Exploring supramolecular self-assembly of tetraarylporphyrins by halogen bonding: Crystal engineering with diversely functionalized six-coordinate tin(L)2-porphyrin tectons

Article abstract of DOI:10.1002/chem.201301857

This study targets the construction of porphyrin assemblies directed by halogen bonds, by utilizing a series of purposely synthesized Sn(axial ligand)₂-(5,10,15,20-tetraarylporphyrin) [Sn(L)₂-TArP] complexes as building units. The porphyrin moiety and the axial ligands in these compounds contain different combinations of complimentary molecular recognition functions. The former bears p-iodophenyl, p-bromophenyl, 4'-pyridyl, or 3'-pyridyl substituents at the meso positions of the porphyrin ring. The latter comprises either a carboxylate or hydroxy anchor for attachment to the porphyrin-inserted tin ion and a pyridyl-, benzotriazole-, or halophenyl-type aromatic residue as the potential binding site. The various complexes were structurally analyzed by single-crystal X-ray diffraction, accompanied by computational modeling evaluations. Halogen-bonding interactions between the lateral aryl substituents of one unit of the porphyrin complex and the axial ligands of neighboring moieties was successfully expressed in several of the resulting samples. Their occurrence is affected by structural (for example, specific geometry of the six-coordinate complexes) and electronic effects (for example, charge densities and electrostatic potentials). The shortest intermolecular I×××N halogen-bonding distance of 2.991A was observed between iodophenyl (porphyrin) and benzotriazole (axial ligand) moieties. Manifestation of halogen bonds in these relatively bulky compounds without further activation of the halophenyl donor groups by electron-withdrawing substituents is particularly remarkable. Porphyrins in tandem: Porphyrin-based network materials can been supramolecularly organized into one-dimensional and two-dimensional structures by means of cooperative directional halogen bonding (see figure). Copyright

Full text of DOI:10.1002/chem.201301857

FULL PAPER
DOI: 10.1002/chem.201301857
Exploring Supramolecular Self-Assembly of Tetraarylporphyrins by Halogen
Bonding: Crystal Engineering with Diversely Functionalized Six-Coordinate
Tin(L)2–Porphyrin Tectons
Hatem M. Titi, Ranjan Patra, and Israel Goldberg*[a]
Abstract: This study targets the con-
struction of porphyrin assemblies di-
rected by halogen bonds, by utilizing a
series of purposely synthesized Sn(axial
ligand)2–(5,10,15,20-tetraarylporphyrin)
[Sn(L)2-TArP] complexes as building
units. The porphyrin moiety and the
axial ligands in these compounds con-
tain different combinations of compli-
mentary molecular recognition func-
tions. The former bears p-iodophenyl,
p-bromophenyl, 4’-pyridyl, or 3’-pyridyl
substituents at the meso positions of
the porphyrin ring. The latter compris-
es either a carboxylate or hydroxy
anchor for attachment to the porphyr-
in-inserted tin ion and a pyridyl-, ben-
zotriazole-, or halophenyl-type aromat-
ic residue as the potential binding site.
The various complexes were structural-
ly analyzed by single-crystal X-ray dif-
fraction, accompanied by computation-
al modeling evaluations. Halogen-
bonding interactions between the later-
al aryl substituents of one unit of the
porphyrin complex and the axial li-
gands of neighboring moieties was suc-
cessfully expressed in several of the re-
sulting samples. Their occurrence is af-
fected by structural (for example, spe-
cific geometry of the six-coordinate
complexes) and electronic effects (for
example, charge densities and electro-
static potentials). The shortest intermo-
lecular I···N halogen-bonding distance
of 2.991 ꢀ was observed between iodo-
phenyl (porphyrin) and benzotriazole
(axial ligand) moieties. Manifestation
of halogen bonds in these relatively
bulky compounds without further acti-
vation of the halophenyl donor groups
by electron-withdrawing substituents is
particularly remarkable.
Keywords: halogens · porphyrins ·
self-assembly
·
supramolecular
chemistry · tin
Introduction
chemical, catalytic, and electronic properties,[6] and they are
particularly interesting from a functional point of view.
When suitably “activated” by complimentary molecular rec-
ognition functions, they represent ideal organic building
blocks for the designed construction of extended supramo-
lecular architectures.[7] Chemical modifications can be readi-
ly performed both in the core and on the porphyrin periph-
ery, as well as by the addition of reactive axial ligands
bound to the metal centers with preferred five- or six-coor-
dination features.[7,8] A wide variety of novel porphyrin-
based supramolecular architectures assembled primarily
through intermolecular coordination and hydrogen-bonding
interactions has been reported in recent years. Programmed
functionalization of tetraarylporphyrin building blocks has
led in recent years to the discovery by others and us of a
large number of supramolecular porphyrin-based networks
and frameworks, formed by direct interporphyrin hydrogen
bonding and coordination or tessellated by external metal-
ion or organic-ligand bridging auxiliaries.[7–11] Some of these
tunable heterometallic frameworks constructed from
paddle-wheel-type dinuclear metal-ion connectors, and with
open access to the porphyrin metal centers, can be particu-
larly useful for heterogeneous-catalysis applications[11] and
as active materials for visible-light catalysis.[12] Recent work
in this area also involves highly stable mesoporous metallo-
porphyrin frameworks tessellated by polynuclear zirconium-
ion clusters, which are of potential utility in biomimetic cat-
alysis[13] and as permanently microporous materials with se-
Halogen bonds are attractive noncovalent supramolecular
interactions for the rational design of new crystalline materi-
als.[1–3] Various applications of halogen bonding have been
reviewed recently,[2,3] and these include the production of su-
pramolecular nanowires, supramolecular gel formation, pro-
duction of liquid crystals, and crystal engineering.[4,5] The
common halogen-bonding interaction can be explained by
electrostatic terms because it involves the positive electro-
static potential of the halogen atom and an electron-rich
atom (for example, N, O, I, or Br) that acts as a Lewis base.
Halogen interactions are specific and directional, and they
have been proven to play a crucial role in crystalline materi-
als.[1–5] Investigation of the various and articulated halogen-
bonding schemes in porphyrins offers a new challenge in
crystal engineering. Porphyrin compounds are important en-
tities in materials chemistry because of their unique photo-
[a] H. M. Titi,+ Dr. R. Patra,+ Prof. I. Goldberg
School of Chemistry, Sackler Faculty of Exact Sciences
Tel-Aviv University, Ramat-Aviv, 69978 Tel-Aviv (Israel)
E-mail: goldberg@post.tau.ac.il
[+] These authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201301857. It contains X-ray
crystallographic details in CIF format for compounds 1–9 and experi-
mental data and additional figures and tables related to the computa-
tional modeling.
Chem. Eur. J. 2013, 19, 14941 – 14949
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lective carbon dioxide uptake.[14] The formation of hybrid
metalloporphyrin frameworks not only in crystalline bulk
but also as highly crystalline nanofilms on surfaces has been
reported as well.[15] On the other hand, multiporphyrin as-
sembly with the aid of halogen bonds has received only
minor attention until now. In a number of pioneering re-
ports on this subject, we have demonstrated the effective-
ness of I···N and Br···N halogen bonds in directing the self-
organization of porphyrin entities in crystals.[16] Porphyrin
units with mixed meso substitutions of 4-pyridyl and either
4-iodo- or 4-bromophenyl residues have been used to this
end, which has resulted in the formation of halogen-bonded
layered assemblies (Figure 1). Moreover, asymmetric substi-
tution of the porphyrin scaffolds led to the formation of
chiral architectures.
Crystallographic characterization of the assembly patterns
formed by the six-coordinate [(SnIV-TArP)2+(LÀ)2] (TArP:
5,10,15,20-tetraarylporphyrin; L: axial ligand) complexes
throws light on the optimal supramolecular interaction of
these bipyramidal metalloporphyrin units and the utilization
of their halogen-bonding molecular recognition features.
The porphyrins used in this study are 5,10,15,20-tetra(4’-io-
dophenyl)porphyrin (TAr1P), 5,10,15,20-tetra(4’-bromophe-
nyl)porphyrin (TAr2P), 5,10,15,20-tetra(3’-pyridyl)porphyrin
(TAr3P), and 5,10,15,20-tetra(4’-pyridyl)porphyrin (TAr4P).
The ligand reactants are isonicotinic acid (L1), nicotinic acid
(L2), 1-hydroxybenzotriazole (L3), 3-hydroxypyridine (L4),
4-iodobenzoic acid (L5), and 4-bromobenzoic acid (L6;
Scheme 1). The peripheral pyridyl N sites on the porphyrin
Figure 1. Layered self-assembly of tris(4-iodophenyl)-mono(4-pyridyl)-
À
À
porphyrin by linear C I···N (blue dotted lines) and perpendicular C I···p
halogen bonds (red dotted lines).[9c] I and N sites are depicted by pink
and blue colors, respectively.
In the present report, we demonstrate yet another meth-
odology for assembling tetraarylporphyrin-based networks
tessellated by intermolecular halogen bonds, which involves
the use of six-coordinate tin–porphyrin complexes with axial
ligands that incorporate molecular recognition functionali-
ties suitable for halogen bonding. To this end, we have in-
corporated mutlidentate carboxylic acid or alcohol entities
that may readily coordinate (in the deprotonated form) to
the central tin ion as axial ligands. If the axial ligands bear
additional functions such as nitrogen or halogen sites, they
may engage effectively in halogen bonding with complemen-
tary sites on the periphery of the porphyrin entity. The ex-
pression of directional but relatively weak halogen bonding
in the crystalline self-assembly of organic species in
common reaction environments is not straightforward, in
view of the competing intermolecular binding forces of simi-
lar strength, such as hydrogen bonds or p–p interactions.
The latter, along with molecular-shape considerations, are of
particular significance in systems with extended aromatic
cores. Several theoretical investigations have probed the
electrostatic potentials of a number of halogen-bonded sys-
tems, because they are a key feature affecting the formation
of such interactions.[17–19]
Scheme 1. Schematic illustration of the metalloporphyrin scaffolds used
in this study. Ar is 4-iodophenyl in compounds 1–4, 4-bromophenyl in 5,
3-pyridyl in 6 and 7, and 4-pyridyl in 8 and 9. L is isonicotinic acid in 1
and 5 (L1), nicotinic acid in 2 (L2), 1-hydroxybenzotriazole in 3 (L3), 3-
hydroxypyridine in 4 (L4), 4-iodobenzoic acid in 6 and 8 (L5), and 4-bro-
mobenzoic acid in 7 and 9 (L6).
and ligand components are essentially inert to potential co-
ordination to the metal centers of another metalloporphyrin
species in the reaction process (see below), due to the
strong oxophilicity (and preferential affinity for the carboxy
or hydroxy coordination sites of the ligands) of the tin(IV)
ions; thus, the pyridyl functionalities are exposed to possible
intermolecular halogen and hydrogen bonding. Correspond-
ingly, we describe below the self-assembled architectures of
the following complexes (some in their solvated forms;
DMF: N,N-dimethylformamide): [Sn
[Sn(TAr1P) (TAr1P)
(L2À)2] (2), [Sn (L3À)2]·
(TAr1P) (DMF)2 (4), [Sn(TAr2P)
(L4À)2]· (L1À)2] (5), [Sn-
(TAr3P) (TAr3P) (DMF)2·A(H2O) (7),
(L5À)2] (6), [Sn (L6À)2]·
[Sn(TAr4P) (TAr4P)
(L5À)2] (8), and [Sn (L6À)2] (9). Struc-
tures 1–5 exhibit prominent halogen-bonding intermolecular
A
(L1À)2] (1),
ACHTUNGTRENNUNG
A
E
E
N
ACHTUNGTRENNUNG
A
E
E
N
ACHTUNGTRENNUNG
A
E
E
G
N
CHTUNGTRENNUNG
A
E
N
ACHTUNGTRENNUNG
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Supramolecular Self-Assembly of Tetraarylporphyrins by Halogen Bonding
FULL PAPER
interactions, which are not detected, however, in 6–9. This
diversity is interpreted below by structure and charge-densi-
ty considerations. It reflects also on the delicate balance be-
tween competing halogen bonding and other noncovalent
interactions during the self-assembly of the porphyrin com-
plexes.
nate tin–porphyrin complexes with suitably activated axial
ligands were found to be perfect model compounds for dem-
onstrating this crystal-engineering methodology.
The halogen-bonding associations in compounds 2 and 4,
with the tin–tetra(4-iodophenyl)porphyrin derivative and
nicotinic acid or 3-hydroxypyridine, respectively, as the axial
ligands, are illustrated in Figure 2. They represent primarily
(additional interactions in 2 are discussed below) continuous
self-assembly in one direction, which utilizes the laterally di-
rected I sites on the porphyrin and the meta-positioned
(with respect to the hydroxy or carboxy anchor) N sites on
the axial ligands of adjacent porphyrin species. Every por-
phyrin complex is thus linked to its two neighbors by a pair
of linear I···Npyridyl interactions on each side. In the triclinic
structure 4, all of the porphyrin scaffolds along the halogen-
bonded chains (propagating along the a–b axis of the crys-
tal) are parallel to one another. In the monoclinic crystal 2,
consecutive porphyrin units along the chains (which extend
along the c axis) are arranged in a herring-bone fashion,
with the halogen-bonding pattern still preserved. In the two
structures, only two of the iodophenyl residues are involved
in such I···N halogen bonds (being thus well ordered); the
other non-interacting iodophenyl substituents are directed
outwards (roughly perpendicular to the chains). The
I···Npyridyl interaction distances are 3.080 and 3.086 ꢀ in 2
and 3.122 ꢀ in 4, which are considerably shorter than the
sum of the corresponding van der Waals radii of N and I
atoms (approximately 3.5–3.8 ꢀ).[22] In the crystal structure
of 4, the porphyrin chains described above are aligned paral-
lel to each other and their arrangement creates interstitial
channel voids, which propagate along the c axis. These voids
are lined with the nonbonded iodophenyl residues and ac-
commodate molecules of the DMF crystallization solvent.
The interporphyrin assembly mode of 2 and 4 into linear ag-
gregates, through directional interactions between the aryl
arms of one porphyrin unit and the axial ligands of the
neighboring porphyrin entities, resembles that observed ear-
lier in other six-coordinate metalloporphyrins functionalized
with hydrogen-bonding sites.[9c,23]
Results and Discussion
It has been demonstrated that halogen bonds of strength
comparable to those of hydrogen bonds are attainable in
crystalline systems.[1–5,20] Most prominent intermolecular in-
teractions have been observed between an aromatic iodine
atom, highly activated by electron-attracting substituents,
and strong and sterically unhindered Lewis bases. The crys-
tal structures of tetraarylmetalloporphyrins are dominated
to a major extent by dispersion forces between aromatic
fragments and related molecular-shape effects,[21] and halo-
gen and hydrogen bonding provide only a secondary cohe-
sive contribution to the stabilization of the corresponding
crystalline solids. Nevertheless, the ability to direct the su-
pramolecular organization in crystals (in deviation from dis-
persion-driven packing) is associated with features of preor-
ganization, as well as complimentarity and multiplicity of
the molecular recognition sites on the component building
blocks. In this work, we attempted to construct crystalline
multiporphyrin architectures affected by halogen bonding,
utilized effectively in the crystal engineering of other sys-
tems.[1–5] Different strategies can be applied to this end. The
first one is to introduce the potential electrophilic “donor”
(halogen atom) and “acceptor” (lone-pair-possessing elec-
tron-rich partner) sites on a single porphyrin macrocycle.
Indeed, it has been shown that the assembly of porphyrins
with mixed iodophenyl or bromophenyl and pyridyl meso
substituents, through either I···N or Br···N halogen bonds,
can be materialized successfully with both free base and
metallated entities (Figure 1).[16a,c] Manifestation of the halo-
gen-bonding interactions occurred even without activation
of the halophenyl rings by electron-attracting substituents.
Herein, we extend this approach with six-coordinate metal-
loporphyrin scaffolds by placing the electrophilic and elec-
tron-rich partners one on the tetraarylporphyrin macrocycle
and the other on the axial ligand (or vice versa). Six-coordi-
In 2 and 4, the iodine atoms not involved in the I···Npyridyl
halogen bonds, and pointing sideways with respect to the
halogen-bonded chain (Figure 2), reveal partial disorder.
This is indicative, perhaps, of the rather weak packing forces
that they are associated with. Indeed, in 4, these I atoms are
Figure 2. One-dimensional arrays of multiply halogen-bonded metalloporphyrin scaffolds in a) triclinic compound 4 (the DMF crystallization solvent is
omitted) and b) monoclinic compound 2. The I···Npyridyl halogen bonds are indicated by blue dotted lines. Note that every porphyrin unit is involved in
four halogen bonds, irrespective of whether the molecules are arranged in a parallel or herring-bone manner.
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I. Goldberg et al.
surrounded by, and in van der Waals contact with, the DMF
crystallization solvent trapped in the crystal lattice. On the
other hand, a more detailed inspection of the intermolecular
organization in 2 reveals that the interface between neigh-
boring chains in a layer parallel to the ac plane of the crystal
is lined with direct I···I contacts at 3.856 ꢀ distance. (This is
the shortest intermolecular I···I distance observed between
the major sites of these disordered I atoms; for simplicity,
longer interactions were not considered.) The layered grid-
type arrangement of the porphyrin entities (Figure 3) exhib-
Figure 4. Crystal structure of 1, which shows two interwoven halogen-
bonded networks of the porphyrin units. The I···N halogen bonds are in-
dicated by dotted lines; the corresponding distances are 3.295 ꢀ for those
shown by the red color in the shaded nets and 3.150 and 3.340 ꢀ for the
blue ones in the other network.
bly. The grids interweave into one another to form a flat en-
tangled layer of the two grids, which extends parallel to the
bc plane of the unit cell. The surfaces of these layers above
and below are lined with the nonbonded I sites, which stick
outwards and interdigitate between similar sites of neighbor-
ing layers stacked along the a axis of the crystal. The forma-
Figure 3. Two-dimensional array (sql) of multiply halogen-bonded metal-
loporphyrin moieties (4-c nodes) in 2 aligned parallel to the ac plane of
the crystal. The I···Npyridyl (along the nearly vertical direction; see Fig-
ure 2b) and I···I (along the horizontal direction at 3.865 ꢀ) halogen
bonds are indicated by blue and red dotted lines, respectively.
À
tion of the grids by nearly linear C I···N halogen bonds is
associated with a herring-bone-type arrangement of the por-
phyrin entities.
The topology of the interporphyrin arrangement descri-
bed above and in Figure 4 is therefore best represented by
twofold interpenetration of the two sql-type (with 4-c nodes,
as in compound 2) nets of the halogen-bonded entities.[24]
An in-depth inspection of the intermolecular contacts in 1
reveals, however, that a third (partly disordered) iodine sub-
stituent of the porphyrin units (in the unshaded network in
Figure 4) is involved in yet another considerably longer I···N
interaction at a distance of 3.628 ꢀ. If this is taken into ac-
count, it changes the topology of this network from sql to a
2-periodic (4,4)IIb (6-c nodes) net with point symbol
(413.62).[24] This additional contact is rather close to the van
its two opposite edges double bridged by the I···N bonds,
with the two other edges having the I···I halogen–halogen
interaction (the corresponding distance being shorter than
the sum of van der Waals radii of the I atoms, ꢀ4.2 ꢀ).[22]
The topology of the two-dimensional porphyrin array thus
formed has been analyzed with the TOPOS (Ver. 4.0) soft-
ware, by reducing the complex structure to a simpler node-
and-linker net.[24a] Accordingly, the network depicted in
Figure 3 is best described as a square layer, sql,[24b] in which
the porphyrin complex is considered to be four connected
(4-c).
In compounds 1 and 5, we introduced isonicotinic acid as
an axial ligand. The N site is placed now in a para (rather
than meta) position on the aryl residue and is thus exposed
to interactions directed perpendicular to the porphyrin ring.
In 1, there are two crystallographically independent por-
phyrins in the asymmetric unit, with each being involved in
a two-dimensional grid of halogen-bonded metalloporphyrin
species (Figure 4). In such grids, every unit interacts with its
four neighbors along the four lateral directions, by utilizing
two Iphenyl and two Npyridyl sites to this end. Two adjacent spe-
cies in each grid are connected by a single halogen bond.
The corresponding I···N distances are 3.295 ꢀ in the centro-
symmetric array and 3.150 and 3.340 ꢀ in the other assem-
À
der Waals limit, and the corresponding C I···N interaction is
far from linear, so the more complex description of the net-
working in 1 is of little relevance in the context of this
paper and will not be discussed further.
A slightly distorted assembly is formed if the iodophenyl
functional groups are replaced on the porphyrin macrocycle
by the analogous bromophenyl residue, as in compound 5.
The bromine electrophile is characterized by lower polariza-
bility and is less prone to engage in halogen bonds. Never-
theless, the Br···Npyridyl interaction in porphyrin assemblies
has been observed before,[16a] and it is apparent in 5 as well
(Figure 5). Indeed, the structure of 5 reveals a square-grid-
type sql assembly[24] of the porphyrin molecules located on
centers of inversion, as in 1, with roughly linear Br···Npyridyl
halogen bonds at 3.305 ꢀ. The second porphyrin species of
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Supramolecular Self-Assembly of Tetraarylporphyrins by Halogen Bonding
FULL PAPER
Figure 5. Crystal structure of 5, viewed approximately down the a axis
and with the two halogen-bonding schemes in the structure shown: the
grid and the zigzag-chain (shaded) patterns. The linear Br···N halogen
bonds are indicated by dotted lines; the corresponding distances are
Figure 6. Supramolecular self-assembly in 3 into 2D halogen-bonding
square-grid-type ꢂpolymersꢃ. The I···N bonds (at 2.991 ꢀ) are indicated by
blue dotted lines, whereas the I···I interactions (at 3.941 ꢀ) are depicted
by pink dotted lines. Note that every porphyrin unit is involved in four
I···N bonds and four I···I contacts.
3.156 ꢀ for those shown by the red color in the shaded nets and 3.305 ꢀ
for the blue ones in the grid network.
the asymmetric unit associates into zigzag-chain assemblies
À
by additional linear C Br···Npyridyl bonds at 3.156 ꢀ through
one of the bromophenyl residues and one pyridyl site. Simi-
lar chain arrays are part of the second sql grid in structure
1, the formation of which didn’t occur in 5 (see below).
These two ensembles pack together in a tight manner, paral-
lel to the bc plane, and the molecular units of the zigzag
chain protrude into the interporphyrin voids of the square-
grid array. The indicated Br···N distances are slightly shorter
than the sum of the van der Waals radii of the two atoms
(approximately 3.4 ꢀ) and the interaction energy is estimat-
ed to be only about 9 kJmolÀ1.[1–5,22,25] It is not surprising
therefore that the electrostatic potential of the Br electro-
phile has also been utilized in 5 in competing intermolecular
interactions of the Br···Br and Br···p types. Incidentally,
there are additional Br···Npyridyl interactions at distances of
>3.5 ꢀ from the zigzag-chain porphyrins to neighboring en-
tities (however, being longer than the van der Waals dis-
tance, they seem to be of minor significance), which, if con-
sidered, would change the topological description from a 1D
zigzag assembly along with the in-contact species to a dis-
torted sql pattern, which interdigitates into the first flat sql
net (the unshaded net in Figure 5).
gen–halogen interactions between perpendicularly oriented
À
C I groups, at 3.941 ꢀ, are also apparent within these as-
semblies (Figure 6). The corrugated nature of the layers
thus formed allows tight stacking of the layers in the normal
direction, with convex surfaces of one layer (represented by
the axial ligands) fitting into the concave areas (formed by
the peripheral fragments of the porphyrins) of two adjacent
layers from above and below. Narrow interstitial channel
voids extending along the c axis of the structure accommo-
date disordered DMF solvent. The topology of the formed
network is a 2-periodic hxl net (the triangular lattice), in
which the six-coordinate metalloporphyrin entity is consid-
ered as a 6-c node. The resulting extended grid assembly in
3 closely resembles the supramolecular organization ob-
served earlier for the [SnACTHUNRGTNE(NUG tetra-(4-pyridyl)porphyrin)(5-
amino isophthalic acid)2] compound, which was directed in a
similar way by COOH···Npyridyl and NH2···Npyridyl hydrogen
bonds.[9c]
In structures 1–5 the lateral iodophenyl sites that are not
involved in the linear I···N halogen bonds reveal partial dis-
order (particularly of the terminal iodine atoms). This pro-
vides another indication of the significance of the halogen
bonds incorporating the other (well-ordered) iodine electro-
philes for the intermolecular organization.
In the next step we have examined several examples of
systems in which the disposition of the halogen (I or Br)
electrophiles and Npyridyl sites was reversed. In compounds
6–9, the halogen sites are placed on the axial ligands (4-
bromo- or 4-iodobenzoic acid), whereas the Npyridyl sites are
incorporated into the porphyrin macrocycle (by using the
tetra(3-pyridyl)- or tetra(4-pyridyl)porphyrin scaffolds). In
this alternative approach, however, the expression of the
The most pronounced manifestation of interporphyrin
halogen bonding in this study has been observed in com-
pound 3 with the tetra(iodophenyl)porphyrin derivative and
1-hydroxybenzotriazole as an axial ligand. The latter is char-
acterized by an increased electron density at the N binding
site, due to the presence of two additional N sites, which fa-
cilitates its direct interaction with the iodine electrophile.
(L3À)2] entities in 3
The self-assembly of the [SnACTHNUGRTENNUG(TAr1P)CAHTUNGTRENNUGN
(Scheme 1) results in two-dimensional layers of the porphyr-
in units interconnected to one another along the four equa-
À
torial directions by nearly linear C I···Ntriazole halogen bonds
À
(Figure 6). Two iodophenyl and two triazole functions of a
given molecule take part in these bonds, with the I···N dis-
tance being 2.991 ꢀ (markedly shorter than the sum of the
van der Waals radii of the two atoms). Secondary I···I halo-
linear C Hal···N bonds in directing the self assembly of the
porphyrin complexes was considerably less effective. Thus,
structure 6 can be best described as composed of linear por-
phyrin chains sustained by other forms of intermolecular at-
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I. Goldberg et al.
tractions that involve the halogen sites (Figure 7). In this
À
case, the C I group doesn’t point at the Npyridyl site in a
linear manner (and in a direction parallel to the pyridine
ring) but is oriented half-way perpendicular to the pyridyl
Figure 7. Linear chains of the porphyrin complexes in 6 tessellated by
weak I···N interactions (3.342 ꢀ; blue dotted lines). Dispersion forces
dominate the crystal packing in the other directions.
Figure 8. a) Crystal packing in compound 7, which shows the close per-
group. This indicates an “intermediate” interaction between
À
pendicular approaches between the C Br groups of neighboring mole-
À
À
cules. b) Crystal packing in
8
(9 is isomorphous), which resembles
the C I···N type and the C I···pACTHNUTRGNEUGN(pyridyl) type. The inclina-
common crystal packing in porphyrin compounds stabilized primarily by
tion angle between the corresponding iodophenyl and pyrid-
yl rings is about 458 and the I···N distance is 3.342 ꢀ.
dispersion forces.[21,26]
Structure 7, in which the p-iodobenzoic acid ligand was
replaced by its bromo analogue, lacks any direct Br···N in-
teraction. Instead, the crystal packing reveals secondary in-
termolecular Br···Br contacts at 3.943 ꢀ (slightly longer than
the sum of two van der Waals radii of bromine atoms). As
commonly observed in crystal structures containing bromo-
structures 1 and 2. These interactions can be analyzed by
2D fingerprint plots. In this representation, the red spots on
the Hirshfeld surface evidence the appearance of close inter-
molecular interactions, such as hydrogen and halogen bonds,
whereas areas without close contacts are shown as blue
spots. Correspondingly, the halogen-bonding sites can be
seen in the Hirshfeld surface as large red spots, whereas
smaller red areas indicate the sites of the peripheral H
atoms on the molecular surface. Two fingerprint plots are
shown in Figure 9. The ones in the center reflect the total in-
termolecular interactions, whereas those on the right exhibit
the contributions of the I···N halogen bonds only. In com-
pound 1, the halogen substituents contribute 27.6% to the
total intermolecular surface-contact interaction, but only
2.8% of the interactions are attributable to direct I···N at-
tractions. The tips of the spikes on the rightmost graph in
Figure 9a are located at di=1.80 ꢀ and de=1.35 ꢀ or di=
À
phenyl groups, the corresponding C Br bonds are aligned in
a nearly perpendicular orientation, which allows the elec-
tron-rich side of one bromo substituent to point at the elec-
tron-deficient pole of the other Br site (Figure 8a).[26,27] Fur-
ther efforts to probe the nature of intermolecular interac-
tions with the same halobenzoic acid ligands but the 4-pyrid-
yl (instead of 3-pyridyl) derivative of the tin–porphyrin com-
pound showed that the crystal packing in the two
isomorphous compounds 8 and 9 is dominated by common
dispersion forces (as in many six-coordinate complexes of
tetraarylmetalloporphyrins), without any marked expression
of specific halogen bonding of one type or another (Fig-
ure 8b).[21,26]
As the largest contribution
to halogen bonding comes
from coulombic plus first-order
polarization terms,[20] we at-
tempted, at this stage, to apply
computational
electrostatic
considerations to sample sys-
tems in order to throw more
light on the above empirical
observations. The Hirshfeld
surface[28,29] is an illustration of
the three-dimensional electron
density of a molecule in a crys-
tal. Mapping the Hirshfeld sur-
face (by using CrystalExplor-
er[30]) is a useful tool for under-
standing and visualization of
intermolecular
For example, Figure 9 depicts
intermolecular interactions in
interactions.
Figure 9. Calculated Hirshfeld surfaces for a) compound 1 and b) compound 2 are shown on the left side, the
fingerprint plots of all of the intermolecular contacts are given in the center, and the fingerprints plots of the
specific I···N interactions are given on the right for each of the two compounds.
14946
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Chem. Eur. J. 2013, 19, 14941 – 14949

Supramolecular Self-Assembly of Tetraarylporphyrins by Halogen Bonding
FULL PAPER
1.35 ꢀ and de=1.80 ꢀ, with di+de representing the shortest
distance between atoms inside the molecular surface and
outside the surface, correspondingly. Thus, the shortest cal-
culated interaction of about 3.15 ꢀ is in perfect agreement
with the shortest I···N bond observed in 1. Similarly, in 2,
the halogen atoms contribute 28.5% to the total interaction,
whereas the specific I···N contacts contribute only 2.0%.
The minimal de+di calculated distance between the tips of
the spikes in Figure 9b is near 3.10 ꢀ, which is in agreement
with the observed 3.080 and 3.086 ꢀ values in the crystal
structure. The larger I···N halogen-bond contribution in 1
than in 2 is most probably a result of better accessibility.
The calculated Hirshfeld surfaces for compounds 3 and 4
are shown in Figure 1S in the Supporting Information.
À
À
In compounds containing peripheral Caryl I or Caryl Br po-
larizable residues, there is a region of positive electrostatic
potential at the tip/cap of this bond, known as the s hole
(the presence of positive electrostatic potential regions on
the outermost portion of covalently bonded halogen atoms
was confirmed by high-level theoretical calculations of elec-
trostatic potentials in halogen-substituted compounds by Po-
litzer and co-workers),[17] which is prone to attractively bind
to electron-rich atoms/sites (Lewis bases).[17–19] Hence, there
Figure 10. Electrostatic potential isosurface maps based on the experi-
mentally derived structural models of a) 1 (with isonicotinic acid as the
axial ligand), b) 2 (with the nicotinic acid ligand), c) 6, and d) 8 (both
with the 4-iodobenzoic acid ligand). The modeled charge densities are
drawn at an isosurface value of Æ0.02 electronsbohrÀ3. The blue, green,
and red colors represent electropositive, neutral, and electronegative re-
gions, respectively. Similar results are also obtained if the same calcula-
tions are done by using optimized geometries (see Figure 2S in the Sup-
porting Information).
À
À
is preferential formation of C I/Br···Npyridyl or C I/Br···p
halogen bonds of linear or perpendicular orientation, re-
spectively, of the interacting components (direct halogen···-
halogen contacts fall in the latter category). In the above
context, we mapped isosurfaces of electrostatic potential for
compounds 1, 2, 6, and 8 with the aid of density functional
theory (see the Supporting Information for details). This in-
volved geometry optimizations, in which the starting atomic
coordinates were taken from the corresponding crystal
structures, followed by calculations of the molecular electro-
static potential surfaces based on both the optimized geome-
tries and the crystallographic structural models. The calcu-
lated isosurface areas are illustrated in Figure 10 with sur-
face contours of the molecular electronic densities, in the
range of Æ0.02 electronsbohrÀ3.
sitions of the tetraarylporphyrin macrocycle. For example,
the overall positive charge density on an iodine atom in
compounds 1 and 2 is +0.26 au, whereas the positive charge
density on an iodine atom in 8 is only +0.24 au (see Fig-
ure 3S in the Supporting Information). In 8, the perpendicu-
larly oriented axial ligand falls optimally in the ring-current
region of the aromatic porphyrin, which causes a decrease
of positive charge density on the halogen atom. It is reason-
able to assume that the same applies to compounds 7 and 9.
The situation is different in compound 6 because the 4-iodo-
phenyl ligand in inclined by about 458 towards the porphyrin
framework, which places the terminal iodine atom in the
outer region of the porphyrin ring current (Figure 7) and
doesn’t affect its positive charge density to the same extent
as that in compounds 7–9. Similar reasoning and ring-cur-
rent effects apply to the negative charge densities of the
Npyridyl atoms. They increase if the N atoms fall in the ring-
current region; the nitrogen negative charge density in com-
pounds 1 and 2 is À0.54 au, whereas it is À0.52 au in com-
pound 8. The above considerations indicate that systems
with halogen sites on the porphyrin component and accessi-
ble pyridyl sites on the axial ligand component (as in 1, 2, 4,
and 5) have a priori higher odds of exhibiting directional
halogen-bonding interactions than systems with reversely
substituted functions (as in 7–9). Minor fluctuations of the
charge density on the interacting partners and the potential
presence of competing interactions, such as hydrogen bond-
ing, may bias the delicate balance in favor or against the ex-
pression of the directional halogen bonding in the self-as-
sembly process.
As indicated in Figure 10, the iodine atoms in all four
compounds are positively polarized at their cap, whereas the
Npyridyl sites (as well as the sterically hindered O sites) are
negatively polarized. In compounds 1 and 2 and partly in 6,
these electrostatic features were well expressed by linear
halogen bonds in which the electrophilic (d+) region of the
iodine atom points toward the nucleophilic (dÀ) pyridyl ni-
trogen atom. The molecular electrostatic potential surface
À
of 8 (in the crystals of which no halogen bonds of the C
I···Npyridyl linear type were observed) exhibits less pro-
nounced positive charge density at the axially oriented
iodine atom (Figure 10d) than the three other molecules.
Further insight is gained from computational analysis of
Mulliken charge densities on the particular halogen and N
atoms in question (see Figure 3S and Tables 2S and 3S in
the Supporting Information). These calculations reveal that
the atomic positive charge density on the iodine atom when
attached to the para position of the axial ligand is smaller
than that of an iodine atom substituted at the equatorial po-
Chem. Eur. J. 2013, 19, 14941 – 14949
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14947

I. Goldberg et al.
Crystal structure determinations: The X-ray measurements (Nonius-Kap-
paCCD (1, 3, 4, 8, 9) and Bruker-ApexDuo (2, 5–7) diffractometers,
MoKa radiation) were carried out at approximately 110(2) K on crystals
coated with a thin layer of amorphous oil to minimize crystal deteriora-
tion, possible structural disorder, and related thermal motion effects and
to optimize the precision of the structural results. These structures were
solved by direct and Fourier methods and refined by full-matrix least-
squares methods (by using standard crystallographic software: SIR97,
DIRDIF-96, SHELXTL-2012).[31a–c] All non-hydrogen atoms were re-
fined anisotropically. The hydrogen atoms were located in idealized/cal-
culated positions and were refined by using a riding model. Compound 3
was found to contain molecules of severely disordered DMF crystalliza-
tion solvent, which could not be reliably modeled by discrete atoms. The
contribution of these molecules was subtracted from the diffraction pat-
tern by the SQUEEZE procedure and PLATON software.[31d] Com-
pounds 8 and 9 were found to be isomorphous, and therefore, only struc-
ture 8 has been analyzed in full detail, with structure 9 being of low qual-
ity. The crystallographic and experimental data for 1–9 are given in
Table 1S in the Supporting Information. CCDC-939013 (1), CCDC-
939014 (2), CCDC-939015 (3), CCDC-939016 (4), CCDC-939017 (5),
CCDC-939018 (6), CCDC-939019 (7), CCDC-939020 (8), and CCDC-
939021 (9) contain the supplementary crystallographicdata for this paper.
These data can be obtained free of charge from The Cambridge Crystal-
lographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Conclusion
The above results demonstrate new types of porphyrin-
based network materials supramolecularly organized into
one-dimensional and two-dimensional architectures, mainly
by means of cooperative directional halogen bonding. They
confirm that a concerted utilization of halogen-bonding at-
tractions between large and rigid building blocks (such as
the tetraarylporphyrins) often has adequate driving force
during the nucleation stage in solution for the preferred con-
struction of supramolecular frameworks with long-range
order. Six-coordinate complexes of tin–tetrarylporphyrins
with suitably functionalized sites (iodophenyl, bromophenyl,
or pyridyl groups) on the porphyrin and on the axial ligand
provide excellent model compounds for probing halogen-
bonding interactions. Structure 3 represents an optimal real-
ization of this crystal-engineering concept, because it reveals
networking in two dimensions and the shortest linear I···N
halogen bond of 2.991 ꢀ. In this compound, the reactive N
sites of the triazole axial ligand are characterized by a
higher electron density than the pyridyl sites in the other an-
alyzed compounds. Although the specific halogen bonds
present only a small contribution to the total enthalpy of
the intermolecular packing and are not a major cohesive
factor, they can play a significant role in directing the supra-
molecular organization of molecular scaffolds with preor-
ganized and sterically unhindered molecular recognition
functions. The formation of network arrangements can be
realized with multiply functionalized complementary com-
ponents of the halogen electrophiles (even in cases in which
the electrophilicity of the aromatic iodine atom is not en-
hanced by electron-withdrawing substituents) and electron-
rich Lewis base sites. Expression of halogen bonds in por-
phyrin assemblies is not easy to obtain and requires careful
design of the molecular scaffolds and optimal experimental
conditions to avoid interference from other reagents and the
dominance of competing intermolecular interactions (includ-
ing features of molecular shape[21]).
Acknowledgements
This research was supported by The Israel Science Foundation (grant no.
108/12).
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Experimental Section
Material syntheses: Full details on the preparation of materials are given
in the Supporting Information. In all reactions, commercially available
reagents of analytical grade were used without further purification, in-
cluding the free base meso-tetrakis(3-pyridyl)porphyrin and meso-tetra-
kis(4-pyridyl)porphyrin. The meso-tetrakis(4-iodophenyl)- and meso-tet-
rakis(4-bromophenyl)porphyrin variants were prepared through common
procedures of porphyrin synthesis by condensing iodobenzaldehyde or
bromobenzaldehyde with hot propionic acid. In the next stage, tin inser-
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an appropriate environment (DMF). The resulting products were cooled
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Supramolecular Self-Assembly of Tetraarylporphyrins by Halogen Bonding
FULL PAPER
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Received: May 14, 2013
Revised: July 24, 2013
Published online: September 13, 2013
Chem. Eur. J. 2013, 19, 14941 – 14949
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14949