Self-assembly of heteroleptic [2 x 2] and [2 x 3] nanogrids

Article abstract of DOI:10.1039/b312807e

Using the HETPHEN concept a general and quantitative approach to the formation of heteroleptic nanogrids is illustrated.

Full text of DOI:10.1039/b312807e

Self-assembly of heteroleptic [2 3 2] and [2 3 3] nanogrids†
Michael Schmittel,*a Venkateshwarlu Kalsani,a Dieter Fenskeb and Andreas Wiegrefea
a Center of Micro and Nanochemistry and Engineering, Organische Chemie I, Universität Siegen,
Adolf-Reichwein-Str., D-57068 Siegen, Germany. E-mail: schmittel@chemie.uni-siegen.de; Fax: (+49) 271
740 3270; Tel: (+49) 271 740 4356
b Institut für Anorganische Chemie, Universität Karlsruhe, Engesserstr. 3045, D-76128 Karlsruhe, Germany
Received (in Cambridge, UK) 14th October 2003, Accepted 12th December 2003
First published as an Advance Article on the web 29th January 2004
Using the HETPHEN concept a general and quantitative
approach to the formation of heteroleptic nanogrids is illus-
trated.
presence of [Cu(MeCN)4]PF6 by ESI. A mixture of the homoleptic
[2 3 2] grids [Cu4(2)4]4+ and [Cu4(3)4]4+ was formed as well as
about an equal amount of the heteroleptic grid [Cu4(2)2(3)2]4+
.
Interestingly, 3 : 1 combinations, such as [Cu4(2)3(3)]4+ or
[Cu4(2)(3)3]4+, were not detected. The need for additional control
becomes even more evident when ligands 2 and 4 were treated in
The design and engineering of nanoscale supramolecular devices
possessing unique functionalities has become an active field of
research:1 cylinders,2 squares, polyhedra1,3 and grids4 are among
the most targeted assemblies.
Seminal work by Lehn et al. has shown how to engineer
homoleptic and even multimetallic metallosupramolecular grid
architectures.4 Due to the desire to explore highly functionalised
assemblies, however, a strategy to prepare heteroleptic nanogrids
would be quite welcome in order to combine different sets of
functionalities. While a first example of a heteroleptic grid5 was
explored by Lehn et al. the chosen approach proved not to be
general.
In recent years we have developed the HETPHEN concept‡6 and
demonstrated its potential for the quantitative formation of
nanoscale heteroleptic assemblies, e.g. the formation of nanoboxes7
and ring-in-ring8 structures. Herein, we describe the synthesis of
four truly nanoscale metallosupramolecular grids along this
concept and their characterisation.
The rigid bis- and trisphenanthroline ligands 1–5 were syn-
thesised utilising sequential Sonogashira coupling protocols that
will be published elsewhere. Ligands 1,5 were encoded with the
required control features to furnish exclusively heteroleptic assem-
blies, i.e. the 2,9 positions of each phenanthroline unit in 1,5 are
shielded with bulky methylaryl groups thus preventing any
association to homoleptic complexes. The appropriate combination
presence of the Cu( ) salt to explore the formation of the [2 3 3]
I
grid. By ESI no [2 3 3] or [3 3 3] grids were observed. As the
major product the homoleptic [2 3 2] grid [Cu4(2)4]4+ was afforded
along with small mononuclear complexes. Apparently, entropic
reasons favor formation of the [2 3 2] grid over any [2 3 3] and [3
3 3] grids. From the above results it is clear that it is not possible
to build heteroleptic nanoscale grids just relying on maximum site
occupancy and cooperativity motifs.
In contrast, treatment of HETPHEN ligand 1 with 1–2 equiv. of
Cu( ) salt resulted in a yellow solution, the analysis of which by
I
1
UV–vis, ESI MS and H NMR indicated [(Cu)2(1)]2+ as the only
species. Obviously, formation of any homoleptic complex of 1 is
prevented. Addition of two equiv. of the parent phenanthroline (6)
led to a red solution which by ESI, 1H-NMR and UV–vis analysis
only contained the bisheteroleptic complex [Cu2(1)(6)2]2+ demon-
strating the validity of the HETPHEN concept.6 The single crystal
structure is presented in Fig. 1.§ Accordingly, the molecule has a
transoid conformation with C2 symmetry.
Each copper( ) centre exhibits a pseudotetrahedral coordination
I
geometry (N2–Cu1–N3, 116.1° and N1–Cu–N4, 120.5°) and finds
itself encapsulated by two duryl rings of the bisphenanthroline. The
duryl groups and the second ligand (phenanthroline 6) are oriented
face-to-face separated by 3.4 Å, suggesting p-stacking. As such,
compound [Cu2(1)(6)2]2+ represents a rigid anti-rack complex.
HETPHEN ligand 1 was now reacted separately with ligands 2 or
3 in the presence of [Cu(MeCN)4]PF6 leading to the instant
formation of the corresponding [2 3 2] nanogrids [Cu4(1)2(2)2]4+
and [Cu4(1)2(3)2]4+ as sole products. Each of them displays a sharp
and single set of signals in the 1H NMR, indicating a highly
symmetric species. The chemical shifts of the mesityl protons (d ≈
7.0 in ligand, d ≈ 6.0 ppm in complex) are most diagnostic for
heteroleptic complex formation as demonstrated earlier.6 In
summary, the observed 1H NMR, COSY, ESI-MS and the
elemental analysis data are all consistent with the formation of
single, highly symmetric heteroleptic assemblies. Though different
configurational isomers should be present in solution, it was not
possible to quantify them because of their similar chemical
shifts.
of this ligand with metal ions such as Cu(
heteroleptic grids over homoleptic ones.
I) is expected to yield
As a test case for the necessity to apply the HETPHEN concept
we first monitored the self-assembly behaviour of 2 and 3 in the
† Electronic supplementary information (ESI) available: experimental
details, including ESI MS and 1H NMR data of grids. See http://
www.rsc.org/suppdata/cc/b3/b312807e/
Fig. 1 Solid state structure of [Cu2(1)(6)2]2+; ball and stick (left) and space
filling (middle) representation.
490
C h e m . C o m m u n . , 2 0 0 4 , 4 9 0 – 4 9 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

providing complexation constants for two intermediate complexes
and the final grid.
It has to be mentioned that these truly nanoscopic grids do not
readily crystallise due to their large voids. Dimensions of the [2 3
2] grids are 2.9 nm along the edge and 2.5 nm for the Cu–Cu
diagonal. The larger [2 3 3] grids exhibit a length of 6.0 nm and a
Cu–Cu diagonal of 5.0 nm (Fig. 3).
In conclusion, the formation of four heteroleptic nanogrids using
the HETPHEN concept‡ has been unambiguously demonstrated.
This concept is very powerful in preparing heteroleptic supramo-
lecular architectures, which have been impossible to prepare using
simply maximum site occupancy and cooperativity motifs.
We are indebted to the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie for financial support.
To probe the HETPHEN concept for [2 3 3] grids we reacted 1
with 4 and 3 with 5. In contrast to the immediate formation of the
[2 3 2] grids we noted a sluggish reaction over two days until
finally the [2 3 3] grids [Cu6(1)3(4)2]6+ and [Cu6(3)3(5)2]6+ had
formed. Again, as before the spectroscopic data (1H NMR,
elemental analysis, ESI-MS), in particular isotopic splitting,
indicated clean formation of the desired nanogrids.
Notes and references
‡ HETPHEN concept: quantitative approach to heteroleptic bisphenanthro-
line metal complexes.6 This approach utilizes steric and electronic effects
originating from bulky aryl substituents at the bisimine coordination sites
(as seen in 1 and 5) to control the coordination equilibrium both kinetically
and thermodynamically.
§
Crystal data for [Cu2(1)(6)2]2+. Crystals were obtained by slow
diffusion of toluene into methylene chloride solution of the complex. Only
poor quality crystals could be obtained so the crystal analysis is poor.
Solvent molecules are severely disordered. Formula sum
C
96H76Br2Cu2F12N8P2, Formula weight 1918.49, Crystal system: triclinic,
¯
Space group: P1, Unit cell dimensions a
= 10.4460(21) Å, b =
14.0790(28) Å, c = 16.8010(34) Å, a = 84.70(3) °, b = 80.32(3) °, g =
88.63(3) °, Cell volume: 2425.23(1407) Å3, density, calculated: 1.348 g
cm23, Pearson code: aP142, formula type: NOPQ4R58, Wyckoff sequence:
i71
CCDC 222763. See http://www.rsc.org/suppdata/cc/b3/b312807e/ for crys-
tallographic data in .cif or other electronic format.
To obtain more insight into the mechanistic scenario leading to
these assemblies, formation of [Cu4(1)2(3)2]4+ was monitored by
both ESI MS (qualitative analysis) and UV–vis (quantitative
analysis). Two series of titrations were performed. In the first series
1 and 3 were titrated with Cu(
I
) salt and in the second series 1 and
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indicated a three step process on the way to the grid (Fig. 2)
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Fig. 2 UV–vis titration of 1 and 3 with Cu+.
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Fig.
3
Hyperchem representations of [Cu4(1)2(3)2]4+ (left) and
[Cu6(1)3(4)2]6+
.
C h e m . C o m m u n . , 2 0 0 4 , 4 9 0 – 4 9 1
491