Synthesis , Spectral Characterization , and Thermal and Cytotoxicity Studies of Cr ( III ) , Ru ( III ) , Mn ( II ) , Co ( II ) , Ni ( II ) , Cu ( II ) , and Zn ( II ) Complexes of Schiff Base Derived from 5-Hydroxymethylfuran-2-carbaldehyde

1Department of Chemistry, College of Science, Princess Nourah Bint Abdulrahman University, Riyadh, Saudi Arabia 2Department of Chemistry, Faculty of Science, Jazan University, Jazan, Saudi Arabia 3Department of Chemistry, Faculty of Science (Boys), Al-Azhar University, Cairo, Egypt 4Department of Chemistry, College of Sciences Al ImamMohammad Ibn Saud Islamic University (IMSIU), Riyadh 11623, Saudi Arabia 5Department of Chemistry, Faculty of Science (Girls), Al-Azhar University, Cairo, Egypt 6Department of Botany and Microbiology, Faculty of Science, King Saud University, Riyadh 11541, Saudi Arabia


Introduction
Transition metal complexes derived from the Schiff base ligands with biological potency have been broadly studied.Schiff bases appear to be an important intermediate in a number of enzymatic reactions including interaction of an enzyme with an amino or a carbonyl group of the substrate.The biochemical process which involves the condensation of a primary amine in an enzyme may be one of the most important catalytic mechanisms kinds [1].Transition metal complexes with different oxidation states have a strong role in bioinorganic chemistry and may give the models basis for active sites of biological systems [2][3][4].For a long time, it had been identified as a serious cofactor in biological compounds, either as a compositional template in protein folding or as a Lewis acid catalyst which can readily adopt the coordination numbers 4, 5, or 6 [5,6].Coordination chemistry of cobalt complexes was a substantial importance as a result of the toxic environmental effect of cobalt.The cobalt mobilization and immobilization in the climate, organisms, and approximately technical systems (such as in ligand exchange chromatography) have been presented manifestly to depend on the complexation of the metal center by chelating nitrogen donor ligands [7].The size and shape of the nanomaterials are considered the key factors for shaping their characteristics such as electrical, optical, magnetic, antimicrobial, and catalytic potency.Metal and metal oxide nanoparticles have found a wide range of uses, including heterogeneous catalysts, environmental remediation, electronics, chemical sensing devices, medicinal fields, separations, thin films, inks, disinfection, and antimicrobial activity [8].These different applications altered with morphology and size of those metal and metal oxide nanoparticles [9].Detection of metal ions of biological importance has attracted much attention.Like Zn 2+ ion fluorescent probes or sensors have achieved special interest.Zn 2+ is an essential trace element and the second (after Cu 2+ ) most abundant metal ion in humans [10,11].In our present work, novel [M(MFMAQ)Cl 2 ] (M = Cr(III) and Ru(III)) and [M(MFMAQ)Cl(H 2 O)]⋅H 2 O (M = Mn(II);  = 1, Co(II);  = 0, Ni(II);  = 2, Cu(II);  = 0 and Zn(II);  = 2.5) complexes were discussed using different studding techniques such as elemental analyses, molar conductance, magnetic moment, and UV-Vis., IR, NMR ( 1 H, 13 C and 15 N), mass, EPR, XRD, SEM, TEM, EDX, and TGA behavior.Also, the ligand (H-MFMAQ) and its complexes show remarkable cytotoxicity against human breast (MCF-7) and lung cancer (A549) cell lines.

Experimental
2.1.Materials and Methods.All the metal salts were gained from E-Merck and used without further purification.Solvents MeOH, EtOH, DMF, DMSO, and agar were procured from Hi-media chemicals.However, the solvents were purified by the standard procedures.Elemental analyses were performed on a Perkin Elmer 2400 CH/N Analyzer.Metal contents were determined complexometrically by standard EDTA titration and the Cl was tested gravimetrically using AgNO 3 .Electronic spectra of the complexes were recorded on a Shimadzu Model 1601 UV-Visible Spectrophotometer.Infrared spectroscopy measurements were performed on an Agilent Cary 630 FTIR spectrometer, using the Attenuated Total Reflectance (ATR) method, with a diamond cell.Spectra were recorded from 4000 to 400 cm -1 , with 64 scans and resolution of 4 cm -1 .The 1 H and { 15 N, 1 H} correlation NMR spectra of samples were recorded in a Bruker Avance III 500 MHz (11.7 T) spectrometer.The 1 H NMR spectrum of samples was recorded in a Bruker Avance III 600 MHz (14 T) spectrometer.The { 15 N, 1 H} NMR correlation spectrum of samples was recorded in a Bruker Avance III 400 MHz (9.4 T) spectrometer.The samples were analyzed in a d6-DMSO solution and the chemical shifts were given relative to tetramethylsilane (TMS).The 13 C and 15 N solid state NMR (SSNMR) spectra were recorded in a Bruker 300 MHz spectrometer, using the combination of cross-polarization, proton decoupling, and magic angle spinning (CP/MAS) at 10 kHz.Electrospray ionization mass spectrometry (ESI-MS) measurements were carried out using a Waters Quattro Micro API.Samples were evaluated in the positive mode.Ligand was analyzed in a 1 : 1 methanol : water solution with addition of 0.10% (v/v) formic acid; Cu(II) complex was analyzed in a 1 : 1 acetonitrile : water with 0.10% (v/v) formic acid; Co(II) and Ru(III) complexes were dissolved in a minimum amount of DMF and further dissolved in a 1 : 1 methanol : water solution with addition of 0.10% (v/v) formic acid immediately before the experiments.Each solution was directly infused into the instrument's ESI source and analyzed in the positive mode, with capillary potential of 3.00 kV, trap potential of 2 kV, source temperature of 150 ∘ C, and nitrogen gas for desolvation.Room temperature magnetic susceptibility measurements were carried out on a modified Gouy-type magnetic balance, Hertz SG8-5HJ.The room temperature molar conductivity of the complexes in MeOH, EtOH, and DMSO solutions (0,001 mol⋅L −1 ) was measured using a deep vision 601 model digital conductometer.The X-band EPR spectrum was performed at LNT (77 K) using TCNE as the g-marker.Powder X-ray diffraction patterns were recorded with a X'Pert PRO Diffractometer using CuK 1 radiation ( = 1.54060Å) with operating voltage 40 kV and a current of 30 mA.Thermal studies were carried out using Q 600SDT and Q 20 DSC thermal analyzer.SEM images were recorded in a Hitachi SEM analyzer and transmission electron microscopy (TEM) images were taken by Zeiss-EM10C-100 KV.Energy Dispersive X-Ray Analysis (EDX) (EDAX Falcon System) was conducted to analyze the presence of elements in the specimens that have been sputtered with carbon black.The cytotoxic activity of the inspected free ligand and complexes (1-7) was studied towards human breast (MCF-7) and lung cancer (A549) cell lines at the Regional Center for Microbiology and Biotechnology, Al-Azhar University as well as the other biological activities.

Synthesis of Schiff Base Ligand (H-MFMAQ).
New Schiff base ligand (H-MFMAQ) (Scheme 1) was prepared when   3(a)) does not show any band at 1700 cm -1 , 3380 cm -1 , and 3250 cm -1 corresponding to the carbonyl groups and free primary amino groups; this confirms the complete condensation between keto and amino groups.The band at 1637 cm -1 is corresponding to ] (CH=N) stretching vibration.The IR spectrum of furan Schiff base ligand (H-MFMAQ) displays a broad band at 3483 cm -1 , which can be assigned to ](OH) group.The   DMF, and DMSO are in the range 7.58-15.24Ω -1 cm 2 mol -1 indicating the nonelectrolytic nature of all complexes [12].This finding is consistent with the infrared spectral data that showed the coordinated nature of chloride anions.

Electronic Absorption, Magnetic Measurements, and
Ligand Field Parameter.The electronic absorption spectra of the free Schiff base ligand and its complexes (Cr(III), Ru(III), Mn(II), Co(II), Ni(II), Cu(II), and Zn(II)) were studied in Nujol mull.
where  (free ion) for Co(II) is 996 cm −1 .The 10, , and  values are 7032 cm −1 , 698 cm −1 , and 0.68, respectively.These results show that the interelectronic repulsion of d-electrons in a complex is less than in the free ion.The value of  in a complex is 78% of the free ion value.The  value is related directly to covalence.The reduction of  is caused by complex formation by the delocalization of the d-electron cloud on the ligand, which in turn causes the covalent bond formation.The data shows the Co(II) complex has covalent character in the metal ligand "" bond [15].

Infrared Spectra of Complexes.
The study of infrared spectra of the furan Schiff base H-MFMAQ comparing to their metal complexes (1-7) (Table 4; Figure 3) foremost revealed that the ligand is tetradentately coordinated to the metal ions.The bands were symbolized at 1712 and 1637 cm -1 due to the carbonyl and azomethine stretching vibration which was shifted to lower frequency by 14-19 and 12-16 cm -1 , suggesting oxygen carbonyl and nitrogen azomethine involvements in complexity.At 1618 cm -1 , the band was assigned to the furan ring ](C-O-C) vibrations which is also shifted to lower frequency by 13-18 cm -1 , which is suggestive to involvement of the furan ring in chelation.Also, at 3483 cm -1 , the band was attributed to ]OH in the ligand (H-MFMAQ) which disappeared in its metal complexes indicating deprotonation of the OH moiety during coordination [17].The new bands at 543-552, 433-438, and 412-416 cm −1 were assigned to M-O (carbonyl), M-O (phenol), M-N (azomethine) and (M-Cl) in the metal complexes spectra were observed [15,17].The IR results showed that the metal was harmonized through one nitrogen atom (azomethine group) and three oxygen atoms (deprotonated hydroxyl group, carbonyl group, and furan ring) besides chlorine atoms.

NMR Spectra Investigation.
The proton NMR spectroscopy of H-MFMAQ ligand (Figure 4(a)) and its [Zn(MFMAQ)Cl(H 2 O)]2.5H 2 O complex (Figure 4(b)) was evident in DMSO-d 6 solution employing tetramethylsilane (TMS) as internal standard.OH signal was found at 11.35 ppm in the spectrum of the ligand H-MFMAQ which completely disappeared in the spectrum of the [Zn(MFMAQ)Cl(H 2 O)]2.5H 2 O complex.This suggests the sharing of the OH group in chelation with Zn(II) through isolation of the OH proton.The azomethine proton signal was shifted to high field in the spectrum of [Zn(MFMAQ)Cl(H 2 O)]2.5H 2 O complex.It was looked at 8.13 ppm as compared to 8.67 ppm in the Schiff's base H-MFMAQ.This refers to the complexity of the zinc atom through nitrogen atom azomethine.However, multiple bands assigned to the aromatic protons were found at 6.92-7.97 and 6.98-7.96ppm in the free Schiff 22 base ligand and Zn(II) complex, respectively.The signal observed at 3.2 ppm with an integration corresponding to seven protons in the case of Zn(II) complex was assigned one coordinate water molecule and half past two hydrate water molecules.
In 13 C NMR of ligand (Figure 5(a)) (-C=O) carbonyl carbon showed signal at 166.47 ppm, (-C=N-) azomethine carbon at 162.12 ppm, (-C-O) phenolic group carbon at 158.27 ppm, and (C-O) furan ring at 170.12.The signals due  and N 1 (quinoline), respectively.In the spectrum of Zn(II) complex only one broad signal is observed for the nitrogen atoms at 162.7 ppm.This behavior led us to assign the broad signal in the spectra of the complexes as both N 12 and N 1 atoms.With this assignment, N 12 shifted 81.6 ppm downfield upon coordination to Zn(II).On the other hand, the nitrogen N 1 shifts only 0.6 ppm upon coordination with Zn(II).This minor shift indicates that this atom is not involved in coordination as already evidenced by other techniques.

ESR Spectra.
The ESR spectrum of the [Cr(MFMAQ)Cl 2 ] complex (Table 5) has been recorded as polycrystalline sample at room temperature.No hyperfine interaction was observed in the ESR spectra of the [Cr(MFMAQ)Cl 2 ] complex at room temperature.The -values are calculated by using the expression,  = 2.0023(1 − 4/10), where  is the spin-orbit coupling constant for the metal ion in the complex.Owen [18] gives the reduction of the spin-orbit coupling constant from the free ion value; 90 cm −1 for chromium(III) can be employed as a measure of metal ligand covalency (Figure 7(a)).It is possible to define a covalency parameter analogues to the Nephelauxetic parameter which is the ratio of the spin-orbit coupling constant for the complex and the free Cr(III) ions.
The solid state ESR spectrum of [Cu(MFMAQ)Cl] complex (Figure 7(b)) is displayed at room temperature.The shape of the spectrum is consistent with octahedral environment around Cu(II) ion and the higher  value for the investigated [Cu(MFMAQ)Cl] complex (Table 5), when compared to that of free electron ( = 2.24) revealing an appreciable covalency of metal ligand bonding with  2 - 2 as the ground-state characteristic of octahedral stereochemistry [19].Also, the ‖/‖ value 143 for the [Cu(MFMAQ)Cl] complex lies just within the range expected for octahedral complex [19].The decrease of the -value by 9 compared to that of the free-electron value (2.07) is an approximate measure of the ligand field strength; the stronger the furan Schiff 's base ligand field, the smaller the decrease in the  value and vice versa.)] complexes, respectively.The consistency and resemblance in between the particle forms of synthesized dimeric complexes suggest that structural phases have a similar template.The particles diameter is found in nano range as follows: Cr(II), 62-224 nm; Ni(II), 18-23 nm; Cu(II), 12-32 nm.Nanoparticle-size complexes may act strong in different application areas in a biological one.

Thermal Decomposition.
To evaluate the thermal behavior of metal complexes, the thermogravimetric (TG/ DTG) curves for the complexes are represented in Figure 11 and weight loss at different decomposition stages, temperatures ranges with DTG peaks, assignments, and the final pyrolysis product observed in the present studies are summarized in Table 7.
In TGA curve of the [Cr(MFMAQ)Cl 2 ] complex does not show any weight loss up to 385 ∘ C; this indicates the absence of lattice and coordinated water molecules in it.The [Cr(MFMAQ)Cl 2 ] complex shows two-step decomposition process in the range of 372-423 ∘ C and 473-798 ∘ C, respectively; the first step is relatively fast and a total of 26.84% weight loss has been observed which corresponds to the nonchelated part of the ligand moiety.The remaining chelated part of the ligand is decomposed in the second step which is 39.64% (calc.38.46%).The final pyrolysis product, projected as chromium(III) oxide, and some unreacted residue had an observed mass of 33.52% (calc.33.34%).
The TG curve of the [Ni(MFMAQ)Cl(H 2 O)]⋅2H 2 O complex shows the three-step decomposition process; in the first step 4.52% (calc.4.23%) weight loss has been observed in the range of 92-124 ∘ C, which indicates the presence of one lattice water molecule in the complex.Another 4.86% (calc.4.23%) weight loss corresponds to one coordinated water molecule which has been observed in the range of 124-173 ∘ C. In the second step of decomposition starting from 313 ∘ C to 442 ∘ C, nonchelated part of the ligand is eliminated which has been found to be 24.82%(calc.26.04%).In the last step starting from 462 ∘ C to the final temperature, 46.84% (calc.46.28%) weight loss corresponds to the remaining part of the ligand which has been noticed and the ultimate pyrolysis product is obtained as nickel(II) oxide of mass 18.96% (calc.18.03%).
Quite similar results have been recorded for [Cu(MFMAQ)Cl(H 2 O)] where decomposition takes place in three steps in the range of 192-273 ∘ C, 293-484 ∘ C, and 506-798 ∘ C, respectively, and copper (II) oxide; mass of 21.19% (calc.19.78%) seems to be the final product.The only difference is that no lattice water molecule is presented in [Cu(MFMAQ)Cl(H 2 O)] complex.On the basis of TG analysis the increasing order of the stability of complexes is as follows: . Thus, the Co(II) complex shows excellent thermal stability in all the complexes when subjected to higher temperature; hence it is used in such types of applications.

Structure of the Complexes.
Based on the above studies, the following structure (Scheme 6) may suggest these complexes.
3.2.9.Cytotoxicity Studies.The anticancer activities of the H-MFMAQ Schiff base ligand and its metal(II/III) complexes against the human breast (MCF-7) and lung cancer (A549) cell lines were screened using MTT assay.The results were analyzed by cell viability curves and expressed as IC 50 values.The maximal inhibition concentrations (IC 50 ) given in Table 8 showed that the cytotoxicity efficiencies of the compounds under investigation follow the order: Cr(III)  complex > Ru(III) complex > H-MFMAQ > Mn(II) complex > Co(II) complex > Ni(II) complex > Cu(II) complex > Zn(II) complex.From the results, it is evident that the Cr(III) and Ru(III) complexes exhibited higher in vitro cytotoxicity against both the selected cell lines when compared to the Schiff base ligand.Also, the cytotoxicity efficiency of the Cr(III) and Ru(III) complexes is comparable with that of the standard drug, cis-platin, while the Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes showed lower anticancer activities when compared to that of the ligand.The cytotoxicity of metal complexes is depending on their ability to bind DNA and damage its structure resulting in the impairment of its function, which is followed by replication and transcription processes inhibition and eventually cell death that is what we can suppose [22][23][24][25][26]. Thus, the relatively higher cytotoxicity exhibited by the Cr(III) and Ru(III) complexes may be due to the relatively stronger binding ability of the complexes with DNA as shown in the DNA binding studies.
Complexes.The ESI mass spectrum of the Co(II) complex [Co(MFMAQ)Cl(H 2 O)] (Figure 1(b)) shows the parental ion peak at / = 379 corresponding to (CoC 15 H 13 ClN 2 O 4 ) + .The other fragments of the complex give the peak with various intensities at different / values like at 74 (CoO) + , 77 (C 6 H 5 ) + , 144 (CoC 4 H 7 NO) + , and 326 (CoC 15 H 11 N 2 O 3 ) + .This schematic mass spectral fragmentation pattern of ligand is consistent with its structure which is depicted in Scheme 3. The ESI mass spectrum of the Cu(II) complex [Cu(MFMAQ)Cl(H 2 O)] (Figure 1(c)) shows the parental ion peak at / = 384 corresponding to (CoC 15 H 13 ClN 2 O 4 ) + .The different fragments of the complex with different / values are present at 97 (C 5 H 7 NO) + , 120 (C 7 H 8 N 2 ) + , 202 (CuC 6 H 7 N 2 O 2 ) + , and 326 (CuC 13 H 11 ClN 2 O 2 ).This schematic mass spectral fragmentation pattern of ligand is consistent with its structure which is depicted in Scheme 4. The ESI mass spectrum of the Ru(III) complex [Ru(MFMAQ)Cl 2 ] (Figure 1(d)) shows the molecular ion peak at / = 439 corresponding to (RuC 15 H 11 Cl 2 N 2 O 3 )+.The other fragments of the complex show the peaks at different / values like at 56 (C 4 H 8 ) + , 210 (RuC 6 H 7 NO) + , 158 (C 10 H 10 N 2 ) + , 263 (RuC 8 H 6 N 2 O 2 ) + , and 326 (RuC 11 H 14 N 2 OCl) + .This schematic mass spectral fragmentation pattern of ligand is consistent with its structure which is depicted in Scheme 5.These facts were matched to the suggested molecular formula for these complexes, that is, [Co(MFMAQ)Cl(H 2 O)], [Cu(MFMAQ)Cl(H 2 O)], and [Ru(MFMAQ)Cl 2 ], where MFMAQ is the ligand.This confirms the Schiff base frame formation.Elemental analysis values were appropriate with those values calculated from the molecular formulae assigned to these complexes which are further supported by mass studies.
to (-C=N-) azomethine, (-C-O) phenolic, and (C-O) furan carbons were slightly shifted downfield in comparison to the corresponding signals of these groups in the ligand thereby confirming the complexation (Figure 5(b)) with zinc metal ion.The 15 N NMR spectra of the free ligand (Figure 6(a)) and the Zn(II) complex (Figure 6(b)) were also obtained.The spectrum of the ligand shows two signals centered at 244.3 and 163.3 ppm, which were assigned to N 12 (azomethine)

Table 3 :
Ligand field parameters of the complexes.

Table 4 :
IR spectral data (cm −1 ) of the H-MFMAQ ligand and its metal complexes.

Table 5 :
The spin-Hamiltonian parameters of Cr(III) and Cu(II) Schiff base complexes in DMSO at 300 and 77 K.

Table 7 :
Thermal decomposition steps of the Cr(III), Ni(II), and Cu(II) complexes in TG plots.