Homo- and heterometallic polynuclear transition metal catalysts for alkane C H bonds oxidative functionalization: Recent advances

A Family of Hexacopper Phenylsilsesquioxane/Acetate Complexes: Synthesis, Solvent-Controlled Cage Structures, and Catalytic Activity

Convenient self-assembly synthesis of copper(II) complexes via double (phenylsilsesquioxane and acetate) ligation allows to isolate a family of impressive sandwich-like cage compounds. An intriguing feature of these complexes is the difference in the structure of a pair of silsesquioxane ligands despite identical (Cu6) nuclearity and number (four) of acetate fragments. Formation of particular combination of silsesquioxane ligands (cyclic/cyclic vs condensed/condensed vs cyclic/condensed) was found to be dependent on the synthesis/crystallization media. A combination of Si4-cyclic and Si6-condensed silsesquioxane ligands is a brand new feature of cage metallasilsesquioxanes. A representative Cu6-complex (4) (with cyclic silsesquioxanes) exhibited high catalytic activity in the oxidation of alkanes and alcohols with peroxides. Maximum yield of the products of cyclohexane oxidation attained 30 %. The compound 4 was also tested as catalyst in the Baeyer-Villiger oxidation of cyclohexanone by m-chloroperoxybenzoic acid: maximum yields of 88 % and 100 % of ϵ-caprolactone were achieved upon conventional heating at 50 °C for 4 h and MW irradiation at 70 or 80 °C during 30 min, respectively. It was also possible to obtain the lactone (up to 16 % yield) directly from the cyclohexane via a tandem oxidation/Baeyer-Villiger oxidation reaction using the same oxidant.

Fe(III)-Based Phenylsilsesquioxane/Acetylacetonate Complexes: Synthesis, Cage-like Structure, and High Catalytic Activity

Unprecedented iron-based silsesquioxane/acetylacetonate complexes were synthesized. The intriguing cage-like structure of compounds is alkaline metal-dependent: the Fe2Li2 complex includes condensed Si6-silsesquioxane and four acetylacetonate ligands; the Fe4Na4 complex exhibits two cyclic Si4-silsesquioxane and eight acetylacetonate ligands, while the Fe3K3 complex features two cyclic Si3-silsesquioxane and six acetylacetonate ligands. The latter case is the very first observation of small trimeric silsesquioxane ligands in the composition of cage-like metallasilsesquioxanes. The Fe4Na4-based complex exhibits a record high activity in the oxidation of inert alkanes with peroxides (55% yield of oxygenates in cyclohexane oxidation). It also acts as a catalyst in the cycloaddition of CO2 with epoxides, leading to cyclic carbonates in good yields (58-96%).

Halogen-Dependent Diversity and Weak Interactions in the Heterometallic Ni/Cd Complex Solids: Structural and Theoretical Investigation

Three novel heterometallic Ni/Cd coordination compounds [Ni(en)3][CdCl4]∙3dmso (1), [Ni(en)2(dmf)2][CdBr4] (2), and [Ni(en)3]2[CdI4](I)2 (3) have been synthesized through the self-assembly process in a one-pot reaction of cadmium oxide, nickel salt (or nickel powder), NH4X (X = Cl, Br, I), and ethylenediamine in non-aqueous solvents dmso (for 1) or dmf (for 2 and 3). Formation of the one- (1) or three-dimensional (2 and 3) hydrogen-bonded frameworks has been observed depending on the nature of the [CdX4]2- counter-anion, as well as on the nature of the solvent. The electronic structures of [Ni(en)3]2+ and [Ni(en)2(dmf)2]2+ cations were studied at the DFT and CASSCF levels, including the ab initio ligand field theory (AILFT) calculations. The non-covalent intermolecular contacts between the cationic nickel and anionic cadmium blocks in the solid state were investigated by the QTAIM analysis. The mechanism of ligand substitution at the nickel center in [Ni(en)2(dmf)2]2+ was theoretically investigated at the ωB97X-D4/ma-def2-TZVP//DLPNO-CCSD(T)/ma-def2-TZVPP level. The results demonstrate that thermodynamic factors are structure-determining ones due to low energy barriers of the rotation of dmf ligands in [Ni(en)2(dmf)2]2+ (below 3 kcal mol-1) and the reversible transformation of [Ni(en)2(dmf)2]2+ into [Ni(en)3]2+ (below 20 kcal mol-1).

Cage-like Cu5Cs4-Phenylsilsesquioxanes: Synthesis, Supramolecular Structures, and Catalytic Activity

A small family of nonanuclear Cu5Cs4-based phenylsilsesquioxanes 1-2 were prepared by a convenient self-assembly approach and characterized by X-ray diffraction studies. The compounds 1 and 2 show some unprecedented structural features such as the presence of a [Ph14Si14O28]14- silsesquioxane ligand and a CuII5CsI4 nuclearity in which the metal cations occupy unusual positions within the cluster. Copper ions are "wrapped" into a silsesquioxane matrix, while cesium ions are located in external positions. This resulted in cesium-involved aggregation of coordination polymer structures. Both compounds 1 and 2 realize specific metallocene (cesium-phenyl) linkage between neighboring cages. Compound 2 is evaluated as a catalyst in the Baeyer-Villiger (B-V) oxidation of cyclohexanone and tandem cyclohexane oxidation/B-V oxidation of cyclohexanone with m-chloroperoxybenzoic acid (mCPBA) as an oxidant, in an aqueous acetonitrile medium, and HNO3 as the promoter. A quantitative yield of ε-caprolactone was achieved under conventional heating at 50 °C for 4 h or MW irradiation for 30 min (for cyclohexanone as substrate); 17 and 19% yields of lactone upon MW irradiation at 80 °C for 30 min and heating at 50 °C for 4 h, respectively (for cyclohexane as a substrate), were achieved. Complex 2 was evaluated as a catalyst for the oxidation of alkanes to alkyl hydroperoxides and alcohols to ketones with peroxides at 60 °C in acetonitrile. The maximum yield of cyclohexane oxidation products was 30%. Complex 2 exhibits high activity in the oxidation of alcohols.

Study of Cyclohexane and Methylcyclohexane Functionalization Promoted by Manganese(III) Compounds

Alkane functionalization using safe and low-energy processes is of great interest to industry and academia. Aiming to contribute to the process of saturated hydrocarbon functionalization, we have studied a set of three manganese(III) complexes as catalysts for promoting the oxidation of saturated hydrocarbons (cyclohexane and methylcyclohexane) in the presence of hydrogen peroxide or trichloroisocyanuric acid (TCCA). The mononuclear manganese(III) compounds were prepared using the ligands H2LMet4 (6,6’-((1,4-diazepane-1,4-diyl)bis(methylene))bis(2,4-dimethylphenol), H2salen (2,2’-((1E,1’E)-(ethane-1,2-diylbis(azaneylylidene))bis(methaneylylidene))diphenol) and H2salan (2,2’-((ethane-1,2-diylbis(azanediyl))bis(methylene))diphenol). The catalytic processes were carried out in acetonitrile at 25 and 50 °C for 24 h. The increase in the temperature was important to get a better conversion. The compounds did not promote cyclohexane oxidation in the presence of H2O2. However, they were active in the presence of TCCA, employing a ratio of 1000:333:1 equivalents of the substrate:TCCA:catalyst. The best catalytic activity was shown by the compound [Mn(salen)Cl], reaching conversions of 14.5 ± 0.3% (25 °C) and 26.3 ± 1.1% (50 °C) (yield for chlorocyclohexane) and up to 12.1 ± 0.5% (25 °C) and 29.8 ± 2.2% (50 °C) (total yield for the mixture of the products 1-chloro-4-methylcyclohexane, 3-methylcyclohexene and 1-methylcyclohexene). The interaction of the catalysts with TCCA was studied using electron paramagnetic resonance (EPR), suggesting that the catalysts [Mn(LMet4)Cl] and [Mn(salan)Cl] act via a different mechanism from that observed for [Mn(salen)Cl].

Peroxidative Oxidation of Cyclohexane Using 3d Metal Complexes with Hydrazone-Derived Ligands as Catalysts: Exploring (Un)Conventional Conditions

Two tetranuclear and two mononuclear Cu(II) complexes with arylhydrazones of malononitrile derived ligands (compounds 1–2 and 3–4, respectively), one trinuclear Co(II/III) complex with an arylhydrazone of acetoacetanilide (5) and one tetranuclear Zn(II) complex of 3-(2-carboxyphenyl-hydrazone)pentane-2,4-dione (6) were screened as potential catalysts in the peroxidative oxidation of cyclohexane by aqueous H2O2 in acetonitrile. The best results were attained in the presence of pyrazine-2-carboxylic acid (PCA) with 1 (26% yield, TON = 52.0) and with 2 (24%, TON = 48.0) after 4 h at 40 °C. In the presence of complexes 5 and 6, no oxygenated products were detected in the studied conditions. The employment of non-conventional conditions like supercritical carbon dioxide (scCO2) as reaction medium or microwave (MW) irradiation was assessed for complexes 1 and 2. After 6 h in acetonitrile–scCO2, at 50 °C and with HNO3 as promoter, only 17% yield was achieved using 1 as catalyst, and 21% using 2. Total yields of oxygenates up to 14 (with 1) and 13% (2) and TOFs of 56.0 and 52.0 h−1, respectively, were obtained working under MW irradiation at 70 °C and for the much shorter time of 0.5 h.

Supramolecular Diversity, Theoretical Investigation and Antibacterial Activity of Cu, Co and Cd Complexes Based on the Tridentate N,N,O-Schiff Base Ligand Formed In Situ

The four new complexes, [Cu(HL¹)(L²)Cl] (1), [Cu(HL¹)(L¹)]∙Cl∙2H₂O (2), [Co(L¹)₂]∙Cl (3) and [Cd(HL¹)I₂]∙dmso (4), have been prepared by one-pot reactions of the respective chloride or iodide metal salt with a non-aqueous solution of the polydentate Schiff base, HL¹, resulted from in situ condensation of benzhydrazide and 2-pyridinecarboxaldehyde, while a ligand HL², in case of 1, has been formed due to the oxidation of 2-pyridinecarboxaldehyde under reaction conditions. The crystallographic analysis revealed that the molecular building units in 1-4 are linked together into complex structures by hydrogen bonding, resulting in 1D, 2D and 3D supramolecular architectures for 1, 2 and 4, respectively, and the supramolecular trimer for 3. The electronic structures of 1-4 were investigated by the DFT theoretical calculations. The non-covalent interactions in the crystal structures of 1-4 were studied by means of the Hirshfeld surface analysis and the QTAIM theory with a special focus on the C-H⋯Cl bonding. From the DFT/DLPNO-CCSD(T) calculations, using a series of charged model {R₃C-H}⁰⋯Cl^- assemblies, we propose linear regressions for assessment of the interaction enthalpy (ΔH, kcal mol^-1) and the binding energy (BE, kcal mol^-1) between {R₃C-H}⁰ and Cl^- sites starting from the electron density at the bond critical point (ρ(r_BCP), a.u.): ΔH = -678 × ρ(r) + 3 and BE = -726 × ρ(r) + 4. It was also has been found that compounds 1, 3 and 4 during in vitro screening showed an antibacterial activity toward the nine bacteria species, comprising both Gram-positive and Gram-negative, with MIC values ranging from 156.2 to 625 mg/L. The best results have been obtained against Acinetobacter baumannii MβL.

Heterometallic Molecular Architectures Based on Fluorinated β-Diketone Ligands

This review summarizes the data on the synthesis of coordination compounds containing two or more different metal ions based on fluorinated β-diketonates. Heterometallic systems are of high interest in terms of their potential use in catalysis, medicine and diagnostics, as well as in the development of effective sensor devices and functional materials. Having a rich history in coordination chemistry, fluorinated β-diketones are well-known ligands generating a wide variety of heterometallic complexes. In this context, we focused on both the synthetic approaches to β-dicarbonyl ligands with additional coordination centers and their possible transformations in complexation reactions. The review describes bi- and polynuclear structures in which β-diketones are the key building blocks in the formation of a heterometallic framework, including the examples of both homo- and heteroleptic complexes.

Mechanistic Study for the Reaction of B12 Complexes with m-Chloroperbenzoic Acid in Catalytic Alkane Oxidations

The oxidation of alkanes with m-chloroperbenzoic acid (mCPBA) catalyzed by the B₁₂ derivative, heptamethyl cobyrinate, was investigated under several conditions. During the oxidation of cyclohexane, heptamethyl cobyrinate works as a catalyst to form cyclohexanol and cyclohexanone at a 0.67 alcohol to ketone ratio under aerobic conditions in 1 h. The reaction rate shows a first-order dependence on the [catalyst] and [mCPBA] while being independent of [cyclohexane]; V_obs = k₂[catalyst][mCPBA]. The kinetic deuterium isotope effect was determined to be 1.86, suggesting that substrate hydrogen atom abstraction is not dominantly involved in the rate-determining step. By the reaction of mCPBA and heptamethyl cobyrinate at low temperature, the corresponding cobalt(III)acylperoxido complex was formed which was identified by UV-vis, IR, ESR, and ESI-MS studies. A theoretical study suggested the homolysis of the O-O bond in the acylperoxido complex to form Co(III)-oxyl (Co-O^•) and the m-chlorobenzoyloxyl radical. Radical trapping experiments using N-tert-butyl-α-phenylnitrone and CCl₃Br, product analysis of various alkane oxidations, and computer analysis of the free energy for radical abstraction from cyclohexane by Co(III)-oxyl suggested that both Co(III)-oxyl and the m-chlorobenzoyloxyl radical could act as hydrogen-atom transfer reactants for the cyclohexane oxidation.

Integration of Trinuclear Triangle Copper(II) Secondary Building Units in Octacyanidometallates(IV)-Based Frameworks

The design and synthesis of high-dimensional materials based on secondary building blocks (SBUs) play a pivotal role in the further development of functional molecular materials. Herein, the self-assembly of Cu(II) ions, pyrazole (Hpz), and octacyanidometallate(IV) anions in the presence of water produced two new isostructural three-dimensional systems {[Cu₃(μ₃-OH)(μ-pz)₃(H₂O)₃]₂[M(CN)₈]}·nH₂O (M = W, 1, and Mo, 2). 1 and 2 consist of trinuclear triangle copper(II) (TTC) SBUs and octacyanidometallates(IV). At room temperature, both assemblies display strong antiferromagnetic interactions within the TTC entities with an average Cu^II···Cu^II isotropic magnetic coupling constant of about -145 cm^-1. Moreover, a detailed analysis of magnetic data revealed the presence of spin frustration with antisymmetric magnetic exchange-coupling constants of around +32 and +46 cm^-1 for 1 and 2, respectively. Finally, quantum chemical calculations explained their magnetic and optical properties.

Synthesis and Reactivity of Triangular Heterometallic Complexes Containing Zn-Zn Bond

This work provides a facile access to a series of triangular [Zn₂M] (M = group 10 and 11 metals) clusters. Treatment of Zn-Zn-bonded compounds [LZn-ZnL] (L = CH₃C(2,6-ⁱPr₂C₆H₃N)CHC(CH₃)(NCH₂CH₂PR₂); R = Ph, ⁱPr) with zero-valent transition-metal reagents selectively afforded the corresponding triangular clusters [Zn₂M], where M = Ni(0), Pd(0), and Pt(0). Notably, the isoelectronic triangular clusters [Zn₂M]⁺, where M = Ag(I) and Cu(I), could also be obtained by reactions of [LZn-ZnL] with AgOTf and CuOTf, respectively. The [Zn₂Ag]⁺ complex containing elusive Zn-Ag bonds was investigated by density functional theory analysis, showing a 3c-2e bonding feature in the metallic ring. The electrochemical behaviors of [Zn₂M] complexes were examined and revealed the donation of electron density from the Zn-Zn σ-bond to the metal centers. Reaction of the [Zn₂Ni] complex with isocyanide gave heterometallic species by coordination of isocyanide to the nickel center, keeping the trimetallic ring core structure intact. In contrast, the Zn-Zn bond was rapidly cleaved upon treatment of the [Zn₂Ni] complex with dihydrogen or phenyl acetylene, generating the hydride- or acetylide-bridged heterotrimetallic complex.

Hydrocarbon Oxidation Depth: H2O2/Cu2Cl4·2DMG/CH3CN System

The oxidation of hydrocarbons of different structures under the same conditions is an important stage in the study of the chemical properties of both the hydrocarbons themselves and the oxidation catalysts. In a 50% H2O2/Cu2Cl4·2DMG/CH3CN system, where DMG is dimethylglyoxime (Butane-2,3-dione dioxime), at 50 °C under the same or similar conditions, we oxidized eleven RH hydrocarbons of different structures: mono-, bi- and tri-cyclic, framework and aromatic. To compare the composition of the oxidation products of these hydrocarbons, we introduced a new quantitative characteristic, “distributive oxidation depth D(O), %” and showed the effectiveness of its application. The adiabatic ionization potentials (AIP) and the vertical ionization potentials (VIP) of the molecules of eleven oxidized and related hydrocarbons were calculated using the DFT method in the B3LYP/TZVPP level of theory for comparison with experimental values and correlation with D(O). The same calculations of AIP were made for the molecules of the oxidant, solvent, DMG, related compounds and products. It is shown that component X, which determines the mechanism of oxidation of hydrocarbons RH with AIP(Exp) ≥ AIP(X) = 8.55 ± 0.03 eV, is a trans-DMG molecule. Firstly theoretically estimated experimental values of AIP(trans-DMG) = 8.53 eV and AIP(cis-DMG) = 8.27 eV.

A Unique P450 Peroxygenase System Facilitated by a Dual-Functional Small Molecule: Concept, Application, and Perspective

Cytochrome P450 monooxygenases (P450s) are promising versatile oxidative biocatalysts. However, the practical use of P450s in vitro is limited by their dependence on the co-enzyme NAD(P)H and the complex electron transport system. Using H₂O₂ simplifies the catalytic cycle of P450s; however, most P450s are inactive in the presence of H₂O₂. By mimicking the molecular structure and catalytic mechanism of natural peroxygenases and peroxidases, an artificial P450 peroxygenase system has been designed with the assistance of a dual-functional small molecule (DFSM). DFSMs, such as N-(ω-imidazolyl fatty acyl)-l-amino acids, use an acyl amino acid as an anchoring group to bind the enzyme, and the imidazolyl group at the other end functions as a general acid-base catalyst in the activation of H₂O₂. In combination with protein engineering, the DFSM-facilitated P450 peroxygenase system has been used in various oxidation reactions of non-native substrates, such as alkene epoxidation, thioanisole sulfoxidation, and alkanes and aromatic hydroxylation, which showed unique activities and selectivity. Moreover, the DFSM-facilitated P450 peroxygenase system can switch to the peroxidase mode by mechanism-guided protein engineering. In this short review, the design, mechanism, evolution, application, and perspective of these novel non-natural P450 peroxygenases for the oxidation of non-native substrates are discussed.

Synthesis and reactivity of heterometallic complexes containing Mg- or Zn-metalloligands

Heteronuclear metal complexes comprising main group metals and transition metals have attracted widespread attention from researchers due to their applications in stoichiometric and catalytic activation of small molecules with possible cooperative effects. Herein, the advances of heterometallic complexes containing Mg- or Zn-metalloligands over the past ten years are reviewed. They consist of two parts: (i) synthetic approaches to heterometallic complexes. Only a brief discussion is made on the different Mg/Zn precursors since they have been summarized before. (ii) Stoichiometric and catalytic reactivities of heterometallic complexes containing Mg/Zn metalloligands. The exploration of the cooperative catalytic reaction of heterometallic complexes is still in its infancy, promising but challenging; thus, further investigations are required in the future.

Synthesis and crystal structures of two tri- and tetra-heterometallic Ni(ii)–Mn(ii)/Ni(ii)–Co(iii) complexes from two different Ni(ii)-containing metalloligands: effective catalytic oxidase activity and Schottky device approach

The development of tri- and tetra-nuclear heterometallic Ni(ii)–Mn(ii)/Ni(ii)–Co(iii) complexes with effective catalytic oxidase activity and the Schottky device approach.

Templated synthesis of Ni(ii) complexes of unsymmetrical Schiff base ligands derived from 1,3-diamino-2-propanol: structural diversity and magnetic properties

A Ferromagnetically coupled tetranuclear, an antiferromagnetically coupled hexanuclear and a mononuclear Ni(II) complex of new unsymmetrical Schiff base ligands derived from 1,3-diamino-2-propanol have been obtained with a slight change in the carbonyl compounds.

Structural topologies involving energetically significant antiparallel π-stacking and unconventional N(nitrile)⋯π(fumarate) contacts in dinuclear Zn(ii) and polymeric Mn(ii) compounds: antiproliferative evaluation and theoretical studies

Anticancer activities considering cell viability, apoptosis and molecular docking have been explored in dinuclear Zn(ii) and polymeric Mn(ii) compounds involving energetically significant unconventional N(nitrile)⋯π(fum) contacts.

Homogeneous oxidation of C–H bonds with m-CPBA catalysed by a Co/Fe system: mechanistic insights from the point of view of the oxidant

A Co/Fe system efficiently catalyses the oxidation of C–H bonds with m-CPBA. The nitric acid promoter hampers the m-CPBA homolysis, suppressing the free radical activity. Experimental and computational data evidence a concerted oxidation mechanism.

Nickel-catalyzed synthesis of Zn(I)-Zn(I) bonded compounds

This work reports the first catalyzed synthesis of d-block metal-metal bonded complexes. The treatment of terminal zinc hydrides [LZnH] [L = CH₃C(2,6-ⁱPr₂C₆H₃N)CHC(CH₃)(N(CH₂)_nCH₂PR₂); n = 1, 2; R = Ph, ⁱPr] in the presence of 5 mol% Ni(CO)₂(PPh₃)₂ afforded Zn(I)-Zn(I) bonded compounds [L₂Zn₂] in high isolated yields with concomitant elimination of dihydrogen. Stoichiometric reactions, kinetic studies and DFT calculations were conducted to elucidate the reaction mechanism.

Heterometallic Al6Zn12 nano-plate with π-conjugated ligand: synthesis and nonlinear absorption properties

Presented herein is the synthesis, structure, and optical properties of the aluminum(III)-zinc(II) heterometallic compound AlOC-57. This compound was found to form a large unit cell (approximately sixteen thousand atoms) and a three-shell nano-plate structure. Based on the Z-scan patterns, the third-order nonlinear optical response of the heterometallic nano-plate was mainly attributed to its nonlinear absorption (reverse saturable absorption).

A novel o-vanillin Fe(III) complex catalytically active in C-H oxidation: exploring the magnetic exchange interactions and spectroscopic properties with different DFT functionals

The novel complex [Fe^IIICl(L)₂(H₂O)] (1) was synthesized by interaction of iron(III) chloride with ethanol solution of o-vanillin (HL) and characterized by IR, UV/Vis spectroscopy, thermogravimetry and single crystal X-ray diffraction analysis. The molecules of 1 in the solid state are joined into supramolecular dimeric units, where a set of strong hydrogen bonds predefines the structure of the dimer according to the "key-lock" principle. From the Hirshfield surface analysis the contribution of π⋯π stacking to the overall stabilization of the dimer was found to be negligible. Broken symmetry DFT calculations suggested the presence of long-range antiferromagnetic interactions (J = -0.12 cm^-1 for H = -JS₁S₂ formalism) occurring through the Fe-O⋯O-Fe pathway, as evidenced by the studies of the model dimers where the water molecules were substituted by acetonitrile and acetone ones. The benchmark studies using a set of literature examples and various DFT functionals revealed the hybrid-GGA B3LYP as the best one for prediction of Fe^III⋯Fe^III antiferromagnetic exchange couplings of small magnitude. Magnetic susceptibility measurements confirmed antiferromagnetic coupling between the metal atoms in 1 with a coupling constant of -0.35 cm^-1. Catalytic studies demonstrated that 1 acts as an efficient catalyst in the oxidation of cyclohexane with hydrogen peroxide in the presence of nitric acid promoter and under mild conditions (yield up to 37% based on the substrate), while tert-butylhydroperoxide (TBHP) and m-chloroperoxybenzoic acid (m-CPBA) as oxidants exhibit less efficiency. Combined UV/TDDFT studies evidence the structural rearrangement of 1 in acetonitrile with the formation of [Fe^IIICl(L)₂(CH₃CN)] species. The TDDFT benchmark using nine common DFT functionals and two model compounds (o-vanillin and [Fe^III(H₂O)₆]³⁺ ion) support the hybrid meta-GGA M06-2X functional as the one most correctly predicting the excited state structure for the Fe(III) complexes, under the conditions studied.

Catalytic Oxidations with Meta-Chloroperoxybenzoic Acid (m-CPBA) and Mono- and Polynuclear Complexes of Nickel: A Mechanistic Outlook

Selective catalytic functionalization of organic substrates using peroxides as terminal oxidants remains a challenge in modern chemistry. The high complexity of interactions between metal catalysts and organic peroxide compounds complicates the targeted construction of efficient catalytic systems. Among the members of the peroxide family, m-chloroperoxybenzoic acid (m-CPBA) exhibits quite complex behavior, where numerous reactive species could be formed upon reaction with a metal complex catalyst. Although m-CPBA finds plenty of applications in fine organic synthesis and catalysis, the factors that discriminate its decomposition routes under catalytic conditions are still poorly understood. The present review covers the advances in catalytic C–H oxidation and olefine epoxidation with m-CPBA catalyzed by mono- and polynuclear complexes of nickel, a cheap and abundant first-row transition metal. The reaction mechanisms are critically discussed, with special attention to the O–O bond splitting route. Selectivity parameters using recognized model hydrocarbon substrates are summarized and important factors that could improve further catalytic studies are outlined.

A 3D MOF based on Adamantoid Tetracopper(II) and Aminophosphine Oxide Cages: Structural Features and Magnetic and Catalytic Properties

This work describes an unexpected generation of a new 3D metal-organic framework (MOF), [Cu₄(μ-Cl)₆(μ₄-O)Cu(OH)₂(μ-PTA═O)₄]_n·2nCl-EtOH·2.5nH₂O, from copper(II) chloride and 1,3,5-triaza-7-phosphaadamantane 7-oxide (PTA═O). The obtained product is composed of diamandoid tetracopper(II) [Cu₄(μ-Cl)₆(μ₄-O)] cages and monocopper(II) [Cu(OH)₂] units that are assembled, via the diamandoid μ-PTA═O linkers, into an intricate 3D net with an nbo topology. Magnetic susceptibility measurements on this MOF in the temperature range of 1.8-300 K reveal a ferromagnetic interaction (J = +20 cm^-1) between the neighboring copper(II) ions. Single-point DFT calculations disclose a strong delocalization of the spin density over the tetranuclear unit. The magnitude of exchange coupling, predicted from the broken-symmetry DFT studies, is in good agreement with the experimental data. This copper(II) compound also acts as an active catalyst for the mild oxidation and carboxylation of alkanes. The present study provides a unique example of an MOF that is assembled from two different types of adamantoid Cu₄ and PTA═O cages, thus contributing to widening a diversity of functional metal-organic frameworks.

Traceless Directing Groups in Sustainable Metal-Catalyzed C–H Activation

Sustainable transformations towards the production of valuable chemicals constantly attract interest, both in terms of academic and applied research. C–H activation has long been scrutinized in this regard, given that it offers a straightforward pathway to prepare compounds of great significance. In this context, directing groups (DG) have paved the way for chemical transformations that had not been achievable using traditional reactions. Few steps, high yields, selectivity and activation of inert substrates are some of the invaluable assets of directed catalysis. Additionally, the employment of traceless directing groups (TDG) greatly improves and simplifies this strategy, enabling the realization of multi-step reactions in one-pot, cascade procedures. Cheap, abundant, readily available transition metal salts and complexes can catalyze a plethora of reactions employing TDGs, usually under low catalyst loadings—rarely under stoichiometric amounts, leading in greater atom economy and milder conditions with increased yields and step-economy. This review article summarizes all the work done on TDG-assisted catalysis with manganese, iron, cobalt, nickel, or copper catalysts, and discusses the structure-activity relationships observed, by presenting the catalytic pathways and range of transformations reported thus far.

Naphthalimide-phenanthroimidazole incorporated new fluorescent sensor for "turn-on" Cu2+ detection in living cancer cells

In recent years, fluorescent sensors have emerged as attractive imaging probes due to their distinct responses toward bio-relevant metal ions. However, the bioimaging application main barrier is the 'turn-off' response toward paramagnetic metal ions such as Cu²⁺ under physiological conditions. Herein, we report a new sensor (2-methyl(4-bromo-N-ethylpiperazinyl-1,8-naphthalimido)-4-(1H-phenanthro[9,10-d]imidazole-2-yl) phenol)NPP with multifunctional (Naphthalimide, Piperazine, Phenanthroimidazole) units for fluorescent and colourimetric detection of Cu²⁺ in an aqueous medium. Both absorption and fluorescence spectral titration strategies were used to monitor the Cu²⁺-sensing property of NPP. The NPP displays a weak emission at ca. 455 nm, which remarkably enhances (⁓3.2-fold) upon selective binding of Cu²⁺ over a range of metal ions, including other paramagnetic metal ions (Mn²⁺, Fe³⁺, Co²⁺). The stoichiometry, binding constant (K_a) and the LOD (limit of detection) of NPP toward Cu²⁺ ions were found to be 1:1, 5.0 (± 0.2) × 10⁴ M^-1 and 6.5 (± 0.4) × 10^-7 M, respectively. We have also used NPP as a fluorescent probe to detect Cu²⁺ in live (human cervical HeLa) cancer cells. The emission intensity of NPP was almost recovered in HeLa cells by incubating 'in situ' the derived Cu²⁺ complex (NPP-Cu²⁺) in the presence of a benchmark chelating agent such as EDTA (ethylenediaminetetraacetate). The fluorescent emission of NPP was reverted significantly in each cycle upon sequencial addition of Cu²⁺ and EDTA to the NPP solution. Overall, NPP is a novel, simple, economic and portable sensor that can detect Cu²⁺ in biological and environmental scenarios.

Synthesis of Ni(ii)-Mn(ii) complexes using a new mononuclear Ni(ii) complex of an unsymmetrical N2O3 donor ligand: structures, magnetic properties and catalytic oxidase activity

A new Ni(ii) complex [NiL] (complex 1) of an asymmetrically di-condensed N₂O₃ donor Schiff base ligand, N-salicylidene-N'-3-methoxysalicylidene-1,3-propanediamine (H₂L), has been synthesized and utilized for the synthesis of three heterometallic complexes, [(NiL)₂Mn(NCS)₂(CH₃OH)₂]·CH₃OH (2) [(NiL)₂Mn(N(CN)₂)₂(CH₃OH)₂]·CH₃OH (3) and [(NiL)₂Mn₂(N₃)₂(μ_1,1-N₃)₂(CH₃OH)₂] (4). Single crystal X-ray diffraction analyses show that complexes 2 and 3 have linear trinuclear structures where two tridentate O₃ donor (NiL) units are coordinated to the central octahedral Mn(ii) centre, whereas complex 4 has a centrosymmetric tetranuclear structure where two binuclear (NiL)Mn units are linked via two phenoxido and two μ_1,1-N₃ bridges. Among the heterometallic complexes (2-4), only 4 is active towards the catalytic oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone. The turnover number for the aerobic oxidation of 3,5-DTBC is 935 h^-1. ESI-mass spectra have been recorded to scrutinize the mechanistic pathway of this catalytic reaction. Variable temperature magnetic susceptibility measurements suggest that complexes 2-4 are antiferromagnetically coupled with coupling constants (J) of -4.84 and -5.23 cm^-1 for complexes 2 and 3, respectively and J₁ = -2.20 cm^-1, J₂ = 1.13 cm^-1 and J₃ = -1.12 cm^-1 for complex 4. DFT calculations have been used to rationalize the magnetic super-exchange in complexes 2-4, by computing the theoretical coupling constants and analyzing the spin density plots.

Hierarchical Construction and Magnetic Difference of {Co12M12} (M = Co2+/Cd2+) Aggregates from {Co14} Cluster-Based Precursors in the Presence of Homo/Heterometal Salts

The controllable construction and function expansion of some sophisticated aggregations represent a current hot topic in scientific research. In this paper, using a prefabricated {Co₁₄} cluster as a synthetic precursor, a homometallic {Co₂₄} and a heterometallic {Co₁₂Cd₁₂} giant cluster possessing similar dual-[M₁₂] (M = Co/Cd) skeletons was prepared by reacting the precursor with excess CoCl₂ and Cd(OAc)₂ salts, respectively. The detailed structural information on {Co₂₄} and {Co₁₂Cd₁₂} was characterized by single-crystal X-ray diffraction and further analyzed by X-ray photoelectron spectroscopy, inductively coupled plasma-mass spectroscopy, and scanning electron microscopy with energy dispersive X-ray (EDX) spectroscopy in the solid state. Compared to the {Co₁₄} precursor, magnetic difference revealed that spin-canting and magnetic ordering had been enhanced in {Co₂₄} and suppressed in {Co₁₂Cd₁₂} when dotted with diamagnetic Cd²⁺ ions.

Novel H-Bonded Synthons in Copper Supramolecular Frameworks with Aminoethylpiperazine-Based Ligands. Synthesis, Structure and Catalytic Activity

New Schiff base complexes [Cu2(HL1)(L1)(N3)3]∙2H2O (1) and [Cu2L2(N3)2]∙H2O (2) were synthesized. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis. The HL1 ligand results from the condensation of salicylaldehyde and 1-(2-aminoethyl)piperazine, while a new organic ligand, H2L2, was formed by the dimerization of HL1 via a coupling of two piperazine rings of HL1 on a carbon atom coming from DMF solvent. The dinuclear building units in 1 and 2 are linked into complex supramolecular networks through hydrogen and coordination bondings, resulting in 2D and 1D architectures, respectively. Single-point and broken-symmetry DFT calculations disclosed negligible singlet-triplet splittings within the dinuclear copper fragments in 1 and 2. Catalytic studies showed a remarkable activity of 1 and 2 towards cyclohexane oxidation with H2O2 in the presence of nitric acid and pyridine as promoters and under mild conditions (yield of products up to 21%). Coordination compound 1 also acts as an active catalyst in the intermolecular coupling of cyclohexane with benzamide using di-tert-butyl peroxide (tBuOOtBu) as a terminal oxidant. Conversion of benzamide at 55% was observed after 24 h reaction time. By-product patterns and plausible reaction mechanisms are discussed.

New members of the polynuclear manganese family: MnMn single-molecule magnets and MnMn antiferromagnetic complexes. Synthesis and magnetostructural correlations

The synthesis, crystal structures and magnetic properties are reported for three novel mixed-valence tetranuclear [MnII2MnIII2(HBuDea)2(BuDea)2(EBA)4] (1), [MnII2MnIII2(HBuDea)2(BuDea)2(DMBA)4] (2) and undecanuclear [MnII3MnIII8O4(OH)2(BuDea)6(DMBA)8] (3) clusters, where H2BuDea is N-butyldiethanolamine, HEBA is 2-ethylbutyric acid and HDMBA is 2,2-dimethylbutyric acid. The compounds have been prepared through self-assembly reactions of manganese(ii) chloride with H2BuDea and respective carboxylic acid in methanol solution in air, affording 1 with HEBA, and 2 or 3 with HDMBA, depending on the experimental conditions. The single crystal X-ray analysis reveals that 1 and 2 have similar centrosymmetric structures based on the {M4(μ3-O)2(μ-O)4} core, while 3 discloses the unprecedented {M11(μ-O)4(μ3-O)12} one. The Mn4 complexes display single-molecule magnet (SMM) behavior with a S = 9 spin ground state and a high energy barrier Ueff/kB of up to 51 K. The magnetic properties of 2 are successfully modeled with JMnIII-MnIII/hc = 25.7 cm-1 and two JMnIII-MnII/hc constants of 3.1 and -0.93 cm-1 (data correspond to the Ĥ = -Jŝ1·ŝ2 formalism). The Mn11 cluster exhibits a paramagnetic behavior with dominant antiferromagnetic coupling. A possible influence of intermolecular effects and of different peripheries of the magnetic cores designed by using 2-ethylbutyrate (in 1) or 2,2-dimethylbutyrate (in 2) on the magnetic properties of 1 and 2 is discussed. The experimental magnetostructural correlations for the {MnII2MnIII2(μ3-O)2(μ-O)4} cores, supported by broken symmetry DFT calculations, disclose the X-MnIIIMnIII angle and MnIII-O distance (where MnIII-X and MnIII-O are axial Jahn-Teller bonds) as the structural factors having the strongest influence on JMnIII-MnIII exchange coupling. It is shown that two JMnIII-MnII constants are necessary for the correct description of magnetic exchange couplings in the {MnII2MnIII2(μ3-O)2(μ-O)4} tetranuclear unit.

Phenoxazinone synthase-like catalytic activity of novel mono- and tetranuclear copper(ii) complexes with 2-benzylaminoethanol

Three novel coordination compounds, [Cu(ca)2(Hbae)2] (1), [Cu(va)2(Hbae)2] (2) and [Cu4(va)4(bae)4]·H2O (3), have been prepared by self-assembly reactions of copper(ii) chloride (1 and 2) or tetrafluoroborate (3) and CH3OH (1 and 3) or CH3CN (2) solution of 2-benzylaminoethanol (Hbae) and cinnamic (Hca, 1) or valeric (Hva, 2 and 3) acid. Crystallographic analysis revealed that both 1 and 2 have mononuclear crystal structures, wherein the complex molecules are H-bonded forming extended supramolecular chains. The tetranuclear structure of 3 is based on the {Cu4(μ3-O)4} core, wherein the metal atoms are bound together by μ3 oxygen bridges from 2-benzylaminoethanol forming an overall cubane-like configuration. The strong hydrogen bonding in 1-3 leads to the joining of the neighbouring molecules into 1D chains. Concentration-dependent ESI-MS studies disclosed the equilibria between di-, tri- and tetranuclear species in solutions of 1-3. All three compounds act as catalysts for the aerobic oxidation of o-aminophenol to the phenoxazinone chromophore (phenoxazinone synthase-like activity), with the maximum reaction rates of 4.0 × 10-7, 2.5 × 10-7 and 2.1 × 10-7 M s-1 for 1, 2 and 3, respectively, supported by the quantitative yield of the product after 24 h. The dependence of the reaction rates on catalyst concentrations is evidence of reaction orders higher than one relative to the catalyst. Kinetic and ESI-MS data allowed us to assume that the tetranuclear species, originating from 1, 2 and 3 in solution, possess considerably higher activity than the species of lower nuclearity. Mechanistic and isotopic 18O-labelling experiments suggested that o-aminophenol coordinates to CuII species with the formation of reactive intermediates, while the oxygen from 18O2 is not incorporated into the phenoxazinone chromophore.

Synthesis, Structural, and Cytotoxic Properties of New Water-Soluble Copper(II) Complexes based on 2,9-Dimethyl-1,10-Phenanthroline and Their One Derivative Containing 1,3,5-Triaza-7-Phosphaadamantane-7-Oxide

A series of water-soluble copper(II) complexes based on 2,9-dimethyl-1,10-phenanthroline (dmphen) and mixed-ligands, containing PTA=O (1,3,5-triaza-7-phosphaadamantane-7-oxide) have been synthesized and fully characterized. Two types of complexes have been obtained, monocationic [Cu(NO₃)(O-PTA=O)(dmphen)][PF₆] (1), [Cu(Cl)(dmphen)₂][PF₆] (2), and neutral [Cu(NO₃)₂(dmphen)] (3). The solid-state structures of all complexes have been determined by single-crystal X-ray diffraction. Magnetic studies for the complex 1–3 indicated a very weak antiferromagnetic interaction between copper(II) ions in crystal lattice. Complexes were successfully evaluated for their cytotoxic activities on the normal human dermal fibroblast (NHDF) cell line and the antitumor activity using the human lung carcinoma (A549), epithelioid cervix carcinoma (HeLa), colon (LoVo), and breast adenocarcinoma (MCF-7) cell lines. Complexes 1 and 3 revealed lower toxicity to NHDF than A549 and HeLa cells, meanwhile compound 2 appeared to be more toxic to NHDF cell line in comparison to all cancer lines. Additionally, interactions between the complexes and human apo-transferrin (apo-Tf) using fluorescence and circular dichroism (CD) spectroscopy were also investigated. All compounds interacted with apo-transferrin, causing same changes of the protein conformation. Electrostatic interactions dominate in the 1/2 – apo- Tf systems and hydrophobic and ionic interactions in the case of 3.