Dendrimers as Ligands: An Investigation into the Stability and Kinetics of Zn<SUP>2+</SUP> Complexation by Dendrimers with 1,4,8,11-Tetraazacyclotetradecane (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 ¹H 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 1 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<INF>1</INF>=44×10⁵ <SC>M</SC>⁻¹ s⁻¹) is followed by two consecutive first-order steps (k<INF>2</INF>=0.53 s⁻¹ and k<INF>3</INF>=0.10 s⁻¹). For 2, a slower second-order process (k<INF>1</INF>=4.9×10⁵ <SC>M</SC>⁻¹ s⁻¹) is followed by a slow first-order step (k<INF>2</INF>=0.40 s⁻¹). In the case of 3, only a very slow second-order process was observed (k<INF>1</INF>=1.2×10⁵ <SC>M</SC>⁻¹ s⁻¹). The different metal–ion incorporation rates for model compound 1 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.