Md Ashraful Alam¹, Pradip Paul¹, Ratul Kumar Shil^1,2, Avijit Chakraborty¹, Saswata Rabi^1,2 and Tapashi Ghosh Roy¹*

¹Department of Chemistry, University of Chittagong, Bangladesh
²Department of Chemistry, Chittagong University of Engineering and Technology, Bangladesh

*Corresponding Author: Tapashi Ghosh Roy, Department of Chemistry, University of Chittagong, Bangladesh.

Received: July 08, 2021; Published: July 26, 2021

Abstract

  Biologically important macrocyclic compounds are getting a mentionable position in the research world due to their exhaustive applications in different sectors. On the other hand, chromium(III) is playing an analytical role by maturing the transition metal photochemistry and a biological role as an essential element for usual human body metabolism. Considering these facts, we carried out some studies on a number of chromium(III) complexes of an isomeric macrocrocyclic ligand and reported herein. Thus, two isomeric ligands, tet-a and tet-b were produced by the reduction of hexamethyl tetraazacyclotetradecadiene diperchlorate, Me₆[14]diene.2HClO₄. Reaction of tet-b with chromium(III) chloride yielded cis-[Cr(tet-b)Cl₂]Cl which on substitution reaction afforded cis-[Cr(tet-b)(NCS)₂](SCN), cis-[Cr(tet-b)(acac)](ClO₄)₂, cis-[Cr(tet-b)(ox)](ClO₄) and cis-[Cr(tet-b)Br₂]Br respectively. Axial substitution reactions on cis-[Cr(tet-b)Cl₂]Cl with KI, NaNO₂ and KNO₃ in methanol furnished six coordinated greenish blue cis-[Cr(tet-b)I₂]I and reddish violet trans-[Cr(tet-b)(NO₂)₂](NO₂) and trans-[Cr(tet-b)(NO₃)₂](NO₃) respectively. The complex, cis-[Cr(tet-b)Cl₂]Cl on reaction with NaOH followed by the addition of HClO₄ and NaClO₄ in aqueous solution produced trans-[Cr(tet-b)(ClO₄)₂](ClO₄). In all cases, the complexes were of six coordinated octahedral geometry. Different analytical and spectroscopic techniques were used for the characterization of the complexes. Antibacterial activities of the concerned ligand and complexes have been studied against four selected bacteria.

Keywords: Tetraazamacrocyclic Ligand; Chromium(III) Complexes; Spectral Studies; Antibacterial Activities

Introduction

  Development of azamacrocyclic complexes are undoubtedly one of the greatest invention in the research field of coordination chemistry [1,2]. Research interest in these compounds has grown during the past decades due to their wide range of applications in the advancement of pharmaceutical sciences [3,4]. In advance medical practice, macrocyclic complexes can be used as therapeutic agents [5], MRI contrast agents [6] and artificial restriction enzymes for cleaving DNA and RNA [7,8]. The complexes also act as biochemical tools, such as they function as antibacterial [9], antifungal [10], anticancer [11] antitumor [12] and anti-proliferative [13] drugs in fighting against different infectious diseases. Studies on macrocyclic complexes also prove their unique properties, most importantly spectral, structural, electrochemical, kinetic and thermodynamic aspects [14,15].

  Studies on copper(II), nickel(II), zinc(II), cadmium(II) and palladium(II) complexes of the concerned ligand, tet-b are available in literature [16-21]. Moreover, the preparation and characterization of several cis-[Cr([14]-decane)(La)_m](ClO₄)_n complexes containing auxiliary ligands, La (La= benzoate, chlorobenzoate, citrate, acetylacetonate, malonate or oxalate anion) [22,23] have been carried out. Ferromagnetic dihydroxo-bridged Cr(III)-Cu(II) and Cr(III)-Cu(II)-Cr(III) complexes, [CR(tet-b)(OH)₂Cu(bpy)(CH₃OH)](ClO₄)₃ and {Cu[Cr(tet-b)(OH)₂]₂}(ClO₄)₄ have also been reported in an earlier study [24]. Axial cis-substitution reaction on cis-[Cr(tet-b)Cl₂]Cl to produce cis-[Cr(tet-b)(NCS)₂](SCN), cis-[Cr(tet-b)(acac)](ClO₄)₂, cis-[Cr(tet-b)(ox)]ClO₄ and cis-[Cr(tet-b)Br₂]Br was carried out as per literature [25] but the antibacterial activities of them were not investigated before. So in continuation of these studies, we prepared some new cis and trans-substituted chromium(III) complexes of the concerned macrocyclic ligand, tet-b and studied the antibacterial activities of all the mentioned Cr(III) complexes. Thus the present work concerns the synthesis and characterization of six-coordinated anion-bound Cr(III) complexes, cis-[Cr-(tet-b)I₂]I and trans-[Cr(tet-b)(Y)₂](Y) (Y=NO₂, NO₃ or ClO₄) and antibacterial activities of all the prepared Cr(III) complexes.

Experimental Study

  Analytical reagent grade (Sigma Aldrich, USA) chemicals were used for the preparation of the complexes. Microanalysis of C, H and N have been carried out on a Leco CHNS-932 elemental analyzer (LECO Corporation, St. Joseph, MI) at the Department of Chemistry, Friedrich Schiller University, Jena, Germany. SYSTONIC electrothermal melting point apparatus was used to find melting points of the compounds. The infrared spectra of the metal complexes were taken as KBr disc in the range 4000-400 cm-1 on a Shimadzu IR 20 spectrophotometer at the Department of Chemistry, University of Chittagong, Chattogram, Bangladesh. Conductance measurements of the metal complexes were done using HANNA instrument with HI 8820N conductivity cell. Electronic spectra of the samples were recorded on a Shimadzu UV-visible spectrophotometer. Magnetic measurements have been carried out by SHERWOOD SCIENTIFIC magnetic susceptibility balance.

  Synthesis of the ligand salt, hexamethyl-tetraazacyclotetradecadiene dihydroperchlorate, Me₆[14]diene.2HClO₄ (L×2HClO₄) and its reduced isomeric ligands, tet-a and tet-b (Scheme 1) were carried out as procedure adopted in the literature [26].

Scheme 1

  The complex cis-[Cr(tet-b)Cl₂]Cl (S₁) and its axial cis-substitution products cis-[Cr(tet-b)(NCS)₂](SCN) (S₂), cis-[Cr(tet-b)(acac)](ClO₄)₂ (S₃), cis-[Cr(tet-b)(ox)](ClO₄) (S₄) and cis-[Cr(tet-b)Br₂]Br (S₅) were prepared as per literature [25].

  The complex cis-[Cr(tet-b)Cl₂]Cl (0.0570 g, 0.13mmol) was suspended in hot methanol (20 mL). Then KI (0.0431 g, 0.26 mmol), NaNO₂ (0.0179 g, 0.26 mmol) and KNO₃ (0.101g, 1mmol) was added separately to the hot suspension. After concentrating the mixtures on water bath, greenish blue cis-[Cr(tet-b)I₂]I (S₆), reddish violet trans-[Cr(tet-b)(NO₂)₂](NO₂) (S₇) and trans-[Cr(tet-b)(NO₃)₂](NO₃) (S₈) were formed respectively. The precipitates were filtered and washed with ethanol followed by diethylether and finally dried in vacuum.

  For cis-[Cr(tet-b)I₂]I (S₆) (717.2): Color: Greenish blue. Anal. Calcd.(%): C, 26.80; H, 5.06; N, 7.81. Found (%) C, 26.83; H, 5.03; N, 7.79. IR(KBr disc,cm-1): ν(N-H), 3165 s, ν(C-H), 2973 s, ν(CH₃), 1383 s, ν(C-C), 1171 s, n(Cr- N), 430 s. Molar conductivity Ω(ohm^-1cm²mole^-1): in DMSO, 77, in CH₃CN 260. Magnetic moment µ_eff(BM): 3.87. UV vis[_max in nm (_max)]: in DMSO, 584(106), 419(101), 385(90).

  For trans-[Cr(tet-b)(NO₂)₂]NO₂ (S₇) (474.5): Color: Reddish violet. Anal. Calcd.(%): C, 40.50; H, 7.65; N, 20.66. Found (%) C, 40.47; H, 7.60; N, 20.61. IR(KBr disc,cm-1): ν(N-H), 3195 w, ν(C-H), 2972 s, ν(CH₃), 1383 s, ν(C-C), 1171 s, n(Cr- N), 440 s, n_asym(NO₂), 1441 m, n_sym(NO₂), 1315 m, n(N=O), 1485, n(NO), 1102. Molar conductivity Ω(ohm^-1cm²mole^-1): in DMSO, 77; in CHCl₃, 0; in H₂O, 290. Magnetic moment µ_eff(BM): 3.87. UV vis[_max in nm (_max)]: in DMSO, 531(120), 457(77), 371(254); in CHCl₃, 553(89), 371(236); in H₂O, 542(66), 389(85).

  For trans-[Cr(tet-b)(NO₃)₂](NO₃) (S₈) (522.5): Color: Reddish violet. Anal. Calcd.(%): C, 36.78; H, 6.94; N, 18.76. Found (%) C, 36.81; H, 6.98; N, 18.75. IR(KBr disc,cm-1): ν(N-H), 3240 vs, ν(C-H), 2973 s, ν(CH₃), 1383 s, ν(C-C), 1129 s, n(Cr- N), 451 s, n(NO₃), 1456m, 1340 m. Molar conductivity Ω(ohm^-1cm²mole^-1): in DMSO, 98; in H₂O, 232; in CH₃CN, 262. Magnetic moment µ_eff(BM): 3.85. UV vis[_max in nm (_max)]: in DMSO, 586(46), 411(29), 384(137); in H₂O, 535(69), 386(55), 309(162).

  cis-[Cr(tet-b)Cl₂]Cl (0.0265 g, 0.06 mmol) complex was suspended in 10 mL of water. Then 1 mL of 0.1 M NaOH was added to the solution while the solution turned blue. This blue solution was neutralized by adding small amount of HClO₄. Then excess NaClO₄ (0.3 g) was added to the solution and after concentrating the solution reddish product, trans-[Cr(tet-b)(ClO₄)₂](ClO₄) (S₉) was filtered off, washed with cold water, then with methanol followed by diethylether and finally dried in vacuum.

  For trans-[Cr(tet-b)(ClO₄)₂](ClO₄) (S₉) (634.8): Color: Red. Anal. Calcd.(%): C, 30.27; H, 5.72; N, 8.83. Found (%) C, 30.34; H, 5.77; N, 8.87. IR(KBr disc,cm-1): ν(N-H), 3170 s, ν(C-H), 2975 s, ν(CH₃), 1393 s, ν(C-C), 1168 s, n(Cr- N), 447 s, n(ClO₄), 1083m, 624 s. Molar conductivity Ω(ohm^-1cm²mole^-1): in DMSO, 34; in H₂O, 103. Magnetic moment µ_eff(BM): 3.84. UV vis[_max in nm (_max)]: in DMSO, 551(153), 385(132); in H₂O, 521(19), 383(138).

  Study on antibacterial activity of isomeric macrocyclic ligand, tet-b and its chromium(III) complexes were performed by using disc diffusion method for controlling pathogenic microorganisms of animals and plants. Solutions of known concentration (1.0 mg/mL) of the test samples were made by dissolving measured amount of the samples in definite volume of solvents. Dried and sterilized filter paper disc (4 mm in width) are then impregnated with known amounts of test substances by using micropipette and these were then allowed to dry. Discs containing the test material were placed on nutrient agar medium uniformly seeded with the test microorganism. Discs absorbed in respective solvent without test material are used as negative control. These plates were then kept at low temperature (4˚C) for 2-4 hours to allow maximum diffusion. During this time dried discs absorb water from the surrounding media and then the test materials are dissolved and diffused out of the media. The diffusion happens as per the physical law that controls the diffusion of molecules through agar gel. Therefore, there is a continuous difference in test materials concentration in the media surrounding the discs. The plates were then incubated at 37˚C for 48 hours to allow maximum growth of the organisms. If the test materials have any antibacterial action, it would inhibit the growth of the microorganisms giving a clear distinct zone called, “zone of inhibition”. The antibacterial activities of the test agents were determined by measuring the zone of inhibition expressed in millimeter in diameter. The experiment was carried out thrice and mean of the reading was taken.

Results and Discussion

The isomeric ligand ‘tet-b’, (Scheme 1) was prepared and characterized as per literature [26].

  The complex cis-[Cr(tet-b)Cl₂]Cl and some cis-substitution products of cis-[Cr(tet-b)Cl₂]Cl: cis-[Cr(tet-b)(NCS)₂](SCN), cis-[Cr(tet-b)(acac)](ClO₄)₂, cis-[Cr(tet-b)(ox)](ClO₄) and cis-[Cr(tet-b)Br₂]Br were prepared and characterized as per literature [25].

  Interaction of cis-[Cr(tet-b)Cl₂]Cl with KI in methanol produced six coordinated octahedral greenish blue cis-[Cr(tet-b)I₂]I complex. The Infrared spectrum of this complex displayed characteristic band at 3165 cm⁻¹, 2973 cm⁻¹, 1383 cm⁻¹, 1171 cm⁻¹ and 430 cm⁻¹ for n(N-H), n(C-H), n(CH₃), n(C-C) and n(Cr-N) respectively. Since the IR spectral band could not be run below 400 cm^-1, so Cr-I stretch could not have been detected. The molar conductivity value 77 ohm^-1cm²mol^-1 in DMSO indicates that the complex is 1:1 electrolyte as expected for the formula assigned. However the conductance value of 260 ohm^-1cm²mole^-1 in acetonitrile corresponding to 1:2 electrolytes is probably due to the replacement of iodide ion by solvent molecule from the coordination sphere [27]. This replacement reaction can be expressed by the following expression:

  In electronic spectra of macrocyclic complexes of Cr(III) exhibit three absorption bands at 598-583, 390-380 and 345-340 nm due to ⁴B_1g®⁴E_g(a), ⁴B_1g®⁴E_g(b) and ⁴B_1g®⁴A_2g transitions respectively [28]. In DMSO the present complex, cis-[Cr(tet-b)I₂]I displayed a sharp d-d band at 584 nm, a medium band at 419 nm and a very small band at around 385 nm. The positions of these bands are consistent with a distorted octahedral geometry with D_4h symmetry [28]. The bands at 584 nm and 385 nm may be assigned to ⁴B_1g®⁴Eg_(a) and ⁴B_1g®⁴E_g(b) transitions respectively. The magnetic moment value, µ_eff = 3.87 corresponds to d³ system of chromium(III) ion [29]. Thus, the structure-I (Chart 1) can be assigned to cis-[Cr(tet-b)I₂]I.

  The complex cis-[Cr(tet-b)Cl₂]Cl underwent axial substitution reactions with NaNO₂ and KNO₃ in methanolic solution to afford reddish violet trans-[Cr(tet-b)(NO₂)₂](NO₂) and trans-[Cr(tet-b)(NO₃)₂](NO₃) respectively. Infrared spectra of these complexes showed n(N-H), n(C-H), n(CH₃), n(C-C) and n(Cr-N) stretching bands at the expected regions. The complex trans-[Cr(tet-b)(NO₂)₂](NO₂) displayed the n_asym(NO₂) band at 1441 cm^-1, n_sym(NO₂) at 1320 cm^-1. Moreover the complex exhibited two bands; one at 1485 cm^-1 due to n(N=O) and the other at 1102 cm^-1 assignable to n(NO), which are as expected for O-bonded nitrito complexes [30]. It may be noted that folding of a 14-membered macrocycle is possible only along a fold axis passing through diagonally opposite nitrogen atoms having their hydrogens lying on the same side [31]. The macrocycle can then fold away from the direction in which the NH hydrogens lie. Though some of the structures do have fold axes but due to steric constrains of two non-linear NO₂^- groups, accommodation of two NO₂^- groups in the cis position is not favorable [25,32]. Thus in this case a trans configuration is suggested for this complex. The other complex trans-[Cr(tet-b)(NO₃)₂](NO₃), displayed bands at 1456 cm^-1 and 1340 cm^-1 attributed to coordinated NO₃^- group. The separation of these bands by 116 cm^-1 can be accounted for unidentate mode of coordination [33,34]. The spectrum further exhibited a band around 1383 cm^-1 (overlapped with CH₃ band) due to ionic NO₃^- group which supports the presence of an ionic nitrate [34]. For both complexes the expected band at around 360- 250 cm^-1 due to n(Cr-O) [35,36] could not have been detected as the spectra could not be measured below 400 cm^-1. For these complexes, molar conductivity values 77-98 ohm^-1cm²mol^-1 in DMSO (the color remained intact) indicated that these complexes are 1:1 electrolytes as expected for the formulae assigned. However the conductivity value 0 ohm^-1cm²mole^-1 for trans-[Cr(tet-b)(NO₂)₂](NO₂) in chloroform corresponding to nonelectrolyte apparently indicates the incorporation of outer NO₂^- in coordination sphere which should increase the coordination number of Cr(III) form 6 to 7, but practically it rather indicates the anion association in the coordination sphere [37]. On the other hand, the conductance values of 232-290 ohm^-1cm²mole^-1 in water and 262 ohm^-1cm²mole^-1 in acetonitrile for trans-[Cr(tet-b)(NO₃)₂](NO₃) corresponding to 1:2 electrolytes are probably due to the replacement of Y^- ion by solvent molecule while the water or acetonitrile molecules force the anion to come out from the coordination sphere [27]. This replacement reaction can be expressed by the following expression:

  UV-Vis spectra of these complexes explored three absorption bands at 531-586 nm, 411-457 nm and 371-389 nm for respective transitions of octahedral geometry as expected [28]. The magnetic moment values of 3.85 and 3.87 of these complexes are an indication of paramagnetism corresponding to three unpaired electrons as expected for octahedral chromium(III) complexes with d²sp³ hybridization [29]. Thus, the structure-II and structure-III (Chart 1) can be assigned to trans-[Cr(tet-b)(NO₂)₂](NO₂) and trans-[Cr(tet-b)(NO₃)₂](NO₃) respectively.

  The complex cis-[Cr(tet-b)Cl₂]Cl interacted simultaneously with NaOH, HClO₄ and NaClO₄ in aqueous solution to yield trans-[Cr(tet-b)(ClO₄)₂](ClO₄). IR spectrum of this complex exhibited n(N-H), n(C-H), n(CH₃), n(C-C) and n(Cr-N) stretching bands at the expected regions. However the bands at 1083 cm^-1 and 624 cm^-1 indicated the presence of ClO₄^- ion. The position as well as spliting of the band at 1083 cm^-1 support the unidentate mode of coordination [38]. The molar conductivity value 103 ohm^-1cm²mol^-1 in water (in which color remained intact) of this complex indicated that this complex is 1:1 electrolyte [27] as expected for the formula assigned. However the conductivity value 34 ohm^-1cm²mole^-1 determined for trans-[Cr(tet-b)(ClO₄)₂](ClO₄) in DMSO corresponding to nonelectrolyte may be due to anion (ClO₄^-) association in the coordination sphere [37] as discussed in earlier section. From electronic spectral analysis three absorption bands were observed at the expected regions which support octahedral geometry of the complex [28]. The magnetic moment value of this complex shows paramagnetism corresponding to three unpaired electrons as expected [29]. By considering all these analyses it can be assumed that accommodation of such two big ClO₄^- ions in cis position is unfavorable [25,32]. Therefore, the structure-IV (Chart 1) can be assigned to trans-[Cr(tet-b)(ClO₄)₂](ClO₄).

Chart 1: Chromium(III) complexes of tet-b.

  Antibacterial activities of the concerned isomeric macrocyclic ligand, tet-b and some of its complexes have already been studied [19-21]. It is noteworthy to examine whether the concerned ligand (tet-b) and its chromium(III) complexes against the concerned bacteria show any such activity or not. Reasonably, investigation on the antibacterial activities of the concerned metal salts, isomeric ligand, tet-b, its Cr(III) complexes and standard antibacterial agent have been carried out against two important gram-positive and two gram-negative bacteria which cause different fatal diseases. As per our earlier studies [20,21], the ligand, tet-b did not show any activity against the tested bacteria, but most of the complexes exhibited mentionable antibacterial activities. From this study, it is evident that most of the complexes were effective against both gram-positive and gram-negative bacteria (Table 1) but exception was found in case of diisothiocynato (S₂), acetylacetonato (S₃) and dinitrato (S₈) complexes against Pseudomonas aeruginosa and dichlorido complex (S₁) against Klebsiella pneumonia as they were fully inactive against the mentioned bacteria. On the other hand, dibromido (S₅), dinitrito (S₇) and diperchlorato (S₉) complexes showed remarkable activity against almost all bacteria. The antibacterial activity of the metal salt and experimental solvent as control were also tested for the comparison. It was found that the experimental solvent has no effect on the growth of bacteria but metal salt showed the activity to a little extent against two gram positive and one gram negative bacteria. Though the metal salt exhibited some activity, but the activity explored by complexes is not due to metal ions. Because the complexes are very much stable. Thus there is no possibility of dissociation of complexes to release metal ions [39]. So based on the positive results of this investigation it can be concluded that further studies are warranted with the concerned macrocyclic ligands and their chromium(III) complexes.

Table 1: Antibacterial activities of ligand and its chromium(III) complexes.

Conclusion

  This study exposes that the concerned isomeric ligand, tet-b undergoes facile complexation with CrCl₃.6H₂O in DMF solvent to produce cis-[Cr(tet-b)Cl₂]Cl which on substitution reactions furnished cis-[Cr(tet-b)(NCS)₂](NCS), cis-[Cr(tet-b)(acac)](ClO₄)₂, cis-[Cr(tet-b)(ox)](ClO₄), cis-[Cr(tet-b)Br₂]Br, cis-[Cr(tet-b)I₂]I, trans-[Cr(tet-b)(NO₂)₂]NO₂, trans-[Cr(tet-b)(NO₃)₂](NO₃) and trans-[Cr(tet-b)(ClO₄)₂](ClO₄) complexes. It is interesting to note that trans-derivatives could be prepared from cis-complex. All the prepared complexes were tested against bacteria and most of the complexes exhibited good activity.

Acknowledgement

  The authors would like to extend their sincerest gratefulness to University Grant Commission (UGC), Bangladesh to sanction a research grant to Professor Dr. Tapashi Ghosh Roy, Department of Chemistry, University of Chittagong, Chattogram-4331, Bangladesh.

Conflict of Interest

There is no conflict of interest.

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Citation

Citation: Tapashi Ghosh Roy., et al. “Chromium(III) Complexes of a Tetraazamacrocyclic Ligand: Synthesis, Characterization and Antibacterial Investigation". Acta Scientific Pharmaceutical Sciences 5.8 (2020): 89-96.

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Copyright: © 2021 Tapashi Ghosh Roy., et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.