Controlling the Extent of Diradical Character by Utilizing Neighboring Group Interactions

Abstract

The extent of diradical character of a recently reported “localized singlet diradical that is indefinitely stable at room temperature” (R 4 P 2 B 2 R‘ 2 ) is assessed by electronic structure calculation of orbital occupation numbers compared to other well-studied diradicaloid systems. Our study shows that it has significantly less diradical character (and much more bonding character) than other typical organic diradicals. How this molecule, R 4 P 2 B 2 R‘ 2, attains this stability (bonding character) despite the long (2.60 Å) B−B distance is satisfactorily explained using a simplified model compound, H 4 P 2 B 2 H 2, and frontier orbital mixing ideas. Increasing bond length usually makes a molecule more diradicaloid. For example, as the H−H bond stretches in a homolytic single-bond breaking process, H 2 becomes more and more diradical-like, eventually becoming a pure diradical at complete separation. A counter-example is presented in this paper in which the molecule with a longer B−B “bond” distance (2.60 Å), H 4 P 2 B 2 H 2, represents less diradical character than the molecule with a shorter B−B distance (2.04 Å), H 4 N 2 B 2 H 2 . This novel observation is also explained in terms of molecular orbital mixing and second-order perturbation theory. Electronic structure calculations reveal that the coupling of radical centers to the adjacent groups via through-bond interaction, as well as the distance between the two radical centers, is critical in determining the extent of diradical character.