A Novel Centrosymmetric Fe(III) Complex with Anionic Bis-pyrazolyl-s-triazine Ligand; Synthesis, Structural Investigations and Antimicrobial Evaluations

Reaction of 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy-1,3,5-triazine (MBPT) pincer ligand with FeCl3 in acidic medium (1:1 v/v) afforded the [Fe(BPT)Cl2(CH3OH)] complex of the hydrolyzed monobasic ligand: 4,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazin-2(1H)-one (HBPT). In this complex, the Fe(III) ion is hexacoordinated with one anionic pincer ligand unit (BPT−1), two chloride ions, and one coordinated methanol molecule. It crystallized in the monoclinic crystal system and centrosymmetric P21/c space group with Z = 2 and unit cell parameters of a = 7.309(2) Å, b = 25.461(8) Å, c = 9.918(3) Å and β = 102.646(7)◦. The structure of this complex is stabilized by C–H . . . Cl intramolecular hydrogen bonding interactions with donor acceptor distances ranging from 3.577(3)–3.609(3) Å while its supramolecular structure is controlled by intermolecular O–H . . . O, O–H . . . N, and C–H . . . Cl hydrogen bonding interactions. Hirshfeld analysis of molecular packing indicates that the percentages of the Cl . . . H, C . . . O, O . . . H, C . . . C, H . . . C, and N . . . H contacts are 21.1, 1.7, 10.2, 2.1, 8.6, and 10.4%, respectively. The nature and relative strength of the different coordination interactions in the [Fe(BPT)Cl2(CH3OH)] complex are discussed based on atoms in molecules theory. Antimicrobial evaluations indicated that the [Fe(BPT)Cl2(CH3OH)] complex showed moderate antibacterial and antifungal activities compared to amoxicillin and ampicillin antibiotics as standard drugs.


Introduction
Iron is considered as a readily available and cheap element with little hazardous effects on the environment. Iron-containing compounds play a crucial role in ammonia production by the Haber-Bosch process and in homogenous molecular catalysis [1][2][3][4]. Iron has a great importance in a variety of biological systems of mammals and other simple microorganisms. Generally, it is considered to have low toxicity, but high iron concentrations in living cells can cause some harmful effects, as can iron deficiency [5][6][7]. High iron levels in the body can catalyze the generation of harmful free radicals [8,9]. In this regard, powerful chelators can be used to control this risk by controlling iron reactivity. Also, iron (III) complexes with N-donor ligands have been reported to have promising anticancer activity [10][11][12][13].
2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methoxy-1,3,5-triazine (MBPT, Figure 1) pincer ligand is a powerful nitrogen tridentate chelator that can form metal complexes by the reaction with a diversity of metal salts [14][15][16]. The self-assembly of this functional ligand with metal salts such as Ni(II), Mn(II), Co(II), Zn(II) and Cd(II) comprising different anions (Cl − , NO 3 − , ClO 4 − ) has afforded a variety of mononuclear homo and heteroleptic metal (II) complexes with coordination numbers varying from five to eight [14][15][16]. Using the same technique, the same ligand underwent metal ion-mediated hydrolysis in the presence of CuCl 2 or Cu(ClO 4 ) 2 , leading to the formation of one-dimensional Cu(II) coordination polymers [17]. Similar hydrolytic reactions were detected in the presence of other metal (II) salts such as ZnCl 2 , PdCl 2 , and PtCl 2 [15,16,18]. It was confirmed that the role of the metal ion is to increase the acidity of the water molecule, which eases the hydrolytic reaction, as was confirmed by the acid-mediated hydrolysis of MBPT in the presence of aqueous hydrochloric acid solution to afford HBPT (Figure 1) [17]. In continuation of this work, we tested the reaction of this pincer ligand with FeCl 3 in acidic medium. The structure of the new [Fe(BPT)Cl 2 (CH 3 OH)] complex was confirmed using elemental analysis, FTIR spectra, and single crystal X-ray diffraction. Evaluation of the antimicrobial activity of the studied complex was also presented.
pincer ligand is a powerful nitrogen tridentate chelator that can form metal complexes by the reaction with a diversity of metal salts [14][15][16]. The self-assembly of this functiona ligand with metal salts such as Ni(II), Mn(II), Co(II), Zn(II) and Cd(II) comprising dif ferent anions (Cl¯, NO3¯, ClO4¯) has afforded a variety of mononuclear homo and hetero leptic metal (II) complexes with coordination numbers varying from five to eight [14][15][16] Using the same technique, the same ligand underwent metal ion-mediated hydrolysis in the presence of CuCl2 or Cu(ClO4)2, leading to the formation of one-dimensional Cu(II coordination polymers [17]. Similar hydrolytic reactions were detected in the presence o other metal (II) salts such as ZnCl2, PdCl2, and PtCl2 [15,16,18]. It was confirmed that th role of the metal ion is to increase the acidity of the water molecule, which eases the hy drolytic reaction, as was confirmed by the acid-mediated hydrolysis of MBPT in th presence of aqueous hydrochloric acid solution to afford HBPT ( Figure 1) [17]. In con tinuation of this work, we tested the reaction of this pincer ligand with FeCl3 in acidi medium. The structure of the new [Fe(BPT)Cl2(CH3OH)] complex was confirmed using elemental analysis, FTIR spectra, and single crystal X-ray diffraction. Evaluation of th antimicrobial activity of the studied complex was also presented.

Materials and Physical Measurements
Chemicals were purchased from the Sigma-Aldrich Company (Chemie GmbH 82024 Taufkirchen, Germany). CHN analyses were performed using a Perkin Elmer 240 Elemental Analyzer (PerkinElmer, Inc., 940 Winter Street, Waltham, MA, USA). Iron content was determined using the Shimadzu atomic absorption spectrophotomete (AA-7000 series, Shimadzu, Ltd., Tokyo, Japan). An Alpha Bruker spectrophotomete (Billerica, MA, USA) was used to measure the FTIR spectra in KBr pellets (Figure S1 Supplementary Data).

Synthesis of S-Triazine-Based Ligand
The ligand MBPT was prepared as previously reported [14] (Supplementary Data Method S1, Figures S2 and S3

Materials and Physical Measurements
Chemicals were purchased from the Sigma-Aldrich Company (Chemie GmbH, 82024 Taufkirchen, Germany). CHN analyses were performed using a Perkin Elmer 2400 Elemental Analyzer (PerkinElmer, Inc., 940 Winter Street, Waltham, MA, USA). Iron content was determined using the Shimadzu atomic absorption spectrophotometer (AA-7000 series, Shimadzu, Ltd., Tokyo, Japan). An Alpha Bruker spectrophotometer (Billerica, MA, USA) was used to measure the FTIR spectra in KBr pellets ( Figure S1, Supplementary Data).

Synthesis of [Fe(BPT)(CH 3 OH)Cl 2 ] Complex
The studied complex was synthesized using a self-assembly technique by mixing the acidified aqueous (2 drops of 1:1 v/v HCl) solution of FeCl 3 (1 mmol, 162 mg) with an equimolar amount of MBPT ligand in ethanol. The mixture was left for slow evaporation and the resulting brown crystals were collected by filtration.

Antimicrobial Studies
The antimicrobial activities of the [Fe(BPT)(CH 3 OH)Cl 2 ] complex against different microbes were determined following the reported method [27]. More details regarding the antimicrobial assay are found in Supplementary Data (Method S2).

Atoms in Molecules (AIM) and Natural Charge Calculations
Gaussian 09 [28] software was used for all single point calculations using the MPW1PW91/ TZVP [29] method. Charge population and atoms in molecules (AIM) analyses were performed using NBO 3.1 [30] and Multiwfn [31] programs, respectively. Based on literature, s-triazine pincer ligands are well known to have a weak ligand field and form only high spin complexes due to the electron-deficient characters of the s-triazine moiety. Hence, the calculations were performed based on the fact that the presented complex was a high spin iron (III) complex with a total spin of 5/2 and sextet multiplicity [14].

Antimicrobial Studies
The antimicrobial activities of the [Fe(BPT)(CH3OH)Cl2] complex against differen microbes were determined following the reported method [27]. More details regarding the antimicrobial assay are found in Supplementary Data (Method S2).

Atoms in Molecules (AIM) and Natural Charge Calculations
Gaussian 09 [28] software was used for all single point calculations using the MPW1PW91/TZVP [29] method. Charge population and atoms in molecules (AIM analyses were performed using NBO 3.1 [30] and Multiwfn [31] programs, respectively Based on literature, s-triazine pincer ligands are well known to have a weak ligand field and form only high spin complexes due to the electron-deficient characters of the s-triazine moiety. Hence, the calculations were performed based on the fact that the pre sented complex was a high spin iron (III) complex with a total spin of 5/2 and sexte multiplicity [14].

Structure Description
The molecular structure of the heteroleptic [Fe(BPT)(CH 3 OH)Cl 2 ] complex is shown in Figure 2. This Fe(III) complex crystallized in the monoclinic crystal system and P2 1 /c space  Table 1.

Structure Description
The molecular structure of the heteroleptic [Fe(BPT)(CH3OH)Cl2] com in Figure 2. This Fe(III) complex crystallized in the monoclinic crystal sys space group with Z = 2 and unit cell parameters of a = 7.309(2) Å, b = 2 9.918(3) Å, and β = 102.646(7)°, where the asymmetric unit contained formula. The most relevant geometric parameters are listed in Table 1.    In this neutral complex, the Fe(III) was hexacoordinated with a distorted octahedral coordination environment. The Fe(III) was coordinated with one mononegative tridentate BPT −1 and one coordinated chloride ion in the basal plane of the distorted octahedron, while the axial positions were occupied by one chloride anion and a coordinated methanol molecule. Similar to other M(II)-MBPT pincer complexes [14][15][16], there were three Fe-N interactions with the organic pincer ligand where the Fe-N (s-triazine) was generally shorter (Fe1-N5: 2.079(2) Å) than any of the Fe-N (pyrazole) bonds (Fe1-N1: 2.154(2) Å and Fe1-N3: 2.156(2) Å). The equatorial Fe1-Cl1 (2.255(8) Å) was slightly shorter than the axial Fe1-Cl2 (2.302(9) Å), while the Fe1-O2 bond with the coordinated methanol was 2.112(2) Å. As calculated by the continuous shape measure (CShM) tool, the values of the CShM were 2.45 and 13.72 against the perfect octahedron and trigonal prism, respectively indicating a slightly distorted octahedral rather than a trigonal prism [34][35][36][37]. The triazine ring itself was not perfectly planar as indicated from the torsion angles where the maximum deviation from planarity was 7.4(3) • for C11-N6-C13-N7 atoms. The mean plane passing through the triazine ring created angles of 2.96 and 7.24 • with the pyrazolyl moieties N4C8C7C6N3 and C3C2C1N1N2, respectively. The reason for these twists could be attributed to the small size of Fe(III) and its high charge density, along with the steric hinder between the coordinated pyrazolyl moieties [15].
The structure of this complex was stabilized by C4-H4C . . . Cl1 and C9-H9B . . . Cl1 intramolecular hydrogen bonds with donor acceptor distances of 3.609 (3) Table 2. In addition, presentation of the packed molecular units via these noncovalent interactions is shown in Figure 3.

Quantitative Analysis of Molecular Packing
Hirshfeld analysis is a simple tool to investigate the molecular packing in the crystal structure at both the qualitative and quantitative levels. In d norm surfaces shown in Figure 4, there were many red regions representing contacts with shorter distances than the van der Waals radii (vdWs) sum of the interacting atoms. On the other hand, the blue and white regions were related to intermolecular contacts having longer and equal interaction distances than the vdWs radii sum of the interacting atoms, respectively. For simplicity, the most significant contacts are labeled A to F, which correspond to Cl . .

Quantitative Analysis of Molecular Packing
Hirshfeld analysis is a simple tool to investigate the molecular packing in the cry structure at both the qualitative and quantitative levels. In dnorm surfaces shown in Fig  4, there were many red regions representing contacts with shorter distances than the der Waals radii (vdWs) sum of the interacting atoms. On the other hand, the blue a structure at both the qualitative and quantitative levels. In dnorm surfaces shown in Figure  4, there were many red regions representing contacts with shorter distances than the van der Waals radii (vdWs) sum of the interacting atoms. On the other hand, the blue and white regions were related to intermolecular contacts having longer and equal interaction distances than the vdWs radii sum of the interacting atoms, respectively. For simplicity, the most significant contacts are labeled A to F, which correspond to Cl…H, C…O, O…H, C…C, H…C, and N…H interactions, respectively. The hydrogenic H…H intermolecular interactions were the most abundant among the other contacts. They contributed 40.9% to the whole fingerprint area ( Figure S4; Supplementary Data). There were no red regions related to the H…H interactions, indicating longer or equal distances compared to twice the vdW radii of hydrogen atoms. On the other hand, the percentage contributions of the Cl…H, C…O, O…H, C…C, H…C, and N…H contacts were quite less. Their percentages were 21.1, 1.7, 10.2, 2.1, 8.6, and 10.4%, respectively. All these contacts appeared as red regions in the dnorm map ( Figure 4).
The shortest contact distances were 2.656Å (Cl2…H2), 3.056 Å (C11…O1), 1.714 Å (O1…H2A), 2.685 Å (H4A…C13), 3.331 Å (C13…C3), and 2.615 Å (N7…H5A). A summary of these short contacts and their interaction distances is found in Table 3. In the fingerprint plots shown in Figure 5, the de and di are distances from a point on the surface to the nearest nucleus outside and inside the surface, respectively. Hence, the pattern of the fingerprint plot gives strong indication of the strength of interaction. The majority of A summary of these short contacts and their interaction distances is found in Table 3. In the fingerprint plots shown in Figure 5, the d e and d i are distances from a point on the surface to the nearest nucleus outside and inside the surface, respectively. Hence, the pattern of the fingerprint plot gives strong indication of the strength of interaction. The majority of these interactions appeared as sharp spikes in their fingerprint plots, indicating that these contacts were the most important and had short interaction distances ( Figure 5). Also, the total area of the fingerprint plot represents the percentage of the intermolecular interactions that occurred in the crystal. As a result, the decomposition of the fingerprint plot shown in Figure 5 gives the percentages of each intermolecular interaction ( Figure S4; Supplementary Data).

NBO and AIM Studies
The natural charges of the ligand groups and the iron atom were calculated in order to determine the number of charge transferences from the ligand groups as Lewis bases to the Fe(III) as Lewis acid. The charge at the Fe atom decreased to 0.9509 e instead of +3 for the isolated Fe(III) ion. Hence, the amount of negative electron density transferred to Fe(III) from the ligand groups was 2.0492 e. The equatorial chloride (0.5571 e) transferred higher negative electron density to the Fe(III) than the axial one (0.5913 e). The amount of electron density transferred from the mononegative tridentate organic N-chelate to the Fe(III) was 0.7424 e, while the corresponding value for the coordinated methanol was 0.1583 e.
On the other hand, the atom-atom interactions could be classified into two main categories, which were the closed shell and sharing interactions [38][39][40][41]. Based on the topology analysis of the atoms in molecules (AIM) theory presented in Table 4, the Fe-N (s-triazine) bond (Fe1-N5) had higher electron density (ρ(r)) than any of the Fe-N (pyrazole) bonds (Fe1-N1 and Fe1-N3). Hence, the Fe-N (s-triazine) was stronger and with higher covalent characters than the Fe-N (pyrazole) bonds, which was further confirmed by the negative H(r) values and V(r)/G(r) > 1. In contrast, the Fe-O bond with the coordinated Symmetry 2021, 13, 1247 8 of 11 methanol had low ρ(r), positive H(r) value, and V(r)/G(r)~1, indicating little covalent character of this bond [38][39][40][41].
these interactions appeared as sharp spikes in their fingerprint plots, indicating that these contacts were the most important and had short interaction distances ( Figure 5). Also, the total area of the fingerprint plot represents the percentage of the intermolecular interactions that occurred in the crystal. As a result, the decomposition of the fingerprint plot shown in Figure 5 gives the percentages of each intermolecular interaction ( Figure S4 Supplementary Data).

NBO and AIM Studies
The natural charges of the ligand groups and the iron atom were calculated in order to determine the number of charge transferences from the ligand groups as Lewis bases to the Fe(III) as Lewis acid. The charge at the Fe atom decreased to 0.9509 e instead of +3 for the isolated Fe(III) ion. Hence, the amount of negative electron density transferred to Fe(III) from the ligand groups was 2.0492 e. The equatorial chloride (0.5571 e) transferred higher negative electron density to the Fe(III) than the axial one (0.5913 e). The amount of electron density transferred from the mononegative tridentate organic N-chelate to the Fe(III) was 0.7424 e, while the corresponding value for the coordinated methanol was 0.1583 e.
On the other hand, the atom-atom interactions could be classified into two main categories, which were the closed shell and sharing interactions [38][39][40][41]. Based on the

Antimicrobial Studies
The biological activity of the [Fe(BPT)(CH 3 OH)Cl 2 ] complex was evaluated against selected Gram-positive (Staphylococcus aureus, Bacillus cereus, and Bacillus subtilis) and Gram-negative (Escherichia coli and Pseudomonas aeruginosa) bacteria as well as the fungus Candida albicans. The inhibition zone diameters were found to be in the range 0.9-1.0 mm at 10 µg/mL of the [Fe(BPT)(CH 3 OH)Cl 2 ] complex. Similar Fe(III) complexes with monoand bis-pyrazolyl s-triazine ligands showed larger inhibition zone diameters (12-25 mm) against the same microbes [42]. It is worth concluding that the antimicrobial activities depend not only on the ligand and nature of the coordination environment around the metal ion but also on the type of substituent attached to the triazine moiety.
In addition, the Minimum Inhibitory Concentrations (MICs) were determined against these microbes ( Table 5). The results indicated broad spectrum antimicrobial action against

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.