Elsevier

Inorganica Chimica Acta

Volume 481, 1 September 2018, Pages 143-150
Inorganica Chimica Acta

Research paper
Tuning the locus of oxidation in Cu-diamido-diphenoxo complexes: From Cu(III) to Cu(II)-phenoxyl radical

https://doi.org/10.1016/j.ica.2017.09.049Get rights and content

Abstract

While N2O2 tetraanionic ligands containing a strong N-amidate σ-donor are generally assumed to stabilise metal high valence states, we herein have shown that, in dianionic Cu(II)-diamido-diphenoxo complexes, H-bonding and electronic effects on the phenolate groups may modulate the electronic structure of their oxidised species from Cu(III) to Cu(II)-phenoxyl radical complexes; and so in the negative potential range. We observe that electron-poor phenolate complexes 22− and 32− oxidise to Cu(III) species, whereas electron rich phenolate complex 12− oxidises to a Cu(II)-phenoxyl radical. Our DFT results suggest that π-electron-rich phenolate rings in 12− are responsible for an increase of the HOMO orbital energy, bringing the HOMO-SOMO gap small enough to favour a ligand-based oxidation process. Further DFT-calculations have also shown that upon changing the o,p-phenol substituent from electron-widthdrawing groups (NO2) to electron-donating ones (OMe), the favoured oxidised state switches from Cu(III) to Cu(II)-radical. These results emphasize the use of the versatile diamido-diphenoxo backbone as a promising way to novel GO-chemical models, as well as molecular switches.

Introduction

Tyrosyl radicals are found to be crucial for the catalytic activity of many metalloenzymes involved in important biological processes [1]. Galactose oxidase (GO) utilises a Cu(II)-tyrosyl radical redox pair to catalyse the two-electron oxidation of a wide range of primary alcohols to their corresponding aldehydes [2]. In the last fifteen years [3], chemists have succeeded in generating and characterising persistent Cu(II)-phenoxyl radical complexes, providing a better understanding of the electronic, chemical, and even structural [4], [5](a) properties of the active form of GO. Recently, in [CuII-salen]radical dot+ species, a tuning of the locus of the radical has been achieved through ligand backbone substitutions, yielding Class III (fully delocalised π-radical) or Class II (localised phenoxyl radical) Cu(II)- radical complexes [3](e), [5]. Remarkably, in the case of Cu(1,2-salen), the one-electron oxidation results in a high-valent, diamagnetic Cu(III) species in the solid state; whilst in solution a temperature-dependent valence tautomerism exists between the Cu(III) and the Cu(II)-radical oxidised forms [5b].

The ease of formation of the phenoxyl radical is a crucial parameter in designing GO-like catalysts. The oxidation of the phenolate ligand in neutral Cu(II)-N2O2 GO-models is found to be in the potential range 0.1–0.6 V (E1/2 vs. Fc+/Fc), above that observed in GO (ca. 0.01 V vs. Fc+/Fc) [2]. To date, the formation of Cu(II)-phenoxyl radical complexes at low negative potential has yet to be reported. Noteworthy, in the cases of dianionic Cu(II)-N2O2 complexes bearing diamido-diphenoxo ligands developed by Collins [6], an oxidation process at low negative potential is observed. However, the process is attributed to a metal-based oxidation leading to a Cu(III) species [6]. Clearly, the duality between the redox-innocent strong σ-donor N-amidate known to stabilise high-valence state metal complexes [7], and the non-innocent O-phenolate donor makes the locus of the oxidation of dianionic bis-salicylamide complexes non-trivial and deserving of further attention.

Recently, we have reported a unique dianionic square planar Cu(II)-diamido-diphenoxo complex (32−, Scheme 1) in which the coordinated phenolates are strongly H-bonded [8]. A recent report on its Ni(II) analogue shows that its oxidation is metal-based yielding Ni(III) species [9]. Herein we report a detailed study of the one-electron oxidised species of 32−, together with those of the non-H-bonded analogue complexes 12− and 22− bearing either tBu- or chloro- substituents at the o- and p-positions of the phenol rings, respectively (Scheme 1). Our combined experimental and theoretical study strongly suggests that H-bonding and electronic effects on the phenolate groups influence greatly the electronic structure of the oxidised species. Thus, whilst the oxidation of 22− and 32− yield Cu(III) species, that of 12− produces a Cu(II)-phenoxyl radical species, and so at low negative potential.

Section snippets

Experimental section

The synthesis of 3LH6, [NBu4]2[3LH4], [NBu4]2[Cu3LH2] (32−) has been carried out as previously reported [8]. The pro-ligands 1LH2, 2LH2 were prepared as previously reported [6], [10].

Physical methods

Elemental analyses of the compounds isolated in this study were accomplished in the University of Bar Ilan. EI mass spectra were recorded on a Q-Tof micro (UK)-micromass-waters spectrometer. 1H NMR spectra were recorded on a Bruker DPX300. UV/Vis spectra were recorded on a Varian Cary 5000 UV/Vis/NIR spectrophotometer. The measurements were carried out using a quartz cuvette with optical pathlength of 0.1 cm.

Cyclic voltammetry measurements were carried out using a Bio Logic SAS Sp-150

DFT calculations

All theoretical calculations were performed with the ORCA program package [11a]. Full geometry optimizations were carried out for all complexes using the GGA functional BP86 [11](b), [11](c), [11](d) in combination with the TZV/P [11e] basis set for all atoms and by taking advantage of the resolution of the identity (RI) approximation in the Split-RI-J variant [11f] with the appropriate Coulomb fitting sets [11g]. To model accurately the H bonding interactions in the complexes, the TZV/P++

Redox potentials: DFT methodology

The thermodynamics of redox reactions is quantified by redox potentials, E, that can be measured electrochemically and are related to the Gibbs free energy change (ΔG) with E = −ΔG/nF where n is the number of electrons transferred in the cell and F is the Faraday constant. Theoretical predictions of redox potentials require evaluation of ΔG for the oxidation or reduction process in question, e.g. Red(aq) → Ox(aq) for the oxidation reaction. A common strategy for calculating the free energy change

The dianionic complexes 12−, 22− and 32−

The diamido-diphenoxo pro-ligands, 1LH2, 2LH2 and 3LH4 were synthesised as previously reported [6], [8], [10]. The dianionic diphenolate salts [1L][NBu4]2, [2L][NBu4]2 and [3LH2][NBu4]2 were made by reacting them with two equivalents of [NBu4][OH] in methanol. The complexes (1)[NBu4]2 and (2)[NBu4]2 were prepared in the same manner as (3)[NBu4]2 [8], by reacting the proligand with [Cu(OAc)2]·(H2O) in MeOH in a 1:1 ratio in the presence of 4 equivalents of [OH][NBu4] in MeOH (see Experimental

Concluding remarks: tuning the locus of oxidation in Cu-phenolate complexes: From Cu(III) to Cu(II)-phenoxyl radical

While N2O2 tetraanionic ligands containing N-amidate strong σ-donors are generally assumed to stabilise metal higher valence state upon oxidation [6], [7], our present study appears to indicate that, when the O-donor is a redox-active phenolate group, this might not be necessarily the case.

We have reported a detailed study of three dianionic square planar Cu(II)-diamido-diphenoxo complexes (12−, 22− and 32−) and their corresponding one-electron oxidised species. Compounds 12− and 22− possess

Acknowledgements

The authors gratefully acknowledge the support of this work by the CNRS and the COST Action CM1305 ECOSTBio (Explicit Control Over Spin-States in Technology and Biochemistry).

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