Multicomponent host-guest chemistry of carboxylic acid and phosphonic acid based guests with dendritic hosts: An NMR study

A new way to analyze supramolecular dendritic architectures is reported by making use of C-13 NMR and P-31 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 C-13 NMR and P-31 NMR next to 1D and 2D H-1 NMR techniques. Different amounts of guest 2 have been added to fifth generation dendrimer le, and the observed chemical shift values in C-13 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 le, two different signals are observed in P-31 NMR. Deconvolution gives the fractions of free and bound guest, resulting in an association constant of (4 3) x 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.