Rational Design of a Phosphorus‐Centered Disbiradical

Abstract

Phosphorus‐centered disbiradicals, in which the radical sites exist as individual spin doublets with weak spin‐spin interaction have not been known so far. Starting from monoradicals of the type [•P(μ‐NTer)2P–R], we have now succeeded in linking two such monoradical phosphorus centers by appropriate choice of a linker. To this end, biradical [•P(μ‑NTer)2P•] (1) was treated with 1,6‐dibromo­hexane, affording the brominated species {Br[P(μ‐NTer)]2}2C6H12 (3). Subsequent reduction with KC8 led to the formation of the disbiradical {•[P(μ‐NTer)]2}2C6H12 (4) featuring a large distance between the radical phosphorus sites in the solid state and formally the highest biradical character observed in a P‐centered biradical so far, approaching 100%. EPR spectroscopy revealed a three‐line signal in solution with a considerably larger exchange interaction than would be expected from the molecular structure of the single crystal. Quantum chemical calculations revealed a highly dynamic conformational space; thus, the two radical sites can approach each other with a much smaller distance in solution. Further reduction of 4 resulted in the formation of a potassium salt featuring the first structurally characterized P‐centered distonic radical anion (5–). Moreover, 4 could be used in small molecule activation.

On Entangled Singlet Pure Diradicals

“…Another family of almost pure diradicals are bis(monoradical) systems where the two monoradicals are so far that they no longer interact with each other. 37 The challenge of the present work is the tentative conception of conjugated alternant hydrocarbons that (i) would be of singlet ground state and (ii) would be Kekuleízable but (iii) would have zero ionic components. This means that they should have two SOMOs of zero energy in the tight-binding Hamiltonian.…”

“…It is important to notice that the known disjoint diradicals built from conjugated hydrocarbons do not accept on-bond electron pairing; they are not Kekuléizable. Another family of almost pure diradicals are bis­(monoradical) systems where the two monoradicals are so far that they no longer interact with each other …”