Synthesis, characterization and reactivity of vanadium, chromium, and manganese PNP pincer complexes

The synthesis of a series of vanadium, chromium, and manganese PNP complexes of the types [M(PNP) Cl-3] (M = V, Cr) and [M(PNP)Cl-2] (M = Cr, Mn) is reported. Vanadium and manganese PNP pincer complexes are described for the first time. All complexes are characterized by their magnetic moments, elemental analysis, and ESI MS. In addition, some compounds are characterized by X-ray crystallography. In a preliminary study, these complexes catalyze the oxidative homo-coupling of aryl Grignard reagents in the presence of Mel as oxidizing agents to give symmetrical biaryls, but are inactive in Kumada cross coupling reactions. The reactivity of V(III), Cr(III), Cr(II) and Mn(II) is compared with related Fe(II) and Co (II) complexes of the types [Fe(PNP-iPr)Cl-2], and [Co(PNP-iPr)Cl-2]. In all cases, good to excellent isolated yields are obtained. However, since the respective metal chlorides in the absence of PNP ligands exhibited comparable reactivities, the new PNP complexes offer no real advantage for this type of coupling reactions. (C) 2016 Elsevier B.V. All rights reserved.