Modulation of a μ-1,2-Peroxo Dicopper(II) Intermediate by Strong Interaction with Alkali Metal Ions

Abstract

The properties of metal/dioxygen species, which are key intermediates in oxidation catalysis, can be modulated by interaction with redox-inactive Lewis acids, but structural information about these adducts is scarce. Here we demonstrate that even mildly Lewis acidic alkali metal ions, which are typically viewed as innocent “spectators”, bind strongly to a reactive cis-peroxo dicopper­(II) intermediate. Unprecedented structural insight has now been obtained from X-ray crystallographic characterization of the “bare” CuII 2(μ-η1:η1-O2) motif and its Li+, Na+, and K+ complexes. UV–vis, Raman, and electrochemical studies show that the binding persists in MeCN solution, growing stronger in proportion to the cation’s Lewis acidity. The affinity for Li+ is surprisingly high (∼70 × 104 M–1), leading to Li+ extraction from its crown ether complex. Computational analysis indicates that the alkali ions influence the entire Cu-OO-Cu core, modulating the degree of charge transfer from copper to dioxygen. This induces significant changes in the electronic, magnetic, and electrochemical signatures of the Cu2O2 species. These findings have far-reaching implications for analyses of transient metal/dioxygen intermediates, which are often studied in situ, and they may be relevant to many (bio)­chemical oxidation processes when considering the widespread presence of alkali cations in synthetic and natural environments.

Antisymmetric Spin Exchange in a μ-1,2-Peroxodicopper(II) Complex with an Orthogonal Cu–O–O–Cu Arrangement and S = 1 Spin Ground State Characterized by THz-EPR

“…Following the reaction by UV/vis absorption spectroscopy (Figure S8) shows the emergence of a dominant peak at 520 nm (ε ≈ 5500 M –1 cm –1 ) with shoulders at 617 nm (ε ≈ 3300 M –1 cm –1 ) and 790 nm (ε ≈ 1500 M –1 cm –1 ), as well as a less pronounced shoulder at higher energy around 430 nm (ε ≈ 2000 M –1 cm –1 ). These distinct features are characteristic of μ-η 1 :η 1 -peroxodicopper­(II) complexes, mostly due to O 2 2– → Cu II charge transfer transitions. , In particular, the spectrum of 2 is very similar to that of complex B (Figure ), whereas complex A reveals a distinct maximum at 648 nm that was recently attributed by TD-DFT calculations to excitations within the π-manifold on the Cu–O 2 –Cu core. , Comparison of the three spectra indicates subtle electronic differences within this series of peroxodicopper­(II) complexes based on the dinucleating pyrazolate/tacn ligands.…”

“…Examples of isolated Cu 2 /O 2 adducts with a cis -μ-η 1 :η 1 peroxo motif ( C P ) are relatively scarce, and structural characterization has been achieved only recently. 10 − 12 While the S P core represents the key intermediate identified in oxygenated type III copper proteins, species with a μ-η 1 :η 1 -peroxo unit have been proposed along the trajectory of O 2 binding/release at these biological dicopper sites, based on computational work by Solomon and co-workers. 13 − 15 In this scenario, binding of triplet dioxygen initially proceeds via simultaneous electron transfer from the two Cu I ions to the two orthogonal dioxygen π* orbitals, whereby 3 O 2 is reduced to 1 O 2 2– .…”

“…trans -μ-η 1 :η 1 - ( T P ) and μ-η 2 :η 2 -peroxodicopper­(II) ( S P ) complexes have been quite well studied, − and they are generally found to have a strongly stabilized total spin S t = 0 ground state due to antiferromagnetic coupling between the two Cu II ions. Examples of isolated Cu 2 /O 2 adducts with a cis -μ-η 1 :η 1 peroxo motif ( C P ) are relatively scarce, and structural characterization has been achieved only recently. − While the S P core represents the key intermediate identified in oxygenated type III copper proteins, species with a μ-η 1 :η 1 -peroxo unit have been proposed along the trajectory of O 2 binding/release at these biological dicopper sites, based on computational work by Solomon and co-workers. − In this scenario, binding of triplet dioxygen initially proceeds via simultaneous electron transfer from the two Cu I ions to the two orthogonal dioxygen π* orbitals, whereby 3 O 2 is reduced to 1 O 2 2– . Delocalization of the unpaired spins to the copper­(II) ions results in sizeable ferromagnetic coupling (Scheme ).…”

Altering oxygen binding by redox-inactive metal substitution to control catalytic activity: oxygen reduction on manganese oxide nanoparticles as a model system

“…Importantly, as described above, this was achieved without changing the crystal structure, particle morphology, nor the valence state of manganese. This suggests that there exist direct correlations between certain intrinsic properties of the substituting elements and the catalytic performance of Mn3O4:M. In fact, the influence of the incorporation of redox-inactive substituents on the redox properties of biological systems 45 and molecular complexes has been reported [46][47][48][49][50] and the observed trends have been rationalized typically by an inductive effect triggered by the substituent/dopant altering the covalency of the bonds between the parent metal centers and adjacent oxygen . 40 More recently, the concept of the inductive effect was extended further to explain the effect of substituents on the redox properties (and in turn the catalytic performance) of complex oxide materials.…”

A Binuclear Cobalt Complex for Molecular CO2 Electrocatalysis

“…[26,27] In our search for new molecular systems that can electrochemically activate and reduce CO2, [28,29] we decided to synthesize a new bimetallic molecular complex bearing a pyrazole-core substituted with terpyridine groups at the 3,5-positions (Figure 1). Although this approach blocks the possibility of exobimetallic substrate activation, [30] ligands bearing a terpyridine fragment have shown the ability to reduce the overpotential for CO2 electroreduction through metal-ligand cooperativity. [31][32][33] Additionally, the new synthesized ligand structure would generate a complex with structural similarities to the [Co II (qpy)(H2O)2] 2+ electrocatalyst (qpy = 2,2':6',2'':6'',2'''-quaterpyridine), which our group has thoroughly studied, [34][35][36] and that could serve as a mononuclear comparative system (Figure 1).…”