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

A new pincer ligand, (P(2)(Ph)Pyr)(-), 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(P(2)(Ph)Pyr)center dot 2BH(3) (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(P(2)(Ph)Pyr)]. Salt metathesis reactions of the chloro complex afford new compounds including [Ni(CH3)(P(2)(Ph)Pyr)] and [Ni(NCCH3)(P(2)(Ph)Pyr)](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.