Stable Nickel(I) Complexes with Electron-Rich, Sterically-Hindered, Innocent PNP Pincer Ligands

The electronic and steric properties of a new class of electron-rich and sterically hindered tertamethylated PNP pincer ligands (Me4PNPR = 2,6-bisRdialkylphosphino)propylipyridine with R = Pr-i, Bu-t) are discussed. Introducing the methyl groups on the pincer arm prevents dearomatization of the pincer framework and increases the bulkiness and electron-donating capacity of the ligand. Highly reactive Ni-I species are thus prevented from dimerizing and can be analyzed by a wide variety of spectroscopic methods. X-ray diffraction study shows that steric bulk has an important influence on the resulting geometric and spectroscopic properties of the Ni-I complexes. Complexes S and 6, which contain Pr-i groups on the phosphorus atoms, show a very rare seesaw geometry around the metal center, while Bu-t complexes 7 and 8 show a distorted square-planar geometry. Computational analysis reveals that the SOMO for all complexes has a d(x)(-y)(2)(2) character with the spin density mostly residing on the nickel.