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.

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We report the reactivity and characterization of hydride, methyl, and bromo Ni II complexes with a new class of electron-rich and sterically hindered PNP pincer ligands, Me 4 PNP R (R = i Pr, t Bu), in which a classical metal−ligand cooperative mode of reactivity via CH 2 arm deprotonation is blocked by methylation. This enables new, uncommon modes of PNP ligand dearomatization that involve reactivity in the para position of the pyridine ring. In particular, the reduction of [(Me 4 PNP iPr )Ni II Me]B(Ar F ) 4 with KC 8 leads to the formation of a new C−C bond via dimerization of two complexes through the para position. This reactivity stands in sharp contrast to the previously reported bromo or chloro complexes, where stable Ni I halogen moieties are formed. Computational analysis showed a greater propensity for ligand-centered radical formation for the presumed intermediate one-electron-reduced species. UV-induced homolysis of the Ni II −Me bond in [(Me 4 PNP iPr )Ni II Me]B(Ar F ) 4 leads to the formation of a Me radical detected by radical traps and Ni I intermediates that can be trapped in the presence of halide ions to give previously characterized, stable Ni I halogen complexes. In addition, treatment of the bromo complexes [(Me 4 PNP R )Ni II Br]BPh 4 with a powerful hydride source, LiBEt 3 H, leads to the reduction of the pyridine ring and the formation of Ni II complexes with an anionic amide donor reduced pincer ligand, although aromatic Ni II hydride complexes could also be obtained with a weaker hydride source. We have observed that steric bulk at the phosphine donors controls the reactivity of the resulting Ni II H complexes. While t-Bu-substituted [(Me 4 PNP tBu )Ni II H]Y (Y = BPh 4 , B(Ar F ) 4 ) does not react with O 2 , the less sterically hindered i Pr-substituted [(Me 4 PNP iPr )Ni II H]Y reacts instantaneously to give an unstable superoxide adduct that can be observed by EPR.

C−X bond reductive elimination and oxidative addition are key steps in many catalytic cycles for C−H functionalization catalyzed by precious metals; however, engaging first row transition metals in these overall 2e − processes remains a challenge. Although high-valent Mn aryl species have been implicated in Mn-catalyzed C−H functionalization, the nature and reactivity of such species remain unelucidated. In this work, we report rare examples of stable, cyclometalated monoaryl Mn III complexes obtained through clean oxidative addition of Ar−Br to Mn I (CO) 5 Br. These isolated Mn III −Ar complexes undergo unprecedented 2e − reductive elimination of the Ar−X (X = Br, I, and CN) bond and Mn II induced by 1e − oxidation, presumably via transient reactive Mn IV species. Mechanistic studies suggest a nonradical pathway.

The macrocyclic ligand conformational behavior in solution, solid-state structures and the photophysical properties of copper(I) cationic and neutral mononuclear complexes supported by tetradentate N, N'-dialkyl-2,11-diaza[3.3](2,6)-pyridinophane ligands N4 (R = H, Me,Bu, Bu,Pent, Pr, Ts) were investigated in detail. Steric properties of the alkyl group at the axial amine in theN4 ligand were found to strongly affect the conformational preferences and dynamic behavior in solution. Several types of conformational exchange processes were revealed by variable-temperature NMR and 2D exchange spectroscopy, including degenerative exchange in a pseudotetrahedral species as well as exchange between two isomers with different conformers of tri- and tetracoordinate N4 ligands. These exchange processes are slower for the complexes containing bulky alkyl groups at the amine compared to less sterically demanding analogues. A clear correlation is also observed between the steric bulk of the alkyl substituents and the photoluminescent properties of the derived complexes, with less dynamic complexes bearing bulkier alkyl substituents exhibiting higher absolute photoluminescence quantum yield (PLQY) in solution and the solid state: PLQY in solution increases in the order Me< Pent< Bu< Bu ≈Pr < Bu. The electrochemical properties of the cationic complexes [(N4)Cu(MeCN)]X (X = BF, PF) were also dependent on the steric properties of the amine substituent.