Dynamic and fluorescent nanoscale phenanthroline/terpyridine zinc(II) ladders. Self-recognition in unlike ligand/like metal coordination scenarios

Article abstract of DOI:10.1021/ja0525096

A new coordination kit, the HETTAP approach, is described, which allows one to prepare supramolecular heteroleptic metal phenanthroline/terpyridine racks and ladders in basically quantitative yield. Due to the dynamic nature of the nanoladder structures, a two-step double-transmetalation and scenarios with like metal/unlike ligand recognition were realized. Copyright

Full text of DOI:10.1021/ja0525096

Published on Web 07/28/2005
Dynamic and Fluorescent Nanoscale Phenanthroline/Terpyridine Zinc(II)
Ladders. Self-Recognition in Unlike Ligand/Like Metal Coordination Scenarios
Michael Schmittel,*,‡ Venkateshwarlu Kalsani,‡ Ravuri S. K. Kishore,‡ Helmut Co¨lfen,§ and
Jan W. Bats†
Center of Micro and Nanochemistry and Engineering, Organische Chemie I, UniVersita¨t Siegen,
Adolf-Reichwein-Str., D-57068 Siegen, Germany, Max-Planck-Institut fu¨r Kolloid und Grenzfla¨chenforschung,
Golm, 14424 Potsdam, Germany, and Institut fu¨r Organische Chemie und Chemische Biologie, Johann Wolfgang
Goethe-UniVersita¨t, Marie-Curie-Strasse 11, D-60439 Frankfurt am Main, Germany
Received April 18, 2005; E-mail: schmittel@chemie.uni-siegen.de
Chart 1. Ligands Used in This Study
The availability of reliable metal-ligand combinations, such as
Pt(Pd)-N(P), Pt-CtC, etc., is a conditio sine qua non for the
erection of intricate metallosupramolecular architectures.1 While
eye-catching but mostly single component aggregates have been
widely prepared,1 our tools to assemble well-defined and robust
multicomponent structures are still limited. Thus, homoleptic
assemblies, such as grids1e and squares,1a-d,g predominate in the
literature,1f while ladder motifs, due to the lack of heteroleptic metal
ligand combinations and thus of control in dynamic multicomponent
aggregation, are restricted to a few reports.2
So far, five-coordinated metal centers with phenanthrolines (or
Scheme 1. Multicomponent Self-Assembly of Rack R1 and
Ladders L1,L2 (cartoon representation)
2,2′-bipyridines) and terpyridines as ligands have been of little utility
in dynamic heteroleptic aggregation,3 except in highly restricted
topologies4c or cooperative helicates.4a,b Clearly, a quantitative yield
can only be realized if the formation of four- and six-coordinated
complexes is suppressed. Herein, we show that properly designed
phenanthrolines can be instructed for quantitative heteroleptic
complexation with terpyridine ligands in the presence of Zn2+, Hg2+
,
and even Cu+. Their superior value is illustrated by the synthesis
of dynamic and fluorescent nanoscale ladders, by facile metal
exchange escorted by OFF-ON-OFF fluorescence signaling, and
by the realization of an unprecedented like metal/unlike ligand
recognition scenario.
with Zn(OTf)2 (1:1:2 equiv, Scheme 1b). All spectroscopic data
confirmed the clean formation and integrity of the ladders L1,L2
that exhibited a distinct fluorescence (λem ) 463 and 468 nm,
respectively [(λexc ) 389 nm]) in acetonitrile at room temperature
(Supporting Information). The 1H NMR showed a single set of sharp
signals, with the mesityl protons being high field shifted (in 2,3: δ
∼7 ppm; in L1,L2: δ ∼6.1 ppm). Peak assignments were
corroborated by COSY experiments. 13C NMR showed single sets
of the expected signals. ESI MS analysis of L1,L2 further
substantiated the proposed structures by showing isotopically
resolved signals, for example, for ladder L1 at m/z ) 485.1,
539.2, 653.9, 814.4, 1055.3, 1455.8, corresponding to [Zn4(2)2(5)2-
(OTf)n](8-n)+ with n ) 0, 1, ...5 (see Supporting Information). Strong
confirmation for the integrity of L1,L2 in solution was obtained
using analytical ultracentrifugation.9 Through sedimentation velocity
experiments of L1, a single species with a molecular weight of
5020 ( 500 Da (measured at 45 µg mL-1) was detected in KOTF-
saturated acetone in good agreement with the expected weight of
L1 (4817.5 Da; see Supporting Information).
Ultimate structural proof was acquired by a single-crystal X-ray
analysis of L2.10 L2 possesses a nanoscale ladder structure, in which
two 3 and 5 assemble about four Zn2+ ions (Zn-Zn diagonals are
2.2 and 2.5 nm; Figure 1). The terpyridine units of 5 are sandwiched
between the 2,9-aryl groups of the phenanthrolines of 3, allowing
for stabilizing π-π interactions (d ) 3.4 Å). Hence, presence of
the 2,9-steric stoppers in 3 forces Zn(II) ions to adapt a heavily
distorted trigonal bipyramidal pentacoordination, with three posi-
tions being filled from the terpyridine unit and two from the
Phenanthrolines designed with steric stoppers according to the
HETPHEN concept5 (e.g., with 2,9-dimesityl/2,9-diduryl; cf. 2, 3)
are instructed to avoid self-association in the presence of many
metal ions, such as Cu(I), Ag(I), and Zn(II) ions (Chart 1).6 Thus,
in combination with sterically innocent phenanthrolines, merely
heteroleptic bisphenanthroline complexes form.6 A like protocol
was now evaluated with terpyridines as partners for HETPHEN
ligands. Hence, 1-37 were tested for their ability to recognize
ligands 4-5 in the presence of zinc(II) ions.8 With 1a, 5, and zinc-
(II) ions, only homoleptic combinations were detected by ESI MS
and 1H NMR (see Supporting Information). Evidently, self-
recognition is the dominating phenomenon, as also recognized by
other authors.3 In contrast, from the reaction of 2, 4, and Zn(OTf)2,
only the rack assembly R1 ) [Zn2(2)(4)2]4+ emerged, as evidenced
1
by ESI MS, H NMR, 13C NMR, and UV/vis (Scheme 1a). For
example, the 1H NMR showed the expected high field shift (∆δ )
0.8 ppm) for the mesityl protons of 2.6 Rack R1 proved to be
fluorescent (λem ) 456 nm) upon excitation at 389 nm (see
Supporting Information).
Along the same strategy, ladders L1 ) [Zn4(2)2(5)2]8+ and L2
) [Zn4(3)2(5)2]8+ were furnished upon reaction of 2 or 3 and 5
‡ Universita¨t Siegen.
§ Max-Planck-Institut fu¨r Kolloid und Grenzfla¨chenforschung.
† Johann Wolfgang Goethe-Universita¨t.
9
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J. AM. CHEM. SOC. 2005, 127, 11544-11545
10.1021/ja0525096 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3. (a) A Two-Step Transmetalation off-on-off
Fluorescence System. (B) Photo Taken under UV Lamp (254 nm)
Figure 1. (a) Single-crystal structure stick representation of L2. (b) Space
filling representation. For clarity, H atoms, solvent molecules, and anions
are omitted.
addition of Hg(II) (Scheme 3). The transmetalations were conve-
niently followed by ESI MS and fluorescence spectrometry (see
Supporting Information).
Table 1. Binding Constants for Various Heteroleptic Metal
Phenanthroline (L) Terpyridine (T) Complexes [M(4)(6)]n+ in
Dichloromethane
In conclusion, a new coordination toolkit, the HETTAP approach,
was developed. It allows one to prepare heteroleptic metal phenan-
throline/terpyridine aggregates in basically quantitative yield. Due
to the dynamic nature of the nanoladder structures, a two-step
double-transmetalation and a like metal/unlike ligand recognition
scenario were realized. Further studies are in progress to study
host-guest complexation and photophysical properties of these
unique multicomponent structures.
M
log KM
log KM
log â21
ML
MLT
Cu+
5.0 ( 0.2
5.9 ( 0.6
6.7 ( 0.6
4.3 ( 0.9
6.5 ( 0.2
8.0 ( 0.1
9.3
Zn2+
Hg2+
12.4a
14.7a
a With 0.1 vol % of methanol.
Scheme 2. Cartoon Representation of the Like Metal/Unlike
Ligand Recognition Scenario
Acknowledgment. We are very much indebted to the DFG for
continued financial support, and to Antje Vo¨lkel for experimental
help with the analytical ultracentrifugation experiments.
Supporting Information Available: 1H and 13C spectra for all
compounds and crystallographic data for L2 (cif). This material is
available free of charge via the Internet at http://pubs.acs.org.
phenanthroline ligand. To our knowledge, this is the first structurally
characterized nanoscale multicomponent ladder assembly combining
both bi- (2 or 3) and tridentate ligands (5).
References
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species L3 ) ([Zn4(2)(3)(5)2]8+) built up beside L1 and L2, as
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1
verified by ESI-MS and H NMR (see Supporting Information).
Hence, the above results demonstrate that a robust and reliable
approach to dynamic heteroleptic terpyridine and phenanthroline
(HETTAP) complexation has been developed. Analogously, we
prepared other metal ladders, such as L4 ) [Cu4(2)2(5)2]4+ and L5
) [Hg4(2)2(5)2]8+. The low binding constant in Table 1 suggests
that Cu+ is only four-coordinated in [Cu(4)(6)]+, while the five-
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Combination of the structural robustness of L1,L2 with its
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unprecedented level of recognition.11 Such phenomena had earlier
been studied by Lehn,11a who was able to demonstrate that from a
mixture of oligobipyridine strands and a mixture of metal ions [Cu-
(I) and Ni(II)] only helicates composed of like ligands and like metal
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from this cocktail of ligands and metal ions, that is, Zn-ladder L1
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1
and Cu-grid G16c (Scheme 2). Despite extensive ESI MS and H
NMR checks, no other aggregates were detectable. Hence, this
reaction is a unique example of a 2-fold like metal + unlike ligand
recognition scenario that leads to the clean formation of two
dynamic nanoaggregates.
L4 and L5 did not fluoresce quite in contrast to L1 (see
Supporting Information). On the basis of the binding constants (see
Table 1), a two-step double-transmetalation off-on-off fluores-
cence system was designed. Upon addition of Zn(II) salt to L4
(nonfluorescent; OFF), L1 was afforded (fluorescent; ON) that could
be further transformed to L5 (nonfluorescent; OFF) upon the
(10) The crystals were extremely solvent-sensitive and diffracted only poorly.
Crystal data of L2: C216H156F24N20O28S8Zn4, M ) 4453.57, monoclinic,
space group P21/n, a ) 21.715(6) Å, b ) 19.196(6) Å, and c )
28.620(7) Å, â ) 94.851(17)°, V ) 11887(5) A3, T ) 157(2) K, Z ) 2,
Dc ) 1.244 g/cm3, λ(Mo KR) ) 0.71073 Å, 101 150 reflections measured,
17 215 unique (Rint ) 0.2147) which were used in all calculations. R1 )
0.2315 (I >2θ (I)) and wR2 ) 0.4456, GOF ) 1.638; max/min residual
density 1.272/-0.556 e Å-3. See also Supporting Information.
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