Multicomponent Host−Guest Chemistry of Carboxylic Acid and Phosphonic Acid Based Guests with Dendritic Hosts: An NMR Study

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Abstract
A new way to analyze supramolecular dendritic architectures is reported by making use of 13C NMR and 31P NMR. Two ethylene glycol guest molecules have been synthesized containing a 13C labeled carboxylic acid headgroup (2) and a phosphonic acid headgroup (3). The binding of these guests to urea-adamantyl modified poly(propylene imine) dendrimers has been investigated with 13C NMR and 31P NMR next to 1D and 2D 1H NMR techniques. Different amounts of guest 2 have been added to fifth generation dendrimer 1e, and the observed chemical shift values in 13C NMR were fitted to a model that assumes 1:1 binding between guest and binding site. An association constant of 400 ± 95 M-1 is obtained for guest 2 with 41 binding sites per dendrimer. When different amounts of phosphonic acid guest 3 are added to dendrimer 1e, two different signals are observed in 31P NMR. Deconvolution gives the fractions of free and bound guest, resulting in an association constant of (4 ± 3) × 104 M-1 and 61 ± 1 binding sites. A statistical analysis shows that guest 2 forms a “polydisperse supramolecular aggregate”, while guest 3 is able to form a “monodisperse supramolecular aggregate” when the amount of guest is high enough. The NMR results are compared with dynamic light scattering experiments, and a remarkable agreement is found. Phosphonic acid guest 3 is able to exchange with guest 2, which is in agreement with the obtained association constants, and shows that these techniques can be used to analyze multicomponent dendritic aggregates.