A comprehensive overview of the orientation of tetradentate N2O2 donor Schiff base ligands in octahedral complexes of trivalent 3d metals

An insight into the hydrogen bonding, halogen bonding and chalcogen bonding interactions in manganese(iii) complexes with N2O2donor salicylidine Schiff base ligands

Four manganese(iii) complexes, [MnL¹(H₂O)₂]ClO₄·H₂O (1), [MnL²(H₂O)₂]ClO₄ (2), [MnL³(DMSO)(H₂O)]ClO₄ (3) and [MnL⁴(DMSO)(H₂O)]ClO₄ (4), where H₂L¹ = N,N'-bis(5-bromosalicylidene)-1,3-diaminopropane, H₂L² = 2,2-dimethyl-N,N-bis(3-methyloxysalicylidene)-1,3-diaminopropane, H₂L³ = N,N'-bis(5-chlorosalicylidene)-2,2-dimethyl-1,3-diaminopropane and H₂L⁴ = 2-hydroxy-N,N'-bis(3-ethyloxysalicylidene)-1,3-diaminopropane are tetradentate N₂O₂-donor ligands and DMSO = dimethyl sulfoxide, have been synthesized and characterised by elemental analysis, IR and UV-vis spectroscopy and single-crystal X-ray diffraction studies. All are monomeric complexes. Complex 1 crystallises in orthorhombic space group P2₁2₁2₁, complex 3 crystallises in triclinic space group P-1, whereas complexes 2 and 4 crystallize in monoclinic space groups, C2/c and C2/m respectively. In all the complexes, manganese(iii) has a six-coordinated pseudo-octahedral geometry in which imine nitrogen atoms and phenolate oxygen atoms of the deprotonated di-Schiff base constitute the equatorial plane. In complexes 1 and 2, water molecules are present in the fifth and sixth coordination sites in the axial positions while in complexes 3 and 4 they are occupied by one water and one DMSO. The coordinated water molecules initiate hydrogen-bonded networks in all complexes. DFT calculations have been carried out to analyze two aspects of these complexes viz. the formation of halogen (HaB) and chalcogen bonding (ChB) interactions in complexes 1 and 3 where the electron donor is the perchlorate anion and the acceptor either bromine or chlorine atoms for the HaBs and the sulfur atom of the coordinated DMSO for the ChB. In addition, other intermolecular effects are discussed in the solid state for complexes 1, 2 and 4, where the hydrogen atoms of the coordinated water molecules interact with the electron rich cavities formed by the phenolate and alkyloxy oxygen atoms of the Schiff-base ligand.

Synthesis, Spectral Characterization, and Biological Activities of Some Metal Complexes Bearing an Unsymmetrical Salen-Type Ligand, (Z)-1-(((2-((E)-(2-Hydroxy-6-methoxybenzylidene)amino)phenyl)amino) methylene) Naphthalen-2(1H)-one

An unsymmetrical salen-type Schiff base ligand, (Z)-1-(((2-((E)-(2-hydroxy-6-methoxybenzylidene)amino)phenyl)amino)methylene)naphthalen-2(1H)-one, and its Zn(II), Cu(II), Co(II), Mn(II), and Fe(III) complexes were synthesized and characterized by mass (MS), nuclear magnetic resonance (NMR), infrared (IR), ultraviolet-visible (UV-Vis) spectra, and effective magnetic moments. The thermal analyses of the obtained ligand and metal complexes were conducted by thermogravimetric analysis (TGA). Antimicrobial activity of the unsymmetrical Schiff base ligand and its metal complexes were examined for Staphylococcus aureus as Gram-positive bacteria and Escherichia coli as Gram-negative bacteria. In vitro anticancer property of synthetic compounds was estimated against human cancer cell lines, a subline of Hela tumor cell line (KB), and a human liver cancer cell line (HepG-2) as well.

Large interaction energy for the homodimer and the heterodimer extracted from the supramolecular chain of a bent trinuclear zinc(ii) complex with a reduced Schiff base ligand

The coordinated amino groups in a trinuclear zinc complex participate in strong H-bonding interactions that have been analysed. The dimerization energy is very large for both the homodimer and the heterodimer in the 1D supramolecular chain.

Revisiting salicylidene-based anion receptors

Several salicylidene-based colorimetric and fluorimetric anion sensors are known in the literature. However, our 1H-NMR experimental results (in DMSO-d6) showed hydrolysis of imine (-N[double bond, length as m-dash]CH-) bonds in salicylidene-based receptors (SL, CL1 and CL2) in the presence of quaternary ammonium salts (n-Bu4N+) of halides (Cl- and Br-) and oxo-anions (H2PO4 -, HSO4 - and CH3COO-). The mono-salicylidene compound CL1 showed the most extensive -N[double bond, length as m-dash]CH- bond hydrolysis in the presence of anions. In contrast, the di-salicylidene compound CL2 and the tris-salicylidene compound SL showed comparatively slow hydrolysis of -N[double bond, length as m-dash]CH- bonds in the presence of anions. Anion-induced imine bond cleavage in salicylidene compounds could easily be detected in 1H-NMR due to the appearance of the salicylaldehyde -CHO peak at 10.3 ppm which eventually became more intense over time, and the -N[double bond, length as m-dash]CH- peak at 8.9-9.0 ppm became considerably weaker. Furthermore, the formation of the salicylidene O-H⋯X- (X- = Cl-/Br-) hydrogen-bonded complex, peak broadening due to proton-exchange processes and keto-enol tautomerism have also been clearly observed in the 1H-NMR experiments. Control 1H-NMR experiments revealed that the presence of moisture in the organic solvents could result in gradual hydrolysis of the salicylidene compounds, and the rate of hydrolysis has further been enhanced significantly in the presence of an anion. Based on 1H-NMR results, we have proposed a general mechanism for the anion-induced hydrolysis of imine bonds in salicylidene-based receptors.

Nickel(II)-Based Building Blocks with Schiff Base Derivatives: Experimental Insights and DFT Calculations

We have recently reported a series of neutral square planar tridentate Schiff base (L) complexes of the general formula [(L)M(py)], showing relatively high first-order hyperpolarizabilities and NLO redox switching behavior. In the present study, new members of this family of compounds have been prepared with the objective to investigate their potential as building blocks in the on-demand construction of D-π-A push–pull systems. Namely, ternary nickel(II) building blocks of general formula [(L^A/D)Ni(4-pyX)] (4–7), where L^A/D stands for an electron accepting or donating dianionic O,N,O-tridentate Schiff base ligand resulting from the monocondensation of 2-aminophenol or its 4-substituted nitro derivative and β-diketones R-C(=O)CH₂C(=O)CH₃ (R = methyl, anisyl, ferrocenyl), and 4-pyX is 4-iodopyridine or 4-ethynylpyridine, were synthesized and isolated in 60–78% yields. Unexpectedly, the Sonogashira cross-coupling reaction between the 4-iodopyridine derivative 6 and 4-ethynylpyridine led to the formation of the bis(4-pyridyl) acetylene bridged centrosymmetric dimer [{(L^D)Ni}₂(µ²-py-C≡C-py)] (8). Complexes 4–8 were characterized by elemental analysis, FT-IR and NMR spectroscopy, single crystal X-ray diffraction and computational methods. In each compound, the four-coordinate Ni(II) metal ion adopts a square planar geometry with two nitrogen and two oxygen atoms as donors occupying trans positions. In 8, the Ni…Ni separation is of 13.62(14) Å. Experimental results were proved and explained theoretically exploiting Density Functional Theory calculations.

Synthesis, DFT Calculations, Antiproliferative, Bactericidal Activity and Molecular Docking of Novel Mixed-Ligand Salen/8-Hydroxyquinoline Metal Complexes

Despite the common use of salens and hydroxyquinolines as therapeutic and bioactive agents, their metal complexes are still under development. Here, we report the synthesis of novel mixed-ligand metal complexes (MSQ) comprising salen (S), derived from (2,2′-{1,2-ethanediylbis[nitrilo(E) methylylidene]}diphenol, and 8-hydroxyquinoline (Q) with Co(II), Ni(II), Cd(II), Al(III), and La(III). The structures and properties of these MSQ metal complexes were investigated using molar conductivity, melting point, FTIR, ¹H NMR, ¹³C NMR, UV–VIS, mass spectra, and thermal analysis. Quantum calculation, analytical, and experimental measurements seem to suggest the proposed structure of the compounds and its uncommon monobasic tridentate binding mode of salen via phenolic oxygen, azomethine group, and the NH group. The general molecular formula of MSQ metal complexes is [M(S)(Q)(H₂O)] for M (II) = Co, Ni, and Cd or [M(S)(Q)(Cl)] and [M(S)(Q)(H₂O)]Cl for M(III) = La and Al, respectively. Importantly, all prepared metal complexes were evaluated for their antimicrobial and anticancer activities. The metal complexes exhibited high cytotoxic potency against human breast cancer (MDA-MB231) and liver cancer (Hep-G2) cell lines. Among all MSQ metal complexes, CoSQ and LaSQ produced IC₅₀ values (1.49 and 1.95 µM, respectively) that were comparable to that of cisplatin (1.55 µM) against Hep-G2 cells, whereas CdSQ and LaSQ had best potency against MDA-MB231 with IC₅₀ values of 1.95 and 1.43 µM, respectively. Furthermore, the metal complexes exhibited significant antimicrobial activities against a wide spectrum of both Gram-positive and -negative bacterial and fungal strains. The antibacterial and antifungal efficacies for the MSQ metal complexes, the free S and Q ligands, and the standard drugs gentamycin and ketoconazole decreased in the order AlSQ > LaSQ > CdSQ > gentamycin > NiSQ > CoSQ > Q > S for antibacterial activity, and for antifungal activity followed the trend of LaSQ > AlSQ > CdSQ > ketoconazole > NiSQ > CoSQ > Q > S. Molecular docking studies were performed to investigate the binding of the synthesized compounds with breast cancer oxidoreductase (PDB ID: 3HB5). According to the data obtained, the most probable coordination geometry is octahedral for all the metal complexes. The molecular and electronic structures of the metal complexes were optimized theoretically, and their quantum chemical parameters were calculated. PXRD results for the Cd(II) and La(III) metal complexes indicated that they were crystalline in nature.

Non-sterical stabilization of one-electron-oxidized NiSalen complex by thiophene core

A combined theoretical/experimental study of the new thiophene-based NiSalen complex with unconjugated bridging fragment. This complex demonstrates unusual stability of the oxidized form, which is not typical for this class of compounds.

A Series of Manganese(III) Salen Complexes as a Result of Team-Based Inquiry in a Transnational Education Programme

The development of a team-based approach to research-led transnational practical chemistry teaching is described in which a team of five Chinese students on an articulated transnational degree programme, supported by a team of academic and technical staff, carried out a study examining the structural chemistry of a series of manganese(III) salen complexes. A series of four crystallographically characterized manganese(III) salen complexes with ancillary carboxylate ligands are reported here. The carboxylate coordination modes range from the bridging syn-anti μ² -κO : κO' mode observed in the predominant cyclohexanoate and isobutyrate species, to a capping terminal monodentate mode for the adamantanoate species, and an unusual mixture of bridging and terminal coordination modes observed in a second minor phase of the cyclohexanoate species. The variation on extended structures based on the weakly interacting aliphatic backbones may provide a useful basis for further structural studies.

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118-119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N2O2-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed.

Relative stability of the cis and trans isomers of octahedral cobalt(iii) complexes of the type [Co(N2O2)X2] with tetradentate salen type Schiff bases: a combined theoretical and experimental study

Energies of cis and trans isomers of a series of octahedral cobalt(iii) complexes with salen type Schiff bases have been estimated by DFT calculations.