Influence of the amine donor on hybrid guanidine-stabilized Bis(μ-oxido) dicopper(III) complexes and their tyrosinase-like oxygenation activity towards polycyclic aromatic alcohols
Graphical abstract
Synopsis
New aromatic hybrid guanidine amine ligands with varying amine donor function stabilize bis(μ-oxido) dicopper(III) species at low temperatures. Small structural changes in the ligand sphere were found to influence the spectral and geometric features of the bis(μ-oxido) species. All three complexes demonstrated their activity in catalytic oxygenations of aromatic alcohols.
Introduction
The activation of molecular oxygen for the application as oxidation catalyst states one major purpose in chemical research [[1], [2], [3]]. In nature, copper enzymes set an example by their ability to activate dioxygen and to catalyze organic transformation reactions [4,5]. Tyrosinase represents a copper-containing enzyme, which is responsible for the hydroxylation of monophenols and the oxidation of ortho-catechols to generate ortho-quinones [6,7]. The enzyme is present in numerous organisms, controlling the production of the pigment melanin from l-tyrosine [8,9]. The molecular mechanisms of the activation and transfer of dioxygen still remain challenging, although many efforts were made to understand the structure-activity relationship of the active sites of the enzyme [5,[10], [11], [12], [13], [14], [15], [16], [17]]. Some extensively studied copper-dioxygen model complexes are the μ-η2:η2-peroxido dicopper(II) [P] and the bis(μ-oxido) dicopper(III) [O] species, which are able to interconvert into each other due to their small isomerization barrier [5,18,19]. In [O] species, the Cu2O2 rhomb is significantly influenced by electronic and steric properties of the supporting N-donor ligand system. Systematic studies on the spectroscopic properties of the [O] species by small modifications of the N-donor moieties are available, which involve amine, histidine, imidazole and guanidine ligands developed by the groups of Tolman [20], Stack [[21], [22], [23], [24], [25], [26], [27]], Solomon [21,23], and Herres-Pawlis [[28], [29], [30], [31]] (Fig. 1).
Geometric elongation and compression of the Cu2O2 rhomb was identified by X-ray diffraction and EXAFS analysis as well as density functional theory (DFT) calculations in those studies. The perturbations of the Cu2O2 geometry were primarily attributed to the steric bulk of the ligand system: the higher the steric demand of the ligand sphere, the more elongated the Cu2O2 rhomb, and vice versa [22]. Also, the twist angle of the CuN2 plane relative to the Cu2O2 plane was found to play an important role in the stabilization of the highly reactive [O] species and their ligand-to-metal charge transfer (LMCT) energies [29,32]. Besides the common direct synthesis of [O] species, Stack and co-workers demonstrated that [O] species can be synthesized indirectly by ligand exchange reactions driven by the more donating and thermodynamically supporting ligand system, which also influence the LMCT transition energies of the [O] species [26,27].
Most of the amine-stabilized [O] species are limited to the stoichiometric hydroxylation of exogenous substrates. Reported synthetic model systems mimicking the functionality of the active site of tyrosinase mostly involve the [P] core [5,15,16,33], but a few [O] species were also reported over the last years [31,34,35]. Catalytic oxygenation features were first presented by Réglier and co-workers in 1990 by using an imine-pyridine ligand system to support the copper-catalyzed oxygenation of 2,4-di-tert-butyl phenol to 3,5-di-tert-butyl quinone [36]. Another binuclear catalyst was shown by Casella and co-workers employing benzimidazole ligand L66 [37]. Further systems were developed in recent years by the groups of Lumb and Stack [[38], [39], [40]], Tuczek [[41], [42], [43], [44], [45], [46]] and Herres-Pawlis [31,34,35,[47], [48], [49], [50]], demonstrating the diversity of stabilizing N-donor ligand systems in copper-catalyzed hydroxylation reactions of (poly)cyclic aromatic alcohols. Besides pyridinyl, pyrazolyl, imine and amine donor units, guanidines are known for their high basicity, enabling the stabilization of high oxidation states of copper complexes such as bis(μ-oxido) and superoxido motifs [28,[30], [31], [32],34,[51], [52], [53], [54], [55], [56], [57], [58]].
Recently, we reported the stabilization of an [O] species by the hybrid guanidine amine ligand TMGbenza (L1) in the presence of weakly-coordinating anions at low temperatures [31]. The bis(μ-oxido) complex depicted high tyrosinase-like reactivity towards diverse phenolic substrates. The high selectivity of the catalyst in the oxygenation reactions was achieved by a beneficial interplay of the strong, sterically demanding guanidine function and the spatially smaller, weaker amine donor. The formed quinones were directly transformed into more stable phenazine derivatives, which are of major interest due to their antibacterial, antitumor and antimalarial features [[59], [60], [61], [62], [63], [64], [65], [66]]. A recent study on a L1-stabilized [O] species in the presence of coordinating halide anions revealed the existence of a supporting iodide-bridge between the formally copper(III) centers and the formation of iodidocuprate anions, yielding a room temperature stable [O] complex with catalytic activity [34].
Aiming to inhibit the formation of unreactive bischelate copper(I) species, we herein report the synthesis and characterization of two novel hybrid guanidine amine ligands with modified amine donor moiety to increase the steric demand of the ligand system (Fig. 1). The monochelate copper(I) complexes stabilized by L2 and L3 are tested towards their ability to activate molecular oxygen to form reactive [O] complexes. The formally copper(III) species is evaluated as catalyst in oxygenation reactions of polycyclic aromatic alcohols, including naphthols and quinolinols. Generated quinones are transformed in a one-pot reaction into phenazines by using 1,2-phenylenediamine. The structure-reactivity is compared to related hybrid guanidine systems published previously and the influence of different amine donor abilities of the ligand system on the bis(μ-oxido) complex and its catalytic activity are evaluated.
Section snippets
General remarks
All synthetic procedures were performed under an inert atmosphere of nitrogen with the use of standard Schlenk or glovebox techniques. All chemicals were purchased commercially (Table S1 in the Supporting Information) and used without further purification unless otherwise noted. Solvents were purified under nitrogen atmosphere via distillation from CaH2 or sodium/benzophenone ketyl radical. Copper salts [67], Vilsmeier salt chloro-N,N,N′,N′-tetramethylformamidinium chloride [[68], [69], [70]]
Synthesis of ligands and Cu(I) complexes
In analogy to our previously reported hybrid guanidine ligand system TMGbenza (L1) [31], two related ligands were developed by modification of the amine donor unit to evaluate the influence on the steric effects and the donor strength of the ligand system on dioxygen activation and transfer processes (Scheme 1). Ligand L1 [31] contains a permethylated amine moiety, which is substituted by a diethyl or di-isopropyl group to give the new hybrid guanidine ligands TMGbenzNEt2 (L2, IUPAC name:
Conclusion
In summary, we designed new aromatic hybrid guanidine ligands with varied amine donor moiety, which exclusively formed monochelate copper(I) complexes due to the increased steric demand of the ligand system, thus eliminating a predominant equilibrium between monochelate and bischelate copper(I) complex. Oxygenation of the copper(I) complexes allowed the formation of the corresponding bis(μ-oxido) complexes in the presence of different weakly-coordinating anions, which were characterized by
Authors statement
Melanie Paul: Conceptualization, Synthetic and UV/Vis spectroscopic and MS spectrometric investigations, Writing, Data curation.
Melissa Teubner: Raman investigation, Writing.
Benjamin Grimm-Lebsanft: Raman investigation, Writing.
Sören Buchenau: Raman investigation.
Alexander Hoffmann: Conceptualization, DFT investigation, formal analysis, Writing, Data curation,
Michael Rübhausen: Supervision, formal analysis.
Sonja Herres-Pawlis: Conceptualization, Writing, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Electronic supplementary material The online version of this article (doi:https://doi.org/10.1016/j.jinorgbio.2021.XXX) contains supplementary material, which is available to authorized users.
Acknowledgements
The authors gratefully acknowledge financial support provided by the German Research Foundation (DFG), in framework of the Priority Program “Reactive Bubbly Flows” SPP 1740 (HE 5480/10-2, http://www.dfg-spp1740.de/), and further projects (RU 773/8-1). This work was also funded by the Bundesministerium für Bildung und Forschung (BMBF, project 05K19GU5). We furthermore thank the Regional Computing Center of the University of Cologne (RRZK) for providing computing time on the DFG-funded
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