Radical-Cation Dimerization Overwhelms Inclusion in [n]Pseudorotaxanes

• Published in: Chemistry : a European journal 2014-06, Vol.20 (24), p.7334-7344
• Main Authors: Nchimi-Nono, Katia, Dalvand, Parastoo, Wadhwa, Kuldeep, Nuryyeva, Selbi, Alneyadi, Shaikha, Prakasam, Thirumurugan, Fahrenbach, Albert C., Olsen, John-Carl, Asfari, Zouhair, Platas-Iglesias, Carlos, Elhabiri, Mourad, Trabolsi, Ali
• Format: Article
• Language: English
• Subjects: Cations; Chemistry; Complexation; Dimerization; electrochemistry; host-guest systems; Models, Molecular; molecular recognition; Molecular Structure; Nuclear magnetic resonance; Phosphazene; pseudorotaxanes; radical ions; Radicals; Reduction; Rotaxanes - chemical synthesis; Rotaxanes - chemistry; Spectroscopy
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201400069

Suppression of the dimerization of the viologen radical cation by cucurbit[7]uril (CB7) in water is a well‐known phenomenon. Herein, two counter‐examples are presented. Two viologen‐containing thread molecules were designed, synthesized, and thoroughly characterized by 1H DOSY NMR spectrometry, UV/Vis absorption spectrophotometry, square‐wave voltammetry, and chronocoulometry: BV4+, which contains two viologen subunits, and HV12+, which contains six. In both threads, the viologen subunits are covalently bonded to a hexavalent phosphazene core. The corresponding [3]‐ and [7]pseudorotaxanes that form on complexation with CB7, that is, BV4+⊂(CB7)2 and HV12+⊂(CB7)6, were also analyzed. The properties of two monomeric control threads, namely, methyl viologen (MV2+) and benzyl methyl viologen (BMV2+), as well as their [2]pseudorotaxane complexes with CB7 (MV2+⊂CB7 and BMV2+⊂CB7) were also investigated. As expected, the control pseudorotaxanes remained intact after one‐electron reduction of their viologen‐recognition stations. In contrast, analogous reduction of BV4+⊂(CB7)2 and HV12+⊂(CB7)6 led to host–guest decomplexation and release of the free threads BV2(.+) and HV6(.+), respectively. 1H DOSY NMR spectrometric and chronocoulometric measurements showed that BV2(.+) and HV6(.+) have larger diffusion coefficients than the corresponding [3]‐ and [7]pseudorotaxanes, and UV/Vis absorption studies provided evidence for intramolecular radical‐cation dimerization. These results demonstrate that radical‐cation dimerization, a relatively weak interaction, can be used as a driving force in novel molecular switches. Multimeric threads in which two and six viologen subunits are covalently bonded to a phosphazene core form the corresponding [3]‐ and [7]pseudorotaxanes on complexation with cucurbit[7]uril. In aqueous solution, these systems can be electrochemically switched between the complexed and uncomplexed states (i.e., the separate components). The driving force for dethreading is intramolecular dimerization of the viologen radical cation (see figure).