Tetradentate N2S2 Ligands Research Articles

A new bis(thioether)-dipyrrin N2S2 ligand and its coordination behaviors to nickel, copper and zinc.

Tetradentate N2S2 coordination platforms are widespread in biological systems and have endowed metalloenzymes and metalloproteins with abundant reactivities and functions. However, there are only three types of N2S2 scaffolds respectively based on the bipyridine, aryl and alkyl amine derivatives, which are significantly underdeveloped for coordination chemistry. With the objective of developing a new N2S2 coordination platform to assemble a series of first-row transition metal complexes, we have designed a novel tetradentate N2S2 ligand containing a central dipyrrin donor functionalized with two thioether-substituted aryl units. Interestingly, complexation of the N2S2 ligand with the chloride salts of Ni(II), Cu(II) and Zn(II) yields various geometries with various coordination numbers. The reaction between the ligand and NiCl2 readily forms two chloride-bridged centrosymmetric dinickel complexes in which the nickel centers are hexacoordinated by an N2S2Cl2 coordination environment in distorted octahedron geometry. In contrast, metalation of the ligand with CuCl2 gives a mononuclear copper complex consisting of a pentacoordinated copper center in a trigonal bipyramidal geometry with an N2S2Cl coordination sphere. Unexpectedly, the complexation of the ligand with ZnCl2 forms a homoleptic zinc complex in which the zinc center is surrounded by an N4 coordination sphere from two dipyrrin units in a non-planar pseudo-tetrahedral geometry despite the steric hindrance of two bulk thioether-substituted aryl units. These various geometries illustrate the potential structural flexibility of this new ligand. In addition, the optical properties of these compounds were also examined. This work thus provides a new N2S2 coordination platform with geometric flexibility.

Copper Complexes of Benzoylacetone Bis-Thiosemicarbazones: Metal and Ligand Based Redox Reactivity

Bis-thiosemicarbazones derived from the β-diketone benzoylacetone (H3banR, R=Me, Et, Ph) are potentially tetradentate N2S2 ligands whose coordination chemistry with copper is reported. In the absence of oxygen and in the presence of base they form anionic CuII complexes of the fully deprotonated ligands [CuII(banR)]–. Upon exposure to atmospheric oxygen they undergo a complex series of reactions leading to two types of products; one a ligand oxidised ketone complex [CuII(banRO)] and the other an unprecedented dimeric di-CuIII complex [(CuIII(banR))2] depending on the R substituent. Time-resolved UV-vis spectroscopy, cyclic voltammetry, spectroelectrochemistry, and electron paramagnetic resonance (EPR) spectroscopy have been used to identify intermediates on the way to stable products formed under both anaerobic and aerobic conditions. It is found that both ligand-centred and Cu-centred oxidation reactions are occurring in parallel leading to this unusually complicated mixture of products.

Spectroscopic Characterisation of a Bio-Inspired Ni-Based Proton Reduction Catalyst Bearing a Pentadentate N2 S3 Ligand with Improved Photocatalytic Activity.

Inspired by the sulfur-rich environment found in active hydrogenase enzymes, a Ni-based proton reduction catalyst with pentadentate N2 S3 ligand was synthesised. When coupled with [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine) as photosensitiser and ascorbate as electron donor in a 1:1 mixture of dimethylacetamide and aqueous ascorbic acid/ascorbate buffer, the catalyst showed improved photocatalytic activity compared with a homologous counterpart bearing a tetradentate N2 S2 ligand. The mechanistic pathway of photoinduced hydrogen evolution was comprehensively analysed through optical transient absorption and time-resolved X-ray absorption spectroscopy, which revealed important electronic and structural changes in the catalytic system during photoirradiation. The NiII catalyst undergoes a photoinduced metal-centred reduction to form a NiI intermediate with distorted square-bipyramidal geometry. Further kinetic analyses revealed differences in charge-separation dynamics between the pentadentate and tetradentate forms.

Connecting Main-Group Metals (Al, Ga, In) and Tungsten(0) Carbonyls via the N2S2 Metallo-Ligand Strategy

Tetradentate N2S2 ligands (such as bismercaptoethanediazacycloheptane in this study) have seen extensive use in combination with transition metals. Well-oriented N2S2 binding sites are ideal for d8 transition metals with square planar preferences, especially NiII, but also as a square pyramidal base for those metals with pentacoordinate preferences, such as [V≡O]2+, [Fe(NO)]2+, and [Co(NO)]2+. Further reactivity at the thiolate sulfurs generates diverse bi, tri, and tetra/heterometallic compounds. Few N2S2 ligands have been explored to investigate the possibility of binding to main group metals, especially group III (MIII) metals, and their utility as synthons for main group/transition metal bimetallic complexes. To open up this area of chemistry, we synthesized three new five-coordinate main group XMN2S2 complexes with methyl as the fifth binding ligand for M = Al, and chloride for M = Ga and In. The seven-membered diazacycle, dach, was engaged as a rigid stabilized connector between the terminal thiolate sulfurs. The pentacoordinate XMN2S2 complexes were characterized by 1H-NMR, 13C-NMR, +ESI-Mass spectra, and X-ray diffraction. Their stabilities and reactivities were probed by adding NiII sources and W(CO)5(THF). The former replaces the main group metals in all cases in the N2S2 coordination environment, demonstrating the weak coordinate bonds of MIII–N/S. The reaction of XMN2S2 (XM = ClGaIII or ClInIII) with the labile ligand W(0) complex W(CO)5(THF) resulted in Ga/In–W bimetallic complexes with a thiolate S-bridge. The synthesis of XMN2S2 complexes provide examples of MIII–S coordination, especially Al–S, which is relatively rare. The bimetallic Ga/In–S–W complex formation indicates that the nucleophilic ability of sulfur is retained in MIII–S–R, resulting in the ability of main group MIII–N2S2 complexes to serve as metalloligands.

X-ray crystal structural and spectral studies of copper(II) and nickel(II) complexes of functionalized bis(thiosemicarbazone) ligands and investigation of their electrochemical behavior

Neutral bivalent copper and nickel bis(thiosemicarbazone) complexes bearing the methoxy and bromine substituents on the dialdehyde-backbone of the ligand have been synthesized. All of them have been characterized by spectroscopic methods and in two cases by X-ray crystallography. Upon formation of these complexes, the each two bis(thiosemicarbazone) arms is deprotonated, acting as dianionic, tetradentate N2S2 ligands. The geometrical distortion from square planar in the copper complex is very greater than the nickel complex. Cyclic voltammetry studies of Cu(II) and Ni(II) complexes in DMF show that all complexes are able to stabilize low oxidation states of Cu(I) and Ni(I). The introduction of different substitutions on the aromatic ring attached to the imine carbon led to two consequences. First, The E1/2 values for the CuI/CuII and NiI/NiII redox couples were cathodically shifted in comparison with the unsubstituted analog. Second, the copper complexes underwent a quasi-reversible one-electron oxidation (CuII/CuIII).

Nickel(II) complexes of N2S2 donor set ligand and halide/pseudohalides: Synthesis, crystal structure, DNA and bovine/human serum albumin interaction

A series of neutral hexacoordinated nickel(II) complexes of formula [Ni II(L)X2] (where L =3,4-bis(2-pyridylmethylthio)toluene with tetradentate N2S2 donor set and X = chloride (1), azide (2), cyanate (3) and isothiocyanate anion (4)) have been synthesized and isolated in pure form. The complexes were characterized by physicochemical and spectroscopic methods along with detailed structural characterization of 1,2 and 3 by single crystal X-ray diffraction analyses. The structural study showed that the nickel(II) ion has a distorted octahedral geometry being chelated by the tetradentate N2S2 ligand and bound to cis- located choride or pseudohalide anions. In dimethylformamide solution the complexes showed quasi-reversible Ni II/NiIII redox couples in cyclic voltammograms with E1/2 values of + 0.723, + 0.749, + 0.768 and + 0.868 V for 1, 2, 3 and 4, respectively. The study of interaction of the complexes with calf thymus DNA, bovine serum albumin (BSA) and human serum albumin (HSA) using spectroscopic and physicochemical tools clearly indicates that the complexes interact with DNA via groove binding mode. Four new neutral distorted octahedral nickel(II) complexes of general formula, [NiII(L)X2] have been synthesized and structurally characterized. The spectroscopic and physicochemical studies of interaction of the complexes with calf thymus DNA, BSA and HSA have also been included.

First example of bridge mono-coordination mode for the ligand 1,8-bis-(2-pyridyl)-3,6-dithiaoctane (pdto) in a Co(II) tetrahedral complex

The reaction between CoCl2·6H2O and the ligand 1,8-bis(2-pyridyl)-3,6-dithiaoctane (pdto) in acetonitrile leads to the formation of the tetrahedral Co(II) complex, [Co(κ-1N-pdto)Cl2]. This compound presents an unusual bridge mono-coordination mode of the tetradentate N2S2 ligand, pdto, not observed with other transition metals before. It was found that the complex presents a similar coordination environment around the Co(II) ion, either in solid sate or in solution, with an important role of chloride ions. Single crystal X-ray diffraction analysis of the compound shows a polymeric structure, where pdto acts as a bimetallic bridge with supramolecular array via intermolecular contacts C–H⋯X, with X=Cl, N and S.

Helical metallomacrocyclic nickel(II) complexes of a tetradentate N2S2 ligand derived from N-methyldithiocarbazic acid

The structure of the tetradentate sulphur-nitrogen chelating agent, 1,2-phenylenebis(methylene) bis(1-methylhydrazinecarbodithioate) (NSSN) has been determined by X-ray diffraction. In the solid state, the two arms of the ligand remain in E configurations about the hydrazinic C-N bonds. The ligand reacts with hydrated nickel(II) salts in a 3:2 ligand-to-metal ratio yielding dimeric triple helical complexes of general formula, [Ni2(NSSN) 3]X4•nH2O (X = Cl, Br, I, NCS; n = 5, 6, 9) which contain two nickel(II) ions in distorted octahedral environments. An X-ray crystallographic structure determination of the [Ni2(NSSN) 3]Cl4•9H2O complex shows that it is a metallomacrocycle in which each ligand acts as a bis-bidentate chelating agent providing NS donors of one arm to one nickel(II) ion and NS donors of the second arm to a second nickel(II) ion. Each nickel(II) ion adopts a distorted octahedral geometry with a NiN3S3 coordination core as shown by their common electronic spectra.

Spectral and Structural Studies of the Copper(II) Complexes of 3, 4‐Hexanedione Bis(3‐azacyclothiosemicarbazones)

AbstractCopper(II) complexes of 3, 4‐hexanedione bis(piperidyl‐ and bis(hexamethyleneiminylthiosemicarbazone), H2Hxpip and H2Hxhexim, respectively, have been prepared and studied spectroscopically. The bis(thiosemicarbazones) have been characterized by their melting points, as well as IR, electronic and 1H NMR spectra. Upon formation of their copper(II) complexes, loss of the hydrazinic hydrogen atoms occurs, and the ligands coordinate as dianionic, tetradentate N2S2 ligands. The crystal structures of H2Hxpip, its 4‐coordinate copper(II) complex, [Cu(Hxpip)], and the related [Cu(Hxhexim)] have been determined by single crystal x‐ray diffraction. The nature of the four‐coordinate copper(II) complexes have also been characterized by ESR, IR, and electronic spectroscopy, as well as magnetic moments and elemental analyses.

Synthesis and spectral characterization of 1-phenylglyoxal bis{N(4)ethylthiosemicarbazone} and its metal complexes

Nickel(II), copper(II), zinc(II), and cadmium(II) complexes of 1-phenylglyoxal bis{N(4)-ethylthiosemicarbazone} have been synthesized and characterized spectroscopically. The bis(thiosemicarbazone) and its diamagnetic complexes have been characterized by their melting points, i.r., electronic, mass and 1H n.m.r. spectra. The four-coordinate copper(II) complex was also studied by e.s.r. spectroscopy. Upon coordination, the loss of a proton from each thiosemicarbazone moiety occurs, and the bis(thiosemicarbazone) coordinates to nickel(II) and copper(II) as a dianionic, tetradentate N2S2 ligand; but for zinc(II) and cadmium(II), mass spectral results suggest the formation of binuclear complexes.

Copper(II) and nickel(II) complexes of the bis{N(3)-substituted thiosemicarbazones} of phenylglyoxal and 1-phenylpropane-1,2-dione

Copper(II) and nickel(II) complexes of phenylglyoxal and phenylpropane-1,2-dione bis{N(3)-methyl-, bis{N(3)-dimethyl-, bis{piperidylthiosemicarbazone} have been prepared and studied spectroscopically. The six bis(thiosemicarbazones) have been characterized by their melting points, as well as i.r., electronic and 1H-n.m.r. spectra. The four-coordinate copper(II) complexes have been studied by e.s.r. spectroscopy, and the copper(II) and nickel(II) complexes by spectroscopic techniques mentioned above. Upon formation of these complexes, the loss of protons from each thiosemicarbazone moiety occurs, and the bis(thiosemicarbazones) coordinate to the metal centres as dianionic, tetradentate N2S2 ligands.

Copper(II) and nickel(II) complexes of glyoxaldehyde bis{N(3)-substituted thiosemicarbazones}

Copper(II) and nickel(II) complexes of glyoxaldehyde bis{N(3)-methyl-, bis{N(3)-ethyl-, bis{N(3)-dimethyl-, bis{piperidyl- and bis{hexamethyleneiminylthiosemicarbazone} have been prepared and studied spectroscopically. The five bis(thiosemicarbazones) have been characterized by their melting points, as well as i.r., electronic and 1H-n.m.r. spectra. The four-coordinate copper(II) complexes have been studied by e.s.r. spectroscopy, and the copper(II) and nickel(II) complexes by a number of the spectroscopic techniques mentioned above. Upon formation of these complexes, loss of protons from each thiosemicarbazone moiety occurs, and the bis(thiosemicarbazones) coordinate to the metal centres as dianionic, tetradentate N2S2 ligands.

Crystal structure of a molybdenum(VI) compound with a tetradentate N2S2 ligand: re-refinement in a higher symmetry space group

Crystal structure of a molybdenum(VI) compound with a tetradentate N2S2 ligand: re-refinement in a higher symmetry space group