Helical, nonracemic inorganic-organic hybrid polymers of cadmium halides with pentadentate bis(oxazoline) ligands

Article abstract of DOI:10.1021/ja0469784

A chiral, nonracemic pentadentate pyridine bis(oxazoline) ligand forms unprecedented P-helical inorganic-organic hybrid polymers with cadmium halides. The one-dimensional chains consist of Λ₂-configured pentagonal-bipyramidal cadmium complexes with metal-centered chirality bridged by [CdX₄]^2- tetrahedra via shared halide atoms. In the solid state, the overall helicity exhibits strongly directed orientation parallel to the crystallographic axis a. Copyright

Full text of DOI:10.1021/ja0469784

Published on Web 08/27/2004
Helical, Nonracemic Inorganic-Organic Hybrid Polymers of Cadmium Halides
with Pentadentate Bis(oxazoline) Ligands
Michael Seitz, Anja Kaiser, Sabine Stempfhuber, Manfred Zabel, and Oliver Reiser*
Institute for Organic Chemistry, UniVersity of Regensburg, UniVersit a¨ tsstrasse 31, D-93053 Regensburg, Germany
Received May 22, 2004; E-mail: oliver.reiser@chemie.uni-regensburg.de
Helical, nonracemic structures play a fundamental role in many
natural systems, perhaps most prominently represented by R-helical
polypeptides or double-stranded helices of nucleic acids. In recent
years, much effort has been devoted to the controlled construction
of inorganic coordination compounds with enantiopure topology,
stimulated by their potential impact on important areas of research
like enantioselective catalysis, molecular recognition, etc. Although
predetermination of chirality in mononuclear metal complexes is
1
quite common today, the extension to polymeric metal-organic
coordination compounds remains challenging. In the field of
coordination polymers2 (infinitely extending metal-ligand as-
semblies with bridging organic ligands), induction of nonracemic
helicity is known in only a few cases. In contrast, the domain of
3
inorganic-organic hybrid polymers (metal centers, modified by
organic ligands and connected by inorganic bridges) has seen only
very recently a first example of the generation of an enantiopure,
one-dimensional helical chain with a chiral M-O-M subnetwork.4
Figure 1. CD spectra for 2 and 3 in CH3CN.
complexes are not stable in DMF or DMSO, yielding NMR spectra
identical to the spectrum of the free ligand.
To investigate the structure of the species in solution further,
especially the stereoselectivity of the complexation, CD spectra were
We recently reported chiral, C
2
-symmetric pentadentate bis-
(oxazoline) ligands for the stereoselective transfer of carbon-
centered, tetrahedral chirality to metal centers with various coor-
dination geometries, thereby creating metal complexes with
recorded in CH CN (Figure 1). 2 and 3 show essentially the same
3
5
predetermined helicity. Extending this concept from the creation
spectra, exhibiting positive peaks for the n-π* transitions of the
of mononuclear species to the construction of polymeric structures
led us to investigate the corresponding cadmium halide species,
known for their tendency to form halogen bridged one- and two-
dimensional assemblies.6
pyridine chromophore around 263 nm and a negative minimum at
209 nm. The shape is very characteristic for a pentagonal Λ2
coordination geometry, as seen in a number of analogous mono-
nuclear complexes of ligand 1.8
The synthesis of the complexes proved to be straightforward
Scheme 1). Refluxing equimolar amounts of CdX and the novel
2
ligand 1 in methanol resulted in colorless precipitates, isolated in
Last but not least, it was possible to obtain crystals suitable for
X-ray analysis by slow vapor diffusion of diethyl ether into a
solution of the complexes in acetonitrile (Figures 2 and 3).
(
9
analytically pure form by a simple filtration and drying in vacuo.
The solid-state structures reflected all the results obtained in
solution. In 2, two unique formula units were found in the
asymmetric unit, which differ, however, only slightly from each
other. The structures of 2 and 3 are very similar and follow the
same design principles.7 They consist of unprecedented one-
dimensional chains of pentagonal-bipyramidal coordinated Cd
Scheme 1
4
centers, connected by only slightly distorted tetrahedral CdX units
The two compounds were only poorly (2) or slightly soluble
(
3) in CH
nuclear, such as [Cd(1)X] and [CdX
spectrometry (ESI-MS). Despite the low solubility, it was possible
3
CN. The species in solution are most probably mono-
+
-
3
] , also supported by mass
1
13
to obtain H NMR and C NMR spectra in CD
only one species and a downfield shift of most of the well-resolved
signals, indicative of strong complexation. In addition, the C
3
CN, clearly showing
2
-
symmetry of the free ligand is preserved in the complexes, as could
be deduced from the number of signals in the NMR spectra. The
Figure 2. Asymmetric unit of 2 (two independent formula units present).
Hydrogen atoms omitted for clarity.
7
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J. AM. CHEM. SOC. 2004, 126, 11426-11427
10.1021/ja0469784 CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Figure 5. View normal to (a) the crystallographic axis a (along the chain
axis) and (b) the axis b for 2 (in analogy for 3).
Figure 3. Asymmetric unit of 3. Hydrogen atoms omitted for clarity.
hybrid polymers with unprecedented design and complete stereo-
selectivity. This constitutes, to the best of our knowledge, the first
example of homochiral, nonracemic, one-dimensional helices with
an infinite backbone exclusively made up by bridging halogens.
Acknowledgment. This work was supported by the German
Research Foundation (DFG) as part of the program SP1118 and
the Fonds der Chemischen Industrie (Ph.D. fellowship A.K. and
Forschungsbeihilfe).
Supporting Information Available: Experimental details for the
1
13
preparation of 2 and 3, as well as H and C NMR spectra, CD spectra,
and crystallographic data (PDF, CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
References
(
1) Reviews: (a) Knof, U.; von Zelewsky, A. Angew. Chem., Int. Ed. 1999,
3
8, 302-322. (b) Brunner, H. Angew. Chem., Int. Ed. 1999, 38, 1194-
Figure 4. Part of the 1D chain subunits of (a) 2 and (b) 3.
1208.
(
2) Reviews: (a) Janiak, C. J. Chem. Soc., Dalton Trans. 2003, 2781-2804.
(
b) Kitagawa, S.; Kitaura, R.; Noro, S.-I. Angew. Chem., Int. Ed. 2004,
via bridging halogens (Figure 4). The inorganic Cd-(µ-X) backbone
itself is achiral but accounts for the infinite polymeric character,
while the helical structure is exclusively made up by the twist of
the organic ligand. This is different from the previously known
related examples for one-dimensional homochiral polymeric helices
4
3, 2334-2375. (c) Kesanli, B.; Lin, W. Coord. Chem. ReV. 2003, 246,
3
05-326. Selected very recent examples: (d) Wang, R.; Xu, L.; Li, X.;
Li, Y.; Shi, Q.; Zhou, Z.; Hong, M.; Chan, A. S. C. Eur. J. Inorg. Chem.
2
004, 1595-1599. (e) Anthony, S. P.; Radhakrishnan, T. P. Chem.
Commun. 2004, 1058-1059. (f) Siemeling, U.; Scheppelmann, I.;
Neumann, B.; Stammler, A.; Stammler, H.-G.; Frelek, J. Chem. Commun.
2003, 2236-2237. (g) Niklas, N.; Hampel, F.; Alsfasser, R. Chem.
Commun. 2003, 1586-1587. (h) Khlobystov, A. N.; Brett, M. T.; Blake,
A. J.; Champness, N. R.; Gill, P. M. W.; O’Neill, D. P.; Teat, S. J.; Wilson,
C.; Schr o¨ der, M. J. Am. Chem. Soc. 2003, 125, 6753-6761. (i) Cui, Y.;
Ngo, H. L.; White, P. S.; Lin, W. Inorg. Chem. 2003, 42, 652-654.
4
with an extended chiral inorganic subnetwork or discrete (µ-X)-
bridged Cd centers infinitely connected by organic ligands2 and
i
constitutes the novel aspect of the present compounds.
All bond lengths and angles are in the normal range. An obvious
difference between the two structures is the increase in the helical
pitch (8.67 Å for 2 vs 9.05 Å for 3), resulting from the different
bridging X-Cd bond length (Cd-Cl, 2.503-2.601 Å vs Cd-Br,
(3) Reviews: (a) Forster, P. M.; Cheetham, A. K. Top. Catal. 2003, 24, 79-
8
6. (b) Hagrman, P. J.; Hagrman, D.; Zubieta, J. Angew. Chem., Int. Ed.
1999, 38, 2638-2684.
(4) Anokhina, E. V.; Jacobson, A. J. J. Am. Chem. Soc. 2004, 126, 3044-
3045.
(
5) Seitz, M.; Kaiser, A.; Powell, D. R.; Borovik, A. S.; Reiser, O. AdV. Synth.
2
2.636-2.729 Å). Regarding stereochemistry, only Λ -configured
Catal. 2004, 346, 737-741.
(
6) Cotton, F. A.; Wilkinson, G. W.; Murillo, C. A. AdVanced Inorganic
Chemistry, 6th ed.; Wiley-Interscience: New York, 1999.
metal centers are present, yielding exclusively right-handed helical
chains. The stacking of the individual chains is parallel to each
other and to the crystallographic axis a, thereby creating a highly
directed, helically ordered assembly. In both compounds, every
other chain is stacked upside-down, resulting in a deviation of the
connecting Cd tetrahedra alternately to the right and to the left with
respect to the plane bisecting the pyridine ring. Obviously, this does
not change the overall helicity of the assembly. In both cases, the
columns are ordered in a zigzag fashion (in the plane normal to
the axis a), and each neighboring chain is shifted by half of the
helix pitch (normal to axis b) (Figure 5).
(
(
7) For more details, see the Supporting Information.
II
8) For example, we have synthesized Λ
2
-[Fe (1)(H
2
O)
2
4
](ClO )
2
, having
pentagonal-bipyramidal geometry (X-ray) and exhibiting CD signals very
similar to those of 2 and 3.
(9) X-ray single-crystal diffraction data for 2 and 3 were collected on a STOE-
IPDS diffractometer. Crystal data for 2:
colorless rod, monoclinic, space group P2 , a ) 8.6650(9) Å, b ) 12.2190-
(8) Å, c ) 29.130(3) Å, â ) 95.297(12)°, V ) 3071.0(5) Å , Z ) 4, F )
27 2 4 3 4
C H27Cd Cl N O (824.24),
1
3
-3
-1
1
.783 g‚cm , µ(Mo KR) ) 1.770 mm , T ) 173(1) K, R1 ) 0.0313,
wR2 ) 0.0575, GOF ) 0.826, abs. structure param. ) 0.00(3). Crystal
data for 3: C27 Cd (1001.94), colorless prism, orthorhombic,
space group P2 , a ) 9.0518(7) Å, b ) 12.5611(7) Å, c ) 29.0084-
H
27Br
4
2 3 4
N O
1 1 1
2 2
3
-3
(
15) Å, V ) 3298.3(4) Å , Z ) 4, F ) 2.018 g‚cm , µ(Mo KR) ) 6.170
-1
mm , T ) 173(1) K, R1 ) 0.0344, wR2 ) 0.0634, GOF ) 0.820, abs.
structure param. ) -0.011(14).
In conclusion, we have shown that the new multidentate ligand
1
is capable of inducing helical topology in inorganic-organic
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