Rational Design of a Phosphorus-Centered Disbiradical

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 [& sdot;P(mu-NTer)2P-R], we have now succeeded in linking two such monoradical phosphorus centers by appropriate choice of a linker. To this end, biradical [& sdot;P(mu-NTer)2P & sdot;] (1) was treated with 1,6-dibromohexane, affording the brominated species {Br[P(mu-NTer)]2}2C6H12 (3). Subsequent reduction with KC8 led to the formation of the disbiradical {& sdot;[P(mu-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. The first synthesis of a phosphorus-centered disbiradical that exhibits highly dynamic behavior in solution leading to isomerization is presented. Among the various conformers, those that have significantly shorter distances between radical sites and thus can interact weakly through space dominate.image