High-valent copper in biomimetic and biological oxidations

Abstract

A long-standing debate in the Cu-O2 field has revolved around the relevance of the Cu(III) oxidation state in biological redox processes. The proposal of Cu(III) in biology is generally challenged as no spectroscopic or structural evidence exists currently for its presence. The reaction of synthetic Cu(I) complexes with O2 at low temperature in aprotic solvents provides the opportunity to investigate and define the chemical landscape of Cu-O2 species at a small molecule level of detail; eight different types are characterized structurally, three of which contain at least one Cu(III) center. Simple imidazole or histamine ligands are competent in these oxygenation reactions to form Cu(III) complexes. The combination of synthetic structural and reactivity data suggests (i) that Cu(I) should be considered as either a one or two electron reductant reacting with O2, (ii) that Cu(III) reduction potentials of these formed complexes are modest and well within the limits of a protein matrix and (iii) that primary amine and imidazole ligands are surprisingly good at stabilizing Cu(III) centers. These Cu(III) complexes are efficient oxidants for hydroxylating phenolate substrates with reaction hallmarks similar to that performed in biological systems. The remarkable ligation similarity of the synthetic and biological systems makes it difficult to continue to exclude Cu(III) from biological discussions.

A Structurally Characterized CuIII Complex Supported by a Bis(anilido) Ligand and Its Oxidative Catalytic Activity

“…Altogether, these results show that two‐electron chemistry at a mononuclear copper site is possible without the assistance of a ligand radical, suggesting that Cu III must not be excluded when dealing with biological oxidations, even those that do not involve an oxygen atom transfer. Finally, given the modularity of the synthesis of polynuclear Schiff base complexes (involving “classical” phenol donors), we envision that our aniline alternative would be a very powerful route towards a new generation of dinuclear model compounds of the pMMO active site.…”

Electrocatalytic Properties of Cuprous Delafossite Oxides for the Alkaline Oxygen Reduction Reaction

“…Looking to nature for inspiration, several biomolecules and their model complexes have mono‐, bi‐, or trinuclear copper sites that have been shown to bind and activate O 2 . For example, binuclear Cu I sites bind to O 2 to form a side‐on peroxo‐bridged structure with an interatomic copper distance near 3.6 Å .…”

“…Important to this structure is that the unoccupied peroxide σ* orbital interacts with occupied copper d orbitals, which significantly weakens the O−O bond (Figure ) . Multicopper oxidase enzymes containing a trinuclear copper site with a neighboring fourth copper can also directly catalyze the four‐electron reduction of O 2 to water . The catalytic activity of these biomolecular copper active sites has been extensively studied with several examples of their direct use or use of their model complexes as heterogeneous ORR catalysts.…”

“…[10] Looking to nature for inspiration,s everal biomolecules and their model complexes have mono-, bi-, or trinuclear copper sites that have been shown to bind and activateO 2 . [11][12][13][14][15][16] For example, binuclear Cu I sites bind to O 2 to form a side-on peroxo-bridged structure with an interatomic copperd istance near 3.6 . [17] Important to this structure is that the unoccupied peroxide s*o rbitali nteracts with occupied copperdo rbitals, which significantly weakens the OÀOb ond ( Figure 1).…”

“…[17,18] Multicoppero xidase enzymes containing at rinuclear coppers ite with an eighboring fourth copper can also directly catalyze the four-electron reduction of O 2 to water. [11][12][13][14][15][16] The catalytic activity of these biomolecular copper active sites has been extensively studied with several examples of their direct use [19][20][21][22][23][24][25][26] or use of their model complexes [27][28][29][30] as heterogeneous ORR catalysts. Despite the biological precedence, oxygen electrocatalysis at coppers ites in oxides has only recently been explored, and the alkaline catalytic activity of binary coppero xidesa nd doped cobaltite oxides has been reported.…”