Copper Coordination Research Articles

Anion-Induced Structural Transformation of a Cage-Based Metal–Organic Framework

Cage-based metal–organic frameworks (MOFs) have large pore spaces but small pore windows, endowing them with unique potential in the fields of gas adsorption, separation, and so on. Here, we successfully synthesized a series of isostructural caged-based MOFs with different counteranions, i.e., {[Cu3(TPA)4(BF4)6]n·(solvent)x} (Cu-TPA–BF4), {[Cu3(TPA)4(ClO4)6]n·(solvent)x} (Cu-TPA-ClO4), and {[Cu3(TPA)4(NO3)6]n·(solvent)x} (Cu-TPA-NO3), which are formed by the coordination of tridentate nitrogen-containing ligands tri(pyridin-4-yl)amine (TPA) and copper(II) cations. Nevertheless, after placing Cu-TPA–BF4 in a glass tube for a long period, a more stable violet crystal {[Cu3(TPA)4(SiF6)3]n·(solvent)x} (Cu-TPA-SiF6) is generated, while no transformation is observed for Cu-TPA-ClO4 and Cu-TPA-NO3. Cu-TPA–BF4 belongs to a tbo topology and possesses octahedral cages and two kinds of cuboctahedral cages, with the uncoordinated BF4– anions filling in the channels. In comparison, as a two-connected node, SiF62– anions participate in the construction of the tetrahedral cages and icosahedral cages, resulting in conversion to Cu-TPA-SiF6 with an ith-d topology. Further investigation indicates that the hydrolysis of BF4– anions and further reaction with SiO2 will afford SiF62– anions, which can be directly involved in the formation of Cu-TPA-SiF6 to induce the transformation. Additionally, the stability and adsorption behaviors of Cu-TPA–BF4 and Cu-TPA-SiF6 are also investigated.

Mechanochemical Preparation, Solid-State Characterization, and Antimicrobial Performance of Copper and Silver Nitrate Coordination Polymers with L- and DL-Arginine and Histidine.

The antimicrobial activity of the novel coordination polymers obtained by co-crystallizing the amino acids arginine or histidine, as both enantiopure L and racemic DL forms, with the salts Cu(NO3)2 and AgNO3 has been investigated to explore the effect of chirality in the cases of enantiopure and racemic forms. The compounds [Cu·AA·(NO3)2]CPs and [Ag·AA·NO3]CPs (AA = L-Arg, DL-Arg, L-His, DL-His) were prepared by mechanochemical, slurry, and solution methods and characterized by X-ray single-crystal and powder diffraction in the cases of the copper coordination polymers, and by powder diffraction and by solid-state NMR spectroscopy in the cases of the silver compounds. The two pairs of coordination polymers, [Cu·L-Arg·(NO3)2·H2O]CP and [Cu·DL-Arg·(NO3)2·H2O]CP, and [Cu·L-Hys·(NO3)2·H2O]CP and [Cu·DL-His·(NO3)2·H2O]CP, have been shown to be isostructural in spite of the different chirality of the amino acid ligands. A similar structural analogy could be established for the silver complexes on the basis of SSNMR. The activity against the bacterial pathogens Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus was assessed by carrying out disk diffusion assays on lysogeny agar media showing that, while there is no significant effect arising from the use of enantiopure or chiral amino acids, the coordination polymers exert an appreciable antimicrobial activity comparable, when not superior, to that of the metal salts alone.

Facile One-Step Access to Pyrrole-Based 1,4-Diphosphabarrelenes.

1,4-Diphosphabarrelenes are bicyclic diphosphines relevant, for example, for the generation of polymetallic coordination compounds. However, current synthetic protocols either suffer from low yields or require multiple reaction steps. Herein, we report the one-step synthesis of pyrrole-based 1,4-diphosphabarrelenes that are obtained in very good yields from the reaction of 1,2,5-trimethylpyrrole with 1,2-bis(dichlorophosphino)ethane or 1,2-bis(dichlorophosphino)benzene. The new caged diphosphines are strong donor ligands and act as bridging ligand in nickel(0), rhodium(I), iridium(I) and copper(I) coordination compounds.

Synthesis, Characterization and Antibacterial Investigation of Mononuclear Copper (II) Complexes of Amine-phenolate Based Ligands

The antibacterial activities of two previously reported copper (II) coordination complexes [Cu(HL1)Cl2] and [Cu(HL2)Cl2], and the two new complexes [Cu(HL3)Cl2] and [Cu(HL4)Cl2], where HL1 is 2-(((pyridin-2-ylmethyl)amino)methyl)phenol, HL2 is 2-((benzyl(pyridine-2-ylmethyl)amino)methyl)phenol, HL3 is 2-(((pyridin-2-ylethyl)amino)methyl)phenol and HL4 is 2-(((pyridin-2-ylmethyl)(quinolin-2-ylmethyl)-amino)methyl)phenol were investigated using agar well plate diffusion assay. The ligands were characterized using elemental analysis, IR, 1H-NMR, 13C-NMR spectroscopy and thermal analysis. The corresponding copper (II) complexes of each ligand were synthesized and characterized using elemental analysis, FT-IR and UV-Visible spectroscopy, electronic spectra and magnetic moment measurements, and thermal analysis. Additionally, a thorough investigation of the copper complexes as potent drug candidates was performed using web-based software SwissADME to predict their physicochemical and pharmacokinetics properties. All copper complexes were stable to air and moisture and showed excellent stability up to 200 °C. Electronic spectra of all copper complexes in methanol consisted of single band at 14,490–15,260 cm−1 depicting ligand field excitation (Cu (II) d–d band). Ligands were completely ineffective on gram positive bacteria; HL1 and HL3 showed moderate to high activity at concentration range of 0.2–1 mg/ml. Copper complexes were found to exhibit moderate to high activity against bacteria as compared to the ligands. SwissADME analysis of all copper (II) complexes justified their candidature as potential candidate for pharmaceutical applications.

Copper(II) coordination polymer based on l-arginine as a supramolecular hybrid inorganic–organic material: synthesis, structural, spectroscopic and magnetic properties

We report the synthesis and structural, spectroscopic and magnetic properties of new 1D coordination polymeric complex {[Cu(μ-l-Arg)2]SO4⋅1.5H2O}n (1) that contains asymmetric μ−O,O’ carboxylic bridge linking distorted square-pyramidal [Cu(μ-l-Arg)2]2+ coordination units. In 1D, the syn−anti−μ2−η1:η1zigzag polymer conformation, the adjacent Cu(II) ions are distanced by 5.707 Å, and the subsequent Cu∙∙∙Cu proximity in 1D-coordination chain equals 6.978 Å. Detailed interpretation of IR and Raman spectra of l-arginine and 1 was performed. The principal components of the g tensor determined from EPR experiments (gx = 2.059, gy = 2.075, gz = 2.228) indicate nearly axial symmetry of Cu(II) coordination sphere and correspond to the unpaired electron occupying the dx2–y2 orbital. The single broad band at 16,200 cm–1, characteristic of d−d transition, is assigned to the dominant dublet-dublet 2B1g(dx2–y2)→ 2Eg(dyz≈dxz) transition. Magnetic susceptibility measurements have revealed ferromagnetic coupling between the Cu(II) ions within the 1D-coordination chain, while the intermolecular coupling is antiferromagnetic.Graphical

Conductivity in Thin Films of Transition Metal Coordination Complexes.

Two coordination complexes have been made by combining the dithiolene complexes [M(mnt)2]2- (mnt = maleonitriledithiolate; M = Ni2+ or Cu2+) as anion, with the copper(II) coordination complex [Cu(Stetra)] (Stetra = 6,6'-bis(4,5-dihydrothiazol-2-yl)-2,2'-bipyri-dine) as cation. The variation of the metal centers leads to a dramatic change in the conductivity of the materials, with the M = Cu2+ variant (Cu-Cu) displaying semiconductor behavior with a conductivity of approximately 2.5 × 10-8 S cm-1, while the M = Ni2+ variant (Ni-Cu) displayed no observable conductivity. Computational studies found Cu-Cu enables a minimization of reorganization energy losses and, as a result, a lower barrier to the charge transfer process, resulting in the reported higher conductivity.

Copper Binding and Redox Activity of α-Synuclein in Membrane-Like Environment.

α-Synuclein (αSyn) constitutes the main protein component of Lewy bodies, which are the pathologic hallmark in Parkinson's disease. αSyn is unstructured in solution but the interaction of αSyn with lipid membrane modulates its conformation by inducing an α-helical structure of the N-terminal region. In addition, the interaction with metal ions can trigger αSyn conformation upon binding and/or through the metal-promoted generation of reactive oxygen species which lead to a cascade of structural alterations. For these reasons, the ternary interaction between αSyn, copper, and membranes needs to be elucidated in detail. Here, we investigated the structural properties of copper-αSyn binding through NMR, EPR, and XAS analyses, with particular emphasis on copper(I) coordination since the reduced state is particularly relevant for oxygen activation chemistry. The analysis was performed in different membrane model systems, such as micellar sodium dodecyl sulfate (SDS) and unilamellar vesicles, comparing the binding of full-length αSyn and N-terminal peptide fragments. The presence of membrane-like environments induced the formation of a copper:αSyn = 1:2 complex where Cu+ was bound to the Met1 and Met5 residues of two helical peptide chains. In this coordination, Cu+ is stabilized and is unreactive in the presence of O2 in catechol substrate oxidation.

Stabilizing copper sites in coordination polymers toward efficient electrochemical C-C coupling

Electroreduction of carbon dioxide with renewable electricity holds promise for achieving net-zero carbon emissions. Single-site catalysts have been reported to catalyze carbon-carbon (C-C) coupling—the indispensable step for more valuable multi-carbon (C2+) products—but were proven to be transformed in situ to metallic agglomerations under working conditions. Here, we report a stable single-site copper coordination polymer (Cu(OH)BTA) with periodic neighboring coppers and it exhibits 1.5 times increase of C2H4 selectivity compared to its metallic counterpart at 500 mA cm−2. In-situ/operando X-ray absorption, Raman, and infrared spectroscopies reveal that the catalyst remains structurally stable and does not undergo a dynamic transformation during reaction. Electrochemical and kinetic isotope effect analyses together with computational calculations show that neighboring Cu in the polymer provides suitably-distanced dual sites that enable the energetically favorable formation of an *OCCHO intermediate post a rate-determining step of CO hydrogenation. Accommodation of this intermediate imposes little changes of conformational energy to the catalyst structure during the C-C coupling. We stably operate full-device CO2 electrolysis at an industry-relevant current of one ampere for 67 h in a membrane electrode assembly. The coordination polymers provide a perspective on designing molecularly stable, single-site catalysts for electrochemical CO2 conversion.

Structural Diversity, XAS and Magnetism of Copper(II)-Nickel(II) Heterometallic Complexes Based on the [Ni(NCS)6]4- Unit.

The new heterometallic compounds, [{Cu(pn)2}2Ni(NCS)6]n·2nH2O (1), [{CuII(trien)}2Ni(NCS)6CuI(NCS)]n (2) and [Cu(tren)(NCS)]4[Ni(NCS)6] (3) (pn = 1,2-diaminopropane, trien = triethylenetetramine and tren = tris(2-aminoethylo)amine), were obtained and characterized by X-ray analysis, IR spectra, XAS and magnetic measurements. Compounds 1, 2 and 3 show the structural diversity of 2D, 1D and 0D compounds, respectively. Depending on the polyamine used, different coordination polyhedron for Cu(II) was found, i.e., distorted octahedral (1), square pyramidal (2) and trigonal bipyramidal (3), whereas coordination polyhedron for nickel(II) was always octahedral. It provides an approach for tailoring magnetic properties by proper selection of auxiliary ligands determining the topology. In 1, thiocyanate ligands form bridges between the copper and nickel ions, creating 2D layers of sql topology with weak ferromagnetic interactions. Compound 2 is a mixed-valence copper coordination polymer and shows the rare ladder topology of 1D chains decorated with [CuII(tren)]2+ antennas as the side chains attached to nickel(II). The ladder rails are formed by alternately arranged Ni(II) and Cu(I) ions connected by N2 thiocyanate anions and rungs made by N3 thiocyanate. For the Cu(I) ions, the tetrahedral thiocyanate environment mixed N/S donor atoms was found, confirming significant coordination spheres rearrangement occurring at the copper precursor together with the reduction in some Cu(II) to Cu(I). Such topology enables significant simplification of the magnetic properties modeling by assuming magnetic coupling inside {NiIICuII2} trinuclear units separated by diamagnetic [Cu(NCS)(SCN)3]3- linkers. Compound 3 shows three discrete mononuclear units connected by N-H…N and N-H…S hydrogen bonds. Analysis of XAS proves that the average ligand character and the covalency of the unoccupied metal d-based orbitals for copper(II) and nickel(II) increase in the following order: 1 → 2 → 3. In 1 and 2, a weak ferromagnetic coupling between copper(II) and nickel(II) was found, but in 2, additional and stronger antiferromagnetic interaction between copper(II) ions prevailed. Compound 3, as an ionic pair, shows, as expected, a spin-only magnetic moment.

Green Emissive Copper(I) Coordination Polymer Supported by the Diethylpyridylphosphine Ligand as a Luminescent Sensor for Overheating Processes.

Tertiary diethylpyridylphosphine was synthesized by the reaction of pyridylphosphine with bromoethane in a suberbasic medium. The reaction of phosphine with the copper(I) iodide led to the formation of a copper(I) coordination polymer, which, according to the X-ray diffraction data, has an intermediate structure with a copper-halide core between the octahedral and stairstep geometries of the Cu4I4 clusters. The obtained coordination polymer exhibits a green emission in the solid state, which is caused by the 3(M+X)LCT transitions. The heating up of the copper(I) coordination polymer to 138.5 °C results in its monomerization and the formation of a new solid-state phase. The new phase exhibits a red emission, with the emission band maximum at 725 nm. According to the experimental data and quantum chemical computations, it was concluded that depolymerization probably leads to a complex that is formed with the octahedral structure of the copper-halide core. The resulting solid-state phase can be backward-converted to the polymer phase via recrystallization from the acetone or DMF. Therefore, the obtained coordination polymer can be considered a sensor or detector for the overheating of processes that should be maintained at temperatures below 138 °C (e.g., engines, boiling liquids, solar heat systems, etc.).

Copper coordination compounds based on bis-quinolylhydrazone of 2,6-diacetylpyridine: Synthesis, structure and cytotoxic activity

Bis-hetarylhydrazone H2L (1), a condensation product of 2,6-diacetylpyridine with 2-hydrazinoquinoline, has been synthesized. By reaction of 1 with Cu(II) nitrate and perchlorate binuclear mixed-valence coordination compounds [CuICuII(H2L)2](NO3)3⋅2H2O (2) and [CuICuII(H2L)2](ClO4)3 (3) were obtained. Cu(I) Ion in the compounds 2, 3 is in the distorted tetrahedral N-donor environment, while Cu(II) center is in the distorted octahedral one. Cu(II) Chloride and bromide form with 1 asymmetric mononuclear compounds with composition [Cu(H2L)Cl]Cl (4) and [Cu(H2L)Br]Br (5) with tetragonal–pyramidal coordination polyhedron. The structures of the compounds were studied by means of NMR, IR, ESR, UV–vis, X-ray absorption spectroscopy and X-ray single crystal diffraction methods. The effect of compounds on viability of the Hep-2 and HepG2 cell lines was investigated in vitro. The study showed that 1 is non-toxic at tested concentrations (1–100 μM), while all the complexes exhibit significant dose-dependent cytotoxic effect. It is shown that compounds 2–5 demonstrate higher cytotoxicity towards considered cancer cell cultures than widely used commercial anticancer pharmaceuticals – cisplatin and carboplatin. Quantum-chemical modeling of the structure of the ligand 1 and its complexes was performed using density functional theory at B3LYP/6-311G(d) level of theory. Molecular electrostatic potential (MEP) and average local ionization energy (ALIE) surfaces were calculated to rationalize the reactive properties of the compounds.

One-Dimensional copper(II) coordination polymer based on 1,2-bis-(4-pyridyl) ethane and 1,10-phenathroline

The one dimensional copper(II) coordination polymer {[Cu(phen)(bpe)(NO3)(H2O)](NO3)(H2O)}n (1) has been synthesized using [Cu(phen)2(NO3)2] (where phen = 1′10-phenanthroline and linker bpe = 1,2-bis-(4-pyridyl) ethane. Complex 1 was characterised by using elemental analysis, IR spectroscopy and single crystal X-ray diffraction analysis. The copper(II) ion is octahedrally coordinated with two nitrogen donor atoms from two bpe, two nitrogen donor atoms from phen, one oxygen donor atom from water and one oxygen donor atom from nitrato. Each 1D polymeric chain is connected to another chain via hydrogen bonding and p....p stacking interaction to form 3D network structures.

Conformational dynamicity in a copper(II) coordination complex.

The geometries of copper coordination complexes are intricately related to their electron transfer capabilities, but the role of dynamics in these processes are not fully understood. We have previously reported CuCl(dpaOMe), a complex exhibiting conformational fluxionality in its CuI state and rigidity upon oxidation to CuII. Here, we report the synthesis and characterization of [CuCl(dpaSMe)]+/0, a complex exhibiting relative rigidity in its CuI state and structural dynamics upon oxidation to CuII. The dynamics of [CuCl(dpaSMe)]+ were characterized via X-ray diffraction, cyclic voltammetry, and EPR spectroscopy, where temperature-dependent interconversion between trigonal bipyramidal and square pyramidal geometries is observed. Coupling these solid and solution-state characterization data enabled assignment of the coordination geometries involved. Factors impacting these dynamics and their potential implications for electron transfer are discussed.

Heteroleptic copper(I) complexes [Cu(dmp)(N^P)]BF4 for photoinduced atom-transfer radical addition reactions.

Earth-abundant copper(I) coordination complexes of an imine-phosphine and a diimine have been developed as visible-light photocatalysts. Reaction of [Cu(MeCN)4]BF4 with hetero-bidentate phosphinopyrazole (phpz) ligand R1R2C3HN2PPh3 (R1 = R2 = H (1a); R1 = H, R2 = Me (1b); R1 = H, R2 = Ph (1c); R1 = R2 = Me (1d)) and 2,9-dimethyl-1,10-phenanthroline (dmp) gave four heteroleptic bis-chelate Cu(I) complexes [Cu(dmp)(R1R2C3HN2PPh3)]BF4 (R1 = R2 = H (2a); R1 = H, R2 = Me (2b); R1 = H, R2 = Ph (2c); R1 = R2 = Me (2d)) with distorted tetrahedral geometries. Complexes 2a-2d exhibited broad absorption in the visible spectrum and could facilitate photochemical intermolecular atom-transfer radical addition reactions of CBr4, or CCl3Br, CHI3 to styrenes in yields up to 91% and with a broad substrate scope. The absorption, emission, redox potential and photocatalytic activity were dependent on the substituents on the phpz ligand. Mechanistic studies supported an atom-transfer radical addition (ATRA) mechanism.

Copper(II) Coordination Compound with 2-Oxonicotinate: Synthesis, Spectroscopic and Electrochemical Studies

Copper(II) Coordination Compound with 2-Oxonicotinate: Synthesis, Spectroscopic and Electrochemical Studies

Polymeric copper(ii) and dimeric oxovanadium(v) complexes of amide-imine conjugate: bilirubin recognition and green catalysis.

An exceptionally simple amide-imine conjugate, (E)-N'-(4-(diethylamino)-2-hydroxybenzylidene)-4-methylbenzohydrazide (L), derived by the condensation of 4-methyl-benzoic acid hydrazide (PTA) with 4-(diethylamino)-2-hydroxybenzaldehyde was utilized to prepare a dimeric oxo-vanadium (V1) and a one-dimensional (1D) copper(ii) coordination polymer (C1). The structures of L, V1 and C1 were confirmed by single crystal X-ray diffraction analysis. The experimental results indicate that V1 is a promising green catalyst for the oxidation of sulfide, whereas C1 has potential for a C-S cross-coupling reaction in a greener way. Most importantly, C1 is an efficient 'turn-on' fluorescence sensor for bilirubin that functions via a ligand displacement approach. The displacement equilibrium constant is 7.78 × 105 M-1. The detection limit for bilirubin is 1.15 nM in aqueous chloroform (chloroform/water, 1/4, v/v, PBS buffer, and pH 8.0).

Degradable copper(ii)-doped starch-based biopolymeric films with antibacterial activity

New copper(ii) coordination compounds bearing ammonia and carboxylate ligands were assembled, fully characterized, and applied as antimicrobial dopants for producing sustainable starch-based biopolymeric films with remarkable antibacterial activity.

A colorimetric investigation of copper(II) solutions

Complex ions containing transition metals, elements with incompletely filled d-orbitals, are usually colored, while similar ions from non-transition metals are not. Copper (Cu) is a transition metal with a flexible coordination sphere, meaning it can adopt many coordination geometries, and therefore have various colored complexes. Addition of Chloride (Cl-) to a CuCl2 solution causes color change away from blue toward a yellowish green. We hypothesized that adding acetone to an CuCl2-water solution would cause a similar shift in solution color. We made aqueous CuCl2 solutions with increasing acetone concentrations ranging from 0 to 100 percent and compared the resulting colors to aqueous CuCl2 solutions with increasing added amounts of NaCl to find the closest visual fit in color from the acetone solutions for each added NaCl equivalent. Our experiment showed that increasing the percent acetone in aqueous CuCl2 solutions gave rise to a color change parallel to that of adding Cl-, shifting solution color away from blue and toward yellow. We propose an explanation of this relationship by considering Cu2+ reduction in high acetone concentrations and discussing ligand field strength and the possible effects of tetrahedral versus octahedral geometries on splitting: weaker splitting may similarly affect color through equilibria leaning towards green/yellow products. This investigation could support our understanding of copper coordination and copper complex color behavior with future implications for other copper complex experiments or, in a broader perspective, detection and evaluation of Cu2+ levels in environmental and biological systems.

Cupric coordination compounds with multiple anions: a promising strategy for the regulation of energetic materials.

To seek new high energetic materials, N-methylene-C-bridged nitrogen-rich heterocycle 1-((4,5-diamino-4H-1,2,4-triazol-3-yl)methyl)-1H-1,2,4-triazol-3,5-diamine (DATMTDA) (2) was first synthesized, and two copper coordination compounds ([Cu12(OH)4(ClO4)4(H2O)4(DATMTDA)12](ClO4)16·12H2O (3) and [Cu3(OH)(ClO4)(DATMTDA)3](ClO4)3(NO3) (4)) based on 2 were formed by introducing different anions. These compounds were characterized by elemental analysis, IR spectroscopy and single-crystal X-ray diffraction analysis. The crystal structures of compounds 3 and 4 are similar and crystallize in monoclinic systems with the P21/c space group, while the central copper atoms show different coordination behaviors. However, the structure of compounds 3 and 4 is analogous to a three dimensional structure owing to the O atom of OH-, forming coordinate bonds with three copper cations. The NBO charge of 2 was calculated using density functional theory to understand its coordination modes. The Hirshfeld surface calculation reveals that 3 and 4 have strong intermolecular interactions. The thermal decomposition processes, non-isothermal kinetics, and enthalpies of formation and sensitivities of these compounds were investigated. By introducing one NO3- of compound 4 to replace one ClO4- in compound 3, compound 4 shows lower density and lower decomposition peak temperature but lower sensitivity and a higher formation enthalpy than compound 3. The complex 4 possesses an outstanding catalytic effect for the decomposition of AP than that of complex 3. The results illustrate the possibility of introducing various anions into energetic coordination compounds for the regulation of energetic materials.

Addressing the gaps in homeostatic mechanisms of copper and copper dithiocarbamate complexes in cancer therapy: a shift from classical platinum-drug mechanisms.

The platinum drug, cisplatin, is considered as among the most successful medications in cancer treatment. However, due to its inherent toxicity and resistance limitations, research into other metal-based non-platinum anticancer medications with diverse mechanisms of action remains an active field. In this regard, copper complexes feature among non-platinum compounds which have shown promising potential as effective anticancer drugs. Moreover, the interesting discovery that cancer cells can alter their copper homeostatic processes to develop resistance to platinum-based treatments leads to suggestions that some copper compounds can indeed re-sensitize cancer cells to these drugs. In this work, we review copper and copper complexes bearing dithiocarbamate ligands which have shown promising results as anticancer agents. Dithiocarbamate ligands act as effective ionophores to convey the complexes of interest into cells thereby influencing the metal homeostatic balance and inducing apoptosis through various mechanisms. We focus on copper homeostasis in mammalian cells and on our current understanding of copper dysregulation in cancer and recent therapeutic breakthroughs using copper coordination complexes as anticancer drugs. We also discuss the molecular foundation of the mechanisms underlying their anticancer action. The opportunities that exist in research for these compounds and their potential as anticancer agents, especially when coupled with ligands such as dithiocarbamates, are also reviewed.