Proton Switch in the Secondary Coordination Sphere to Control Catalytic Events at the Metal Center: Biomimetic Oxo Transfer Chemistry of Nickel Amidate Complex

Abstract

High‐valent metal‐oxo species are key intermediates for the oxygen atom transfer step in the catalytic cycles of many metalloenzymes. While the redox‐active metal centers of such enzymes are typically supported by anionic amino acid side chains or porphyrin rings, peptide backbones might function as strong electron‐donating ligands to stabilize high oxidation states. To test the feasibility of this idea in synthetic settings, we have prepared a nickel(II) complex of new amido multidentate ligand. The mononuclear nickel complex of this N5 ligand catalyzes epoxidation reactions of a wide range of olefins by using mCPBA as a terminal oxidant. Notably, a remarkably high catalytic efficiency and selectivity were observed for terminal olefin substrates. We found that protonation of the secondary coordination sphere serves as the entry point to the catalytic cycle, in which high‐valent nickel species is subsequently formed to carry out oxo‐transfer reactions. A conceptually parallel process might allow metalloenzymes to control the catalytic cycle in the primary coordination sphere by using proton switch in the secondary coordination sphere.

A Contrasting Effect of Acid in Electron Transfer, Oxygen Atom Transfer, and Hydrogen Atom Transfer Reactions of a Nickel(III) Complex

“…A nickel­(II) complex, [Ni II (PaPy 3 *)] + ( 2 ), was synthesized by mixing an equal amount of Ni II (ClO 4 ) 2 ·6H 2 O and H-PaPy 3 * (Supporting Information Figure S1) in acetonitrile (MeCN), followed by adding 1 equiv of triethylamine …”

“…The crystal structure of 2 showed a one-dimension polymeric structure as shown in Figure S2 in the Supporting Information, in which the oxygen atom in the carbonyl group (O carbonyl ) of the PaPy 3 * ligand binds with the next Ni­(II) center with a Ni II –O carbonyl bond distance of 2.124 Å, and the crystallographic data and selected bond distances and angles for 2 are listed in Tables S1 and S2 in the Supporting Information. After 2 was slowly dissolved in MeCN, the weak Ni II –O carbonyl bond would dissociate and 2 in MeCN probably gave a vacant coordination site at the Ni­(II) center . The simplified X-ray crystal and chemical structures of 2 are shown in Figure .…”

“…The UV–vis spectrum of 2 shows the typical weak absorption peaks around 525 nm (ε 1 = 3.4 × 10 M –1 cm –1 ) and 810 nm (ε 2 = 3.2 × 10 M –1 cm –1 ) due to the Ni­(II) complex (Figure S3b in the Supporting Information). , The electron paramagnetic resonance (EPR) spectrum of 2 was silent (Figure S3c in the Supporting Information). The cyclic voltammetry (CV) of 2 showed a reversible redox couple of Ni II/III with the one-electron oxidation potential ( E 1/2 vs SCE) of 0.84 V (Figure S4 in the Supporting Information).…”

Preparation and Characterization of a Formally NiIV–Oxo Complex with a Triplet Ground State and Application in Oxidation Reactions

“…Finally, a mixture of Ni IV (O) and Ni III (O) species was reported by Kim and co-workers in 2016 as intermediates in the catalytic epoxidation of olefins by [Ni II (dpaq)­Cl] treated with the oxidant m -CPBA (Scheme E) . Since our 2020 review, there have been a few proposed Ni­(O) intermediates, − but these lack in-depth spectroscopic characterization. Recently, O–NiF 2 has been prepared in a solid neon matrix by Riedel and co-workers.…”

Silver Cyanide Powder‐Catalyzed Selective Epoxidation of Cyclohexene and Styrene with its Surface Activation by H2O2(aq) and Assisted by CH3CN as a Non‐Innocent Solvent

“…The use of highly active hydroperoxyl species such as H 2 O 2(aq) , m ‐chloroperoxybenzoic acid ( m ‐CPBA), and t ‐butyl hydroperoxide (TBHP) for the activation of olefins in a liquid phase are common approaches for selective epoxidation [2] . In most cases, metal catalysts are required, and CH 3 CN often serves as a popular solvent for the corresponding conversions [2a–c,i–l] …”