Synthesis of the first 1,3-disubstituted 1,3-butadienyl heteroannular bridged [4]ferrocenophane

Article abstract of DOI:10.1016/j.jorganchem.2010.12.032

An unusual heteroannular cyclization of methyl 4-(((1′- (trimethylsilyl)ethynyl)ferrocenyl)ethynyl)benzoate (11) under the basic desilylation condition yielded 1,1′-(l-methoxy-3-(4′- (methoxycarbonyl)phenyl)-1,3-butadienylene)ferrocene (7), the first repo

Full text of DOI:10.1016/j.jorganchem.2010.12.032

Journal of Organometallic Chemistry 696 (2011) 1491e1495
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Synthesis of the first 1,3-disubstituted 1,3-butadienyl heteroannular
bridged [4]ferrocenophane
,b,c,
Kai-Qiang Wu a, Li-Li Xie a, Yong-Fan Zhang a, Feng-Bo Xu b, Yao-Feng Yuan a
*
a Department of Chemistry, Fuzhou University, Qi Shan Campus, 2 Xue Yuan Road, Fuzhou, Fujian 350108, China
b State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
c State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China
a r t i c l e i n f o
a b s t r a c t
An unusual heteroannular cyclization of methyl 4-(((10-(trimethylsilyl)ethynyl)ferrocenyl)ethynyl)
benzoate (11) under the basic desilylation condition yielded 1,10-(l-methoxy-3-(40-(methoxycarbonyl)
phenyl)-1,3-butadienylene)ferrocene (7), the first reported 1,3-disubstituted 1,3-butadienyl hetero-
annular bridged [4]ferrocenophane. Compound 7 has been characterized by elemental analysis, IR
spectra, 1H NMR spectroscopy and X-ray diffraction analysis. The electrochemical behavior has been
investigated by cyclic voltammetry. The DFT calculation results of 7 are also reported.
Ó 2011 Elsevier B.V. All rights reserved.
Article history:
Received 12 November 2010
Received in revised form
13 December 2010
Accepted 24 December 2010
Keywords:
[4]Ferrocenophane
Alkynylferrocene
Desilylation
Heteroannular cyclization
1. Introduction
through a heteroannular cyclization reaction under basic condition
(aqueous KOH in methanol) [25,26]. All of the resulting [4]ferroce-
Ferrocenophanes have attracted great attention due to their
unique structures and properties, as well as their potential applica-
tions in the preparation of organometallic polymers [1e10]. Strained
ferrocenophanes are one of the most important classes of such
compounds which have been used in the preparation of high
molecular weight polymers via ring-opening polymerization (ROP)
[11e16] or ring-opening metathesis polymerization (ROMP) [17e21]
with different monomers.
nophanes were “1-monosubstituted”, bearing an alkyl (t-butyl)
[22,23] or aryl (phenyl or mesityl) [24] group at the 1-position of the
1,3-butadienyl bridging unit (see 3, 4 and 5 in Chart 1). ROMP of these
monomers afforded soluble polymeric materials with high molecular
weight. Unfortunately, the conductivities upon doping remained
relatively low, merely on the order of 10ꢀ5 S cmꢀ1
.
More recently, Butenschön et al. reported the transannular addition
of phenols to 1,10-dialkynylferrocenes under acidic alkyne metathesis
condition, leading to the isolation of a series of “1,2,3-trisubstituted”
phenoxy[4]ferrocenophanedienes (6) (Chart 1) [27]. The ROMP of
these [4]ferrocenophanes were not examined.
The ROMP products containing
p-conjugated organic spacers
are of particular interest because of their promising electronic,
optical, and magnetic properties afforded by potential interactions
of the iron centers through the
p
-network.
To the best of our knowledge, “disubstituted” 1,3-butadienyl het-
eroannular bridged [4]ferrocenophane has not previously been
reported. We are interested in developing an easily adaptable synthetic
route to “disubstituted” 1,3-butadienyl heteroannular bridged [4]fer-
rocenophanes with donor or acceptor group attached to the butadienyl
A pioneeringstudyperformedbyGrubbs andco-workersshowed
that the ROMP of 1,3-butadiene heteroannular bridged [4]ferroce-
nophanes 1,4-(1,10-ferrocenediyl)-1,3-butadiene (1) and 1,4-(1,10-
ferrocenediyl)-1-methoxy-1,3-butadiene (2) (Chart 1) produced
polymers with poor solubility and low molecular weight [21].
In recent years, great efforts have been made to enhance the
solubility of the conjugated polymers. Lee et al. conducted several
structural modifications [22e24] on the 1,3-butadienyl bridging unit
oftheparent compound2, first synthesized byPudelskiand Callstrom
bridging unit, which upon ROMP may provide p-conjugated organo-
metallic polymers with novel optoelectronic properties.
In this contribution, we present the synthesis, electrochemistry
and theoretical calculation results of the first “1,3-disubstituted”
1,3-butadienyl heteroannular bridged [4]ferrocenophane 1,10-(1-
methoxy-3-(40-(methoxycarbonyl)phenyl)-1,3-butadienylene)ferr-
ocene (7) (Scheme 1), an unexpected species obtained during our
preparation of alkynyl-bridged multiferrocenyl molecular wires.
Compound 7 was formed through an intramolecular heteroannular
* Corresponding author. Department of Chemistry, Fuzhou University, Qi Shan
Campus, 2 Xue Yuan Road, Fuzhou, Fujian 350108, China. Tel./fax: þ86 5912286 6161.
E-mail address: yaofeng_yuan@fzu.edu.cn (Y.-F. Yuan).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.12.032

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K.-Q. Wu et al. / Journal of Organometallic Chemistry 696 (2011) 1491e1495
Chart 2.
Chart 1.
Adopting the synthetic route illustrated in Scheme 1, many
structural analogs of 7 can be prepared by changing the haloarene
coupling partner of 8 in the first step. Moreover, taking advantage
of the vinyl ether group, further transformation of those [4]ferro-
cenophanes would be convenient as previously reported in detail
by others [22e24].
Single crystals of 7 suitable for X-ray diffraction studies were
obtained by slow diffusion of hexane into its CH2Cl2 solution. Fig. 1
shows the molecular structure of 7 [35].
cyclization reaction when the desilylation reaction of the (trime-
thylsilyl)ethynyl substituted ferrocene derivative was conducted
under a basic condition. X-ray crystallographic analysis provided
further confirmation of the structure of 7.
2. Results and discussion
As shown in Scheme 1, the Sonogashira coupling reaction
[28,29] between 1-ethynyl-10-iodoferrocene (8) and methyl 4-
iodobenzoate (9) catalyzed by 1 mol% of Pd(Ph3P)2Cl2 and 2 mol%
CuI in THF/diisopropylamine gives 10 in 78% yield after column
chromatographic purification over silica gel. 8 is used in excess to
ensure the complete consumption of 9, and consequently simplifies
the purification process. In a similar fashion, the coupling reaction
of 10 and excess trimethylsilylacetylene gives 11 in 56% yield after
column chromatography.
Treatment of a deoxygenated solution of 11 in methanol/diethyl
ether with potassium carbonate at room temperature [30,31] was
expected to yield a terminal ferrocenyl alkyne for further frame-
work extension. However, in contrast to our expectation, the
reaction affords air and moisture stable compound 7 as the only
detectable product in 66% isolated yield. We note a recent publi-
cation of Sita et al. describing the frustrating desilylation outcome
of (trimethylsilyl)ethynyl ferrocene derivative 12, the reaction
mixture had not been separated and characterized because of its
complexity [32]. Therefore, terminal alkyne with a skeleton like 13
depicted in Chart 2 is difficult to synthesize via conventional
desilylation of its trimethylsilyl-protected precursor [25,26,32]. In
addition, it is worthwhile to mention that steric hindrance may
stabilize the terminal alkyne as it cumbered the approach of the
two 1,10-alkynyl portions and subsequent cyclization to afford fer-
rocenophanes. Two examples formerly reported are shown in Chart
2 (14 and 15, both bearing bulky groups) [33,34].
The cyclopentadienyl (Cp) rings are planar to ꢁ0.009 Å. CeC
bond lengths for the two Cp rings fall in a range from 1.398 Å to
1.435 Å. The Cp rings are staggered as indicated by a mean twist
angle of 30.37ꢂ found for torsion angle of type C(n)eRC(1)eRC(2)e
C(11ꢀn) (for the definition of RC(1) and RC(2), see the caption of
Fig. 1). The Fe1eRC(1) and Fe1eRC(2) distances are 1.6337(4) and
1.6370(4) Å, respectively, both slightly shorter than the distance
found in ferrocene [36e38]. The lengths of CeC single bond con-
necting unsaturated groups (C6eC11, C1eC14, C12eC13, C12eC16,
C19eC22) are all very close to the standard value of a simple
sp2esp2 CeC single bond (1.48 Å). The dihedral angle between the
benzene ring and the ethylenic plane defined by the phenyl-
substituted C]C double bond is about 39.97ꢂ. The benzene ring is
nearly perpendicular to the Cp rings, as shown by the dihedral
angles between the benzene ring and the two Cp rings, 84.82(18)ꢂ
for Cp(1) ring and 80.7(2)ꢂ for Cp(2) ring.
The 1,3-butadienyl heteroannular bridge imposes ring tilt in 7,
as manifested by the dihedral angle between the Cp rings of 10.8
(2)ꢂ. Comparison of Fe1ering-carbon distances shows that the Cp
rings separation is smallest at the heteroannular bridge. Bond angle
distortion of the 1,3-butadienyl bridging unit is listed in the caption
of Fig. 1 (four CeCeC bond angles consist of C1, C6, C11, C12, C13,
C14). An average bond angle of about 129ꢂ indicates a considerable
expansion from the ideal sp2-hybridized angle of 120ꢂ. Twist in the
heteroannular bridge is revealed by analysis of the torsional
angle data. While torsional angles of C1eC14eC13eC12 and
C13eC12eC11eC6 are nearly negligible, 7 exhibits a considerable
C14eC13eC12eC11 torsional angle (ꢀ44.2(5)ꢂ), hence the carbon
chain connecting the Cp rings is helically distorted [27].
A possible reaction pathway for the formation of 7 is illustrated in
Scheme 2 [25,26]. Desilylation of 11 generates ferrocenyl alkyne 16 as
intermediate, which subsequently suffers methoxide attack at an
ethynyl carbon
a to the Cp ring followed by intramolecular cyclization
onto the -carbon of the ethynyl group on the opposing Cp ring. Proton
b
The Cp ring tilt angle and bond angle distortion in the hetero-
annular bridge have been suggested to be indications of strain in
ferrocenophanes [39]. These two structure features of 7 as des-
cribed above are similar to 2, 4 and 5 [24,26], thus the strain is
reserved after the introduction of 40-(methoxycarbonyl)phenyl,
making 7 a promising candidate for ROMP.
abstraction from methanol gives 7 and regenerates methoxide ion.
Scheme 1. Synthetic route to compound 7. (a) Pd(Ph3P)2Cl2, CuI, THF, i-Pr2NH, 60 ꢂC,
60 h (b) HC^CSi(CH3)3, Pd(Ph3P)2Cl2, CuI, THF, i-Pr2NH, 50 ꢂC, 36 h (c) K2CO3, CH3OH,
Et2O, rt, 24 h.
Scheme 2. Proposed mechanism of the heteroannular cyclization reaction.

K.-Q. Wu et al. / Journal of Organometallic Chemistry 696 (2011) 1491e1495
1493
Fig. 1. ORTEP view of 7 (50% probability level). Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å), bond angles (deg) and torsional angles (deg):
Fe1eC1 ¼ 2.000(3), Fe1eC6 ¼ 2.013(3), C6eC11 ¼ 1.466(5), C11eC12 ¼ 1.338(4),
C12eC13 ¼ 1.477(4), C13eC14 ¼ 1.339(4), C1eC14 ¼ 1.478(4), C12eC16 ¼ 1.491(4),
C19eC22
C6eC11eC12
C12eC13eC14
¼
1.480(5), O1eC14
¼
1.372(4); RC(1)eFe1eRC(2)
128.1(3), C11eC12eC16
128.4(3); C6eC11eC12eC13
2.8(5); RC(1) and RC(2)
¼
172.85(9),
119.4(3),
¼ ꢀ0.8
¼
128.4(3), C11eC12eC13
131.4(3), C13eC14eC1
¼
¼
¼
¼
Fig. 3. Diagrams of the surfaces and energies of HOMO and LUMO for 2 and 7.
(5),C11eC12eC13eC14
denote the centroids of Cp(1) rings (atoms C1 to C5) and Cp(2) rings (atoms C6 to C10),
respectively.
¼ ꢀ44.2(5),C12eC13eC14eC1 ¼
Density functional theory (DFT) calculations of 2 and 7 have
been carried out to gain a deeper insight into their electronic
properties. 2 was chosen for comparison because of its similarity in
structure with 7. The geometric parameters employed in the
calculations were based on their crystal structural data. The
surfaces of HOMO and LUMO along with the orbital energy for each
level are depicted in Fig. 3.
The electrochemistry of 1 mM solutions of 7 and ferrocene in
acetonitrile was studied. Tetrabutylammonium hexafluorophos-
phate was used as the electrolyte in these experiments in which the
data were collected at various scan rates.
The electrochemistry of
7 was substantially irreversible
between 0.00 and þ0.80 V, unlike most of the 1-monosubstituted
1,3-butadienyl heteroannular bridged [4]ferrocenophanes. Cyclic
voltammogram of 7 obtained at a scan rate of 10 mV sꢀ1 shows
a similar feature to the unsubstituted [4]ferrocenophanes 1 [21].
The cathodic peak current diminished relative to the anodic peak
current (Fig. 2). As the scan rate increased, the reduction peak
became more significant. The lower amount of reduction current is
likely due to the instability of the oxidized form of 7 in acetonitrile
solvent.
The E1/2 value of 7 was determined to be 0.57 V vs Ag/AgCl
electrode from data collected at a scan rate of 200 mV sꢀ1 with
peak-to-peak separation of 95 mV, although a ratio of cathodic peak
current to anodic peak current of only 0.5 was measured at this
The HOMOs of both 2 and 7 involve
1,3-butadienyl bridging unit with significant contribution from
iron. The LUMO is dominated by the
* orbitals of, for 2, the 1,3-
p orbitals localized on the
p
butadienyl bridging unit with little contribution from iron, while
for 7, the 40-(methoxycarbonyl)phenyl group and the C]C double
bond it attached to. Compared with 2, the incorporation of 40-
(methoxycarbonyl)phenyl group in 7 lowers the energy levels
of the frontier molecular orbitals (FMOs), especially the LUMO.
Therefore, the band gap between FMOs (4.22 eV for 2 and 3.65 eV
for 7) is reduced, probably due to the extension of conjugation.
The difference of Mulliken atomic charges of iron in 2 and 7 (0.958
and 0.954, respectively) is negligible, however, cyclic voltam-
metric experiments revealed a modest potential difference of
40 mV (vide supra). The discrepancy between experimental
deductions and calculation results is believed to be a reflection
of the environmental variation of practical electrochemical
measurement.
scan rate (Fig. 2). Compared with ferrocene (E1/2 ¼ 0.44 V), a
DE1/2
value of 130 mV for 7 was obtained, significantly larger than that of
2 (90 mV) [26].
3. Conclusions
We have discovered a heteroannular cyclization reaction of
methyl 4-(((10-(trimethylsilyl)ethynyl)ferrocenyl)ethynyl)benzoate
(11) that leads to the formation of novel 1,3-disubstituted 1,3-
butadienyl heteroannular bridged [4]ferrocenophane 7. X-ray
crystal structure study revealed modest strain in 7. Further studies
of the cyclization reaction, including the extension of the reaction
to prepare analogous [4]ferrocenophanes, as well as the synthetic
transformation and ROMP of the resulting [4]ferrocenophanes, are
currently in progress.
4. Experimental
4.1. General
All manipulations involving air-sensitive materials were carried
out by using standard Schlenk techniques under an atmosphere of
nitrogen. Chromatography was performed on silica gel (100e200
Fig. 2. Cyclic voltammograms of 1 mM solutions of 7 in 0.10 M tetrabutylammonium
hexafluorophosphate in acetonitrile recorded at different scan rates using a glassy
carbon working electrode.

1494
K.-Q. Wu et al. / Journal of Organometallic Chemistry 696 (2011) 1491e1495
mesh). Pd(Ph3P)2Cl2 was purchased from Acros and used as
received. Solvents were dried by standard methods and distilled
under nitrogen before use. 8 [40] and 9 [41] were prepared
according to literature procedures.
organic layer was dried over Na2SO4. After evaporation of the
solvent, the crude product was purified by column chromatography
using a 5:1 petroleum ether/CH2Cl2 solvent system. Thesecond band
was collected and evaporated to give 10 mg (66%) of 7 as an
orangeered powder. mp 137.3e139.2 ꢂC. IR (KBr) 3094, 3008, 2950,
2928, 2849,1713,1630,1600,1462,1437,1406,1310,1282,1224,1202,
1177, 1143, 1106, 1016, 983, 830, 804, 774 cmꢀ1. 1H NMR (400 MHz)
1H NMR spectra were recorded in CDCl3 solutions using a Bruker
AVANCE 400 spectrometer. Chemical shifts are given in ppm rela-
tive to internal tetramethylsilane. IR spectra were recorded in KBr
pellets with a PerkineElmer Spectrum 2000 spectrophotometer
(4000e400 cmꢀ1). Melting points were determined on a Shen-
guang WRS-1B apparatus and are uncorrected. Mass spectra (ESI)
were obtained from a ThermoFinnigan DECAX-30000 LCQ Deca XP
spectrometer. Elemental analysis was performed on an Elementar
Vario MICRO instrument. Cyclic voltammetry was performed on
a CHI 620C electrochemical analyzer (CH Instruments, Inc.) in
a standard three-electrode system. A glassy carbon working elec-
trode was employed in conjunction with an Ag/AgCl (3.0 M KCl)
d
8.04 (d, J ¼ 8.4 Hz, 2H, PhH), 7.70 (d, J ¼ 8.4 Hz, 2H, PhH), 6.19 (s,1H,
CpCH ¼ CPh), 4.69 (s, 2H, CpH), 4.61 (s, 2H, CpH), 4.31 (s, 2H, CpH),
4.28 (s, 2H, CpH), 3.93 (s, 3H, PhCOOCH3), 3.81 (s, 1H, CpC
(OMe) ¼ CH), 3.49 (s, 3H, CpC(OCH3)). Anal. Calcd for C23H20FeO3: C,
69.02; H, 5.04. Found: C, 69.34; H, 5.08. Mass (m/z): 400.6 [Mþ].
4.5. X-ray structure determination
Suitable single crystal of 7 was mounted on glass fiber for X-ray
measurement. Data were collected at 293(2) K on a Rigaku Saturn
reference electrode and
a platinum wire counter electrode.
Geometry optimizations were performed at the B3LYP/6-311G(d)
DFT level using the Gaussian 03 program [42].
724 CCD diffractometer with graphite monochromatized Mo K
a
radiation (
l
¼ 0.71073 Å) following standard procedures. The struc-
ture was solved and refined using the SHELX suite of programs [43].
4.2. Methyl 4-((10-iodoferrocenyl)ethynyl)benzoate (10)
Acknowledgments
Toadeoxygenatedsolutionof957mg(2.85mmol)of8and731mg
(2.79 mmol) of 9 in 25 mL of THF/diisopropylamine (4:1, v/v) solvent
mixture was added 20 mg (0.028 mmol) of Pd(Ph3P)2Cl2 and 11 mg
(0.058 mmol) of CuI. The resulting mixture was heated at 60 ꢂC for
60 h and then cooled to room temperature. The solvent was removed
in vacuo, and the residue was redissolved in CH2Cl2 and filtered
through a short silica gel column. The crude product was purified by
column chromatography. An orange band eluting with 4:1 petroleum
ether/CH2Cl2 afforded 1023 mg (78%) of 10 as an orangeered oil. IR
(KBr) 3441, 2948, 2206,1720, 1708, 1603,1435,1405,1307,1275,1171,
This work was supported financially by the National Natural
Science Foundation of China (Grant No. 20772016), the Natural
Science Foundation of Fujian Province (No. 2010J01036) and the
Project Sponsored by the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, State Education Ministry of
China.
Appendix A. Supplementary material
1107,1016, 811, 767, 694 cmꢀ1.1H NMR (400 MHz)
d
8.00 (d, J ¼ 8.4 Hz,
CCDC-770371 contains the supplementary crystallographic data
for compound 7. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
2H, PhH), 7.56 (d, J ¼ 8.4 Hz, 2H, PhH), 4.59e4.25 (m, 8H, CpH), 3.93 (s,
3H, OCH3). Mass (m/z): 472.2 [(M þ 2)þ].
4.3. Methyl 4-(((10-(trimethylsilyl)ethynyl)ferrocenyl)ethynyl)
benzoate (11)
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[35] Crystal data for 7: C23H20FeO3, Mw ¼ 400.24 g molꢀ1, T ¼ 293(2) K, monoclinic,
space group C2/c, a ¼ 37.625(8) Å, b ¼ 7.3748(15) Å, c ¼ 14.197(3) Å, ¼ 110.86
b
(3)ꢂ, V ¼ 3681.1(13) Å3, Z ¼ 8, rcalcd ¼ 1.444 g cmꢀ3
,
m
¼ 0.840 mmꢀ1, F
(000) ¼ 1664;
q
range 3.1e27.5ꢂ; reflections, 14784 measured, 4227 indepen-
dent (Rint ¼ 0.0360), final R indices [I > 2 (I)] R1 ¼ 0.0564, wR2 ¼ 0.1500, R
s
indices (all data) R1 ¼ 0.0630, wR2 ¼ 0.1593. The goodness of fit on F2 was 1.290.
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