Torands revisited: Metal sequestration and self-assembly of cyclo-2,9-tris-1,10-phenanthroline hexaaza macrocycles

Article abstract of DOI:10.1002/chem.201406602

A series of novel toroidal cyclo-2,9-tris-1,10-phenanthroline macrocycles with an unusual hexaaza cavity are reported. Nickel-mediated Yamamoto aryl-aryl coupling was found to be a versatile tool for the cyclotrimerization of functionalized 1,10-phenathroline precursors. Due to the now improved processability, both liquid-crystalline behavior in the bulk phase and two-dimensional self-assembly at the molecular level could be studied, for the first time, for a torand system. The macrocycles exhibited a strong affinity for the complexation of different metal cations, as evidenced by MALDI-TOF analysis and spectroscopic methods. Experimental results were correlated to an extensive computational study of the cyclo-2,9-tris-1,10-phenanthroline cavity and its binding mode for metal cations. Due to the combination of several interesting features, toroidal macrocycles may find future applications in the field of ion and charge transport through molecular channels, as well as for chemical sensing and molecular writing in surface-confined monolayers under STM conditions.

Full text of DOI:10.1002/chem.201406602

DOI: 10.1002/chem.201406602
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Liquid Crystals |Hot Paper|
Torands Revisited: Metal Sequestration and Self-Assembly of
Cyclo-2,9-tris-1,10-phenanthroline Hexaaza Macrocycles
Matthias Georg Schwab,[a, f] Masayoshi Takase,[b] Alexey Mavrinsky,[a] Wojciech Pisula,[a]
Xinliang Feng,[a] JosØ A. Gµmez,[c] Walter Thiel,[d] Kunal S. Mali,[e] Steven de Feyter,[e] and
Klaus Müllen*[a]
Abstract: A series of novel toroidal cyclo-2,9-tris-1,10-phe-
nanthroline macrocycles with an unusual hexaaza cavity are
reported. Nickel-mediated Yamamoto aryl–aryl coupling was
found to be a versatile tool for the cyclotrimerization of
functionalized 1,10-phenathroline precursors. Due to the
now improved processability, both liquid-crystalline behavior
in the bulk phase and two-dimensional self-assembly at the
molecular level could be studied, for the first time, for
a torand system. The macrocycles exhibited a strong affinity
for the complexation of different metal cations, as evidenced
by MALDI-TOF analysis and spectroscopic methods. Experi-
mental results were correlated to an extensive computation-
al study of the cyclo-2,9-tris-1,10-phenanthroline cavity and
its binding mode for metal cations. Due to the combination
of several interesting features, toroidal macrocycles may find
future applications in the field of ion and charge transport
through molecular channels, as well as for chemical sensing
and molecular writing in surface-confined monolayers under
STM conditions.
Introduction
Nitrogen-containing macrocycles represent an important class
of compounds in organic chemistry and materials science. Few
cyclic systems with a hexagonal arrangement of nitrogen
atoms and an 18-membered cavity have been reported
(Scheme 1). Due to their ring-shaped structure, these com-
pounds were termed torands.[1,2] Examples of toroidal macrocy-
cles involve a number of conjugated cyclic Schiff bases derived
from the parent hexa[18]azaannulene system A.[1,3–6] Depend-
[a] Dr. M. G. Schwab, Dr. A. Mavrinsky, Dr. W. Pisula, Prof. Dr. X. Feng,
Prof. Dr. K. Müllen
Max-Planck-Institut für Polymerforschung
Ackermannweg 10, 55128 Mainz (Germany)
E-mail: muellen@mpip-mainz.mpg.de
[b] Dr. M. Takase
Department of Chemistry, Graduate School of Science and Engineering,
Tokyo Metropolitan University, Hachioji, Tokyo 192-0397 (Japan)
Scheme 1. The molecular structures of representative hexaaza macrocycles
with an 18-membered cavity: Schiff base macrocycles A–C, cyclosexipyridine
D, dodecahydrohexaazakekulene E, and target compound cyclo-2,9-tris-1,10-
phenanthroline F.
[c] Dr. J. A. Gµmez
Institute of Technical and Macromolecular Chemistry, Division of Technical
Chemistry and Petrol Chemistry, RWTH Aachen University
Worringerweg 1, 52074 Aachen (Germany)
ing on the choice of the precursors, pyridine[3–6] B or 1,10-phe-
nanthroline C[1] contribute to the sixfold binding geometry of
these cyclic ligands, which are typically synthesized through
[2+2] template-assisted cyclocondensation. However, signifi-
cant deviations from planarity will be encountered and prevent
efficient face-to-face packing of these molecules in the solid
state.[5,6] On the contrary, cyclosexipyridine D[7,8] is a hexaaza
macrocycle, in which the aromatic units are connected through
aryl–aryl bonds with the six nitrogen atoms directed towards
the center of the cavity. Additional fusion of the six pyridine
[d] Prof. Dr. W. Thiel
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim (Germany)
[e] Dr. K. S. Mali, Prof. Dr. S. de Feyter
Katholieke Universiteit Leuven
Celestijnenlaan 200 F, 3001 Leuven (Belgium)
[f] Dr. M. G. Schwab
Present Address:
BASF SE, Carl-Bosch-Straße 38, 67056 Ludwigshafen (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406602.
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units of D through ethyl linkages will further increase both ri-
gidity and planarity.[9–12] As such, dodecahydrohexaazakekulene
E[9] and its derivatives[10–12] were obtained previously.
With the toolbox of modern aryl–aryl cross-coupling reac-
tions to hand, alternative pathways towards the construction
of novel torands can be thought of. Instead of assembling the
cyclic system from two or more complementarily functional-
ized monomers,[13–15] cyclotrimerization of suitable precursors
will significantly shorten the synthetic route. Examples of suc-
cessful cyclotrimerization reactions involve the copper(II)-medi-
ated reaction of Grignard reagents derived from biphenyl,[16]
ortho-terphenyl,[17] and phenanthrene[18,19] and the trimeriza-
tion of Lipshutz cuprates.[20] We have recently introduced a Ya-
mamoto-based concept, which relies on the selective assembly
of functionalized phenanthrene[21,22] and triphenylene[23–25a] de-
rivatives to form a new class of C3 symmetric macrocycles.
Lately, this concept was adopted to synthesize a series of qui-
noxaline-based cyclo(oligophenylenes).[25b]
tocol. To ensure processability, substitution of the targeted
1,10-phenanthroline macrocycle with solubilizing side chains
was also taken into consideration. We discuss the metal-com-
plexation behavior of these novel compounds, which has been
screened with the assistance of various physicochemical meth-
ods and correlated to results from computational analysis. For
the first time, the liquid-crystalline properties of torand–metal
complexes could be investigated in the bulk state by using
XRD techniques. The self-assembly of these torands was then
studied at the organic liquid–solid interface by applying high-
resolution STM.
Results and Discussion
The prerequisite for a smoothly occurring cyclotrimerization is
a 1208 substitution pattern on the molecular building block, as
reflected by our previous investigations.[21–25] Thus, a five-step
synthetic route from parent 1,10-phenanthroline (1) to the
target 2,9-dichloro-5,6-dialkoxy-1,10-phenanthroline precursors
6a–c was developed (Scheme 2). In the key step, compounds
1,10-Phenanthroline[26–28] was identified as a structural motif
that would yield a novel hexaaza macrocycle of intrinsic rigidi-
ty upon macrocycle formation.
The chelating ability of 1,10-phe-
nanthroline makes it a frequently
applied bidentate ligand in coor-
dination chemistry.[26–28] Transi-
tion-metal cations, such as ruth-
enium(II),[29] and members of the
lanthanide series[30] easily form
complexes with the molecule
and have been studied, for ex-
ample, with respect to their pho-
tophysical properties.
It is believed that the binding
of metal guests within a 1,10-
phenanthroline-surrounded
cavity will be unique because it
is based on pure ion-induced-
dipole electrostatic interactions.
Scheme 2. Synthetic route to macrocycles 7a–c. Reagents and conditions: i) 1,3-dibromopropane, 1208C, 95%;
ii) K3Fe(CN)6, NaOH, À108C, 41%; iii) POCl3, PCl5, reflux, 82%; iv) H2SO4, HNO3, KBr, 808C, 88%; v) tetrabutylammoni-
um bromide (TBAB), Na2S2O4, H2O/THF, alkyl bromide, KOH, 408C, 60–70%; vi) bis(cycloocta-1,5-diene)nickel(0), cy-
cloocta-(1,5)-diene, 2,2’-bipyridine, toluene/DMF, 608C, 15–20%.
Such
a
“electrically neutral”
cyclic ligand is in contrast to
porphyrins and phthalocyanines,
which are typically deprotonated
at the pyrrole site when binding to a metal guest.[31–33]
Previous investigations into metal complexation of related
toroidal macrocycles, such as cyclosexipyridine D[7,8] and do-
decahydrohexaazakekulene E,[9–12] have been hampered by
their poor solubility, in combination with the high polarity of
the hexaaza cavity.[7–12] Regarding self-assembly, understanding
of this class of ligands has so far been limited to the crystalline
state[3,5,6,11] and thin films.[12] Liquid crystallinity, which can be
anticipated from their flat doughnut-like geometry; surface-as-
sisted 2D self-assembly; and the impact of metal guests on the
supramolecular chemistry remain important issues yet to be
explored.
6a–c were obtained in good yield from 2,9-dichloro-1,10-phe-
nanthrolin-5,6-dione (5)[34] through a reductive alkylation as
off-white waxy powders. The side-chain length was varied
from octyl (6a), dodecyl (6b), to hexadecyl (6c) to compare
the effect of the substituent on the supramolecular behavior
of the compounds. 5,6-Bis(dodecyloxy)-1,10-phenanthroline (9)
was prepared as a model compound in an analogous fashion
from commercially available 1,10-phenanthroline-5,6-dione (8).
For the cyclotrimerization of precursors 6a–c towards mac-
rocycles 7a–c, a standard Yamomoto protocol[21–23,35,36] was car-
ried out in an overall 3/1 mixture of toluene/DMF at 608C
under pseudo-dilution conditions (cf. Scheme 2, for the pro-
posed reaction pathway towards 7a–c, see Scheme S2 in the
Supporting Information). The products were isolated by using
a combination of column chromatography on silica gel and re-
Herein, we present a series of unreported torands, namely,
cyclo-2,9-tris-1,10-phenanthroline hexaaza macrocycles F,
which have been prepared by a straightforward synthetic pro-
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cycling gel permeation chromatography to yield 7a–c as
orange waxy solids in respectable overall yields between 15
and 20%. All compounds displayed high solubility in polar
aprotic solvents, such as CH2Cl2, THF, and Et2OAc. However,
they were poorly soluble in nonpolar media, such as alkanes,
and could be precipitated into alcohols. As described in detail
below, the Na+ complexes of 7a–c were used as reference
points for all experiments.
with our results. Therefore, it can be anticipated that only
large alkaline,[3–5,8,11] alkaline earth,[5,10] and transition-metal cat-
ions[3,6] will be suitable guests for 7a–c.
Metal complexation
Analysis by MALDI-TOF mass spectrometry confirmed that the
as-synthesized macrocycles 7a–c were typically found as mix-
tures of various alkaline-metal complexes after their isolation
from the crude reaction product; the Na+ and K+ adducts ex-
hibited the strongest peak intensity (Figure 1b). The high affin-
ity of hexaaza macrocycles with similar cavity sizes towards al-
kaline-metal cations were reported previously.[3–5,8,11] It is rea-
sonable to presume that, in the present case, these guest ions
are entrapped from the glassware and solvents during the re-
action and workup procedure. The tendency towards complex-
ation with alkaline-metal cations was further reflected by the
fact that the preparation of the metal-free macrocycle by acid
treatment was unsuccessful. On the other hand, as depicted in
Figure 1b for 7b, the Na+ complex can be prepared in virtual-
ly quantitative yield by washing an organic layer containing
the compound with an aqueous solution of sodium hydroxide.
Therefore, this compound was chosen as the starting point for
all following metal sequestration experiments and is labeled as
7b-Na.
The hexaaza cavity of the new ligand is unique, in contrast
to well-established macrocyclic ligands such as porphyrins and
phthalocyanines, because it does not offer any NÀH bonds
that could be deprotonated upon guest inclusion.[31–33] Instead,
weaker binding through ion-induced-dipole electrostatic inter-
actions is expected to govern the host–guest system in the
case of cyclo-2,9-tris-1,10-phenanthroline. Hence, the metal
complexing and exchange ability of 7b-Na was studied as
a function of the ionic diameter of various metal cations.[41]
Successful sequestration was possible for a number of large
transition-metal guests, including Pb2+, Ag+, Cd2+, Zn2+, and
Cu2+, as confirmed by MALDI-TOF analysis (Figures S9–S24 in
the Supporting Information). In all resulting spectra, the com-
plete disappearance of the initial Na+ signal of 7b-Na was ob-
served. For Ag+ and Cu2+, the corresponding 1:1 adducts
The successful buildup of the cyclo-2,9-tris-1,10-phenanthro-
1
1
line system was first confirmed by H, 13C, and H–13C COSY
NMR spectroscopy in CD2Cl2 at room temperature. As shown
for 7b-Na, two characteristic doublets from the AB spin system
on the 1,10-phenanthroline backbone are found at d=8.98
1
(Ha) and 8.75 ppm (Hb) in the H NMR spectrum. Correlation of
1
the H resonances to the signals of the spin-echo 13C spectrum
unambiguously proves the presence of the desired cyclic
trimer and the absence of acyclic byproducts (Figure S4 in the
Supporting Information). The UV/Vis absorption spectra for
7b-Na were recorded in CH2Cl2 and compared with data ob-
tained for the corresponding monomeric model compound 9
at a concentration of 1”10À5 m. With respect to the absorption
spectrum of pristine 1,10-phenanthroline, which essentially
contains two major absorption bands at l=205 and
263 nm,[37] the signals of macrocycle 7b-Na (as well as those
for model compound 9) are significantly shifted to longer
wavelengths. This can be rationalized as a result of increased
conjugation and the electron-donating effect of the alkoxy
substituents. The spectrum of the macrocycle contains a broad
absorption maximum centered at l=309 nm (e=
78313mÀ1 cmÀ1) and two minor resolved bands at l=359 and
403 nm (Figure S25 in the Supporting Information).
Structure analysis
Due to the prefused aromatic rings of the 1,10-phenanthroline
building block, less conformational freedom is expected for
cyclo-2,9-tris-1,10-phenanthroline F in comparison to cyclosexi-
pyridine D,[7,8] whereas dodecahydrohexaazakekulene E[9–12] ex-
hibits yet higher structural rigidity than the present compound
(Scheme 1). DFT calculations indicate a minimum-energy pro-
peller-like structure of C2 symmetry with dihedral angles of
26.7 and À21.38 between two neighboring 1,10-phenanthro-
line units and cavity axes of 5.614 and 5.574 Š, respectively
(Figure 1a). A C3 symmetric structure would require all NCCN
dihedrals to have the same sign. This, in addition to rendering
a structure that is slightly higher in energy, is statistically less
probable than allowing one of the dihedral angles to have
a different sign (Figure S32 in the Supporting Information).
Indeed, an alternation of the sign of the dihedral angles has al-
ready been reported in X-ray crystal structures of related tor-
ands.[38]
were detected as exclusive species, whereas, for Pb2+, Cd2+
,
and Zn2+, water and counterions participate in complex forma-
tion.
Complementary computational simulations indicate that the
Na+ adduct displays the lowest binding energy (Table S3 in
the Supporting Information); thus explaining its easy replace-
ment by other metal species. The same argument applies to
any alkaline and alkaline-earth metal, for which the interaction
between metal and ligand is essentially electrostatic. On the
contrary, a significantly stronger covalent contribution to the
bond was calculated for other metal species. Energy decompo-
sition analysis reveals that metals with accessible empty s (e.g.,
Ag+) or p orbitals (e.g., Pb2+) allow for covalent ligand-to-
metal dative bonds. This reinforces the electrostatic compo-
nent; thus resulting in stronger metal–ligand interactions. Fur-
thermore, when the cation is small enough (e.g., Zn2+), it may
leave the center of the macrocycle cavity, resulting in increased
Theoretical calculations on related Schiff base macrocy-
cles,[39] as well as single-crystal XRD data from dodecahydro-
hexaazakekulene derivatives[11] and bis(1,10-phenanthrolinyl-
2,5-pyrrole),[40] revealed that the distance between two oppo-
site nitrogen atoms of the hexagonal cavity was between 5.0
and 6.0 Š for these compounds; this is in good agreement
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Figure 1. a) Geometric dimensions of the cyclo-2,9-tris-1,10-phenanthroline macrocycle 7a–c, as derived from DFT analysis. b) MALDITOF spectrum of 7bNa
after treatment with NaOH; the inset shows the distribution of protonated 7b and its Na+ and K+ complexes after synthesis before (green) and after (blue)
exposure to 2m NaOH. c) The evolution of the fluorescence excitation and emission spectra of 7bNa upon titration with silver triflate. d) The reduction of
7bAg with sodium borohydride monitored by UV irradiation at l=366 nm.
electrostatic and covalent strength of the bond (Figures S35–
S38 in the Supporting Information, and discussion herein).
The inclusion of the new metal guests was monitored spec-
troscopically. We observed that the rapid replacement of the
cationic species was accompanied by significant changes in
the photoluminescence (PL) excitation and emission spectra
of 7b-Na (Figure 1c). The PL spectrum of 7b-Na shows
a broad emission between l=380 and 650 nm with the maxi-
mum located at l=444 nm and two minor peaks resolved at
l=420 and 467 nm. The vibronic fine structure of the emis-
sion spectrum is mirrored by the corresponding excitation
spectrum, as expected for a rigid molecular backbone. Step-
wise addition of zinc acetate to the parent solution of 7b-Na
was followed by a progressive bathochromic shift of the emis-
sion maximum by 16 nm to a final value of l=460 nm after ti-
tration with one equivalent of salt (Figure S26 in the Support-
ing Information). While a similar trend was confirmed by titra-
tion with silver triflate, the treatment resulted in a much stron-
ger bathochromic shift of the main emission to a final value of
l=474 nm and a 90% quenching of emission after the addi-
tion of two equivalents (Figure 1c). Most likely, the quenching
of the fluorescence in the silver complex originates from fast
population transfer to the triplet state. This is a result of the
so-called heavy-atom effect.[42, 43] Heavy atoms induce high
spin-orbit coupling, which allows fast and efficient population
transfer between different spin states. The migration of the
emission band upon titration with a metal source has been
described previously studies and the corresponding cyclic[44–46]
and acyclic ligands[47, 48] were thus applied as efficient chemo-
sensors for the detection of trace amounts of metal cations in
solution. In contrast, the direct and quantitative replacement
of a metal guest by another cation, as seen in the present
case, is a rare feature for a cyclic system. Whereas exchange of
the metal guest was also previously achieved for other tor-
ands, their cavity needed to be “emptied” by an intermediate
acid treatment step.[9–12]
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Moreover, the exchange of Na+ for Ag+ also resulted in an
lations and the lack of higher-order reflections indicate moder-
ate macroscopic orientation of the macrocycles along the ex-
trusion direction. The weak tendency of Na+ to assemble pro-
vided motivation to examine possible stabilization of the mes-
ophase by larger, electron-rich transition-metal cations and dif-
ferent counterions.
1
overall line broadening of the H NMR spectrum of 7b-Ag in
[D2]1,1,2,2-tetrachloroethane ([D2]TCE).[44] This effect was partic-
ularly pronounced for the two doublets that corresponded to
the aromatic protons of the macrocycle, which were not re-
solved for 7b-Ag, even at higher temperatures than those of
the pristine 7b-Na system (Figure S6 in the Supporting Infor-
mation).
Similar observations were made previously for liquid-crystal-
line crown ether derivatives and their corresponding metal
complexes.[60–62] To this effect, the Ag+ and Pb2+ complexes,
7b-Ag and 7b-Pb, respectively, were investigated by tempera-
ture-dependent 2D-WAXS experiments. In the corresponding
complexes, the triflate and acetate ions, which originate from
the salts used to introduce the cation, counterbalance the pos-
itive charges and are probably sandwiched between the neigh-
boring macrocycles.[63] Figure 2b and c reveals that both metal
complexes again exhibit a hexagonal columnar organization
after extrusion of the filaments; the degree of molecular order
is still moderate. The two samples were then subjected to an
annealing procedure with a maximum temperature of 1508C.
It should be noted that none of the compounds displayed
phase transitions in the differential scanning calorimetry (DSC)
scans in this temperature regime. Nevertheless, this treatment
leads to improved self-organization of the macrocycles. At
a temperature of 1508C, sharp meridional reflections are well
resolved in the pattern of 7b-Ag (Figure 2b). Additional higher
order reflections in the equatorial plane of the pattern also
confirm an enhancement of the hexagonal columnar organiza-
tion. Also, the high degree of order is preserved upon cooling
to 308C, as seen by the prevalence of these reflections. The
inter- and intracolumnar parameters of the mesophase can be
also used as a descriptor to illustrate the improved order of
7b-Ag compared with that of the Na+ complex. The Ag+ com-
plex exhibits an intercolumnar separation of ahex =3.02 nm (cf.
More experiments were then carried out to examine possi-
ble re-exchange of the Ag+ cation by a Na+ guest. To make
the opposite pathway favorable, the Ag+ cation first needed
to be irreversibly removed from the macrocycle. This was trig-
gered by the addition of excess sodium borohydride to a solu-
tion of 7b-Ag, leading to the re-formation of 7b-Na and the
precipitation of colloidal silver nanoparticles, as observed by
electron microscopy (Figure S41 in the Supporting Informa-
tion). As illustrated in Figure 1d, the reduction experiment can
be easily monitored under UV radiation (l=366 nm). Upon
mixing the reducing agent with the solution of macrocycle, an
instant reappearance of the characteristic bright turquoise
fluorescence of 7b-Na was noted, along with the evolution of
hydrogen gas bubbles.
Three-dimensional self-assembly in the bulk phase
In principle, the stacking of rigid macrocyclic mesogens results
in the formation of tubular superstructures, as observed for
macrocyclic polyethers,[49] polyamines,[50] phthalocyanines,[51,52]
and various shape-persistent phenylene–ethynylene macrocy-
cles.[53,54] It can be expected that the presence of intraannular
aza or oxo coordination sites will further enable the formation
of nanoscale tubes; thus holding promise for the transport of
ions, charges, and small molecules.[51] Useful applications could
involve membranes that contain ion-selective channels,[55] am-
phiphilic metallomesogens with enhanced charge-carrier mobi-
lities for organic field-effect transistors,[56] and conducting
wires.[51,57]
a
hex =3.75 nm for Na+) and a typical intramolecular stacking
distance of p=0.35 nm, which is indicative of strong p–p inter-
actions within the columns. Concerning the bulk packing of
7b-Pb, the metal guest was also found to efficiently help in
the columnar arrangement of the cyclic metal complexes (Fig-
ure 2c). However, the improvement of the molecular order al-
ready occurred at a lower temperature. In analogy to 7b-Ag,
pronounced meridional reflections evolved at 1108C, which
proved the presence of a well-arranged hexagonal columnar
mesophase. Further increasing the temperature to 1508C
leads, however, to a decrease in the molecular order for this
compound. After the thermal cycle, self-organization is not as
pronounced as that for 7b-Ag, but the patterns allow for the
The supramolecular organization of the cyclo-2,9-tris-1,10-
phenanthroline compounds 7a–c and the transition-metal
complexes of 7b in the bulk state were studied by 2D wide-
angle X-ray scattering (2D-WAXS).[52–54,58] This method examines
the aggregation behavior in bulk by collecting X-ray scattering
patterns from an extruded filament of the material under in-
vestigation.[59] The reflection positions in the 2D-WAXS patterns
can then be correlated to the intra- and intercolumnar supra-
molecular organization. Because the fibers are oriented verti-
cally with respect to the incident X-ray beam, the meridional
reflections correspond to the correlations that result from disk
stacking and the equatorial reflection distribution contains in-
formation on the order between columns.
determination of the packing parameters, which are ahex
=
3.10 nm and p=0.37 nm, respectively. In summary, our 2D-
WAXS experiments illustrate the ability of the cyclo-2,9-tris-
1,10-phenanthrolines to self-assemble into liquid-crystalline
hexagonal columnar mesophases (as schematically depicted in
Figure 2d); this has not yet been observed for a toroidal mac-
rocycle. The packing tendency itself is moderate; this can be
rationalized by the relatively small p surface of the compound
compared with typical liquid-crystalline derivatives of coronene
or hexa-peri-hexabenzocoronene (HBC).[59,64,65]
The measurements on 7a–c indicated a liquid-crystalline
hexagonal columnar arrangement of the macrocycles and a co-
axial orientation of the discs in the stacks. The intercolumnar
distance, ahex, between the molecular stacks increased succes-
sively with the length of the alkoxy chains from ahex =3.32 (7a-
Na) to 3.75 (7b-Na) and 4.22 nm (7c-Na; Figure 2a). The rather
isotropic small-angle reflections related to intercolumnar corre-
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Figure 2. a) Linear relationship between alkoxy chain length and intercolumnar spacing for macrocycles 7a-Na, 7b-Na and 7c-Na; the inset shows the 2D-
WAXS pattern of 7b-Na (308C). b) Temperature-dependent 2D-WAXS patterns of 7b-Ag. c) Temperature-dependent 2D-WAXS patterns of 7b-Pb. d) Schematic
illustration of the molecular packing of the metal complexes after annealing; the stabilizing metal guest is indicated as a green sphere.
Two-dimensional self-assembly on surfaces
form high-density structures on the HOPG surface, for which
the alkoxy chains of adjacent molecules interdigitate with each
other. In the present case, however, the chains simply adsorb
next to each other without forming these typical interdigita-
tion patterns.[24,66] While the reason for such packing is not
clear, one may consider the contribution of an electrostatic
component to the assembly process due to the presence of
metal ions captured in the cavity of cyclo-2,9-tris-1,10-phenan-
throline. The alkoxy chains run parallel to the main graphite
symmetry axes, whereas the rows of the 1,10-phenanthroline
cores are oriented approximately along the normal to the
main symmetry axis. A striking detail of the high-resolution
STM images is the presence of curved alkoxy chains, as indicat-
ed by white arrows in Figure 3a–c. This phenomenon results
from the filling of free volume by the dynamic rearrangement
of the alkoxy chains, and has also been reported for HBC deriv-
atives.[67]
STM was then used for real-space visualization of the self-as-
sembled monolayers of 7b-Na, 7b-Ag, and 7b-Pb at submo-
lecular resolution. At the 1-phenyloctane/highly ordered pyro-
lytic graphite (HOPG) interface, all three molecules formed
highly ordered, 2D superstructures, in which the macrocycles
were arranged in the form of a characteristic row pattern. Fig-
ure 3a–c contains high-resolution STM images of the monolay-
ers, and representative large-scale STM images are shown in
the Supporting Information (Figures S39 and S40 in the Sup-
porting Information). The p-conjugated 1,10-phenanthroline
moieties of the macrocycle appear bright, whereas the stripy
features between the macrocycles correspond to the peripher-
al dodecyloxy chains. Although the computational analysis pre-
sented above indicates a propeller-like conformation for the ar-
omatic core, there is no clear indication of nonplanarity in the
high-resolution STM images. The densely packed structures of
these molecules are peculiar and differ from those of typical C3
symmetric building blocks substituted with peripheral alkoxy
chains reported previously.[24,66] For example, cyclo-tris(7,9-tri-
phenylenylene) and dehydrobenzo[12]annulene derivatives
Having discussed the similarities between the STM images
of monolayers formed by different metal-ion-containing mac-
rocycles, we now look at the differences. The STM images pro-
vided in Figure 3a–c indicate a visible depression in the center
of the cyclic system for 7b-Na and 7b-Ag, which leads to the
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“shines through” the ligand;
thus imparting more apparent
height to the metal.[68–70]
Although detailed electronic
calculations are not within the
scope of this work, we believe
that similar considerations apply
for the present system as well.
The distinct STM appearance of
Pb2+ in the STM images[71] is
possibly related to d-orbital-
mediated tunneling, whereas the
lack of STM signature for Ag+ re-
lates to a negligible contribution
of the d orbitals to the Fermi
energy surface. On the other
hand, Na+, which is a relatively
small ion, does not possess d or-
bitals. A comparison of the STM
results with those of 2D-WAXS
discussed above clearly reveals
that self-assembly in the bulk
differs from that at the solution–
solid interface; the reasons for
this have been discussed in
detail by us elsewhere.[72]
Figure 3. High-resolution STM images of physisorbed monolayers of a) 7bNa, b) 7bAg, and c) 7bPb at the 1-
phenyloctane/HOPG interface. The white arrows indicate the presence of unusually curved alkoxy chains. The unit
cell parameters for the three molecules are identical: a=(2.8Æ0.1) nm, b=(4.6Æ0.2) nm, and a=(75.0Æ1.8). The
graphite symmetry axes are indicated in the bottom-left corner of each image. d)–f) 3D rendering of slightly
larger scale STM images to highlight the difference in the appearance of the aromatic core of the three macrocy-
cles, depending on the central metal ion.
impression that these are empty. On the other hand, STM
Conclusion
images of 7b-Pb reveal a protrusion that indicates the pres-
ence of the metal ion. The 3D rendering provided in Fig-
ure 3d–f graphically visualizes this difference; the centers of
7b-Na (Figure 3d) and 7b-Ag (Figure 3e) appearing to be fea-
tureless and those of 7b-Pb (Figure 3 f) show pronounced
spikes. It must be noted, however, that the lack of a signature
in the STM images does not necessarily imply the absence of
the metal guest in the molecules. Given that STM maps the
local density of states, the apparent “height” in constant cur-
rent mode is a complex function of the density of electrons
that possess an energy near that of the Fermi level. As a conse-
quence, the local density of states around a metal atom/ion
may deviate significantly from the overall electron density of
the ligand (the cyclo-2,9-tris-1,10-phenanthroline backbone in
this case). Earlier reports have addressed this issue in great
detail by systematically studying the metal-ion-dependent STM
contrast of phthalocyanines.[68–70] High-resolution STM images
of copper (CuPc) and cobalt phthalocyanine (CoPc) adsorbed
on Au(111) surfaces revealed that the central metal appeared
as a depression in the case of CuPc, whereas a protrusion was
observed for CoPc. This difference in the STM signature of the
two metals originates from the dissimilar d orbitals that medi-
ate tunneling: whereas the CoII d9 system has significant d-or-
bital contribution to the Fermi level, the CuII d9 system does
not. Electronic structure calculations further revealed that the
half-filled CoPc dz2 orbital lay very close to the HOMO. On the
contrary, there is no significant d orbital contribution within
a band 1 eV above and 2 eV below the HOMO of the host in
CuPc. As a consequence the cobalt surface density of states
New toroidal macrocycles with high structural rigidity were
synthesized through the Yamamoto cyclotrimerization of func-
tionalized 1,10-phenathrolines. The unusual hexaaza cavity of
these cyclic ligands accommodated a number of alkaline-, tran-
sition-, and heavy-metal cations and was not deprotonated
upon guest inclusion. The geometric parameters and the spe-
cial binding mode between the macrocycle and several of its
metal guests were analyzed in detail by computational meth-
ods. Metal inclusion was accompanied by pronounced changes
in the spectroscopic signature of the cyclo-2,9-tris-1,10-phe-
nanthroline macrocycle; this makes it a useful candidate for
sensing applications. In combination with the liquid-crystalline
behavior of these disc-shaped molecules, the metal complexa-
tion ability could be exploited for the buildup of artificial mo-
lecular channels suitable for the transport of electrically
charged guest species. Along these lines, the fixation and thus
stabilization of the mesophase through a polymerization step
remains an important synthetic challenge.[52,28b] The tendency
towards self-organization the macrocycles and their metal
complexes is further illustrated by the observation of well-or-
dered monolayers at the solid/liquid interface through STM;
this was achieved, for the first time, for a torand macrocycle.
Further attempts are in progress to reduce the metal ions se-
questered within the cavities of the macrocycle to their neutral
state under STM control. It is expected that this transformation
will be accompanied by a strong increase in the STM contrast
compared with their ionic state. In this regard, it is envisaged
to apply the STM tip as a “pencil” to selectively address individ-
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ual molecules; thus realizing storage of data on the nanoscale
by “binary writing”.
Keywords: liquid crystals · macrocycles · phenanthrolines ·
scanning probe microscopy · self-assembly
[1] T. W. Bell, A. Firestone, F. Guzzo, L. Y. Hu, J. Inclusion Phenom. 1987, 5,
149–152.
Experimental Section
[2] T. W. Bell, P. J. Cragg, M. G. B. Drew, A. Firestone, A. D. I. Kwok, J. I. A. Liu,
R. T. Ludwig, A. T. Papoulis, Pure Appl. Chem. 1993, 65, 361–361.
[3] M. G. B. Drew, J. de O. Cabral, M. F. Cabral, F. S. Esho, S. M. Nelson, J.
Chem. Soc. Chem. Commun. 1979, 1979, 1033–1035.
[4] T. W. Bell, F. Guzzo, J. Am. Chem. Soc. 1984, 106, 6111–6112.
[5] T. W. Bell, F. Guzzo, M. G. B. Drew, J. Am. Chem. Soc. 1991, 113, 3115–
3122.
[6] F. Benetollo, G. Bombieri, L. De Cola, A. Polo, D. L. Smailes, L. M. Vallari-
no, Inorg. Chem. 1989, 28, 3447–3452.
[7] G. R. Newkome, H. W. Lee, J. Am. Chem. Soc. 1983, 105, 5956–5957.
[8] J. L. Toner, Tetrahedron Lett. 1983, 24, 2707–2710.
[9] J. E. Butler Ransohoff, H. A. Staab, Tetrahedron Lett. 1985, 26, 6179–
6182.
[10] T. W. Bell, A. Firestone, J. Am. Chem. Soc. 1986, 108, 8109–8111.
[11] T. W. Bell, P. J. Cragg, M. G. B. Drew, A. Firestone, D. I. A. Kwok, Angew.
Chem. Int. Ed. Engl. 1992, 31, 348–350; Angew. Chem. 1992, 104, 321–
323.
[12] L. Boguslavsky, T. W. Bell, Langmuir 1994, 10, 991–993.
[13] D. Zhao, J. S. Moore, Chem. Commun. 2003, 2003, 807–818.
[14] W. Zhang, J. S. Moore, Angew. Chem. Int. Ed. 2006, 45, 4416–4439;
Angew. Chem. 2006, 118, 4524–4548.
2,9-Dichloro-5,6-bis(dodecyloxy)-1,10-phenanthroline (6b)
Compound 5 (1.00 g, 3.61 mmol),[34] tetrabutylammonium bromide
(TBAB, 0.75 g, 2.33 mmol), and sodium dithionite (3.77 g,
21.66 mmol) were added to a mixture of water (10.0 mL) and
(10.0 mL) THF. Then, dodecyl bromide (3.0 g, 12.0 mmol) was
added to the mixture. A solution of potassium hydroxide (3.0 g,
54.15 mmol) in water (20.0 mL) was subsequently added, and the
reaction mixture turned black. The reaction mixture was stirred for
48 h at 408C. After dilution with water, the reaction mixture was
extracted with ethyl acetate. The crude product was purified by
column chromatography (CH2Cl2/ethyl acetate=50:1) to yield 6b
as
a
light yellow powder (1.49 g, 2.41 mmol, 67%). 1H NMR
(250 MHz, CD2Cl2, 258C): d=8.53 (d, J=8.6, 2H), 7.64 (d, J=8.6,
2H), 4.22 (t, J=6.7, 4H), 1.86 (p, J=7.0, 4H), 1.61–1.10 (m, 36H),
0.87 ppm (t, J=6.9, 6H); 13C NMR (75 MHz, CD2Cl2, 258C): d=150.8,
143.2, 143.1, 134.3, 126.4, 124.9, 74.8, 32.5, 30.9, 30.3, 30.2, 30.21,
30.19, 30.05, 29.95, 26.7, 23.3, 14.5 ppm; MS (8 kV): m/z (%): 616.1
(100.0) [M+].
[15] C. Grave, A. D. Schlüter, Eur. J. Org. Chem. 2002, 2002, 3075–3098.
[16] H. Bräunling, F. Binnig, H. A. Staab, Chem. Ber. 1967, 100, 880–888.
[17] H. Meyer, H. A. Staab, Liebigs Ann. Chem. 1969, 724, 30–33.
[18] H. A. Staab, H. Bräunling, Tetrahedron Lett. 1965, 6, 45–49.
[19] H. A. Staab, U. E. Meissner, B. Meissner, Chem. Ber. 1976, 109, 3875–
3885.
[20] M. J. Rahman, J. Yamakawa, A. Matsumoto, H. Enozawa, T. Nishinaga, K.
Kamada, M. Iyoda, J. Org. Chem. 2008, 73, 5542–5548.
[21] W. Pisula, M. Kastler, C. Yang, V. Enkelmann, K. Müllen, Chem. Asian J.
2007, 2, 51–56.
[22] J. Zhang, X. Wang, Q. Su, L. Zhi, A. Thomas, X. Feng, D. S. Su, R. Schlçgl,
K. Müllen, J. Am. Chem. Soc. 2009, 131, 11296–11297.
[23] M. G. Schwab, T. Qin, W. Pisula, A. Mavrinskiy, X. Feng, M. Baumgarten,
H. Kim, F. Laquai, S. Schuh, R. Trattnig, E. J. W. List, K. Müllen, Chem.
Asian J. 2011, 6, 3001–3010.
Cyclo-2,9-tris-5,6-bisdodecyloxy-1,10-phenanthroline (7b)
The catalyst solution was prepared inside a glove box by adding
DMF (20.0 mL) and toluene (40.0 mL) to a mixture of bis(cycloocta-
diene)nickel(0) (0.43 g, 1.55 mmol), bipyridine (0.24 g, 1.55 mmol),
and cyclooctadiene (0.19 mL, 1.55 mmol). The resulting solution
was stirred for 30 min at 608C. Then, a solution of 6b (0.40 g,
0.65 mmol) in toluene (20.0 mL) was added dropwise over 30 min.
The reaction mixture was stirred for 72 h at 608C under the exclu-
sion of light. After cooling, the mixture was diluted with diethyl
ether and washed with diluted hydrochloric acid (2m). The crude
product was purified by column chromatography on silica gel
(CH2Cl2/methanol=10/1). Further purification was achieved by
preparative gel permeation chromatography (chloroform) to yield
7b as an orange waxy solid (67 mg, 0.041 mmol, 19%). 1H NMR
(500 MHz, CD2Cl2, 258C): d=8.98 (d, J=8.7, 6H), 8.79 (d, J=8.8,
6H), 4.37 (t, J=6.6, 12H), 1.86 (p, J=7.0, 12H), 1.75–1.10 (m,
108H), 0.89 ppm (t, J=6.9, 18H); 13C NMR (126 MHz, CD2Cl2, 258C):
d=153.8, 144.6, 143.0, 133.2, 127.5, 123.0, 74.9, 32.6, 31.1, 30.4,
30.3, 30.2, 30.0, 26.9, 23.3, 14.8 ppm; MS (MALDI-TOF): m/z (%):
1663.24 (100.0), 1662.25 (82.3), 1664.25 (62.4), 1665.26 (29.5).
[24] K. S. Mali, M. G. Schwab, X. Feng, K. Müllen, S. De Feyter, Phys. Chem.
Chem. Phys. 2013, 15, 12495–12503.
[25] a) K. Cui, F. Schlütter, O. Ivasenko, M. Kivala, M. G. Schwab, S.-L. Lee,
S. F. L. Mertens, K. Tahara, Y. Tobe, K. Müllen, K. S. Mali, S. De Feyter,
Chem. Eur. J. 2015, 21, 1652–1659; b) L. Marin, J. Kudrjasova, P. Verstap-
pen, H. Penxten, K. Robeyns, L. Lutsen, D. J. M. Vanderzande, W. Maes, J.
Org. Chem. 2015, 80, 2425–2430.
[26] P. G. Sammes, G. Yahioglu, Chem. Soc. Rev. 1994, 23, 327–334.
[27] G. Accorsi, A. Listorti, K. Yoosaf, N. Armaroli, Chem. Soc. Rev. 2009, 38,
1690–1700.
[28] A. Bencini, V. Lippolis, Coord. Chem. Rev. 2010, 254, 2096–2180.
[29] S. Campagna, F. Puntoriero, F. Nastasi, G. Bergamini, V. Balzani, Top. Curr.
Chem. 2007, 280, 117–214.
[30] S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli, B. Ventura, F.
Barigelletti, Inorg. Chem. 2005, 44, 529–537.
Acknowledgements
M.G.S. thanks the “Graduate School Materials Science in Mainz”
for a scholarship. K.S.M. thanks the “Fonds Wetenschappelijk
Onderzoek Vlaanderen”. J.A.G. acknowledges the Alexander
von Humboldt Foundation for a postdoctoral fellowship. K.S.M.
and S.d.F. thank the Fund of Scientific Research-Flanders
(FWO), KU Leuven (GOA 11/003), and Belgian Federal Science
Policy Office (IAP-7/05) for financial support. This research was
also funded by the DFG Priority Program SPP 1459, DFG Proj-
ect MU 334/36-1, the ERC grant on NANOGRAPH, Graphene
Flagship (No. CNECT-ICT-604391), and the European Union
Projects UPGRADE, GENIUS, and MoQuaS.
[31] A. Jasat, D. Dolphin, Chem. Rev. 1997, 97, 2267–2340.
[32] I. Beletskaya, V. S. Tyurin, A. Y. Tsivadze, R. Guilard, C. Stern, Chem. Rev.
2009, 109, 1659–1713.
[33] J. Elemans, R. van Hameren, R. Nolte, A. Rowan, Adv. Mater. 2006, 18,
1251–1266.
[34] M. Yamada, Y. Tanaka, Y. Yoshimoto, S. Kuroda, I. Shimao, Bull. Chem.
Soc. Jpn. 1992, 65, 1006–1011.
[35] T. Yamamoto, Prog. Polym. Sci. 1992, 17, 1153–1205.
[36] T. Yamamoto, Bull. Chem. Soc. Jpn. 1999, 72, 621–638.
[37] M. S. Henry, M. Z. Hoffman, J. Phys. Chem. 1979, 83, 618–625.
[38] T. W. Bell, P. J. Cragg, A. Firestone, D.-I. A. Kwok, Angew. Chem. Int. Ed.
Engl. 1992, 31, 345–347; Angew. Chem. 1992, 104, 319–321.
[39] C. L. Honeybourne, Tetrahedron 1973, 29, 1549–1557.
Chem. Eur. J. 2015, 21, 8426 – 8434
www.chemeurj.org
8433
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
[40] A. McSkimming, S. Shrestha, M. M. Bhadbhade, P. Thordarson, S. B. Col-
bran, Chem. Asian J. 2014, 9, 136–145.
[58] B. Schmaltz, A. Rouhanipour, H. J. Räder, W. Pisula, K. Müllen, Angew.
Chem. Int. Ed. 2009, 48, 720–724; Angew. Chem. 2009, 121, 734–738.
[59] W. Pisula, Z. Tomovic, C. Simpson, M. Kastler, T. Pakula, K. Müllen, Chem.
Mater. 2005, 17, 4296–4303.
[60] V. Percec, G. Johansson, J. Heck, G. Ungarb, S. V. Battyb, J. Chem. Soc.
Perkin Trans. 1 1993, 1411–1420.
[61] M. Kaller, S. Tussetschläger, P. Fischer, C. Deck, A. Baro, F. Giesselmann,
S. Laschat, Chem. Eur. J. 2009, 15, 9530–9542.
[62] M. Kaller, C. Deck, A. Meister, G. Hause, A. Baro, S. Laschat, Chem. Eur. J.
2010, 16, 6326–6337.
[41] R. D. Shannon, Acta Crystallogr. Sect. A 1976, 32, 751–767.
[42] J. C. Koziar, D. O. Cowan, Acc. Chem. Res. 1978, 11, 334–341.
[43] N. Armaroli, L. De Cola, V. Balzani, J.-P. Sauvage, C. O. Dietrich-Buchecker,
J.-M. Kern, A. Bailal, J. Chem. Soc. Dalton Trans. 1993, 3241–3247.
[44] B. Jousselme, P. Blanchard, E. Levillain, J. Delaunay, M. Allain, P. Ri-
chomme, D. Rondeau, N. Gallego-Planas, J. Roncali, J. Am. Chem. Soc.
2003, 125, 1363–1370.
[45] P. N. W. Baxter, J. Org. Chem. 2004, 69, 1813–1821.
[46] L. Tian, C. Wang, S. Dawn, M. D. Smith, J. A. Krause, L. S. Shimizu, J. Am.
Chem. Soc. 2009, 131, 17620–17629.
[47] J. H. Huang, W. H. Wen, Y. Y. Sun, P. T. Chou, J. M. Fang, J. Org. Chem.
2005, 70, 5827–5832.
[48] Y. Fu, H. Li, W. Hu, Eur. J. Org. Chem. 2007, 2007, 2459–2463.
[49] J. P. Behr, J.-M. Lehn, A. C. Dock, D. Moras, Nature 1982, 295, 526–527.
[50] J.-M. Lehn, J. MalthÞte, A. M. Levelut, J. Chem. Soc. Chem. Commun.
1985, 1985, 1794–1796.
[63] Ref. [11].
[64] C. D. Simpson, J. Wu, M. D. Watson, K. Müllen, J. Mater. Chem. 2004, 14,
494–504.
[65] P. Herwig, C. W. Kayser, K. Müllen, H. W. Spiess, Adv. Mater. 1996, 8,
510–513.
[66] K. Tahara, S. Lei, J. Adisoejoso, S. de Feyter, Y. Tobe, Chem. Commun.
2010, 46, 9125–9127.
[67] A. Stabel, P. Herwig, K. Müllen, J. P. Rabe, Angew. Chem. Int. Ed. Engl.
1995, 34, 1609–1611; Angew. Chem. 1995, 107, 1768–1770.
[68] K. W. Hipps, X. Lu, X. D. Wang, U. Mazur, J. Phys. Chem. 1996, 100,
11207–11210.
[69] X. Lu, K. W. Hipps, X. D. Wang, U. Mazur, J. Am. Chem. Soc. 1996, 118,
7197–7202.
[70] X. Lu, K. W. Hipps, J. Phys. Chem. B 1997, 101, 5391–5396.
[71] K. S. Mali, L. Zçphel, O. Ivasenko, K. Müllen, S. de Feyter, Chem. Asian J.
2013, 8, 2497–2505.
[51] C. F. van Nostrum, R. J. M. Nolte, Chem. Commun. 1996, 1996, 2385–
2392.
[52] J. F. Van der Pol, E. Neeleman, J. C. Van Miltenburg, J. W. Zwikker, R. J. M.
Nolte, W. Drenth, Macromolecules 1990, 23, 155–162.
[53] M. Fischer, G. Lieser, A. Rapp, I. Schnell, W. Mamdouh, S. de Feyter, F. C.
de Schryver, S. Hçger, J. Am. Chem. Soc. 2004, 126, 214–222.
[54] D. Zhao, J. S. Moore, J. Org. Chem. 2002, 67, 3548–3554.
[55] U. Beginn, G. Zipp, A. Mourran, P. Walther, M. Mçller, Adv. Mater. 2000,
12, 513–516.
[72] L. Zçphel, K. S. Mali, P. S. Reddy, M. Wagner, S. de Feyter, W. Pisula, K.
Müllen, Chem. Eur. J. 2012, 18, 3264–3276.
[56] Y. Chen, W. Su, M. Bai, J. Jiang, X. Li, Y. Liu, L. Wang, S. Wang, J. Am.
Chem. Soc. 2005, 127, 15700–15701.
[57] C. F. van Nostrum, S. J. Picken, R. J. M. Nolte, Angew. Chem. Int. Ed. Engl.
1994, 33, 2173–2175; Angew. Chem. 1994, 106, 2298–2300.
Received: December 23, 2014
Published online on April 13, 2015
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