Synthesis and characterization of polyamidoamine dendrimers surface-functionalized with bromotricarbonylpyridyliminerhenium(I) units

A novel type of rhenium-containing dendrimers has been prepared and characterized. A series of Schiff base-terminated PAMAM dendrimers were prepared by condensing the terminal amine groups of PAMAM dendrimers with pyridine-2-carboxaldehyde. Complete condensation of the terminal amines was confirmed by ¹H NMR spectroscopy. Bromotricarbonylrhenium(I) moieties were introduced onto the surface of these modified PAMAM derivatives by refluxing with bromopentacarbonylrhenium(I). These complexes had been characterized by a variety of analytical and spectroscopic techniques and their IR, NMR, and mass spectra discussed. The crystal structure of the model compound [(CH₃CONHCH₂CH₂NCHPy)ReBr(CO)₃] confirms a facial configuration of the three carbonyl ligands. UV-Vis absorption spectroscopy suggests that the {Re(CO)₃} moieties are quite far apart even in the second generation PAMAM dendrimers and do not interact with one another. In fact, the intensity of the metal-to-ligand (dπ-pπ^∗) charge-transfer is a linear function of the number of {Re(CO)₃} chromophores.