Controlling the extent of diradical character by utilizing neighboring group interactions

The extent of diradical character of a recently reported localized singlet diradical that is indefinitely stable at room temperature (R4P2B2R'(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, R4P2B2R'(2), attains this stability (bonding character) despite the long (2.60 Angstrom) B-B distance is satisfactorily explained using a simplified model compound, H4P2B2H2, 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 Angstrom), H4P2B2H2, represents less diradical character than the molecule with a shorter B-B distance (2.04 Angstrom), H4N2B2H2. 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.