New dendrimers containing a single cobaltocenium unit covalently attached to the apical position of Newkome dendrons: Electrochemistry and guest binding interactions with cucurbit[7]uril

Two new dendrimer series were prepared and characterized. These dendrimers contain a single bis(cyclopentadienyl)-cobalt(III) (cobaltocenium, Cob ⁺) unit covalently attached to the apical (focal) position of Newkome-type dendrons, ranging in size from first to third generation. The dendrimers in the first series (1ECob⁺ - 3ECob⁺) are hydrophobic and have 3, 9, and 27 tert-bulyl esters on their peripheries, whereas the dendrimers in the second series (1Cob⁺ - 3Cob ⁺) are hydrophilic with 3, 9, and 27 carboxylic acid groups on their surfaces, respectively. In voltammetric experiments, all dendrimers showed the expected one-electron reversible reduction of the cobaltocenium center, and the heterogeneous rate of electron transfer decreased with generation in both dendrimer series. The host - guest binding interactions between water-soluble dendrimers 1Cob⁺ - 3Cob⁺ and the cucurbit[7]uril (CB7) host were investigated using ¹H NMR spectroscopy, MALDI-TOF mass spectrometry, and electrochemical techniques. The association equilibrium constants (K) for all dendrimer guests were significantly lower than that measured for the inclusion complex between underivatized Cob⁺ and CB7 (K = 5.7 × 10⁹ M^-1)- Nonetheless, among the three dendrimers surveyed, the second-generation dendrimer, 2Cob⁺, afforded optimum stabilization for the CB7 inclusion complex.