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 31PNMR spectrum (C6D6), diphosphene 1 showed a low-fielded AB quartet at δ 501.7 and 479.5 ppm with the coupling constant 1JPP = 546 Hz, which is characteristic of an unsymmetrically substituted transdiphosphene. The molecular structure of 1 was established by X-ray crystallographic analysis, which showed a transdiphosphene 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 +0.34V 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.