Liquid Crystalline Copper(II) Complexes of Poly(propylene imine) Dendrimers
In the last years hybrid systems consisting of organic materials and metal centres have attracted considerable attention since they combine characteristics of metals with those of organic compounds. Particularly interesting are liquid crystalline systems because they cause to supramolecular ordered systems with taylored physical properties. Dendrimers with donor sites can be used as chelates for metal complexation and their well defined structure enables control over the number and topology of the metal centres. With this motivation new liquid crystalline, polynuclear complexes of mesogenic 3,4-decyloxybenzoyl-functionalized poly(propylene imine) PPI dendrimers from 1st to 5th generation as ligands and copper(II) as paramagnetic centres have been synthesised. The dendrimeric complexes were prepared by coordination of copper(II) to the corresponding ligands. The maximum loadings (average number of copper centres per dendrimer) reached for generations 1st to 5th are 1.9, 6.3, 12.0, 24.2, and 45.9, respectively. In order to investigate the influence of the copper loading on the properties of the complexes, a series of 2nd generation complexes, covering the whole range of copper loadings (0.3 to 6.3), has been prepared by varying either the stoichiometry of the reaction or the working-up procedure. TEM of the dendrimeric copper(II) complexes revealed relatively monodisperse particles with circular structures have been observed with mean particle diameter of 2 nm for the 2nd up to 3 nm for the 5th generation. According to estimations from X-ray experiments, these sizes correspond roughly to the dimensions of the dendrimeric polar core (tertiary amine scaffold and amidobenzoyl groups in the outer part), where the coordination of copper takes place. EELS spectroscopy has been also performed on the complexes and confirms the presence of copper. The complexes up to 3rd generation were identified by MALDI-ToF-MS as single molecules corresponding to a dendrimeric ligand with different intramolecular copper loadings. For the complexation the copper was used as its nitrate trihydrate Cu(NO3)2.3H2O. Under the reaction conditions nitric acid developes and takes part in the composition of almost every complex as indicated by the N and O contents, which were determined by elemental analysis. In the FTIR spectra of the complexes characteristic bands of quaternary ammonium salts are observed which gives evidence for the protonation of the tertiary amines and excludes them as complexation sites for copper in the dendrimers. FTIR analysis revealed the amide groups as the coordinating groups of copper(II) and gives evidence of the coexistence of N- and O-amido copper coordination in the complexes. Indications for copper-coordinated bidentate nitrate have been observed. The 1st generation complexes can be separated in two types of complexes (´blue´, ´green´), which can be selectively prepared and are related to each other. The individual structures have been studied by EPR spectroscopy (by Dr. N. E. Domracheva), which revealed the coexistence of both types of complexes within the dendrimers of the higher generations and showed a N2O3 (NO4) coordination for copper(II) in the ´blue´ (´green´) complexes. At low temperatures an antiferromagnetic exchange has been observed. This behaviour is caused by a mu-1,3 nitrate group bridging two copper centres, which gives rise to highly ordered structures with an estimated copper to copper distance of 6.9 Å. All investigated PPI copper (II)-complexes are liquid crystalline. For both the complexes and the ligands the segregation of the polar core and the apolar alkoxy terminal chains in different domains is responsible for the appearance of mesophases. The complexation of copper(II) together with the protonation of the tertiary amine groups contribute to the formation of broader mesophases for the complexes as compared to that of the ligands. The complexes exhibit a hexagonal columnar (p6mm) mesophase. Moreover, for complexes up to the 4th generation a columnar rectangular mesophase (probably p2mg) is induced below the hexagonal mesophase, if a certain copper content is exceeded. By wide-angle X-ray diffraction of the complexes highly ordered, columnar mesophases with stacking distances of ca. 6.9 Å were identified, which is in agreement with the EPR data. Hence, it is assumed that copper centres, belonging to different dendrimeric ligand molecules, can coordinate by a nitrate in a mu-1,3 bridging mode, which causes the ordered stacking of the dendrimer units along the columns in the mesophases.
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