Dendrimers as ligands:: An investigation into the stability and kinetics of Zn2+ complexation by dendrimers with 1,4,8,11-tetraazacyclotetiradecane (Cyclam) cores

We have investigated the complexation of Zn2+ with 1,4,8,11-tetrakis(naphthylmethyl) cyclam (1; cyclam 1,4,8,11-tetraazacyclotetradecane) and with two dendrimers consisting of a cyclam core with four dimethoxybenzene and eight naphthyl appendages (2), and twelve dimethoxybenzene and sixteen naphthyl appendages (3). An important, common feature of model compound 1 and dendrimers 2 and 3 is that their potentially fluorescent naphthyl units are quenched by exciplex formation with the cyclam nitrogen atoms. Complexation with Zn2+, however, prevents exciplex formation and results in the appearance of an intense naphthyl fluorescence signal that can be used for monitoring the complexation process. Luminescence titration, together with competition experiments and H-1 NMR titration, have shown that 1:1 and 1:2 (metal/ligand) complexes are formed in the cases of 2 and 3, whereas model compound I gives only a 1:1 complex. We have also investigated the 1:1 complexation kinetics by the stopped-flow technique. In the case of 1, a second-order process (k(1)=44x10(5)M(-1)s(-1)) is followed by two consecutive first-order steps (k(2)=0.53s(-1) and k(3)=0.10 s(-1)). For 2, a slower second-order process (k(1) = 4.9x10(5)M(-1)s(-1)) is followed by a slow first-order step (k(2) = 0.40 s(-1)). In the case of 3, only a very slow second-order process was observed (k(1) = 1.2x10(5)M(-1)s(-1)). The different metal-ion incorporation rates for model compound I and dendrimers 2 and 3 have been discussed in terms of conformational changes of the dendron subunits affecting the chelating properties of the cyclam core. This work reports the first kinetic study on metal-ion coordination by dendrimers with a well-defined coordination site.