Diffusion and NOE NMR studies on multicationic DAB-organoruthenium dendrimers: size-dependent noncovalent self-assembly to megamers and ion pairing.
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.
Dipartimento di Chimica, Università degli Studi di Perugia, Via Elce di Sotto 8-06123 Perugia, Italy.
Stefania
S
Pettirossi
Gianfranco
G
Bellachioma
Gianluca
G
Ciancaleoni
Cristiano
C
Zuccaccia
Daniele
D
Zuccaccia
Alceo
A
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