Rapid Hydrogen and Oxygen Atom Transfer by a High-Valent Nickel–Oxygen Species

Abstract

Abstract.Terminal high-valent metal-oxygen species are key reaction intermediates in the catalytic cycle of both enzymes (e.g., oxygenases) and synthetic oxidation catalysts. While tremendous efforts have been directed towards the characterization of the biologically relevant terminal manganese-oxygen and iron-oxygen species, the corresponding analogues based on late-transition metals such as cobalt, nickel or copper are relatively scarce. This is in part related to the "Oxo Wall" concept, which predicts that late transition elements cannot support a terminal oxido ligand in a tetragonal environment. Here, the nickel(II) complex (1) of the tetradentate macrocyclic ligand bearing a 2,6-pyridinedicarboxamidate unit is shown to be an effective catalyst in the chlorination and oxidation of C-H bonds with sodium hypochlorite as terminal oxidant in the presence of acetic acid (AcOH). Insight into the active species responsible for the observed reactivity was gained through the study of the reaction of 1 with ClO -at low temperature by UV/Vis absorption, resonance Raman, EPR, ESI-MS, and XAS analyses. DFT calculations aided the assignment of the trapped chromophoric species (3) as a nickel-hypochlorite species. Despite the fact that the formal oxidation state of the nickel in 3 is +4, experimental and computational analysis indicate that 3 is best formulated as a Ni III complex with one unpaired electron delocalized in the ligands surrounding the metal center. Most remarkably, 3 reacts rapidly with a range of substrates including those with strong aliphatic C-H bonds, indicating the direct involvement of 3 in the oxidation/chlorination reactions observed in the 1/ClO -

Modulation of Nickel Pyridinedicarboxamidate Complexes to Explore the Properties of High‐valent Oxidants

“…[5] Te rminal Ni III =Oa nd Ni IV = Oa re frequently invokeda si ntermediates in these reactions, and their existence is supported by mechanistic experimental [6] and computational studies, [7] as well as by modelc omplexes. [9] These include Ni II (O 2 C), [10] Ni III (O 2 ), [10d, 11] Ni II (OOR), [11c, 12] Ni IV ÀOH, [13] Ni II À OOCOR, [14] Ni IV ÀOCl, [9] [Ni III 2 (m-O) 2 ], [10c, 15] and [Ni III (m-O) 2 Cu III ]. [9] These include Ni II (O 2 C), [10] Ni III (O 2 ), [10d, 11] Ni II (OOR), [11c, 12] Ni IV ÀOH, [13] Ni II À OOCOR, [14] Ni IV ÀOCl, [9] [Ni III 2 (m-O) 2 ], [10c, 15] and [Ni III (m-O) 2 Cu III ].…”

Transient Formation and Reactivity of a High-Valent Nickel(IV) Oxido Complex

“…2 Recently, attention has turned to synthetic nickel-based complexes due to both enzymes such as NOD (nickel oxide dismutase) 3 and their potential in the activation of small molecules, including H 2 O 2 , 4 mCPBA, 5 and NaOCl. 6 In the latter case, such complexes open the possibility to achieve selective alkane chlorination and alkane and alkene oxygenation.…”

Dinuclear Iron(III) and Nickel(II) Complexes Containing N‐(2‐Pyridylmethyl)‐N′‐(2‐hydroxyethyl)ethylenediamine: Catalytic Oxidation and Magnetic Properties

“…These results indicate that the Ni II −OOC(O)R intermediate 3 underwent the partition of heterolysis (55.8 %) and homolysis (44.2 %), affording high‐valent complexes. Importantly, the presence of high‐valent Ni III =O and Ni III –oxyl species has been proposed in only few studies . Notably, the high‐valent complexes (Ni III =O and Ni IV =O) are observed using the mechanistic probe PPAA.…”