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

Abstract

The electronic and steric properties of a new class of electron-rich and sterically hindered tertamethylated PNP pincer ligands (Me 4 PNP R = 2,6-bis[(dialkylphosphino)propyl]pyridine with R = i Pr, t Bu) 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 5 and 6, which contain i Pr groups on the phosphorus atoms, show a very rare seesaw geometry around the metal center, while t Bu complexes 7 and 8 show a distorted square-planar geometry. Computational analysis reveals that the SOMO for all complexes has a d x 2 −y 2 character with the spin density mostly residing on the nickel.

The Electronic Properties of Ni(PNN) Pincer Complexes Modulate Activity in Catalytic Hydrodehalogenation Reactions

“…The next biggest difference occurs in the Ni1‐Cl1 distance: the triclinic form measures 2.181 Å, slightly smaller than the average in the monoclinic form 2.185 Å. The Ni1‐Cl1 distances are at the longer end while the Ni1–P1 distances are at the shorter end of the ranges found in other (PNN)NiCl [ 26,28,29 ] or (PNP)NiCl [ 32,33,41 ] complexes. In 1‐Me , the five‐membered NC 2 PNi chelate ring adopts an envelope conformation (fold angles = 16.3° for P 1, 11.0° and 20.1° for rings involving Ni1 and N1a, respectively, in the monoclinic form) and the six‐membered NC 2 N 2 Ni chelate ring adopts a half‐boat conformation to give an overall structure that distinguishes the phenyl rings of the PPh 2 group (Figure 2, right), with a pseudoaxial ring (type A) being closer to the toluidinyl ring than the pseudoequatorial ring (type B).…”

Bulky PNP Ligands Blocking Metal-Ligand Cooperation Allow for Isolation of Ru(0), and Lead to Catalytically Active Ru Complexes in Acceptorless Alcohol Dehydrogenation

“…Ligands L1 and L2 were prepared according to the literature procedure. 1 Synthesis of complexes 1, 2 2, 3 and 15, 4 was previously described in the literature.…”

“…The solids were filtered, washed twice with cold pentane and dried under high vacuum for 7 hours to obtain complex 3 as a yellow solid (108 mg, 82 % yield). NMR Characterization: 1 H NMR. (400 MHz, THF-d8, 23 ℃) δ 7.69 (t, 1H, J = 7.9 Hz, py-H), 7.08 (d, 2H, J = 7.9 Hz, py-H), ), 3.01 (m, 2H, i Pr-H), 2.49 (m, 2H, i Pr-H), 1.66 (m, 18H, i Pr-Me and P−C(Me)), 1.46 (dd, 6H, JHP = 10.9 Hz, JHH = 5.6 Hz, i Pr-Me), 1.15 (dd, 6H, JHP = 13.9 Hz, JHH = 7.3 Hz, i Pr-Me), 0.79 (dd, 6H, JHP = 13.9 Hz, JHH = 7.3 Hz, i Pr-Me), -14.60 (v.t, 1H, JHP = 16.1 Hz, Ru-H).…”

“…C{ 1 Under an inert atmosphere, complex 3 (50.7 mg, 0.09 mmol) was dissolved in super dehydrated benzene (3 ml) and potassium bis(trimethylsilyl)amide (18 mg, 0.09 mmol) was added. The color immediately changes to dark red, after which the solution was further stirred at room temperature for 30 min.…”

“…This complex was characterized on the same day it was obtained by NMR only due to instability in solution and in the solid state. 1 H NMR spectrum of 6 in toluene-d8 at -13 ℃.…”