Diffusion and NOE NMR Studies on Multicationic DAB-Organoruthenium Dendrimers: Size-Dependent Noncovalent Self-Assembly to Megamers and Ion Pairing

New multicationic organoruthenium dendrimers (RuPF6-Dabit, n=2, 4, 8, 16) have been synthesized by coupling of [Ru(eta(6)-p-cymene)(kappa(3)-dpk-OCH2CH2OH)]X (1PF(6), dpk=2,2'-dipyridyl ketone, X=PF6) with 1,4-diaminobutane (DAB) and polypropylenimine dendrimer DAB-dendr-(NH2)(n), {n=4, 8, 16} mediated by 1,1'-carbonyldiimidazole (CDI). The intermediate in the synthesis, [Ru(eta(6)-p-cymene)(kappa(3)-dpk-OCH2CH2OC(O)]X (2PF, Im=imidazole, X=PF6) has been isolated and characterized by single-crystal XRD. The intra- and supramolecular structures in a solution of RuPF6-Dabn dendrimer have been investigated by multidimensional and multinuclear NMR techniques. Diffusion NMR experiments oil dilute solutions indicated that the linear distance between two metal centers (14.9-22.1 angstrom depending oil the dendrimer generation) is much greater than the diameter of 1PF(6) (9-9 angstrom). F-19,H-1-HOESY NMR experiments (HOESY = heteronuclear Overhauser effect spectroscopy) showed that the counterion is positioned on the surface of the dendrimers and assumes the same relative anion-cation orientation as in 2PF(6). Diffusion NMR experiments on RuPF6-Dabn dendrimers in CD2Cl2 at different concentrations revealed a process of supramolecular assembly of dendrimers to megamers that is strongly favored for the highest generations. Megamer formation is coupled with an increased fraction of free ions (alpha) and a consequent reduction in ion-paired ruthenium centers. Graphs of alpha versus C-Ru (the concentration of ruthenium centers) showed a minimum for RuPF6-Dab(4), RuPF6-Dab(8), and RuPF6-Dab1(6) at a position coinciding with the significant presence of supramolecular dendritic dimers. The tendency to ion pairing decreases as the dendrimer generation increases.