CuII–salan compounds: Synthesis, characterization and evaluation of their potential as oxidation catalysts

Biological activity of mixed chelate copper(II) complexes, with substituted diimine and tridentate Schiff bases (NNO) and their hydrogenated derivatives as secondary ligands: Casiopeína's fourth generation

We synthesize and characterize nine copper(II) compounds. Four with general formula [Cu(NNO)(NO₃)] and five mixed chelates [Cu(NNO)(N-N)]⁺, where NNO corresponds to asymmetric salen ligands (E)-2-((2-(methylamino)ethylimino)methyl)phenolate (L1) and (E)-3-((2-(methylamino)ethylimino)methyl)naphthalenolate (LN1); and their hydrogenated derivatives 2-((2-(methylamino)ethylamino)methyl)phenolate (LH1) and 3-((2-(methylamino)ethylamino)methyl)naphthalenolate (LNH1); and N-N correspond to 4,4'-dimethyl-2,2'-bipiridyne(dmbpy) or 1,10-phenanthroline (phen). Using EPR, the geometries of the compounds in solution in DMSO were assigned, [Cu(LN1)(NO₃)] and [Cu(LNH1)(NO₃)] a square-planar, [Cu(L1)(NO₃)], [Cu(LH1)(NO₃)], [Cu(L1)(dmby)]⁺ and [Cu(LH1)(dmby)]⁺ a square-based pyramid; and [Cu(LN1)(dmby)]⁺, [Cu(LNH1)(dmby)]⁺ and [Cu(L1)(phen)]⁺ and elongated octahedral. By X-ray it was observed that [Cu(L1)(dmby)]⁺ and. [Cu(LN1)(dmby)]⁺ presented a square-based pyramidal, and [Cu(LN1)(NO₃)]⁺ a square-planar geometry. The electrochemical study showed that copper reduction process is a quasi-reversible system, where the complexes with hydrogenated ligands were less oxidizing. The cytotoxicity of the complexes was tested by MTT assay, all the compounds showed biological activity in HeLa cell line, the mixed compounds were the more active ones. Naphthalene moiety, imine hydrogenation and aromatic diimine coordination, increased biological activity. A structure-activity relationships were found: Log(IC₅₀) =  - 1.01(E_pc) - 0.35(Conjugated Rings) + 0.87, for Schiff base complexes and Log(IC₅₀) = 0.078(E_pc) - 0.32(Conjugated Rings) + 1.94, for hydrogenated complexes; the less oxidizing species with a great number of conjugated rings presented the best biological activity. Complexes-DNA binding constants were obtained by uv-vis studies using CT-DNA, the results suggested that the complexes can interact through the grooves, except the phenanthroline mixed complex that intercalate with DNA. Gel electrophoresis study with pBR 322 showed that compounds can produce changes in the form of DNA and some complexes can cleave DNA in the presence of H₂O₂.

Fully Chelating N3O2-Pentadentate Planar Ligands Designed for the Strongest and Selective Capture of Uranium from Seawater

Based on the unique fivefold equatorial coordination of UO₂²⁺, water-compatible pentadentate planar ligands, H₂saldian and its derivatives, were designed for the strong and selective capture of UO₂²⁺ in seawater. In the simulated seawater condition (0.5 M NaCl + 2.3 mM HCO₃^-/CO₃^2-, pH 8), saldian^2- shows the strongest complexation with UO₂²⁺ to form UO₂(saldian) (log β₁₁ = 28.05 ± 0.07), which is more than 10 order of magnitude greater than amidoxime-based or -inspired ligand systems most commonly employed for U capture from seawater. Good selectivity for UO₂²⁺ from other metal ions coexisting in seawater was also demonstrated.

A bis-chelate o-vanillin-2-ethano-lamine copper(II) complex bearing both imine and amine forms of the ligand

The mol-ecular bis-chelate complex (2-{[(2-hy-droxy-ethyl-κO)amino-κN]meth-yl}-6-meth-oxy-phenolato-κO)(2-{[(2-hy-droxy-eth-yl)imino-κN]meth-yl}-6-meth-oxy-phenolato-κO)copper(II), [Cu(C10H14NO3)(C10H12NO3)] or [Cu(HL im)(HL am); HL im = C10H14NO3; HL am = C10H12NO3, represents the first compound containing a salicyl-idene-2-ethano-lamine type ligand in both imino HL im (Schiff base) and amino HL am (reduced Schiff base) forms that has been structurally characterized on the basis of X-ray data. Two mol-ecules of the monodeprotonated ligands coordinate the CuII ion in an (N,O phen)-bidentate and an (N,O phen,O alc)-tridentate fashion in the case of the imino and amino forms, respectively. The shape of the CuN2O3 coordination polyhedron is a distorted square-pyramid (geometry index τ5 = 0.26). Inter-molecular N-H⋯O and O-H⋯O hydrogen bonds, involving H atoms of the amino and hy-droxy-ethyl groups, create a two-dimensional supra-molecular array extending parallel to (010).

Room-Temperature Cu(II) Radical-Triggered Alkyne C-H Activation

A dimeric Cu(II) complex [Cu(II)2L2(μ2-Cl)Cl] (1) built from an asymmetric tridentate ligand (2-(((2-aminocyclohexyl)imino)methyl)-4,6-di-tert-butylphenol) and weakly coordinating anions has been synthesized and structurally characterized. In dichloromethane solution, 1 exists in a monomeric [Cu(II)LCl] (1') (85%)-dimeric (1) (15%) equilibrium, and cyclic voltammetry (CV) and electron paramagnetic resonance (EPR) studies indicate structural stability and redox retention. Addition of phenylacetylene to the CH2Cl2 solution populates 1' and leads to the formation of a transient radical species. Theoretical studies support this notion and show that the radical initiates an alkyne C-H bond activation process via a four-membered ring (Cu(II)-O···H-Calkyne) intermediate. This unusual C-H activation method is applicable for the efficient synthesis of propargylamines, without additives, within 16 h, at low loadings and in noncoordinating solvents including late-stage functionalization of important bioactive molecules. Single-crystal X-ray diffraction studies, postcatalysis, confirmed the framework's stability and showed that the metal center preserves its oxidation state. The scope and limitations of this unconventional protocol are discussed.

Copper(II) Complexes of Sulfonated Salan Ligands: Thermodynamic and Spectroscopic Features and Applications for Catalysis of the Henry Reaction

Copper(II) complexes formed with sulfonated salan ligands (HSS) have been synthesized, and their coordination chemistry has been characterized using pH-potentiometry and spectroscopic methods [UV-vis, electron paramagnetic resonance (EPR), and electron-electron double resonance (ELDOR)-detected NMR (EDNMR)] in aqueous solution. Several bridging moieties between the two salicylamine functions were introduced, e.g., ethyl (HSS), propyl (PrHSS), butyl (BuHSS), cyclohexyl (cis-CyHSS, trans-CyHSS), and diphenyl (dPhHSS). All of the investigated ligands feature excellent copper(II) binding ability via the formation of a (O^-,N,N,O^-) chelate system. The results indicated that the cyclohexyl moiety significantly enhances the stability of the copper(II) complexes. EPR studies revealed that the arrangement of the coordinated donor atoms is more symmetrical around the copper(II) center and similar for HSS, BuHSS, CyHSS, and dPhHSS, respectively, and a higher rhombicity of the g tensor was detected for PrHSS. The copper(II) complexes of the sulfosalan ligands were isolated in solid form also and showed moderate catalytic activity in the Henry (nitroaldol) reaction of aldehydes and nitromethane. The best yield for nitroaldol production was obtained for copper(II) complexes of PrHSS and BuHSS, although their metal binding ability is moderate compared to that of the cyclohexyl counterparts. However, these complexes possess larger spin density on the nitrogen nuclei than that for the other cases, which alters their catalytic activity.

Shedding light on the use of Cu(ii)-salen complexes in the A3 coupling reaction

One Cu(ii) complex, {Cu(ii)L} (1S), has been synthesised, in two high yielding steps under ambient conditions, and characterised by single-crystal X-Ray diffraction (SXRD), IR, UV-Vis, circular dichroism (CD), elemental analysis, thermogravimetric analysis (TGA) and electron spray ionization mass spectroscopy (ESI-MS). This air-stable compound enables the generation, at room temperature and in open-air, of twenty propargylamines, nine new, from secondary amines, aliphatic aldehydes and alkynes with a broad scope with yields up to 99%. Catalyst loadings can be as low as 1 mol%, while the recovered material retains its structural integrity and can be used up to 5 times without loss of its activity. Control experiments, SXRD, cyclic voltammetry and theoretical studies shed light on the mechanism revealing that the key to success is the use of phenoxido salen based ligands. These ligands orchestrate topological control permitting alkyne binding with concomitant activation of the C-H bond and simultaneously acting as a template temporarily accommodating the abstracted acetylenic proton, and continuously generating, via in situ formed radicals and a Single Electron Transfer (SET) mechanism, a transient Cu(i) active site to facilitate this transformation. The scope and limitations of this protocol are discussed and presented.

Unusual chemistry of Cu(ii) salan complexes: synthesis, characterization and superoxide dismutase activity

Cu(ii)-salan complexes: structural and spectral characterization, solvent assisted ring cleavage and correlation of superoxide dismutase activity with cyclic voltammetry data and steric effects.

Fluorescence Spectroscopic Insight into the Supramolecular Interactions in DNA-Based Enantioselective Sulfoxidation

Interactions of copper(II)-bipyridine cofactors and thioanisole substrate with human telomeric G-quadruplex DNA were studied by UV/Vis absorption, circular dichroism, and fluorescence quenching titration. Three copper(II)-bipyridine complexes are equivalently anchored to the G-quadruplex scaffold at all five fluorescently labeled sites. Thioanisole interacts with the DNA architecture at both the second loop and 3' terminus in the absence or presence of copper(II)-bipyridine complexes. These nonspecificities in the weak interactions of Cu^II complexes and thioanisole with G-quadruplex might explain why DNA only affords a modest enantioselectivity in the oxidation of thioanisole. These findings provide insights toward the construction of highly enantioselective DNA-based catalysts.

Chiral sulfoxides: advances in asymmetric synthesis and problems with the accurate determination of the stereochemical outcome

Chiral sulfoxides are in extremely high demand in nearly every sector of the chemical industry concerned with the design and development of new synthetic reagents, drugs, and functional materials. The primary objective of this review is to update readers on the latest developments from the past five years (2011-2016) in the preparation of optically active sulfoxides. Methodologies covered include catalytic asymmetric sulfoxidation using either chemical, enzymatic, or hybrid biocatalytic means; kinetic resolution involving oxidation to sulfones, reduction to sulfides, modification of side chains, and imidation to sulfoximines; as well as various other methods including nucleophilic displacement at the sulfur atom for the desymmetrization of achiral sulfoxides, enantioselective recognition and separation based on either metal-organic frameworks (MOF's) or host-guest chemistry, and the Horner-Wadsworth-Emmons reaction. A second goal of this work concerns a critical discussion of the problem of the accurate determination of the stereochemical outcome of a reaction due to the self-disproportionation of enantiomers (SDE) phenomenon, particularly as it relates to chiral sulfoxides. The SDE is a little-appreciated phenomenon that can readily and spontaneously occur for scalemic samples when subjected to practically any physicochemical process. It has now been unequivocally demonstrated that ignorance in the SDE phenomenon inevitably leads to erroneous interpretation of the stereochemical outcome of catalytic enantioselective reactions, in particular, for the synthesis of chiral sulfoxides. It is hoped that this two-pronged approach to covering the chemistry of chiral sulfoxides will be appealing, engaging, and motivating for current research-active authors to respond to in their future publications in this exciting area of current research.

New Heterodinuclear Zn/Ln (Ln = Gd, Tb, Er, Yb) Complexes of Hexadentate N,N′-Bis(3-alkoxy-2-hydroxybenzyl)cyclohexane-1,2-diamines: Synthesis and Structure

Two N,N′-bis(3-alkoxy-2-hydroxybenzyl)cyclohexane-1,2-diamine proligands, H2L1 (R = OCH3) and H2L2 (R = OC2H5), and five heterodinuclear ZnII/LnIII complexes, [Zn(L)(µ-CH3COO)Ln(NO3)2], containing [L1]2– and Gd3+, Tb3+, Er3+, or Yb3+ and [L2]2– and Yb3+ have been synthesised and structurally characterised. The complexes are isostructural and crystallise in the P21/n monoclinic space group. Zinc(ii) is coordinated by the inner N2O2 donor set of the ligand and an oxygen of the bridging acetate anion; the lanthanide(iii) ions possess an O9 coordination environment involving the interaction with the ligand’s outer O4 donor set, two bidentate nitrate ions, and the bridging acetate.

Ligand free copper(I)-catalyzed synthesis of diaryl ether with Cs2CO3 via a free radical path

Complexes [Cu(I)(2,4-dimethylphenoxy)2](-) (A) and [Cu(II)(2,4-dimethylphenoxy)2(p-tolyl)](-) (B) were observed by in situ electrospray ionization mass spectrometry (ESI-MS) analysis of the ligand free copper(I)-catalyzed C-O coupling reaction using Cs2CO3 under the catalytic reaction conditions indicating that they could be intermediates in the reaction. The radical scavenger cumene retarded the reaction. Catalytic cycles involving a free radical path are proposed based on these observations.

Heptacoordinate Heteroleptic Salan (ONNO) and Thiosalan (OSSO) Titanium(IV) Complexes: Investigation of Stability and Cytotoxicity

Seven heptacoordinate titanium(IV) complexes were synthesized based on the concept of hetero-bis-chelate stabilization of salan (ONNO) and thiosalan (OSSO) titanium(IV)alkoxides with 2,6-pyridinedicarboxylic acid (dipic) and derivatives thereof. The resulting compounds were investigated in a solid by X-ray diffraction and in solution by NMR spectroscopy. A thiosalan (OSSO) titanium(IV) complex could be isolated and its conformational stabilization by dipic was shown by (1)H NMR spectroscopy to lead to nonfluxional behavior even at room temperature. The stability of selected complexes was assessed at pH 1.9, 6.8, and 12.1 by an UV-vis monitored hydrolysis study with >5 Mio. equivalents of water. Even at pH 12.1 [L(1)Ti(dipic)(1)] showed t1/2 of more than 2 days. The cytotoxicity of all compounds was investigated in two human carcinoma cell lines. IC50-values in the range of cisplatin were achieved by all tested compounds except for [L(4)Ti(dipic)(1)], which was proven to be nontoxic. The functionalization of dipic was thus well tolerated and did neither interfere with the stability nor the cytotoxicity of the heteroleptic complexes.

Thirty years through vanadium chemistry

The relevance of vanadium in biological systems is known for many years and vanadium-based catalysts have important industrial applications, however, till the beginning of the 80s research on vanadium chemistry and biochemistry did not receive much attention from the scientific community. The understanding of the broad bioinorganic implications resulting from the similarities between phosphate and vanadate(V) and the discovery of vanadium dependent enzymes gave rise to an enormous increase in interest in the chemistry and biological relevance of vanadium. Thereupon the last 30years corresponded to a period of enormous research effort in these fields, as well as in medicinal applications of vanadium and in the development of catalysts for use in fine-chemical synthesis, some of these inspired by enzymatic active sites. Since the 80s my group in collaboration with others made contributions, described throughout this text, namely in the understanding of the speciation of vanadium compounds in aqueous solution and in biological fluids, and to the transport of vanadium compounds in blood plasma and their uptake by cells. Several new types of vanadium compounds were also synthesized and characterized, with applications either as prospective therapeutic drugs or as homogeneous or heterogenized catalysts for the production of fine chemicals. The developments made are described also considering the international context of the evolution of the knowledge in the chemistry and bioinorganic chemistry of vanadium compounds during the last 30years. This article was compiled based on the Vanadis Award presentation at the 9th International Vanadium Symposium.

Oxygen activation by copper camphor complexes

In situreduction of CuCl₂enables formation of Cu(i) complexes prone to activate molecular oxygen from air towards catalytic oxidation of 1,3-dicarbonyls.

Amino acid derived CuII compounds as catalysts for asymmetric oxidative coupling of 2-naphthol

Several chiral amino acid-derived Cu^II complexes were prepared and used as catalysts in the aerobic oxidative coupling of 2-naphthol. Moderate homocoupling product yields and enantiomeric excesses were obtained under mild, aqueous conditions.

Possible intermediates of Cu(phen)-catalyzed C-O cross-coupling of phenol with an aryl bromide by in situ ESI-MS and EPR studies

The C-O coupling reaction between 2,4-dimethylphenol and 4-bromotoluene catalyzed by the CuI/K2CO3/phen system can be inhibited by the radical scavenger cumene. Complexes [Cu(i)(phen)(1-(2,4-dimethylphenoxy)-4-methylbenzene)](+) (denoted as A), {H[Cu(i)(phen)(2,4-dimethylphenoxy)]}(+) and [Cu(i)(2,4-dimethylphenoxy)2](-) (denoted as B) were observed by in situ electrospray ionization mass spectrometry (ESI-MS) analysis of the copper(i)-catalyzed C-O coupling reaction under the catalytic reaction conditions indicating that they could be intermediates in the reaction. The in situ EPR study of the reaction solution detected the Cu(ii) species with a fitted g value of 2.188. A catalytic cycle with a single electron transfer (SET) step was proposed based on these observations.