Smaragdyrin-azobenzene conjugates: Syntheses, structure, and spectral and electrochemical properties

Article abstract of DOI:10.1002/ejoc.200600719

The syntheses, characterization, and spectral properties of smaragdyrin-azobenzene conjugates are reported. Our synthetic strategy involves linking the azobenzene group in one of the precursors to a dipyrromethane subunit, which was achieved by reaction o

Full text of DOI:10.1002/ejoc.200600719

FULL PAPER
DOI: 10.1002/ejoc.200600719
Smaragdyrin–Azobenzene Conjugates: Syntheses, Structure, and Spectral and
Electrochemical Properties
Sabapathi Gokulnath,[a] Viswanathan Prabhuraja,[a] Jeyaraman Sankar,[a] and
Tavarekere K. Chandrashekar*[a][‡]
Keywords: Porphyrins / Dipyrromethane / UV/Vis spectroscopy / Isomerization / Electrochemistry
The syntheses, characterization, and spectral properties of
smaragdyrin–azobenzene conjugates are reported. Our syn-
thetic strategy involves linking the azobenzene group in one
of the precursors to a dipyrromethane subunit, which was
achieved by reaction of azobenzenecarbaldehyde with pyr-
role under TFA catalysis. A [3+2] acid-catalyzed oxidative
coupling of this precursor with the tripyrrane moiety gave
the expected smaragdyrin–azobenzene conjugates. The azo-
benzene is in the (E) conformation both in the precursor and
in the smaragdyrin conjugates, as revealed by its single-crys-
tal X-ray structure. Electronic absorption and emission spec-
tral studies reveal the presence of a moderate electronic in-
teraction between the azobenzene and smaragdyrin π-sys-
tems. Irradiation experiments at 360 nm reveal the presence
of a reversible (E)/(Z) transformation of the azobenzene moi-
ety in the precursors 3b and 4b. However, in the smaragdyrin
conjugates the formation of the (Z) conformer leads to the
decomposition of the macrocycle upon prolonged irradiation.
Excitation at the azobenzene absorption results in the ap-
pearance of the emission band of smaragdyrin, thereby sug-
gesting an energy transfer. The electrochemical data reveal
that ring oxidations of the smaragdyrin macrocycle become
harder upon azobenzene introduction, which suggests the
electron-withdrawing nature of the azobenzene in these con-
jugates. A significant shortening of the N–N bond (0.067 Å)
and elongation of the C–N bonds (0.055 and 0.069 Å) in 7a
relative to the azobenzene–dipyrromethane precursor 4a
clearly reveal a rearrangement of the electronic π-delocaliza-
tion pathway in the smaragdyrin–azobenzene conjugates.
RhI binds to one amino and one imino nitrogen atom in the
smaragdyrin cavity to form a 1:1 complex.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
taining azobenzenes linked to (porphyrin)Zn complexes
through a ferrocenyl spacer. They have shown that this sys-
tem can, in principle, be used as a molecular motor due to
a light-induced scissor-like conformational change that
leads to the mechanical twisting of a noncovalently bound
guest molecule. Thus, an understanding of the spectral,
photochemical, and electrochemical properties of the
ground and excited states is an important first step towards
building photoresponsive molecular devices. Most of the
systems studied in the literature are porphyrins and phthal-
ocyanines. To the best of our knowledge, however, there are
no reports on azobenzene-linked expanded porphyrin sys-
tems. The availability of larger delocalized π-electron sys-
tems and larger cavities in the expanded porphyrins[8] may
lead to better interactions with the azobenzene partner rela-
tive to the simple porphyrin systems. Hence, in this paper
we wish to report a series of smaragdyrin–azobenzene con-
jugates in which the azobenzenes are covalently linked at
one of the meso positions of the 22π-electron smaragdyrin
macrocycle. The spectral and electrochemical properties re-
veal a moderate electronic interaction between the sma-
ragdyrin π-system and the azobenzene moiety, and it is
shown that the azobenzene is in the (E) conformation in
the conjugates.
Introduction
Increasing attention has recently been devoted to materi-
als containing photochromic azobenzene moieties because
of interest in their application in fields such as photomemo-
ries,[1] optical switching,[2] and optoelectronics.[3] The intro-
duction of azobenzene units into various molecular systems
leads to an alteration in the properties of both the azoben-
zene and the molecular system.[4–6] This change in the prop-
erties can be utilized to build molecular photoswitches. Por-
phyrins[5] and phthalocyanines[6] are some of the most pop-
ular molecular systems for azobenzene linking. Specifically,
azobenzene units are covalently linked either at the meso
positions of porphyrin[5d] or through the meso-phenyl rin-
g.[5a,5b] There are also examples where azobenzenes are
linked through axial bonding involving a central metal ion
coordinated to the porphyrin system.[5c] Very recently, Aida
and co-workers[7] have reported a molecular system con-
[a] Department of Chemistry, Indian Institute of Technology,
Kanpur 208016, India
Fax: +91-512-259-7436
E-mail: tkc@iitk.ac.in
[‡] Present address: Director, Regional Research Laboratory, Triv-
andrum, Kerala 695019, India
Supporting information for this article is available on the
WWW under http://www.eurjoc.org or from the author.
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Results and Discussion
Syntheses
ylrhodium(I)] in the presence of sodium acetate[11]
(Scheme 3). The purification was performed by silica gel
chromatography. Complexes 8–11 were also found to be
stable at room temperature. To the best of our knowledge,
this is the first example of an expanded porphyrin contain-
ing an azobenzene moiety at the meso position.
The introduction of an azobenzene moiety at the meso
position of smaragdyrin requires a precursor containing
such a moiety. We therefore decided to introduce azoben-
zene into a dipyrromethane moiety, which is one of the pre-
cursors generally used for smaragdyrin synthesis.[9] This was
achieved by performing the diazotization of p-aminobenzal-
dehyde (1) followed by coupling with phenol to give p-hy-
droxyazobenzenecarbaldehyde (2), which was further
treated with methyl iodide and potassium carbonate in dmf
to give the corresponding p-methoxy derivative 3, which, on
reaction with excess pyrrole under TFA catalysis, yielded
the required dipyrromethane-containing azobenzene
(Scheme 1).
4
The synthesis of the smaragdyrins was achieved by the
usual acid-catalyzed [3+2] oxidative coupling of 4a,b with
para-substituted 5,10-diphenyl-16-oxatripyrranes 5a–d. The
major products – the expected smaragdyrin–azobenzene
conjugates
–
were isolated in about 25% yield
(Scheme 2).[10] A trace amount of corrole was also formed
in the reaction. The rhodium complexes were synthesized
Scheme 3. Syntheses of rhodium complexes of azobenzene–sma-
by treating the free base with di-µ-chloridobis[dicarbon- ragdyrin conjugates.
Scheme 1. Synthesis of dipyrromethanes 4a,b.
Scheme 2. Synthesis of azobenzene–smaragdyrin conjugates.
192
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Smaragdyrin–Azobenzene Conjugates
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1
Figure 1. H-1H COSY spectrum of 7a with assignments in the aromatic region.
The composition of the molecules was confirmed by b attached to the azobenzene are deshielded by 1.06 and
FAB mass spectrometry and CHN analysis. The solution 0.86 ppm in 7a relative to the dipyrromethane-containing
1
structure of the smaragdyrin was elucidated by detailed H azobenzene precursor 4b (see Supporting Information).
and 2D NMR studies. As a typical example, the 1H and 1H/ Furthermore, protons c and g experience a small deshield-
1H COSY spectrum in the aromatic region of 7a is shown ing, the extent of which depends on the distance of these
in Figure 1. The assignments and correlations observed are protons from the porphyrin π-system.
marked. The bipyrrole protons (3–10) resonate as four well-
resolved doublets in the region δ = 8.5–10 ppm. The outer
biyrrole protons (3, 6, 7, and 10) are more shielded as com-
pared to the inner bipyrrole protons (4, 5, 8, and 9) because
Electronic Absorption Spectra
of the upfield ring-current contribution of the meso-aryl
The electronic absorption spectra of 7a, its rhodium
ring. The β-CH protons of the furan ring (1, 2) appear as complex 10, and meso-tolylsmaragdyrin (6e) are shown in
a sharp singlet at δ = 8.8 ppm. Only two of the three inner Figure 2. They show a typical intense Soret-like band in the
NH protons are observed at room temperature (δ = –1.68 region 440–460 nm and four Q-bands in the region 550–
and –2.64 ppm). The third NH proton is involved in rapid 725 nm, thus confirming the porphyrin nature.[10] The UV/
tautomerism between the imino and amino nitrogen atoms Vis data are listed in Table 1. The presence of the azoben-
at room temperature and hence is not seen. The observation zene chromophore is confirmed by the band at around
of a singlet for the β-CH protons of the furan ring suggests 340 nm (ε = 4ϫ104 –1 cm–1), which is attributed to a π–π*
a twofold symmetry bisecting the furan oxygen atom and transition in its (E) form.[5a] The introduction of azoben-
the dipyrromethane unit. This is possible if the NH protons zene at the meso position results in the following changes
of the pyrrole in the dipyrromethane are exchanging with in the absorption spectra: (a) a redshift and broadening and
the nitrogen sites, thus supporting the presence of the tau- splitting of the Soret band (544 cm–1); (b) a redshift (125–
tomerism and being consistent with our earlier observa- 480 cm–1) of the Q-bands; (c) on protonation both the Soret
tion.[10] The presence of tautomerism was also confirmed and Q-bands are redshifted and the number of Q-bands is
1
by inserting RhI into the cavity. The H NMR spectrum also reduced, which is characteristic of meso-arylporphyrin
shows only one NH signal at δ = –1.89 ppm at room tem- systems[8] (see Supporting Information); (d) on metalation
perature, thus confirming the binding of RhI to the amino with RhI there is a 15-nm redshift in the Soret band and
and imino nitrogen atoms, as observed previously by our the Q-bands are also slightly redshifted; (e) the ε values of
group.[11]
the metalated derivative are also reduced by 25–30%. These
The effect of azobenzene introduction on the expanded observations clearly suggest a weak electronic interaction
1
porphyrin skeleton is also observed in the H NMR spec- between the azobenzene moiety and the porphyrin π-sys-
trum. For example, the chemical shifts of protons a and tem.
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S. Gokulnath, V. Prabhuraja, J. Sankar, T. K. Chandrashekar
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observations indicate the conversion of the (E) isomer into
the (Z) isomer. However, upon continuous irradiation for
approximately 5 min, the green color of the solution started
to fade and the absorption spectrum of smaragdyrin com-
pletely disappeared, thus suggesting the decomposition of
the complex. These observations prompted us to look into
the (E)/(Z) isomerization process in the precursor aldehyde
3b and dipyrromethane-containing azobenzene 4b. Figure 3
shows the results of these experiments. Typically, irradiation
at 360 nm results in a gradual decrease in the absorption at
350 nm with time and a small increase at about 450 nm,
which suggests the presence of an (E)/(Z) conversion under
photolytic conditions.
Figure 2. Electronic absorption spectra of 6e, 7a, and 10 in CH2Cl2
at a concentration of 0.5ϫ10–6 .
Table 1. UV/Vis absorption and emission data for smaragdyrin–
azobenzene conjugates in dichloromethane.
λmax [nm] (εϫ10–4 [–1 cm–1])
λem [nm]
Azo group Soret
Q-bands
6a
346 (4)
445 (21.3) 557 (2), 597 (1.7),
456 (19.9) 641 (1.4), 707 (2.1)
445 (25) 555 (2.1), 595 (1.7),
720
721
720
722
721
725
721
723
724
722
724
723
701
6b
6c
6d
7a
7b
7c
7d
8
342 (3.8)
343 (6.1)
349 (6.9)
340 (4)
456 (19.7)
640 (1.4), 705 (2)
444 (21.5) 559 (2.7), 602 (2.7),
456 (22.1) 647 (2.6), 719 (3.6)
445(28.1)
558(2.7), 601(2.8),
456 (29.6) 647 (2.7), 715 (3.8)
444 (19) 556 (1.7), 597 (1.5),
455 (19)
641 (1.2), 707 (1.9)
445 (19) 558 (1.9), 600 (2.1),
456 (19.4) 647 (1.9), 712 (2.4)
444 (23.5) 559 (2.8), 602 (2.9),
456 (23.9) 647 (2.7), 719 (3.9)
444 (33.6) 558(3.2), 601(3.2),
346 (5.5)
343 (6.8)
344 (8.4)
297 (3.0)
346 (2.8)
300 (2.8)
455 (34.7)
647(3.0), 717(4.7)
460 (11.3) 579 (1.0), 618 (0.9),
478 (8.2) 639 (1.5), 704 (1.4)
461 (13.8) 579 (1.0), 619 (1.0),
9
351 (3.3.) 478 (10.5) 644 (1.0), 707 (1.7)
300 (3.5)
345 (4.6)
298 (3.8)
344 (4.6)
459(15.8) 578 (1.2), 618 (1.2),
477(11.8) 638 (1.8), 705 (2)
461 (17.3) 578 (1.3), 618 (1.3),
478 (13.2) 638 (1.3), 705 (2.3)
554 (2.3), 593 (2.3),
10
11
6e
–
444 (30)
634 (0.5), 695 (0.4)
Figure 3. UV/Vis absorption spectral changes of the reversible (E)/
(Z) isomerization of (a) 3b and (b) 4b in toluene. The irradiation
time is shown in seconds.
Irradiation Studies
The formation of (E) and (Z) isomers was monitored by
irradiation experiments at approximately 360 nm for 7a and
These observations suggest the attainment of a photo-
the precursors 3b and 4b.[12] In the case of 7a, irradiation stationary state. The irradiated sample was kept in the dark
at 360 nm resulted in a decrease in the absorption band for several hours, and the absorption spectra recorded after
centered at 350 nm and a small increase in the absorption this time showed the emergence of a 350-nm band, thus
band at 450 nm. A sharp decrease in intensity was also ob- suggesting the conversion of the (Z) into the (E) isomer in
served in the Soret region of the absorption spectra. These the dark. Thus, in precursors 3b and 4b the (E)/(Z) conver-
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Smaragdyrin–Azobenzene Conjugates
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sion is reversible, while in the case of 7a, even though an azobenzene substituent, with 7a suggests a redshift of
(E)/(Z) isomerization takes place, the instability of the (Z) the emission maxima by about 20 nm followed by a broad-
isomer results in the decomposition of the smaragdyrin ening of the emission band. The full width at half maximum
macrocycle.
(FWHM) value for 7a is 3261 cm–1, which is 1.6-times
higher than that of 6e. These observations also suggest elec-
tronic communication between the azobenzene moiety and
the porphyrin π-system. In order to prove the possible en-
ergy transfer from azobenzene to the porphyrin π-system,
the fluorescence spectrum of 7a was recorded by exciting
at two different wavelengths. Thus, excitation at the Soret
maximum (444 nm) resulted in the expected emission at
around 721 nm, whereas excitation at 325 nm, which corre-
sponds to the absorption of azobenzene, resulted in the ap-
pearance of a weak emission band (Figure 4a) with reduced
intensity, which clearly suggests an energy transfer from the
azobenzene moiety to the porphyrin π-system. A recent re-
port from the Hecht group[13] also supports this energy-
transfer hypothesis.
Emission Studies
The fluorescence spectra of smaragdyrin–azobenzene
conjugates were recorded in toluene. Typically, excitation
of the Soret band (444 nm) of smaragdyrin results in the
observation of an emission band centered around 721 nm
(Figure 4a). The emission data are listed in Table 1. A com-
parison of the emission spectrum of 6e, which does not have
Electrochemical Studies
The redox behavior of the azobenzene-containing sma-
ragdyrins was studied by cyclic voltammetry using 0.1 
tetra-n-butylammonium hexafluorophosphate (TBAPF6) as
supporting electrolyte in CH2Cl2, in the potential range
–1.5 to 1.5 V vs. SCE. On scanning to negative potential,
no proper reduction waves were observed. However, on
scanning towards positive potential, two reversible oxi-
dations corresponding to the smaragdyrin ring were ob-
served for all the free bases and the Rh complexes. A com-
parison of the cyclic voltammogram of a smaragdyrin with-
out an azobenzene moiety (6e) and one with an azobenzene
moiety (7a) and its Rh complex (10) are shown in Figure 5.
The differential pulse voltammogram is also shown. The
electrochemical data are listed in Table 2. In all the cases
two reversible oxidations (∆Ep ≈ 75–100 mV) were ob-
served, except for 7c, where the oxidations are quasi-revers-
ible (∆Ep = 120 mV). The introduction of azobenzene shifts
the oxidation potential towards more positive values, which
suggests more difficult oxidation. Finally, insertion of RhI
into the smaragdyrin cavity further shifts the potentials
towards more positive values. Earlier studies on porphyrins
containing azobenzene revealed that the azobenzene moiety
in these systems acts as an electron acceptor that attracts
electron density from the porphyrin π-system.[5a] The obser-
vation of more difficult oxidations in the present study also
reveals the electron-attracting nature of azobenzene in the
smaragdyrin–azobenzene conjugates. Attempts to study the
reduction of azobenzene in the negative potential region re-
sulted in decomposition of the complex.
Figure 4. (a) Emission spectra of 7a (bold line: λexc = 444 nm; dot-
ted line: λexc = 325 nm); (b) emission spectrum of 6e (λexc
=
444 nm).
Table 2. Electrochemical data for azobenzene–smaragdyrin conjugates.
6a
6b
6c
6d
6e
7a
7b
7c
7d
8
10
Eox1 [V]
Eox2 [V]
0.368
0.671
0.345
0.601
0.396
0.740
0.384
0.710
0.329
0.638
0.349
0.663
0.343
0.611
0.403
0.756
0.390
0.705
0.412
0.696
0.428
0.769
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S. Gokulnath, V. Prabhuraja, J. Sankar, T. K. Chandrashekar
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were solved. Selected crystallographic parameters are given
in the Experimental Section. The structure of 4a (Figure 6)
clearly reveals the thermodynamically more stable azoben-
zene moiety in the (E) form. The azobenzene groups are
almost perpendicular to the dipyrromethane moiety (dihe-
dral angles of 69.04° for the pyrrole containing C1, C2, C3,
C4, and N1 and 72.39° for the pyrrole ring containing C6,
C7, C8, C9, and N2 with respect to the phenyl ring bonded
to the dipyrromethane carbon). Compound 7a (Figure 7)
shows an almost flat structure with small deviations of the
macrocycle from the mean plane. The mean plane is defined
by the three meso carbon atoms C5, C14, and C23. These
deviations are 18.19°, 11.21°, 6.83°, 12.95° for the pyrrole
N1, N2, N3, and N4 rings, respectively, and 17.41° for the
furan ring. The dihedral angle (58.75°) between the mean
plane of the macrocycle and the meso-phenyl group of the
azobenzene unit is slightly higher than that of the meso-
tolyl group (51.05°, 42.27°). A molecule of methanol is
trapped inside the cavity of the macrocycle and the side
view of the structure clearly shows that the solvent meth-
anol is held by two hydrogen-bonding interactions between
the methanol oxygen atom and the hydrogen atoms at-
tached to the pyrrole N1 and N4 rings (O3···H102 2.025 Å,
128.2°; O3···H103 2.121 Å, 94.97°). Some selected bond
length data which reveal the changes in the electronic struc-
ture of the azobenzene moiety upon binding with sma-
ragdyrin π-system are given in Table 3. For example, the
length of the C14__C31 bond, which links the azobenzene
moiety to the meso carbon atom, is reduced by 0.03 Å in
7a compared to that in 4a. On the other hand, the carbon–
nitrogen bonds of azobenzene (C34__N5 and C37–N6) are
longer in 7a by 0.055 and 0.069 Å, respectively, relative to
4a, thus clearly suggesting a change in the electron delocal-
ization pathway. This is also supported by a significant re-
duction (0.067 Å) in the N–N bond length of the azoben-
zene moiety in 7a compared to 4a. These observations also
Figure 5. Cyclic voltammograms (solid line) and differential pulse
voltammograms (dashed line) of (a) 6e, (b) 7a, and (c) 10 in CH2Cl2
containing 0.1  TBAPF6, recorded at a scan speed of 100 mVs–1.
Crystallographic Characterization
In order to confirm the geometry of the azobenzene moi-
ety in the smaragdyrin–azobenzene conjugates, the crystal
structures of the precursor dipyrromethane-containing azo-
benzene 4a and the smaragdyrin–azobenzene conjugate 7a Figure 6. Single-crystal X-ray structure of 4a.
196 Eur. J. Org. Chem. 2007, 191–200
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Smaragdyrin–Azobenzene Conjugates
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confirm the electronic communication between the azoben- propriate precursors. The spectroscopic and electrochemical
zene moiety and the smaragdyrin π-system observed by measurements clearly show that the azobenzene moiety di-
spectroscopic studies.
rectly covalently linked to the meso carbon atom of the
macrocycle leads to a moderate interaction between the azo-
benzene moiety and the macrocyclic π-system. These stud-
ies also reveal partial electron transfer from smaragdyrin
to the electron-withdrawing azobenzene moiety and energy
transfer from the azobenzene moiety to the smaragdyrin π-
system. The facile reversible (E)/(Z) isomerization for azo-
benzene-containing precursors 3b and 4b has been demon-
strated by irradiation studies. Similar attempts were also
made for the smaragdyrin–azobenzene conjugates. The so-
lid-state structures of 4a and 7a further confirmed the pro-
posed structure, where azobenzene is in its (E) form. Efforts
to prepare a series of covalently linked azobenzene moieties
with other expanded porphyrins and to study their coordi-
nation chemistry are currently underway in our group.
Experimental Section
Instrumentation: Electronic spectra were recorded with a Perkin–
Elmer Lambda 20 UV/Vis spectrophotometer. 1H NMR spectra
were recorded with a 300 MHz JEOL spectrometer with samples
in CDCl3. FAB mass spectra were recorded with a JEOL-SX-120/
DA6000 spectrometer. The fluorescence spectra were recorded with
a SPEX-Fluorolog F112X spectrofluorimeter. Cyclic voltammetric
studies were performed with a CH instrument (CH, 620B). A three-
electrode system consisting of a platinum working electrode, a plati-
num mesh counter electrode, and a commercially available satu-
rated calomel electrode (SCE) as the reference electrode were used.
Irradiation studies were carried out using the output from a 200-
W high-pressure mercury lamp, which was passed through a 360-
nm Oriel band-pass filter.
Chemicals: All NMR solvents were used as received. Solvents like
dichloromethane, tetrahydrofuran, and n-nexane were purified and
distilled by standard procedures. Tetra-n-butylammonium hexaflu-
orophosphate from Fluka was used as the supporting electrolyte
for cyclic voltammetric studies. 2,5-Bis[hydroxy(phenyl)methyl]-
furan and 5,10-diphenyl-16-oxatripyrranes 5a–5d were prepared ac-
cording to the published procedure[14] and stored under an inert
gas at –10 °C.
4-Aminobenzaldehyde (1): 4-Aminobenzaldehyde (1) was prepared
by suitable modification of a literature method.[15] A suspension of
4-nitrobenzaldehyde (7.6 g, 50 mmol) and Zn dust (3.9 g, 60 mmol)
was stirred with ammonium formate (4.7 g, 75 mmol) at room tem-
perature. The reaction was monitored by TLC and, after comple-
tion of the reaction, the mixture was filtered. The organic phase
was washed successively with brine, water (2 times), and dried with
Na2SO4. The solvent was removed under reduced pressure to afford
1 as an orange semi-solid (yield: 4.8 g, 80%).
Figure 7. Single-crystal X-ray structure of 7a with one trapped
methanol molecule: (a) top view; (b) side view (meso-phenyl rings
have been omitted for clarity; dotted lines show the hydrogen bond
between methanol and the pyrrole NH groups).
Table 3. Selected bond lengths [Å] for 4a and 7a.
4a
7a
C5–C10
C13–N3
C16–N4
N3–N4
1.526(2)
1.427(2)
1.423(2)
1.258(2)
C14–C31
C34–N5
C37–N6
N5–N6
1.495(5)
1.479(7)
1.496(7)
1.191(5)
4-Formyl-4Ј-hydroxyazobenzene (2a): A solution of 4-aminobenzal-
dehyde (1) (4.8 g, 40 mmol) in acetone (40 mL) was added to a
stirred mixture of concd. HCl (7 mL in 20 mL of water) at –15 °C.
NaNO2 (2.7 g in 15 mL water, 39 mmol) was added dropwise to
this solution and stirring was continued for 15 min. A solution of
phenol (3.73 g, 40 mmol) in 30 mL of 10% NaOH was added very
slowly to the diazonium salt solution over a period of 20 min. The
color of the solution changed from orange to red and the mixture
was stirred for a further 30 min to attain room temperature. The
Conclusions
We have synthesized smaragdyrin–azobenzene conju-
gates by [3+2] acid-catalyzed oxidative coupling of the ap- organic phase was extracted with CH2Cl2, washed with NaHCO3,
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and dried with Na2SO4. The solvent was removed in a rotary evap-
orator and the resulting red solution was purified by column
chromatography [silica gel (100–200 mesh), ethyl acetate/petroleum
ether, 20:80]. The first moving orange fraction was identified as
containing 4-formyl-4Ј-hydroxyazobenzene (2a). Yield: 3.2 g
1 mmol) were dissolved in dry dichloromethane (250 mL) and the
mixture was stirred under nitrogen for 5 min. TFA (0.0068 mL,
0.1 mmol) was added and stirring was continued for a further
90 min. Chloranil (0.738 g, 3 mmol) was added and the reaction
mixture was exposed to air and refluxed for a further 90 min. The
solvent was then evaporated in vacuo. The residue was purified by
1
(35%). FAB-MS: m/z (%) = 226 (100) [M+]. H NMR (300 MHz,
CDCl3, 25 °C, TMS): δ = 7.02 (d, J = 9 Hz, 2 H), 7.95 (m, 6 H), column chromatography on basic alumina, and the green band
10.09 (s, 1 H) ppm. C13H10N2O2 (226.23): calcd. C 69.02, H 4.46, which eluted with dichloromethane gave 0.195 g of 6a (25% yield),
N 14.14; found C 69.12, H 4.36, N 14.21. This procedure was re-
peated with 2,6-dimethylphenol instead of phenol to synthesize 2b.
Compound 2b: FAB-MS: m/z (%) = 254 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 2.35 (s, 6 H), 7.62 (s, 2 H),
8.03 (m, 4 H), 10.09 (s, 1 H) ppm. C15H14N2O2 (254.28): calcd. C
70.85, H 5.55, N 12.58; found C 70.63, H 5.72, N 12.47.
which decomposed above 320 °C. This procedure was applied to
synthesize azobenzene–smaragdyrin conjugates 6b–d and 7a–d
from the corresponding precursors.
Compound 6a: FAB-MS: m/z (%) = 754 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 2.74 (s, 6 H), 3.97 (s, 3 H),
7.13 (d, J = 8.8 Hz, 2 H), 7.62 (d, J = 7.8 Hz, 4 H), 8.02 (d, J =
4.2 Hz, 2 H), 8.12 (d, J = 8.4 Hz, 4 H), 8.38 (d, J = 8.4 Hz, 2 H),
8.47 (d, J = 4.2 Hz, 2 H), 8.55 (d, J = 8.4 Hz, 2 H), 8.80 (s, 2 H),
9.03 (d, J = 4.2 Hz, 2 H), 9.42 (d, J = 4.4 Hz, 2 H), 9.49 (d, J =
4-Formyl-4Ј-methoxyazobenzene (3a):
A solution of 2a (3 g,
13 mmol) in dmf (30 mL) was added dropwise to a suspension of
K2CO3 (3.7 g, 27 mmol) in dmf (15 mL) and the mixture stirred
under N2 for 30 min. CH3I (1.65 mL, 27 mmol) was added drop-
wise to this solution at 0 °C. The resulting mixture was stirred at
room temperature for 24 h and poured onto crushed ice to remove
excess K2CO3 and dmf. The organic phase was extracted with
CH2Cl2 (5ϫ150 mL), washed twice with water, and dried with
Na2SO4. The solvent was removed in a rotary evaporator and the
resulting orange solution was purified by column chromatography
[silica gel (100–200 mesh), ethyl acetate/petroleum ether, 8:92]. The
first moving orange fraction was identified as containing 4-formyl-
4Ј-methoxyazobenzene (3a). Yield: 2.1 g (65%). FAB-MS: m/z (%)
= 241 (90) [M+]. 1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ =
3.89 (s, 3 H), 7.02 (d, J = 9 Hz, 2 H), 7.98 (m, 6 H), 10.08 (s, 1 H)
ppm. C14H12N2O2 (241.26): calcd. C 69.99, H 5.03, N 11.66; found
C 69.86, H 5.28, N 12.01. This procedure was repeated with 2b to
synthesize 3b.
4.2 Hz, 2 H) ppm. UV/Vis (CH2Cl2): λmax (εϫ10–4
(4), 382 (4), 445 (21.3), 557 (2), 597 (1.7), 641 (1.4), 707 (2.1) nm;
(CH2Cl2/TFA): λmax (εϫ10–4 –1 cm–1) = 348 (3.6), 455 (14.5), 482

–1 cm–1) = 346

(sh, 10.8), 615 (1.3), 673 (1.9), 738 (4.3) nm. C50H38N6O2 (754.88):
calcd. C 79.55, H 5.07, N 11.13; found C 79.46, H 5.34, N 11.31.
Compound 6b: FAB-MS: m/z (%) = 787 (80) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 3.97 (s, 3 H), 4.12 (s, 6 H),
7.13 (d, J = 9.0 Hz, 2 H), 7.62 (d, J = 7.8 Hz, 4 H), 8.02 (d, J =
4.2 Hz, 2 H), 8.12 (d, J = 8.4 Hz, 4 H), 8.36 (d, J = 8.4 Hz, 2 H),
8.47 (d, J = 4.2 Hz, 2 H), 8.54 (d, J = 8.2 Hz, 2 H), 8.80 (s, 2 H),
9.02 (d, J = 6.0 Hz, 2 H), 9.41 (d, J = 4.2 Hz, 2 H), 9.46 (d, J =
4.2 Hz, 2 H) ppm. UV/Vis (CH2Cl2): λmax (εϫ10–4
(3.8), 383 (3.8), 445 (25), 555 (2.1), 595 (1.7), 640 (1.4), 705 (2) nm;
(CH2Cl2/TFA): λmax (εϫ10–4 –1 cm–1): 352 (3.4), 453 (15.2), 485

–1 cm–1) 342

(sh, 10.2), 613 (1.5), 662 (2), 737 (4.7) nm. C50H38N6O4 (786.88):
calcd. C 76.32, H 4.87, N 10.68; found C 76.25, H 4.98, N 10.55.
Compound 3b: FAB-MS: m/z (%) = 268 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 2.39 (s, 6 H), 3.80 (s, 3 H),
7.66 (s, 2 H), 8.01 (s, 4 H), 10.09 (s, 1 H) ppm. C16H16N2O2
(268.21): calcd. C 71.62, H 6.01, N 10.44; found C 71.89, H 5.85,
N 10.21.
Compound 6c: FAB-MS: m/z (%) = 834 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 1.93 (s, 12 H), 2.65 (s, 6 H),
3.96 (s, 3 H), 7.34 (s, 4 H), 8.12 (d, J = 8.7 Hz, 2 H), 8.38 (m, 4
H), 8.54 (d, J = 8.1 Hz, 4 H), 8.60 (s, 2 H), 9.01 (d, J = 4.2 Hz, 2
H), 9.39 (d, J = 4.2 Hz, 2 H), 9.47 (d, J = 4.2 Hz, 2 H) ppm.
Compound 4a: A mixture of pyrrole (14.4 mL, 0.21 mol) and 3a
(2 g, 8.3 mmol) was degassed by bubbling argon through it for
10 min. Trifluoroacetic acid (0.064 mL, 0.83 mmol) was then added
and the mixture was stirred at room temperature for 20 min, after
which time it was diluted with CH2Cl2 (50 mL) and then washed
with 0.1  NaOH. The organic layer was dried with anhydrous
Na2SO4. The solvent was removed under reduced pressure and the
unreacted pyrrole was removed by vacuum distillation at room tem-
perature. The resulting viscous liquid was purified by column
chromatography [silica gel (100–200 mesh), ethyl acetate/petroleum
ether, 15:85; yield: 1.3 g, 45%]. FAB-MS: m/z (%) = 356 (50) [M+].
1H NMR (300 MHz, CDCl3, 25 °C, TMS): δ = 3.85 (s, 3 H), 5.47
(s, 1 H), 5.91 (m, 2 H), 6.15 (m, 2 H), 6.66 (m, 2 H), 6.99 (d, J =
7.2 Hz, 2 H), 7.28 (d, J = 7.8 Hz, 2 H), 7.78 (d, J = 6.6 Hz, 2 H),
7.88 (d, J = 7.2 Hz, 2 H), 7.95 (br. s, 2 H) ppm. C22H20N4O
(356.42): calcd. C 74.14, H 5.66, N 15.72; found C 74.27, H 5.45,
N 15.51. This procedure was repeated with 3b to synthesize 4b.
UV/Vis (CH2Cl2): λmax (εϫ10–4
(21.5)456 (22.1), 559 (2.7), 602 (2.7), 647 (2.6), 719 (3.6) nm;
(CH2Cl2/TFA): λmax (εϫ10–4 –1 cm–1) = 344 (4.9), 456 (18.0), 484

–1 cm–1) = 343(6.1), 383 (3.8), 444

(sh, 11.4), 613 (1.3), 668 (2), 738 (4.8) nm. C54H46N6O2 (834.37):
calcd. C 79.97, H 5.72, N 10.36; found C 79.85, H 5.92, N 10.25.
Compound 6d: FAB-MS: m/z (%) = 838 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 1.74 (s, 18 H), 3.87 (s, 3 H),
7.23 (d, J = 9.0 Hz, 2 H), 7.72 (d, J = 7.8 Hz, 4 H), 8.12 (d, J =
4.2 Hz, 2 H), 8.22 (d, J = 8.4 Hz, 4 H), 8.35 (d, J = 8.4 Hz, 2 H),
8.57 (d, J = 4.2 Hz, 2 H), 8.64 (d, J = 8.2 Hz, 2 H), 8.87 (s, 2 H),
9.02 (d, J = 6.0 Hz, 2 H), 9.51 (d, J = 4.2 Hz, 2 H), 9.56 (d, J =
4.2 Hz, 2 H) ppm. UV/Vis (CH2Cl2): λmax (εϫ10–4
(6.9), 445 (28.1), 456 (29.6), 558 (2.7), 601 (2.8), 647 (2.7), 715 (3.8)
nm; (CH2Cl2/TFA): λmax (εϫ10–4 –1 cm–1) = 332 (4.5), 456 (16.2),

–1 cm–1) = 349

484 (sh, 11.9), 613 (1.3), 673 (1.9), 738 (4.8) nm. C56H50N6O2
(838.40): calcd. C 80.16, H 6.01, N 10.02; found C 80.09, H 6.21,
N 10.14.
Compound 4b: FAB-MS: m/z (%) = 384 (50) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 2.36 (s, 6 H), 3.76 (s, 3 H),
5.45 (s, 1 H), 5.91 (m, 2 H), 6.16 (m, 2 H), 6.64 (m, 2 H), 7.29 (d,
J = 7.2 Hz, 2 H), 7.61 (s, 2 H), 7.81 (d, J = 6.6 Hz, 2 H), 7.92 (br.
s, 2 H) ppm. C24H24N4O (384.47): 74.97, H 6.29, N 14.57; found
C 74.67, H 5.98, N 15.87.
Compound 7a: FAB-MS: m/z (%) = 782 (60) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = –2.69 (br. s, 1 H), –1.81 (br.
s, 1 H), 2.46 (s, 6 H), 2.73 (s, 6 H), 3.86 (s, 3 H), 7.61 (d, J = 7.8 Hz,
4 H), 7.82 (s, 2 H), 8.09 (d, J = 7.8 Hz, 4 H), 8.37 (d, J = 8.1 Hz,
2 H), 8.48 (d, J = 4.2 Hz, 2 H), 8.54 (d, J = 8.4 Hz, 2 H), 8.80 (s,
2 H), 9.02 (d, J = 4.2 Hz, 2 H), 9.40 (d, J = 4.2 Hz, 2 H), 9.49 (d,
Typical Procedure for the Synthesis of Smaragdyrin: The oxatripyr-
rane 5a (0.407 g, 1 mmol) and dipyrromethane 4b (0.385 g,
J = 4.2 Hz, 2 H) ppm. UV/Vis (CH2Cl2): λmax (εϫ10–4 –1 cm–1) =

198
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Eur. J. Org. Chem. 2007, 191–200

Smaragdyrin–Azobenzene Conjugates
FULL PAPER
340 (4), 444 (19.0), 455 (19.0), 556 (1.7), 597 (1.5), 641 (1.2), 707 2 H), 8.89 (s, 2 H), 9.24 (d, J = 4.5 Hz, 2 H), 9.54 (m, 2 H) ppm.
(1.9) nm; (CH2Cl2/TFA): λmax (εϫ10–4 –1 cm–1) = 343 (4.5), 455 UV/Vis (CH2Cl2): λmax (εϫ10–4 –1 cm–1) = 300 (2.8), 351 (3.3.),
(20), 484 (sh, 14.8), 613 (1.7), 668 (2.5), 738 (6) nm. C52H42N6O2 461 (13.8), 478 (10.5), 579 (1.0), 619 (1.0), 644 (1.0), 707 (1.7) nm.


(782.93): calcd. C 79.77, H 5.41, N 10.73; found C 79.61, H 5.53,
N 10.59.
C58H49N6O4Rh (996.95): calcd. C 69.87, H 4.95, N 8.43; found C
66.95, H 4.82, N 8.54.
Compound 7b: FAB-MS: m/z (%) = 815 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 2.45 (s, 6 H), 3.91 (s, 3 H),
4.14 (s, 6 H), 7.61 (d, J = 7.8 Hz, 4 H), 7.82 (s, 2 H), 8.09 (d, J =
7.8 Hz, 4 H), 8.37 (d, J = 8.1 Hz, 2 H), 8.48 (d, J = 4.2 Hz, 2 H),
8.54 (d, J = 8.4 Hz, 2 H), 8.80 (s, 2 H), 9.02 (d, J = 4.2 Hz, 2 H),
9.39 (d, J = 4.2 Hz, 2 H), 9.46 (d, J = 4.2 Hz, 2 H) ppm. UV/Vis
Compound 10: FAB-MS: m/z (%) = 940 (60) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = –1.95 (br. s, 2 H), 2.48 (s, 6
H), 2.74 (s, 6 H), 3.86 (s, 3 H), 7.62 (d, J = 7.8 Hz, 4 H), 7.82 (s,
2 H), 8.06 (d, J = 4.2 Hz, 2 H), 8.22 (d, J = 8.4 Hz, 2 H), 8.36 (d,
J = 8.4 Hz, 2 H), 8.47 (d, J = 4.2 Hz, 2 H), 8.54 (m, 2 H), 8.76 (d,
J = 6.0 Hz, 2 H), 8.83 (s, 2 H), 9.16 (d, J = 4.2 Hz, 2 H), 9.49 (m,
(CH2Cl2): λmax (εϫ10–4
455 (19.4), 558 (1.9), 600 (2.1), 647 (1.9), 712 (2.4) nm; (CH2Cl2/
TFA): λmax (εϫ10–4 –1 cm–1) = 349 (4.8), 455 (17.1), 483 (sh, 12.4),

–1 cm–1) = 346 (5.5), 381 (4.6), 447 (19),
2 H) ppm. UV/Vis (CH2Cl2): λmax (εϫ10–4 –1 cm–1) = 300 (3.5),

345 (4.6), 459 (15.8), 477 (1.8), 578 (1.2), 618 (1.2), 638 (1.8), 705
(2) nm. C54H41N6O4Rh (940.85): calcd. C 68.94, H 4.39, N 8.93;
found C 68.81, H 4.57, N 8.79.

615 (1.8), 661 (2.1), 738 (4.9) nm. C52H42N6O4 (814.92): calcd. C
76.64, H 5.19, N 10.31; found C 76.73, H 5.05, N 10.12.
Compound 11: FAB-MS: m/z (%) = 1024 (50) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = –1.92 (br. s, 2 H), δ = 1.74 (s,
18 H), 2.75 (s, 6 H), 3.86 (s, 3 H), 7.80–7.85 (m, 6 H), 8.10 (d, J =
7.8 Hz, 4 H), 8.26 (d, J = 8.1 Hz, 2 H), 8.34 (d, J = 8.4 Hz, 2 H),
8.47 (d, J = 7.2 Hz, 2 H), 8.58 (m, 2 H), 8.84 (d, J = 4.3 Hz, 2 H),
9.25 (d, J = 4.5 Hz, 2 H), 9.56 (m, 2 H) ppm. UV/Vis (CH2Cl2):
Compound 7c: FAB-MS: m/z (%) = 837 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 1.85 (s, 12 H), 2.56 (s, 6 H),
2.74 (s, 6 H), 3.97 (s, 3 H), 7.35 (s, 4 H), 7.82 (s, 2 H), 8.37 (m, 4
H), 8.54 (d, J = 8.4 Hz, 2 H), 8.61 (s, 2 H), 9.01 (d, J = 4.2 Hz, 2
H), 9.39 (d, J = 4.2 Hz, 2 H), 9.48 (d, J = 4.2 Hz, 2 H) ppm. UV/
Vis (CH2Cl2): λmax (εϫ10–4
(23.9), 559 (2.8), 602 (2.9), 647 (2.7), 719 (3.9) nm; (CH2Cl2/TFA):
λmax (εϫ10–4 –1 cm–1) = 353 (4.3), 455 (15.2), 483 (sh, 9.7), 613

–1 cm–1) = 343 (6.8), 444 (23.5), 456
λmax (εϫ10–4 –1 cm–1) = 298 (3.8), 344 (4.6), 461 (17.3), 478 (13.2),

578 (1.3) 618 (1.3), 638 (1.3), 705 (2.3) nm. C60H53N6O4Rh
(1024.32): C70.31, H 5.21, N 8.20; found C 70.31, H 4.97, N 8.48.

(1.5), 673 (1.9), 738 (5.2) nm. C56H50N6O2 (837.04): calcd. C 80.16,
H 6.01, N 10.02; found C 80.25, H 5.89, N 10.14.
X-ray Crystallographic Studies: The crystals were immersed in hy-
drocarbon oil (Paraton N®), a single crystal selected, mounted on
a glass fiber, and placed in the low-temperature N2 stream. Inten-
sity data were collected at 210 K with a Stoe IPDS2 system utilizing
Mo-Kα radiation (λ = 0.71073 Å). The intensities were corrected
for Lorentz and polarization effects. The structures were solved by
direct methods using the SHELXTL PLUS program system[16] and
refined against |F2| with the program SHELXL-97 using all data.[16]
Compound 7d: FAB-MS: m/z (%) = 866 (100) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = 1.75 (s, 18 H), 2.45 (s, 6 H),
3.91 (s, 3 H), 7.61 (d, J = 7.8 Hz, 4 H), 7.82 (s, 2 H), 8.09 (d, J =
7.8 Hz, 4 H), 8.37 (d, J = 8.1 Hz, 2 H), 8.48 (d, J = 4.2 Hz, 2 H),
8.54 (d, J = 8.4 Hz, 2 H), 8.80 (s, 2 H), 9.02 (d, J = 4.2 Hz, 2 H),
9.39 (d, J = 4.2 Hz, 2 H), 9.46 (d, J = 4.2 Hz, 2 H) ppm. UV/Vis
(CH2Cl2): λmax (εϫ10–4
(34.7), 558 (3.2), 601 (3.2), 647 (3.0), 717 (4.7) nm; (CH2Cl2/TFA):
λmax (εϫ10–4 –1 cm–1) = 348 (3.6), 455 (14.5), 482 (sh, 10.8), 615

–1 cm–1) = 344 (8.4), 444 (33.6), 455

Table 4. Crystallographic data for azobenzene-containing dipyrro-
(1.3), 673 (1.9), 738 (4.3) nm. C58H54N6O2 (866.43): calcd. C 80.34,
H 6.28, N 9.69; found C 80.42, H 6.12, N 9.74.
methane 4a and azobenzene–smaragdyrin conjugate 7a.
4a
7a
Rhodium(I) Complex 8: Compound 6b (0.030 g, 0.037 mmol) was
dissolved in alcohol-free dichloromethane (40 mL). Anhydrous so-
dium acetate (0.030 g, 0.37 mmol) was added to the solution fol-
Solvent for crystallization
Empirical formula
T [K]
Crystal system
Space group
V [Å3]
a [Å]
b [Å]
c [Å]
CH2Cl2/hexane CH2Cl2/MeOH
C22H20N4O
100(2)
monoclinic
P21/c
C53H46N6O3
100(2)
lowed
by
di-µ-chloridobis[dicarbonylrhodium(I)]
(0.020 g,
triclinic
¯
0.05 mmol), and the mixture was stirred under reflux for 2 h. The
solvent was evaporated and the residue was column-chromato-
graphed on silica gel with dichloromethane as eluent. Removal of
the solvent gave 8 as a green solid (0.032 g, 95%), which was recrys-
tallized from a dichloromethane/n-hexane mixture. FAB-MS: m/z
P1
1808.0(3)
7.5731(15)
40.991(8)
5.8368(12)
90
93.79(3)
90
2093.5(5)
9.5647(19)
14.347(3)
15.660(3)
84.033(3)
84.715(3)
79.165(3)
2
α [°]
β [°]
γ [°]
1
(%) = 944 (60) [M+]. H NMR (300 MHz, CDCl3, 25 °C, TMS): δ
= –1.97 (br. s, 2 H), 3.98 (s, 3 H), 4.14 (s, 6 H), 7.13 (d, J = 9.0 Hz,
2 H), 7.62 (d, J = 7.8 Hz, 4 H), 8.02 (d, J = 4.2 Hz, 2 H), 8.12 (d,
J = 8.4 Hz, 4 H), 8.36 (d, J = 8.4 Hz, 2 H), 8.47 (d, J = 4.2 Hz, 2
H), 8.54 (m, 2 H), 8.76 (d, J = 6.0 Hz, 2 H), 8.83 (s, 2 H), 9.16
Z
4
ρcalcd. [Mgm–3]
1.309
1.293
14034/9945
0.0370
Reflections measured/unique 12055/4473
(d, J = 4.2 Hz, 2 H), 9.49 (m, 2 H) ppm. UV/Vis (CH2Cl2): λmax R (in)
0.0359
(εϫ10–4 –1 cm–1) = 297 (3.0), 346 (2.8), 460 (11.3), 478 (8.2), 579

F(000)
Limiting indices
752
860
–10 Յ h Յ 9
–54 Յ k Յ 47
–7 Յ l Յ 5
1.028
–12 Յ h Յ 12
–19 Յ k Յ 9
–20 Յ l Յ 20
1.152
(1.0), 618 (0.9), 638 (1.0), 705 (1.4) nm. C52H37N6O6Rh (944.79):
calcd. C 66.11, H 3.95, N 8.90; found C 66.42, H 3.78, N 8.79. This
procedure was applied for azobenzene–smaragdyrin conjugates 6d,
7a, and 7d to give the rhodium complexes 9–11, respectively.
GoF (F2)
Final R indices [I Ͼ 2σI]
Compound 9: FAB-MS: m/z (%) = 996 (50) [M+]. 1H NMR
(300 MHz, CDCl3, 25 °C, TMS): δ = –1.92 (br. s, 2 H), δ = 1.74 (s,
18 H), 3.97 (s, 3 H), 7.13 (d, J = 9 Hz, 2 H), 7.8–7.85 (m, 4 H),
8.09–8.14 (m, 4 H), 8.22 (d, J = 8.1 Hz, 2 H), 8.34 (d, J = 8.4 Hz,
2 H), 8.47 (d, J = 7.2 Hz, 2 H), 8.58 (m, 2 H), 8.84 (d, J = 4.3 Hz,
R1
wR2
0.0559
0.1274
0.1054
0.2138
R indices all data
R1
wR2
0.0755
0.1374
0.1356
0.2305
Eur. J. Org. Chem. 2007, 191–200
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
199

S. Gokulnath, V. Prabhuraja, J. Sankar, T. K. Chandrashekar
FULL PAPER
T. E. O. Screen, I. M. Blake, L. H. Rees, W. Clegg, S. J. Bor-
wick, H. L. Anderson, J. Chem. Soc., Perkin Trans. 1 2002,
320–329.
J. L. Rodríguez-Redondo, Á. Sastre-Santos, F. Fernández-
Lázaro, D. Soares, G. C. Azzellini, B. Elliott, L. Echegoyen,
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Hydrogen atoms were placed geometrically and refined using a ri-
ding model. See Table 4 for further details. CCDC-603853 (4a)
and -603854 (7a) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/
data_request/cif.
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Supporting Information (see footnote on the first page of this arti-
1
cle): FAB mass, UV/Vis, H NMR spectra of selected compounds.
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This work was supported by grants from the Department of Sci-
ence and Technology (DST) and the Council of Scientific and In-
dustrial Research (CSIR), Government of India, New Delhi. S. G.,
V. P. R., and J. S. thank CSIR for fellowships.
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Received: August 16, 2006
Published Online: November 17, 2006
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