Dicopper Complex Research Articles

Mechanistic Insights into the ortho-Defluorination-Hydroxylation of 2-Halophenolates Promoted by a Bis(μ-oxo)dicopper(III) Complex.

C-F bonds are one of the most inert functionalities. Nevertheless, some [Cu2O2]2+ species are able to defluorinate-hydroxylate ortho-fluorophenolates in a chemoselective manner over other ortho-halophenolates. Albeit it is known that such reactivity is promoted by an electrophilic attack of a [Cu2O2]2+ core over the arene ring, the crucial details of the mechanism that explain the chemo- and regioselectivity of the reaction remain unknown, and it has not being determined either if CuII2(η2:η2-O2) or CuIII2(μ-O)2 species are responsible for the initial attack on the arene. Herein, we present a combined theoretical and experimental mechanistic study to unravel the origin of the chemoselectivity of the ortho-defluorination-hydroxylation of 2-halophenolates by the [Cu2(O)2(DBED)2]2+ complex (DBED = N,N'-di-tert-butylethylenediamine). Our results show that the equilibria between (side-on)peroxo (P) and bis(μ-oxo) (O) isomers plays a key role in the mechanism, with the latter being the reactive species. Furthermore, on the basis of quantum-mechanical calculations, we were able to rationalize the chemoselective preference of the [Cu2(O)2(DBED)2]2+ catalyst for the C-F activation over C-Cl and C-H activations.

Sensing and Liquid-Liquid Extraction of Dicarboxylates Using Dicopper Cryptates.

We report the investigation of dicopper(II) bistren cryptate, containing naphthyl spacers between the tren subunits, as a receptor for polycarboxylates in neutral aqueous solution. An indicator displacement assay for dicarboxylates was also developed by mixing the azacryptate with the fluorescent indicator 5-carboxyfluorescein in a 50:1 molar ratio. Fluorimetric studies showed a significant restoration of fluorophore emission upon addition of fumarate anions followed by succinate and isophthalate. The introduction of hexyl chains on the naphthalene groups created a novel hydrophobic cage; the corresponding dicopper complex was investigated as an extractant for dicarboxylates from neutral water into dichloromethane. The liquid–liquid extraction of succinate—as a model anion—was successfully achieved by exploiting the high affinity of this anionic guest for the azacryptate cavity. Extraction was monitored through the changes in the UV–visible spectrum of the dicopper complex in dichloromethane and by measuring the residual concentration of succinate in the aqueous phase by HPLC-UV. The successful extraction was also confirmed by 1H-NMR spectroscopy. Considering the relevance of polycarboxylates in biochemistry and in the environmental field, e.g., as waste products of industrial processes, our results open new perspectives for research in all contexts where recognition, sensing, or extraction of polycarboxylates is required.

Exchange Spin Coupling from Gaussian Process Regression

Heisenberg exchange spin coupling between metal centers is essential for describing and understanding the electronic structure of many molecular catalysts, metalloenzymes, and molecular magnets for potential application in information technology. We explore the machine-learnability of exchange spin coupling beyond linear regression, which has not been studied yet. We employ Gaussian process regression, since it can potentially deal with small training sets (as likely associated with the rather complex molecular structures required for exploring spin coupling) and since it provides uncertainty estimates ("error bars") along with predicted values. We compare a range of descriptors and kernels for 257 small dicopper complexes and find that a simple descriptor based on chemical intuition, consisting only of copper-bridge angles and copper-copper distances, clearly outperforms several more sophisticated descriptors when it comes to extrapolating toward larger experimentally relevant complexes. Exchange spin coupling is similarly easy to learn as the polarizability, while learning dipole moments is much harder. The strength of the sophisticated descriptors lies in their ability to linearize structure-property relationships, to the point that a simple linear ridge regression performs just as well as the kernel-based machine-learning model for our small dicopper data set. The superior extrapolation performance of the simple descriptor is unique to exchange spin coupling, reinforcing the crucial role of choosing a suitable descriptor and highlighting the interesting question of the role of chemical intuition vs systematic or automated selection of features for machine learning in chemistry and material science.

Hexanuclear Copper(I) Hydride from the Reduction-Induced Decarboxylation of a Dicopper(II) Formate.

Copper(I) hydride complexes represent a promising entry into formic acid dehydrogenation catalysis. Herein we present the spontaneous decarboxylation of a μ1,3-formate-bridged dicopper(II) complex (1H) to a hexacopper(I) hydride cluster (2H) upon reduction. Isotopic labeling studies revealed that both the H- and CO2 originate from the bound μ1,3-formate in 1H, which represents a key step of the metal-mediated formic acid dehydrogenation. The full reaction equation for the conversion of 1H to 2H is established. The structure of 2H features two Cu3 triangles, each capped by a hydride ligand. Typical hydride reactivity of 2H is demonstrated by the addition of phenylacetylene, leading to the replacement of the hydrides by alkynide ligands -C≡CPh (3) while retaining the hexacopper(I) core. Temperature-dependent dynamic behavior in solution on the NMR time scale was observed for both 2H and 3, reflecting the rich structural landscape of the bis(pyrazolate)-bridged hexacopper(I) core (four isomers each for 2H and 3) predicted by DFT calculations.

Structure and Reactivity of Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-Ion-Exchanged Chabazite Catalyst for NH3-Mediated Selective Catalytic Reduction.

The NH3-mediated selective catalytic reduction (NH3-SCR) of NOx over Cu-ion-exchanged chabazite (Cu-CHA) catalysts is the basis of the technology for abatement of NOx from diesel vehicles. A crucial step in this reaction is the activation of oxygen. Under conditions for low-temperature NH3-SCR, oxygen only reacts with CuI ions, which are present as mobile CuI diamine complexes [CuI(NH3)2]+. To determine the structure and reactivity of the species formed by oxidation of these CuI diamine complexes with oxygen at 200 °C, we have followed this reaction, using a Cu-CHA catalyst with a Si/Al ratio of 15 and 2.6 wt% Cu, by X-ray absorption spectroscopies (XANES and EXAFS) and diffuse reflectance UV-Vis spectroscopy, with the support of DFT calculations and advanced EXAFS wavelet transform analysis. The results provide unprecedented direct evidence for the formation of a [Cu2(NH3)4O2]2+ mobile complex with a side-on μ-η2,η2-peroxo diamino dicopper(II) structure, accounting for 80–90% of the total Cu content. These [Cu2(NH3)4O2]2+ are completely reduced to [CuI(NH3)2]+ at 200 °C in a mixture of NO and NH3. Some N2 is formed as well, which suggests the role of the dimeric complexes in the low-temperature NH3-SCR reaction. The reaction of [Cu2(NH3)4O2]2+ complexes with NH3 leads to a partial reduction of the Cu without any formation of N2. The reaction with NO results in an almost complete reduction to CuI, under the formation of N2. This indicates that the low-temperature NH3-SCR reaction proceeds via a reaction of these complexes with NO.

Selective colorimetric sensing and perfect linear recognition of azide: Formation of Cu-azide-Cu cascade complex within the cavity of cryptand.

Bis-trpn [tris(3-aminopropyl)amine], capped dicopper complex of bicyclic cryptand L, 1, became a potential selective colorimetric chemosensor for azide anion. Complex 1 is generating a space inside the cylindrical cavity which will be opt for perfect linear recognition of azide anion through as N4-Cu⋯N3-⋯Cu-N4 axle. Naked eye colorimetric and UV-Vis spectrometric investigations shows the complex 1 has the capability of selective sensing of azide anion. The association constant and limits of detection (LoD) of complex 1 towards azide are found to be 2.754 × 103 M-1 and 1.91 × 10-6 M. To the best of our knowledge, this is the first example of selective colorimetric sensing of azide by a bis‑copper cryptate 1 via ideal linear orientation of N4-Cu⋯N3⋯Cu-N4 axle inside the cylindrical shaped cavity.

The formation of di-copper (II) complexes with a hetero-site cyclopeptide–spectroscopic and potentiometric studies

The interest in cyclic peptide research has increased in the last decade. These compounds are resistant to enzymatic degradation and possess a more rigid scaffold than their linear analogues. This paper presents the results of combined potentiometric and spectroscopic studies on the coordination properties of hetero-site cyclopeptide (CDP) with the c(HHKHHPHDHKHP) sequence. Thermodynamic stability constants for ligands and complex forms have been determined in a system with a double excess of copper ions, i.e. with a molar ratio to ligand L:Cu(II) of 1:2. The interaction was characterised by UV–Vis, CD, EPR and ESI-MS methods over a wide pH range. CDP is a good chelator for divalent metal ions, it creates mono-, di- and trinuclear complexes with copper(II) ions over a wide range of pH. It was found that promotion of di-copper(II) complexes is connected with a higher flexibility of the cyclopeptide chain.

Towards Building Blocks for Supramolecular Architectures Based on Azacryptates.

In this work, we report the synthesis of a new bis(tris(2-aminoethyl)amine) azacryptand L with triphenyl spacers. The binding properties of its dicopper complex for aromatic dicarboxylate anions (as TBA salts) were investigated, with the aim to obtain potential building blocks for supramolecular structures like rotaxanes and pseudo-rotaxanes. As expected, UV-Vis and emission studies of [Cu2L]4+ in water/acetonitrile mixture (pH = 7) showed a high affinity for biphenyl-4,4′-dicarboxylate (dfc2−), with a binding constant of 5.46 log units, due to the best match of the anion bite with the Cu(II)-Cu(II) distance in the cage’s cavity. Compared to other similar bistren cages, the difference of the affinity of [Cu2L]4+ for the tested anions was not so pronounced: conformational changes of L seem to promote a good interaction with both long (e.g., dfc2−) and short anions (e.g., terephthalate). The good affinity of [Cu2L]4+ for these dicarboxylates, together with hydrophobic interactions within the cage’s cavity, may promote the self-assembly of a stable 1:1 complex in water mixture. These results represent a good starting point for the application of these molecular systems as building units for the design of new supramolecular architectures based on non-covalent interactions, which could be of interest in all fields related to supramolecular devices.

Tuning the catecholase activity of bis(pyrazolyl)methane-based copper(II) complexes by substitutions of the ligand core: unraveling a dual O2/H2O2 oxidation mechanism

Five new dinuclear copper(II) complexes based on dicompartmental ligands of the general formula R1R2R1-C6(OH)[C(O)-NH-CH2-CH(pz)2]2 (pz = pyrazolyl ring, R1 = R2 = H: HHH-Cu2; R1 = H, R2 = t-Bu: HButH-Cu2; R1 = Me, R2 = H: MeHMe-Cu2; R1 = Me, R2 = Br: MeBrMe-Cu2; R1 = Me, R2 = NO2: MeNO2Me-Cu2) were prepared and characterized in both solution and solid state. The phenoxide moiety acts as a bridge between two copper(II) centers; the remaining coordination sites of the metals are filled by the deprotonated N-amide atoms, the nitrogen atoms originating from the bis(pyrazolyl)methane groups and one exogenous hydroxide bridge. The catecholase-type activity of these dicopper complexes was studied in methanol, using a combination of aerobic and anaerobic methods. These investigations revealed that the substitution pattern of the central arene ring has an impact on the catalytic activity of our complexes, with the catalytic activity increasing in the following order: HHH-Cu2 <HButH-Cu2 <MeNO2Me-Cu2 <MeBrMe-Cu2 <MeHMe-Cu2. H2O2 was identified as the byproduct of O2 reduction. Its influence on the catalytic mechanism has been examined and found to have an impact on the catecholase-type reaction using these catalysts. Based on these findings, a dual mechanism for the catechol oxidation is proposed, where both molecular oxygen and the formed hydrogen peroxide act concurrently as oxidants.

Tuning the Bonding of a μ-Mesityl Ligand on Dicopper(I) through a Proton-Responsive Expanded PNNP Pincer Ligand

We report the synthesis and characterization of a series of cationic, neutral, and anionic dicopper(I) complexes featuring a μ-mesityl ligand and a naphthyridine-derived PNNP expanded pincer ligand. Structural characterization showed that the protonation state of the dinucleating ligand has a pronounced effect on the bending and tilting of the μ-mesityl ligand. DFT calculations indicate that the varying orientations of the μ-mesityl ligand are inherent due to changes in electronic structure rather than crystal-packing effects. NBO analysis reveals how the interactions that contribute to the three-center–two-electron bond between the μ-mesityl ligand and the dicopper core change for the various degrees of observed bending and tilting.

METHODS FOR THE SYNTHESIS OF IMMUNOSTIMULANT PLERIXAFOR

This minireview discusses methods for the synthesis of the immunostimulant plerixafor (Mozobil®, AMD3100). The information presented covers the period from the publication of the first work devoted to the synthesis of plerixafor and the study of the electrochemical properties of its dinickel(II) and dicopper(II) complexes in 1987 to the present.

Schiff base triggering synthesis of copper(II) complex and its catalytic fate towards mimics of phenoxazinone synthase activity

This research article reports the synthesis, structural characterization and phenoxazinone synthase activity of a new phenoxido-bridged dicopper(II)-Schiff base complex, [Cu2(L)3]ClO4(1), [HL = (Z)-2-methoxy-6-(((2-methoxyphenyl)imino) methyl)phenol]. X-ray structure of the Cu(II) complex reveals that each of the two Cu(II) centres in 1 adopts different coordination geometries (distorted octahedral and distorted square planar) and phenoxido bridges couple two Cu(II) centres to form a dinuclear copper complex with Cu⋯Cu distance 3.182 Å. The dicopper(II) complex catalyzes the aerobic oxidation of 2-aminophenol (2-AP) to aminophenoxazinone species in acetonitrile with good turnover number, 78.14 h−1. Electro-chemical and electrospray ionization mass spectrometry analysis of 1 in presence of 2-AP ensure the involvement and key role of the copper(II) centres in formation of enzyme-substrate adduct. Subsequently, electron paramagnetic resonance study confirms the generation of radical species in the course of catalysis. Finally, density functional theoretical calculations well reproduce the experimental geometrical parameters and spectroscopic behaviours of 1 and decisively argue in favour of the proposed catalytic pathways.

Electronic Structures and Reactivity Profiles of Aryl Nitrenoid-Bridged Dicopper Complexes.

Dicopper complexes templated by dinucleating, pacman dipyrrin ligand scaffolds (Mesdmx, tBudmx: dimethylxanthine-bridged, cofacial bis-dipyrrin) were synthesized by deprotonation/metalation with mesitylcopper (CuMes; Mes: mesityl) or by transmetalation with cuprous precursors from the corresponding deprotonated ligand. Neutral imide complexes (Rdmx)Cu2(μ2-NAr) (R: Mes, tBu; Ar: 4-MeOC6H4, 3,5-(F3C)2C6H3) were synthesized by treatment of the corresponding dicuprous complexes with aryl azides. While one-electron reduction of (Mesdmx)Cu2(μ2-N(C6H4OMe)) with potassium graphite initiates an intramolecular, benzylic C-H amination at room temperature, chemical reduction of (tBudmx)Cu2(μ2-NAr) leads to isolable [(tBudmx)Cu2(μ2-NAr)]- product salts. The electronic structures of the thermally robust [(tBudmx)Cu2(μ2-NAr)]0/- complexes were assessed by variable-temperature electron paramagnetic resonance spectroscopy, X-ray absorption spectroscopy (Cu L2,3/K-edge, N K-edge), optical spectroscopy, and DFT/CASSCF calculations. These data indicate that the formally Class IIIA mixed valence complexes of the type [(Rdmx)Cu2(μ2-NAr)]-feature significant NAr-localized spin following reduction from electronic population of the [Cu2(μ2-NAr)] π* manifold, contrasting previous methods for engendering iminyl character through chemical oxidation. The reactivity of the isolable imido and iminyl complexes are examined for prototypical radical-promoted reactivity (e.g., nitrene transfer and H-atom abstraction), where the divergent reactivity is rationalized by the relative degree of N-radical character afforded from different aryl substituents.

Copper(I) complexes bearing mesoionic carbene ligands: influencing the activity in catalytic halo-click reactions.

Metal complexes of mesoionic carbene (MIC) ligands are known to catalyze a variety of chemical transformations. In this contribution, we report on the synthesis of a dicationic dicopper(i) complex containing a di-MIC ligand. Two routes are presented for the synthesis of the dicopper complex: Ag-mediated transmetalation and direct deprotonation. For the Ag-mediated transmetalation route, the detection and isolation of several Ag-containing intermediates that are relevant for the final formation of the aforementioned dicopper complex are reported. We then investigate a series of copper(i) complexes based on MIC ligands as precatalysts for the azide halo-alkyne (Click) cycloaddition reaction. In a comparative study, three different halide-containing (I, Br, Cl) substrates have been investigated with different catalysts to survey the behaviour for mono/di-copper-MIC complexes as well as neutral, mono- and di-cationic complexes. The cationic complexes proved to have superior activities compared to the neutral species. These are the first reports on the use of Cu-MIC complexes as precatalysts for the halo-Click reaction.

Isomerism and dynamic behavior of bridging phosphaalkynes bound to a dicopper complex.

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu2(μ-η 1 :η 1 -MeCN)DPFN](NTf2)2, was treated with phosphaalkynes (RC[triple bond, length as m-dash]P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η 2:η 2 binding coordination modes. X-ray crystallography revealed that these unusual "tilted" structures exist in two isomeric forms (R "up" vs. R "sideways"), depending on the steric profile of the phosphaalkyne's alkyl group (R = Me, Ad, or t Bu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and t Bu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution, and NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu2(μ-η 1:η 1-MeCN)DPFN](NTf2)2 with 2-butyne affords [Cu2(μ-η 2:η 2-(MeC[triple bond, length as m-dash]CMe))DPFN](NTf2)2: its similar ligand geometry demonstrates that the tilted μ-η 2:η 2 binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding.

Formation of a four-bladed waterwheel-type chloro-bridged dicopper(ii) complex with dithiamacrocycle via double exo-coordination.

A combination of O3S2-macrocycles incorporating different sulfur-to-sulfur separations (S-(CH2)n-S, L1: n = 2, L2: n = 3) and copper(ii) nitrate afforded new types of both monocopper(ii) and dicopper(ii) complexes, respectively. L1 gave a 1D coordination polymer [Cu2(L1)2(NO3)4]n (1) based on a convergent exo-coordination mode while L2 resulted in the formation of a divergent exo/exo-coordinated dicopper(ii) complex, [Cu2(L2ox)4(μ-Cl)](NO3)4 (2), whose shape resembles a four-bladed waterwheel in which in situ oxidized macrocycles (L2ox) act as the blades and a CuII-(μ-Cl)-CuII entity corresponds to the axle shaft. The chloro-bridging ligand is derived from the dichloromethane solvent and its arrangement is held together by C-HCl- H-bonds. Compound 2 shows a weak antiferromagnetic property via the CuII-(μ-Cl)-CuII entity.

Dinuclear copper(ii) complexes containing oxamate and blocking ligands: crystal structure, magnetic properties, and DFT calculations

Dicopper(ii) complexes were prepared as promising platforms for the design of polynuclear systems providing an avenue toward new molecule-based materials.

Methods for the synthesis of immunostimulant plerixafor

This minireview discusses methods for the synthesis of the immunostimulant plerixafor (Mozobil®, AMD3100). The information presented covers the period from the publication of the first work devoted to the synthesis of plerixafor and the study of the electrochemical properties of its dinickel(II) and dicopper(II) complexes in 1987 to the present.

SOD activity of new copper II complexes with ligands derived from pyridoxal and toxicity in Caenorhabditis elegans

This work presents the synthesis, characterization of copper(II) complexes (C1-C6) and the potential of these compounds to mimic the catalytic activity of the enzyme superoxide dismutase (SOD). The copper(II)complexes were obtained by reaction between the aldol condensation between substituted aromatic hydrazides and aromatic aldehydes (salicylic aldehyde and pyridoxal hydrochloride), forming two new ligands (L1 to L6), resulting in new dimeric dicopper (II) complexes (C1 and C2), new three monomeric CuII derivatives (C3, C4 and C6) and a polymeric complex (C5). The CuII complexes were fully characterized by X-ray diffraction, spectroscopic and electrochemical analysis. Subsequently, CuII derivatives were evaluated for their antioxidant activities, using the NBT (Nitro blue tetrazolium chloride) photoreduction methodology.After evaluating the antioxidant activity in vitro, it was observed that the best inhibition rates of the superoxide ion are associated to the C4 and C5 complexes. Computational analysis via molecular docking and quantum chemical calculation (Fukui map) offered a molecular level explanation on the biological activity of CuII complexes. Additionally, cytotoxicity of C1-C6 was tested in the first time in vivo in nematodes Caenorhabditis elegans, corroborating with the results identified for C4 and C5.

Copper complexes of 1,4-diazabutadiene ligands: Tuning of metal oxidation state and, application in catalytic C-C and C-N bond formation

Reaction of 1,4-diazabutadiene (p-RC6H4N = C(H)(H)C = NC6H4R-p; R = OCH3, CH3, H and Cl; abbreviated as L-R) with CuCl2·2H2O in methanol at ambient temperature (25 °C) affords a group of doubly chloro-bridged dicopper complexes of type [{CuI(L-R)Cl}2], designated as 1-R. Similar reaction carried out in acetonitrile furnishes a family of doubly chloro-bridged dicopper complexes of type [{CuII(L-R)Cl2}2], designated as 2-R. Molecular structures of 1-OCH3 and 2-OCH3 have been determined by X-ray crystallography. While copper(I) is having a nearly tetrahedral N2Cl2 coordination sphere in 1-OCH3, the N2Cl3 coordination sphere around copper(II) is distorted square pyramidal in nature in 2-OCH3. Isolated 2-R complexes, on dissolution in methanol, are found to undergo facile reduction of the metal center to generate the corresponding 1-R complexes. The 1-R and 2-R complexes show intense absorptions in the visible and ultraviolet regions. Cyclic voltammetry on the 1-R and 2-R complexes shows both metal-centered and ligand centered redox responses. The 1-R complexes are found to efficiently catalyze C-N cross-coupling reactions between arylboronic acids and aryl amines; while the 2-R complexes display notable catalytic efficiency for nitroaldol reactions.