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

Abstract

New multicationic organoruthenium dendrimers (RuPF(6)-Dabn, n = 2, 4, 8, 16) have been synthesized by coupling of [Ru(eta(6)-p-cymene)(kappa(3)-dpk-OCH(2)CH(2)OH)]X (1PF(6), dpk = 2,2'-dipyridyl ketone, X = PF(6)) with 1,4-diaminobutane (DAB) and polypropylenimine dendrimer DAB-dendr-(NH(2))(n) {n=4, 8, 16} mediated by 1,1'-carbonyldiimidazole (CDI). The intermediate in the synthesis, [Ru(eta(6)-p-cymene)(kappa(3)-dpk-OCH(2)CH(2)OC(O)Im]X (2PF(6), Im = imidazole, X = PF(6)) has been isolated and characterized by single-crystal XRD. The intra- and supramolecular structures in a solution of RuPF(6)-Dabn dendrimer have been investigated by multidimensional and multinuclear NMR techniques. Diffusion NMR experiments on dilute solutions indicated that the linear distance between two metal centers (14.9-22.1 A depending on the dendrimer generation) is much greater than the diameter of 1PF(6) (9.9 A). (19)F,(1)H-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 RuPF(6)-Dabn dendrimers in CD(2)Cl(2) 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 RuPF(6)-Dab4, RuPF(6)-Dab8, and RuPF(6)-Dab16 at a position coinciding with the significant presence of supramolecular dendritic dimers. The tendency to ion pairing decreases as the dendrimer generation increases.