Infinite Three-Dimensional Polymeric Metalloporphyrin Network via Six-Coordinate Zn(II) and Two Axial Oxygen Donors

Article abstract of DOI:10.1021/ol035665v

(Equation presented) An X-ray crystallographic study of zinc(II) 5,15-di-(2-methoxymethylphenyl)-porphyrin indicates that it forms a coordination polymer through ligation of the ether oxygen atoms on the porphyrin peripheries to the metal centers of two i

Full text of DOI:10.1021/ol035665v

ORGANIC
LETTERS
2003
Vol. 5, No. 22
4207-4210
Infinite Three-Dimensional Polymeric
Metalloporphyrin Network via
Six-Coordinate Zn(II) and Two Axial
Oxygen Donors
Tang-Lin Teo, Muthalagu Vetrichelvan, and Yee-Hing Lai*
Department of Chemistry, National UniVersity of Singapore, 3 Science DriVe 3,
Singapore 117543
chmlaiyh@nus.edu.sg
Received September 1, 2003
ABSTRACT
An X-ray crystallographic study of zinc(II) 5,15-di-(2-methoxymethylphenyl)-porphyrin indicates that it forms a coordination polymer through
ligation of the ether oxygen atoms on the porphyrin peripheries to the metal centers of two identical adjacent porphyrins. This gives a novel
extensively linked, three-dimensional polymeric structure in which the zinc(II) metal forms a six-coordination center. The uniquely structured
network has cavities between 4.81 and 9.27 Å, which makes it resemble molecular sieve materials.
Multiporphyrin architectures reveal enormous versatility in
photoactivity, electron transfer, and redox properties.1-3 In
particular, functional assemblies of multiporphyrin arrays
have been designed for the development of new molecular
electronic devices,4 molecular machines, and catalysts.5,6
Although many elegant structures of such arrays have been
constructed via direct covalent bonding, they are difficult to
realize on a practical scale.7 Molecular self-assembly offers
a good alternative to a wide variety of one-, two-, and three-
dimensional arrays through various associations and orienta-
tions. Self-assembly of zinc porphyrins via a molecular
recognition approach proved to be successful when external
nitrogen donors were used to form a series of intermolecular
Zn-N coordinations.8 Extension into a one-dimensional (1D)
network was achieved in a series of tetra(4-pyridyl)-
porphyrins.9 A five-coordinate Zn(II) with one intermolecular
Zn-N(pyridyl) coordination was preferred due to steric
reasons. Three-dimensional (3D) coordination polymers
involving six-coordinate Zn(II) with two axial N(pyridyl)
donors was only formed after more extensive crystallization
experiments.9 Coordination polymers of Zn(II) porphyrins
with one axial O donor from functional groups attached to
peripheral phenyl rings have also been reported.10 These
examples are, however, limited to five-coordinate Zn(II), thus
restricting the formation to only a 1D polymer network. To
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10.1021/ol035665v CCC: $25.00 © 2003 American Chemical Society
Published on Web 09/24/2003

the best of our knowledge, six-coordinate Zn(II) porphyrin
complexes with two axial O donors forming a 3D polymer
network were not known before our work. The ability of
the central metal ion to accommodate two axial ligands is
important in obtaining extensive multilayered arrays. From
molecular modeling observations, we predicted that the
porphyrin 5 (Scheme 1) with ortho-substituted benzylic
latter was reduced by DIBAL to provide the aldehyde
function in 3 needed for the construction of the porphyrin
template. The other building block, namely, 4, was formed
by a condensation reaction between pyrrole and paraform-
aldehyde according to a literature method.12 The porphyrins
5 were prepared from 3 and 4 via a MacDonald [2 + 2]
condensation reaction13 and subsequent oxidation of the
intermediate porphyrinogens by chloranil.
Earlier work14 on the synthesis of 5,15-diarylporphyrins
indicated the presence of atropisomers in examples involving
ortho substitutents in the aryl rings. More recently, discrete
atropisomers of a series of meso-tetraarylporphyrins having
ortho-aryl rings were successfully separated by chromatog-
Scheme 1. Synthetic Route to Polymeric Metalloporphyrin 6a
1
raphy.15 The H NMR (CDCl3) spectrum of the product
mixture immediately isolated from the reaction between 3
and 4 clearly suggested the presence of trans- and cis-6. Two
separate signals 0.07 ppm apart in a 1:1 ratio were observed
for the methylene protons of the two isomers, respectively,
although their corresponding pairs of other proton signals
are identical. Use of a 1:1 solvent mixture of cyclohexane/
chloroform allowed the isolation of a pure sample of trans-5
by fractional crystallization. The mother liquor was further
purified by chromatography on silica gel to provide a pure
sample of cis-5 that eluted slower than the trans isomer. The
two isomers show almost identical 13C NMR and EI-mass
spectra. Use of their methyl protons as a probe gave results
from dynamic 1H NMR studies of separate isomers of 5 that
indicated no isomeric interchange up to 100 °C.
Suitable single crystals of trans-5 could be obtained for
X-ray crystallographic analysis. The asymmetric unit of
trans-5 consists of half of the molecule, and its ORTEP
diagram is shown in Figure 1. The porphyrin ring is nearly
a Reaction conditions: (i) Na/CH3OH; (ii) DIBAL/benzene; (iii)
HCl/H2O; (iv) CF3COOH/ CH2Cl2; (v) chloranil; (vi) Zn(OAc)2‚
2H2O/ethanol.
oxygen donors (earlier examples involved phenolic oxygen
donors) should have an appropriate metal-oxygen distance
suitable for rigid, stable, and extensively linked 3D metal-
loporphyrin arrays as in 6 with tolerable steric demand. The
ether function is preferred over a hydroxyl group, as the latter
may favor intermolecular hydrogen bonding instead of metal
coordination.11
Figure 1. ORTEP drawing of anti-5 (thermal ellipsoids are drawn
at their 30% probability level).
The synthesis of 6 involved a five-step reaction pathway
as summarized in Scheme 1. The methoxymethyl group was
introduced by a nucleophilic substitution in 1 to give 2. The
planar, and the two substituted aromatic rings are confirmed
to be in trans conformation. All the hydrogen atoms are
(12) Lee, C. H.; Lindsey, J. S. Tetrahedron 1994, 50 (39), 11427.
(13) Arsenault, G. P.; Bullock, E.; MacDonald, S. F. J. Am. Chem. Soc.
1960, 82, 4384.
(11) Bhyrappa, P.; Wilson, S. R.; Suslick, K. S. J. Am. Chem. Soc. 1997,
119, 8492.
4208
Org. Lett., Vol. 5, No. 22, 2003

located directly, and the thermal ellipsoids indicate a
relatively rigid core structure with somewhat dangling side
chains. The two aryls rings are oriented at an angle of 71.1°
with respect to the molecular plane of the porphyrin ring
with only a negligible deviation of 0.0024 Å. This orientation
is comparable to values reported for tetraaryl-substituted
porphyrin systems.16
Table 1. Selected Proton Chemical Shifts (δ) of anti-5 and 6
PhHa
CH2O
OCH3
anti-5
6
7.68
6.89
4.11
2.61
2.80
1.50
a Proton on carbon adjacent to carbon carrying CH2OCH3.
Complex formation was attempted by stirring a concen-
trated ethanol solution of trans-5 and Zn(OAc)2‚2H2O over
5 days. The product isolated was purified on a short
chromatographic column of silica gel using a mixture of
hexane/dichloromethane as the eluant. This sample exhibited
interesting time-dependent 1H NMR spectra as illustrated in
Figure 2. The initial spectrum showed that the benzylic,
alone. These results we believe were derived from coordina-
tion of O donor to zinc, thus locating the protons concerned
within the shielding zone of the adjacent aromatic porphyrin
macroring, accounting for the strong shielding effect ob-
served. Such geometric arrangement(s) in solution could be
similar to that of the polymeric 6 (Figures 3 and 4). The
Figure 3. ORTEP view of the asymmetric unit of complex 6.
1
time-dependent H NMR spectra could suggest an initial
solution of monomer and different conformational framework
Figure 2. Time-dependent 1H NMR spectra (in CDCl3) of a freshly
isolated sample of polymeric complex 6.
methyl, one of the pyrrolic, and phenyl protons appeared as
significantly broadened signals. As time progressed, these
signals shifted to higher fields and sharpened. There were
essentially no further changes observed after the final
spectrum was taken after 10 h. In fact, this spectrum was
reproducible even after solvent removal and redissolving the
residue in the same solvent. A similar phenomenon was
observed in experiments using deuterated toluene as a
solvent.
By comparing the final 1H NMR spectrum of the product
in the time-dependent study and that of the ligand anti-5,
the methylene and methyl protons, and the aryl proton on
the carbon next to that carrying the CH2OCH3 group of the
product, are significantly shifted upfield (Table 1). The
observed shifts are too large to be due to solvent effects
(14) (a) Manka, J. S.; Lawrence, D. S. Tetrahedron Lett. 1989, 30, 6989.
(b) Osuka, A.; Nagata, T.; Kobayashi, F.; Maruyama, K. J. Heterocyclic
Chem. 1990, 27, 1657.
(15) (a) Setsune, J.; Hashimoto, M.; Shiozawa, K.; Hayakawa, J.; Ochi,
T.; Masuda, R. Tetrahedron 1998, 54, 1407. (b) Ruzie´, C.; Michaudet, L.;
Boitrel, B. Tetrahedron Lett. 2002, 43, 7423.
(16) Balaban, T. S.; Eichho¨fer, A.; Lehn, J.-M. Eur. J. Org. Chem. 2000,
4047.
Figure 4. Segment of the coordination polymer 6.
Org. Lett., Vol. 5, No. 22, 2003
4209

of oligomers as reflected in the rather broad and relatively
less shielded signals. Slow aggregation processes would
eventually result in an optimum and relatively more rigid
polymeric framework that was still soluble, allowing rapid
diffusion of solvent in and out of the network, resulting in
Along the a-axis of the polymeric structure of 6 (Figure
5a), two different alignments of the units are visible. One
1
sharp averaged signals in the H NMR spectrum. In the
recrystallization process, concentration of the solution was
associated with further aggregation to form the infinite 3D
network as described for 6 in the solid state (refer to later
discussion on X-ray crystallographic analysis). Redissolving
the infinite coordination polymer in the same solvent gave
an identical 1H NMR spectrum due to breaking down of the
polymeric framework into the soluble aggregate as observed
earlier.
The aggregation pattern of the Zn(II) porphyrin complex
6 was elucidated by X-ray crystallographic study. Intensely
pink single crystals of 6 were successfully grown by slow
evaporation of a solution of 6 in a cyclohexane/chloroform
mixture. Complex 6 is a 3D coordination polymer, and the
ORTEP drawing of its fundamental building unit is shown
in Figure 3. Each Zn(II) metal ion forms a square plane
(angles are perfect at 180.0°) with the four pyrrolic nitrogen
atoms, and coordination to two neighboring oxygen atoms
of the methoxymethyl functions completes a distorted
octahedral geometry of the metal ion with angles ranging
from 87.58 to 92.42°. The average equatorial Zn-N distance
is 2.049 Å. This value is very similar to those reported for
related porphyrin systems,8-10 thus indicating that there is
little geometric strain/changes in the porphyrin units of the
polymeric framework of 6. The two axial methoxy groups
are in trans orientation with an identical Zn-O bond distance
of 2.434 Å. This value, as expected, is slightly longer than
those reported for a number of five-coordinated zinc por-
phyrins,10 but it is shorter than that (2.536 Å) of the Zn-O
bond distance reported for a six-coordinated Zn(THF)2
complex.17 Thus, the identical Zn-O distances in 6 undoubt-
edly correspond to actual Zn-O coordination bonds. This
serves as evidence for a true six-coordinated system in 6
and rules out any solid-state phenomenon based on some
cooperative interaction that physically places the oxygen
groups above and below the plane of the porphyrin. A
segment of the coordination polymer 6 is shown in Figure
4.
Figure 5. View of 6 along the (a) a-axis and (b) c-axis.
array is flat, and another array is slanted; thus, a 3D
coordination network polymer resulted. There is no ap-
preciable aromatic interaction in the chains on the basis of
the estimated inter-ring distances. Along the c-axis, a clearer
overview of the 3D network architecture is observed (Figure
5b) and the packing creates the flowerlike cavities as shown.
The shortest and longest non-hydrogen atomic distances
across the channel are about 4.81 and 9.27 Å, respectively
(Figure 5b). These cavities and channels should be involved
in the solvent diffusion phenomenon observed in the time-
dependent NMR study described earlier. Applications of the
3D polymer 6 in selective trapping of organic molecules and
inorganic ions and serving as a polymer assembly for
catalytic organic reactions are in progress in our laboratory.
For Zn(II) metalloporphyrins, five-coordinate examples8-10
are common, and the six-coordinate zinc(II) polymer 6 is,
we believed, the first zinc(II) porphyrin 3D framework
successfully assembled through two axial Zn(II)-O coor-
dination.10 Examples reported earlier involved phenoxy
functions with a Zn(II)-O distance too short for construction
of a 3D framework. Our modeling studies indicated that
extending the oxygen function to a methoxymethyl group at
the ortho position should have the [Zn(II)-O]-[Zn(II)-O]′
coordination from two adjacent units adopting an ideal
“square planar” arrangement with appropriate Zn(II)-O
distances. This is clearly confirmed in the crystallographic
3D framework of 6 (Figures 3 and 4). Compared to anti-5,
the phenyl rings in 6 are tilted at a relatively larger angle
(76.7°) with respect to the porphyrin molecular plane to
facilitate coordination to the central metal.
Acknowledgment. This work was supported by the
National University of Singapore (NUS). The authors thank
the staff at the Chemical and Molecular Analysis Center,
Department of Chemistry, NUS, for their technical assistance.
Supporting Information Available: CIF files for anti-5
and 6. This material is available free of charge via the Internet
at http://pubs.acs.org.
(17) DiMangno, S. G.; Lin, V. S.-Y.; Therien, M. J. J. Am. Chem. Soc.
1993, 115, 2513.
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