Iron(III) Complexes on a Dendrimeric Basis and Various Amine Core Investigated by Mössbauer Spectroscopy

Dendrimers of various generations were synthesized by the divergent method. Starting from various amine cores (G0a, G0b, G0c) the generations were built by reaction of the amine with acrylnitrile followed by hydrogenation with DIBAL-H. Treatment with salicylaldehyde creates a fivefold coordination sphere for iron in the molecular periphery. The resulting multinuclear coordination compounds are investigated by Mossbauer spectroscopy.


Introduction
Dendrimers represent a novel type of polymeric materials that have been of increasing interest in recent years revealing applications in supramolecular chemistry, nanoscience, medicine and catalysis [1]. These macromolecules are defined as highly branched, spherical and monodisperse molecules composed of monomers that spread from a central core [2]. The dendrimers are produced in iterative sequences of reactions steps, whereby any further sequence leads to a higher generation dendrimer. (See Fig. 1) In this study, we focused on divergent approach, where the molecule grows from the core to the periphery and whereas convergent synthesis first builds up the dendrons called arms, wich were connected in a merging step to the core fragment. The extraordinary topology of the dendrimers was used to yield multinuclear iron(III) complexes by functionalizing the terminal amine groups and subsequently to observe the influence of the generation growth on the spin state of the resulting metal complex with Mössbauer spectroscopy [3][4][5][6][7].

Higher generation dendrimeric nitrile and amine
The dendrimeric nitriles and dendrimeric amines of the second (G 2 ), third (G 3 ) and fourth (G 4 ) generation were synthesized in analogy to first generation dendrimeric nitriles and amines starting from the respective amines and niriles of the previous generation. (See Fig. 2)

Functionalization and complexation of dendrimers
The amine G 0a (2 mmol) was dissolved in methanol. The mixture was heated under reflux for 10 min and salicylaldehyde (6.6 mmol) was added dropwise. After 30 min of stirring at room temperature a solution of anhydrous iron(III) chloride (2 mmol) in methaol was added slowly. The resulting solution was heated to 50 °C for 5 min and triethylamine (4 mmol) was added. Reducing the solvent and cooling precipitates the product. A solid G 0a -Cl was collected and dried under reduced pressure. Yield: 55 %, (M= 546.87 g/mol). IR (KBr): ν (cm -1 ) = 3347 cm -1 (N-R, val.), 2935 cm -1 (CH 2 , val.), 1619 cm -1 (C꞊N, val.). The synthesis of the dendrimeric compounds of the first (G 1 ), second (G 2 ), third (G 3 ) and fourth (G 4 ) generations were performed in analogy to the zeroth generation. The synthesis of dendremeric compounds with a different coreG b and G c were performed in analogy to G a . (See Fig. 3

Structure Investigation of the Dendrimeric Compounds
All compounds were characterized by IR spectroscopy and ESI-MS. The IR spectra were measured in KBr matrix on a Brucker Optics Tensor in the 4000-400 cm -1 region at ambient temperatures. Infrared spectroscopy has proved to be a very useful tool in the structural analysis and identification of different dendrimeric compounds [8,9]. For the synthesized amines, the IR spectra show a band in the range of 1654- in the ESI-MS despite its gentle ionisation method. Nevertheless, a tendency for a relative increase in the number of fragments with high mass-to-charge (m/z) ratios can be observed for higher generation numbers. Table 3. Selected examples of infrared spectroscopy bands and electrospray ionisation mass spectrometry (ESI MS) signals of different dendrimeric complexes

Mössbauer Results and Discussion
The Mössbauer spectra were recorded using a conventional transmission spectrometer at T = 20 K and T = 300 K. 57 Co/Rh was used as the source of the radiation and the isomeric shift data are given relative to Fe in Rh at room temperature. Fig. 6 and Table 4 show the Mössbauer spectra and the parameter of the dendrimeric complexes from zeroth to fourth generation. Mössbauer spectra of all dendrimeric compounds at 300K and at 20 K exhibit the shape of a simple line doublet indicating Fe(III) in high-spin state (S=5/2). Some spectra could be described best by a fit with 2 or 3 lorentzian doublets. Since one would not expect discrete positions for the iron centres, for both isomeric shift (IS) and quadrupole splitting values the centre of gravity was determined for the complete signal in order to achieve better comparability within this discussion.
At 300 K all spectra exhibit typical values for the IS in the range of 0.237(22)-0.341(13) mm/s. The observed higher values at 20 K are well in accordance with the "Second-Order Doppler Shift" (in the range of0.295(37)-0.440(11) mm/s). There seems to be no consistent trend of an influence of the generation growth on the IS. Deviations of individual values could be caused by agglomeration effects. (See Tab. 4) The QS values exhibit a slight temperature dependence. Values in a range of 0.554(21)-1.300(30) mm/s indicate a deviation from an ideal octahedral coordination sphere. The differences occurring on generation growth could be explained by influences of different conformations and agglomeration degrees on the symmetry of the coordination sphere and therefore the electric field gradient. (See Fig. 5& Tab. 4) In case of the G c -Dendrimers strong asymmetry of the Mössbauer doublet is observed (see fig. 6). This asymmetry is particularly pronounced for the first and third generation and seems to be temperature dependent. In general, an asymmetric absorption signal in the Mössbauer spectroscopy can be caused by a texture, Goldanski-Karyagin or relaxation effect. The temperature dependence as well as the similar areas of the two resonant lines lead to the assumption that this asymmetry is caused by (spin-spin) relaxation.     (78) 100 100 -----------