Characterization of the structure and reactivity of monocopper–oxygen complexes supported by β‐diketiminate and anilido‐imine ligands

Abstract

Copper-oxygen complexes supported by beta-diketiminate and anilido-imine ligands have recently been reported (Aboelella et al., J Am Chem Soc 2004, 126, 16896; Reynolds et al., Inorg Chem 2005, 44, 6989) as potential biomimetic models for dopamine beta-monooxygenase (DbetaM) and peptidylglycine alpha-hydroxylating monooxygenase (PHM). However, in contrast to the enzymatic systems, these complexes fail to exhibit C--H hydroxylation activity (Reynolds et al., Chem Commun 2005, 2014). Quantum chemical characterization of the 1:1 Cu-O(2) model adducts and related species (Cu(III)-hydroperoxide, Cu(III)-oxo, and Cu(III)-hydroxide) indicates that the 1:1 Cu-O(2) adducts are unreactive toward substrates because of the weakness of the O--H bond that would be formed upon hydrogen-atom abstraction. This in turn is ascribed to the 1:1 adducts having both low reduction potentials and basicities. Cu(III)-oxo species on the other hand, determined to be intermediate between Cu(III)-oxo and Cu(II)-oxyl in character, are shown to be far more reactive toward substrates. Based on these results, design strategies for new DbetaM and PHM biomimetic ligands are proposed: new ligands should be made less electron rich so as to favor end-on dioxygen coordination in the 1:1 Cu-O(2) adducts. Comparison of the relative reactivities of the various copper-oxygen complexes as hydroxylating agents provides support for a Cu(II)-superoxide species as the intermediate responsible for substrate hydroxylation in DbetaM and PHM, and suggests that a Cu(III)-oxo intermediate would be competent in this process as well.

Quantum mechanical calculations suggest that lytic polysaccharide monooxygenases use a copper-oxyl, oxygen-rebound mechanism

“…Thus, we consider C-H oxidation by a Cu(II)-oxyl ROS, dubbed the "oxyl mechanism" (Fig. 3) as this species has been previously predicted to exhibit a more strongly oxidative character than Cu(II)-superoxo (31,32 (20,33,34). From this system, we test two different geometries for oxidative attack at the C1 and C4 carbons to evaluate different enzyme-substrate poses (SI Appendix, Fig.…”

Theoretical Exploration of the Oxidative Properties of a [(trenMe1)CuO2]+ Adduct Relevant to Copper Monooxygenase Enzymes: Insights into Competitive Dehydrogenation versus Hydroxylation Reaction Pathways

“…This suggests that a balance should be found in the charge transfer from copper to dioxygen when designing ideal biomimetic systems: reactive complexes should permit sufficiently strong coordination of dioxygen but should also retain superoxide character to promote C À H abstraction. [31][32][33][34][35] A first illustration of this aspect was recently reported in a work dedicated to the oxidation of an exogenous substrate by a Cu I mononuclear complex relying on a neutral but strongly donating (N 4 ) ligand. [36][37][38] This shows that the catalysis of oxygen incorporation by a copper center might not be as unusual as thought.…”

“…Even though these values appear scattered, they are within the range observed upon variation of the substrates. [32] Starting from A F , the first step involves transfer from the terminal CH 3 group to distal oxygen atom O d . The geometric parameters of the C···H···O arrangement in TSA C H T U N G T R E N N U N G (A F -F) ( Table 1) are within 0.01 compared to those of triplet TSA C H T U N G T R E N N U N G (A C -C).…”

A new copper-oxo player in methane oxidation

“…Intriguingly, the Cu-ZSM-5 site evolves along the reaction coordinate with a nonuniform allocation of electron density from the lattice ligands, such that the electronic structure of the core becomes asymmetric. One of the copper sites develops significant copper-oxyl (radical), i.e., ''cupryl'' character (1), a species described only theoretically but attributed with high reactivity toward C-H bonds (1,13,14). A theoretical picture of methane oxidation at copper-oxo sites emerges in which unpaired electron density roughly localized on a Cu-bound oxo ligand may be critical for reactivity (see Fig.…”