Tetradentate Research Articles

Engineered organotin(IV) and vanadium(V) derivatives with distinct coordination modes and luminescent properties for the efficient detection and quantification of permanganate ions.

The distinction in coordination modes of metal complexes leads to their versatile structural features and unique properties. Here, we report two tetradentate Schiff base ligands (H2L1 and H2L2) bearing N2O2 donor sets, tactically selected to provide distinct coordination modes with different metal ions. The ligands were utilized to synthesize their organotin(IV) (1-4) and vanadium(V) (5) derivatives. The synthesized compounds were characterized using elemental analysis, FT-IR spectroscopy, multi-nuclei NMR (1H, 13C, and 119Sn) spectroscopy, mass spectrometry, and single-crystal X-ray diffraction. The organotin(IV) derivatives (1-4) displayed hepta-coordination around both the Sn centres as they were achieved in their dimeric form. Contrariwise, the vanadium(V) compound (5) was isolated as a mononuclear entity exhibiting penta-coordinated geometry around the vanadium centre. The variation in the coordination modes was evident in their UV-vis and fluorescence spectra. The organotin(IV) compounds (1-4) exhibited a strong emission band centred at 468nm when excited at a wavelength of 360nm whereas the vanadium(V) (5) derivative displayed poor fluorogenic response. Compound 1 was further explored for the fluorogenic chemo-sensing of permanganate ions (MnO4-) amongst various anions by quenching response. A detailed investigation of the recognition of permanganate ions was accomplished by spectrofluorometric, spectroscopic (119Sn NMR), mass spectrometric, and computational studies.

Homogeneous Ruthenium catalyzed Hydrogenation of Furanoate Esters and Ether‐containing aliphatic Esters into primary Alcohols

AbstractFuranoate esters have been hydrogenated successfully with ruthenium complexes bearing tetradentate ligands in high yield, high selectivity and with a low catalyst loading. The catalysts and the reaction conditions described herein enabled the facile reduction of a variety of substituted 2‐ and 3‐furanoate esters, with the latter being never reduced before under catalytic hydrogenation conditions. Furthermore, several well‐defined ruthenium complexes were evaluated and compared, with bis‐imino complexes being the most versatile in terms of reactivity and selectivity.

Order-Disorder, Symmetry Breaking, and Crystallographic Phase Transition in a Series of Bis(trans-thiocyanate)iron(II) Spin Crossover Complexes Based on Tetradentate Ligands Containing 1,2,3-Triazoles.

We report herein a series of neutral trans-thiocyanate mononuclear spin crossover (SCO) complexes, [FeIIL(NCS)2] (1-4), based on tetradentate ligands L obtained by reaction of N-substituted 1,2,3-triazolecarbaldehyde with 1,3-propanediamine or 2,2-dimethyl-1,3-diaminopropane [L = N1,N3-bis((1,5-dimethyl-1H-1,2,3-triazol-4-yl)methylene)propane-1,3-diamine/-2,2-dimethylpropane-1,3-diamine, 1/2 and N1,N3-bis((1-ethyl/1-propyl-1H-1,2,3-triazol-4-yl)methylene)-2,2-dimethylpropane-1,3-diamine, 3/4]. The thermal-induced SCO behavior is characterized by abrupt transitions with an average critical temperature (ΔT1/2)/hysteresis loop width (ΔThyst) in the range 190-252/5-14 K, while the photo-generated metastable high-spin (HS) phases are characterized by TLIESST temperatures in the range 44-59 K. Single crystal analysis shows that except 1, all compounds experience reversible symmetry breaking coupled with the thermal SCO. Furthermore, 4 experiences an additional phase transition at ca. 290 K responsible for the coexistence of two HS phases quenched at 10 K through LIESST and TIESST effects. The molecules form hexagonally packed arrays sustained by numerous weak CH···S and C···C/S···C/N···C bonds involving polar coordination cores, while non-polar pendant aliphatic substituents are segregated inside, occupying hexagonal channels. Energy framework analysis of complexes with one step SCO transition (1, 2, and 4) shows a correlation between the cooperativity and the amplitude of changes in the molecule-molecule interactions in the lattice at the SCO transition.

Study of the Degradation of Dyes by a Mononuclear Copper Complex Forming a Zigzag Chain

The titled mononuclear complex [Cu (L)] with the formally tetradentate schiff base ligand, [N,N’-bis (2-hydroxy-3-methoxybenzylidene)-1,3-diaminopropan-2-ol] has been synthesized and characterized using IR spectroscopy and elemental analysis. Crystal structure has been determined by XRD which reveals the pseudo octahedral complex geometry and the use of hydroxo bridges between two neighbor units to form a 1D coordination polymer. The use of this complex as a catalyst for the degradation of two textile dyes, in the presence of the appropriate amount of H2O2 and a UV light source has given very good results with yields exceeding 98% after 50 minutes.

Synthesis and Characterization of New Tetradentate N2O2-Based Schiff’s Base Cu (II) Complexes for Dye Photodegradation

We have reported tetradentate ligands (salophen) coordinated with N and O atoms that led to the Cu (II) complexes. These Cu (II) complexes (C-1 and C-2) were firstly established by using elemental analysis and confirmed by mass spectra. At the same time, the characterization of C-1 and C-2 complexes is performed by using several spectroscopic methods and morphological analysis. The bandgap values of the C-1 and C-2 complexes are evaluated with UV-vis DRS spectra. The PL spectral data and photocurrent curves clearly indicated the small recombination rate of the hole–electron pair. The synthesized C-1 and C-2 complexes’ photocatalytic properties were examined for the degradation of cationic dyes such as methylene blue (MB λmax.= 654 nm) and methyl violet (MV λmax.= 590 nm) below visible-light action. The C-2 complex is more active than the C-1 complex because of its high photostability, small band-gap energy, and low recombination rate for hole–electron pair separation, and improved visible-light character, which encourages the generation of hydroxyl radical species throughout the photodegradation process. Scavenger probes were used to identify the dynamic species for the photodegradation of dyes, and a mechanism investigation was established.

New Mn(III) complexes with tetradentate cis-N2O2, acacen-type ligands. Synthesis and characterization, single-crystal X-ray diffraction and DFT studies

One of the challenges of manganese(III) magnetochemistry is the design and synthesis of new building block systems to study and develop molecular magnets. In this context, two new Mn(III) complexes were synthesized using new tetradentate cis-N2O2 acacen ligands from the condensation of 1-anisyl-1,3-butanedione and 1,2-diaminoethane (2:1 ratio). All compounds were fully characterized by conventional spectroscopic techniques, such as FT-IR, HRMS, and UV–Vis, and rationalized with DFT and TD-DFT studies. The resulting cationic complexes were composed of N4O2 coordination spheres using a tetradentate ligand (N2O22−) in the equatorial plane and two axial pyridines with general formulae as [MnIII(L)(Py)2](X) (X = BF4−; ClO4−). Single-crystal X-ray diffraction revealed that both complexes are isostructural with typical axial anisotropy due to the Jahn-Teller effect by tetragonal distortion. A complete analysis of the crystalline structures revealed a nonclassical intermolecular interaction of CH···F and CH···O. Supported by Hirshfeld surfaces and 2D- fingerprint plots, it was possible to isolate and calculate the contribution of intermolecular interactions in the supramolecular arrangement. For both complexes, the magnetic susceptibility confirmed a high spin state (HS) with 4.8 µB and 4.9 µB related to the spin-only contribution to the magnetic moment.

Mono- and Bimetallic Aluminum Complexes Supported by Thioether-Amide Ligands: Synthesis, Characterization, and Application in the Ring Opening Polymerization of l-Lactide

Mono- and bimetallic aluminum complexes bearing bidentate NS or tetradentate NSSN ligands have been synthesized via reaction of AlMe3 with the corresponding proligand and spectroscopically characterized. In these complexes the aluminum centers are surrounded by anionic nitrogen and neutral sulfur atoms that, together with the carbon atoms of the alkyl groups, produce tetrahedral coordination geometries. In the bimetallic complexes the two metal centers are linked by a thioether-bridge of different lengths. The solid state structure of the complexes featuring the thioethyl or thiopropyl bridge were determined by X-ray diffraction studies. Both mono- and bimetallic aluminum complexes display fluxional behaviors in solution. After proper activation, all complexes were able to promote the ring opening polymerization of l-lactide in toluene solution. The polymerizations were well controlled as testified by the linear increase of the molecular weights with the conversions and the narrow polydispersity indexes. The highest activity was observed for the bimetallic complex bearing the NSSN ligand with the shortest bridge (ethyl bridge) with a turnover frequency of 11.6 molLA·molAl–1·h–1 (toluene, 80 °C). The activity decreased as the distance between the two metal centers increased. Moreover, the activity of the bimetallic complex with the ethyl bridge was higher than those of the monometallic ones. These findings suggest that a cooperative interaction between two aluminum centers is operative in the bimetallic complex with the shortest bridge.

Synthesis of Cytotoxic Iron‐Containing Macrocycles Through Unexpected C(sp2)−C(sp2) Bonds Formation

AbstractIn previous reports, it has been demonstrated that cyclometalated iron(II/III) complexes can be prepared by reacting iron(0) precursors and mercurated or brominated derivatives. However, in this report, the reaction between [Fe3(CO)12] and mercurated 6‐phenyl‐2,2′‐bipyridine or brominated 2,6‐diphenyl‐pyridine pincer derivatives led to compounds in which C(sp2)−C(sp2) bonds have been formed between two ligands. A 16‐electron iron(II) complex (1Cl) bearing a tetradentate ligand originating from the dimerization of 6‐phenyl‐2,2′‐bipyridine was isolated, while a protonated 14‐membered macrocycle with [FeBr4]− as counterion (2) was obtained from 2,6‐diphenyl‐pyridine. Studies by X‐ray diffraction crystallography, NMR, UV‐vis and cyclic voltammetry confirmed the structures. Additionally, the cytotoxicity of the new compounds toward gastric cancer cell lines was evaluated, and it was established that the presence of the iron(II) center was crucial for an elevated activity.

Trading Symmetry for Stereoinduction in Tetradentate, non-C2 -Symmetric Fe(II)-Complexes for Asymmetric Catalysis.

A series of stereogenic-at-metal iron complexes comprising a non-C2 -symmetric chiral topology is introduced and applied to asymmetric 3d-transition metal catalysis. The chiral iron(II) complexes are built from chiral tetradentate N4-ligands containing a proline-derived amino pyrrolidinyl backbone which controls the relative (cis-α coordination) and absolute metal-centered configuration (Λ vs. Δ). Two chloride ligands complement the octahedral coordination sphere. The modular composition of the tetradentate ligands facilitates the straightforward incorporation of different terminal coordinating heteroaromatic groups into the scaffold. The influence of various combinations was evaluated in an asymmetric ring contraction of isoxazoles to 2H-azirines revealing that a decrease of symmetry is beneficial for the stereoinduction to obtain chiral products in up to 99 % yield and with up to 92 % ee. Conveniently, iron catalysis is feasible under open flask conditions with the bench-stable dichloro complexes exhibiting high robustness towards oxidative or hydrolytic decomposition. The versatility of non-racemic 2H-azirines was subsequently showcased with the conversion into a variety of quaternary α-amino acid derivatives.

Synthesis and crystal structure of a PdII complex of ortho-xylylenebis(pyridyl-triazole).

A tetra-dentate ligand, namely, ortho-xylylenebis(pyridyl-triazole), o-xpt, was synthesized using the 'click' method and complexed with Pd(BF4)2. In the title com-plex, bis-{μ-1,1'-(o-xylylene)bis-[4-(pyridin-2-yl)triazole]-κN 3:N 3'}dipalladi-um(II) tetra-kis-(tetrafluoridoborate)-di-methyl-formamide-diethyl ether (1/2/1), as the BF4 salt, and including di-methyl-formamide and diethyl ether solvent mol-ecules, with stoichiometry [Pd2(C22H18N8)2](BF4)4·2C3H7NO·C4H10O, the Pd complex and the disordered diethyl ether mol-ecule lie on inversion centers. The ligand coordinates to the PdII centers with square-planar geometry, forming a dimeric macrocycle. The Pd⋯Pd separation in the complex [Pd2(o-xpt)2]4+ cation is 3.6184 (4) Å. In the crystal, the complex mol-ecules are stacked along the b axis, with π-π inter-actions between the pyridyl-triazole ligands of two mol-ecules.

Mechanochemical Synthesis and Reactivity of a Stable Nickel Borohydride

AbstractWe report the solvent‐assisted mechanochemically synthesised mononuclear Ni(II) borohydride complex supported by a macrocyclic tetradentate ligand. It was characterised as stable high‐spin [(LMe)Ni(η2‐BH4)]+ cation in solid‐state and in solution by various physicochemical methods and capable of transferring hydride. The stoichiometric reactivity of [(LMe)Ni(η2‐BH4)]+ cation was examined with trityl cation, alkyl halide, disulphide, thiol, carboxylic acid and enol at room temperature.

Unfolding the extraction and coordination behaviors of trivalent lanthanides by two novel cyclohexyl o-oxydiamides ligands

In this work, two cyclohexyl o-oxydiamides ligands: 2,2′-(cyclohexane-1,2-diylbis(oxy))bis(N,N-dioctylacetamide)(L1: Octy-CDA) and 2,2′-(cyclohexane-1,2-diylbis(oxy))bis(N,N-bis(2-ethylhexyl)acetamide) (L2: 2-ethylhexyl-CDA) have been synthesized to study the extraction and coordination behaviors with trivalent lanthanides in HNO3 media. The extractability of ligands increases with the atomic number of lanthanides increasing. The straight alkyl chain demonstrated better extraction ability than the branched chain, which was also most likely caused by the impact of steric hindrance effects. Coordination chemistry studies were evaluated through slope analysis, ultraviolet–visible spectroscopy, nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry, Fourier transform infrared spectroscopy and fluorescence, and all the results indicating that lanthanides with the above ligands can form both 1:1 and 1:2 metal/ligand complexes during extraction, and two ligands are tetradentate ligands. Density functional theory (DFT) calculations shed light that the interaction between the carbonyl oxygen atom and Eu(III) in the ligand is much stronger than that of the ether-bonded oxygen atom, and the analysis of the bond-forming properties shows that the stability of the 1:2 complex is stronger than that of the 1:1 complex. This work enriched the coordination study of diamide with lanthanides, which is expected to improve our understanding of the chemical properties of lanthanides during coordination and provide new insight into the design and synthesis of novel ligands.

Molybdenum(VI) complexes with a chiral l-alanine bisphenol [O,N,O,O’] ligand. Synthesis, structure, spectroscopic properties and catalytic activity

Dioxidomolybdenum(VI) compound [MoO2Cl2(dmso)2] reacts with a chiral tetradentate O3N-type l-alanine bisphenol ligand precursor (Et3NH)H2Lala to form an oxidochloridomolybdenum(VI) complex [MoOCl(Lala)] (1) as two separable geometric isomers with phenolate groups in cis or trans positions. The single crystal X-ray and NMR analyses of cis- and trans-1 reveal that the complexes are formed of monomeric molecules, in which the ligand has a tetradentate coordination through three oxygen donors and one nitrogen donor. The reaction of Na2MoO4·2H2O with the same ligand precursor in an acidic methanol solution leads to the formation of an anionic dioxido complex (Et3NH)[MoO2(Lala)] (2) with a trans coordination of the tetradentate ligand. Trans-1 and 2 were studied as active catalysts for olefin epoxidation: i.e. styrene, cyclohexene, S(–)-limonene and (–)-α-pinene using H2O2 and tBuOOH as oxidants.

Template Synthesis of Ni(II) and Zn(II) Complexes Derived from N2O2 Donor Symmetrical Tetradentate Ligands System: Comparative Antioxidant, Xanthine inhibitory, and Computational Studies

Four mononuclear symmetric coordination compounds of Ni(II) and Zn(II) (C1–C4), obtained from N2O2 donor Schiff base ligand systems were synthesized in-situ and characterized with a variety of analytical and spectroscopic techniques such as elemental CHN-analysis, 1H-NMR, 13C{H}NMR, UV-Visible, Infrared, powder X-ray diffraction, scanning electron, and high-resolution mass spectroscopies. These analyses revealed that the ligands chelate the metal ions through the oxygen and nitrogen (N2O2) atoms of the phenolate and azomethine functional groups, resulting in distorted and perfect square planar geometry around Zn(II) and Ni(II) ions, respectively. The 2,2-diphenylpicrylhydrazyl (DPPH) radical scavenging and xanthine oxidase activity of the complexes were investigated using in vitro assay techniques. The studies revealed that the complexes possess moderate radical scavenging activity with no or very weak xanthine oxidase activity, in which the controls show better activity than the complexes in both assays. However, C3 and C1 demonstrated some level of activity at higher concentrations but not commensurate with the controls. The DFT calculation of the complexes using the M06- 2X/B3LYP-311G9d.p) level of the theory presented electronic properties that supported the experimental findings on the complexes

Electroluminescence of Tetradentate Pt(II) Complexes: O^N^N^O versus C^N^N^O Coordination.

Alkylation of one of the phenolic hydroxyl groups in a salen-type tetradentate ligand changes the coordination mode from O^N^N^O to the cyclometallating C^N^N^O type. The ligand was used to synthesize a new cyclometalated luminescent Pt(II) complex 2. While in solution the complex is poorly luminescent, in the solid state the emission is reinstated, which allowed one to evaluate complex 2 as a phosphorescent emitter in organic light-emitting diodes. 2 displays external quantum efficiency (EQE) = 9.1% and a maximum luminance of 9000 cd m-2 in a vacuum-deposited device. We carried out comparative analysis of photo- and electroluminescence of complex 2 with O^N^N^O complex 1 and demonstrated that the similar luminescent properties of the O^N^N^O and C^N^N^O complexes are rather coincidental because they display different excited-state landscapes. Surprisingly, the two complexes display a dramatically different electrochemical behavior, with O^N^N^O coordination leading to the formation of a stable electropolymer but C^N^N^O coordination fully preventing electropolymerization.

Copper and Nickel Complexes of Oxamate−Phenol Containing Ligands: A Structural Dichotomy in Oxidized Species

AbstractThe copper and nickel complexes of two tetradentate ligands derived from bis(aminophenol) and bis(phenol) architectures connected by an oxamate linker were isolated. Depending on the metal and ligand, the complex is isolated with either an intact (deprotonated) ligand (12−), one‐electron oxidized ligand (2−) or quinone form (3). Surprisingly, the Mannich base is easier to oxidize than the amidophenol derivatives. The complexes were characterized by X‐ray diffraction, cyclic voltammetry, UV‐Vis‐NIR and EPR spectroscopies. Complex 1 shows two reversible oxidation waves assigned to the successive iminosemiquinone/aminophenolate redox systems. Complex 2− shows an intense NIR feature, as well as an EPR signal at giso=2.043, consistent with a metallic contribution to the main ligand radical SOMO. Complex 3 shows the typical feature of an isolated Cu(II) complex. Spectro‐electrochemistry coupled to DFT calculations demonstrate a ligand‐centered oxidative redox chemistry for all the complexes.

One-pot synthesis, characterization, photocatalytic activity and biological studies of Co(II), Ni(II) and Cu(II) complexes of a tetraazamacrocyclic Schiff base

Template condensation between pentane-1,5-dial and triethylenetetramine in the presence of divalent metal salts in methanolic solution resulted in mononuclear 15-membered tetraazamacrocyclic Schiff base metal complexes, [M(C11H22N4)X2], where M = Co(II), Ni(II) and Cu(II), X = Cl− and C11H22N4 is the tetradentate macrocyclic ligand. The bonding and overall geometry of the complexes are inferred through elemental analysis, FT-IR, UV–vis, 1H/13C NMR, ESI-mass, molar conductance, magnetic susceptibility and TG measurements. On the basis of the absorption bands in UV–vis spectra and magnetic moment analyses, a six-coordinate octahedral geometry is proposed for all the metal complexes. The formulation of the metal chelates is compatible with the thermal decomposition profiles. Photocatalytic efficiency of the macrocyclic Ni(II) complex was studied using solar water-splitting. The in vitro antimicrobial activity of metal complexes has been tested against certain bacterial and fungal species. The scavenging ability of DPPH free radicals was used to test the antioxidant activity of the complexes. In addition, MTT assay was used to assess the cytotoxicity of Cu(II) chloro complex against L929 mouse fibroblast cell lines.

Stereochemical Properties of Two Schiff-Base Transition Metal Complexes and Their Ligand by Using Multiple Chiroptical Spectroscopic Tools and DFT Calculations.

Two transition metal complexes were synthesized with Ni(II) and Cu(II) using a tetradentate Schiff-base ligand, (R,R) and (S,S)-N,N'-Bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine. The stereochemical properties of the ligand and the metal complexes were investigated using a combined experimental and theoretical approach. Multiple spectroscopic techniques, which include IR, vibrational circular dichroism (VCD), UV-Vis and electronic circular dichroism (ECD), as well as Raman and the newly discovered ECD-circularly polarized Raman (i.e., eCP-Raman) spectroscopies were utilized. The good agreement achieved between the experimental and simulated IR, VCD, UV-Vis and ECD spectra of the ligand allowed one to identify the presence of three main ligand conformers in solution, thanks, especially to the high VCD sensitivity to the conformations associated with the tertbutyl groups. The helicity of the metal complexes was identified to be M and P for those with the (R,R) and (S,S) ligands, respectively. Furthermore, eCP-Raman measurements were carried out for the two metal complexes under (near) resonance. Their induced solvent chiral Raman features were explained, and the potential application of eCP-Raman was discussed.

Li vs Na: Divergent Reaction Patterns between Organolithium and Organosodium Complexes and Ligand-Catalyzed Ketone/Aldehyde Methylenation.

Organosodium chemistry is underdeveloped compared with organolithium chemistry, and all the reported organosodium complexes exhibit similar, if not identical, reactivity patterns to their lithium counterparts. Herein, we report a rare organosodium monomeric complex, namely, [Na(CH2SiMe3)(Me6Tren)] (1-Na) (Me6Tren: tris[2-(dimethylamino)ethyl]amine) stabilized by a tetra-dentate neutral amine ligand Me6Tren. Employing organo-carbonyl substrates (ketones, aldehydes, amides, ester), we demonstrated that 1-Na features distinct reactivity patterns compared with its lithium counterpart, [Li(CH2SiMe3)(Me6Tren)] (1-Li). Based on this knowledge, we further developed a ligand-catalysis strategy to conduct ketone/aldehyde methylenations, using [NaCH2SiMe3]∞ as the CH2 feedstock, replacing the widely used but hazardous/expensive C═O methylenation methods, such as Wittig, Tebbe, Julia/Julia-Kocieński, Peterson, and so on.

Recent Development of Copper (II) Complexes of Polypyridyl Ligands in Chemotherapy and Photodynamic Therapy.

Cancer is a deadly disease associated with abnormal cell growth and invasion to various parts of the body. Many drugs such as cisplatin and carboplatin have been used for the treatment of cancer over the years. However, a lack of selectivity toward cancer cells and associated side effects with current treatment options has fueled continued efforts throughout the world to find better anticancer drugs. Over the past decades, copper-containing compounds have shown excellent anticancer activity. Cu(II) complexes are well studied and show broad-spectrum pharmacological activity. The redox activity of copper ions contributes to cytotoxic activity in combination with ligands that possess bioactivity. Binding of copper with various types of bidentate, tridentate, and tetradentate ligands can activate the processes of necrosis, apoptosis, and angiogenesis. Copper induces the formation of reactive oxygen species to cleave DNA. Similarly, light-assisted excitation of Cu(II) complexes in the red region of the electromagnetic spectrum helps produce different reactive species, thereby inducing anticancer activity through a photodynamic mechanism. In general, in vivo and in vitro studies have demonstrated that the administration of copper ions is effective against various cancer cell lines. Herein we discuss the past ten years of research into copper complexes of terpyridine-, 2,2'-bipyridine-, and 1,10-phenanthroline-based ligands as potential anticancer agents. The aspects of chemotherapy by redox-mediated pathways and photodynamic therapy using light-assisted excitation are reviewed, with a focus on mechanisms of activity, details of important experimental procedures, as well as drawbacks in the design of copper-containing drugs.