The Mechanochemistry of Carboranes

Abstract

We developed o-carborane as a new mechanophore by showing that the o-carborane cluster is the preferred scission site in chain-centered polymers through ultrasonication mechanochemistry. Mechanistic studies are consistent with a predominately homolytic mechanism of chain scission. The mechanically generated monocarbaborane fragments are highly reactive toward alcohol nucleophiles. By contrast, carborane with a different regiochemistry (m-carborane) maintained its high mechanical stability. DFT simulations provide insights into the origins of carborane's mechanical lability. This fundamental research provides a new stimulus for carborane cage activation.

Theoretical and Experimental Investigations of Stable Arylfluorene‐Based Radical‐Type Mechanophores

“…Two methyl groups were selected as anchor atoms, and potential energy curves were calculated at a B3LYP level with a 6-31G(d) basis set which is a much prevailing calculation level for CoGEF simulation. [5,[54][55][56][57][58] The values of F max of DFSN, BTAF, BPyF, and BPF were 4.0, 3.8, 3.7, and 3.5 nN, respectively (Figures S30-S33). The order was in good accordance with that of thermostability, implying that the nature of EWGs at the sp 2hybridized carbon-centered radical would contribute to not only the thermal stability but also the mechanical responsiveness of the mechanophores.…”

“…In order to gain the theoretical homolysis of the central CÀ C bond of RMs through mechanical force, we carried out the constrained geometries simulate external force (CoGEF) simulations, [53] which is a strong protocol to predict mechanochemical reactivity by estimating the maximum force (F max ). [5,[54][55][56][57] This theoretical methodology, which is still an infant, would be one of the strong solutions for the molecules that we could not address due to the tolerance for oxygen like BPyF. Two methyl groups were selected as anchor atoms, and potential energy curves were calculated at a B3LYP level with a 6-31G(d) basis set which is a much prevailing calculation level for CoGEF simulation.…”

“…This theoretical methodology, which is still an infant, would be one of the strong solutions for the molecules that we could not address due to the tolerance for oxygen like BPyF. Two methyl groups were selected as anchor atoms, and potential energy curves were calculated at a B3LYP level with a 6‐31G(d) basis set which is a much prevailing calculation level for CoGEF simulation [5,54–58] . The values of F max of DFSN, BTAF, BPyF, and BPF were 4.0, 3.8, 3.7, and 3.5 nN, respectively (Figures S30–S33).…”

“…The development of mechanophores, that is, molecular units that respond to exposure to mechanical energy, has been established as the central avenue of research in the field of mechanoresponsive materials. For example, mechanophores can endow materials with fluorescence, the ability to change color, and/or the ability to release small molecules such as acids, carbocations, and ketenes through chemical reactions in response to being exposed to mechanical forces [4–26] …”

Mechanochemistry of Cubane

“…As a result, mechanical activation leads smoothly to the corresponding syn -tricyclooctadiene, a product that eludes isolation upon thermal activation and otherwise requires rhodium­(I) catalysis for its generation . Simple computational models of mechanochemical coupling show how the structure of what might be described as a three-dimensional cluster-type mechanophore contributes to the observed behavior and has inspired us to think about how molecular designs that lead to similar types of opening and unfolding might be harnessed to access otherwise hard-to-reach intermediates. In addition, cyclobutane mechanophores have been used to probe structure–activity relationships in macroscopic networks, ,, and cubane offers similar mechanochemical reactivity but with lower force, smaller molecular extension, products with different subsequent reactivity, and greater exothermicity than are otherwise available among the cyclobutane family.…”