A R T I C L E S
Misra et al.
Scheme 1. Syntheses of New Aldehydes
using TPA cross section measurements. In these systems the
σ(2) values are in the range of 100-15000 GM. Very recently
Kim and Osuka and co-workers have reported σ(2) values for
fused tetramericporphyrinic sheet and porphyrin dimers and
trimers where the large σ(2) value of the order of 93600 GM
has been observed.10 For a better understanding of the correlation
between TPA cross section values and the molecular structure
of porphyrins and expanded porphyrins, more studies on diverse
porphyrin architecture are essential. Very recently, we have
reported large σ(2) values of expanded porphyrins11 containing
26π and 34π electrons. In continuation of our studies on the
structure-function correlation, in this paper we wish to report
the syntheses, characterization, spectrochemical, and electro-
chemical properties as well as TPA cross section values of a
series of 22π smaragdyrin with diverse meso substituents.
We have chosen two different types of meso substituents in
this study. In the first category a series of phenylacetylene and
diphenylacetylene spacers are linked at the meso position of
the 22π smaragdyrin to understand the effect of enhanced π
conjugation on σ(2) values.12 In the second category an elec-
troactive group like ferrocene is either linked directly to the
meso position of the smaragdyrin skeleton or the linking has
been done through phenylacetylene spacers. It has been shown
that the linking of meso substituents enhances the π-electron
conjugation between the smaragdyrin π system and the meso
substituents resulting in enhanced TPA cross section values.
Furthermore, metalation of the smaragdyrin skeleton with
Rh(I) also enhances the σ(2) values due to increased aromatic
character. Geometry optimization and calculation of molecular
electrostatic potential (MESP) support the increased aromatic
character upon the introduction of Rh(I).
with silica gel column chromatography, where the intense green
color fraction was isolated as a pure product. The metal
complexes were quite stable at room temperature.
B. Structural Analysis by 1H NMR. The structure elucida-
tion of the various smaragdyrin molecular conjugates in solution
1
has been done through a detailed analysis of H and 2D NMR
1
spectra. In general, H NMR spectra of conjugates were well
resolved both in the free base as well as in the metallated form.
A typical spectra of 4a is shown in Figure 1, and the assignments
are marked. Briefly, the bipyrrole protons (b, b′; c, c′; d, d′; e,
e′) resonate as four well-resolved doublets in the region of 8-10
ppm with coupling constants of 4-4.4 Hz. The outer pyrrolic
protons (b, b′; e, e′) are more shielded compared to the inner
pyrrolic protons (c, c′; d, d′) because of the upfield ring current
contribution of the meso aryl ring. The â-CH protons of the
furan ring (a, a′) appear as a sharp singlet at 8.54 ppm, while
the meso hydrogen (f) resonates as sharp singlet at 9.83 ppm.
The equivalence of these pyrrolic protons (b, b′; c, c′; d, d′; e,
e′) in the 1H NMR spectrum suggests that the molecule adopts
a symmetric conformation in solution with respect to the mirror
plane passing through the methine bridge connecting the meso
free hydrogen and the furan oxygen atom. This is possible only
if there is a rapid tautomerism between the inner -NH protons
indicating the three -NH protons exchange sites between four
bipyrrole nitrogen centers. Upon lowering the temperature to
240 K, no -NH signals were observed, indicating tautomerism
even at 240 K.
II. Results
A. Syntheses. For the synthesis of smaragdyrins with spacers,
two new aldehydes which are unknown in the literature were
synthesized using the Sonogashira coupling reaction (Scheme
1). Typically, ethynylferrocene 6 and ethynylphenylferrocene
7 were reacted with 3-iodobenzaldehyde under the sonogashira
condition to get aldehyde 1i and 1k, respectively, in 89% yield.
Further, the required smaragdyrin 4 was synthesized by a
well-known acid-catalyzed [3 + 2] oxidative coupling meth-
odology13 using dipyrromethane 2 with the appropriate R group
and oxatripyrrane 3 followed by chloranil oxidation (Scheme
2). The desired product was separated by silica gel column
chromatography using dichloromethane as solvent. The sma-
ragdyrins 4a-4k were found to be stable both in solid and
solution phase in their free-base form. The Rh(I) complexes
5a-5k were synthesized by the reaction of the corresponding
free bases 4a-4k with di-µ-chlorobis[dicarbonylrhodium(I)] in
the presence of sodium acetate. The purification was performed
Treatment of the free-base smaragdyrins with di-µ-chlorobis-
[dicarbonylrhodium(I)] gives the corresponding Rh(I) salt of
the smaragdyrins, where Rh is bound in an η2 fashion to the
pyrrole nitrogens of dipyrromethane unit, with the other two
coordination sites on Rh taken by ancillary carbonyl groups.16
1
A representative H/1H COSY spectrum observed for 5c is
depicted, and assignments are marked (Figure 2). The metalation
with rhodium arrests the NH tautomerism and the -NH protons,
which are not coordinating to rhodium metal, now appear as a
singlet at -1.60 ppm, supporting the presence of NH tautom-
erism in the free base.
After metalation the pyrrolic protons (b, b′; c, c′) appear as
quartets (J ) 2.2 Hz), which in the free base was a doublet,
(8) Ogawa, K.; Ohashi, A.; Kobuke, Y.; Kamada, K.; Ohta, K. J. Am. Chem.
Soc. 2003, 125, 13356.
(9) Collini, E.; Ferrante, C.; Bozio, R. J. Phys. Chem. B 2005, 109, 2.
(10) (a) Ahn, T. K.; Kim, K. S.; Kim, D. Y.; Noh, S. B.; Aratani, N.; Ikeda, C.;
Osuka, A.; Kim, D. J. Am. Chem. Soc. 2006, 128, 1700. (b) Nakamura,
Y.; Aratani, N.; Shinokuba, H.; Takagi, A.; Kawai, T.; Matsumoto, T.;
Yoon, Z. S.; Kim, D. Y.; Ahn, T. K.; Kim, D.; Muranaka, A.; Kobayashi,
N.; Osuka, A. J. Am. Chem. Soc. 2006, 128, 4119.
(11) Rath, H.; Sankar, J.; Prabhuraja, V.; Chandrashekar, T. K.; Nag, A.;
Goswami, D. J. Am. Chem. Soc. 2005, 127, 11608.
(14) Chandrashekar, T. K.; Venkatraman, S. Acc. Chem. Res. 2003, 36, 676-
691.
(12) Misra, R.; Kumar, R.; Chandrashekar, T. K.; Nag, A.; Goswami, D. Org.
Lett. 2006, 8, 629.
(15) Melinger, J. S.; Pan, Y.; Kleiman, V. D.; Peng, Z.; Davis, B. L.; McMorrow,
D.; Lu, M. J. Am. Chem. Soc. 2002, 124, 12002.
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(16) This conclusion is based on our earlier single-crystal X-ray structural studies
on the RhI complex of oxa smaragdyrins; for details see ref 17.
9
16084 J. AM. CHEM. SOC. VOL. 128, NO. 50, 2006