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 KC(8) in the presence of excess PMe(3) afforded the Fe(i) complex 3,5-Pr(i)(2)-Ar'Fe(PMe(3)) (1) (Ar'-3,5-Pr(i)(2) = C(6)H-2,6-(C(6)H(3)-2,6-Pr(i)(2))-3,5-Pr(i)(2)), which has a structure very different from the previously reported, linear Cr(i) species 3,5-Pr(i)(2)-Ar*Cr(PMe(3)) (3,5-Pr(i)(2)-Ar* = C(6)H-2,6-(C(6)H(2)-2,4,6-Pr(i)(3))(2)-3,5-Pr(i)(2)) and features a strong Fe-eta(6)-aryl interaction with the flanking aryl ring of the terphenyl ligand. In sharp contrast, the reduction of {ArCoCl}(2) (Ar = 3,5-Pr(i)(2)-Ar' and Ar') afforded the allyl complexes Co(eta(3)-{1-(H(2)C)(2)C-C(6)H(3)-2-(C(6)H(2)-2,4-Pr(i)(2)-5-(C(6)H(3)-2,6-Pr(i)(2)))-3-Pr(i)})(PMe(3))(3) (4) and Co(eta(3)-{1-(H(2)C)(2)C-C(6)H(3)-2-(C(6)H(4)-3-(C(6)H(3)-2,6-Pr(i)(2)))-3-Pr(i)})(PMe(3))(3) (5) formed by an unusual triple dehydrogenation of an isopropyl group. It is proposed that the reduction initially generates an intermediate 3,5-Pr(i)(2)-Ar'Co(PMe(3)), which is similar in structure to , followed by 3,5-Pr(i)(2)-Ar'Co(PMe(3)) decomposition to a cobalt hydride intermediate and dehydrogenation of the isopropyl group via remote C-H activation induced by PMe(3) 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.