Hydrogen‐Atom Transfer (HAT) Initiated by Intramolecular Ligand–Metal Electron Transfer

Abstract

Hydrogen-atom transfer (HAT) is of key importance for several catalytic and biological processes, and provides an elegant access to C-H activation. In synthetic chemistry, a photoactivated metal complex is often employed to abstract an oxygen- or nitrogen-bound hydrogen, and the as-generated oxygen- or nitrogen-centered radical is the hydrogen-atom acceptor for HAT. Here, we report the first examples for HAT processes initiated by one-electron oxidation of urea azines. A further novelty is that the HAT-initiating oxidation can be realized by intramolecular ligand-metal electron transfer in copper(II)-urea azine complexes. These complexes are first characterized in the solid state, in which they are stable. Electron-transfer-initiated HAT processes are observed upon dissolving the complexes in organic solvents, and the kinetics of these processes varies with the solvent polarity. The carbon-centered radicals formed by HAT can either be trapped with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) or undergo radical recombination reactions with itself, yielding diamagnetic end-products.

Tuneable Redox Chemistry and Electrochromism of Persistent Symmetric and Asymmetric Azine Radical Cations

“…Colourless crystals grown from a MeCN/Et 2 O solution of 8 and CuCl 2 were identified as salt ( 8 +H)(CuCl 2 ). The formation of the reduced, protonated RAA compound indicates decomposition initiated by intramolecular HAT (as observed earlier for compound 1 , see Scheme ) …”

“…An electron paramagnetic resonance (EPR) study for the compound in the solid state showed that the spin-density is predominantly located on the copper atom. [10] In solution, the copper complexes slowly decomposedw ithin minutes,aprocess that is initiated by intramolecular electron transfer from the electron-rich urea azine ligand to the Cu II atom (intermediate 1-2). The electron transfer is followed by decomplexation and formation of the free radicalc ation 1C + + (species 1-3)t ogether with aC u I counter-ion.M onitoring the reactionb yU V/Vis and EPR spectroscopy showedt hat the decomposition kineticsd epend on the solventp olarity and the choiceo fh alide co-ligands.…”

“…Hydrogen loss combined with dimerisation leads to formation of a 12-membered-ring product (compound 1-7)a si llustrated in Scheme 1a.T he radical 1-4 could be trappedb ya ddition of (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO). [10] Furthermore, it was shown that one-electrono xidation of 2 with AgBF 4 is accompanied by dehydrogenation of an ethylene bridge in the backbone to give (2-2H)C + (compound 2-2, Scheme 1b). The isolation of 2C + (compound 2-1)w as finally possible by reactionw ith tetracyanoquinodimethane (TCNQ), yieldingt he donor-acceptor material 2(TCNQ) (compound 1-3).…”

Guanidines as Reagents in Proton-Coupled Electron-Transfer Reactions and Redox Catalysts

“…The first step was shown to be an intramolecular hydrogen atom transfer (HAT), as sketched in Scheme 3 (a) for compound 10. 88 The resulting carbon-centered radical could be trapped with TEMPO. Without a trapping reagent, it reacts further with itself to give the protonated compound, [10+H] + , and a 12-membered ring dimer [(10-H) 2 ] 2+ (Scheme 3, a).…”

On the Metal Cooperativity in a Dinuclear Copper–Guanidine Complex for Aliphatic C−H Bond Cleavage by Dioxygen

“…[19,35] In recent years, our group has extensively explored the chemistry of copper complexes bearing guanidino-substituted aromaticsa sl igands. [49] We have also demonstrated their use as catalysts for oxidationr eactions with dioxygen. [50] Herein, we report two new copper complexes with the bisguanidinel igands L 1 and L 2 (Scheme 2).…”