Spectral, thermal, optical and biological studies on (E)-4-[(2-hydroxyphenyl)imino]pentan-2-one and its complexes J. Serb. Chem. Soc. 81 (1) 57–66 (2016) UDC 542.9+547.571+547.551:546.562’732’742’ JSCS–4827 472:547.292:543.57:615.281/.282–188 Original scientific paper 57 Spectral, thermal, optical and biological studies on (E)-4-[(2-hydroxyphenyl)imino]pentan-2-one and its complexes NASSER M. HOSNY1*, REYAD IBRAHIM and AHMED A. EL-ASMY 1Chemistry Department, Faculty of Science, Port Said University, Port Said, Egypt, 2Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt and 3Chemistry Department, Faculty of Science, Kuwait University, Kuwait (Received 1 February, revised 24 April, accepted 7 July 2015) Abstract: Metal complexes derived from the reaction of Cu(II), Co(II), Ni(II) and Zn(II) acetates and (E)-4-[(2-hydroxyphenyl)imino]pentan-2-one (H2L) were synthesized and characterized by elemental analyses, MS, IR, UV–Vis and 1H-NMR spectroscopy, thermogravimetry (TG) and differential TG (DTG), and magnetic measurements. In all complexes except for the Zn(II) complex, the Schiff base ligand acts as a mono-negative tridentate (NOO) donor, through the azomethine nitrogen, the hydroxyl oxygen and the enolic carbonyl oxygen. The structure of the Cu(II) complex is square planar, the Co(II) is octahedral while, the Ni(II) and Zn(II) are tetrahedral. Optical band gap measurements indicated the semi-conducting nature of these complexes. The biological activities were screened against two bacteria and two fungi. Some of the studied complexes showed activity against bacteria and fungi. Keywords: Schiff bases; band gap; anti-fungal and anti-bacterial agents. INTRODUCTION Complexes of Schiff bases represent an important class of compounds. The enol form of Schiff bases of mono- and diamines have attracted much attention because of their advantages, such as ease of synthesis and the presence of variety of coordination sites that make them invaluable in coordination chemistry.1,2 Schiff base complexes have many applications, e.g., they are used as catalyst in oxygenation, decomposition, electro-reduction and hydrolysis reactions.3,4 Schiff bases of β-diketones stabilize different metals in various oxidation states in many useful catalytic transformations.5 Moreover, Schiff base complexes showed optical6 and magnetic properties7 that enabled them to be used as good candi- dates in modern technologies. Furthermore, Schiff-base complexes have found biological applications as antibacterial,8–11 anti-HIV12 and anti-inflammatory * Corresponding author. E-mail: nasserh56@yahoo.com doi: 10.2298/JSC150201058H 58 HOSNY, IBRAHIM and El-ASMY agents.13 The biological activity of Schiff bases comes from their ability to bind with transition metal ions in living systems.14,15 This work describes the synthesis and spectral, thermal, optical and bio- logical activities of Cu(II), Co(II), Ni(II) and Zn(II) complexes with (E)-4-[(2- hydroxyphenyl)imino]pentan-2-one with the aim of shedding more light on their structures and biological application. RESULTS AND DISCUSSION All the isolated complexes were colored and soluble in coordinating solvents such as dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) but insol- uble in H2O. The ligand can exist either in the keto (Fig. 1, A) or in the enol form (Fig. 1, B). Fig. 1. The keto (A) and enol (B) tautomeric forms of the ligand. IR spectra of the complexes The most important IR bands of the ligand and its complexes are given in the Supplementary material to this paper. The bands at 3435 and 3380 cm–1 in the spectrum of the ligand are attributed to phenolic and enolic (OH), respectively, while the bands due to (C=N), (CH=C), δ(OH), (C–O) and (C–N) appear at 1598, 1546, 1033, 1315 and 1238 cm–1 respectively.16 The absence of the band assigned to (C=O) of pen- tanedione and the appearance of a new band at 1546 cm–1 due to (HC=C) con- firmed that the free ligand exists in the enol form. The IR spectra of the metal complexes showed the disappearance of the enolic OH band, indicating the par- ticipation of this group in bonding after deprotonation. The shift of (C=N) to 1586–1615 cm–1 confirmed that the azomethine group coordinated to the metal ion. The spectra of Cu(II), Co(II) and Ni(II) complexes exhibited bands in the region 3440–3469 cm–1, assigned to the phenolic OH. Furthermore, these com- plexes showed two new bands assigned to as(COO–) and s(COO–) at 1534 and 1333 cm–1 in the Co(II) complex and at 1565 and 1375–1345 cm–1 in the Cu(II) and Ni(II) complexes, respectively. These bands did not appear in the spectrum of the Zn(II) complex. The difference between as(COO–) and s(COO–) bands indicates the bidentate nature in the Co(II) complex and monodentate in the SCHIFF BASE COMPLEXES 59 Cu(II) and Ni(II) complexes.17–20 The shift of the bands at 1315 and 1284 cm–1, due to the phenolic and enolic C–O, supports the participation of the phenolic and enolic oxygens in the bonding. The spectra showed new bands in the regions 530–540 and 426–435 cm–1, assigned to (M–O) and (M–N), respectively.21 These findings suggest the involvement of C=N, C–O and the phenolic hyd- roxyl groups in the coordination (Figs. 2–4).   N M O H3C CH3 OH nH2O xEtOH OAc .. Fig. 2. Suggested structure of the Cu(II) and Ni(II) complexes, n = 1.5 and x = 0 for the Cu(II), and n = 1, x = 1/2 for the Ni(II) complex. Fig. 3. Suggested structure for the Zn(II) complex.   N Co O H3C CH3 1/2 EtOH O O CH3 OH2 O H . Fig. 4. Suggested structure for the Co(II) complex. 1H-NMR spectra The 1H-NMR spectrum of the ligand in DMSO-d6 showed two singlet sig- nals at 2.34 and 2.50 ppm downfield from TMS, assigned to the protons of the two CH3 groups. In addition, another singlet signal at 5.20 ppm corresponded to the proton of the HC=C group.22 The multiplet signal in the region 6.75–7.18 ppm was assigned to the phenyl protons. The two singlet signals at 9.93 and 12.15 ppm were attributed to the phenolic and enolic OH protons, respectively.23 The presence of the enolic proton together with HC=C confirmed that the ligand exists in the enol form. 60 HOSNY, IBRAHIM and El-ASMY The 1H-NMR spectrum of the Zn(II) complex in DMSO-d6 showed two singlet signals at 2.23 and 2.43 ppm due to the protons of the two CH3 groups. The multiplet signal in the region 6.96–7.11 ppm corresponded to the phenyl protons. The spectrum showed another singlet signal at 7.6 ppm, assigned to the proton of the HC=C group. The shift of the latter signal in comparison with its position in the free ligand indicates the participation of the enolic group in the bonding. The disappearance of the enolic and phenolic OH protons confirmed the participation of these groups in the bonding. Mass spectra of the complexes The mass spectral fragmentation mode of H2L was investigated. It showed an intense molecular ion peak at m/z 191, corresponding to the formula C11H13NO2. A stable peak at 176 was due to the loss of CH3. The loss of hyd- roxyl group gives a peak at m/z 160. The latter peak loses C2H2 with rear- rangement to give the base peak at m/z 134. The base peak undergoes loss of CH3 to give a peak at m/z 120, which undergoes loss of CH, OH, OH and CH4 to give peaks at m/z 109, 93, 80 and 63, respectively, as indicated in Scheme S-1 of the Supplementary material to this paper. The MS of the Cu(II) complex exhibited the molecular ion peak at m/z 340, in good agreement with the formula [Cu(HL)(OAc)]·1.5H2O. Its fragmentation pattern is shown in Scheme S-2 of the Supplementary material. The mass spectrum of the Co(II) complex showed the molecular ion peak at m/z 350, corresponding to the formula [Co(HL)(OAc)(H2O)]·0.5EtOH. Its frag- mentation pattern is indicated in Scheme S-3 of the Supplementary material. The Ni(II) complex has a mass spectrum with the molecular ion peak at m/z 350, corresponding to [Ni(HL)(OAc)(H2O)]·0.5EtOH. The fragmentation pattern of this formula is illustrated in Scheme S-4 of the Supplementary material. Thermal gravimetric analysis Thermal gravimetric analysis gives useful data about the thermal stability of the metal complexes. The TG and DTG curves were recorded within the tem- perature range 25–800 °C. The TG curves for the complexes are given in Figs. S-1–S-4 and the results are tabulated in Table S-I of the Supplementary material to this paper. The results indicated that [Cu(HL)(OAc)]·1.5H2O decomposed in several steps. The first occurred in the range 26–133 °C, corresponding to the loss of 1.5 H2O (calcd. 7.9 %; found 8.0 %). The final step leaves CuO as a residue (calcd. residue 23.4 %; found 22.6 %). The TG curve of the Co(II) complex showed a decomposition step at 22–104 C, assigned to the loss of 0.5C2H5OH (calcd. 6.5 %; found 5.7 %). The last step could leave Co2O3 as the residue (calcd. residue 47.4 %; found 46.5 %). SCHIFF BASE COMPLEXES 61 The TG curve of the Ni(II) complex showed a decomposition step at 54–130 C due to the loss of 0.5C2H5OH (calcd. 6.5 ; found 6.0 %). The last step can generate NiO as the residue (calcd. residue 19.8 %; found 20.0 %). The TG curve of the Zn(II) complex showed a decomposition step at 26–200 C, assigned to the loss of coordinated water (calcd. 6.5 %; found 5.6 %). The third step could leave ZnO as the residue (calcd. residue 29.6 %; found 28.7 %). The intermediate steps for all four complexes appear to represent the decomposition of the ligand to different organic moieties.2,10 Electronic spectra and magnetic moments The electronic spectrum of the Cu(II) complex in DMF showed a broad band centered at 14005 cm–1, attributed to 2B1g  2A1g and 2B1g  2Eg transitions in a square-planar geometry.24 The band recorded at 26775 cm–1 was assigned to ligand to metal charge transfer (LMCT) (Table S-II of the Supplementary mat- erial). The magnetic moment (2.1 B) of the complex was in the range expected for monomeric Cu(II) complexes. The high value of the square–planar complex compared with those reported for the octahedral may be taken as supporting evidence for the presence of a square-planar geometry.25 The electronic spectrum of the Co(II) complex in DMSO showed three bands at 22222, 19379 and 15948 cm–1 due to LM charge-transfer, 4T1g(F)   4T1g(P) and 4T1g(F)  4A2g transitions, respectively. The ligand field para- meters (B = 878 cm–1,  = 0.90 and 10Dq = 8340 cm–1) correspond with those reported for Co(II) complexes with octahedral geometry.25,26 The value of the magnetic moment (5.2 B) supports the presence of octahedral geometry around the Co(II) ion (Table S-II). The spectrum of the Ni(II) complex, in DMSO, exhibits three bands at 15128, 23201 and 25000 cm–1 assigned to 3T1  3T1(P), 3T1  1T2 and LMCT transitions, respectively, in a tetrahedral geometry around Ni(II) ion.25 The value of magnetic moment (3.9 B) falls in the range reported for tetrahedral geometry around a Ni(II) ion.26 Optimized modeled structures of the complexes were proposed based on the previous analytical and spectral data. The experimental and theoretical electronic spectra of the optimized structures are collected in Table S-III. Selected bond lengths and angles are presented in Table S-IV of the Supplementary material. In case of Co(II) complex, the (N–M–O enolic) angle was 93.67°, which is slightly greater than is normal for an octahedral structure. The corresponding values for the Ni(II), Cu(II) and Zn(II) complexes were 102.59, 102.40 and 99.4°, respectively, deviate from the angle of normal tetra- hedral structures. The optimization was performed for single molecules and the interactions between neighboring molecules were not taken into consideration; this may be the cause of the deviations. 62 HOSNY, IBRAHIM and El-ASMY Optical band gap (Eg) The aim of measuring the conductive and semi-conductive properties of metal complexes is to reveal the feasibility of their potential use in molecular electronics. According to the conducting properties, metal complexes are applic- able in optical devices, organic transistors and sensors.27,28 To clarify the con- ductivity of the isolated complexes, the optical band gaps (Eg) of the Co(II), Ni(II), Cu(II) and Zn(II) complexes was determined from the absorption spectra of these complexes. The absorption coefficient (α) was determined from the relation: 1 ln A d   (1) where d is the path length of the sample. The optical band gap (Eg) was cal- culated from the relation:  g mh A h E    (2) where m is equal to 1/2 and 2 for direct and indirect transition, respectively, and A is an energy independent constant.29,30 A plot of (αhν)2 vs. hν was used to determine the direct band gap (Fig. 5) by extrapolating the linear portion of the curve to (αhν)2 = 0. The curves revealed that the values of the direct band gap (Eg) were equal to 3.37 (calcd. 4.2), 3.38 (calcd. 3.9), 3.74 (calcd. 3.6) and 3.58 (calcd. 6.0) eV for the Co(II), Ni(II), Cu(II) and Zn(II) complexes, respectively. The band gap values suggest that these complexes may behave as semi-con- ductors. In addition, the values of Eg are in the same range as those of highly Fig. 5. Optical band gap calculations for the isolated complexes. SCHIFF BASE COMPLEXES 63 efficient photovoltaic materials. The present compounds could be used in har- vesting solar radiation.31 Biological activity The biological activities of the isolated complexes were tested against two bacteria (Staphylococcus aureus NCMB 6571 and Vibrio cholerae TCBS) and two fungi (Aspergillus niger and A. flavus) by the disc diffusion method. The activities were measured as the diameter of inhibition zone (Table I). It is clear that, the four complexes had anti-bacterial activities. The highest anti-bacterial activity was registered for the Cu(II) complex. Only the Co(II) complex showed both antibacterial and antifungal activity and hence, it could be used as a wide spectrum antibiotic. The Zn(II) complex showed antifungal activity against A. niger only and hence, it could be considered as a selective antibiotic for this microorganism. The Cu(II) and Ni(II) complexes did not show any effect against the two fungi. TABLE I. Anti-microbial activity presented as the diameter of the inhibition zone, cm; –: negative Metabolite Complex with: Co Ni Cu Zn S. aureus 0.7 0.75 0.9 0.85 V. cholerae 1.0 1.0 1.1 0.7 A. niger 2.9 – – 3.0 A. flavus 1.75 – – – EXPERIMENTAL Reagents All the employed chemicals were of analytical grade and were used without further purification. Equipment and measurements The carbon and hydrogen contents were determined using a CHN analyzer (Perkin– –Elmer model 2400). Infrared spectra were measured as KBr discs on a Mattson 5000 FTIR spectrometer. Electronic spectra were recorded on a UV2 Unicam UV–Vis spectrometer using 1 cm silica cells. Thermogravimetric measurements (TG) were performed on a Shimadzu model 50 H instrument with a nitrogen flow rate and heating rate of 20 cm3 min-1 and 10 °C min-1, respectively. The 1H-NMR spectra were recorded on a Bruker Avance III 400 MHz instrument using TMS as an internal reference and DMSO-d6 as the solvent. The physical, analytical and spectral data for the ligand and its complexes are listed in the Supplementary material to this paper. Synthesis of (E)-4-[(2-hydroxyphenyl)imino]pentan-2-one (H2L) 2,4-Pentanedione (0.01 mol, 1.4 mL) was added dropwise to 0.01 mol (1.19 g) of a hot ethanolic solution of 2-hydroxyaniline. The reaction mixture was heated under reflux for 6 h. 64 HOSNY, IBRAHIM and El-ASMY A brown crystalline precipitate formed on cooling. The precipitate was filtered off and washed with ethanol and diethyl ether. Synthesis of metal complexes A general procedure was followed for the synthesis of all complexes. An aqueous solution of 0.01 mol of metal acetates was added to a hot ethanolic solution of 0.01 mol of (E)-4-[(2-hydroxyphenyl)imino]pentan-2-one (H2L) and refluxed for 8 h. The precipitated metal complexes were filtered off, washed with ethanol, and preserved over anhydrous cal- cium chloride. Computational details Molecular mechanics and the semi-empirical methods in the Hyperchem series of prog- rams were used to investigate the geometries of the complexes.32 AM1 and PM3 methods were used to optimize the molecular geometries of complexes. The low lying conformers obtained by the molecular mechanics technique (MM+ force field) were then optimized at AM1, PM3 (Polak–Ribiere) RMS 0.01 kcal.33 Antimicrobial assay The microbial strains were selected based on their clinical importance in causing disease in humans. Two bacteria (S. aureus NCMB 6571 and V. cholerae) and two fungi (A. niger and A. flavus), ear pathogens isolated from a clinical culture and a laboratory culture of Suez Canal University Center for Environmental Studies and Consultations were used for the evalu- ation of the antimicrobial activity of the synthesized compounds. The bacteria were sub- cultured at 37 °C for 6 h on nutrient agar, whereas the fungi were subcultured on Sabouraud dextrose agar (subcultured at 28 C for 4–7 days on malt agar). Samples (20 μL) of the tested compound at a concentration of 50 mg 10 mL-1 in 0.7 cm holes were incubated against the strains at 37 °C for 48 h using the “disc diffusion method”.34 CONCLUSIONS New biologically active metal complexes of Cu(II), Co(II), Ni(II) and Zn(II) with (E)-4-[(2-hydroxyphenyl)imino]pentan-2-one (H2L) were synthesized and characterized by different physicochemical techniques. Optical band gap mea- surements indicated that the isolated complexes are semi-conductors. The optical band gap values showed that the compounds could be used as harvesting mat- erials of the sun radiation in the UV and visible regions. The anti-fungal and anti- -bacterial tests showed that the synthesized complexes are biologically active against both bacteria and fungi. One of the investigated compounds could be used as a broad-spectrum antibiotic. SUPPLEMENTARY MATERIAL Physical, analytical and spectral data for the ligand and its complexes, MS fragmentation patterns, thermogravimetry results, magnetic moments, electronic bands and ligand field parameters, experimental and calculated electronic spectra and selected bond lengths and angles for the optimized ligand and its metal complexes are available electronically from http://www.shd.org.rs/JSCS/, or from the corresponding author on request. SCHIFF BASE COMPLEXES 65 И З В О Д СПЕКТРАЛНА, ТЕРМАЛНА, ОПТИЧКА И БИОЛОШКА ИСПИТИВАЊА (E)-4-[(2-ХИДРОКСИФЕНИЛ)ИМИНО]ПЕНТАН-2-ОНА И ЊЕГОВИХ КОМПЛЕКСА СА НЕКИМ ЈОНИМА ПРЕЛАЗНИХ МЕТАЛА NASSER M. HOSNY1, REYAD IBRAHIM и AHMED A. EL-ASMY 1 Chemistry Department, Faculty of Science, Port Said University, Port Said, Egypt, 2 Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt и 3 Chemistry Department, Faculty of Science, Kuwait University, Kuwait Комплекси прелазних метала, синтетисани у реакцији између Cu(II)-, Co(II)-, Ni(II)- и Zn(II)-ацетата и (E)-4-[(2-хидроксифенил)имино]пентан-2-она (H2L) као ли- ганда, окарактерисани су помоћу елементалне микроанализе, MS, IR, UV–Vis и 1H-NMR спектроскопије, термалне анализе (TG и DTG) и магнетних мерења. У испитиваним комплексима, осим у случају Zn(II) јона, наведена Шифова база се тридентатно (NOO) координује преко азометинског атома азота, хидроксилног атома кисеоника и енолног атома кисеоника из карбонилне групе. Нађено је да Cu(II) комплекс има квадратно-пла- нарну геометрију, док је геометрија Co(II) комплекса октаедарска. За испитиване ком- плексе Ni(II) и Zn(II) нађено је да имају тетраедарску геометрију. Испитивана је био- лошка активност комплекса на две врсте бактерија и гљивица. Нађено је да неки од испитиваних комплекса имају одређену активност према бактеријама и гљивицама. (Примљено 1. фебруара, ревидирано 24. априла, прихваћено 7. јула 2015) REFERENCES 1. X. Pang, X. Chen, H. Du, X. Wang, X. Jing, J. Organomet. Chem. 692 (2007) 5605 2. N. M. Hosny, M. A. Hussien, F. M. Radwan, N. Nawar, Spectrochim. Acta, A 132 (2014) 121 3. N. Sohrabi, N. Rasouli, M. Kamkar, Bull. Korean Chem. Soc. 35 (2014) 2523 4. S. Sathiyaraj, K. Sampath, C. Jayabalakrishnan, Synth. React. Inorg., Met.-Org., Nano- -Met. Chem. 44 (2014) 1261 5. S. Kumar, D. N. Dhar, P. N. Saxena, J. Sci. Ind. Res. 68 (2009) 181 6. R. A. Sheldon, J. K. 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