The Niecke Biradicals and Their Congeners – The Journey from Stable Biradicaloids to Their Utilization for the Design of Nonlinear Optical Properties

Abstract

Biradicals within organic chemistry are known to be of fleeting existence. They can be traced only by femtosecond spectroscopy and/or corresponding trapping experiments. The conceptual understanding of a biradical originates mainly from gas phase experiments. These shaped the biradicals as highly unstable species, their lifetimes determined by reactions, which are merely entropy controlled. On the other hand, biradicals in π‐systems of non‐Kekule type, lead to species, which either have distinct singlet or triplet ground states. Yet they are still species confined to spectroscopic investigations. The matter changed by the introduction of sterically protecting groups into chemistry, in order to stabilize hitherto highly reactive compounds. With the new growing field of phosphorus organic chemistry, both fields merged and the concept of a stable biradical came to light. The concept matured in subsequent years and it is now a flowering field with promising aspects in materials science. The Niecke‐type biradicaloid, which is a basic unit in this area, owes its stability by protection of the phosphorus position with sterically demanding substituents, in particular with the “super‐mesityl” (Mes*) group. The electron withdrawing TMS groups at the carbon positions enhance the singlet stability. On this basis it was possible to isolate and structurally characterize these non‐Kekule type structures. The description of these systems as biradicals or biradicaloids is determined by their high reactivity and they offer versatility for a variety of chemical reactions. From the point of theory this refers to small energy gaps between adiabatic singlet and triplet states. However, the understanding as biradicals is different to the Doering concept of a biradical vs. a biradicaloid, as anticipated in the transition state of the Cope automerization. In the Niecke‐type biradicaloid a corresponding biradical structure is promoted with enhanced pyramidalization at the phosphorus atoms or in more general by a decrease of the Lewis basicity of the lone pairs. The Niecke‐type biradical can be converted into a Bertrand‐type biradical with trans‐annular bonding. This requires changing the donor‐ability at the phosphorus centers. This prediction could be proven by either addition with Lewis acids or complexation with transition metal fragments. In the last part of this article, it is briefly shown how biradicals can be utilized as units for building extended π‐systems. They feature an interesting target for the design of interesting materials for nonlinear optics.