Activation of Molecular Oxygen by a Cobalt(II) Tetra‐NHC Complex**

Abstract

The first dicobalt(III) μ2‐peroxo N‐heterocyclic carbene (NHC) complex is reported. It can be quantitatively generated from a cobalt(II) compound bearing a 16‐membered macrocyclic tetra‐NHC ligand via facile activation of dioxygen from air at ambient conditions. The reaction proceeds via an end‐on superoxo intermediate as demonstrated by EPR studies and DFT. The peroxo moiety can be cleaved upon addition of acetic acid, yielding the corresponding CoIII acetate complex going along with H2O2 formation. In contrast, both CoII and CoIII complexes are also studied as catalysts to utilize air for olefin and alkane oxidation reactions; however, not resulting in product formation. The observations are rationalized by DFT‐calculations, suggesting a nucleophilic nature of the dicobalt(III) μ2‐peroxo complex. All isolated compounds are characterized by NMR, ESI‐MS, elemental analysis, EPR and SC‐XRD.

Late Lanthanide Macrocyclic Tetra-NHC Complexes

“…This geometry is structurally similar to the distorted square-pyramidal tetra­ethyl­ammonium carbonyl­bis­[difluoro­(glyoximato)­borato]­copper­(I), which features a Cu I center displaced 1.03 Å from the N atoms in the basal plane . To our knowledge, this is the most out-of-plane position for a metal atom on a monomeric five-coordinate macrocyclic tetra-NHC complex to date. − ,,,,− Indeed, the metal atom is so far out of the plane that the trans C–Gd–C bond angles are comparable to the N–Gd–C angles, all of which are near 120°.…”

Organometallic 3d transition metal NHC complexes in oxidation catalysis

“…Upon addition of acetic acid, the formed peroxo moiety can be cleaved, yielding a Co(II) acetate complex 41, by simultaneously releasing H 2 O 2 (Scheme 23). 187 As described before, oxoiron(IV) or (V) species are considered to be key intermediates in the catalytic cycle of numerous (non)heme iron enzymes and complexes allowing for the insertion of an oxygen atom into unreactive C-H bonds. Mössbauer spectra reveal the cyclic tetra-NHC as an extremely strong equatorial σ-donor with high stabilizing capacity towards high valent metals.…”

“…Meyer et al , 185 Radius et al 186 and our group 187 worked on the selective activation of dioxygen from air utilizing cobalt NHC complexes and their potential use as artificial oxygen carriers. Ligands that enforce a tripodal topology on coordinated metal ions are known to provide powerful platforms for small molecule activation and functionalization.…”

“…Upon addition of acetic acid, the formed peroxo moiety can be cleaved, yielding a Co( ii ) acetate complex 41 , by simultaneously releasing H 2 O 2 (Scheme 23). 187…”

Hydrogen Bonding Effect on the Oxygen Binding and Activa-tion in Cobalt(III)-peroxo Complexes

“…The [Co III (O2)] complexes have a high thermodynamic stability and a kinetic stability while they are electronically comparable to other metal-dioxygen species present in enzymes. 6,7 Moreover, they are readily prepared at ambient conditions via the reaction between a cobalt(II) salt and hydrogen peroxide (H2O2). 8 In this report, we propose tripodal [Co III (O2)] complexes as models to investigate the effect of the hydrogen bonding on the metal-dioxygen binding and on the oxygen-oxygen bond activation (Figure 1c.)…”