Synthesis and Characterization of New Star-Shaped Metal-Containing Polymers for Optical Applications
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al characteristics is realistically achievable here, since almost all metals except the alkali metals form stable complexes with this bidentate ligand. Furthermore, the metal complex aluminum tris(8-hydroxyquinoline), or Alq3, is a prototype electroluminescent material. It would be intriguing to see how does this EL behavior evolve with the metal anchored to a polymeric backbone [6]. It is worth noting here also that once we have developed an understanding of the general features of structure/ property relations for such polymers, one could in fact apply these strategies for the preparation and characterization for polymers incorporating other bidentate ligands. We have recently reported on the synthesis and characterization of linear coordination polymers, prepared by coordination polymerization of bis(8-hydroxyquinoline) ligands separated by a flexible hydrocarbon spacer group [7]. For the sake of simplicity, we will limit our discussion here to 8-hydroxyquinoline. We would like to emphasize the large scope of this study and the wide range of possible structures accessible through this methodology. 4
6
3
4
7
2 N 1
6
5
5
7
3
8 OH
8 2
N
N
1
10
9
Figure 1: Structure and Numbering System of 8-Hydroxyquinoline and 1,10-Phenanthroline
Our goal was to prepare a new set of tris- bidentate ligands, which would be suitable for the binding of metal ions to give the desired star-shaped metallated macrmolecules. Our initial efforts concentrated on the preparation of the ligands shown below:
N HO
OH
X N
X
X
X
N
X
X=N, CH
X
N OH
The chemistry involved proved to be rather complicated. As of now, we were unable to prepare such ligands. We always get the mono- (major) or at best small amounts of the di- substituted product. An alternative synthetic strategy is being pursued. In this report a rather simpler tris bidentate ligand is describes as shown in reaction scheme 1. EXPERIMENTAL The synthetic strategy is shown in scheme 1. The key compound is the acid chloride prepared form compound 1, which in turn was prepared by direct coupling between 8hydroxyquinoline and the diazonium salt derived form p-aminobenzoic acid. The acid chloride was then reacted with triethanolamine to give the triester tris bidentate ligand. This ligand was then used to prepare the coordination polymers by reaction with the divalent metal salts (usually the acetate) in boiling ethanol. All products were characterized by FTIR, 1H NMR, and elemental analyses. The detailed synthetic procedures will be described elsewhere. A representative example will be given here.
COOH
COOH
HO NaNO2/HCl 0 C0
N
O N
8-hydroxyquinoline
N
OH
(1) N2+Cl -
NH2
SOCl2
HO
OH N
2+
Coordination Polymers
M
Tris(bidentate) Ligand
OH
HO
N
O N
N
Cl
Scheme (I): Synthesis of tris bidentate triester and the corresponding star-shaped coordination polymers SYNTHESIS OF THE TRIESTER TRIS BIDENTATE LIGAND The acid chloride of compound 1, (6.00 mmol) and triethanolamine (Aldrich) (2.00 mmol) were heated at reflux in a 10.0 mL pyridine for 30 min
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