18,49] show other spectral
properties and much higher electronic coupling parameters
(2250 cmꢁ1 (NPh), 1100 cmꢁ1 (S), 1350 cmꢁ1 (O)) could be calcu-
lated [19]. Within these studies, the estimation of rab has been car-
ried out by a method, which shows a linear relationship between
core. A T-shaped
ical studies (cyclic and square wave voltammetry) show that single
redox waves with high Ep values (140 mV, 3b; 144 mV, 3c) occur,
p–p stacking network occurs in 3a. Electrochem-
D
the
D
E°0 values and the oscillator strength under the assumption
indicating that two individual oxidation processes take place in a
close potential range. In contrast to the results of the electrochem-
ical measurements, in in situ spectroelectrochemical UV–Vis/NIR
and IR studies mixed-valent species and partial delocalization
was observed. This reveals that, despite having quite small redox
splitting between the first and the second oxidation, the 2,5-
diethynylferrocenyl heterocycles can be classified as class II sys-
tems according to Robin and Day [70]. Furthermore the calculation
of the electronic coupling parameter Hab makes a further assess-
ment as weak coupled class II systems possible. In comparison to
the redox behavior of 2,5-Fc2-cC4H2E (E = NPh, S, O) [18,49], the
influence of the ethynyl bridging unit, which causes a larger
metal–metal distance, on the redox separation and the band shape
and intensity of the IVCT transition could be demonstrated.
that a series of molecules with similar geometries and properties
is considered [18,19]. In accordance with these studies it is found
that the electronic coupling for the pyrrole containing molecule
3a is increased, when compared with furan 3c and thiophene 3b,
despite the fact that the electronic trend within the series 3a–c is
reversed in contrast to 2,5-Fc2-cC4H2E (E = NPh, S, O).
The ethynyl fragment with its characteristic m(C„C) vibration
allows to investigate electron transfer processes and hence the
formation of a mixed-valent species by in situ IR spectroelectro-
chemistry [23,71]. The typical m(C„C) band at 2205 cmꢁ1 of the
diethynylferrocenyl groups in neutral 3a-c could not have been de-
tected in solution (dichloromethane, 0.1 mol Lꢁ1 [NnBu4][B(C6F5)4]
as supporting electrolyte) prior to measurement, due to the low
analyte concentration (10.0 mmol Lꢁ1 of 3a–c, limit of the OTTLE
cell). Hence the data in Table 4 for the neutral species are collected
from IR measurements in KBr. During oxidation (from ꢁ200 to
510 mV) IR bands at 2167 and 2184 cmꢁ1 (symmetric and asym-
metric) could be detected for in situ generation of 3a+ (Table 4,
Fig. 6). The absorptions are shifted hypsochromically as oxidation
Acknowledgement
We are grateful to the Fonds der Chemischen Industrie (FCI) for
financial support. M. K. thanks the Fonds der Chemischen Industrie
for a fellowship.
proceeded (3a+ ? 3a2+).
A similar electronic behavior was
observed for 3b and 3c, whereby pyrrole 3a showed with
27 cmꢁ1 the largest shift from 3a+ to 3a2+. On the basis of the IR
data, it can be concluded that the transition from neutral 3a-c to
dicationic 3a–c2+ proceeds clearly over mixed-valent species 3a–
c+ with a partially delocalization of the in situ generated charge.
Appendix A. Supplementary data
CCDC 964521 and 964522 contains the supplementary crystal-
lographic data for 3c and 3a. These data can be obtained free of
Please cite this article in press as: U. Pfaff et al., Polyhedron (2014), http://dx.doi.org/10.1016/j.poly.2014.02.008

8
U. Pfaff et al. / Polyhedron xxx (2014) xxx–xxx
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