Bis-pyrazolyl-s-triazine Ni(II) pincer complexes as selective gram positive antibacterial agents; synthesis, structural and antimicrobial studies

Deciphering the Mechanism of MRSA Targeting Copper(II) Complexes of NN2 Pincer-Type Ligands

WHO lists AMR as one of the top ten global public health issues. Therefore, constant effort is needed to develop more efficient antimicrobial drugs. As a result, earth-abundant transition-metal complexes have emerged as an excellent solution. In this regard, new aminoquinoline-based copper(II) pincer complexes 1-3 were designed, synthesized, and characterized by modern spectroscopic techniques. It is worth mentioning that, at the highest concentration (1024 μg/mL) of complexes (1-3), the hemolysis was found to be <15%, implying their less toxicity. Further, the complexes effectively interfered with the growth of Gram positive MRSA and the fungus Candida albicans. Among them, complex 2 was promising (MIC = 16 μg/mL) against MRSA, which was better than the known antibacterial drug kanamycin (64 μg/mL) under identical conditions. The Alamar blue cell viability test and the MBC/MFC identified by spot assay were in accordance with MIC values. Moreover, the insilico studies explained the most probable mechanism of action as inhibition of cell wall biosynthesis and dysfunction of antibiotic sensing proteins. Similarly, the antifungal action might be due to the cell surface adhesion protein dysfunction by the complexes. Furthermore, we are expecting to draw these compounds for clinical applications.

Pincer Complexes Derived from Tridentate Schiff Bases for Their Use as Antimicrobial Metallopharmaceuticals

Within the current challenges in medicinal chemistry, the development of new and better therapeutic agents effective against infectious diseases produced by bacteria, fungi, viruses, and parasites stands out. With chemotherapy as one of the main strategies against these diseases focusing on the administration of organic and inorganic drugs, the latter is generally based on the synergistic effect produced by the formation of metal complexes with biologically active organic compounds. In this sense, Schiff bases (SBs) represent and ideal ligand scaffold since they have demonstrated a broad spectrum of antitumor, antiviral, antimicrobial, and anti-inflammatory activities, among others. In addition, SBs are synthesized in an easy manner from one-step condensation reactions, being thus suitable for facile structural modifications, having the imine group as a coordination point found in most of their metal complexes, and promoting chelation when other donor atoms are three, four, or five bonds apart. However, despite the wide variety of metal complexes found in the literature using this type of ligands, only a handful of them include on their structures tridentate SBs ligands and their biological evaluation has been explored. Hence, this review summarizes the most important antimicrobial activity results reported this far for pincer-type complexes (main group and d-block) derived from SBs tridentate ligands.

Synthesis, Structure and Biological Evaluations of Zn(II) Pincer Complexes Based on S-Triazine Type Chelator

2,4-bis (3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy-1,3,5-triazine (BPMT) pincer ligand was used to synthesize the new [Zn(BPMT)(NCS)2] (1) and [Zn(BPMT)(Br)2] (2) complexes by a reaction with Zn(NO3)2.6H2O in the presence of either KSCN or KBr, respectively. The structure of complex 1 has been exclusively confirmed using single crystal X-ray diffraction. In this neutral heteroleptic complex, the BPMT is a pincer chelate coordinating the Zn(II) ion via three interactions with the two pyrazole moieties and the s-triazine core. Hence, BPMT is a tridentate NNN-chelate. The coordination environment of Zn(II) is completed by two strong interactions with two terminal SCN− ions via the N-atom. Hence, the Zn(II) is penta-coordinated with a distorted square pyramidal coordination geometry. Hirshfeld analysis indicated the predominance of H…H, H…C and N…H intermolecular interactions. Additionally, the S…H, S…C and S…N contacts are the most significant. The free ligand has no or weak antimicrobial, antioxidant and anticancer activities while the studied Zn(II) complexes showed interesting biological activity. Complex 1 has excellent antibacterial activity against B. subtilis (2.4 μg/mL) and P. vulgaris (4.8 μg/mL) compared to Gentamycin (4.8 μg/mL). Additionally, complex 1 (78.09 ± 4.23 µg/mL) has better antioxidant activity than 2 (365.60 ± 20.89 µg/mL). In addition, complex 1 (43.86 ± 3.12 µg/mL) and 2 (30.23 ± 1.26 µg/mL) have 8 and 12 times the anticancer activity of the free BPMT ligand (372.79 ± 13.64 µg/mL).

Co (II) Complexes Based on the Bis-Pyrazol-S-Triazine Pincer Ligand: Synthesis, X-ray Structure Studies, and Cytotoxic Evaluation

Four pincer-type Co (II) complexes of the 2,4-bis (3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy-1,3,5-triazine ligand (L) were evaluated for their cytotoxic activities against lung and breast cancer cell lines using cell viability assay. The X-ray single crystal structure of [Co(L)(H2O)2Br]Br (1) confirmed the pincer coordination behavior of the ligand L as an N-tridentate chelate. The hexa-coordination environment of Co (II) is completed by one bromide ion completing the equatorial plane of the octahedral structure and two trans water molecules at the axial positions. It crystallized in the monoclinic crystal system and P21/m space group with crystal parameters of a = 11.3170(10) Å, b = 7.4613(7) Å, c = 12.6917(12) Å and β = 95.927(3)°. Based on Hirshfeld analysis, the most dominant contacts are H…H (48.8%), Br…H (17.6%), H…C (11.2%) and O…H (10.1%), where the Br…H interactions are the most significant. The cytotoxic evaluation of the studied systems indicated that complex [Co(L)(NO3)2] (4) has the highest activity against lung (A-549) and breast (MCF-7) cell lines. In contrast, complex [Co(L)(H2O)3](ClO4)2.H2O(3) has the lowest cytotoxic activity against both cell lines.

Syntheses and Structural Investigations of Penta-Coordinated Co(II) Complexes with Bis-Pyrazolo-S-Triazine Pincer Ligands, and Evaluation of Their Antimicrobial and Antioxidant Activities

Two penta-coordinated [Co(^MorphBPT)Cl₂]; 1 and [Co(^PipBPT)Cl₂]; 2 complexes with the bis-pyrazolyl-s-triazine pincer ligands ^MorphBPT and ^PipBPT were synthesized and characterized. Both ^MorphBPT and ^PipBPT act as NNN-tridentate pincer chelates coordinating the Co(II) center with one short Co-N(s-triazine) and two longer Co-N(pyrazole) bonds. The coordination number of Co(II) is five in both complexes, and the geometry around Co(II) ion is a distorted square pyramidal in 1, while 2 shows more distortion. In both complexes, the packing is dominated by Cl…H, C-H…π, and Cl…C (anion-π stacking) interactions in addition to O…H interactions, which are found only in 1. The UV-Vis spectral band at 564 nm was assigned to metal–ligand charge transfer transitions based on TD-DFT calculations. Complexes 1 and 2 showed higher antimicrobial activity compared to the respective free ligand ^MorphBPT and ^PipBPT, which were not active. MIC values indicated that 2 had better activity against S. aureus, B. subtilis, and P. vulgaris than 1. DPPH free radical scavenging assay revealed that all the studied compounds showed weak to moderate antioxidant activity where the nature of the substituent at the s-triazine core has a significant impact on the antioxidant activity.

Synthesis, Structure and In Vitro Anticancer Activity of Pd(II) Complex of Pyrazolyl-s-Triazine Ligand; A New Example of Metal-Mediated Hydrolysis of s-Triazine Pincer Ligand

The square planar complex [Pd(PT)Cl(H2O)]*H2O (HPT: 6-(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazine-2,4(1H,3H)-dione) was obtained by the reaction of 2-methoxy-4,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazine (MBPT) pincer ligand with PdCl2 in a molar ratio (1:1) under thermal conditions and using acetone as a solvent. The reaction proceeded via C-N cleavage of one C-N moiety that connects the pyrazole and s-triazine combined with the hydrolysis of the O-CH3 group. The reaction of the chloride salt of its higher congener (PtCl2) gave [Pt(3,5-dimethyl-1H-pyrazole)2Cl2]. The crystal structure of [Pd(PT)Cl(H2O)]*H2O complex is stabilized by inter- and intra-molecular hydrogen bonding interactions. Hirshfeld analysis revealed that the H...H (34.6%), O...H (23.6%), and Cl...H (7.8%) interactions are the major contacts in the crystal. The charges at Pd, H2O, Cl and PT are changed to 0.4995, 0.2216, −0.4294 and −0.2917 instead of +2, 0, −1 and −1, respectively, using the MPW1PW91 method. [Pd(PT)Cl(H2O)]*H2O complex has almost equal activities against MDA-MB-231 and MCF-7 cell lines with IC50 of 38.3 µg/mL.

Fe(III) Complexes Based on Mono- and Bis-pyrazolyl-s-triazine Ligands: Synthesis, Molecular Structure, Hirshfeld, and Antimicrobial Evaluations

The self-assembly of iron(III) chloride with three pyrazolyl-s-triazine ligands, namely 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)-1,3,5-triazine (^PipBPT), 4-(4,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazin-2-yl)morpholine (^MorphBPT), and 4,4’-(6-(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazine-2,4-diyl)dimorpholine (^bisMorphPT) afforded [Fe(^PipBPT)Cl₂][FeCl₄] (1), [Fe(^MorphBPT)Cl₂][FeCl₄] (2), and [H(^bisMorphPT)][FeCl₄]. ^bisMorphPT.2H₂O (3), respectively, in good yield. In complexes 1 and 2, the Fe(III) is pentacoordinated with three Fe-N interactions from the pincer ligand and two coordinated chloride anions in the inner sphere, and FeCl₄¯ in the outer sphere. Complex 3 is comprised of one protonated ligand as cationic part, one FeCl₄¯ anion, and one neutral ^bisMorphPT molecule in addition to two crystallized water molecules. Analysis of molecular packing using Hirshfeld calculations indicated that H…H and Cl…H are the most important in the molecular packing. They comprised 40.1% and 37.4%, respectively in 1 and 32.4% and 37.8%, respectively in 2. Complex 1 exhibited the most bioactivity against the tested microbes while 3 had the lowest bioactivity. The ^bisMorphPT and ^MorphBPT were inactive towards the tested microbes while ^PipBPT was active. As a whole, the Fe(III) complexes have enhanced antibacterial and antifungal activities as compared to the free ligands.