Multicomponent assembly of heterometallic isosceles triangles

Article abstract of DOI:10.1021/ic8021084

The multicomponent synthesis and solution-state characterization of three supramolecular bis-heterometallic isosceles triangles are elaborated. The triangular assemblies are isosceles both geometrically and chemically; they comprise multiple ligands, meta

Full text of DOI:10.1021/ic8021084

Inorg. Chem. 2009, 48, 822-824
Multicomponent Assembly of Heterometallic Isosceles Triangles
Michael Schmittel* and Kingsuk Mahata
Center of Micro- and Nanochemistry and Engineering, Organische Chemie I, UniVersita¨t Siegen,
Adolf-Reichwein-Strasse 2, D-57068 Siegen, Germany
Received November 4, 2008
The multicomponent synthesis and solution-state characterization
of three supramolecular bis-heterometallic isosceles triangles are
elaborated. The triangular assemblies are isosceles both geo-
metrically and chemically; they comprise multiple ligands, metals,
and binding motifs. Variation of the length of one side of the triangle
by changing the number of phenyl spacers n ) 0, 1, and 2
influences the redox potential of the opposing copper(I) center,
allowing translation of the nanomechanical changes into electroni-
cally readable values.
The deficiencies and limitations of the presently used
metal-coordination toolkits are so pronounced that even the
clean preparation of a supramolecular triangle, formally the
simplest and smallest member of the two-dimensional family
of macrocycles, has been generating immense problems.
Typically, supramolecular triangles show up in equilibrium
with larger aggregates, such as squares or other polygons,7
so that only in very few cases exclusive formation of the
triangle was ascertained.8,9 Furthermore, the reported tri-
angles so far are usually homometallic as well as equilateral,
both geometrically and chemically (Figure 1a).10 In the
following, we will demonstrate that the until now unknown
geometrically and chemically isosceles supramolecular tri-
angles (Figure 1d) can be prepared in a clean heterometallic
form. This progress is a promising step toward the geo-
metrically and chemically scalene triangle (Figure 1e), the
holy grail of supramolecular triangles.
Over the last 2 decades, the design and fabrication of
supramolecular nanoarchitectures using self-assembly have
received considerable attention.1 Among the various non-
covalent binding tools to build intricate assemblies, metal
coordination has turned out to be one of the most successful
protocols. Even multifaceted construction algorithms can
easily be stored as information in the metal centers and
molecular components, allowing reliable readout of the
encoded instructions. Thus, metallosupramolecular ap-
proaches have been utilized to forge a large assortment of
fascinating two- and three-dimensional nanoarchitectures.1
The majority thereof, however, is comprised of only two
different components and a single binding motif. This
limitation suggests the need to explore ways to increase
complexity and diversity,2 particularly as higher-order mul-
ticomponent architectures2b using coordination-driven self-
assembly remain rare.3-6
To overcome the problems associated with angular com-
pression at acute angles, an increased challenge for geo-
(4) Chichak, K. S.; Cantrill, S. J.; Pease, A. R.; Chiu, S.-H.; Cave,
G. W. V.; Atwood, J. L.; Stoddart, J. F. Science 2004, 304, 1308.
(5) Christinat, N.; Scopelliti, R.; Severin, K. Angew. Chem., Int. Ed. 2008,
47, 1848.
(6) Kishore, R. S. K.; Paululat, T.; Schmittel, M. Chem.sEur. J. 2006,
12, 8136.
(7) (a) Sautter, A.; Schmid, D. G.; Jung, G.; Wu¨rthner, F. J. Am. Chem.
Soc. 2001, 123, 5424. (b) Kraus, T.; Budeˇsˇínsky´, M.; Cvacˇka, J.;
Sauvage, J.-P. Angew. Chem., Int. Ed. 2006, 45, 258. (c) Uehara, K.;
Kasai, K.; Mizuno, N. Inorg. Chem. 2007, 46, 2563. (d) Ferrer, M.;
Gutie´rrez, A.; Mounir, M.; Rossell, O.; Ruiz, E.; Rang, A.; Engeser,
M. Inorg. Chem. 2007, 46, 3395. (e) Champin, B.; Sartor, V.; Sauvage,
J.-P. New J. Chem. 2008, 32, 1048. (f) Sun, Q.-F.; Wong, K. M.-C.;
Liu, L.-X.; Huang, H.-P.; Yu, S.-Y.; Yam, V. W.-W.; Li, Y.-Z.; Pan,
Y.-J.; Yu, K.-C. Inorg. Chem. 2008, 47, 2142. (g) Coronado, E.; Galan-
Mascaros, J. R.; Gavin˜a, P.; Mart´ı-Gastaldo, C.; Romero, F. M.; Tatay,
S. Inorg. Chem. 2008, 47, 5197. (h) Weilandt, T.; Troff, R. W.; Saxell,
H.; Rissanen, K.; Schalley, C. A. Inorg. Chem. 2008, 47, 7588.
(8) (a) Kryschenko, Y. K.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J. Am.
Chem. Soc. 2003, 125, 5193, and references cited therein. (b) Vicente,
J.; Chicote, M.-T.; Alvarez-Falco´n, M. M.; Jones, P. G. Chem.
Commun. 2004, 2658. (c) Cotton, F. A.; Murillo, C. A.; Wang, X.;
Yu, R. Inorg. Chem. 2004, 43, 8394. (d) Mimassi, L.; Guyard-
Duhayon, C.; Rager, M. N.; Amouri, H. Inorg. Chem. 2004, 43, 6644.
(e) Heo, J.; Jeon, Y.-M.; Mirkin, C. A. J. Am. Chem. Soc. 2007, 129,
7712. (f) Ghosh, S.; Turner, D. R.; Batten, S. R.; Mukherjee, P. S.
Dalton Trans. 2007, 1869. (g) Mukherjee, P.; Drew, M. G. B.; Estrader,
M.; Ghosh, A. Inorg. Chem. 2008, 47, 7784.
* To whom correspondence should be addressed. E-mail: schmittel@
chemie.uni-siegen.de.
(1) (a) Lehn, J.-M. Chem.sEur. J. 2000, 6, 2097. (b) Kovbasyuk, L.;
Kra¨mer, R. Chem. ReV. 2004, 104, 3161. (c) Drain, C. M.; Goldberg,
I.; Sylvain, I.; Falber, A. Top. Curr. Chem. 2005, 245, 55. (d) Fiedler,
D.; Leung, D. H.; Bergman, R. G.; Raymond, K. N. Acc. Chem. Res.
2005, 38, 351. (e) Schmittel, M.; Kalsani, V. Top. Curr. Chem. 2005,
245, 1. (f) Di Nicola, C.; Karabach, Y. Y.; Kirillov, A. M.; Monari,
M.; Pandolfo, L.; Pettinari, C.; Pombeiro, A. J. L. Inorg. Chem. 2007,
46, 221. (g) Nakabayashi, K.; Ozaki, Y.; Kawano, M.; Fujita, M.
Angew. Chem., Int. Ed. 2008, 47, 2046.
(2) (a) Yang, H.-B.; Ghosh, K.; Northrop, B. H.; Zheng, Y.-R.; Lyndon,
M. M.; Muddiman, D. C.; Stang, P. J. J. Am. Chem. Soc. 2007, 129,
14187. (b) Schmittel, M.; Mahata, K. Angew. Chem., Int. Ed. 2008,
47, 5284.
(9) Schmittel, M.; Mahata, K. Chem. Commun. 2008, 2550.
(10) Definitions: a chemically equilateral triangle is a triangle with the same
ligand; a geometrically equilateral triangle is a triangle with sides of
the same length.
(3) Pentecost, C. D.; Chichak, K. S.; Peters, A. J.; Cave, G. W. V.; Cantrill,
S. J.; Stoddart, J. F. Angew. Chem., Int. Ed. 2007, 46, 218.
822 Inorganic Chemistry, Vol. 48, No. 3, 2009
10.1021/ic8021084 CCC: $40.75  2009 American Chemical Society
Published on Web 01/07/2009

COMMUNICATION
Scheme 2. Self-Assembly of Isosceles Triangles Tn
Figure 1. Cartoon representation of different types of triangles, both homo-
and heterometallic.
Figure 2. Cartoon representation of the self-assembly. A kinetically locked
angular hinge is reacted in a metallosupramolecular heteroleptic aggregation
with the third side.
14
ation with ligand Bn opens a direct venue to a variety of
isosceles triangles Tn (n ) 0, 1, and 2; Scheme 2).
Synthesis of hinge A, similar to an analogous hinge with
phenanthroline terminals, was realized.9 First, a kinetically
locked bis(phenanthroline)copper(I) complex functionalized
with alkynes was prepared that was further equipped in a
Sonogashira coupling with two terpyridine terminals. Fab-
rication of the isosceles triangles Tn was carried out by
adding A to an acetonitrile solution of bisphenanthroline Bn
and Zn2+ at 60 °C for 2 h. In another set of experiments, all
components were taken up together in acetonitrile, refluxing
the solution for 6 h. Exclusive formation of the isosceles
triangle Tn was confirmed in both cases. In the latter case,
we initially observed the formation of Tn along with a
homoleptic complex of A (mainly [Zn2(A)2]6+), but with
elapsed time, self-repair occurred, leading to the exclusive
formation of Tn. Because of the required self-correction, the
reaction time was longer. Each triangle Tn (n ) 0, 1, and 2)
was characterized by clean electrospray ionization mass
spectra (ESI-MS), showing exclusively signals of the ex-
pected triangle (Figure 3, top, and the Supporting Informa-
Scheme 1. Synthesis of Angular Complex A
metrically isosceles supramolecular triangles (Figure 1c,d),
we used benefits from entropy and heteroleptic aggregation
as provided by the PHENLOCK11 and HETTAP12 protocols.
The PHENLOCK concept allows the preparation of a
kinetically locked copper(I) bis(phenanthroline) complex11
that is useful as a moderately flexible hinge unit (Figure 2).
The combination of the latter with a third side through
heteroleptic aggregation along the HETTAP13 concept leads
to the desired geometrically and chemically isosceles tri-
angles. In addition, the use of Cs-symmetric HETTAP
complex units also precludes the formation of diastereomers
as received in our earlier work on geometrically equilateral
triangles.9 Thus, the chiral hinge A (Scheme 1) containing
a kinetically locked copper(I) bis(phenanthroline) complex
and two terpyridine terminals was conceived. Its amalgam-
1
tion), H and 13C NMR, and elemental analysis.
1H NMR data nicely support the formation of the triangles
Tn. The upfield shifts of the mesityl protons (Figure 3,
bottom; δ ≈ 7.0 ppm in ligand Bn, δ ≈ 6.2 ppm in triangle
Tn) are most diagnostic for heteroleptic complex formation
as established earlier.12 Likewise, because of the strong
shielding effect of the bisphenanthrolines Bn, the R protons
(g, g′) of the terpyridine unit in Tn experienced a notable
upfield shift. Downfield shifts of the 4′′′ and 7′′′ protons in
the phenanthroline units of Bn are also clearly indicative of
the self-assembled triangular structure.
(11) Kalsani, V.; Schmittel, M.; Listorti, A.; Accorsi, G.; Armaroli, N. Inorg.
Chem. 2006, 45, 2061.
(12) (a) Schmittel, M.; Kalsani, V.; Kishore, R. S. K.; Co¨lfen, H.; Bats,
J. W. J. Am. Chem. Soc. 2005, 127, 11544. (b) Schmittel, M.; Kalsani,
V.; Mal, P.; Bats, J. W. Inorg. Chem. 2006, 45, 6370. (c) Schmittel,
M.; Mal, P. Chem. Commun. 2008, 960.
(13) The HETTAP approach to heteroleptic terpyridine and phenanthroline
complexes allows one to control the dynamic and heteroleptic
aggregation of phenanthroline and terpyridine ligands at a single metal
center; see ref 12.
The single phenanthroline-proton set is clear evidence for
the existence of just one single triangle in solution; no
isomeric species were detectable in variable-temperature
NMR (+25 to -25 °C; see the Supporting Information).
(14) Schmittel, M.; Michel, C.; Wiegrefe, A.; Kalsani, V. Synthesis 2001,
1561.
Inorganic Chemistry, Vol. 48, No. 3, 2009 823

COMMUNICATION
Figure 4. Energy-minimized structure of isosceles triangle T0 (hydrogen
atoms are removed for clarity). Counteranions are not included because
19F NMR spectra indicated them to be free (see the Supporting Information).
The direct relationship between the geometry and the redox
potential of copper(I) complexes15 prompted us to investigate
the redox behavior of the triangles Tn. Because the bisecting
angle between the two phenanthrolines at the copper(I) center
is expected to vary with the changing length of the
bisphenanthroline Bn, a change in the redox potential was
expected. Differential pulse voltammetry of the triangles Tn
reflected this trend. Although the bulky aromatic groups at
the PHENLOCK hinge unit prevent pronounced geometric
changes, a shift by 14 mV was observed among the triangles
Figure 3. Top: ESI-MS spectrum of T2 in acetonitrile. Bottom: Partial 1H
NMR (600 MHz, 298 K) spectra of A (CD2Cl2), B0 (CD2Cl2), and T0
(CD3CN).
Because of the stereogenic unit in A, protons of the
terpyridine units and the aromatic substituents of bisphenan-
throline are expected to be diastereotopic in Tn. Indeed, this
could be confirmed by 1H NMR, with all assignments being
verified by COSY spectra.
Tn as a function of n ) 0, 1, and 2 [E1/2(T0) ) 0.798 VSCE
;
E1/2(T1) ) 0.784 VSCE; E1/2(T2) ) 0.784 VSCE; Supporting
Information].
Variations in the length of the bisphenanthroline Bn are
expected to cause either a change in the geometry near the
metal centers and/or a bending in the ethynyl moieties. In
In conclusion, we have fabricated the first supramolecular
triangles Tn, which are heterometallic and isosceles, both
geometrically and chemically. They contain two different
metals, two different ligands, and multiple binding motifs.
Despite such complexity, the yield is quantitative. The
angular hinge A is a key to the successful realization of Tn
because it is able to adapt to the length of the bisphenan-
throline Bn, clearly seen from 1H NMR and redox data. Thus,
the assemblies are not only a class of challenging nanoar-
chitectures but also promising test cases for nanomechanical
devices,16 such as adjustable tweezers.17
1
fact, H NMR data hint that both processes play a role.
Because of the reduced steric bulk at the two zinc(II) centers,
their geometric adjustment is more pronounced, as evinced
from the large chemical shift changes of the surrounding
protons. On the other hand, chemical shifts are affected less
near the kinetically inert copper(I) center in Tn as a function
of n ) 0, 1, and 2. Changes in the chemical shifts at the
aromatic spacer in T1 and T2 suggest some bending near
the ethynyl moieties. Changes in the 13C NMR resonance of
the ethynyl carbons [δ(T0) ) 90.9 and 98.1 ppm; δ(T1) )
90.4 and 98.4 ppm; δ(T2) ) 90.5 and 99.9 ppm for the
ethynyl groups near the copper(I) center] also support this
view.
Acknowledgment. We are thankful to the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for financial support.
To have further insight into the geometry of the isosceles
triangles, solid-state structures would certainly be desirable.
Unfortunately, all efforts directed toward the generation of
single crystals suitable for X-ray analysis proved to be
unsuccessful. Alternatively, we performed MM+ computa-
tions, providing ZnII-CuI and ZnII-ZnII distances of 1.59
and 1.28 nm, respectively, for the energy-minimized structure
of T0 (Figure 4). In combination with the numbers (Sup-
Supporting Information Available: Experimental procedures
and spectroscopic data provided for A and Tn. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC8021084
(15) Ambundo, E. A.; Deydier, M.-V.; Grall, A. J.; Aguera-Vega, N.;
Dressel, L. T.; Cooper, T. H.; Heeg, M. J.; Ochrymowycz, L. A.;
Rorabacher, D. B. Inorg. Chem. 1999, 38, 4233.
(16) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007,
46, 72.
(17) (a) Petitjean, A.; Khoury, R. G.; Kyritsakas, N.; Lehn, J.-M. J. Am.
Chem. Soc. 2004, 126, 6637. (b) Abe, A. M. M.; Helaja, J.; Koskinen,
A. M. P. Org. Lett. 2006, 8, 4537.
II
I
II
II
porting Information) for T1 (dZn -Cu ) 1.61 nm; dZn -Zn
)
II
I
II
II
1.61 nm) and T2 (dZn -Cu ) 1.62 nm; dZn -Zn ) 1.94 nm),
a trend becomes apparent: the angular complex A in Tn (n
) 0, 1, and 2) is increasingly bent with smaller n.
824 Inorganic Chemistry, Vol. 48, No. 3, 2009