A heterocyclic N'-(4-(diethylamino)-2-hydroxybenzylidene)-4-oxopiperidine-1-carbohydrazide Schiff base ligand and its metal complexes: Synthesis, structural characterization, thermal behavior, fluorescence properties, and biological activities European Journal of Chemistry 13 (4) (2022) 415-425 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2022 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.13.4.415-425.2337 European Journal of Chemistry View Journal Online View Article Online A heterocyclic N'-(4-(diethylamino)-2-hydroxybenzylidene)-4-oxopiperidine- 1-carbohydrazide Schiff base ligand and its metal complexes: Synthesis, structural characterization, thermal behavior, fluorescence properties, and biological activities Gajanan Mahadu Dongare and Anand Shankarrao Aswar * Department of Chemistry, Sant Gadge Baba Amravati University, Amravati-444602, Maharashtra State, India * Corresponding author at: Department of Chemistry, Sant Gadge Baba Amravati University, Amravati-444602, Maharashtra State, India. e-mail: aswaranand@gmail.com (A. Aswar). 10.5155/eurjchem.13.4.415-425.2337 Received: 22 August 2022 Received in revised form: 30 September 2022 Accepted: 07 October 2022 Published online: 31 December 2022 Printed: 31 December 2022 A new heterocyclic hydrazone Schiff base ligand, N'-(4-(diethylamino)-2-hydroxy benzylidene)-4-oxopiperidine-1-carbohydrazide, (H2L) was derived by a condensation reaction of 4-oxopiperidine-1-carbohydrazide with 4-(diethylamino)-2-hydroxybenz- aldehyde. The ligand reacts with chloride salts of chromium(III), manganese(II), iron(III), cobalt(II), nickel(II), copper(II) and zinc(II) to form metal complexes of [Cr(L)(Cl)(H2O)2], [Mn(HL)(Cl)(H2O)2], [Fe(L)(Cl)(H2O)2], [Co(HL)(Cl)(H2O)2], [Ni(HL)(Cl)(H2O)2], [Cu(HL)(Cl) (H2O)2], [Zn(L)(H2O)], respectively. The structure of the hydrazone ligand was confirmed by elemental analysis and spectroscopic techniques, viz., FT-IR, 1H NMR, 13C NMR, and LC-MS spectroscopy. The newly synthesized ligand behaves as a tridentate ONO donor towards Cr, Mn, Fe, Co, Ni, Cu, and Zn metal ions. The spectral, magnetic moment, and thermal data indicate the octahedral geometry for all metal complexes except for Zn, which has tetrahedral geometry with 1:1 stoichiometry (M:L). ESR study revealed that π-bonding covalency is much stronger than the σ-bonding with axial distortion in the structure. The molar conductivity data suggested the nonelectrolytic nature of the complexes. The powder X-ray diffraction patterns suggest the nanocrystalline nature of the compounds. The SEM micrograph of the ligand significantly differs from its Ni(II) complex indicating coordination of Ni(II) ion to the ligand. The intense fluorescence emitted in the region of λExcitation 521 to 524 nm due to the functional fluorophores of the ligand and its manganese (II), chromium(III), cobalt(II), and zinc(II) complexes. Various kinetic parameters such as Ea, ∆S, ∆H, and ∆G of various decomposition steps were calculated from TGA diagrams using Coats- Redfern method and the thermal stability order was found to be Cr < Fe < Co < Mn = Cu < Zn < Ni. The antibacterial and antifungal activities of the ligand and its divalent and trivalent metal complexes were performed against the various pathogens viz. Escherichia coli, Salmonella typhi, Staphylococcus aureus, Bacillus subtilis, Candida albicans, and Aspergillus niger with reference to standard antibiotics viz. ofloxacin, azithromycin, and fluconazole. All metal complexes showed promising biological activity as compared with their parent ligand and may be used as a potential antimicrobial candidate in biological science. SEM X-ray diffraction Metal complexes Antimicrobial activity Fluorescence emission Heterocyclic hydrazone Cite this: Eur. J. Chem. 2022, 13(4), 415-425 Journal website: www.eurjchem.com 1. Introduction The heterocyclic hydrazone Schiff base ligand plays a significant role in the coordination chemistry, as they easily form more stable complexes with most of the transition metal ions and found extensive applications in the various fields comprising organic synthesis, medicinal chemistry, non- linearity, and superb optical (i.e., absorbance and emission properties) to the supramolecular chemistry [1-4]. The hydrazone Schiff base containing the tridentate moiety of ONO active donor sites displayed a versatile class of ligands which have been studied for a long time as potential multifunctional properties due to their structural diversity, tautomerism, reaction conditions, and with good yield. The heterocyclic tridentate hydrazone contains the flexible ONO donor atoms in the carbonyl ‘O’ and the azomethine ‘N’, and phenolic ‘O’ active donor sites. Aroyl hydrazones are having to possess paramount importance due to their molecular modularity, structural flexibility, straightforward synthesis, and thermal stability have been explored as a reason for their diversified potential application [5,6]. The transition metal complexes derived from aroyl hydrazones exhibit very good enzymatic activity and are found very important in organometallic synthesis, analytical chemistry, and medicinal chemistry [6,7]. Furthermore, the tridentate moieties of the Schiff base of hydrazones and their complexes gained more recognition due to their broad-ranging biological properties, antinociceptive, and anti-inflammatory activities [8-10]. Hydrazone compounds containing the azo- methine group have shown a significant role in the medicinal ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.13.4.415-425.2337 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.13.4.415-425.2337 mailto:aswaranand@gmail.com http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.13.4.415-425.2337&domain=pdf&date_stamp=2022-12-31 416 Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 field for the copper and iron chelators in vivo [11] and in vitro for iron overload treatment [12]. The hydrazone Schiff base ligands derived from salicylaldehyde derivatives are found as well-known as multidentate ligands and possess a broad level of antimicrobial efficiency [13-15]. Recently, the interest has grown up due to the single-step synthetic procedure for hydrazone Schiff base ligands with good yield in the crystalline state and more desirable donating property through the enolate/amide oxygen (nucleophile), azomethine nitrogen (nucleophile), and phenolate oxygen. Additionally, piperidone-based carbohydrazones are known to have strong coordinating properties and complexing ability toward transition-metal ions adopting several geometries, e.g., bidentate, tridentate, or polydentate mode of linkage via NO, ONO, and N2O4. Transition metal complexes of Schiff base with carbohydrazide functionalities have of special research interest because of their roles in numerous physico-chemical and broad spectrum of biological properties, and functional fluorescence. Reviewing all above facts and in view of our continuing interest in the hydrazone of ligand, we designed and synthesized N'-(4- (diethylamino)-2-hydroxybenzylidene)-4-oxopiperidine-1- carbohydrazide ligand (H2L) and its complexes of chromium (III), manganese(II), iron(III), cobalt(II), nickel(II), copper(II), and zinc(II) ions. The structure of the ligand and complexes were fully characterized by spectro/analytical techniques and magnetic moments. After complete spectro/analytical charac- terization, the ligand and its complexes were biologically screened in vitro for their antibacterial efficiency using the bacterial cocci of Escherichia coli (NCBI-0157), Salmonella typhi (NICM-2257), Staphylococcus aureus (NDCM-2257), Bacillus subtilis (NICM-2477) and fungal species of Candida albicans (MTCC-1637) and Aspergillus niger (NCIM-1005). In addition to this, thermo-kinetic data, fluorescence emission properties, and surface morphology of complexes have been reported for applications. 2. Experimental 2.1. Reagents and physical measurement methods Metal chlorides were of AR grade (99%) and purchased from S.D. fine chemicals, India, 4-diethylamino salicyldehyde (99%) (Alpha Aesar, Great Britain), 1-carboethoxypiperid-4- one (99%) (Avra Synthesis, Hyderabad, India) were used without further purification. The solvents were doubly distilled before use. The elemental analysis (CHN) was carried out on a Carlo-Erba analyzer. The metal content of the complexes was analyzed by the gravimetric method after decomposing the organic moiety by a mixture of HClO4, H2SO4, and HNO3 (1:1.5: 2.5) and then igniting metal oxide. The molar conductance measurements of a 1×10-3 M solution of metal complexes in DMSO at room temperature were carried out using a 180-Elico conductivity bridge. FT-IR spectra of ligand and its metal complexes were recorded on an Alpha-II compact Bruker Advance. IR spectrometer. The magnetic susceptibility of the complexes was measured at ambient temperature with a Sherwood MSB-MK-1 balance. The NMR (1H, 13C) spectra of hydrazone ligand were recorded on a Bruker Advance NMR spectrometer (500 MHz). The HR-Mass spectra of the hydrazone ligand was recorded using a micro-mass spectro- photometer. The absorption spectra have been recorded on a UV-Visible-1800 Shimadzu spectrophotometer in the wave- length range (λ) 250 to 1200 nm. The ESR spectrum of the Cu(II) complex was recorded at 77 K in LNT on a JEOL-ESR spectro- photometer using microwave frequency in the range of 8.750- 9.650 GHz. Thermogravimetric analysis (TG/DTG) was carried out in N2 atmosphere on a TGA-4000 Perkin-Elmer thermal analyzer in the temperature range of 40-800 °C, with a linear heating rate 20 °C/min with precision and accuracy of ±0.8 °C controlled by inbuilt Pyris software for TGA application of inbuilt electromagnetic compatibility (EMC) facilitated of stable weight calibration. The powder X-ray diffraction patterns of the compounds were recorded on a Miniflex (600) benchtop Rigaku X-ray diffractometer with a scanning range between 10-80°, the scan-axis 2θ/θ. The surface morphology of the hydrazone ligand and its complexes were recorded in 3D mode on JEOL- 6390LV SEM(X100-X500) scanning electron microscope at 10 μM at 10 kV. The emission spectra were recorded in the wavelength region of 250-800 nm on a photoluminescence spectrophotometer (PL) Hitachi (F-7000) using a 150 W Xenon lamp. The progress of the reaction for the formation of the ligand was checked by using TLC (Merck’s silica plates 60F254) and visualized by UV irradiation (254 nm). 2.2. Synthesis 2.2.1. Synthesis of N'-(4-(diethylamino)-2-hydroxy benzylidene)-4-oxopiperidine-1-carbohydrazide (H2L) The hydrazone Schiff base (H2L) ligand was synthesized by a reaction of 4-oxopiperidine-1-carbohydrazide (0.315 g, 2.0 mmol) with 4-(diethylamino)-2-hydroxybenzaldehyde (0.3865 g, 2.0 mmol) in 20 mL ethanol with continuous stirring. The catalytic amount of sulfuric acid (1-2 drops) was added to the reaction mixture and refluxed for 3 hours in a water bath. After cooling to room temperature, a bright, spongy yellow product was separated out. The resulting product obtained was filtered, washed with dry ethanol, and finally recrystallized from DMSO, then dried over anhydrous CaCl2 in a desiccator (Scheme 1). N'-(4-(Diethylamino)-2-hydroxybenzylidene)-4-oxopiperidine -1-carbohydrazide: Color: Yellow. Yield: 80 %. M.p.: 183-185 °C. FT-IR (KBr, ν, cm-1): 3314 (OH, br, phenolic), 3261 (N-H, amide), 1670 (C=O, amide), 1626 (C=N, azomethine), 1300 (C-O, phenolic), 961 (N-N, azomethine). 1H NMR (500 MHz, DMSO-d6, δ, ppm): 11.48 (s, 1H, Ph-OH), 9.60 (s, 1H, -CH=N-), 8.60 (s, 1H, >NH), 7.41 (d, 1H, Ar-H), 6.55 (d, 1H, Ar-H), 6.11 (s, 1H, Ar-H), 3.49 (dd, 4H, -CH2-C=O), 3.40 (q, J = 3.98 Hz, 4H, >N-CH2-CH3), 3.33 (dd, 4H, -CH2-N), 1.05 (t, J = 6.90 Hz, 6H, >N-CH2-CH3). 13C NMR (125 MHz, DMSO-d6, δ, ppm): 12.30 (1C, CH3), 12.42 (1C, - CH3), 39.23 (1C, -CH2, O=C-CH2), 39.56 (1C, O=C-CH2), 43.74 (1C, N-CH2), 43.99 (1C, N-CH2), 55.89 (2C, N-CH2-), 95.79 (1C, Ar-C), 96.92 (1C, Ar-C), 103.93 (1C, Ar-C), 104.34 (1C, Ar-C), 106.27 (1C, Ar-C), 132.90 (1C, HC=N-), 160.48 (1C, C=O, >NHC=O), 162.00 (1C, Ar-C-OH), 180.01 (1C, C=O, H2C(C=O)CH2). HRMS (EI, m/z) calcd. for C17H24N4O3, 332.18; Found [M]+1 = 333.34. Anal. calcd. for C17H24N4O3: C, 61.43; H, 7.28; N, 16.86. Found: C, 61.72; H, 7.50; N, 16.90%. UV/Vis (DMSO, λmax, nm, ε): 245 (2.31), 331 (1.673), 432 (0.21). 2.2.2. Preparation of metal complexes All metal complexes were synthesized by a conventional method. The H2L ligand (0.6648 g, 2 mmol) and the respective metal chlorides (2 mmol) were dissolved separately in 25 mL of warm DMSO and dry ethanol, respectively. Both solutions were filtered and mixed in warm conditions with continuous stirring. The reaction mixture was then refluxed in an oil bath for about 4 hours and the pH of the reaction mixture was adjusted to ca. ~7.0 by adding ethanol: ammonia solution (7:3). The reaction mixture was cooled to room temperature. The colored solid products isolated were collected by filtration, washed with dry ethanol and petroleum ether, and finally dried in a desiccator over anhydrous calcium chloride. The analytical data of the compounds together with their proposed formula are given in Table 1. Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 417 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 Table 1. Analytical and physical data of the compounds. Compounds Molecular formula Molecular weight (g/mol) Elemental analysis (%) Found, (Calc.) Molar conductance (Ω–1cm2mol–1) C H N Cl Metal H2L C17H24N4O3 332.40 61.72 (61.43) 7.50 (7.28) 16.90 (16.86) - - 0.0 [Cr(L)(Cl)(H2O)2] CrC17H26N4O5Cl 453.86 45.00 (44.99) 6.00 (5.77) 12.50 (12.34) 7.90 (7.81) 12.80 (11.46) 18.20 [Mn(HL)(Cl)(H2O)2] MnC17H27N4O5Cl 457.81 44.76 (44.60) 6.00 (5.94) 12.40 (12.24) 7.84 (7.74) 12.15 (12.00) 15.16 [Fe(L)(Cl)(H2O)2)] FeC17H26N4O5Cl 457.71 44.76 (44.61) 5.85 (5.73) 12.45 (12.24) 7.80 (7.75) 12.30 (12.20) 26.15 [Co(HL)(Cl)(H2O)2] CoC17H27N4O5Cl 461.80 44.30 (44.21) 6.10 (5.89) 12.40 (12.13) 7.75 (7.68) 12.84 (12.76) 17.80 [Ni(HL)(Cl)(H2O)2] NiC17H27N4O5Cl 461.56 44.35 (44.24) 6.00 (5.90) 12.25 (12.14) 7.78 (7.68) 12.80 (12.72) 21.90 [Cu(HL)(Cl)(H2O)2] CuC17H27N4O5Cl 466.42 44.90 (43.78) 6.90 (5.84) 12.15 (12.01) 7.75 (7.60) 13.80 (13.62) 8.15 [Zn(L)(H2O)] ZnC17H24N4O4 413.78 49.50 (49.35) 5.90 (5.85) 13.70 (13.54) - 15.95 (15.80) 6.14 Scheme 1. Synthesis of the ligand and its ketonic-enolic forms. 3. Results and discussion The reaction between 4-oxopiperidine-1-carbohydrazide and 4-(diethylamino)-2-hydroxybenzaldehyde in ethanol pro- duced new hydrazone, N'-(4-(diethylamino)-2-hydroxybenzyli- dene)-4-oxopiperidine-1-carbohydrazide (H2L) (Scheme 1). The formation of the ligand was further confirmed by spectro/ analytical techniques, viz., FT-IR, 1H NMR, 13C NMR, and HR-MS. The reaction of this ligand with metal chlorides formed two types of complex with good yield [Cr(L)(Cl)(H2O)2] (1), [Fe(L) (Cl)(H2O)2)] (2), and [Zn(L)(H2O)] (3) in which the hydrazone ligand acts as a dibasic tridentate (in its enolic state) coordinated by ONO donor atoms, while in [Mn(HL)(Cl)(H2O)2] (4), [Co(HL)(Cl)(H2O)2] (5), [Ni(HL)(Cl)(H2O)2)] (6) and [Cu(HL)(Cl)(H2O)2] (7) hydrazone ligand acts as a monobasic- tridentate (in its ketonic form) coordinated by ONO donor atoms. All synthesized metal (II) and (III) complexes are colored powder, non-hygroscopic, and stable at room tempe- rature. The complexes are insoluble in water and most of the common organic solvents, but sparingly soluble in DMF and DMSO. The analytical data obtained for the synthesized complexes confirmed that the complexes are mononuclear, and the ligand to metal ratio was found to be 1:1 (M:L) in all complexes. The molar conductivity of metal complexes in DMSO solution (1×10–3 M) at room temperature shows low values of 6.14-26.15 Ω–1cm2mol–1 indicating non-electrolytic nature and, moreover, indicated the chloride ion present within the coordination sphere. 3.1. Characterization of the ligand The proton NMR spectrum of ligand shows a dd at δ 3.40 ppm due to (N-CH2) protons and a singlet at δ 1.05 ppm for the methylenic protons at α- and β-positions of the (CH3CH2)2N< group. The heterocyclic ring shows two doublets at δ 3.33 ppm for four protons of the -CH2- group at α-position and another two-doublet appeared at δ 3.49 ppm of four protons another of the -CH2- group at β-position. One phenolic (OH) and another one azomethine proton (-HC=N-) showed downfield singlet appears at δ 11.48 and 9.60 ppm, respectively. Aryl protons were observed in their typical δ 6.11-7.41 ppm region. The 13C NMR spectrum of the hydrazone ligand exhibits four signals at δ 39.23, 39.56, 43.74 and 43.99 ppm due to the four carbons of the heterocyclic ring. The signal observed at δ 180.01 ppm of the saturated carbonyl carbon of the heterocyclic ring located at low intensity whereas another signal at δ 160.48 ppm is due to amide carbonyl carbon. The azomethine carbon (-HC=N-) signal appeared at δ 132.90 ppm. The aryl carbons appear well within the expected region δ 95.79-106.27 ppm. The separate signals that appeared at δ 12.30 and 12.42 ppm are due to methyl carbon. The HR-Mass spectrum of the ligand shows M+ = 333.34 m/z of its molecular ion peak, considered as to its molecular mass. Some stable fragmented molecular ion species are 260.10 m/z (C13H14N3O3+) at 100% intensity regarded as a base peak whereas other fragmented species 206.12 m/z and 164.10 m/z indicating (C11H16N3O+) and (C10H14NO+) molecular ions, respectively. 3.2. Infrared spectra and bonding mode In order to ascertain the bonding mode of the hydrazone ligand to the metal ion in the complexes, the infrared spectra of the metal complex were compared with that of its parent ligand. The selected IR bands for the ligand and its complexes together with their assignments are listed in Table 2. The IR spectra impart characteristic bands to identify various functional groups such as carbonyl (C=O), imino (=NH), (C=C), (C-C) bonds, crystal/lattice water, halide linkage, tautomerism, enolic, and phenolic group of vibrations in a heterocyclic ring system. The IR spectrum of the ligand shows a medium broad band at 3314 cm–1 due to the stretching frequency of the phenolic (OH) group and this band has been disappeared from the spectra of all complexes, indicating the formation of a coordination bond between the metal ion and the phenolic oxygen atom via deprotonation. It was further confirmed by the upward shifting of stretching frequencies (C–O) by the incre- 418 Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 Table 2. IR spectral data (cm-1) of the hydrazone ligand and its metal complexes. Compounds ν(OH) phen. ν(NH) amide ν (C=O) ν(C=N) azomethine ν(C-O) in phenyl ν(C-O) in enol ν(N-N) Coord. ν(H2O) ν(M-O) ν(M-N) H2L 3314 3261 1670 1626 1300 - 961 - - - [Cr(L)(Cl)(H2O)2] - - 1620 1600 1310 1235 970 850 585 488 [Mn(HL)(Cl)(H2O)2] - 3250 1625 1605 1341 - 958 838 553 440 [Fe(L)(Cl)(H2O)2)] - - 1651 1604 1349 1239 980 840 590 452 [Co(HL)(Cl)(H2O)2] - 3255 1622 1600 1348 - 970 835 560 448 [Ni(HL)(Cl)(H2O)2] - 3248 1630 1600 1312 - 964 820 570 460 [Cu(HL)(Cl)(H2O)2] - 3235 1665 1598 1305 - 972 835 580 474 [Zn(L)(H2O)] - - - 1600 1317 1240 975 850 558 460 Table 3. Magnetic susceptibility and electronic absorbance data of metal complexes. Metal complexes µeffective (B.M.) Maximum absorption v (cm-1) Maximum absorption wavelength v (nm) Spectral transitions Ligand fields Dq. (cm-1) Interelectronic repulsion B (cm-1) Racah parameter Β0 (cm-1) Nephelauxetic ratio (β = B/Β0) [Cr(L)(Cl)(H2O)2] 3.84 16529 19321 26667 605 518 375 4A2g→4T2g 4A2g→4T1g(F) 4A2g→4T1g(P) 1653 976 918 0.74 [Mn(HL)(Cl)(H2O)2] 5.86 10417 11494 20408 960 870 490 6A1g→4Eg(4D) 6A1g→4Eg(G) 6A1g→4T2(G) 1042 667 860 0.77 [Fe(L)(Cl)(H2O)2)] 5.90 10101 13297 18868 990 752 530 6A1g→4T2g(G) 6A1g→4T2g(G) 6A1g→4Eg(D) - - - - [Co(HL)(Cl)(H2O)2] 4.62 9346 10787 17575 1070 927 569 4T1g→4T2g 4T1g→4A2g 4T1g→4A2g(F) 1168 704 971 0.73 [Ni(HL)(Cl)(H2O)2] 2.80 10256 11163 22173 975 880 451 3A2g→3T2g(F) 3A2g→3T1g(F) 3A2g→3T1g(P) 1026 794 1030 0.77 [Cu(HL)(Cl)(H2O)2] 1.71 11073 15384 20408 895 652 490 2B1g→2A1g(v1) 2B1g→2B2g(v2) (CT) - - - - [Zn(L)(H2O)] Dia. 27473 39216 364 255 ILCT (n→π*) ILCT (π→π*) - - - - ments of 5 to 49 cm–1 from 1300-1349 cm-1 in all complexes. The phenolic ‘O’ atom has been involved in the bond formation with metal(II) and metal(III) ions of the complexes. The ligand spectrum displayed a strong characteristic vibrational band at 1626 cm–1, due to ν(-HC=N-, azomethine) and shifting of this band towards lowering frequency by 21-28 cm–1 from 1605- 1598 cm–1 in the spectra of Mn(II), Co(II), Ni(II), and Cu(II) complexes indicates the coordination metal atom through azomethine nitrogen. This has been further strengthened by upward the shifting of ν(N–N) band from 958 to 972 cm–1 in the spectra of the metal complexes. The ligand shows band at 3261 cm–1 due to the stretching vibration of ν(N-H) group which appeared almost about at the same position as in the case of Mn(II), Co(II), Ni(II), and Cu(II) complexes indicating non- involvement in the coordination, which confirm the presence of ligand in its ketonic form. This is also further confirmed by the shifting of amide ν(C=O) to lowering frequency by 5-50 cm–1 from 1625-1665 cm–1 in these complexes, suggesting the coordination of carbonyl oxygen atom to metal ions as such without experiencing the enolization. However, these bands are completely absent in Cr(III), Fe(III), and Zn(II) complexes, which confirmed the enol formation act in accordance with the coordination linkages to the metal ion after deprotonation [16,17]. Furthermore, the existence of coordinated water molecules was confirmed by the presence of non-ligand bands 820-850 cm–1, which was further supported by the thermal analysis. The coordination of ligand was further confirmed by the appearance of new weak intensity bands, non-ligand bands in the region of 553-590 and 440-488 cm–1 in the IR spectra of the complexes are assigned to frequencies of ν(M-O) and ν(M- N) stretching vibrations, respectively. Based on the above data, all metal complexes were identified as a mononuclear compound displayed on the basis of the characteristics of the vibrational band frequencies [18]. 3.3. Room temperature magnetic susceptibility and electronic absorption spectral properties The structural interpretation for the metal complexes was predicted from their magnetic moment and electronic absorp- tion spectra measured at 300.15 K. The synthesized metal complexes were identified by their characteristic absorption of maxima as well as minima and the data are listed in Table 3. The electronic spectrum of the Cr(III) complex shows three bands at 16529, 19231, and 26667 cm-1 which are assigned to the 4A2g → 4T2g (ν1), 4A2g → 4T1g (F) (ν2), and 4A2g → 4T1g (P) (ν3) transitions, respectively, in accordance with octahedral geo- metry around chromium ion. The magnetic moment (µ) value is 3.84 B.M., which is indicative of the octahedral geometry. The ligand field parameters such as the crystal field splitting energy (Dq), the Racah parameter (B), the nephelauxetic ratio (β), and the v2/v1 were evaluated by using the König equation [19] and values are found to be 1653 cm-1, 676 cm-1, 0.74 and 1.16, respectively, which are also in agreement with the octahedral Cr(III) complexes. The value of interelectronic repulsion parameter B was found to be lower (676 cm-1) than that of the free ion, confirming the presence of covalency in the metal- ligand bond. The electronic spectrum of the Mn(II) complex exhibits three bands at 10417, 11494, and 20408 cm-1 attributed to 6A1g → 4Eg (4D) (ν1), 6A1g → 4Eg (G) (ν2), and 6A1g → 4Eg (G) (ν3) transi- tions, respectively, in an octahedral geometry. Additionally, the value of magnetic moment 5.86 B.M. is also supported to confirm an octahedral geometry around the Mn(II) ion. The electronic spectrum of the Fe(III) complex showed three bands located at 10101, 13297, and 18868 cm-1 attributed to the 6A1g → 4T1g (G) (ν1), 6A1g → 4T2g (D) (ν2) and 6A1g → 4Eg (D) (ν3) transitions, respectively, in conformity with octahedral arrangements around the Fe(III) ion and the magnetic moment value is 5.90 B.M. which indicates the presence of the Fe(III) complex in octahedral geometry with d5 high spin electronic configuration. Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 419 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 Table 4. Electron spin resonance spectral parameters of Cu(II) complex. ge g|| (Avg.) g⊥ (Avg.) g (Avg.) Δg = g||-g⊥ G HSC×10-4 (cm-1) HSC Avg. f = g|| /A|| Bonding parameters Orbital reduction A|| A⊥ α2 β2 K2|| K2⊥ 2.0036 2.211 2.092 2.152 0.119 2.327 148 88 118 149.39 0.77 1.46 0.88 1.55 Figure 1. ESR spectrum of Cu(II) complex. The electronic spectrum of the Co(II) complex exhibits three absorption bands at 9346, 10787, and 17575 cm-1, which have been tentatively assigned to 4T1g → 4T2g (ν1), 4T1g → 4A2 (G) (ν2), and 4T1g → 4T1g (F) (ν3) transitions, respectively, which was in accordance with the octahedral geometry around the cobalt ion. The magnetic moment value 4.62 B.M. indicates octahedral geometry with high-spin arrangements around cobalt ion. The different ligand field parameters such as Dq, B, β, and v2/v1 have been evaluated and the values are found to be Dq = 1168 cm-1, Racah parameter value (B) = 704 < 971 cm-1 (in free ion), β = 0.73 cm-1 and v2/v1 = 1.15. The lower B value of the complex than the free ion value suggests the presence of significant covalency in M-L bonding. The absorption spectrum of the Ni(II) complex displayed three bands at 10256, 11163, and 22173 cm-1 due to 3A2g → 3T2g (F) (ν1), 3A2g → 3T1g (F) (ν2) and 3A2g → 3T1g (P) (ν3) transitions, respectively, characteristic of an octahedral environment for Ni(II) complexes. The observed magnetic moment is 2.80 B.M., which is very close to the spin-only value for two unpaired electrons and is supported for octahedral geometry for the complex. The calculated values of various ligand field para- meters were found to be Dq = 1026 cm-1, B = 794 < 1030 cm-1 in free ions, and β = 0.77 cm-1 are applicable with to those of reported for octahedral complexes of Ni(II) [20]. The observed lower value of B than that of the free ion indicates an orbital overlap. The absorption spectrum of the Cu(II) complex shows a broad absorption band at ~11073 cm-1 due to the 2B1g → 2A1g transition for distorted octahedral stereochemistry of Cu(II) complexes. The other weak absorption bands observed at 14663, and 20408 cm-1 correspond to 2B1g → 2Eg and (L → M ion) charge transfer, transitions, respectively. The broadness of band occurs due to the presence of Jahn-Teller distortion, indicating distortion from octahedral geometry. The complex showed a magnetic moment of 1.71 B.M., which indicates the presence of one unpaired electron with the monomeric nature of the complex. The electronic spectrum of the Zn(II) complex shows two bands at 27473 and 39216 cm-1, which may reasonably be assigned to a charge transfer transition from the ligand to a metal ion. The ligand chromophores transition of π → π* and n → π* group in the UV region. The complex is found to be diamagnetic, as expected for the d10 system, which may suggest the tetrahedral geometry around the Zn(II) ion [21]. 3.4. ESR study of [Cu(HL)(Cl)(H2O)2] complex The ESR spectrum of the Cu(II)complex exhibits three prominent locations of (g||) peaks at 2794, 2981, and 3161 G and another two g⊥ peaks at 3336, and 3430 G and the spectrum is shown in Figure 1 and its data are summarized in Table 4. A well-resolved spectrum carrying the electron magnetic tensor components g|| = 2.211 and g⊥ = 2.092, which informs the presence of axial symmetry in the complex. The tensor values are shows a (g|| > g⊥) and the further trend to appears in the order of (g|| > g⊥) more than free electron magnetic tensor ge = 2.0036 shows the configuration in a dx2-dy2 orbital in the ground state containing an unpaired electron. It was reported that the g|| value is sensitive to the covalency nature of the ligand-metal bond (g|| less than 2.3), which indicates that the covalency in the Cu-L bond. The deviation of tensor gave = 2.152 from the free electron tensor value ge = 2.0036 accounted for the covalency in the metal-ligand bond. The geometric parameter was calculated by using Hathway’s equation G = (g|| - 2.0023)/(g⊥ - 2.0023) and found to be 2.327 value which was lower than 4.0 suggesting considerable exchange interaction with the metal center [22]. The tensor (A) displays the most significant tensor properties of magnetic interaction between the electron spin and neighboring nuclear spin, giving rise to the hyperfine structure in the spectrum. The hyperfine splitting constant (HSC) was calculated A|| = 148 cm-1 and A⊥= 88 cm-1, which suggesting that electron interaction within the only single copper nucleus confirmed distortion in the octahedral geometry around the Cu(II) ion center. The tensor component ‘A’ was calculated from the spectra by comparison of the spectral line width and the line shape. The calculated the tensor component (A|| 148 cm-1 > A⊥ 88 cm-1) which predicted the distorted octahedral structure with axial symmetry through the copper-O bond and the copper-N bond. Inverse relation of the tensor components A∥ with g∥ indicates the conformity of the more delocalization of the electrons in the metal-ligand covalent bond. Comparing the increased values of A∥ with declining values of g∥, it clearly indicates more interaction of electrons with its nuclear spin. The ratio (f) = g∥/A∥ was found to be 149.39 and its Aavg. = 118 which inferred the presence of distortion in the structure. The molecular orbital coefficient was calculated β2 (1.46) > α2 (0.77) which represents that π-bonding covalency is stronger than the σ-bonding with axial distortion in the structure. The contribution of the orbital reduction parameters (K2⊥ = 1.55) > (K2|| = 0.88) indicates the greater in-plane π-bonding than the out-of-plane π-bonding [23]. 420 Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 (a) (b) Figure 2. Scanning electron microscopic image of (a) H2L and (b) Ni(II) complex using JEOL-6390L microscope. 3.5. SEM study The scanning electron microscopic (SEM) image was captured to compare the surface morphology of the synthesized ligand and its Ni(II) complex as a representative case and is shown in Figures 2a and 2b. The different characteristic shapes and sizes of the ligand particles and its Ni(II) complex provide evidence of the metal complexation. The unit cell of the ligand appears as uniform platelets with slightly varying lateral dimensions to represent the cuboidal shape without any shadows and this was supported by the monoclinic crystal Bravais lattice from XRD studies. The high level of resolution at 500× SEM image of the Ni(II)complex, revealed that morpho- logy appears as a cauliflower-like shape with upper grooves, non-uniform, irregular in shape with aggregation of molecules [24,25]. The magnified pictogram of ligand at ×400 and complex at ×500 level clearly informed differences in shape and size of the particle giving evidence of the formation of the metal complex. 3.6. X-ray diffraction analysis The powder XRD pattern of H2L ligand and its derived Mn(II), Ni(II), and Zn(II) metal complexes have been recorded by using the X-ray diffractometer with a target source of copper as an anode material, CuKα = 1.540559 Å. The XRD diffract- tograms as representative cases to determine the Bravais lattice system, lattice framework, and volume of newly synthesized compounds [26,27]. The prominent peaks of the X- ray diffraction patterns have been indexed to find crystallite size, nature of compounds, peak intensities, relative intensities, interplanar spacing, and unit cell dimensions. Bragg’s equation (nλ = 2dsinθ) was used for the evaluation of angle in 2θ and interplanar distances as d. The Miller indices were also reflected in a specified direction, which was supported by the crystal lattice parameters of h, k, and l values. The average particle size was calculated from Scherrer’s equation (1918) L = (K×λ(nm))/(β(2θ)×cosθ); Scherer’s constant (K = 0.9), peak position in 2θ (deg.), and β parameters of full width at half maximum (FWHM). The ligand and divalent complexes of Mn(II), Ni(II), and Zn(II) were found to be 21.53, 37.030, 39.4341, and 34.89 nm, respectively, which inferring their nanocrystalline nature [28]. 3.7. Thermogravimetric analysis Thermogravimetric analysis is a tool to evaluate the thermal stability of complexes in terms of mass loss with respect to the variation of temperature and provide infor- mation about compositions, fragmentation patterns, and the presence of water of hydration as well as crystallization in the complexes. In the presence investigation, TG-DTG analysis of the H2L ligand and its metal complexes was carried out under N2 atmosphere in the temperature range of 40-800 °C with a linear heating rate 20 °C/min. Thermogravimetric analysis results are summarised in Table 5. TG plot of ligand shows a one-step decomposition pattern and exhibits a broad endothermic peak (DTGmax), ie, Tmax = 375 °C, ∆H = 196.3 kJ/mol with mass loss of 63.83% (Theor. 63.87%) due to the loss of one fragmented organic species (-C7H24N4O3). Further continuous heating up to 707 °C, the ligand undergoes complete decomposition and the thermogram becomes horizontal, which may possibly be due to the formation of the carbon residue 33.17%, (Theor. 36.13%). The TG/DTG curve of the [Cr(L)(Cl)(H2O)2] complex shows a broad peak (DTGmax = 130 °C) in the first decomposition step with a mass loss of 8.40% (Calc. 8.38%) corresponding to the elimination of two coordinated water molecules. In the second stage, a mass loss of 9.09% occurred (Calc. 8.49%) at ~250 °C (DTGmax) which corresponded to the removal of one mole of chloride ion. In the third stage of degradation, mass loss of the non-coordinated part of the ligand is observed at 62.08% (Calc. 62.04%) at ~450 °C and finally, the TG curve attains a hori- zontal level corresponding to the formation of stable metal oxide (½Cr2O3) [29,30]. The TG/DTG curve of the [Mn(HL)(Cl)(H2O)2] complex accounted for a mass loss of 8.00% (Calc. 7.86%) at ~110 °C (broad, DTGmax), due to the removal of coordinated water (2H2O) molecules per mole of complex, whereas in the second decomposition step (DTG max) at 249 °C complex shows a mass loss of 8.68% (Calc. 8.41%) corresponds to the loss of one chloride ion. The third decomposition step was observed around at ~390 °C with a mass loss of 75.57% (Calc. 74.00%) corresponding to the left of the maximum part of the coordinated ligand. At the end of the thermogram, it attains a horizontal plateau with residue left about 7.75% (Calc. 9.73%) as a stable metal oxide. The thermogram of the [Fe(L)(Cl)(H2O)2] complex exhibits mass loss ~7.86% (calc.7.86 %) at 100 °C broad (DTGmax) which is attributed to the loss of two coordinated water molecules from its coordination, while in 2nd step of decom-position at temperature 260 °C (DTGmax) with a mass loss 7.94% (Calc. 7.92%) corresponding to the elimination of one-mole chloride molecule. The 3rd stage of pyrolysis was observed at 360 °C with a mass loss of 64.24% (Calc. 64.22 %), which was attributed to the elimination of organic parts and was found at 20.01% (Calc. 20.00%) left as a ½Fe2O3 residue. The TG/DTG curve of the [Co(HL)(Cl)(H2O)2] complex shows three steps decomposition in the temperature range of 45-750 °C. The first step of pyrolysis at 130 °C shows a mass loss of 5.98% (Calc. 7.77%) due to the loss of two coordinated molecules from its coordination sphere, whereas, in the second step, DTGmax at ~250 °C with a loss of mass of 7.82% (calc. 8.33%) corresponds to one molecule of chloride ion. With continued further heating, 3rd stage pyrolysis occurred within the temperature range between 320-600 °C with a maximum mass loss of 62.5% (Calc. 61.76%) corresponding to the organic parts of ligand. At the end of the thermal reaction, the formation of cobalt oxide was 23.70% (Calc. 22.14%). Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 421 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 Table 5. Stepwise thermal analysis data of the ligand and its metal(II/III) complexes. Compounds TG range (°C) Derivative TG (°C) Mass loss (%) Fragmented species / Residue assignments Experimental Theoretical H2L 44-375 375 66.83 63.87 Removal of C7H24N4O3 species 375-707 - 33.17 36.13 Organic residues [Cr(L)(Cl)(H2O)2] 45-130 130 8.40 8.38 Removal 2×H2O molecules 130-252 249 9.10 9.09 Removal of Cl ion 252-451 451 62.08 62.04 Removal of organic parts 410-700 - 20.42 20.40 Residue of 1/2Cr2O3 [Mn(HL)(Cl)(H2O)2] 45-110 110 8.00 7.86 Removal of coordinated 2×H2O molecules 110-255 249 8.68 8.41 Removal of one Cl ion 255-380 379 75.57 74.00 Removal of organic parts 380-700 - 7.75 9.73 Residue of MnO [Fe(L)(Cl)(H2O)2] 45-110 100 7.86 7.86 Removal of coordinated 2×H2O molecules 110-390 260 7.94 7.92 Removal of Cl ion 390-610 390 64.24 64.22 Removal of organic parts 610-800 - 20.01 20.00 Residue of 1/2Fe2O3 [Co(HL)(Cl)(H2O)2] 45-110 130 5.98 7.77 Removal of coordinated 2×H2O molecules 110-252 251 7.82 8.33 Removal of Cl ion 252-320 320 62.50 61.76 Removal of organic parts 320-807 23.70 22.14 Residue of CoO [Ni(HL)(Cl)(H2O)2] 45-130 100 7.07 7.05 Removal of coordinated 2×H2O molecules 130-289 270 12.71 9.60 Removal of Cl ion 289-450 390 64.43 64.25 Removal of organic parts 450-753 - 15.78 19.10 Residue of NiO [Cu(HL)(Cl)(H2O)2] 45-130 120 7.50 7.49 Removal of coordinated 2×H2O molecules 130-320 270 8.30 8.22 Removal of Cl ion 320-630 330 64.20 63.99 Removal of organic parts 630-776 - 20.00 19.95 Residue of CuO [Zn(L)(H2O)] 45-240 130 9.97 9.81 Removal of coordinated H2O molecule 240-650 650 69.01 69.00 Removal of organic parts 650-750 - 21.02 20.81 Residue of ZnO [NiC17H27N4O5Cl] [NiC17H23N4O3Cl] Stage 1 -2H2O Stage 2 - Cl NiO Ni(II) complex [NiC17H23N4O3] Stage 3 Org. moieties Scheme 2. Schematic representation of thermal degradation of Ni(II) complex. The TG/DTG curve of the [Ni(HL)(Cl)(H2O)2] complex showed three intense (DTGmax) endothermic peaks at 100, 270, and 390 °C exhibiting three-stage decomposition DTGmax at 100 °C with a loss of mass of 7.07% (Calc. 7.05%) with the removal of two coordinated molecules, in the 2nd step decomposition, at 270 °C with a mass loss 12.71 % (Calc. 9.60%) by the removal of fragments of one chloride ion. The 3rd degradation stage at 390 °C shows a maximum mass loss of 64.43% (Calc. 64.25%) of the organic parts of the ligand. The residue was left as NiO with 15.78% (Calc. 19.10%). The thermal degradation pattern for the Ni complex is presented in Scheme 2. The TG curve of [Cu(HL)(Cl)(H2O)2] complex shows a mass loss of 7.50% (Calc. 7.49%) at 120 °C, which accounted for the loss of two coordinated water molecules. The second step involves a mass loss of 8.30% (Calc. 8.22%) at DTGmax = 270 °C and this may attribute a loss of one chloride ion. In third decomposition step, involves a mass loss of 64.20% (Calc. 63.99%), at 330 °C which confirms the loss of organic parts of ligand, and finally, the thermogram became horizontal which possibly indicates the formation of stable copper oxide residue 20.00% (Calc. 19.95%). The [Zn(L)(H2O)] complex displayed two-stage decom- position in which one small and another broad decomposition DTGmax at 130 °C for loss of one water coordinated/lattice water 9.97% (Calc. 9.81%) and the loss of organic moiety DTGmax at 650 °C was found (69.01% (Calc. 69.00%) in the temperature range of 400-650 °C. The formation of ZnO as the final residue was found to be 21.02%, nearly equal to the theoretical values (20.81%). The presence of water molecules was confirmed by the appearance of IR frequencies in the 820-850 cm-1 in the coordination, as well as the presence of chloride ions, by the Volhard estimation of these metal complexes [31,32]. We have attempted to predict the thermal stability of metal complexes on the basis of third-step decompositions and found to be Co < Cu < Mn < Fe ≌ Ni < Cr < Zn. All the physicochemical studies suggest the structure of three sets of complexes as proposed in Scheme 3. 3.8. Kinetic parameters of thermal decomposition (Coats-Redfern method) The thermo-kinetic parameters activation energy (Ea), entropy (∆S), enthalpy (∆H), and free energy (∆G) for the thermal decomposition reactions of the complexes were evaluated from the TG curves by using the Coats-Redfern relation, and values are reported in Table 6. The decomposition rate of compounds was depending upon the temperature and complexity of the structure [33]. The degree of thermal decomposition (α) calculated on TG curve by means of α = W𝑖𝑖 −W𝑡𝑡 W𝑖𝑖−W𝑓𝑓 (1) whereas Wi, Wf, and Wt are the initial weight, final weight, and weight at a time, respectively. The activation energy (Ea) is calculated by Equation (2). 𝑙𝑙𝑙𝑙𝑙𝑙 [1−𝛼𝛼]1−𝑛𝑛 [1−𝑛𝑛] = 𝐸𝐸𝑎𝑎 2.303×𝑅𝑅×(T2×𝑆𝑆) (2) 422 Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 Table 6. Calculated thermo-kinetic parameters for thermal degradation data for H2L ligand and its di-and trivalent metal complexes (Coat-Redfern method). Compound Decomposition temperature (°C) Ea (kJ/mole) Z (s-1) ∆S (J/mol/k) ∆H (kJ/mole) ∆G (kJ/mole) H2L 375 201.64 1.805×102 -16.50 196.3 207.0 [Cr(L)(Cl)(H2O)2] 130 10.87 2.01×101 -11.90 7.50 12.30 250 23.09 2.11×101 -13.70 18.70 25.90 450 140.94 2.34×101 -15.50 134.90 146.10 [Mn(HL)(Cl)(H2O)2] 110 18.80 2.01×101 -11.50 15.60 20.00 380 186.32 2.05×101 -15.70 180.90 191.20 [Fe(L)(Cl)(H2O)2 ] 100 14.22 2.01×101 -11.30 11.10 15.30 390 167.29 2.05×101 -15.90 161.80 172.30 610 299.00 1.94×101 -18.70 307.00 290.70 [Co(HL)(Cl)(H2O)2] 130 20.81 2.01×101 -11.90 17.50 22.30 250 107.69 2.04×101 -13.90 103.30 110.60 [Ni(HL)(Cl)(H2O)2] 100 18.61 2.01×101 -11.20 15.50 19.70 270 115.91 2.04×101 -14.20 111.40 119.10 390 712.15 2.21×101 -15.20 706.60 716.70 [Cu(HL)(Cl)(H2O)2] 120 12.40 2.01×101 -11.70 9.10 13.70 270 91.46 2.03×101 -14.30 86.90 94.70 [Zn(L)(H2O)] 130 23.09 2.01×101 -11.90 19.70 24.50 650 74.11 2.02×101 -18.80 66.40 83.80 N N N O O O NC2H5 C2H5 M H2O Cl H2O N HN N O O O NC2H5 C2H5 M H2O Cl H2O N N N O O O NC2H5 C2H5 M H2O M = Cr(III), Fe(III) M = Mn(II), Co(II), Ni(II), and Cu(II) M = Zn(II) Scheme 3. Proposed structure of the metal complexes. where ‘α’ denotes the degree of thermal decomposition and its conversion factors for mass losses, ‘n’ denotes the order of reaction, Gas constant R = 8.314 (J/mol) and T2S (K). The entropy of the thermal reaction was calculated by Equation (3), Entropy (∆S) = 2.303 R ×log Z×h K×Tm (3) where ‘Z’ denotes the frequency factor (s-1); h is the Plank’s constant (6.63×10-34 J.s); k denotes Boltzmann constant (1.3806×10-23 J/K); Tm was taken from the peak temperature in K. The enthalpy of reaction (∆H) is indicating the amount of heat released or absorbed during the reaction and is calculated by Equation (4), ∆H = Ea-R×T (4) Another important thermodynamic quantitative property, i.e., free energy change of reaction (∆G) of a system is calculated with this by Equation (5), ∆G = ∆H - T×∆S (5) where ∆H = Heat of reaction, T = Temperature (K), and ∆S = Entropy change. The values of the thermodynamic activation parameters for the ligand and its complexes revealed that the activation energy increased (Ea) 10.87 to 299 kJ/mol, suggesting that the high thermal stability of the compounds may be due to the covalency character of the M-L bond. The negative value of entropy of activation (∆S) from -11.20 to - 18.80 J/mol.K informed that the activated species have more structurally ordered than the dissociated ones and later was slower than the normal reaction rate. The negative values of entropy and the low-frequency factor (Z) were followed in the overall 1st, 2nd and 3rd step and finally resulted in stable metal oxide formation indicating slow decomposition, non-sponta- neous nature and degree of structural ‘complexity’. This was also supported by the positive value of the +ve (ΔG) values. The comparative order of parameters follows as Ea < ΔG ≌ ∆H which correlates with their thermal stability. The values of ΔH and the ΔG of the reaction were found in the range 7.50-706.60, and 13.70-207.00 kJ/mol, respectively, revealing that metal complexes showed more endothermic than exothermic as well as more thermally stable [34]. 3.9. Fluorescence spectral study The fluorescence emission spectra of H2L and its complexes were recorded in the wavelength region of λ = 210-810 nm and the emission spectra are presented in Figure 3 and the data in Table 7. At the excitation state, the emitted band was measured in the emission slits having a width of 1.0 nm with the scan speed at a rate of 240 nm/min. The fluorescence emission intensity of the ligand and its [Cr(L)(Cl)(H2O)2], [Mn(HL)(Cl) (H2O)2], [Co(HL)(Cl)(H2O)2] and [Zn(L)(H2O)2] complexes were clearly indicated due to the active fluorophores group, i.e. an exchangeable azomethine proton (-CH=N-), functional (C=O) group in heterocyclic rings, the combined inhibition effects of photoinduced electron transfer (PET), static interaction by charge transfer from metal ion to fluorophore groups. The fluorescence emission band of the H2L ligand and its metal complexes was observed with the varying wavelengths. The compounds have been induced from hyperchromic (blue shift) to bathochromic (red shift) which was resulting the intense fluorescent intensity (IF) of 120, 109 a.u. at the wavelength λExcitation 527, 524 nm of H2L ligand and Mn complex, respectively. Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 423 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 Table 7. The fluorescence emission spectral data of the H2L ligand and its divalent and trivalent metal complexes. Compounds Wavelength (nm) Fluorescence emission intensity (a.u.) H2L 527 120.0 [Mn(HL)(Cl)(H2O)2] 524 109.0 Cr(L)(Cl)(H2O)2] 527 54.43 [Co(HL)(Cl)(H2O)2] 530 53.61 [Zn(L)(H2O)] 544 22.74 Table 8. Antibacterial and antifungal assays of the H2L and its divalent and trivalent metal complexes (mm.) Compounds Bacteria (Gram –ve) Bacteria (Gram +ve) Fungal species E. coli (NCBI-0157) S. typhi (NICM-2257) S. aureus (NDCM-2257) B. subtilis (NICM-2477) C. albicans (MTCC-1637) A. niger (NCIM-1005) H2L 08 09 10 10 11 09 [Cr(L)(Cl)(H2O)2] 10 10 11 12 12 10 [Mn(HL)(Cl)(H2O)2] 15 11 09 09 09 10 [Fe(L)(Cl)(H2O)2] 11 08 14 10 10 12 [Co(HL)(Cl)(H2O)2] 18 15 15 13 20 12 [Ni(HL)(Cl)(H2O)2] 16 16 14 19 15 15 [Cu(HL)(Cl)(H2O)2] 14 12 12 14 15 18 [Zn(HL)(Cl)(H2O)2] 19 20 14 17 25 13 Ofloxacin 30 30 - - - - Azithromycin - - 15 20 - - Fluconazole - - - - 28 30 DMSO - - - - - - Figure 3. Comparative fluorescence emission spectra of the H2L ligand with its metal(II) and metal(III) complexes. The other three complexes of Cr, Co, and Zn also emitted significant fluorescent intensity (IF) as 54.73, 53.61, and 22.74 a.u. at the wavelength of λExcitation 527, 530, and 544 nm, respectively. On the basis of emission intensity of H2L ligand was shown excellent fluorescent properties than to it’s divalent and trivalent complexes and displayed the increasing order of Zn < Co < Cr < Mn < H2L ligand [35,36]. The experi-mental emission data revealed that the hydrazone compound and its metal(II) and metal(III) complexes are good fluorescent components that may be used in photochemical applications and the detection of heavy metal ions in biological samples. The other Fe, Ni, and Cu complexes of this series were analyzed for fluorescence properties; however, they were referred to as nonfluorescent components due to negligible emission. 3.10. Evaluation of antimicrobial activities The important aim was to the synthesis of N'-(4-(diethyl amino)-2-hydroxybenzylidene)-4-oxopiperidine-1-carbohydra- zide ligand and it’s metal complexes for testing in vitro antibiotic susceptibility for future biological applications. The antimicrobial activities were performed against selected bacterial (cocci) strains (Gram–ve): Escherichia coli (NCBI- 0157), Salmonella typhi (NICM-2257); Gram +ve bacterial cocci: Staphylococcus aureus (NDCM-2257), Bacillus subtilis (NICM- 2477) and fungal species of Candida albicans (MTCC-1637), Aspergillus niger (NCIM-1005) at a concentration of 1.0 mg/mL employing the disc-diffusion antibiotic culture-base test [37,38]. The culture-based microbiological assay was carried out in the diagnostic laboratory. The DMSO solvent was used as –ve control in the biological reaction. The potentiality of newly synthesized compounds was compared with the standard drugs viz. ofloxacin, azithromycin, and fluconazole regarded as a +ve control. The inhibition zone growth of microorganisms was studied after ~24 hours of the incubation period. The signify- cant result was found due to the presence of two ethyl groups in the bonding of nitrogen in the amino group, i.e. (C2H5)2-N- in all the compounds which lead to decelerating the cluster growth of microorganisms [39]. The antibacterial and anti- fungal results data of the H2L hydrazone ligand and its metal(II) and metal(III) complexes are summarized in Table 8. The inhibition zone data suggest pathogenic Gram +ve cocci bacteria, i.e. Staphylococcus aureus and Bacillus subtilis are found more sensitive than Gram -ve cocci bacteria viz. Escherichia coli and Salmonella typhi. Such group specificity may be due to an increase in lipophilicity upon complex formation. The coordination of metal ions increases activity upon coordination with the H2L ligand was described on account of Tweedy’s theory of chelation [40,41]. The partial sharing of the metal(II) and metal(III) ions with the ligand results the in the reduction of polarity which increases lipid solubility and leads to favor in entering into the normal cell of microorganisms [42,43]. Within the fungi, Candida albicans are moderately active compared to Aspergillus niger. The group of azomethine (C=N) acting in the exchanging of the proton to the metal complexes enhances the destroying capacity of the pathogenic microbial colonies of clusters and chain forms. It was found that the Cu complex played as a competent agent among all compounds inhibiting the growth of A. niger from the fungal species. The Co complex showed better activity than the Ni complex, which may be due to the varied morphological structures of the two fungi. In the antibacterial and antifungal studies of the H2L ligand and its transition complexes showed a broad spectrum activity by the growing resistance to micro- 424 Dongare and Aswar / European Journal of Chemistry 13 (4) (2022) 415-425 2022 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.13.4.415-425.2337 organisms and may be used as a potent antimicrobial agent in biological sciences [44-46]. 4. Conclusions The metal(II) and metal(III) complexes were synthesized from the reaction of the heterocyclic ONO donor N'-(4-(diethyl amino)-2-hydroxybenzylidene)-4-oxopiperidine-1-carbohydra- zide ligand and with metal chlorides salts. The structural skeleton of the ligand was confirmed by spectroscopic and analytical methods. The newly synthesized mononuclear transition metal complexes are of three types viz. [M(L)(Cl) (H2O)2] [M = Cr and Fe], [M(HL)(Cl)(H2O)2] (M = Mn, Co, Ni, and Cu) and [Zn(L)(H2O)]. The spectral, analytical, and thermal analysis data have confirmed the suitable geometry of all new complexes. The infrared spectra, absorption-emission spectra, ESR (Cu complex), and magnetic susceptibility data authentic- cate that the bonding mode of the H2L ligand through three donor ONO atoms with the metal ion the structure and assigned octahedral structure to each of [Cr(L)(Cl)(H2O)2], [Mn(HL)(Cl) (H2O)2], [Fe(L)(Cl)(H2O)2], [Co(HL)(Cl)(H2O)2], [Ni(HL)(Cl) (H2O)2], [Cu(HL)(Cl)(H2O)2], complexes whereas tetrahedral geometry to the [Zn(L)(H2O)] complex. On the basis of the ESR spectral data, the Cu(II) complex indicated more the π-bonding covalency than σ-bonding in the octahedral region. The powder XRD study has shown the nanocrystalline nature of the complexes. The kinetic data of thermal decomposition were evaluated from the Coats-Redfern relation, indicating that complexes show more ordered structures and are thermally stable at high temperature. The fluorescence emission spectral data of the four compounds demonstrated good fluorescent components and are recommended for use in photochemical applications. The hydrazone ligand and its metal(II) and (III) complexes show better potential activities in antibacterial screening against species of Staphylococcus aureus, Salmonella typhi, Bacillus subtilis, Escherichia coli, and fungal species of Candida albicans and Aspergillus niger. The zone of inhibition data suggests that all divalent and trivalent metal complexes are exhibiting more broad-spectrum activity than the free hydrazone Schiff base ligand, and are represented as potent antimicrobial agents for biological applications. Acknowledgements The authors are grateful to the authorities of Sant Gadge Baba Amravati University, Amravati, for providing necessary laboratory facilities, FT-IR, and TGA. We are thankful to the authorities of the sophisticated instrumentation facility, Chandigarh, for the analysis of the elements, the recording of NMR- (1H, 13C), HR-Mass spectra, and Scanning microscopic analysis of compounds. We are also grateful to the authorities of the Indian Institute of Technology (IIT), Mumbai, India, for the recording of the ESR spectra and the Central Instrumentation Centre (CIC), Shri Shivaji College of Science, Amravati, for the measurement of UV-Visible and X-ray diffraction spectra. Disclosure statement Conflict of interest: The authors declare that they have no conflict of interest. Ethical approval: All ethical guidelines have been adhered. Sample availability: Sample of the compounds are available from the author. CRediT authorship contribution statement Conceptualization: Anand Shankarrao Aswar; Methodology: Gajanan Mahadu Dongare; Software: Gajanan Mahadu Dongare; Validation: Anand Shankarrao Aswar; Formal Analysis: Gajanan Mahadu Dongare; Investigation: Gajanan Mahadu Dongare; Resources: Gajanan Mahadu Dongare; Data Curation: Anand Shankarrao Aswar; Writing - Original Draft: Gajanan Mahadu Dongare; Writing - Review and Editing: Gajanan Mahadu Dongare; Visualization: Gajanan Mahadu Dongare; Funding acquisition: Gajanan Mahadu Dongare; Supervision: Anand Shankarrao Aswar; Project Administration: Anand Shankarrao Aswar. ORCID and Email Gajanan Mahadu Dongare infogmdongare@gmail.com https://orcid.org/0000-0002-3986-9796 Anand Shankarrao Aswar aswaranand@gmail.com https://orcid.org/0000-0003-4368-496X References [1]. Backes, G. L.; Neumann, D. M.; Jursic, B. S. Synthesis and antifungal activity of substituted salicylaldehyde hydrazones, hydrazides and sulfohydrazides. Bioorg. Med. 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This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (http://www.eurjchem.com/index.php/eurjchem/pages/view/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). http://www.eurjchem.com/index.php/eurjchem/pages/view/terms http://creativecommons.org/licenses/by-nc/4.0 http://www.eurjchem.com/index.php/eurjchem/pages/view/terms 1. Introduction 2. Experimental 2.1. Reagents and physical measurement methods 2.2. Synthesis 2.2.1. Synthesis of N'-(4-(diethylamino)-2-hydroxy benzylidene)-4-oxopiperidine-1-carbohydrazide (H2L) 2.2.2. Preparation of metal complexes 3. Results and discussion 3.1. Characterization of the ligand 3.2. Infrared spectra and bonding mode 3.3. Room temperature magnetic susceptibility and electronic absorption spectral properties 3.4. ESR study of [Cu(HL)(Cl)(H2O)2] complex 3.5. SEM study 3.6. X-ray diffraction analysis 3.7. Thermogravimetric analysis 3.8. Kinetic parameters of thermal decomposition (Coats-Redfern method) 3.9. Fluorescence spectral study 3.10. Evaluation of antimicrobial activities 4. Conclusions Acknowledgements Disclosure statement CRediT authorship contribution statement ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField16: PrintField17: PrintField18: PrintField19: PrintField110: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: PrintField26: PrintField27: PrintField28: PrintField29: PrintField210: