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.


Introduction
Iron is a readily available element, as it is considered to be one of the most abundant. It is cheap and has almost-negligible hazardous effects on the environment as it has low toxicity [1][2][3]. Iron compounds play a crucial role in ammonia production by the Haber-Bosch process. On other hand, iron and its compounds have a key role in homogenous molecular catalysis [4][5][6][7].
Bis-pyrazolyl-s-triazine (BPT) ligands are a class of chelators which have been utilized in the synthesis of several divalent metal ion complexes with interesting molecular and supramolecular structures [8][9][10][11][12][13]. These s-triazine pincer-type complexes can be easily synthesized using self-assembly in a water-alcohol mixture. Additionally, they have extra-stability due to the chelate effect. Although iron has low toxicity, there are many problems due to high iron overload because it plays a major role in the generation of free radicals [14,15]. BPT ligands have key characteristics to act as a solution for this problem because they are powerful chelators.
On other hand, several organic-based antibacterial and antifungal drugs were discovered over the last few years [16]. Many of these antibiotics cannot overcome the problem of multidrug-resistant On other hand, several organic-based antibacterial and antifungal drugs were discovered over the last few years [16]. Many of these antibiotics cannot overcome the problem of multidrug-resistant microbes [17][18][19]. Therefore, the replacement of these traditional antibiotics by other medications that can solve the problem of antibiotic-resistant pathogens has become an urgent need [17][18][19]. In this regard, some Fe(III) complexes have good antibacterial activity against a broad range of bacteria, but not fungi [20]. Others were found to have good antibacterial and moderate antifungal activities [20].

Structure Description
The crystals of the synthesized complexes were simply obtained from the direct reaction of the Fe(III) salt with the functional ligand in water-ethanol solvent mixture at room temperature using self-assembly. The X-ray single-crystal structure of the Fe(III) complexes are presented for the first time and the crystal data are listed in Table 1 4 ] unit. The structure of the inner sphere complex in 1 consists of one cationic complex unit in which the Fe(III) ion is coordinated by Pip BPT in a tridentate pincer fashion and two chloride ions. The outer sphere is an anion: a tetrahedral FeCl 4¯u nit ( Figure 2). The Fe-N distances are significantly shorter for the Fe-N(s-triazine) than the Fe-N(pyrazole), where the two Fe-N(pyrazole) bonds are only slightly different ( Table 2). The two Fe1-Cl1 and Fe1-Cl2 bonds have very close bond distances of 2.1699(6) Å and 2.1766(6) Å, respectively. The coordination geometry of the five-coordinated Fe(III) ion is described using Addison criteria [23]. The coordination geometry, as shown in Figure 2, lies between the square pyramid and the trigonal bipyramid with a N3-Fe1-N2 angle (β) of 146.46(6) • and N1-Fe1-Cl2 angle (α) of 133.34(5) • , giving a τ = ((β − α)/60) value of 0.22. As a result, the coordination geometry around Fe(III) could be described as a distorted square pyramid.   Table 2). The two Fe1-Cl1 and Fe1-Cl2 bonds have very close bond distances of 2.1699(6) Å and 2.1766(6) Å, respectively. The coordination geometry of the five-coordinated Fe(III) ion is described using Addison criteria [23]. The coordination geometry, as shown in Figure 2, lies between the square pyramid and the trigonal bipyramid with a N3-Fe1-N2 angle (β) of 146.46(6)° and N1-Fe1-Cl2 angle (α) of 133.34(5)°, giving a τ = ((β-α)/60) value of 0.22. As a result, the coordination geometry around Fe(III) could be described as a distorted square pyramid.   The molecules of 1 are packed mainly by Cl . . . H hydrogen bonds as shown in Figure 3 (upper part) and listed in Table 3. The donor-acceptor distances are 3.428(2) Å, 3.622(2) Å, and 3.723(2) Å for C8-H8 . . . Cl1, C3-H3 . . . Cl3, and C5-H5B . . . Cl6 hydrogen bonding interactions, respectively. The packing of complex molecules is shown in Figure 4 (upper part). The network connected via Cl . . . H bridge interactions shows a 3D connectivity.  (3) The molecules of 1 are packed mainly by Cl…H hydrogen bonds as shown in Figure 3 (upper part) and listed in Table 3. The donor-acceptor distances are 3.428(2) Å, 3.622(2) Å, and 3.723(2) Å for C8-H8...Cl1, C3-H3...Cl3, and C5-H5B...Cl6 hydrogen bonding interactions, respectively. The packing of complex molecules is shown in Figure 4 (upper part). The network connected via Cl…H bridge interactions shows a 3D connectivity.      Complex 2 has a very close structure to 1, with one [Fe( Morph BPT)Cl 2 ] + as an inner sphere complex and [FeCl 4 ]¯as a counter ion. The major difference is that complex 2 crystallizes in the more symmetric orthorhombic crystal system and space group Pbcm with half molecular formula as asymmetric unit.
The molecule comprises a symmetrical plane passing vertically through the molecule, intersecting the Fe(III) center and the two chloride anions, and splitting the organic ligand into two halves. In this regard, there are two equidistant Fe-N(pyrazole) bonds with iron to nitrogen distance of 2.099(2) Å and one shorter Fe-N(s-triazine) bond (2.036(3) Å). List of the most important bond distances are given in Table 4. The coordination sphere is completed by the two coordinated chloride anions with iron to chlorine distances ranging from 2.090(6)-2.262(5) Å for the two disordered parts ( Figure S4, Supplementary Materials). The Addison criteria τ for the two complex parts are 0.26 and 0.05 for the disordered parts A and B, respectively. These calculations indicate that the two complex parts have different coordination geometries: part B is closer to being a more perfect square pyramid than part A. Regarding the scale factors for the two domains A and B, both are close 0.5. Thus, if the Cl1A and Cl2B atoms, as well as Cl1B and Cl2A, are assumed to belong to the same polyhedron, the τ values are calculated to be same (τ = 0.26 and 0.05), which confirms our conclusion.
The different hydrogen bridge contacts controlling the molecular packing of complex 2 are listed in Table 3

Crystal Structure Description of [H( bisMorph PT)][FeCl 4 ] bisMorph PT.2H 2 O (3)
Attempts to synthesize a coordination complex compound of the bisMorph PT ligand with FeCl 3 have failed so far. The only crystalline compound that was found was a [H( bisMorph PT)][FeCl 4 ] salt with one co-crystallized bisMorph PT ligand and two crystal water molecules in the asymmetric unit (Z = 2) of the triclinic unit cell with the symmetry P-1.
Compound 3 comprises four parts: the protonated organic ligand [H( bisMorph PT)] + as a cationic part, one electrically neutral bisMorph PT molecule, a negatively charged [FeCl 4 ]¯ion, and two crystal water molecules in a void of the packing ( Figure 5). The crystal quality of this compound was not very good. We found some disorder in the organic part of the crystal structure, and the protons of the crystal water molecules were not detectable. For these reasons, we only give the crystallographic data in this Molecules 2020, 25, 5750 7 of 13 publication and do not further describe its molecular and supramolecular aspects in detail. Although our attempts to synthesize a complex containing the bisMorph PT ligand and FeCl 3 were not successful, it does not necessarily mean that such a compound does not exist. In any case, it seemed to be useful to publish the data of compound 3 found in this context in order to create a reference for subsequent work. One possible reason for not obtaining a coordination complex of Fe(III) with the bidentate bisMorph PT ligand is its lower denticity compared to the tridentate Pip BPT and Morph BPT pincer chelates.
Another possible reason is the steric effect resulting from the replacement of one pyrazole moiety by the morpholine one. The latter has no coordinating ability and a more bulky character that prevents the bisMorph PT from coordinating to the Fe(III) ion.
Molecules 2020, 25, x FOR PEER REVIEW 7 of 13 data in this publication and do not further describe its molecular and supramolecular aspects in detail. Although our attempts to synthesize a complex containing the bisMorph PT ligand and FeCl3 were not successful, it does not necessarily mean that such a compound does not exist. In any case, it seemed to be useful to publish the data of compound 3 found in this context in order to create a reference for subsequent work. One possible reason for not obtaining a coordination complex of Fe(III) with the bidentate bisMorph PT ligand is its lower denticity compared to the tridentate Pip BPT and Morph BPT pincer chelates. Another possible reason is the steric effect resulting from the replacement of one pyrazole moiety by the morpholine one. The latter has no coordinating ability and a more bulky character that prevents the bisMorph PT from coordinating to the Fe(III) ion.

Analysis of Molecular Packing
Hirshfeld surfaces mapped over dnorm, shape index (SI), and curvedness for complexes 1 and 2 are shown in Figure S5 (Supplementary Materials). A summary of the most important contacts and their percentages are shown in Figure 6, while the decomposed dnorm maps of the short and most significant contacts in the studied complexes are collected in Figure 7. The decomposed fingerprint plots indicate the same common contacts in both complexes, which are H…H and Cl…H interactions, the most abundant intermolecular interactions in the studied complexes. The percentages of these contacts are 40.1% and 37.4% in complex 1, respectively while they are 32.4% and 37.8% in complex 2, respectively. The Cl…H hydrogen bonds appear as red regions in the Hirshfeld dnorm maps in both complexes and indicate shorter contact distances than the van der Waals (vdW) radii sum of H and Cl atoms. The anion (FeCl4¯)-π stacking interactions are significant in both complexes. Complexes 1 and 2 show significantly short C…Cl and N…Cl contacts, with interaction distances also found to be shorter than the van der Waals radii sum of the two elements sharing this contact (Figure 7). The contact distances of the N…Cl interactions are 3.213 Å and 3.226 Å for complexes 1 and 2, respectively, while the C…Cl contact distances are 3.257 Å and 3.381 Å for complexes 1 and 2, respectively. In complex 1, there is one short Fe1…Cl3 interaction (3.708 Å) between the complex cation and one of the Cl atoms from the complex anion (FeCl4¯).

Analysis of Molecular Packing
Hirshfeld surfaces mapped over d norm , shape index (SI), and curvedness for complexes 1 and 2 are shown in Figure S5 (Supplementary Materials). A summary of the most important contacts and their percentages are shown in Figure 6, while the decomposed d norm maps of the short and most significant contacts in the studied complexes are collected in Figure 7. The decomposed fingerprint plots indicate the same common contacts in both complexes, which are H . . . H and Cl . . . H interactions, the most abundant intermolecular interactions in the studied complexes. The percentages of these contacts are 40.1% and 37.4% in complex 1, respectively while they are 32.4% and 37.8% in complex 2, respectively. The Cl . . . H hydrogen bonds appear as red regions in the Hirshfeld d norm maps in both complexes and indicate shorter contact distances than the van der Waals (vdW) radii sum of H and Cl atoms. The anion (FeCl 4¯) -π stacking interactions are significant in both complexes. Complexes 1 and 2 show significantly short C . . . Cl and N . . . Cl contacts, with interaction distances also found to be shorter than the van der Waals radii sum of the two elements sharing this contact (Figure 7). The contact distances of the N . . . Cl interactions are 3.213 Å and 3.226 Å for complexes 1 and 2, respectively, while the C . . . Cl contact distances are 3.257 Å and 3.381 Å for complexes 1 and 2, respectively. In complex 1, there is one short Fe1 . . . Cl3 interaction (3.708 Å) between the complex cation and one of the Cl atoms from the complex anion (FeCl 4¯) .

Inhibition Zones
In the current study, the antibacterial activity of the free ligands as well as compounds 1-3 were tested against Gram-positive bacteria, namely, Staphylococcus aureus (ATCC 29213) and Staphylococcus epidermidis (ATCC 12228); and Gram-negative bacteria, namely, Escherichia coli (ATCC 25922) and Pseudomonas aeruginosa (ATCC 27853) [24][25][26][27]. The free Morph BPT and bisMorph PT ligands were inactive against the target microbes at the applicable concentration. On other hand, Pip BPT showed good activity against S. aureus, S. epidermidis, P. aeruginosa, and Candida albicans (ATCC 60193) and it was completely inactive against E. coli (Table 5). In contrast, Fe(III) compounds 1-3 showed more potent activities against the target pathogenic microbes than did the corresponding free ligands, as illustrated from the values of the inhibition zones (mm) in Table 5. The values are considered an indicator for the bioactivity of the tested compounds at a concentration of 200 µg/mL. Compounds 1-3 appeared to have a more potent bioactivity against the target Gram-positive pathogenic bacteria than against the Gram-negative ones, and showed potent activity against the tested fungus (C. albicans). Complex 1 showed the most potency as an antibacterial and antifungal agent against all the target microbes, while 3 showed the lowest bioactivity. Additionally, complex 1 (18 mm) had better antifungal activity than the standard fluconazole (14 mm). Complex 1 had better antibacterial action against S. epidermidis (23 mm) and P. aeruginosa (22 mm) than the standard drug gentamycin (22 mm and 19 mm, respectively).
The antimicrobial activities of the studied compounds were also evaluated at different concentrations of 100 µg/mL, 200 µg/mL, and 300 µg/mL per disc, as shown in Table 6. Compounds 1-3 at these concentrations showed moderate to strong activity against all tested microbes, even at the lowest concentration of 100 µg/mL, where the best results were obtained for complex 1. On the other hand, as the concentration of the tested compound increased, the inhibition zone also increased. This result reveals that the presence of the piperidine/bis-pyrazolo combination with Fe(III) in one compound is the key for the bioactivity. These data agree with the literature, where the presence of piperidine enhanced the activity compared to the analogous morpholine derivatives [27].

Minimum Inhibitory Concentration (MIC) and Minimum Bactericidal Concentration (MBC)
The minimum inhibitory concentrations (MIC) of compounds 1-3 are given in Table 7. All tested compounds were active against S. aureus, S. epidermidis, E. coli, P. aeruginosa, and C. albicans. Again, complex 1 had the lowest MIC and MBC values, indicating its higher potency against all tested microbes as compared to 2 and 3 (Table 7).

Materials and Physical Measurements
Chemicals were purchased from Sigma-Aldrich Company. CHN analyses were performed using a Perkin Elmer 2400 Elemental Analyzer.

Synthesis of s-Triazine-Based Ligands
The ligands Pip BPT, Morph BPT, and bisMorph PT were prepared following the method reported by our research group [21,22] (Supplementary Materials, Method S1 and Method S2, Figures S1-S3).

Synthesis of Fe(III) Complexes
All of the studied complexes were synthesized using a self-assembly technique, by mixing the aqueous solution of FeCl 3 (1 mmol, 162 mg) with the ethanolic solution of the functional ligand. The resulting clear solutions were left for slow evaporation until plate-like brown crystals of the target complexes were formed. The resulting crystals were collected by filtration and were found suitable for single-crystal X-ray diffraction measurements.

Antimicrobial Studies
We determined the antimicrobial activities of the free ligands, as well as those of the corresponding Fe(III) complexes, against different microbes [22]. More details regarding the antimicrobial assay are found in Supplementary Materials (Methods S3-S6).

Conclusions
Three self-assembled Fe(III) complexes were synthesized by direct reaction of iron(III) chloride and the functional ligand in a water-alcohol medium. All complexes were obtained in good yield and their structures were confirmed by single-crystal X-ray diffraction. The supramolecular structures of complexes 1 and 2 were analyzed using Hirshfeld calculations with the aid of the CIF data. The Fe(III) complexes were bioactive against the target microbes and generally more active than the functional ligands. It was found that the combination of piperidine and bispyrazolo moieties with Fe(III) in one compound (1) had the best bioactivity in comparison with the corresponding complexes (2 and 3) comprised of morpholine group(s).

Supplementary Materials:
The following are available online. Figure S1: 1 H-NMR and 13 C-NMR of Morph BPT, Figure S2: 1 H-NMR and 13 C-NMR of Pip BPT, Figure S3: 1 H-and 13 C-NMR for bisMorph PT, Figure S4: Structure showing the disordered parts in 2. Figure S5: Hirshfeld surfaces mapped over d norm , shape index, and curvedness. In addition, the detailed synthesis of the ligands and biological experiments are given in the Supplementary data file. Methods S1-S6.