A Kinetically Stabilized Ferrocenyl Diphosphene: Synthesis, Structure, Properties, and Redox Behavior

Abstract

A new, stable ferrocenyl diphosphene [Tbt‐PP‐Fc] (1) (Tbt=2,4,6‐tris[bis(trimethylsilyl)methyl]phenyl, Fc = ferrocenyl) was synthesized by the dehydrochlorination reaction of the corresponding diphosphane, [Tbt‐P(H)‐P(Cl)‐Fc] (8), with 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) in good yield. Diphosphene 1 is very stable in the solid state and also in solution. In the ³¹P NMR spectrum (C₆D₆), diphosphene 1 showed a low‐fielded AB quartet at δ 501.7 and 479.5 ppm with the coupling constant ¹J_PP=546 Hz, which is characteristic of an unsymmetrically substituted trans‐diphosphene. The molecular structure of 1 was established by X‐ray crystallographic analysis, which showed a trans‐diphosphene with a C‐P‐P‐C torsion angle of 177.86(17)°. The phosphorusphosphorus bond length of 1 [2.0285(15) Å] which is considerably shorter than the typical PP single‐bond lengths (ca. 2.22–2.24 Å) and within the range of reported PP double‐bond lengths (1.985–2.051 Å) for diaryl diphosphenes, evidenced the PP double‐bond character of 1 in the solid state. In addition, the cyclic voltammograms of 1 showed reversible reduction and oxidation couples at −1.95 and +0.34 V versus SCE, respectively. The electrochemical results for 1 were reasonably supported by the DFT calculations, which suggested that the LUMO and HOMO orbitals should be mainly π* orbital of the diphosphene moiety and d orbitals of the iron(II) atom, respectively.