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)₂ P-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)₂ P⋅] (1) was treated with 1,6-dibromohexane, affording the brominated species {Br[P(μ-NTer)]₂ }₂ C₆ H₁₂ (3). Subsequent reduction with KC₈ led to the formation of the disbiradical {⋅[P(μ-NTer)]₂ }₂ C₆ H₁₂ (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.

Keywords: Biradical; Diradical; Distonic Radical Anion; EPR Spectroscopy; Phosphorus Chemistry.