Synthesis, X-ray Structure, Hirshfeld Analysis of Biologically Active Mn(II) Pincer Complexes Based on s -Triazine Ligands

: Herein, the synthesis and antimicrobial activities of [Mn( Morph BPT )(H 2 O) 2 NO 3 ]NO 3 ; ( 1 ) and [Mn( Pip BPT )(H 2 O) 2 NO 3 ]NO 3 ; ( 2 ) complexes of the pincer-type tridentate ligands Morph BPT ; 4-(4,6-di(1 H -pyrazol-1-yl)-1,3,5-triazin-2-yl)morpholine and Pip BPT ; 2-(piperidin-1-yl)-4, 6-di(1 H -pyrazol-1-yl)-1,3,5-triazine are presented. Both complexes have slightly distorted octahedral coordination geometry. Their molecular packing depends on O–H ··· O, C–H ··· O hydrogen bonds and anion– π stacking contacts. Hirshfeld analysis was used to quantify the di ﬀ erent contacts. Both complexes exhibited better anti-fungal activity than the standard Fluconazole and comparable antibacterial activity to Gentamycin against Staphylococcus aureus and Escherichia coli microbes. Moreover, complexes 1 and 2 are biologically more active than the free ligands against these microbes.

Of transition metals, manganese is considered an essential micronutrient in living organisms and has an important role in a broad range of enzyme-catalyzed reactions. There is no doubt about the potential use of Mn(II) complexes as catalytic scavengers for H 2 O 2 against oxidative stress [7][8][9][10][11][12][13][14][15]. From this point of view, an increasing interest with the synthesis of more Mn(II) complexes in order to investigate their catalytic and antimicrobial activities. Mn(II) complexes of bipyridine and phenanthroline ligands were found to have promising antifungal activity comparable to the antifungal drug ketoconazole [15,16]. Mn(II) plays a key role in biology as required enzyme activator, which is responsible for metabolism and apoptosis [17,18]. In addition, a Mn(II) complex of the Schiff base ligand derived from 1,4-diaminobutane and pyridoxal hydrochloride showed a great anticancer activity against breast cancer [19]. More recently, a Mn(II) complex of a Schiff base derived from vitamin B6 was found as an apoptosis inducer in human MCF7 and HepG2 cancer cells [20].
In continuation to our interest with the s-triazine pincer complexes [21][22][23][24], and in light of the interesting recently reported data in literature [25][26][27][28][29][30][31], we are presenting here the synthesis of two new Mn(II) complexes with the s-triazine based NNN-pincer ligands shown in Figure 1. The structural In continuation to our interest with the s-triazine pincer complexes [21][22][23][24], and in light of the interesting recently reported data in literature [25][26][27][28][29][30][31], we are presenting here the synthesis of two new Mn(II) complexes with the s-triazine based NNN-pincer ligands shown in Figure 1. The structural features of both complexes are elucidated. In addition, their antimicrobial activities as antibacterial and antifungal agents are presented.

Materials and Methods
Chemicals, reagents, and solvents used in this work were purchased from their commercial suppliers. The CHN analyses were determined using Perkin-Elmer 2400 instrument (PerkinElmer, Inc. 940 Winter Street, Waltham, MA, USA).

Preparation of the Organic Ligands
The organic ligands were prepared using the method reported in literature [32] (Supplementary data, Method S1, Figures S1 and S2).

Materials and Methods
Chemicals, reagents, and solvents used in this work were purchased from their commercial suppliers. The CHN analyses were determined using Perkin-Elmer 2400 instrument (PerkinElmer, Inc. 940 Winter Street, Waltham, MA, USA).

Preparation of the Organic Ligands
The organic ligands were prepared using the method reported in literature [32] (Supplementary data, Method S1, Figures S1 and S2).

Antimicrobial Studies
The bio-activities of the free Morph BPT and Pip BPT ligands, as well as the corresponding Mn(II) complexes against different microbes, were determined [32]. More details regarding the bio-experiments are found in Supplementary data.

X-ray Crystal Structure Description
The structure with atomic numbering of [Mn( Morph BPT)(H 2 O) 2 NO 3 ]NO 3 complex (1) are shown in Figure 2 and list of the most important geometric parameters are given in Table 2. It crystallized in the triclinic crystal system and P-1 space group, and Z = 2. This cationic complex has a hexa-coordinated Mn(II) with one tridentate pincer ligand, one monodentate nitrate ion, and two water molecules in its inner sphere while the outer sphere comprised two halves of nitrate ions. The manganese to nitrogen distance is significantly shorter with s-triazine to perfect octahedron and trigonal prism, respectively were computed. The CShM values revealed slightly distorted octahedral coordination geometry.
order of Mn-O(equatorial water) < Mn-O(nitrate) < Mn-O(axial water). The two bite angles of the tridentate chelate are 69.49(5) and 69.09(5)° for N5-Mn1-N1 and N5-Mn1-N7, respectively while the angle between the two trans Mn-N(pyrazole) bonds is 138.55(5)° for N1-Mn1-N7. The O2-Mn1-O3 and O2-Mn1-O1 bond angles of these cis bonds are 90.33(5) and 80.66(6)°, respectively while the O3-Mn1-O1 trans bond angle is 167.72(5)°. The hexa-coordinated Mn(II) has a distorted octahedral configuration with a distorted square comprised the N1N5N7O2 atoms while the O1 and O3 atoms are located at the apexes. Using Shape 2.1 software (Barcelona, Spain), the continuous shape measure (CShM) values of 4.1 and 11.8 relative to perfect octahedron and trigonal prism, respectively were computed. The CShM values revealed slightly distorted octahedral coordination geometry.     (6) The three dimensional structure of 1 is built by O-H···O hydrogen bonds and C-H···O interactions as shown in the left part of Figure 3. The donor-acceptor distances are generally shorter (2.662(3)-2.924(2) Å) in the former than in the latter (3.271(3)-3.455(3) Å) ( Table 3). The packing of 1 is shown in the upper part of Figure 4. In addition, anion-π contacts play an important role in the packing of 1 ( Figure S3 (Supplementary data) and Table 4). The three dimensional structure of 1 is built by O-H···O hydrogen bonds and C-H···O interactions as shown in the left part of Figure 3. The donor-acceptor distances are generally shorter (2.662(3)-2.924(2) Å) in the former than in the latter (3.271(3)-3.455(3) Å) ( Table 3). The packing of 1 is shown in the upper part of Figure 4. In addition, anion-π contacts play an important role in the packing of 1 ( Figure S3 (Supplementary data) and Table 4).   (3) 154 Symmetry Code: (i) 1-x,1-y,-z; (ii) 1-x,1-y,1-z; (iii) 1+x,y,z; (iv) -x,1-y,-z         . The hexa-coordinated Mn(II) coordination configuration is slightly less distorted than that in 1 where the continuous shape measure values for 2 were computed to be 3.3 and 11.9 with respect to the perfect octahedron and trigonal prism, respectively. The most important O-H···O and C-H···O hydrogen bond contacts as well as the anion-π stacking contacts in 2 are shown in the right part of Figure 3 and Figure S3, respectively. The packing of 2 could be considered as 1D hydrogen bond polymer (Figure 4) while list of the hydrogen bonds is given in Table 3.
A slight structural difference between complexes 1 and 2 is the deviation of the Mn and coordinated equatorial oxygen atoms from the s-triazine plane. The plane passing through the perfectly planar aromatic s-triazine moiety is nearly passing through the center of Mn atom in complex 2 with only Crystals 2020, 10, 931 7 of 14 0.040(2) Å deviation from this mean plane. The corresponding distances in complex 1 is 0.142(2) Å. The equatorial oxygen atom is deviated from this plane by a distance of 0.603(3) and 0.515(3) Å in complexes 1 and 2, respectively. The reason could be simply attributed to the involvement of the s-triazine in larger number of anion-π stacking contacts in 1 compared to 2.
There is another structural difference between complexes 1 and 2. It is the orientation of the nitrate counter anion, with respect to the s-triazine moiety. In complex 1, the nitrate anion is nearly perpendicular to the s-triazine mean plane where the angle between the two planes is 86.67(3) • . It seems that such situation allowed further anion-π stacking interactions in 1 compared to 2. The corresponding angle between the two mean planes in complex 2 is only 26.64(3) • .

Analysis of Molecular Packing
Hirshfeld surfaces for 1 and 2 are given in Figure S4 (Supplementary data) while all possible contacts are shown in Figure 5. The decomposed d norm maps of the short and most significant contacts are collected in Figure 6. The H···H, O···H, N···H and C···H interactions are the most frequent in both complexes. In complex 1, these contacts contributed by 38.4, 37.5, 9.9 and 6.1%, respectively from the whole fingerprint area while the corresponding values in complex 2 are 45.2, 32.8, 8.8 and 5.2%, respectively. In addition, both complexes showed comparable amounts of anion-π stacking interactions with net C(s-triazine)···O(nitrate) and N(s-triazine)···O(nitrate) contacts of 2.8 and 2.5% for complexes 1 and 2, respectively. The latter is weaker in complex 2 and not showed the characteristics of short contacts. The shortest C···O contact is C7···O8 (2.982(2) Å) in 2 while in 1 the shortest contact is C8···O9 (2.889(3) Å). Regarding the N···O contacts in complexes 1 and 2, the shortest contact distances are N5···O9 (2.831(3) Å) and N6···O7 (3.077(2) Å), respectively. The N···O interaction in 2 is slightly longer than the vdW radii sum of nitrogen and oxygen indicating weaker interaction than 1. The O···H contacts appeared strong in both complexes where the O11···H1 (1.742 Å) and O7···H1B (1.770 Å) are the shortest in complexes 1 and 2, respectively. The values are different from those obtained from the CIF data given in Table 3 because in the Hirshfeld analysis the X-H (X = C, O) distances are normalized using the default criteria of Crystal Explorer 17.5 program. Many of the O···H contacts appeared as red regions in the d norm map indicated shorter distance than the vdW radii sum of hydrogen and oxygen. In addition, complex 2 showed H···H and C-H···π interactions as red regions in the d norm map with remarkable short distances of 1.959 Å (H5B···H5B) and 2.646 Å (C6···H5C). The latter occurred between the C-H of one methyl group from a complex unit with the s-triazine π-system from another complex unit. There are no observable π-π stacking interactions from the shape index and curvedness surfaces in both complexes.

AIM Topology Analysis
The nature and strength of Mn-N and Mn-O interactions in the studied complexes were analyzed using atoms in molecules (AIM) calculations [44][45][46][47][48][49][50][51][52][53][54]. The electron density (ρ(r)) of the Mn-O and Mn-N bondings are in the range of 0.028-0.046 and 0.035-0.045 a.u, respectively which are generally lower than 0.1 a.u indicating weak interactions with closed shell characters (Table 5). With the exception of the Mn-N(s-triazine), the positive H(r) and V(r)/G(r) < 1 for the rest of Mn-N and Mn-O interactions are the typical characteristics of the closed shell interactions. The Mn-N(s-triazine) bonds have very slightly small negative H(r) values and V(r)/G(r) very slightly higher than one indicating that the Mn-N(s-triazine) bonds have higher covalent characters than the Mn-N(pyrazole). Among the Mn-O bonds, the equatorial bond which is located trans to the Mn-N(s-triazine) has the highest ρ(r) value and the highest interaction energy. As clearly seen in Figure 7, the Mn-O distances correlate well with the ρ(r) values as well as interaction energies (E int ). Similar observation could be noted for the Mn-N distances where the correlation coefficients (R 2 ) are found to be 0.992 and 0.993, respectively. Crystals 2020, 10, x 9 of 14 Table 5. Atoms in molecules (AIM) indices (a.u.) for the Mn-O and Mn-N bonds.  Bond orbital analysis (Table 6)    Bond orbital analysis (Table 6)   Charge calculations of the free ligands allowed us to investigate the charge variations at the coordinating sites due to the chelation with the Mn(II) cation. It is obvious from the natural charges listed in Table 7 that all the coordinated donor atoms have more negative charge than those in the free ligand. The natural charge variation is higher (0.12-0.13 e) for the s-triazine N-site than the pyrazole (0.08-0.09 e) nitrogen atoms. As a conclusion, the coordination of the pincer ligand with the positively charged Mn(II) ion produced further polarization in the electron density towards the donor atom. Table 7. The calculated natural charge at the N-sites of the free and coordinated pincer ligands using MPW1PW91(WB97XD) methods.

Antimicrobial Activity
In the current study, the bio-activity of Morph BPT and Pip BPT as well as their Mn(II) complexes were tested as antibacterial and antifungal agents (Supplementary data, Method S2) [32,[55][56][57][58]. The Mn(II) complexes showed good bio-activities against the target pathogenic microbes more than original ligand as illustrated from the inhibition zones (mm), which were measured as indicator for bioactivity of the tested compounds (Table 8) at concentration 200 µg/mL. Morph BPT is completely inactive against all tested microbes while Pip BPT showed good activity against Staphylococcus aureus and Candida albicans, while completely inactive against Escherichia coli (Table 8). Both Mn(II) complexes have better antibacterial and antifungal activity than the free ligands against S. aureus, E. coli, and C. albicans. Moreover, complexes 1 and 2 have better antifungal activity than the standard Fluconazole. In addition, the studied complexes have comparable antibacterial activity to Gentamycin against S. aureus and E. coli. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values of all the tested complexes against S. aureus, E. coli, and C. albicans are reported (Table 9). It is clear that the studied Mn(II) complexes showed good bio-activity against S. aureus and E. coli as well as the fungus C. albicans. The MIC values are less for [Mn( Pip BPT)(H 2 O) 2 NO 3 ]NO 3 than [Mn( Morph BPT)(H 2 O) 2 NO 3 ]NO 3 against S. aureus, while both compounds showed similar antifungal actions against C. albicans and almost similar bio-activity against E. coli. Similarly, the MBC revealed these results very well. (2) were synthesized using self-assembly of the pincer Morph BPT and Pip BPT ligands with Mn(NO 3 ) 2 ·4H 2 O in water-alcohol mixture. The molecular and supramolecular structures of complexes 1 and 2 were investigated using X-ray single crystal diffraction combined with Hirshfeld calculations. Their anti-microbial activities were compared with the free ligands and with Fluconazole and Gentamycin as standard agent. Both complexes showed better anti-fungal activity than the standard Fluconazole. Complexes 1 and 2 are biologically more active than the free ligands against S. aureus, E. coli, and C. albicans microbes.