Compartmentalization in Hybrid Metallacarborane Nanoparticles Formed by Block Copolymers with Star-Like Architecture

Boron cluster leveraged polymeric building blocks

Boron cluster compounds (BCCs) are inorganic molecules characterized by their unique physical and chemical properties. Polymeric materials incorporating BCCs exhibit significant chemical and thermal stability, making them valuable for applications in biomedical fields, energy storage, ultrahigh stability materials, and π-conjugated luminochromic polymers. This review article aims to explore the primary methods for integrating these distinctive clusters into traditional carbon-based polymers. Both boron and carbon atoms possess catenation abilities, enabling the formation of extensive macromolecular structures. While carbon forms long linear chains, boron typically leads to three-dimensional polyhedral clusters. We first examine hybrid nanostructures, focusing on weak non-covalent interactions such as dihydrogen bonding, hydrophobic, and chaotropic effects between boron clusters and polymer chains. We then discuss classical chemical bonding approaches. Despite their inorganic nature, boron clusters can undergo exoskeletal substitution akin to organic counterparts, allowing their attachment as side groups to polymer repeating units. Additionally, polyhedral boron clusters can be incorporated into polymer backbones primarily through polycondensation reactions, resulting in hybrid macromolecules with exceptional physical and chemical attributes. Finally, we summarize the applications of BCC-containing polymeric materials, including their use in boron neutron capture therapy (BNCT), solid polymer electrolytes (SPEs) for metal ion batteries, and as electron acceptor groups in stimuli-responsive luminescent materials. In summary, BCC-containing polymeric materials are increasingly considered viable alternatives to traditional hydrocarbon-based polymers for biomedical applications, ion-conducting materials, luminescent materials, and temperature-resistant materials.

Preparing (Metalla)carboranes for Nanomedicine

"There's plenty of room at the bottom" (Richard Feynman, 1959): an invitation for (metalla)carboranes to enter the (new) field of nanomedicine. For two decades, the number of publications on boron cluster compounds designed for potential applications in medicine has been constantly increasing. Hundreds of compounds have been screened in vitro or in vivo for a variety of biological activities (chemotherapeutics, radiotherapeutics, antiviral, etc.), and some have shown rather promising potential for further development. However, until now, no boron cluster compounds have made it to the clinic, and even clinical trials have been very sparse. This review introduces a new perspective in the field of medicinal boron chemistry, namely that boron-based drugs should be regarded as nanomedicine platforms, due to their peculiar self-assembly behaviour in aqueous solutions, and treated as such. Examples for boron-based 12- and 11-vertex clusters and appropriate comparative studies from medicinal (in)organic chemistry and nanomedicine, highlighting similarities, differences and gaps in physicochemical and biological characterisation methods, are provided to encourage medicinal boron chemists to fill in the gaps between chemistry laboratory and real applications in living systems by employing bioanalytical and biophysical methods for characterising and controlling the aggregation behaviour of the clusters in solution.

Reversibly Switchable Fluorescent Molecular Systems Based on Metallacarborane-Perylenediimide Conjugates

Icosahedral metallacarboranes are θ-shaped anionic molecules in which two icosahedra share one vertex that is a metal center. The most remarkable of these compounds is the anionic cobalt-based metallacarborane [Co(C₂ B₉ H₁₁ )₂ ]^- , whose oxidation-reduction processes occur via an outer sphere electron process. This, along with its low density negative charge, makes [Co(C₂ B₉ H₁₁ )₂ ]^- very appealing to participate in electron-transfer processes. In this work, [Co(C₂ B₉ H₁₁ )₂ ]^- is tethered to a perylenediimide dye to produce the first examples of switchable luminescent molecules and materials based on metallacarboranes. In particular, the electronic communication of [Co(C₂ B₉ H₁₁ )₂ ]^- with the appended chromophore unit in these compounds can be regulated upon application of redox stimuli, which allows the reversible modulation of the emitted fluorescence. As such, they behave as electrochemically-controlled fluorescent molecular switches in solution, which surpass the performance of previous systems based on conjugates of perylendiimides with ferrocene. Remarkably, they can form gels by treatment with appropriate mixtures of organic solvents, which result from the self-assembly of the cobaltabisdicarbollide-perylendiimide conjugates into 1D nanostructures. The interplay between dye π-stacking and metallacarborane electronic and steric interactions ultimately governs the supramolecular arrangement in these materials, which for one of the compounds prepared allows preserving the luminescent behavior in the gel state.

Tuning of Thermoresponsivity of a Poly(2-alkyl-2-oxazoline) Block Copolymer by Interaction with Surface-Active and Chaotropic Metallacarborane Anion

Thermoresponsive nanoparticles based on the interaction of metallacarboranes, bulky chaotropic and surface-active anions, and poly(2-alkyl-2-oxazoline) block copolymers were prepared. Recently, the great potential of metallacarboranes have been recognized in biomedicine and many delivery nanosystems have been proposed. However, none of them are thermoresponsive. Therefore, a thermoresponsive block copolymer, poly(2-methyl-2-oxazoline)-block-poly(2-n-propyl-2-oxazoline) (PMeOx-PPrOx), was synthesized to encapsulate metallacarboranes. Light scattering, NMR spectroscopy, isothermal titration calorimetry, and cryogenic TEM were used to characterize all solutions of the formed nanoparticles. The cloud-point temperature (T_CP ) of the block copolymer was observed at 30 °C and polymeric micelles formed above this temperature. Cobalt bis(dicarbollide) anion (COSAN) interacts with both polymeric segments. Depending on the COSAN concentration, this affinity influenced the phase transition of the thermoresponsive PPrOx block. The T_CP shifted to lower values at a lower COSAN content. At higher COSAN concentrations, the hybrid nanoparticles are fragmented into relatively small pieces. This system is also thermoresponsive, whereby an increase in temperature leads to higher polymer mobility and COSAN release.