Syntheses of normal, expanded, strapped and N-confused calixbenzophyrins from a single starting material

Article abstract of DOI:10.1039/c3cc41601a

The syntheses and spectral and structural characterization of hitherto unknown p-benzene incorporated normal, expanded, strapped and N-confused calixbenzophyrins are achieved; altogether five new macrocycles from a single starting material are reported. The binding studies of the strapped macrocycle revealed that the molecule is selective towards Fe(iii) ions.

Full text of DOI:10.1039/c3cc41601a

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Syntheses of normal, expanded, strapped and
N-confused calixbenzophyrins from a single
starting material†
P. S. Salini,a K. S. Anju,a M. L. P. Reddyb and A. Srinivasan*c
Cite this: Chem. Commun., 2013,
49, 5769
Received 2nd March 2013,
Accepted 1st May 2013
DOI: 10.1039/c3cc41601a
www.rsc.org/chemcomm
The syntheses and spectral and structural characterization of hitherto
unknown p-benzene incorporated normal, expanded, strapped and
N-confused calixbenzophyrins are achieved; altogether five new
macrocycles from a single starting material are reported. The binding
studies of the strapped macrocycle revealed that the molecule is
selective towards Fe(III) ions.
Pyrrolic macrocyclic systems have attracted growing interest
due to their diverse application in the areas of anion binding,
cation coordination, conducting polymers, liquid crystals, and
nonlinear optics.1 Among these, the chemistry of porphyrinogens
especially calixphyrins have received renewed interest since
they offer promising perspectives in the field of substrate
recognition.2 A large number of structurally diverse calixphyrins
have been reported in the literature which include normal,
expanded, N-confused, three dimensional and core modified
macrocycles with unexpected coordination properties.3
Scheme 1 Syntheses of calix[n]-p-benzo[m]phyrins. (a) BF3OEt2, pyrrole; (b) CH2Cl2,
C6F5CHO, TFA, DDQ.
Strapped macrocycles have been identified as one of the
more attractive hosts because of the presence of diametrical
bridging that provides a convenient means of introducing starting material. Among these, the strapped derivative repre-
functionalized groups at or near the binding domain. Such sents a new class of calixbenzophyrins bearing a strap with an
type of porphyrins and calixpyrroles are well established in the ether linkage on one side of the molecule which is hitherto
literature.4 However, the corresponding calixphyrins are still unknown in calixphyrin chemistry. The formation of normal
unknown. In the context of ongoing attempts to develop the and strapped calixbenzophyrins was confirmed by single crystal
chemistry of calixphyrins, we report the synthesis of four X-ray diffraction analysis. Further, the strapped calixbenzophyrin
different classes of p-benzene based calixphyrins such as normal, binds selectively to Fe(III) ions.
expanded, strapped and N-confused derivatives from a single
The syntheses of calixbenzophyrins are outlined in Scheme 1.
The precursors for the target macrocycles were obtained by the
acid catalysed condensation of 1,4-bis(1,1-dimethylhydroxy
methyl)benzene with pyrrole.2c The reaction afforded two types
of precursors, a,a0 substituted bispyrrolylphenyl derivative, 1,
and N-confused a,b0 substituted bispyrrolylphenyl derivative, 2
in 32% and 20% yields (Fig. S1 and S2, ESI†). Both of the
a Photosciences and Photonics Group, Chemical Sciences and Technology Division,
CSIR-National Institute for Interdisciplinary Science and Technology (NIIST),
Thiruvananthapuram 695 019, Kerala, India
b Inorganic Chemistry Section, Chemical Sciences and Technology Division,
CSIR-National Institute for Interdisciplinary Science and Technology (NIIST),
Thiruvananthapuram 695 019, Kerala, India
c School of Chemical Sciences, National Institute of Science Education and Research, precursors led to the formation of entirely different classes of
IOP Campus, Sainik School – P. O., Bhubaneswar-751 005, Odisha, India.
calixphyrins, when condensation with pentafluorobenzaldehyde
in dichloromethane in the presence of trifluoroacetic acid (TFA)
was followed by oxidation with 2,3-dichloro-5,6-dicyano-1,4-
E-mail: srini@niser.ac.in; Fax: +91-674-2302436
† Electronic supplementary information (ESI) available: Experimental sections,
synthetic details, supplementary results, CIF files. CCDC 926221 and 926222. For
benzoquinone (DDQ). Precursor 1 afforded the formation of normal
and expanded calix[n]-p-benzo[m]phyrins (n = 2,3,4; m = 4,6,8)
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c3cc41601a
c
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Fig. 2 Single crystal X-ray structure of P-I. (a) Top view and (b) side view. In the
side view the meso groups are omitted for clarity.
Fig. 1 1H NMR spectra of P-I to P-V. The signals correspond to the protons of the
meso groups and solvent peaks are omitted for clarity.
in 20% (P-I), 10% (P-II), and 5% (P-III) yields, respectively, while
2 resulted in the generation of strapped and N-confused
calixbenzophyrins in 20% (P-IV) and 10% (P-V) yields.
All of the macrocycles were characterized using various
spectroscopic techniques such as FAB-MS (Fig. S3–S7, ESI†)
and 1H NMR spectral studies (Fig. S8–S12, ESI†), and the
structure of P-I and P-IV was unambiguously determined
through single-crystal X-ray diffraction analysis. The exact
composition of the macrocycles P-I to P-V was established from
FAB mass spectra which showed molecular ion signals at
m/z 936.66, 1404.55, 1874.19, 1311.54 and 937.79, respectively.
Fig. 3 Single crystal X-ray structure of P-IV. (a) Top view and (b) side view. In the
side view the meso groups are omitted for clarity.
1
In the H NMR spectrum of P-I to P-III, NH protons resonated
as a broad singlet at 12.5 ppm suggesting the intramolecular distance and angle of N2–H2ꢀꢀꢀN3 and N4–H1ꢀꢀꢀN1 being 2.10 Å
hydrogen bonding interaction between the amine and imine and 1291 respectively. The dipyrrin units are perpendicular to the
pyrrolic NHs (Fig. 1). The p-phenyl protons resonated as a meso-pentafluorophenyl unit as well as the p-phenyl rings and
singlet at 7.12 ppm indicating the similar chemical environ- adopt a chair-like conformation (Fig. 2b). The crystal analysis of P-I
ment in the benzene protons, whereas pyrrolic b-CH protons shows two types of 1-D arrays formed from the intermolecular
resonated as two doublets at 6.11 and 6.35 ppm respectively.
C–Hꢀ ꢀ ꢀF interactions between (i) pyrrolic b-CH (C26–H26) and
In the 1H NMR spectrum of P-IV, p-phenyl –CH protons F atom (F4) and (ii) the meso-methyl C–H (C22–H22) units and
resonated as four doublets from 7.63 to 7.04 ppm, which fluorine (F5) of the neighbouring pentafluorophenyl group with
suggests that all of the four phenyl protons are in different the distances and angles of C26–H26ꢀ ꢀ ꢀF4 and C22–H22ꢀ ꢀ ꢀF5
chemical environments. There are two types of pyrrolic b-CH being 2.58, 2.59 Å and 1501, 1541 respectively (Fig. S16, ESI†).
protons, where the two doublets at 6.56–6.41 ppm correspond These two 1-D arrays combined together to generate a 2-D
to normal pyrrolic b-CH protons, while the strapped pyrrolic supramolecular assembly in the solid state (Fig. S17, ESI†).
b-CH protons resonated as a singlet at 6.24 ppm, which is
The single crystal X-ray structure of P-IV is shown in Fig. 3
confirmed from H–1H COSY spectral analysis (Fig. S13, ESI†). where the structure crystallizes in a monoclinic crystal system.
The –CH protons which are close to the oxygen atom in the As observed from the spectral analysis, in addition to the
strap appeared as a singlet at 5.29 ppm. On the other hand, p-phenyl rings in the macrocyclic framework, the dipyrrin
there are two kinds of pyrrolic moieties present in P-V, normal moieties contain both the normal and confused pyrrole rings.
and N-confused. The normal pyrrolic b-CH protons resonated The a-position of the confused pyrrole units are connected by a
as two doublets at 6.29 and 6.18 ppm whereas the confused strap with an ether linkage containing two pentafluorophenyl
pyrrolic a and b-CH protons appeared as two singlets at 7.05 units in it. Both the pyrrole units are involved in intramolecular
and 6.25 ppm, respectively, which were further confirmed by hydrogen bonding interactions with the distances and angles of
1H–1H COSY spectral analysis (Fig. S14, ESI†). The p-phenyl N1–H1ꢀ ꢀ ꢀN4: 2.15 Å, 1271 and N2–H2ꢀ ꢀ ꢀN3: 2.20 Å and 1221
–CH protons resonated as double doublets at 7.23–7.15 ppm. respectively (Fig. 3a). The pentafluorophenyl rings which are in
Finally the NH protons of the strapped and N-confused calix- the strap are pointing downwards, whereas in the meso-
1
benzophyrins appeared as singlets at 12.00 ppm.
positions they are in a side on manner and are perpendicular
The single crystal X-ray structure of P-I is shown in Fig. 2 to the mean dipyrrin plane and adopt a bowl-like conformation
where the macrocycle crystallizes in the triclinic crystal system. (Fig. 3b and Fig. S19, ESI†). The crystal packing revealed
As predicted from the NMR analysis, the dipyrrin unit contains the presence of intermolecular interactions through C–Hꢀ ꢀ ꢀF
both the amine and imine pyrrole nitrogens which are involved hydrogen bonding, Fꢀ ꢀ ꢀF and C–Fꢀ ꢀ ꢀp interactions which leads
in intramolecular hydrogen bonding interactions with the to the formation of two self-assembled dimers (Fig. S19, ESI†)
c
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presence of other metal ions (Fig. 4c). The changes in the
emission-spectral data indicate a 1 : 1 binding mode with an
association constant of 2.8 ꢁ 103 Mꢂ1 (Fig. 4d and Fig. S23,
ESI†). The reversibility of the binding mode was examined by
the addition of EDTA (Fig. S24, ESI†). To understand the role of
the oxygen atom in the strap, the binding ability of P-I and
P-V was evaluated with Fe(III), Hg(II) and Cu(II) under similar
conditions of P-IV, which revealed that P-I and P-V have no
selectivity towards these metal ions as compared to P-IV
(Fig. S25 and S26, ESI†).
In summary, we have demonstrated the synthesis and
spectral and structural characterization of four different classes
of calixphyrins, normal, expanded, strapped and N-confused
calixbenzophyrins, from a single starting material. The reported
strapped calixbenzophyrin is the first of its kind in the calixphyrin
area with a strap in it. Furthermore, the single-crystal X-ray
analysis of normal and strapped calixbenzophyrins reveals the
Fig. 4 (a) The absorption and (b) emission changes in P-IV (15 mM) upon presence of both inter- and intramolecular hydrogen bonding
titration with Fe(III) ions. Inset of (a) shows the color change of P-IV upon
interactions including C–Hꢀ ꢀ ꢀF, Fꢀ ꢀ ꢀF, C–Hꢀ ꢀ ꢀp and C–Fꢀ ꢀ ꢀpF
addition of Fe(III) ions under visible light. (c) Metal ion selectivity of P-IV
interactions and generates two-dimensional supramolecular
(8.7 mM) in the presence of 10 equivalents of various metal ions. (1) Cu(II); (2) Cd(II);
assemblies. Strapped calixbenzophyrin binds selectively to
(3) Ca(II); (4) K(I); (5) Li(I); (6) Ni(II); (7) Pb(II); (8) Hg(II); (9) Zn(II); and (10) Fe(III). The small,
Fe(III) ions over the biologically important different metal ions.
coloured bars represent the changes in the emission intensity of a solution of P-IV with
the cation of interest. The cyan bars show the fluorescence change that occurs upon Studies on the receptor properties of other macrocycles are
addition of 10 equivalents of Fe(III) to the solution containing P-IV and the respective
currently underway in our laboratory.
cation. (d) Jobs plot indicating 1 : 1 stoichiometry.
Dr A. S. thanks the Directors, NIIST-CSIR and NISER for
financial support. S. P. S. thanks CSIR for a research fellowship.
and two 1-D arrays (Fig. S20, ESI†). The two self-assembled We thank Dr Babu Varghese, SAIF, IIT-Chennai, for solving
dimers are generated by the combination of (i) Fꢀ ꢀ ꢀF and the crystal structure of P-I and P-IV. We greatly acknowledge
C–Hꢀ ꢀ ꢀp interaction with the distances of F7ꢀ ꢀ ꢀF13 and Mrs Viji and Mrs Soumini, NIIST-CSIR, for recording the
C41–H41ꢀ ꢀ ꢀPy(p) being 2.09 Å and 2.87 Å and (ii) two C–Hꢀ ꢀ ꢀF FAB-MS and NMR spectra respectively.
and one C–Fꢀ ꢀ ꢀpF interactions with the distances and angles of
C53–H53ꢀ ꢀ ꢀF10, C40–H40ꢀ ꢀ ꢀF2 and C9–F9ꢀ ꢀ ꢀpF being 2.51 Å,
Notes and references
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Chem. Commun., 2013, 49, 5769--5771 5771