Influence of Copper Coordination Spheres on Nitrous Oxide Reductase (N2Or) Activity of a Mixed-Valent Copper Complex Containing a {Cu2S} Core

Construction of eight mixed-valence pentanuclear CuI4CuII clusters using ligands with inhomogeneous electron density distribution: synthesis, characterization and photothermal properties

To enhance light absorption in the visible region for the utilization of sunlight, eight mixed-valence polynuclear Cu^I/Cu^II clusters have been synthesized for evaluating their photothermal conversion performance. They are fabricated considering the ligand's electron density distribution inhomogeneity using 1,2,3-triazole (3N) or tetrazole (4N) and different mono-phosphine ligands. We report here the synthesis, crystal structure, characterization, optical properties, and photothermal conversion performance of these clusters. X-ray crystal structures reveal that those pentanuclear clusters are neutral clusters with octahedrally-coordinated copper(II) ion being surrounded by four tetrahedrally coordinated copper(I) ions. Interestingly, with the introduction of the mixed-valence centers, these compounds show additional light absorption centers in 350-600 nm via the IVCT transition mechanism, compared with our previously reported Cu(II) compounds. These clusters show excellent photothermal conversion performance, with an average equilibrium temperature (∼60 °C) and a temperature increment (∼40 °C), which are also superior to Cu(II) complexes (the average equilibrium temperature ∼55 °C). This work proves that it is possible to design and prepare new polynuclear mixed-valence Cu^I/Cu^II clusters for achieving high-performance photothermal conversion materials.

Coordination chemistry of the CuZ site in nitrous oxide reductase and its synthetic mimics

Atmospheric nitrous oxide (N2O) has garnered significant attention recently due to its dual roles as an ozone depletion agent and a potent greenhouse gas. Anthropogenic N2O emissions occur primarily through agricultural disruption of nitrogen homeostasis causing N2O to build up in the atmosphere. The enzyme responsible for N2O fixation within the geochemical nitrogen cycle is nitrous oxide reductase (N2OR), which catalyzes 2H+/2e- reduction of N2O to N2 and H2O at a tetranuclear active site, CuZ. In this review, the coordination chemistry of CuZ is reviewed. Recent advances in the understanding of biological CuZ coordination chemistry is discussed, as are significant breakthroughs in synthetic modeling of CuZ that have emerged in recent years. The latter topic includes both structurally faithful, synthetic [Cu4(µ4-S)] clusters that are able to reduce N2O, as well as dicopper motifs that shed light on reaction pathways available to the critical CuI-CuIV cluster edge of CuZ. Collectively, these advances in metalloenzyme studies and synthetic model systems provide meaningful knowledge about the physiologically relevant coordination chemistry of CuZ but also open new questions that will pose challenges in the near future.

Water-mediated reduction of [Cu(dmp)2(CH3CN)]2+: implications of the structure of a classical complex on its activity as an anticancer drug

[Cu(dmp)₂(CH₃CN)]²⁺ can be reduced in acetonitrile containing water due to steric constraints of the ligands. Hydroxyl radicals are produced from water oxidation. We take advantage of this reaction to evaluate the anticancer activity of the complex.

Impact of Electronic and Steric Changes of Ligands on the Assembly, Stability, and Redox Activity of Cu4(μ4-S) Model Compounds of the CuZ Active Site of Nitrous Oxide Reductase (N2OR)

Model compounds have been widely utilized in understanding the structure and function of the unusual Cu₄(μ₄-S) active site (Cu_Z) of nitrous oxide reductase (N₂OR). However, only a limited number of model compounds that mimic both structural and functional features of Cu_Z are available, limiting insights about Cu_Z that can be gained from model studies. Our aim has been to construct Cu₄(μ₄-S) clusters with tailored redox activity and chemical reactivity via modulating the ligand environment. Our synthetic approach uses dicopper(I) precursor complexes (Cu₂L₂) that assemble into a Cu₄(μ₄-S)L₄ cluster with the addition of an appropriate sulfur source. Here, we summarize the features of the ligands L that stabilize precursor and Cu₄(μ₄-S) clusters, along with the alternative products that form with inappropriate ligands. The precursors are more likely to rearrange to Cu₄(μ₄-S) clusters when the Cu(I) ions are supported by bidentate ligands with 3-atom bridges, but steric and electronic features of the ligand also play crucial roles. Neutral phosphine donors have been found to stabilize Cu₄(μ₄-S) clusters in the 4Cu(I) oxidation state, while neutral nitrogen donors could not stabilize Cu₄(μ₄-S) clusters. Anionic formamidinate ligands have been found to stabilize Cu₄(μ₄-S) clusters in the 2Cu(I):2Cu(II) and 3Cu(I):1Cu(II) states, with both the formation of the dicopper(I) precursors and subsequent assembly of clusters being governed by the steric factor at the ortho positions of the N-aryl substituents. Phosphaamidinates, which combine a neutral phosphine donor and an anionic nitrogen donor in the same ligand, form multinuclear Cu(I) clusters unless the negative charge is valence-trapped on nitrogen, in which case the resulting dicopper precursor is unable to rearrange to a multinuclear cluster. Taken together, the results presented in this study provide design criteria for successful assembly of synthetic model clusters for the Cu_Z active site of N₂OR, which should enable future insights into the chemical behavior of Cu_Z.

Synthesis and Characterization of Cu(II) and Mixed-Valence Cu(I)Cu(II) Clusters Supported by Pyridylamide Ligands

A group of copper complexes supported by polydentate pyridylamide ligands H₂bpda and H₂ppda were synthesized and characterized. The two Cu(II) dimers [Cu^II₂(Hbpda)₂(ClO₄)₂] (1) and [Cu^II₂(ppda)₂(DMF)₂] (2) were constructed by using neutral ligands to react with Cu(II) salts. Although the dimers showed similar structural features, the second-sphere interactions affect the structures differently. With the application of Et₃N, the tetranuclear cluster (HNEt₃)[Cu^II₄(bpda)₂(μ₃-OH)₂(ClO₄)(DMF)₃](ClO₄)₂ (3) and hexanuclear cluster (HNEt₃)₂[Cu^II₆(ppda)₆(H₂O)₂(CH₃OH)₂](ClO₄)₂ (4) were prepared under similar reaction conditions. The symmetrical and unsymmetrical arrangement of the ligand donors in ligands H₂bpda and H₂ppda led to the dramatic conformation difference of the two Cu(II) complexes. As part of our effort to explore mixed-valence copper chemistry, the triple-decker pentanuclear cluster [Cu^II₃Cu^I₂(bpda)₃(μ₃-O)] (5) was prepared. XPS examination demonstrated the localized mixed-valence properties of complex 5. Magnetic studies of the clusters with EPR evidence showed either weak ferromagnetic or antiferromagnetic interactions among copper centers. Due to the trigonal-planar conformation of the trinuclear Cu(II) motif with the μ₃-O center, complex 5 exhibits geometric spin frustration and engages in antisymmetric exchange interactions. DFT calculations were also performed to better interpret spectroscopic evidence and understand the electronic structures, especially the mixed-valence nature of complex 5.