Reduction of terphenyl iron(II) or cobalt(II) halides in the presence of trimethylphosphine: an unusual triple dehydrogenation of an alkyl group

The reduction of {ArFeBr}₂ (Ar = terphenyl) with KC₈ in the presence of excess PMe₃ afforded the Fe(I) complex 3,5-Prⁱ₂-Ar′Fe(PMe₃) (1) (Ar′-3,5-Prⁱ₂ = C₆H-2,6-(C₆H₃-2,6-Prⁱ₂)-3,5-Prⁱ₂), which has a structure very different from the previously reported, linear Cr(I) species 3,5-Prⁱ₂-Ar*Cr(PMe₃) (3,5-Prⁱ₂-Ar* = C₆H-2,6-(C₆H₂–2,4,6-Prⁱ₃)₂–3,5-Prⁱ₂) and features a strong Fe-η⁶-aryl interaction with the flanking aryl ring of the terphenyl ligand. In sharp contrast, the reduction of {ArCoCl}₂ (Ar = 3,5-Prⁱ₂-Ar′ and Ar′) afforded the allyl complexes Co(η³-{1-(H₂C)₂C-C₆H₃–2-(C₆H₂–2,4-Prⁱ₂–5-(C₆H₃–2,6-Prⁱ₂))-3-Prⁱ})(PMe₃)₃ (4) and Co(η³-{1-(H₂C)₂C-C₆H₃–2-(C₆H₄–3-(C₆H₃–2,6-Prⁱ₂))-3-Prⁱ})(PMe₃)₃ (5) formed by an unusual triple dehydrogenation of an isopropyl group. It is proposed that the reduction initially generates an intermediate 3,5-Prⁱ₂-Ar′Co(PMe₃), which is similar in structure to 1, followed by 3,5-Prⁱ₂-Ar′Co(PMe₃) decomposition to a cobalt hydride intermediate and dehydrogenation of the isopropyl group via remote C–H activation induced by PMe₃ complexation. Complexes 1, 4, and 5 were characterized by X-ray crystallography. In addition, 1 was studied by NMR and EPR spectroscopy; 4 and 5 were characterized by NMR spectroscopy.