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

Abstract

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-butyl 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 x 10(9) M(-1)). Nonetheless, among the three dendrimers surveyed, the second-generation dendrimer, 2Cob+, afforded optimum stabilization for the CB7 inclusion complex.

Dendritic Encapsulation of Redox‐Active Units

“…These values also indicate an important feature of dendritic encapsulation-restriction of the interaction between a core and a relatively small molecule guest. 119 Other investigations included electron transfer rate attenuation and binding between cobaltocenium core dendrons and cucurbit [7]uril, 120 and between viologen (4,4-bipyridinium) core dendrons and cucurbit [7]uril. 121 Kraatz et al also prepared and studied formally "off-center" ferrocene core dendrimers with glutamic acid-based repeat units of the form Core 9 -Rpt 10 -Periph 13 .…”

Electron Transfer and Molecular Recognition in Metallocene‐Containing Dendrimers

“…Once these amine dendrons are available, we can easily tap into the reactivity of the focal amine groups to covalently attach a variety of electroactive and fluorescent groups to the dendrons. Our group has taken advantage of this chemistry to prepare dendrimers with ferrocene, [19,20] dansyl, [21] pyrene, [22] viologen, [23] cobaltocenium, [24] and hydrogen-bonding ureidopyrimidine [25] groups attached to the focal position of the dendron. In this review, we will obviously focus our attention on the properties of dendronized ferrocene and cobaltocenium compounds, but will use some of the data obtained with other functional groups to support specific conclusions along the way.…”

“…[24] As it is the case with the ferrocene derivatives, the dendrimers are initially prepared with tert-butyl ester peripheries (hydrophobic series, 24-26) but hydrolysis with formic acid led to their water-soluble forms (hydrophilic series, 27-29, see Figure 10 for structures). As expected, the cathodic electrochemical behavior of the dendrimers is dominated by the reversible one-electron reduction of cobaltocenium.…”

“…In general terms, however, the trends observed in the K values with CB7 are quite different from what was observed with βCD, as dendrimer growth does not decrease the binding affinity in the quick and pronounced way that was observed with the latter host. We also investigated the binding interactions between CB7 and cobaltocenium-containing dendrimers [24] 27-29 ( Table 4). The K values offer many similarities to those obtained with the dendronized ferrocenes.…”

Inclusion compounds of cucurbit[8]uril with noble metal polyamine complexes

“…14 Recent studies 14-16 have shown that the in clusion of platinum polyamine complexes having antitu mor activity into cucurbit[n]urils leads to a substantial decrease in the toxicity of these complexes. Antitumor drugs containing inclusion compounds of CB[7] and CB [8] with platinum, palladium, and gold complexes were cov ered by patents. 17-20…”