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.
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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).
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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
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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
14947