Tetradentate Research Articles

THE FIRST STRUCTURAL CHARACTERIZATION OF A NI(NCS)64– BRIDGED HOMOMETALLIC NI(II) CHAIN DERIVED FROM A N3O DONOR SCHIFF BASE LIGAND

Thiocyanate bridged homometallic Ni(II) chains have potential applications in areas such as molecular magnetism and spintronics due to their tunable magnetic properties. The magnetic properties of these chains are of particular interest and can lead to intriguing magnetic behaviors, such as antiferromagnetic or ferromagnetic interactions. In this context, we have successfully synthesized a Ni(NCS)64– bridge Ni(II) chain complex [Ni2(H2L)2(m1,3-NCS)2(NCS)4]n∙2nCH3CN (1), derived from a tetradentate N3O donor Schiff base ligand (HL). It has been thoroughly characterized by the help of elemental analysis and IR spectroscopy. Single crystal X-ray crystallography has confirmed the geometry of the chain complexes. Both Ni1 and Ni2 centers exhibit hexa-coordination with slightly distorted octahedral geometries, and their coordination environments differ significantly (NiN2O2S2 for Ni1 and NiN6 for Ni2). Within the solid-state structure of the complex, a noteworthy two-dimensional network of hydrogen bonding is observed. The present complex showing a unique example in the realm of single thiocyanato-bridged Ni(II) chains.

Unusual Ni⋯Ni interaction in Ni(ii) complexes as potential inhibitors for the development of new anti-SARS-CoV-2 Omicron drugs.

Two nickel(ii) coordination complexes [Ni(L)]2(1) and [Ni(L)]n(2) of a tetradentate Schiff base ligand (H2L) derived from 2-hydroxy-1-naphthaldehyde with ethylenediamine were synthesized, designed, and characterized via spectroscopic and single crystal XRD analyses. Both nickel(ii) complexes exhibited unusual Ni⋯Ni interactions and were fully characterized via single-crystal X-ray crystallography. Nickel(ii) complexes [Ni(L)]2(1) and [Ni(L)]n(2) crystallize in monoclinic and triclinic crystal systems with P21/c and P1̄ space groups, respectively, and revealed square planar geometry around each Ni(ii) ion. The structure of both the complexes have established the existence of a new kind of metal system containing nickel(ii)-nickel(ii) interactions with a square planar-like geometry about the nickel(ii) atoms. Both square planar Ni(ii) complexes were often stacked with relatively short Ni⋯Ni distances. The non-bonded Ni-Ni distance (Ni⋯Ni separation) seems to be 3.356 Å and 3.214 Å from the nickel atoms of [Ni(L)]2(1) and [Ni(L)]n(2), respectively. These distances are shorter than the sum of their van der Waals radii (4.80 Å) but longer than the sum of their covalent radii (2.50 Å), indicating that there is a Ni⋯Ni interaction but not a Ni-Ni bond. The discrete molecules are π-stacked and connected via weak intermolecular interactions (C-H⋯O and C-H⋯N). Cyclic voltammetry measurements were obtained for both the complexes, and their pharmacokinetic and chemoinformatics properties were also explored. Detailed structural analysis and non-covalent supramolecular interactions were investigated using single-crystal structure analysis and computational approaches. Both the unique structures show good inhibition performance for the Omicron spike proteins of the SARS CoV-2 virus. To gain insights into potential SARS-CoV-2 Omicron drugs and find inhibitors against the Omicron variants of SARS-CoV-2, we examined the molecular docking of the nickel(ii) complexes [Ni(L)]2(1) and [Ni(L)]n(2) with the SARS-CoV-2 Omicron spike protein (PDB ID: 7WK2 and 7WVO). A strong binding was predicted between Ni(ii) coordination complexes [Ni(L)]2(1) and [Ni(L)]n(2) with the SARS-CoV-2 Omicron variant receptor protein through the negative value of binding affinity. Molecular docking of Nil(ii) complexes [Ni(L)]2(1) and [Ni(L)]n(2) with a DNA duplex (PDB ID: 7D3T) and RNA (PDB ID: 7TDC) binding protein was also studied. Overall, this study suggests that Ni(ii) complexes can be considered as drug candidates against the Omicron variants of SARS-CoV-2.

IMPACT OF METAL SALTS ON COORDINATION CHEMISTRY OF NICKEL(II) WITH A N3O DONOR SCHIFF BASE LIGAND: SYNTHESIS, STRUCTURES AND HERSHFIELD SURFACE ANALYSIS

The key findings and significance of this report is the synthesis and characterization of two mononuclear nickel(II) complexes, [Ni(L)(val)].0.5MeOH (1) and [Ni(HL)2](ClO4)2 (2) derived from a tetradentate Schiff base ligand HL and two different nickel(II) salts. These complexes were thoroughly characterized using techniques such as elemental analysis, IR spectroscopy, single crystal X-ray diffraction, and Hirshfeld surface analysis. The results revealed distinct coordination modes of the ligands in the complexes, leading to different non-covalent interactions and packing arrangements in their crystal structures. Specifically, complex 1 exhibited tetradentate coordination in addition to the bidentate o-vanillinate ligand, while complex 2 featured tridentate coordination with the zwitterionic form of the ligand. Hirshfeld surface analysis provided insights into the relative contributions of various intermolecular interactions, highlighting the significance of hydrogen bonding, tetrel bonding, and other non-covalent interactions in stabilizing the crystal structures. In general, this study contributes to the differences in structural integrity on slide variation in metal salts and the understanding in depth of various non covalent interactions using Hirshfeld surface analysis tool.

Pendulum-like hemilability in a Ti-based frustrated Lewis Trio.

We describe the first experimental example of a theoretically predicted Frustrated Lewis Trio (FLT). A tetradentate PNNP ligand is used to stabilise a highly electrophilic [TiCl3]+ fragment in a way that results in two equally long and frustrated Ti-P bonds. A combined experimental and computational approach revealed a distinct role of each Lewis basic phosphine in the heterolytic activation of chemical bonds. This dual functionality is characterised by a pendulum-like hemilability, where one of the phosphines acts as a nucleophile while the other serves as a hemilabile ligand that dynamically tunes the Ti-P distance as a function of the required electron density at the Ti centre.

A combined theoretical and experimental approach to determine the right choice of co-ligand to impart spin crossover in Fe(II) complexes based on 1,3,4-oxadiazole ligands.

We present the synthesis of two new novel tetradentate ligands based on 1,3,4-oxadiazole, 2-(2-pyridyl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole (LTetraPy-ODA) and 2-(2-phenyl)-5-[N,N-bis(2-pyridylmethyl)aminomethyl]-1,3,4-oxadiazole (LTetraPh-ODA). The ligands were used to prepare six mononuclear complexes [FeII(LTetraPy-ODA)(NCE)] (C1-C3) and [FeII(LTetraPh-ODA)(NCE)] (C4-C6) where E = S, Se or BH3. In addition, the ligand LTetraPy-ODA was employed in the synthesis of a new di-nuclear complex [FeII2(LTetraPh)](ClO4)4·1 CH3NO2·1.5 H2O (C7). Characterization of all complexes was carried out using single-crystal X-ray crystallography, elemental analysis, and infrared spectroscopy. Magnetic susceptibility measurements, performed in the temperature range of 2-300 K using a SQUID magnetometer, revealed spin crossover behaviour exclusively in the mononuclear complexes C3 and C6, in which two monodentate NCBH3- co-ligands coordinate. The presence of the lattice solvent was found to be crucial to the spin transition property, with complex C3 exhibiting a switching temperature (T1/2) of approximately 165 K and C6 approximately 194 K. The other four mononuclear complexes C1, C2, C4, C5, as well as the dinuclear complex C7 are locked in the high spin state over the measured temperature range. Density Functional Theory (DFT) calculations were performed on complexes C1-C6 to rationalise the observed magnetic behaviour, demonstrating the significant effect of the NCS-, NCSe- and NCBH3- co-ligands ligands on the spin-crossover behaviour of the [FeII(L)(NCE)] complexes.

Efficient hydrogenation of ketones over the diaminophosphino manganese complex.

Herein, we synthesized new manganese(I) complexes coordinated with the tetradentate ligand PNNP. The complexes show higher activity and excellent substituent tolerance in contrast to their manganese counterparts and are applicable in the hydrogenation of a wide range of aromatic, aliphatic and heterocyclic ketones to their corresponding alcohols.

Steric influence and controlled dynamic process in a chiral square planar nickel(ii) complex supported by a dipyrromethane-based NNNN tetradentate ligand

The intra-ligand congestion in a square planar nickel(ii) complex of a dipyrromethane-based ligand decides helical chirality and the pair of enantiomers exists in a dynamic equilibrium in solution. A similar complex without congestion is achiral.

Formation of a low-symmetry Pd8 molecular barrel employing a hetero donor tetradentate ligand, and its use in the binding and extraction of C70.

The majority of reported metallo-supramolecules are highly symmetric homoleptic assemblies of M x L y type, with a few reports on assemblies that are obtained using multicomponent self-assembly or using ambidentate ligands. Herein, we report the use of an unsymmetrical tetratopic ligand (Lun) containing pyridyl and imidazole donor sites in combination with a cis-protected Pd(ii) acceptor for the formation of a low-symmetry M8Lun 4 molecular barrel (UNMB). Four potential orientational isomeric (HHHH, HHHT, HHTT, and HTHT) molecular barrels can be anticipated for the M8Lun 4 type metallo-assemblies. However, the formation of an orientational isomer (HHTT) of the barrel was suggested from single-crystal X-ray diffraction and 1H NMR analysis of UNMB. Two large open apertures at terminals and the hydrophobic confined space surrounded by four aromatic panels of Lun make UNMB a potential host for bigger guests. UNMB encapsulates fullerenes C70 and C60 favoured by non-covalent interactions between the fullerenes and aromatic panels of the ligand molecules. Experimental and theoretical studies revealed that UNMB has the ability to bind C70 more strongly than its lower analogue C60. The stronger affinity of UNMB towards C70 was exploited to separate C70 from an equimolar mixture of C70 and C60. Moreover, C70 can be extracted from the C70⊂UNMB complex by toluene, and therefore, UNMB can be reused as a recyclable separating agent for C70 extraction.

Design of rigid chiral bipyridine-2NO tetradentate ligands: application in asymmetric catalysis

A new class of chiral bipyridine-2NO ligands, which incorporate the advantages of both the bipyridine skeleton and the pyrroloimidazolone-based N-oxide moiety, was developed.

Effects of mono- or di-fluoro-substitution on spin crossover behavior in a pair of Schiff base-like FeII-coordination polymers.

Spin crossover (SCO) has long been a hot topic in the field of molecular magnetism owing to its unique bistability character. Rational control of thermal hysteresis and transition temperature (T1/2) is crucial for their practical applications, which rely on precise manipulation of the substituents of SCO coordinating ligands and molecular packing interactions. In this study, we designed two different bridging ligands (2-FDPB: 4,4'-(2-fluoro-1,4-phenylene)dipyridine; 2,3-FDPB: 4,4'-(2,3-difluoro-1,4-phenylene)dipyridine) featuring one and two fluoro substitution on the central benzene ring and applied a Schiff base-like equatorial tetradentate ligand {diethyl(E,E)-2,2'-[4,5-difluoro-1,2-phenyl-bis(iminomethylidyne)]bis(3-oxobutanoate)-(2-)-N,N',O3,O3'} (H2L) to coordinate with the FeII ion. Two FeII-coordination chain polymers [FeII(L)(2,3-FDPB)]·0.25CH2Cl2 (1) and [FeII(L)(2-FDPB)]·0.5CH3OH (2) were obtained. 1 crystallizes in the monoclinic P21/n space group with only one FeII center, while 2 crystallizes in the triclinic P1̄ space group with two independent FeII centers. Unlike the identical 2D layer stacking in 1, 2 exhibited alternating stacking of the extending 2D layers and meshed chains. Magnetic measurements revealed the typical thermally induced spin crossover behavior (SCO): 1 exhibited a 41 K-wide thermal hysteresis with transition temperatures of T1/2↑ = 245 K and T1/2↓ = 204 K, while 2 showed a higher transition temperature (T1/2 = 330 K) with no thermal hysteresis. Magneto-structural correlation studies suggest that the electron-withdrawing effect present in the fluoro substituents does not have a significant impact on the SCO behaviors. Despite the fluoro substituents having a similar atomic radius of hydrogen atoms, variations in the number of these substituents can alter the crystallization behavior of these complexes, which in turn affects the solvents, molecular stacking patterns, and intermolecular interactions, ultimately influencing the SCO behaviors.

New asymmetric tetradentate phenanthroline chelators with pyrazole and amide groups for complexation and solvent extraction of Ln(III)/Am(III).

To tune the complexation and solvent extraction performance of the ligands with a 1,10-phenanthroline core for trivalent actinides (An3+) and lanthanides (Ln3+), we synthesized two new asymmetric tetradentate ligands with pyrazole and amide groups, i.e., L1 (N,N-diethyl-9-(5-ethyl-1H-pyrazol-3-yl)-1,10-phenanthroline-2-carboxamide) and its analogue L2 with longer alkyl chains (N,N-dihexyl). The complexation of the ligands with Ln3+ was confirmed by 1H NMR titration and X-ray crystallography, and stability constants were measured in methanol by spectrophotometric titration. The asymmetric ligands exhibited an improved performance in terms of selective solvent extraction of Am3+ over Eu3+ in strongly acidic solutions compared to their symmetric analogues. The improved selectivity of the asymmetric ligands was interpreted theoretically by density functional theory simulations. This study implies that combining different functional groups to construct asymmetric ligands may be an efficient way to tune ligand performance with regard to An3+ separation from Ln3+.

Design and characterization of Fe(II) complexes with tetradentate ligands exhibiting spin-crossover near room temperature.

Construction of spin-crossover (SCO) materials is very appealing for applications such as molecular switches and information storage. This study focuses on the design of Fe(II) complexes using N,N'-bis(2-pyridinylmethyl)-1,2-ethanediamine-based ligands with an N4 structure for SCO material development. By incorporating para-substituted benzene groups into the ligand's pyridine moiety, two polymorphs, α and β, were obtained, both exhibiting SCO activity. Notably, the β polymorph displayed a spin crossover temperature of 270 K, which is approaching room temperature. Structural analyses were conducted to compare the differences between the polymorphs, along with a literature review of related complexes, providing insights into the characteristics of SCO behavior.

Exploring a prototype for cooperative structural phase transition in cobalt(II) spin crossover compounds.

The creation of magnetically switchable materials that concurrently incorporate spin crossover (SCO) and a structural phase transition (SPT) presents a significant challenge in materials science. In this study, we prepared four structurally related cobalt(II)-based SCO compounds: two one-dimensional (1D) chains of {[(enbzp)Co(μ-L)](ClO4)2·sol}n (L = bpee, sol = 2MeOH·H2O, 1; L = bpea, sol = none, 2; enbzp = N,N'-(ethane-1,2-diyl)bis(1-phenyl-1-(pyridin-2-yl)methanimine); bpee = 1,2-bis(4-pyridyl)ethylene; and bpea = 1,2-bis(4-pyridyl)ethane) and their discrete segments, [{(enbzp)Co}2(μ-L)](ClO4)4·2MeOH (L = bpee, 3; L = bpea, 4). In all of these complexes, each Co(II) center is equatorially chelated by the planar tetradentate ligand enbzp and connected to a chain or dinuclear structure through bpee or bpea ligands along its axial direction. All of the complexes, including their desolvated phases, displayed overall incomplete and gradual SCO properties. Interestingly, the desolvated phase of 1 exhibited an additional non-spin magnetic transition characterized by wide room-temperature hysteresis (>40 K), which was reversible and rate-dependent, showcasing the synergy between SCO and SPT manifested through slow kinetics. We discuss the possible reasons for the distinct features and our findings demonstrate that the combination of a rigid polymeric framework with flexible substituents holds promise for achieving synergy between SCO and SPT.

Phen-2NO, a new C2-symmetric rigid-featured tetradentate ligand, and its application in the asymmetric alkylation reaction of indoles

A new class of C2-symmetric rigid-featured chiral phen ligands that provide the N,N,O,O-tetradentate coordination of the phen moiety and two additional N,N-dioxides were rationally designed and developed.

Selective synthesis of cyclic alcohols from cycloalkanes using nickel(ii) complexes of tetradentate amidate ligands.

Selective functionalisation of hydrocarbons using transition metal complexes has evoked significant research interest in industrial chemistry. However, selective oxidation of unactivated aliphatic C-H bonds is challenging because of the high bond dissociation energies. Herein, we report the synthesis, characterisation and catalytic activity of nickel(ii) complexes ([Ni(L1-L3)(OH2)2](ClO4)2 (1-3)) of monoamidate tetradentate ligands [L1: 2-(bis(pyridin-2-ylmethyl)amino)-N-phenylacetamide, L2: 2-(bis(2-pyridin-2-ylmethyl)amino)-N-(naphthalen-1-yl)acetamide, L3: N-benzyl-2-(bis(pyridin-2-ylmethyl)amino)acetamide] in selective oxidation of cycloalkanes using m-CPBA as the oxidant. In cyclohexane oxidation, catalysts showed activity (TON) in the order 1 (654) > 2 (589) > 3 (359) with a high A/(K + L) ratio up to 23.6. Using catalyst 1, the substrate scope of the reaction was broadened by including other cycloalkanes such as cyclopentane, cycloheptane, cyclooctane, adamantane and methylcyclohexane. Further, the Fenton-type reaction in the catalytic cycle was discarded based on the relatively high 3°/2° ratio of 8.6 in adamantane oxidation. Although the formation of chlorinated products during the reactions confirmed the contribution of the 3-chlorobenzoyloxy radical mechanism, the high alcohol selectivity obtained for the reactions indicated the participation of nickel-based oxidants in the oxidation process.

Design of Azomethine based Metalloorganic Complexes and their Binding Interactions with Nucleic Acid and Protein

Mononuclear nickel, copper, and zinc complexes based on salen type (ONNO) donor acting as a tetradentate ligand obtained by the condensation of 4-methyl-o-phenylenediamine and 5-fluoro salicylaldehyde in 1:2 ratios. The compounds are characterized using UV-visible, FT-IR, 1H, 13C-NMR, HR-MS, Elemental (CHN) analyses ESR spectral techniques, and single crystal X-ray diffraction studies (SC-XRD). The binding interactions of the metal complexes with deoxyribonucleic acid (DNA) and bovine serum albumin (BSA) are monitored using UV absorption titration and tryptophan emission quenching techniques, respectively. The complexes possess strong binding propensities with DNA and prefer groove binding, as indicated by the hyperchromic effect. The complexes quench the intrinsic fluorescence of BSA with increasing amounts of the complex concentration in a dose-dependent manner. The gel electrophoresis was carried out against pBR322 to analyze the cleaving ability of the complexes. The results of the NBO (natural bond orbitals) analysis performed in combination with the Time-dependent Density Functional Theory (TD-DFT) calculations provided a good explanation of their electronic and structural characteristics. Further, the predictions of molecular docking studies support the experimental observations. The Gel electrophoresis study reveals the cleaving ability of the complexes by their interactions with DNA. In addition, the cell viability of the complexes is assessed based on MTT assay and the results indicate that they have a good antiproliferative activity only on liver cancer cell-line HepG-2 and does not show cytotoxic effect on normal vero cell lines with 90% viability.

Synthesis and characterization of Co (II), Ni (II) and Cu (II) complexes with unsymmetrical tetradentate Schiff base ligand

A novel series of cobalt (II), nickel (II) and copper (II) complexes of unsymmetrical Schiff base ligand 1-(2-hydroxynaphthalideneimino)-2-(5 methoxysalicylidene imino)-4 chlorobenzene-1,2-diamine was synthesised by condensation reaction of 1:1:1 molar ratio of 2-hydroxynaphthalene-1-carbaldehyde, 4-chlorobenzene -1,2-diamine and 5-methoxysalicyidehyde in alcoholic medium. The synthesised ligand and its metal complexes were characterised by elemental analysis, molar conductance measurements, magnetic susceptibility measurements, UV-Vis and infrared spectral data. The molar conductance of synthesised metal complexes in DMSO solution indicates their non-electrolytic nature. The analytical data show the general formula for the complexes to be [M(L)2] where L = Schiff base ligand and M = Co (II), Ni (II) and Cu (II). The spectral data and magnetic moment suggested that Schiff base behaves as a tetradentate ligand, coordinating through azomethine nitrogen and phenolic oxygen atoms to the metal ion. Magnetic susceptibility measurements, UV-Vis and infrared spectral data geometry of metal complexes were proposed to be square planar.

Dynamic Configuration on a Chiral-at-Rhodium Catalyst Featuring a Flexible Tetradentate Ligand.

The unique dynamic configuration of an enantioselective chiral-at-metal catalyst based on Rh(III) and a non-chiral tetradentate ligand is described and resolved. At room temperature, the catalyst undergoes a dynamic configuration process leading to the formation of two interconvertible metal-stereoisomers, remarkably without racemization. Density functional theory (DFT) calculations indicate that this metal-isomerization proceeds via a concerted transition state, which features a trigonal bipyramidal geometry stabilized by the tetradentate ligand. Furthermore, the resolved enantiopure complex shows high catalytic enantioinduction in the Friedel-Crafts reaction, achieving enantiomeric ratios as high as 99 : 1.

Differently structural Cu(II) and Ni(II) complexes of a nonsymmetrical tetradentate half-salamo like N3O-donor ligand: Synthesis, crystal structure, spectral properties and theoretical studies

A half-salamo like N3O-donor ligand HL was synthesized and used for the construction of Cu(II) and Ni(II) complexes, namely as [Cu(L)(NO3)] (1) & [Ni2(L)2(NO3)2]·2H2O (2). The constructed complexes were structurally characterized by X‐ray crystallography technique. The results revealed that the complex 1 is the most common mononuclear species, in which Cu(II) ion is penta-coordinate with twisted square pyramid configuration. Unlike complex 1, the central Ni(II) ions of dinuclear complex 2 are hexa-coordinate possessing slightly distorted octahedron. The final supramolecular structures of the two complexes are stabilized and expanded by noncovalent interactions. In addition, spectral characteristics of the title complexes were examined. The frontier molecular orbitals as well as energy gaps of the complexes were explored by density functional theory calculation. The primary aspects of the ultraviolet absorption spectra were interpreted using time-dependent density functional theory computation. Thorough analysis of the Hirshfeld surfaces quantifies the interactions involved in the crystal structures of the Cu(II) and Ni(II) complexes.

Synthesis, spectroscopic elucidation, density functional theory calculation, and molecular docking studies of a novel series of tetradentate macrocyclic Schiff base ligands and their Zn(II) complexes and investigations of their antimicrobial, anti‐inflammatory, and anticancer activities

In the present work, novel Schiff base macrocyclic Zn(II) complexes [Zn(N8O4MacL1)Cl2–Zn(N8O4MacL3)Cl2] were synthesized by the reaction of ZnCl2 and macrocyclic ligands (N8O4MacL1–N8O4MacL3) derived from the condensation of ligand (L) and dicarboxylic acids [HOOC‐(CH2)n‐COOH]. The structural confirmation of the newly synthesized compounds was accomplished through various analytical techniques, including elemental analysis, infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS), UV–visible spectroscopy, and powder X‐ray diffraction (XRD) studies. The spectral data provided evidence that the macrocyclic ligands acted as tetradentate ligands, forming coordination bonds with Zn(II) ions through the nitrogen atom of the imine (>C=N) group. The coordination complexes exhibited an octahedral geometry around the zinc ion, with two chloro groups covalently attached. Density functional theory (DFT) studies of the synthesized Schiff base macrocyclic compounds were carried out to determine their structural and electronic properties. The antimicrobial potential of the macrocyclic compounds was examined against a panel of pathogenic microbes, including four bacterial strains (Staphylococcus aureus, Bacillus subtilis, Escherichia coli, and Xanthomonas campestris) and two fungal strains (Fusarium oxysporum and Candida albicans). Then, the newly synthesized Schiff based macrocyclic compounds were evaluated for their anti‐inflammatory activity using an egg albumin method. The synthesized compounds were subjected to further evaluation to assess their potential as anticancer agents against MCF‐7 (human breast adenocarcinoma cell line), HCT‐116 (human colon cancer cell line), and A549 (human alveolar adenocarcinoma epithelial cell line). In addition, molecular docking studies were conducted to explore the potential interactions between the synthesized compounds and the target proteins VEGFR2 (PDB ID 1YWN), EGFR (PDB ID 1M17), and DNA gyrase B (PDB ID 4URO). It was found that there was a high correlation between the experimental results and the docking calculations.