Rational Design of Persistent Phosphorus-Centered Singlet Tetraradicals and Their Use in Small-Molecule Activation

Biradicalsare important intermediates in the process of bond formationand breaking. While main-group-element-centered biradicals have beenthoroughly studied, much less is known about tetraradicals, as theirvery low stability has hampered their isolation and use in small-moleculeactivation. Herein, we describe the search for persistent phosphorus-centeredtetraradicals. Starting from an s-hydrindacenyl skeleton,we investigated the introduction of four phosphorus-based radicalsites linked by an N-R unit and bridged by a benzene moiety.By varying the size of the substituent R, we finally succeeded inisolating a persistent P-centered singlet tetraradical, 2,6-diaza-1,3,5,7-tetraphospha-s-hydrindacene-1,3,5,7-tetrayl (1), in goodyields. Furthermore, it was demonstrated that tetraradical 1 can be utilized for the activation of small molecules such as molecularhydrogen or alkynes. In addition to the synthesis of P-centered tetraradicals,the comparison with other known tetraradicals as well as biradicalsis described on the basis of quantum mechanical calculations withrespect to its multireference character, coupling of radical electrons,and aromaticity. The strong coupling of radical electrons enablesselective discrimination between the first and the second activationsof small molecules, which is shown by the example of H-2 addition. The mechanism of hydrogen addition is investigated withparahydrogen-induced hyperpolarization NMR studies and DFT calculations.

Automated summary

In this study, a persistent phosphorus-centered tetraradical was isolated and found to be capable of activating small molecules. Quantum mechanical calculations were used to compare its properties and aromaticity with other known tetraradicals and biradicals.