Supramolecular Structure and Antimicrobial Activity of Ni(II) Complexes with s-Triazine/Hydrazine Type Ligand

: The two complexes, [Ni(DPPT) 2 ](NO 3 ) 2 *1.5H 2 O ( 1 ) and [Ni(DPPT)(NO 3 )Cl].EtOH ( 2 ), were synthesized using the self-assembly of ( E )-2,4-di(piperidin-1-yl)-6-(2-(1-(pyridin-2-yl)ethylidene)hydrazinyl)- 1,3,5-triazine ( DPPT ) with Ni(NO 3 ) 2 *6H 2 O in the absence and presence of NiCl 2 *6H 2 O, respectively. In both cases, the neutral tridentate DPPT ligand is found coordinated to the Ni(II) via three N-atoms from the hydrazone, pyridine and s -triazine rings. Hence, the homoleptic complex 1 has a NiN 6 hexa-coordination environment while two NO 3 − are counter anions in addition to one-and-a-half crystallized hydration water molecules are found acting as an outer sphere. The heteroleptic complex 2 has a NiN 3 O 2 Cl coordination sphere where the coordination environment of the Ni(II) is completed by one bidentate nitrate and one chloride ion leading to a neutral inner sphere while the outer sphere contains one crystallized ethanol molecule. Both complexes have distorted octahedral coordination environments around the Ni(II) ion. Using Hirshfeld analysis, the intermolecular contacts H . . . H and O . . . H in 1 and the Cl . . . H, O . . . H, N . . . H, H . . . H, C . . . H and C . . . C in 2 are found to be the most important for crystal stability. The antimicrobial activity of complexes 1 and 2 was assessed against different bacterial and fungal strains, and the results were compared with the free ligand as well as the antibacterial ( Gentamycin ) and antifungal (Ketoconazole) positive controls. Both Ni(II) complexes are better antibacterial and antifungal agents than the free ligand. Interestingly, both Ni(II) complexes have similar antifungal activity against C. albicans compared to Ketoconazole.


Introduction
Increased mortality by pathogenic diseases due to antimicrobial resistance (AMR) has become a serious hazard to human health and economic progress. The World Health Organization (WHO) has listed AMR among the top 10 worldwide public health challenges to humanity [1,2]. For this concern, medicinal inorganic chemistry is crucial for the development of new drugs [3][4][5]. Because of the important roles of transition metal ions and their complexes in biological processes due to their ability to treat a variety of diseases, chemists are investigating these compounds to enhance their pharmacological activities. Numerous studies on antibacterial agents have shown that the potency of the ligand is increased if it is chelated with a metal ion [6][7][8]. The Overtone concept and Tweedy's chelation theory were used to present a potential mechanism for the higher activity of metal chelates [9]. Additionally, chelation could make the core metal more lipophilic to facilitate its penetration across the lipid bilayers of the cell membrane, which would boost the uptake of the metal chelates [10].
Additionally, most 3D-block elements and their coordination compounds have a biological necessity and a diversity of acknowledged bioactivities. Urease is a well-identified nickel enzyme that drives researchers to further investigate the coordination chemistry of Ni(II) complexes [11,12]. These Ni(II) complexes have the ability to permeate the microbial membrane and affect the enzyme activity leading to wide-spectrum action against microbes [13][14][15][16][17][18][19].
On the other hand, nitrogen heterocycles are included in around 75% of small-molecule medicines [20]. This can be attributed to the capacity of the nitrogen atom to quickly form hydrogen bonds with biological targets [21][22][23]. Among these important nitrogen heterocycles, hydrazone Schiff bases have attracted the interest of researchers due to their chelating diversity as a result of their well-known structural flexibility [24][25][26][27][28]. In addition, hydrazones enhance cytotoxicity, which could help in constructing promising anticancer agents [29]. Furthermore, s-triazine-hydrazino derivatives have received considerable attention [30][31][32], with a special interest in coordination and supramolecular chemistry [33,34], as well as their enormous potential in medicines [35].
In our previous work, we reported the synthesis and antimicrobial activity of the Cu(II), Mn(II) and Ni(II) complexes with the s-triazine/hydrazine type ligand, namely, 2,4-bis(morpholin-4-yl)-6-[(E)-2-[1-(pyridin-2-yl)ethylidene]hydrazin-1-yl]-1,3,5triazine, (DMPT) [36,37]. To continue this work, the present study aims to synthesize two nitrogen-containing Ni(II) complexes using the s-triazine ligand DPPT shown in Figure 1. The supramolecular structure of the synthesized complexes has been explored based on single-crystal X-ray diffraction (SCXRD) and Hirshfeld calculations. Additionally, the antimicrobial activity of the Ni(II) complexes against six harmful microorganisms was assessed. Tweedy's chelation theory were used to present a potential mechanism for the higher activity of metal chelates [9]. Additionally, chelation could make the core metal more lipophilic to facilitate its penetration across the lipid bilayers of the cell membrane, which would boost the uptake of the metal chelates [10]. Additionally, most 3D-block elements and their coordination compounds have a biological necessity and a diversity of acknowledged bioactivities. Urease is a well-identified nickel enzyme that drives researchers to further investigate the coordination chemistry of Ni(II) complexes [11,12]. These Ni(II) complexes have the ability to permeate the microbial membrane and affect the enzyme activity leading to wide-spectrum action against microbes [13][14][15][16][17][18][19].
On the other hand, nitrogen heterocycles are included in around 75% of small-molecule medicines [20]. This can be attributed to the capacity of the nitrogen atom to quickly form hydrogen bonds with biological targets [21][22][23]. Among these important nitrogen heterocycles, hydrazone Schiff bases have attracted the interest of researchers due to their chelating diversity as a result of their well-known structural flexibility [24][25][26][27][28]. In addition, hydrazones enhance cytotoxicity, which could help in constructing promising anti-cancer agents [29]. Furthermore, s-triazine-hydrazino derivatives have received considerable attention [30][31][32], with a special interest in coordination and supramolecular chemistry [33,34], as well as their enormous potential in medicines [35].

Synthesis and Characterizations
The organic ligand (DPPT) was synthesized by a reaction of 2-hydrazinyl-4,6-di(piperidin-1-yl)-1,3,5-triazine with 2-acetylpyridine in ethanol by heating under reflux conditions. The target hydrazone was obtained in high yield and with high purity and then used as it is for the preparation of the two Ni(II) complexes, as shown in Scheme 1. The Ni(II) complexes were obtained in a highly crystalline form via self-assembly of DPPT and Ni(NO3)2*6H2O in ethanol in the absence and presence of NiCl2*6H2O affording the monomeric complexes [Ni(DPPT)2](NO3)2*1.5H2O (1) and [Ni(DPPT)(NO3)Cl].EtOH (2), respectively, as shown in Scheme 1. Both complexes were air stable for a long time, and the crystal quality was not changed over time. Addition-

Synthesis and Characterizations
The organic ligand (DPPT) was synthesized by a reaction of 2-hydrazinyl-4,6-di(piperidin-1-yl)-1,3,5-triazine with 2-acetylpyridine in ethanol by heating under reflux conditions. The target hydrazone was obtained in high yield and with high purity and then used as it is for the preparation of the two Ni(II) complexes, as shown in Scheme 1. The Ni(II) complexes were obtained in a highly crystalline form via self-assembly of DPPT and Ni(NO 3 (2), respectively, as shown in Scheme 1. Both complexes were air stable for a long time, and the crystal quality was not changed over time. Additionally, complexes 1 and 2 are soluble in polar protic solvents such as ethanol and methanol, as well as in polar aprotic solvents such as DMSO, DMF and acetonitrile. Their structures were confirmed using single-crystal X-ray crystallography (SCXRD).

X-ray Structure
The structure of the homoleptic complex, [Ni(DPPT)2](NO3)2*1.5H2O (1), was confirmed using single-crystal X-ray diffraction. It crystallized in the monoclinic crystal system and C2/c as a space group. The asymmetric formula of complex 1 contains half of the formula above. In the unit cell, there are two [Ni(DPPT)2](NO3)2*1.5H2O formulas, and the unit cell volume is 4517.8(2) Å 3 while the calculated density is 1.427 Mg/m 3 . The presentation of the coordination sphere of complex 1 is shown in Figure 2.
In complex 1, the Ni(II) is hexa-coordinated with two DPPT ligand units as a neutral tridentate ligand via three N-atoms from the hydrazone, pyridine and s-triazine moieties. Hence, the coordination sphere of this complex is the cationic formula [Ni(DPPT)2] 2+ , while the outer sphere comprises two NO3¯ ions in addition to one-and-a-half crystal water. The respective bond distances and angles are depicted in Table 1. It is clear that the Ni1-N2 bond (2.015(2) Å) with the hydrazone N-atom is the shortest, while the Ni1-N1 (2.087(2) Å) with the pyridine and Ni1-N4 (2.139(2) Å) with the triazine moieties are significantly longer. Due to symmetry consideration, the other three Ni-N bonds with the second DPPT ligand unit are symmetry-related and have similar bond distances. The two bite angles of the tridentate chelate are 77.67(9) and 77.27(9)° for N2-Ni1-N1 and N2-Ni1-N4, respectively. The bond angles of the cis Ni-N bonds are found in the range of 77.27 (9) to 111.45(9)° while the trans N-Ni-N bond angles range from 154.63(9) to 168.65(13)°. Hence, the NiN6 coordination sphere has a distorted octahedral geometry.

X-ray Structure
The structure of the homoleptic complex, [Ni(DPPT) 2 ](NO 3 ) 2 *1.5H 2 O (1), was confirmed using single-crystal X-ray diffraction. It crystallized in the monoclinic crystal system and C2/c as a space group. The asymmetric formula of complex 1 contains half of the formula above. In the unit cell, there are two [Ni(DPPT) 2 ](NO 3 ) 2 *1.5H 2 O formulas, and the unit cell volume is 4517.8(2) Å 3 while the calculated density is 1.427 Mg/m 3 . The presentation of the coordination sphere of complex 1 is shown in Figure 2.   In complex 1, the Ni(II) is hexa-coordinated with two DPPT ligand units as a neutral tridentate ligand via three N-atoms from the hydrazone, pyridine and s-triazine moieties. Hence, the coordination sphere of this complex is the cationic formula [Ni(DPPT) 2 ] 2+ , while the outer sphere comprises two NO 3 ions in addition to one-and-a-half crystal water. The respective bond distances and angles are depicted in Table 1. It is clear that the Ni1-N2 bond (2.015(2) Å) with the hydrazone N-atom is the shortest, while the Ni1-N1 (2.087(2) Å) with the pyridine and Ni1-N4 (2.139(2) Å) with the triazine moieties are significantly longer. Due to symmetry consideration, the other three Ni-N bonds with the second DPPT ligand unit are symmetry-related and have similar bond distances. The two bite angles of the tridentate chelate are 77.67(9) and 77.27(9) • for N2-Ni1-N1 and N2-Ni1-N4, respectively. The bond angles of the cis Ni-N bonds are found in the range of 77.27 (9) to 111.45(9) • while the trans N-Ni-N bond angles range from 154.63(9) to 168.65(13) • . Hence, the NiN 6 coordination sphere has a distorted octahedral geometry.
As clearly seen from Figure 3, the packing view of complex 1 along the crystallographic a-direction shows the cationic complex units and the outer sphere of the complex in an alternating manner. The nitrate anion and the crystal water molecule are found in the spaces between these complex cationic units ( Figure 3A Figure 3B, while the corresponding hydrogen bond parameters are depicted in Table 2.

X-ray Structure of 2
The structure of the heteroleptic complex 2 was found to be [Ni(DPPT)(NO 3 )Cl].EtOH, which represents the asymmetric unit of 2 ( Figure 4). This complex crystallized in the less symmetric triclinic crystal system and P-1 space group. In the unit cell, there are two molecules of the asymmetric formula [Ni(DPPT)(NO 3 )Cl].EtOH and its volume is 1282.43(4) Å 3 , while the calculated density is 1.509 Mg/m 3 .   (Table 2).
The structure of the heteroleptic complex 2 was found to be [Ni(DPPT)(NO3)Cl].EtOH, which represents the asymmetric unit of 2 ( Figure 4). This complex crystallized in the less symmetric triclinic crystal system and P-1 space group. In the unit cell, there are two molecules of the asymmetric formula [Ni(DPPT)(NO3)Cl].EtOH and its volume is 1282.43(4) Å 3 , while the calculated density is 1.509 Mg/m 3 .  (Table 2).  The Ni(II) is coordinated with one tridentate DPPT ligand, one bidentate nitrate ion and one monodentate chloride ion. The hexa-coordination sphere of complex 2 is neutral, and hence there is not any counter anion in the outer sphere of this complex. On the other hand, there is one ethanol molecule as a crystal solvent which plays an important role in  (Ni1-Cl1), which is the longest in the coordination sphere, was also found. As clearly seen from Table 3, all angles deviated significantly from the ideal values for a perfect octahedron (90 and 180 • ). Hence, the coordination geometry of this complex is a distorted octahedron. The  Table 4, while the presentation of the most important contacts is shown in Figure 5A. The N3-H3N . . . O4 H-bond occurs between the N3-H3 group of the hydrazone moiety as a H-bond donor with an O4 atom of ethanol as a H-bond acceptor. The C7-H7A . . . O4 and C7-H7C . . . N3 interactions occurred between the C-H bond from the methyl group as a H-bond donor with the O4 and N3 atoms of the OH and NH groups as a H-bond acceptor. In addition, the coordinated chloride participated in the molecular packing via the formation of an O4-H4O . . . Cl1 hydrogen bond. These non-covalent interactions connect the complex units leading to the hydrogen-bonded dinuclear formula shown in Figure 5B.  (17) 174 (3) #1 -x+1,-y+1,-z+1.

Hirshfeld Analysis of Molecular Packing
In crystalline materials, the intermolecular interactions play a vital role in the crystal stability. Hirshfeld topology analysis is important for predicting all possible intermolecular contacts in the crystal structure. The d norm , curvedness and shape index surfaces of complex 2 are shown in Figure 6 Table 5.   Table 5.   Other contacts are also detected in the crystal structure of this complex but have less significance in the molecular packing as these contacts have long interaction distances. A summary of all contacts contributing to the molecular packing of 2 is presented in Figure 7. Their percentages are depicted in the same illustration. Other contacts are also detected in the crystal structure of this complex but have less significance in the molecular packing as these contacts have long interaction distances. A summary of all contacts contributing to the molecular packing of 2 is presented in Figure  7. Their percentages are depicted in the same illustration.  Similarly, the Hirshfeld surfaces of 1 are shown in Figure 8, where the decomposition of the fingerprint plot revealed the importance of the H . . . H and O . . . H contacts in the molecular packing. A summary of these short interactions is given in Table 6.    The decomposition analysis of all contacts that occurred in the crystal structure of 1 is shown in Figure 9.

Antimicrobial Evaluations
The results of the antimicrobial evaluations of the studied compounds against selected microbes (Table 7). It is clear that the free ligand has no activity against microbes except B. subtilis, where the inhibition zone diameter is only 10 mm while the MIC value is 1250 μg/mL. As a result, the free ligand has weak antimicrobial activity against the studied microbes. On the other hand, the studied Ni(II) complexes have interesting antimicrobial activity against Gram-positive bacteria and fungi while not active against Gram-negative bacteria. In the case of complex 1, the inhibition zone diameters are determined to be 20 and 21 mm against the fungi A. fumigatus and C. albicans while 7 and 19 mm against the Gram-positive bacteria S. aureus and B. subtilis, respectively. The corresponding values for 2 are 18, 19, 8 and 22 mm. Hence, complex 2 has slightly better antifungal activity than 1. In contrast, complex 1 has better antibacterial activity against the two Gram-positive bacteria than 2. The results of the MIC are in accord with the inhibition zone diameters, where the MIC is only 78 μg/mL for complex 2 against B. subtilis. The Ni(II) chelates could affect the respiration of the microbial organisms, which inhibits their ability to produce their own proteins leading to their death [38], which could explain the interesting antimicrobial activity. On the other hand, the studied Ni(II) complexes have moderate activity compared to antifungal (Ketoconazole) and antibacterial (Gentamycin) controls. It is clear that both Ni(II) complexes have similar antifungal activity against C. albicans compared to Ketoconazole, where all have MIC values of 312 μg/mL.

Antimicrobial Evaluations
The results of the antimicrobial evaluations of the studied compounds against selected microbes (Table 7). It is clear that the free ligand has no activity against microbes except B. subtilis, where the inhibition zone diameter is only 10 mm while the MIC value is 1250 µg/mL. As a result, the free ligand has weak antimicrobial activity against the studied microbes. On the other hand, the studied Ni(II) complexes have interesting antimicrobial activity against Gram-positive bacteria and fungi while not active against Gram-negative bacteria. In the case of complex 1, the inhibition zone diameters are determined to be 20 and 21 mm against the fungi A. fumigatus and C. albicans while 7 and 19 mm against the Grampositive bacteria S. aureus and B. subtilis, respectively. The corresponding values for 2 are 18, 19, 8 and 22 mm. Hence, complex 2 has slightly better antifungal activity than 1. In contrast, complex 1 has better antibacterial activity against the two Gram-positive bacteria than 2. The results of the MIC are in accord with the inhibition zone diameters, where the MIC is only 78 µg/mL for complex 2 against B. subtilis. The Ni(II) chelates could affect the respiration of the microbial organisms, which inhibits their ability to produce their own proteins leading to their death [38], which could explain the interesting antimicrobial activity. On the other hand, the studied Ni(II) complexes have moderate activity compared to antifungal (Ketoconazole) and antibacterial (Gentamycin) controls. It is clear that both Ni(II) complexes have similar antifungal activity against C. albicans compared to Ketoconazole, where all have MIC values of 312 µg/mL.
In our previous work, the antimicrobial activity of the Cu(II), Mn(II) and Ni(II) complexes with DMPT was reported [36,37]. It was found that all complexes except the [Mn(DMPT)Cl 2 ] complex have no antifungal activity. In contrast, the Mn(II) and Cu(II) complexes of DMPT showed interesting antimicrobial activity against both Gram-positive and Gram-negative bacteria, while the corresponding Ni(II) complex has less antibacterial potency. Interestingly, the studied Ni(II) complexes 1 and 2 have improved antifungal and antibacterial activities compared to the previously published metal(II) analogues of DMPT. As a result, the modification of the structure of the coordinated ligand by replacement of the morpholine ring with piperidine has a significant impact on the improvement of the antimicrobial activity.

Physical Measurements
All details regarding the instrumentations and chemicals are given in Supplementary Materials. FTIR and NMR spectra of DPPT are shown in Figures S3-S5 (Supplementary Materials). [36,37] The synthetic method for the ligand DPPT is similar to the previously reported method [36,37] in which refluxed in EtOH an equimolar of the 2-acetylpyridine and striazine-hydrazine derivative in the presence of catalytic amount of AcOH. The product obtained is white material (Scheme 1).

Hirshfeld Analysis
The Crystal Explorer Ver. 3.1 program [44] was used to perform this analysis.

Antimicrobial Assay
The methods used for determining antibacterial activity are mentioned in Method S1 (Supplementary Materials) [45].

Conclusions
The supramolecular structure of the newly synthesized complexes, [Ni(DPPT) 2 ](NO 3 ) 2 *1. The coordination environment of Ni(II) in 2 is completed by one bidentate nitrate and one chloride ion. Complexes 1 and 2 have similar antifungal activity against C. albicans compared to Ketoconazole. Additionally, both Ni(II) complexes are better antibacterial and antifungal agents than the free ligand. In comparison with our previous work, the replacement of the morpholine ring by piperidine moiety at the s-triazine core has a significant impact on the improvement of the antimicrobial activity.