Synthesis and properties of 1,1′-bis[p-(N,N-dimethylaminophenyl)- butadiynyl]ferrocene: A methodology for proton-mediated reversible conformation control of two function sites

Article abstract of DOI:10.1016/j.tetlet.2012.10.094

An extended π-electronic conjugation system of the 1,1′-bis[p-(N, N-dimethylaminophenyl)butadiynyl]ferrocene derivative 1 has been synthesized, with our integrated cross-coupling reaction between the corresponding TMS-protected acetylenes in one-pot. Quantitative ¹H NMR and UV-vis spectral studies of 1 have been performed, providing a methodology for molecular functions transformable by a proton-mediated reversible conformation control.

Full text of DOI:10.1016/j.tetlet.2012.10.094

Tetrahedron Letters 54 (2013) 66–71
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis and properties of 1,10-bis[p-(N,N-dimethylaminophenyl)-
butadiynyl]ferrocene: a methodology for proton-mediated reversible
conformation control of two function sites
⇑
Takuya Toyama, Shizuka Komori, Junro Yoshino, Naoto Hayashi, Hiroyuki Higuchi
Graduate School of Science and Engineering, University of Toyama, 3190 Gofuku, Toyama, Toyama 930-8555, Japan
a r t i c l e i n f o
a b s t r a c t
p
-electronic conjugation system of the 1,10-bis[p-(N,N-dimethylaminophenyl)butadiy-
Article history:
An extended
Received 16 September 2012
Revised 16 October 2012
Accepted 22 October 2012
Available online 29 October 2012
nyl]ferrocene derivative 1 has been synthesized, with our integrated cross-coupling reaction between
the corresponding TMS-protected acetylenes in one-pot. Quantitative 1H NMR and UV–vis spectral
studies of 1 have been performed, providing a methodology for molecular functions transformable by
a proton-mediated reversible conformation control.
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Diacetylene
Ferrocene
Dimethylaminobenzene
Proton
Integrated reaction
Conformation
Control
Introduction
(Chart 1).3 Although each diacetylene-group connected Th constit-
uent tightly combined with cyclopentadienyl ring (Cp) rotates
In recent years, a great deal of efforts have been taken for the
construction of well-sophisticated -electronic systems ES)
reversibly transformable between well-defined molecular struc-
tures by means of various operations such as proton-base and me-
tal–ligand interactions, to develop the new organic functional
materials.1 In relation with functional materials science aiming at
along the Fc axis freely, the transformable functionality from the
spectral changes would be brought purposively by control of the
molecular motion and by manipulation of the spatial interactions
between two Cp-Th constituents. In fact, Fc-Th2 proved to be sen-
sitive to Ag+ and Hg2+ ions selectively to change its electronic
absorption spectra proportionally upon adding those metal ions
and newly to afford the corresponding metal–ligand charge trans-
fer (MLCT) transition bands, in striking contrast with the corre-
sponding Fc derivative carrying one Cp-Th constituent (Fc-Th).
This result suggests that two diacetylene-group connected Cp-Th
constituents cooperatively react as ligands with these Ag+ and
Hg2+ ions, and then stop rotation along the Fc axis to face each
other, like a pincette conformation, in a syn or synclinal confor-
mation. The study of Fc-Th2 which is further in progress can be re-
p
(p
such reversibly transformable
pES, we have been also engaged in
the study of our original
pES reversibly transformable between
its molecular functions with various outside stimuli.2
In the research processes, besides the promising functions, it is
important as well that the structure-function relationships are
readily and accurately evaluated. For these purposes, we have
commonly introduced the 1,3-butadiyne linkage (diacetylene
: –C„C–C„C–) into our original
for well-defining the constituents attached at both ends but also
for the extension of conjugation with various -electronic compo-
pES, which is useful not only
garded as
a methodological research for the transformable
p
functionality with conformation control by capturing Ag+ or
Hg2+ ion as an outside stimulus, potentially leading to a new me-
tal–ligand induced sensor.4
nents, potentially leading to the new and curious molecular
functions through one-, two, and/or three-dimensional interac-
tions between them. As an example along this line, we previ-
ously reported the synthesis and properties of the 1,10-bis
(2-thienyldiacetylenyl)ferrocene derivative, described as Fc-Th2
In our continuous investigations of the structure–function
relationships of the 1,10-bis(diacetylene-group)Fc system, the
dimethylaminobenzene (DMAB) derivative 1 has been newly de-
signed (Chart 2). There are two DMAB moieties in 1, on which
the protonation can be expected to proceed stepwise to produce
one ammonium moiety at the first stage and two ammonium
⇑
Corresponding author. Tel./fax: +81 (0)76 445 6616.
E-mail address: higuchi@sci.u-toyama.ac.jp (H. Higuchi).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.10.094

T. Toyama et al. / Tetrahedron Letters 54 (2013) 66–71
67
M+
S
S
S
M+
Fe
Fe
S
M+=Ag+ and Hg2+
Fc-Th2-M+
Fc-Th2
Chart 1.
acceptor side
+
H
NMe2
NMe2
H
NMe2
+
H+
H+
Fe
Fe
Fe
+
Me2N
Me2N
NMe2
H
donor side
Fc-DMAB2(H+)
Fc-DMAB2(2H+)
1: Fc-DMAB2
Chart 2.
moieties at the second stage. In other words, the positive charge is
introduced into 1 at each protonation stage one by one. The posi-
tive charge introduction, however, seems to bring in a different
influence on the molecular motion between both protonation
stages. At the first stage, the positive charge introduced by proton-
ation on one DMAB as Lewis base could play an effective role for
association with the other non-protonated DMAB in 1 for stabiliza-
tion to stop rotation of two Cp-DMAB constituents along the Fc
axis, resulting in a syn or synclinal conformation of those constit-
uents preferably. Such an association between constituents of
Cp-DMAB(H+) and Cp-DMAB in 1 likely arises from a hydrogen-
bonding, to form a proton sponge,5 and/or from a CT interaction
between them (Chart 2). To the contrary, at the second stage, the
repulsive interaction between positive charges on both N/N atoms
of DMABs in 1 should cause dissociation of the two Cp-DMAB(H+)
constituents to restart rotation along the Fc axis, resulting in an
anti or anticlinal conformation of those constituents preferably,
similar to the conformation in neutral solutions due to the steric
repulsion (vide infra). Based on the present speculative guideline,
1H NMR, and electronic spectral changes of Fc-DMAB2 between
neutral and acidic solutions were examined and observed quanti-
tatively. Taking pKa values of 2.73–4.01 for conjugated acids
p-(X)DMAB(H+) into consideration,6 among a variety of acids,
trifluoroacetic acid (TFA, pKa = 0.23) was employed in order to
analyze the spectral changes in this experiment. In fact, TFA was
found a good stimulus candidate to drive the purposive rotation
of the Cp-DMAB constituents along the Fc axis in 1.
cross-coupling reaction between terminal acetylenes of Fc (5,7
1.0 equiv) and DMAB (6,8 1.1 equiv) under the Eglinton conditions9
afforded Fc-DMAB 2,10 Fc2 3,3,12 and DMAB2 410 in 65, 19, and 25%
yields, respectively. On the other hand, Fc-DMAB2 1 cannot be ob-
tained under the same reaction conditions as above, because the
1,10-diethynyl Fc derivative 7 corresponding to 5 is extremely
unstable to be employed as
a
convenient synthon.11 Then,
according to an integrated synthetic method for 1,10-bis
(diacetylene-group) connected Fc derivatives, just recently devel-
oped by us,12 one-pot reaction between trimethylsilyl (TMS)-
protected Fc derivative 813 and ethynyl-DMAB derivative 6 under
the TMS-deprotection and Eglinton-coupling conditions was ap-
plied to the synthesis of 1. As expected, the desired compound
110 could be obtained as a stable substance. But, its yield was
10% at most with very low reproducibility. Under the present reac-
tion conditions, the DMAB dimer 4 formed in large quantity to-
gether with insoluble Fc polymer, indicating the much higher
reactivity of the DMAB terminal acetylene 6 toward oxidative
homo-coupling, as competed with deprotection reaction of the
TMS group from 812b. Therefore, taking this fact into consideration,
the TMS-deprotection and cross-coupling reaction between TMS-
protected reactants 8 and 9 was carried out, consequently afford-
ing 1 in an acceptable yield of 45%. In the present tandem reaction
conditions for Fc-TDMAB2, dimethoxyethane (DME) was found a
3
better co-solvent than CHCl3 somehow, similar to that for Fc-Th2
.
The structures of new compounds including 1 were determined
by Mass, IR, 1H NMR, and UV–vis spectral measurements as well as
their elemental analyses. In 1H NMR spectra of 1–4, even though
the chemical shift differences between corresponding protons in
respective compounds are fairly small, the structure-property
analysis can be performed with confidence, because respective
components are definitely combined with the rigid diacetylene
linkage. In the case of 3 and 4, it is shown that the diacetylene link-
age induces low-field shift of aryl protons (Ar–H) by a maximum of
In this Letter, the synthesis of Fc-DMAB2 1 and its spectral
changes between neutral and acidic solutions will be reported to-
gether with a methodological concept for conformation control of
the two Cp-DMAB constituents with TFA, as compared with those
of the related compounds 2–4 (Chart 3).
The Fc-DMAB2 and Fc-DMAB derivatives, 1 and 2, were synthe-
sized, as shown in Schemes 1 and 2. As conventional, the oxidative
NMe2
Fe
Me2N
NMe2
Fe
Fe
2: Fc-DMAB
3: Fc2
4: DMAB2
Chart 3.

68
T. Toyama et al. / Tetrahedron Letters 54 (2013) 66–71
H
Cu(OAc)2
+
+
2
3
4
+
H
NMe2
Fe
pyridine
MeOH
65%
19%
25%
5
6
Scheme 1.
K2CO3
Cu(OAc)2
R
+
+
1
4
R
NMe2
Fe
pyridine
MeOH
R
CHCl3
DME
6: R=H
9: R=TMS
10%
45%
72% (from 8 and 6)
38% (from 8 and 9)
7: R=H
8: R=TMS
Scheme 2.
0.16 ppm from the corresponding ones of DMAB (d = 7.25 and
6.73 ppm), of methyl protons (Me–H) by 0.05 ppm from that of
DMAB (d = 2.94 ppm), and of Cp-H by a maximum of 0.5 ppm from
that of Fc (d = 4.15 ppm), respectively, apparently due to its induc-
tive and anisotropic effects. In the case of 1 and 2, Cp-H of 1
(d = 4.54 and 4.33 ppm) appeared at the lower field than those of
2 (d = 4.49, 4.25, and 4.23 ppm), while Ar–H of 1 (d = 7.37 and
face-to-face conformation between two Cp-DMAB constituents in
1. In other words, it can be concluded that under the neutral
(non-protonated DMAB) conditions, as naturally deduced, Fc-
DMAB2 1 exists in an anti or anticlinal conformation due to the
steric repulsion (vide infra).
6.58 ppm) showed
a small high-field shift than those of 2
Ha Hb
(d = 7.40 and 6.62 ppm). The former can be explained by the same
effects from the diacetylene linkage as above in consequence. On
the other hand, the latter could be also attributed to an anisotropic
effect from the increased diamagnetic ring current of Cp by con-
nection with the diacetylene linkage, to affect Ar–H of an opposite
DMAB. Otherwise, it suggests the more significance of a syn or syn-
clinal conformation in 1 due to an attractive interaction between
two extended Cp-DMAB conjugation constituents, where another
small anisotropic effect has influence on the opposite Ar–H
mutually more or less (Chart 2).14 However, in practice, no chem-
ical shift difference was observed in Me–H (d = 2.99 ppm) of DMAB
between 1 and 2, clearly indicating a very low possibility of the
NMe2
Fe
CHCl3
Cp-H
Ha
Me-H
Hb
(a)
(b)
(c)
(d)
/105 M-1cm-1
0.5
(e)
(f)
(g)
(h)
400
500
(i)
7.7 7.3
4.4
/ppm
4.2
3.3
2.9
6.9 6.5 4.6
0.0
500
Wavelength/nm
600
400
300
Figure 2. 1H NMR spectral behavior of 2 (7.5 ꢀ 10ꢁ3 Mꢁ1 in CDCl3) by addition of
TFA; (a) 0.0 equiv, (b) 0.2 equiv, (c) 0.4 equiv, (d) 0.6 equiv, (e) 0.8 equiv, (f)
1.0 equiv, (g) 3.0 equiv, (h) 5 equiv, and (i) 25 equiv.
Figure 1. Electronic absorption spectra of 1 (green), 2 (red), 3 (purple), and 4 (blue)
in CHCl3 solution (1.3–1.5 ꢀ 10ꢁ5 mol cmꢁ3).

T. Toyama et al. / Tetrahedron Letters 54 (2013) 66–71
69
The UV–vis spectra of 1–4 in CHCl3 are shown in Figure 1. As is
generally observed in Fc derivatives,15 compounds 1, 2, and 3 char-
acteristically exhibit very broad and weak bands at 400–550 nm
due to the 3d–3d transition in Fe(II). The diacetylene-group con-
In order to examine the possibility of conformational control
between two Cp-DMAB constituents in 1, the changes by TFA addi-
tion into chloroform solution were quantitatively observed by 1H
NMR spectral measurements (Figs. 2 and 3). From the sequential
spectra of 1 and 2, it is apparently indicated that both Ar–H and
Me–H are fairly sensitive to TFA to shift to the lower field more
or less and thus the efficient protonation takes place rightly on
DMAB to introduce the positive charge into these compounds. In
the case of Fc-DMAB 2 (Fig. 2), Ar–H and Me–H as well as Cp–H
simply shifted to the low field, almost proportionally upon adding
TFA into CDCl3 solution, respectively. The low-field shift changes
with TFA contents for Ar–H and Me–H of Fc-DMAB2 1 (Fig. 3),
however, proved not to be regular but rather of characteristic in
behavior. In a range of 0.0–0.2 equiv addition of TFA, both Ar–H
and Me–H swiftly moved down to 7.45 ppm for Ha, to 6.90 ppm
for Hb, and to 3.05 ppm for Me–H (Fig. 3a and b). Unexpectedly,
TFA addition beyond 0.2 equiv to 1 returned both Hb and Me–H
shifting to the high field, with Ha almost remained at around
7.45 ppm similar to Cp–H at the original positions. This phenome-
non was kept maintained successively up to 1.0 equiv TFA addition
to 1 (Fig. 3c–f). Furthermore, TFA addition beyond 1.0 equiv to 1
again affected all protons to shift subtly and yet reversely to the
low field toward 7.60 ppm for Ha, 7.44 ppm for Hb, and
3.31 ppm for Me–H (Fig. 3g–i) in a stationary spectrum, all of
which are finally almost the same as those for corresponding
nected DMAB dimer
4
afforded several vibration bands
(kmax = 327sh, 351, and 377 nm) together with a large bathochro-
mic shift from DMAB (kmax = 251 nm),16 while the diacetylene-
group connected Fc dimer 3 showed almost one broad main band
at kmax = 290 nm similar to that of Fc (kmax = 276 nm).15 It simply
concludes that the energy levels of HOMO and LUMO of the diacet-
ylene linkage are much closer to those of DMAB to interact with
each other efficiently. On the other hand, the derivatives 1 and 2
exhibited the electronic nature of both Fc and DMAB, indicating
an electronic interaction between them through the diacetylene
linkage. In particular, on the basis of Fc-DMAB 2, the Fc-DMAB2
derivative 1 afforded a further combined spectrum between those
of 3 and 4, showing a fairly increased intensity in the main band at
around 340 nm with two shoulders and a small bathochromic shift
(D
k = 18 nm) in the weak 3d–3d transition band.15 These results
conclude that the diacetylene linkage plays an important role as
a transmission mediator to make an electronic communication be-
tween Cp and DMAB rings efficiently. Theoretical studies of 1 and 2
are in progress, for clarification of the absorption band origins be-
tween Cp and
p-electronic components such as benzene, Th, and
DMAB rings in this system.3,10
Ha
Hb
NMe2
Fe
Me2N
CHCl3
Ha
Hb
Cp-H
Me-H
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
7.7 7.3
4.4
4.2
3.3
2.9
6.9 6.5 4.6
/ppm
Figure 3. 1H NMR spectral behavior of 1 (3.8 ꢀ 10ꢁ3 Mꢁ1 in CDCl3) by addition of TFA; (a) 0.0 equiv, (b) 0.2 equiv, (c) 0.4 equiv, (d) 0.6 equiv, (e) 0.8 equiv, (f) 1.0 equiv, (g)
3.0 equiv, (h) 5.0 equiv, and (i) 25 equiv.

70
T. Toyama et al. / Tetrahedron Letters 54 (2013) 66–71
protons of 2 in a CDCl3 solution containing 25 equiv of TFA (Fig. 2i).
From these spectral changes, it is apparently indicated that since
TFA is stronger in acidity than DMAB(H+) by more than 300 times,
the first-step protonation onto DMAB in 1 immediately and stea-
dily takes place, similar to 2. On the other hand, the second-step
protonation in 1 was found fairly slow, especially as made in a
comparison between Figures 2f–i and 3f–i. These results clearly
show that one DMAB(H+) site protonated in a range between 0.0
and 1.0 equiv of TFA interacts attractively for stabilization with
the other non-protonated DMAB in 1 to stop rotation along Fc axis
forming a closely connected conformation preferably, where the
diamagnetic ring current effect of DMAB directly has influence
on each other (Chart 2). To the contrary, two DMAB(H+) sites grad-
ually formed in a range between 1.0 and 25 equiv of TFA interact
repulsively with each other to restart rotation along Fc axis
forming a far disconnected conformation preferably, where both
DMAB(H+) sites of 1 are free from such an anisotropic effect,
independently behaving as the DMAB(H+) site of 2. Also, it might
be noted that in both cases, the peaks owing to Cp–H gradually
broadened and completely disappeared in the baseline in a range
of excessive amount of TFA somehow.10
/105 M-1cm-1
0.6
0.4
0.2
500
400
Similarly, from sequences of UV–vis spectra of 1 and 2 in re-
sponse to TFA contents in CHCl3, it is indicated that their changing
behavior was characteristic (Figs. 4 and 5). In the case of 2, the
spectra changed gradually but explicitly via two isosbestic points
at 305 and 378 nm, accompanied by a simple hypochromic effect
up to 600 equiv addition of TFA. Successively, in a region beyond
600 equiv of TFA, the spectral changes were fairly clear-cut to af-
ford several vibration bands reaching the stationary spectrum at
around 4000 equiv TFA addition. Nearly the same spectral chang-
ing behavior as 2 was observed in Fc-DMAB2 1 via isosbestic points
at 294 and 395 nm, finally affording almost the same-shaped sta-
tionary spectrum as 2 at around 4000 equiv TFA addition. In the
case of 1, distinct from 2, another set of isosbestic points was ob-
served at 435 and 490 nm in the weak 3d–3d transition bands,
which is almost overlapped so much as to be overlooked. These re-
sults apparently indicate that the stationary spectra of 1 and 2 at
around 4000 equiv of TFA are both assigned to the completely pro-
tonated DMAB(H+) species. Both electronic absorption and 1H NMR
spectra of 1 and 2 changed by TFA addition recovered the original
ones entirely by neutralization with triethylamine (NEt3), that is,
these spectral changes are reversibly transformable with TFA and
NEt3 within a region under the applied conditions.
200
300
400
500
600
700
/nm
Figure 4. UV–vis spectral changes of 2 (3.82 ꢀ 10ꢁ5 Mꢁ1 in CHCl3). Arrows show
the changing direction of respective spectra by addition of TFA from 0 equiv,
100 equiv, 200 equiv, 400 equiv, 600 equiv, 800 equiv, 1000 equiv, 2000 equiv,
4000 equiv, 6000 equiv, 8000 equiv, and to 10,000 equiv.
/105 M-1cm-1
0.8
0.6
0.4
Even in such lower-concentration solutions for UV–vis spectral
experiment, as compared with 1H NMR spectral one, the mono-
protonated Cp-DMAB(H+) constituent should evidently be pro-
duced in 1 somewhere in a range between 0 and 600 equiv TFA
addition. And thus the Cp-DMAB(H+) constituent would interact
with the other non-protonated Cp-DMAB constituent inductively
to form a closely connected conformation, as already proved from
the 1H NMR spectral experiment. Further spectral changes in a
range beyond 600 equiv of TFA presumably indicate the repulsive
interaction between two Cp-DMAB(H+) constituents in 1 to recover
a far disconnected conformation, reflecting a unique electronic
structure owing to the completely protonated Cp-DMAB(H+) con-
stituent. Unfortunately, beyond or according to expectation, it
500
400
0.2
200
300
400
500
600
700
/nm
Figure 5. UV–vis spectral changes of 1 (1.92 ꢀ 10ꢁ5 Mꢁ1 in CHCl3). Arrows show
the changing direction of respective spectra by addition of TFA from 0 equiv,
100 equiv, 200 equiv, 400 equiv, 600 equiv, 800 equiv, 1000 equiv, 2000 equiv,
4000 equiv, 6000 equiv, 8000 equiv, and to 10,000 equiv.
proved that no particularly new
p-electronic interaction induces
in the mono-protonated Cp-DMAB(H+) species of 1. This is natu-
rally because the two Cp-DMAB constituents in 1 are fixed and ro-
tate along Fc axis so far at a distance of 4 Å as to be unable to make
a transannular interaction electronically,17 even though the Cp-
DMAB(H+) species of 1 exists in a face-to-face conformation
(Chart 2).
protected acetylenes 8 and 9. 1H NMR and UV–vis spectral studies
of 1 and 2 indicated that the diacetylene linkage works as a medi-
ator for communication with DMAB and Cp rings smoothly. It is
also revealed that TFA–NEt3 combination could drive a purposive
rotation of two Cp-DMAB constituents along the Fc axis in 1,
potentially providing a methodology for reversibly transformable
functions by the conformational control between them. Further
investigations on the spectral changes of 1 by means of other
In conclusion, the 1,10-bis(diacetylene-group) connected Fc-
DMAB2 derivative 1 has been synthesized in a moderate yield
under the improved cross-coupling conditions between TMS-

T. Toyama et al. / Tetrahedron Letters 54 (2013) 66–71
71
7. Ethynylferrocene 5 is a stable compound with mp. 50–54 °C, which can be
purchased from Sigma–Aldrich (CAS No. 1271-47-2). Rosenblum, M.; Brawn,
N.; Papenmeier, J.; Applebaum, M. J. Organomet. Chem. 1966, 6, 173–180.
8. 4-Ethynyl-N,N-dimethylaniline 6 is a stable compound with mp. 51–53 °C,
which can be purchased from Sigma-Aldrich (CAS No. 17573-94-3). (a)
Hoffmann, E. Liebigs Ann. Chem. 1891, 264, 160–169; (b) Takalo, H.; Kankale,
J.; Hanninen, E. Acta Chem. Scand. Ser. B 1988, B42, 448–454.
outside stimuli such as metal-coordination and charge-transfer
(CT) complexation are now in progress.18
Acknowledgments
9. Eglinton, G.; Galbraith, A. R. Chem. Ind. 1956, 737–738.
Financial support by a Grant-in-Aid for Scientific Research from
the Ministry of Education, Science, Sports, Culture and Technology
(Nos. 22106512 and 23550046), is gratefully acknowledged. H.H. is
also grateful to the Center of Instrumental Analyses at the Univer-
sity of Toyama for their support in the measurements of physical
properties of new compounds.
10. 1H NMR (CDCl3) and UV–vis (CHCl3) spectral data are only just preliminarily
shown. Other physical properties will be reported properly in a conclusive
discussion on the structure–property relationship somewhere, in terms of
solvent effect, temperature effect, theoretical interpretation, and so on. 1: 1H
NMR; d = 7.37 (d, J = 8.2 Hz, 4H, Ar–H), 6.58 (d, J = 8.2 Hz, 4H, Ar–H), 4.54 (d,
J = 2.0 Hz, 4H, Cp–H), 4.33 (d, J = 2.0 Hz, 4H, Cp–H), and 2.98 ppm (s, 12H, Me–
H). UV–vis; kmax = 291 (loge 4.46, sh), 341 (4.76), 364 (4.69, sh), and 457 nm
(3.43). 2: 1H NMR; d = 7.40 (d, J = 8.4 Hz, 2H, Ar–H), 6.62 (d, J = 8.4 Hz, 2H, Ar–
H), 4.50 (dd, J = 2.0 and 2.0 Hz, 2H, Cp–H), 4.25 (s, 5H, Cp–H), 4.23 (dd, J = 2.0
and 2.0 Hz, 2H, Cp–H), and 2.99 ppm (s, 6H, Me–H). UV–vis; kmax = 286 (4.22,
sh), 334 (4.58), 355 (4.47, sh), and 439 nm (3.24). 4: 1H NMR; d = 7.39 (d,
J = 8.9 Hz, 4H, Ar–H), 6.61 (d, J = 8.9 Hz, 4H, Cp–H), and 2.99 ppm (s, 12H, Me–
H). UV–vis; kmax = 327 (4.64, sh), 351 (4.71), and 377 nm (4.64).
References and notes
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18. A synthetic method for the Fc derivatives with CT character, described as Fc-(
p-
acceptor)( -donor), has been recently developed. From the viewpoint of the
p
conformation control driven by CT interaction stimulus, the results will be
reported in detail, through the comparative study between intra- and
intermolecular complexation, together with their metal-coordination
complexation.