Anionic Boron Cluster Compounds: Self-Assembly without Head-Tail Structure, and Amphidynamic Nature of their Polymeric Nanocomposites

We report an experimental evidence that the anionic boron clusters form multimolecular aggregates in solution,[1] and are surface active despite an absence of a hydrophobic tail as compared to classical surfactants.[2] The studied compounds comprise the boron clusters of various hydrophile-lipophile balances: from small and hydrophilic decaborate with high charge density (resembling thus hydrotropes) to bulky and hydrophobic metallacarborane (behaving like surfactants). The aggregation takes place regardless of the exoskeletal substitution or the presence of C-H units within the cluster. The driving force for all above-mentioned processes is closely related with a non-classical hydrophobic effect.

New polymeric nanocomposites based on interaction of metallacarborane, cobalt bis(dicarbollide)(-1), trivially called COSAN, with polycations poly(2-vinyl pyridine), P2VP and poly(4-vinyl pyridine), P4VP, and neutral poly(ethylene oxide), PEO, were prepared. While P2VP/COSAN and P4VP/COSAN nanocomposites are fully amorphous, PEO/COSAN exhibits narrow reflextions in WAXS pattern indicateing a certain degree of inner organization. More details on the inner structure and segmental motions were obtained by ss-NMR. The structure of PEO/COSAN is unique, where Na+ and COSAN- are evenly dispersed within the amorphous PEO matrix. Both P2VP and P4VP matrix is fully charged and protonized. Due to the bulkiness of COSAN clusters and their different accessibility to positively charged nitrogens in P2VP and P4VP isomers. The ability of COSAN to rotate is also different for all the studied nanocomposites, the systems can be therefore called as amphidynamic ones resembling molecular rotors.[3]

[1] Dordovic, Z. Tosner, M. Uchman, A. Zhigunov, M. Reza, J. Ruokolainen, G. Pramanik, P. Cigler, K. Kalikova, M. Gradzielski, P. Matejicek, Langmuir, 2016, 32, 6713. [2] Uchman, V. Dordovic, Z. Tosner, P. Matejicek, Angew. Chem. Int. Ed., 2015, 54, 14113. [3] J. Brus, A. Zhigunov, J. Czernek, L. Kobera, M. Uchman, P. Matejicek, Macromolecules, 2014, 47, 6343.