The Hammett substituent constant of the dodecaborate dianion inaugurates a non-coordinating, non-basic, and strongly inductively electron-donating group for organic chemistry

Abstract

The dodecaborate anion (B12H122–, DDB) has been qualitatively considered as a strong electron donor. Here, we report the Hammett substituent constant for DDB and its amino derivative (DDB–NH), which are among the strongest electron donors reported so far, on a scale that employs the 19F chemical shift of meta- and para-substituted fluorobenzenes in DMSO. The dissection of the substituent effect into inductive/field and resonance contributions shows that specifically the DDB group exerts an exceptionally strong and dominant inductive/field effect, which presents another unique feature in comparison to established electron-donating groups. In contrast to the latter, the DDB group does not become readily protonated, it does not tend to coordinate with Lewis acids or transition metals, and it is chemically and photochemically inert. In addition, the electron-donating effect of the DDB group can be combined with other electron donors to afford an even more strongly electron-donating group, DDB–NH. The newly characterized electron-donating groups can be utilized in diverse contexts, among which the maximization of push-pull effects on the optical absorption spectra of chromophores presents one specific application.