Metal-driven and covalent synthesis of supramolecular grids from racks: A convergent approach to heterometallic and heteroleptic nanostructures

Article abstract of DOI:10.1021/ic050088n

Supramolecular nanogrids were prepared from dynamic supramolecular racks through the coupling of terminal alkynes using either a covalent (with CuCl/O₂) or a coordinative (with [trans-(PEt₃) ₂PtCl₂]) approach. Because of the rapid equilibration of the racks (as tested by exchange reactions), oligomeric adducts potentially formed in the coupling process will selectively furnish the nanogrids through an entropically driven self-repair mechanism. To ascertain the structural assignment, the nanogrids were also synthesized by an independent strategy.

Full text of DOI:10.1021/ic050088n

Inorg. Chem. 2005, 44, 4115−4117
Metal-Driven and Covalent Synthesis of Supramolecular Grids from
Racks: A Convergent Approach to Heterometallic and Heteroleptic
Nanostructures
Michael Schmittel,*,† Venkateshwarlu Kalsani,† and Jan W. Bats‡
Center of Micro and Nanochemistry and Engineering, Organische Chemie I, UniVersita¨t Siegen,
Adolf-Reichwein-Strasse, D-57068 Siegen, 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 January 19, 2005
Supramolecular nanogrids were prepared from dynamic supra-
molecular racks through the coupling of terminal alkynes using
either a covalent (with CuCl/O2) or a coordinative (with [trans-
(PEt3)2PtCl2]) approach. Because of the rapid equilibration of the
racks (as tested by exchange reactions), oligomeric adducts
potentially formed in the coupling process will selectively furnish
the nanogrids through an entropically driven self-repair mechanism.
To ascertain the structural assignment, the nanogrids were also
synthesized by an independent strategy.
pramolecular nanoscale structures, such as nanobaskets,7
ring-in-ring structures,8 nanoscaffolds,9 nanoracks, and nano-
grids10 were afforded that, by their heteroleptic nature, are
ideally suited for functional applications.11
We recently reported on dynamic bisphenanthroline cop-
per(I) racks10b that contrast to the large multitude of
kinetically stable ruthenium-based complexes.2,12 Herein, we
describe the self-assembly of functionalized rack motifs that
are equipped with bromo and alkyne termini. The latter
feature suggested to us that we use the racks as versatile
precursors12 for supramolecular nanostructures, such as
pseudorotaxanes/rotaxanes, cylinders, or grids (Figure 1). As
a demonstration of the power of this approach, we present
two strategies, a covalent route and a coordinative route, to
fabricate heteroleptic grids directly from supramolecular
racks.
Ligands 1 and 2 were prepared by known procedures13
using sequential Sonogashira coupling reactions (Chart 1).
The synthesis of phenanthrolines 3-5, whose 2,9-disubsti-
tution with bulky aryl groups (mesityl ) Mes or duryl )
Dur) is dictated by the HETPHEN6 approach, was described
elsewhere.8,13
The masterful use of metal coordination chemistry is a
conditio sine qua non for the preparation of discrete
metallosupramolecular aggregates, such as cylinders,1 racks,2
nanoscaffolds,1 and grids.3 To date, most of these structures
have been assembled using bis- or polyhomoleptic coordina-
tion motifs, but there is growing interest in preparing
heteroleptic aggregates,4 for which several strategies have
been designed.1 In this context, we have recently introduced
the HETPHEN5 approach, allowing heteroleptic bisphenan-
throline metal complexes of tetrahedrally coordinated metal
ions to be built.6 Utilizing this methodology, several su-
The combination of ligand 1 or 2 with 310b or 4 in the
presence of [Cu(CH3CN)4]+ (1:2:2) produced the racks R1-
R4 in basically quantitative yields. For all racks R1-R4,
the ESI-MS exhibited a single set of signals resulting from
the successive loss of counterions (see Supporting Informa-
* To whom correspondence should be addressed. E-mail: schmittel@
chemie.uni-siegen.de. Fax: (+49) 271 740 3270.
† Universita¨t Siegen.
‡ Johann Wolfgang Goethe-Universita¨t.
(1) Schmittel, M.; Kalsani, V. Top. Curr. Chem. 2005, 245, 1-53.
(2) (a) Sleiman, H.; Baxter, P. N. W.; Lehn, J.-M.; Airola, K.; Rissanen,
K. Inorg. Chem. 1997, 36, 4734-4742. (b) Brown, D.; Zong, R.;
Thummel, R. P. Eur. J. Inorg. Chem. 2004, 3269-3272 and references
therein.
(3) Ruben, M.; Rojo, J.; Romero-Salguero, F. J.; Uppadine, L. H.; Lehn,
J.-M. Angew. Chem., Int. Ed. 2004, 43, 3644-3662.
(4) Schalley, C. A. Angew. Chem., Int. Ed. 2004, 43, 4399-4401.
(5) The HETPHEN (heteroleptic bisphenanthroline) approach is based on
steric and electronic effects originating from bulky aryl substituents
at the bisimine coordination sites (as seen in 3-5, 7, and 8) to control
the coordination equilibrium both kinetically and thermodynamically.
(6) (a) Schmittel, M.; Ganz, A. Chem. Commun. 1997, 999-1000.
(b) Schmittel, M.; Lu¨ning, U.; Meder, M.; Ganz, A.; Michel, C.;
Herderich, M. Heterocycl. Commun. 1997, 3, 493-494.
(7) Kalsani, V.; Ammon, H.; Ja¨ckel, F.; Rabe, J. P.; Schmittel, M. Chem.
Eur. J. 2004, 21, 5481-5492.
(8) Schmittel, M.; Ganz, A.; Fenske, D. Org. Lett. 2002, 4, 2289-2292.
(9) Schmittel, M.; Ammon, H.; Kalsani, V.; Wiegrefe, A.; Michel, C.
Chem. Commun. 2002, 2566-2567.
(10) (a) Schmittel, M.; Kalsani, V.; Fenske, D.; Wiegrefe, A. Chem.
Commun. 2004, 490-491. (b) Kalsani, V.; Bodenstedt, H.; Fenske,
D.; Schmittel, M. Eur. J. Inorg. Chem. 2005, 1841-1849.
(11) Schmittel, M.; Kalsani, V.; Kienle, L. Chem. Commun. 2004, 1534-
1535.
(12) Benaglia, M.; Ponzini, F.; Woods, C. R.; Siegel, J. S. Org. Lett. 2001,
3, 967-969.
(13) Schmittel, M.; Michel, C.; Wiegrefe, A.; Kalsani, V. Synthesis 2001,
1561-1567.
10.1021/ic050088n CCC: $30.25
Published on Web 05/12/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 12, 2005 4115

COMMUNICATION
Scheme 1. Synthesis of Grids G1 and G215 under Oxidative
Conditions from Racks R3 and R4, Respectivelya
Figure 1. Cartoon representation of a rack as a potential precursor to
cylinder, pseudorotaxane/rotaxane, and grid motifs.
a ESI-MS signal of G14+ provided as inset.
Chart 1. Ligands Used for the Construction of Heteroleptic,
Functionalized Racks
Figure 2. Single-crystal structure of R5; stick representation.
data15 were equally in accordance with those of previously
reported nanogrid structures.10a
Recently, connecting terminal alkynes through Pt(II) or
Pd(II) centers by a coordinative approach was utilized by
Lin et al.16 for the formation of polygons. As an extension,
we decided to probe the self-recognition of Cu(I) ions and
phenanthrolines and that of Pt(II) ions and alkynyl ligands
in one pot. The mixing of 5, 6, and [Cu(CH3CN)4]+ (1:1:1,
respectively) in dichloromethane at room temperature,7
followed by a posttreatment with [trans-(PEt3)2PtCl2],16
afforded the heterometallic rack R5 exclusively, as verified
tion). Moreover, the characteristic high-field shifts in the
1H NMR spectra for the MesH and DurMe protons of 3 and
4 in R1-R4 were indicative of the formation of a hetero-
leptic bisphenanthroline complex.6
1
by H NMR, ESI-MS, and 31P{H} NMR (δ 13.3 ppm)
analysis. The structure of R5 was confirmed by crystal
structure analysis (Figure 2).17 The molecule is centrosym-
metric with the Pt atom at a crystallographic inversion
center.10 The Cu atom shows a heavily distorted tetrahedral
coordination sphere. N-Cu-N angles range from 82.1(2)°
to 141.5(2)°. The Cu-N distances range from 1.989(5) to
2.088(4) Å. The Pt atom exhibits a square-planar coordina-
tion, and the Pt-P distances of 2.296(2) Å and Pt-C
distances of 1.976(6) Å agree with values found in related
structures.18 The crystal packing showed an intermolecular
As mentioned above, racks with functional groups, such
as terminal alkynes, can serve as precursors for higher
supramolecular aggregates (Figure 1). In a first try, R3 and
R4 were reacted under oxidative conditions typical for the
homocoupling of terminal alkynes (O2, CuCl, NEt3,
CH2Cl2)14 to furnish G1 (89%) and G2 (94%) in high yield
1
(see Supporting Information). H NMR spectra confirmed
(15) Broadening of a few 1H NMR signals tentatively points to the presence
of several isomers, such as cisoid, transoid, and intertwined grid
isomers.
the grid structure, showing a single set of signals character-
ized by the lack of the acetylene proton at δ ∼3.5 ppm and
the presence of a diagnostic signal for the heteroleptic
complex at δ ∼6 ppm (MesH). Moreover, the ESI-MS
analysis showed the signals of the grids G1 and G2 (see
Supporting Information and Scheme 1) with a correct isotopic
distribution of the 3+ and 4+ charged species. All other
(16) Jiang, H.; Lin, W. J. Am. Chem. Soc. 2004, 126, 7426-7427 and
references therein.
(17) Crystal data for R5: Crystals were obtained by slow diffusion of
toluene into a dichloromethane solution of the complex. C100H100N8P2-
Cu2Pt‚2PF6‚2toluene, triclinic, space group P1h, a ) 12.893(3) Å,
b ) 13.701(3) Å, c ) 16.224(3) Å, R ) 76.032(17)°, â ) 80.039-
(17)°, γ ) 73.553(17)°, V ) 2650.4(9) Å3, Z ) 1, Dcalcd ) 1.424
Mg/m3, µ ) 1.847 mm-1, final R indices [I > 2σ(I)] R1 ) 0.0592,
wR2 ) 0.1195, R indices (all data) R1 ) 0.1143, wR2 ) 0.1361.
(18) Osakada, K.; Hamada, M.; Yamamoto, T. Organometallics 2000, 19,
458-468.
(14) Carina, R. F.; Dietrich-Buchecker, C.; Sauvage, J. P. J. Am. Chem.
Soc. 1996, 118, 9110-9116.
4116 Inorganic Chemistry, Vol. 44, No. 12, 2005

COMMUNICATION
Scheme 2. Synthesis of Grids G3 and G415 from R3 and R4,
Respectively, in the Presence of [trans-(PEt3)2PtCl2]a
a ESI-MS signal of G34+ provided as inset.
Figure 3. Space-filling model of G1 and G2 (the two differ only by the
alkoxy chain) optimized with the MM+ force field.22 Alkoxy chains are
omitted for clarity.
Scheme 3. Cartoon Representation of (a) the Equilibration of Racks
at Room Temperature and (b) the Entropically Driven Repair
Mechanism that Will Suppress Formation of Oligomeric Aggregates
Because of the Kinetic Lability of the Rack Assembly
To ascertain the above assignment beyond doubt, we
prepared the ligands 713 and 8 from 4 through oxidative
coupling (O2, CuCl, NEt3) and coordination with [trans-
(PEt3)2PtCl2], respectively.21 In analogy to our recent work
on the synthesis of nanogrids,10 G1-G4 were prepared
independently by combination of 1 and 2 with 7 and 8,
respectively, in the presence of [Cu(CH3CN)4]+. Full agree-
ment of their spectral data with those of the grids prepared
by routes shown in Schemes 1 and 2 was obtained. As in
previous reports, the assignments were further confirmed by
UV/vis and ESI-MS titrations (see Supporting Information).10
Because suitable crystals for X-ray structure determination
could not be obtained, the dimensions of these nanogrids
were assessed from calculated energy-minimized structures.22
As depicted in Figure 3, the dimensions of G1 and G2 are
2.9 nm (extension along the bisphenanthroline) and 2.3 nm
for the Cu-Cu diagonal.
The present results suggest that functionalized racks can
be used as precursors to supramolecular grids and possibly
for even more complex aggregates, such as rotaxanes and
cylinders. As some of the grids are both heterometallic and
heteroleptic, the present approach should be fruitful for the
preparation of highly diversified assemblies. Hence, further
studies are in progress to transform racks into higher
aggregates and to study their properties.
π-π interaction (3.4 Å) between the phenanthrolines (see
Supporting Information).
When R3 or R4 were reacted with [trans-(PEt3)2PtCl2],16
the heterometallic19 and heteroleptic grids G3 and G4 formed
(Scheme 2), as demonstrated by ESI-MS (see Supporting
Information).
The isotopic distributions of the 3+ and 4+ charged
species were in good agreement with the calculated ones.
1H15 and 31P{1H} NMR spectroscopies, ESI-MS, and
elemental analysis point to the formation of a single
heterometallic grid species. For example, the 31P{1H}
NMR spectra of G3 and G4 exhibited a single peak at
δ ∼11.6 ppm with a set of PtII satellites (see Supporting
Information).16
The above results demonstrate that the racks could be
coupled to grids in high yield (∼90%) under both covalent
or coordinative conditions. In contrast to the systems of
Siegel et al.,12 equilibration of the racks at room temperature
is rapid, as demonstrated by the ligand exchange reaction
R1 + R3 h 2R6 (Scheme 3a).20 Hence, such ligand
exchange in potentially formed oligomeric adducts (Scheme
3b) will always lead to G1 and G3 because grid formation
from oligomers is driven by entropy (self-repair!).
Acknowledgment. We are indebted to the Deutsche
Forschungsgemeinschaft and the Fonds der Chemischen
Industrie for financial support.
Supporting Information Available: Experimental details and
characterization for all racks and grids, including crystal data for
R5 (cif file). This material is available free of charge via the Internet
at http://pubs.acs.org.
IC050088N
(19) Petitjean, A.; Kyritsakas, N.; Lehn, J.-M. Chem. Commun. 2004,
1168-1169 and references therein.
(21) The synthesis of a family of Pt-linked bisphenanthrolines and their
complexes will be described in detail in a forthcoming paper. Schmittel,
M.; Kalsani, V., manuscript in preparation. 1H, 13C, 31P NMR, and
ESI-MS data are in agreement with their structure.
(22) Hyperchem 6.02 Release for Windows by Hypercube, Inc., Gainesville,
FL, 2000. MM+ force field.
(20) Full equilibration after 5 min was observed by ESI-MS. 1H NMR
experiments for the R1-R3-R6 equilibration could not be performed
because of signal overlap. However, in a similar Cu(I)-based rack
system, the dynamic nature could readily be observed by 1H NMR
spectroscopy (see R7 in the Supporting Information).
Inorganic Chemistry, Vol. 44, No. 12, 2005 4117