Nickel(II) Complexes Containing a Pyrrole–Diphosphine Pincer Ligand

A new pincer ligand, (P2PhPyr)−, based on the anion of 2,5-bis­[(diphenylphosphino)­methyl]­pyrrole has been prepared in four steps from pyrrole. The ligand undergoes oxidation to diphosphine oxide under ambient conditions and was therefore isolated as its borane adduct, H­(P2PhPyr)·2BH3 (2). Delivery of the ligand to nickel­(II) was accomplished by the direct reaction of NiCl2 with 2 in the presence of Et2NH to afford [NiCl­(P2PhPyr)]. Salt metathesis reactions of the chloro complex afford new compounds including [Ni­(CH3)­(P2PhPyr)] and [Ni­(NCCH3)­(P2PhPyr)]­(OTf). In all cases, the ligand gives rise to diamagnetic square-planar complexes, which have been fully characterized in solution and the solid state. All complexes examined display an irreversible oxidation to nickel­(III) according to cyclic voltammetry. Reduction of the chloro complex in dichloromethane results in an electrocatalytic process, whereas reduction in tetrahydrofuran leads to the irreversible formation of a nickel­(I) species.