Veyan Taher Suleman1* and Abdul Ghany Mohammed Al-Daher2

1Department of Chemistry, College of Science, University of Dohuk, Dohuk, Iraq.

2Al-Noor university college, Mosul, Iraq.

Corresponding Author E-mail: veyan.suleman@uod.ac

Article Publishing History
Article Received on : 08 Sep 2023
Article Accepted on :
Article Published : 06 Dec 2023

ABSTRACT:

Novel hydrazone ligands (HL) derived from 2,5-hexandione and benzoyl hydrazine (HDBH), 2-furoyl hydrazine (HDFH), or 2-picolinoyl hydrazine (HDPH) were used to synthesize mononuclear Mn(II), Co(II), Ni(II), Cu(II), and Zn(II) complexes of the general formula [M(L)2].xH2O (x= 0-3). The hydrazone ligands and the isolated complexes were characterized by, spectral, thermal analysis, molar conductance, and magnetic moment measurements. Beside, density functional theory (DFT) computations have been to study structures and energetic of the ligand and its complexes. The IR spectral information suggests that the ligands exhibit monobasic tridentate behavior through their ONO donor atoms. The metal complexes were determined to possess monomeric octahedral geometry. The DPPH antioxidant capability of both the ligand and their metal complexes was evaluated.

KEYWORDS:

Antioxidant activity; DFT calculation; Hydrazone Complexes

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Suleman V. T, Al-Daher A. G. M. Synthesis, Characterization, Density Functional Theory Studies and Antioxidant Activity of Novel Hydrazone Ligands and their Metal Complexes. Orient J Chem 2023;39(6).

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Suleman V. T, Al-Daher A. G. M. Synthesis, Characterization, Density Functional Theory Studies and Antioxidant Activity of Novel Hydrazone Ligands and their Metal Complexes. Orient J Chem 2023;39(6). Available from: https://bit.ly/4a9lId0

Intruduction

In general, hydrazones are created via a condensation process involving an acid hydrazide and an aldehyde or ketone, conducted within an alcoholic solvent1. Various aspects, including the pH of the environment, substituent characteristics, and the positioning of the hydrazone group concerning other functional groups, impact the coordination tendencies of hydrazones. Furthermore, the removal of a proton from the NH group, a phenomenon possible in the ligand complexes, is produced in the creation of tautomeric anionic entities (=N-N−C=O or =N-N=C-O−), which exhibit unique coordination characteristics2

In recent times, there has been an increasing fascination with investigating hydrazones possessing donating properties. This surge in interest stems from research indicating that their pharmacological effects are linked to their capacity to create chelates with essential metal ions in biological systems. Consequently, a multitude of hydrazones and their complexes were identified to exhibit antibacterial, antifungal, antiviral, ant oxidative, and antitumor activities. The coordination geometry, the nature of donor atoms existing in the ligands, the sort of metallic ion, and its oxidation state all have a significant role in influencing the biological outcomes of these compounds3.

In view of the above mentioned significant importance of hydrazone metal complexes in various fields and on continuation of our studies of hydrazone metal complexes4, we were motivated to synthesis Co(II), Ni(II), Cu(II) and Zn(II) Complexes with novel aroyl hydrazone ligands (Scheme 1) and to describe their structural and antioxidant properties.

Experimental

Equipments

The CHN analysis was carried out at the microanalytical unite elemental analyzer and mass spectroscopy MSDCHEM of the University of Iran. Perkin-Elmar (AA500G) atomic absorption spectrophotometer was used to obtain metal content. FT-IR spectra were obtained for KBr discs containing the organic ligands and their complexes in the 4000-400 cm-1 range using a Perkin-Elmer FT-IR 660 spectrophotometer. The 1H and 13C NMR spectra of the organic ligands were collected using a Bruker Advance II 400 NMR spectrometer. The measurements were carried out in DMSO-d6 or CDCl3 solvents, utilizing a superconducting NMR spectrometer operating at frequencies ranging from 13000 to 64 MHz. The internal standard used was TMS. The UV-visible absorption spectra were measured within a quartz cell with a diameter of 1cm and a concentration of 10–3 M in dimethylsulfoxide within a range of 1100–200nm via a Unicam HEIOS UV-VIS spectrophotometer. Magnetic susceptibility readings were obtained using a Johnson Matthey MSB/AUTO balance. Conductivity assessments for complexes were measured at a concentration of 10–3 M in methanol and DMSO and at a temperature of 25 ᵒC, employing a conductivity meter marked as model 430 PH. DFT calculations were performed utilizing the B3LYP/6-311++G(d,p) methodology. Specifically  in a gas-phase environment. The thermal analyses were performed at department of chemistry, college of basic education , Mosul university using a METTLER TOLEDO TGA\DSC instrument, along with STARe evaluation software version 16.3. The measurements were taken in the temperature range of 25 to 600°C, employing a heating ramp rate of 5°C/min.

Synthetic Methods     

Synthesis of hydrazone ligands

Hydrazide compounds BH, FH, and PH were prepared by refluxing Ethylbenzoate (15g, 0.1 mole) for (BH), Ethyl-2-furate (14.0 g, 0.1 mole) for (FH), or Ethylpicolenate (15.1 g, 0.1 mole) for (PH) in 25 mL of ethanol with a slight excess of hydrazine hydrate NH2NH2.H2O (6.0 g, 0.12 mole)  for 5 hours. The compounds crystallize when left overnight in a freezer, after which they are filtered and subsequently washed with ethanol and ether. The pure hydrazides were obtained through recrystallization from hot ethano5.

The hydrazone ligands HDBH, HDFH, and HDPH were prepared by a condensation reaction between an ethanolic solution (50 mL) of 2,5-hexandione (1.14 g, 0.01 mole) and acid hydrazide (BH, FH, PH) (0.01 mole) in a 1:1 molar ratio. The mixture was magnetically stirred and then refluxed in a water bath for 5 hours. After evaporating the liquid left, a white hydrazone precipitate was obtained. The resulting solid was filtered, rinsed softly with cold ethanol, and dried using a vacuum desiccator containing anhydrous calcium chloride. The substance was then recrystallized from chloroform, and the reactions were verified by employing thin-layer chromatography (TLC) on silica gel sheets (Scheme-1-)

Synthesis of complexes

The compounds were produced by adding 0.23 g (0.002 mole) of triethylammine to a hot solution containing each of the ligands (HDBH, HDFH, or HBPH) (0.002 mole) in 25 mL of methanol .The color of the solution became yellow. A solution containing 0.001 mole of metal acetate hydrate in 25 mL of methanol, maintaining a molar ratio of (1:2) metal:ligand , was then added. The reaction mixtures were then refluxed for 4 hours. Afterward, the volume of the mother liquor was reduced by half and cooled. The obtained solid was filtered, washed repeatedly with methanol and ether, and then dried using a vacuum desiccator along with anhydrous calcium chloride.

Computational methods

DFT calculations were carried out using Gaussian 09 software, utilizing the B3LYP/6-311++G(d,p) approach in a gas-phase context2. These calculations were executed on the refined structures of the generated compounds to evaluate their structural characteristics and stability. This analysis included examining thermodynamic properties, bond lengths, angles, and the characteristics of frontier molecular orbitals.

Antioxidant activity

The free radical scavenging effect was estimated by treating the DPPH radical with a methanolic solution of the tested compounds. Freshly prepared methanolic solution of 2,2-diphenyl-1-picrylhydrazyl (0.5 mmol), giving a violet solution that changes to a faint yellow color when combined with the various concentrations of the investigated substances (20, 40, and 60 ppm). The reaction mixture will be left to incubate in darkness at room temperature for 30 minutes. The absorbance at 517 nm will be gauged using a UV-Vis spectrometer to gauge the effectiveness of DPPH radical scavenging. The percentage inhibition will be computed using the subsequent formula6:

DPPH scavenging ability (%) = (Abs control – Abs sample /Abs control) * 100

Where:  Abs control = Absorbance of the DPPH radical + methanol.

Abs sample = Absorbance of the tested sample with DPPH after 30 minutes.

Table 1:  Physical and elemental analysis results of the ligands and their metal complexes.

No

Comp.

color

M.P. ˚C

Yield%

Ω ohm1cm2mol-1

Elemental analysis% cal./found

 

Me OH

DMSO

C%

H%

N%

M%

HL1

HDBH C13H16O2N2

White

176-178

86.0

—-

—-

67.24/67.20

6.89/6.43

12.06/12.00

      —

1

[Mn(HDBH-H)2].H2O

Reddish-brown

156-157

71.0

17.2

7.4

58.32/58.01

5.98/5.56

10.46/10.01

10.26/10.23

2

[Co(HDBH-H)2].3H2O

Dark- brown

164-166

74.4

13.1

6.7

54.27/54.07

6.26/6.12

9.74/9.04

10.24/10.10

3

[Ni(HDBH-H)2]

Olive green

246-250

77.6

15.2

8.9

59.91/59.33

5.76/5.21

10.75/10.54

11.27/11.02

4

[Cu(HDBH-H)2].H2O

Black-brown

254-256

78.0

11.7

11.4

57.40/57.11

5.88/5.18

10.30/10.21

11.68/11.12

5

[Zn(HDBH-H)2].H2O

Pale yellow

208-210

66.6

17.7

13.0

57.21/57.06

5.86/5.33

10.26/10.17

11.97/11.64

HL2

HDFH C11H14O3N2

White

218-220

85.0

—

—

59.45/59.11

6.30/6.10

12.61/12.23

       —

6

[Mn(HDFH-H)2]

Black

238-240

79.0

9.8

8.6

53.12/53.09

5.23/5.20

11.26/11.45

11.04/11.0

7

[Co(HDFH-H)2].2H2O

Dark brown

244-245

65.0

14.6

13.1

49.17/49.08

5.58/5.36

10.43/10.14

10.97/10.57

8

[Ni(HDFH-H)2].2H2O

Pale-olive-green

251-253

65.0

13.8

10.6

49.18/49.06

5.58/5.43

10.43/10.11

10.93/10.44

9

[Cu(HDFH-H)2].3H2O

green

289-292

72.3

11.6

12.0

47.18/47.09

5.71/5.21

10.00/9.87

11.34/11.15

10

[Zn(HDFH-H)2].H2O

Pale Yellow

290-293

70.0

14.8

10.1

50.25/50.15

5.33/5.18

10.66/10.41

12.43/12.11

HL3

HDPH C12H15O2N3

White

210-212

79.0

—

—

61.80/61.17

6.43/6.11

18.02/18.00

       —

11

[Mn(HDPH-H)2].H2O

Yellow – brown

227-230

69.5

19.0

13.2

53.64/53.17

5.58/5.28

15.64/15.11

10.22/10.08

12

[Co(HDPH-H)2].2H2O

Dark brown

270

77.6

18.2

9.2

51.52/50.57

5.72/5.67

15.02/14.56

10.53/10.20

13

[Ni(HDPH-H)2].2H2O

Dark-brown

223-225

70.0

20.0

8.5

51.54/51.04

5.72/5.45

15.03/14.87

10.50/10.20

14

[Cu(HDPH-H)2]

Olive green

280 d

68.8

16.0

11.5

54.59/54.87

5.30/5.18

15.92/15.48

12.03/11.98

15

[Zn(HDPH-H)2].2H2O

Orange -yellow

251-255

65.0

15.0

16.1

50.94/50.43

5.66/5.27

14.85/14.66

11.55/11.32

d = decomposed

Results and Discussion

The hydrazones of 2,5-hexandion were efficiently synthesized by direct condensation of 2,5-hexandion with benzoyl hydrazine (HDBH), 2-furoyl hydrazine (HDFH), and picolinoyl hydrazine (HDPH) in a 1:1 molar ratio in absolute ethanol (Scheme 1). These hydrazones, when reacted with metal(II) acetate in presence of triethylamine in a 1:2:1 molar ratio in methanol, yielded 1:2 complexes. Analytical data showed that the ligands underwent enolization and deprotonation during complex formation. The complexes display intense colors and maintain their solid state at room temperature, unaffected by air or moisture. Generally, they exhibit low solubility in water and non-polar organic solvents but readily dissolve in ethanol, methanol, DMF, and DMSO. The complexes demonstrate low conductivity values in methanol (9.8-20.0 ohm-1mol-1cm2) and in DMSO (6.7-16.1 ohm-1mol-1cm2) solutions, indicating their non-electrolytic character, some physical properties  as shown in Table17,8.

FT-IR analysis

The significant infrared spectra of the synthesized compounds, as shown in Figures 1and 2, were measured using KBr disks and are outlined in Table 2. The frequencies of functional groups in the free ligands are indicated by bands in the specific region. (3329-3217), (1693-1658) and (1640-1620) cm-1, are consigned to the ʋ(N-H), ʋ(C=O)dione and ʋ(C=O)hydrazide respectively. The disappearance of the ʋ(N-H), and ʋ(C=O)Hydrazide bands of ligands in metal complexes and the appearance of anew  bands ʋ(C-O)enol  on (1188–1138)cm-1 ranges indicating the complexation  of the ligands in enol form resulted the involving  these bands in coordination with metal ions9. The ʋ(C=O)dione were slight shifted to lower frequencies of all metal complexes to (1651-1620)cm-1 confirming the participation of ʋ(C=O)dione in coordination 10 The additional strong bands appeared in the region (1601-1585) cm-1are corresponding to the stretching vibrational of azomethin group ʋ(C=N) of free ligands but are moved to lower frequency ranges (1581-1527) cm-1on their complexes, indicating the participation C=N group in coordination to metal ions 11 The bonding of nitrogen with the metal atom causes a reduction in electron density within the azomethine group, leading to a shift of the ν(C=N) vibration band towards lower frequencies. Interestingly, the stretching vibration of (N-N) in the ligand’s spectrum at (1007-995) cm-1 experiences a shift to higher frequencies by (69-21) cm-1 in their corresponding complexes. This alteration acts as an additional indicator of coordination between the metal and the nitrogen within the azomethine group12. The rise in frequency of the (N-N) stretching vibration results from the diminished repulsion between lone pair electrons of neighboring nitrogen atoms, facilitated by electron sharing with the metal ion. The weak absorption bands around 618 cm-1, attributed to the pyridine ring’s (py) deformation vibration in the spectrum of the free ligand (HDPH), were identified at nearly the same or lower frequencies. This implies that the pyridine nitrogen atom did not engage in coordination within the HDPH complexes13.The appearance of extra bands in the complex spectra spanning the ranges of (556-432) cm-1 and (497-412) cm-1 can tentatively be linked to (M-O) and (M-N) interactions, respectively. This offers supplementary confirmation of the ligand’s bonding with the metal ions14.

Due to the presence of symmetric and asymmetric stretching modes demonstrated by the non-coordinated water molecule, some complexes show a band in their spectra that is centered about 3745–3600 cm-1. Additionally, there are no bands in the areas where coordinated water is bending and deforming. The results of the thermal analysis investigation affirm the existence of lattice water molecules in certain complexes. However, all of these complexes release water when subjected to heating up to 600 °C15.

Table 2: Infrared spectral assignment (cm-1) of the prepared ligands and their complexes.

No.

Comp.

ν(N-H)

ν(C=O)

dione

ν(C=O)

hydrazide

Ʋ(C=N)

ν(C-O) enol

ⱱ(N-N)

⸹(py)

v(M-O)

ν(M-N)

HL1

HDBH C13H16O2N2

3217m

1658sh

1640s

1601sh

–          

1003m

—

—

   —

1

[Mn(HDBH-H)2].H2O

—

1638sh

—

1554s

1169m

1024m

—

513w

435m

2

[Co(HDBH-H)2].3H2O

—

1628sh

—

1573s

1165m

1032m

—

519w

459w

3

[Ni(HDBH-H)2]

—

1651m

—

1546s

1138m

1058m

—

513m

451w

4

[Cu(HDBH-H)2].H2O

—

1631m

—

1581s

1141m

1068m

—

551w

495w

5

[Zn(HDBH-H)2].H2O

—

1643s

—

1547s

1141m

1030m

—

459w

417w

HL2

HDFH C11H14O3N2

3283s

1658sh

1631s

1585sh

–

1007m

—

–

    —

6

[Mn(HDFH-H)2]

—

1640sh

—

1552s

1167m

1038m

—

513w

436m

7

[Co(HDFH-H)2].2H2O

—

1601s

—

1549sh

1180m

1045m

—

556w

497w

8

[Ni(HDFH-H)2].2H2O

—

1643s

—

1527s

1188m

1035m

—

536w

424w

9

[Cu(HDFH-H)2].3H2O

—

1651s

—

1537s

1184m

1061m

—

546w

497w

10

[Zn(HDFH-H)2].H2O

—

1643s

—

1539s

1188m

1053m

—

432w

412w

HL3

HDPH C12H15O2N3

3329s

1693sh

1620s

1585m

–

995m

617m

—

–

11

[Mn(HDPH-H)2].H2O

 

1616 m

—

1550m

1145m

1045m

617w

509w

420m

12

[Co(HDPH-H)2].2H2O

—

1639m

—

1550m

1142m

1044m

613w

520w

418w

13

[Ni(HDPH-H)2].2H2O

—

1632s

—

1536s

1157m

1064m

617m

540w

420w

14

[Cu(HDPH-H)2]

—

1643s

—

1558sh

1168m

1062sh

618m

516m

482w

15

[Zn(HDPH-H)2].2H2O

—

1634s

—

1545s

1143m

1064m

617w

530w

420w

sh=sharp, s = strong, m = medium, w= weak

Analysis of the ligands using 1H and 13C HNMR spectroscopy

In DMSO-d6 and CDCl3 solvents, the 1H-NMR and 13C-NMR spectra of the free ligands HDBH, HDFH, and HDPH were recorded as in Figures 3 and 4. They show singlet signals at (2.043–1.365 ppm) that are attributed to the proton of aliphatic methyl (6H, s, 2CH3). The multiple signals at (3.708–2.106 ppm) are due to the protons of (4H, m, 2CH2) aliphatic , Aromatic protons of the ligands exhibit a multiple of signals throughout the ranges (8.641–6.517ppm) corresponding to aromatic protons of benzene, furan and pyridine rings of the ligands. The ligands also reveals a singlet signal peak at (10.529-10.023ppm) are confirm to the (1H, s, NH) proton.  In13C-NMR, the signals at 18.43–11.2 ppm is attributed to CH3 groups, while the signals at 104.1–22.971 ppm are confirmed to be from the two CH2 groups. The aromatic phenyl carbon atoms of all three ligands occurred within 148.47–126.44 ppm. The signals at 164.39–144.91 ppm and 199.65–184.92 parts per million (ppm) are attributed to (C=N) and (C=O), respectively; these results are consistent with prior reports for similar compounds16,17

Mass spectroscopy

The mass spectra of the hydrazone ligands HDBH, HDFH, and HDPH are illustrated. In the mass spectrum of each ligand, a peak is observed at m/z = 232.4, 221, and 233.2, respectively, corresponding to the molecular ion peak. The fragmentation pattern was investigated, which displayed various peaks. All these results provide evidence of the chemical structure of the prepared ligands and, thus, their successful synthesis as in Figure 5.

Electronic absorption spectra and magnetic moments

The magnetic moments were measured at room temperature, and the electronic absorption bands of all synthesized compounds were recorded in DMSO solution within the specified range of 1100–250 nm using quart cuvettes with a 1 cm path length, as shown in Table 3. The absorption of HDBH, HDFH, and HDPH appeared within regions (37077–32786) and (31645–29154) cm-1, which have been attributed to π→⁕π and n→⁕π transitions, respectively18.

The electronic spectra of the complexes were recorded in DMSO and compared with those of the respective ligands. Mn(II) electronic spectrum complexes 1, 6, and 11 appeared within the specified range (24390–2389 cm-1), referring to the n→⁕π transition; however, d-d transitions were not seen, most likely due to the low intensities of these forbidden transitions . This was matched with magnetic moment values of (5.7–5.5) BM, which confirmed a high-spin d5-system containing five unpaired electrons and suggested octahedral Mn(II) complexes19. The Co(II) complexes 2, 7, and 12 spectra have visible three bands located at (9744-9596) cm-1, (16006-15497.6) cm-1, and (20468-18903) cm-1, which correspond to 4T1g →4T2g(F)(v1), 4T1g →4A2g(F)(v2), and 4T1g(F) →4T2g(P)(v3), respectively, together with a magnetic moment value (5.0-4.5) BM affording the octahedral geometry20. Three absorption peaks were observable within the electronic absorption spectra of the Ni(II) complexes 3, 8, and 13, the peaks at (9794-9531)cm-1, (19551-14300)cm-1, and (23641-22474)cm-1 corresponding to 3A2g →3T2g (F)(v1), 3A2g →3T1g (F)(v2), and 3A2g→ 3T1g (P) (v3) transitions. This was consistent with (3.2-2.7) BM magnetic moment values confirming to two unpaired electrons in the d8-system in octahedral Ni(II) complexes21. Whereas complexes 4,9,14 of Cu(II) displayed a single broad band in the range of (14598-13210) cm-1, which can be attributed to 2Eg→2T2g that exhibit comparable energy levels, resulting in a single broad absorption band. The width of this band might arise from forceful Jahn-Teller distortion and magnetic moment values ranging from (2.3-1.87) BM. Collectively, these data strongly suggest an octahedral geometry round the Cu(II) ion22. In the electronic spectra of the diamagnetic Zn(II) complexes 5,10,15 only one band was observed due to the LMCT transition23.

Table 3:  Magnetic moments and electronic absorption spectra for the complexes.

No.

Compound

μeffc(B.M)

  Band position (cm-1)

Assignment

Geometry

1

[Mn(HDBH-H)2].H2O

5.7

23809.6

 

  n→⁕π

 

Octahedral

6

[Mn(HDFH-H)2]

5.5

24390.8

11

[Mn(HDPH-H)2].H2O

5.7

24096.7

 

 

 

V1

V2

V3

 

 

2

[Co(HDBH-H)2].3H2O

5.0

96333.4

16006.6

19417.5

4T1g →4T2g(F)(ʋ1)

 4T1g →4A2g(F)(ʋ2)

4T1g(F)→4T2g(P)(ʋ3)

 

Octahedral

7

[Co(HDFH-H)2].2H2O

4.8

9596.4

15722.2

20468.4

12

[Co(HDPH-H)2].2H2O

4.5

9794.7

15497.6

1890.8

3

[Ni(HDBH-H)2]

3.2

9784.5

14300.7

23474.8

 

3A2g → 3T2g (F) ʋ1 ,

3A2g → 3T1g (F) ʋ2  

3A2g → 3T1g   (P) ʋ3

 

 

Octahedral

8

[Ni(HDFH-H)2].2H2O

2.7

 9531.4

19551.6

23641.3

13

[Ni(HDPH-H)2].2H2O

3.1

9794.9 

16778.6

22390.7

4

[Cu(HDBH-H)2].H2O

2.2

14598.4

 

2Eg→2T2g

 

Octahedral

9

[Cu(HDFH-H)2].3H2O

1.87

14492.5

14

[Cu(HDPH-H)2]

2.2

13210.9

5

[Zn(HDBH-H)2].H2O

Dia

Dia

Dia

24396.6

 

LMCT

 

Octahedral

10

[Zn(HDFH-H)2].H2O

24390.8

15

[Zn(HDPH-H)2].2H2O

24271.6

 

Thermal analysis

Thermal analysis investigations, encompassing thermogravimetry (TG) and differential thermogravimetry (DTG), were undertaken on several synthesized complexes spanning temperatures from 50-600°C. The decomposition patterns and the associated theoretical percentage mass losses for each complex are detailed in Table 4 as well as visualized in Figure 6 24,25.

The calculated weight loss percentages, derived from the TG data, closely corresponded with the proposed molecular formula of the studied complexes. The thermal decomposition curves of the reported complexes are characterized by multiple steps, indicating a multi-stage decomposition process. The TG curve displays a reduction in weight within the temperature interval of 50–130 °C. This decline in mass corresponds to the elimination of water molecules in hydrated state molecules: 1H2O hydrate molecule for complexes [Zn(HDBH)2].H2O, [Zn(HDFH)2].H2O, and 2H2O molecules for complex [Co(HDPH)2].2H2O. Within the TG curve, the subsequent decrease in weight occurs between 150 -350°C, aligning with the degradation of ligand fragments. Specifically, the fragments are C9H8ON2 for [Zn(HDBH)2].H2O, C9H8O2N for [Zn(HDFH)2].H2O, and C17H17O2N4 for [Co(HDPH)2].2H2O. The third decomposition step occurs between the temperatures of 360 – 600 ˚C, with further weight losses attributed to the remaining organic hydrazone ligands. These fragments are C17H22O2N2 for [Zn(HDBH)2].H2O, C13H18O3N3 for [Zn(HDFH)2].H2O, and C7H11ON2 for [Co(HDPH)2].2H2O. The final residues obtained from the thermal decomposition of the complexes were pure metal oxides, specifically ZnO and CoO. These residues remained stable even at a temperature of 600 °C. Regarding the complex [Cu(HDFH)2].3H2O, its thermal decomposition occurred in a single step, starting at 50 ˚C and ending at 800 ˚C. This decomposition process resulted in the formation of the metal oxide CuO, corresponding to the weight loss of the 3H2O hydrate and the complete degradation of the hydrazone ligand.

Table 4: Thermal degradation of the ligand and  its corresponding metal complexes.

No.

Complexes

Temp. range   (˚C)

Estimated /calculated

Mass loss (%)

   Assignment

5

[Zn(HDBH)2].H2O

C26H32O5N4Zn

50-130

3.3 /3.3

Loss of hydrated water   (1 H2O)

   

150-350

28.8/29.3

C9H8ON2

   

360-600

51.0/52.4

C17H22O2N2

Residu ZnO

9

[Cu(HDFH)2].3H2O

      C22H32O9N4Cu

50-800

85.8/85.7

Losses C22H32O8N4 and (3H2O) hydrate water

 Residue    CuO

10

 

[Zn(HDFH)2].H2O

C22H28O7N4Zn

50-130

3.2 /3.4

Loss of hydrated water (1 H2O)

   

150-350

30.0/30.8

C9H8O2N

   

360-600

49.8/50.2

C13H18O3N3

Residue ZnO

12

 

[Co(HDPH)2].2H2O

C24H32O6N6Co

50-130

5.6/6.4

Loss of hydrated water (2 H2O)

   

150-350

54.4/55.2

C17H17O2N4

   

360-600

25.0/24.8

 

 

Computational studies (DFT)

To enhance our understanding of how metal ions influence the electronic properties of ligands, we performed computational DFT calculations using Gaussian 09 software at the DFT-B3LYP/6-311++G(d,p) method . This computational approach has become a valuable tool for determining various properties, including molecular construction, bond angles, bond lengths, thermodynamic parameters, and electronic properties, which were analyzed using DFT computations Figure 7. The results reveal that changing the central atom significantly affects the energy of the frontier molecular orbitals26

Figure 7: Optimized  Structures of the Ligands and Complexes. Metal atoms are shown in purple, Nitrogen in blue, Oxygen in Red, Carbon in Grey, and Hydrogen in White.

Click here to View Figure

Thermodynamics parameter for prepared compounds

Thermodynamic parameters for the complexes were computed to establish coherent connections between their energetic, structural, and reactive characteristics, and the energy of a system. This fundamental statement is justified by the calculated zero-point energy, Gibbs free energy (∆G), enthalpy (∆H), internal energy (E), entropy (∆S), and specific heat (Cv) for all prepared compounds. These experimental data shown in Table 5 are essential parameters in describing the   reaction pathways and relative energies for the compounds 27. 

Table 5: Thermodynamic parameters of ligands and their complexes were computed using Density Functional Theory (DFT) with the B3LYP method at a temperature of 298.15K and a pressure of 1 atm.

No. Compounds

Zero-point vibrational energies (a.u)

Gibbs free energy (a.u)

Enthalpy (a.u)

Internal energy (a.u)

Entropy (calmol-1K – 1 )

Specific heat, Cv (calmol-1K -1

HL1

-3069.66

-3069.72

-3069.62

-3069.62

199.12

122.73

1

-3124.27

-2381.79

-3497.20

-3487.75

134.76

111.51

2

-2910.88

-2910.93

-2910.85

-2910.85

179.45

114.62

3

-3101.62

-3001.98

-3001.85

-3001.42

201.92

116.21

4

-3168.67

-3168.73

-3168.64

-3168.64

203.32

118.70

5

-3307.54

-3307.62

-3307.51

-3307.51

226.69

131.39

HL2

-762.07

-762.12

-762.05

-762.05

143.06

58.60

6

-2674.76

-2674.83

-2674.72

-2674.72

224.05

123.91

7

-2907.09

-2907.16

-2907.06

-2907.06

224.19

124.43

8

-3032.63

-3032.70

-3032.59

-3032.59

222.00

123.94

9

-3164.26

-3164.31

-3164.23

-3164.23

181.41

105.56

10

-3303.68

-3303.75

-3303.64

-3303.64

228.42

125.47

HL3

-780.92

-780.96

-780.90

-780.90

139.80

61.93

11

-2712.35

-2712.41

-2712.31

-212.32

195.66

119.52

12

-2944.19

-2944.25

-2944.16

-2944.16

201.72

122.03

13

-3069.66

-3069.72

-3069.62

-3069.62

199.12

122.73

14

-3201.88

-3201.94

-3201.84

-3201.94

200.49

118.38

15

-3340.71

-3340.78

-3340.68

-3340.68

216.90

127.71

 

The tables (6 and 7) present data on the bond lengths and angles of prepared combinations. In these complexes, from the chelate ligand, two nitrogen and four oxygen donor atoms create a coordination polyhedron. The observed bond lengths and bond angles are typical of six-coordinated M(II) complexes containing a monohydrazone, which involves the contribution of six donor atoms, complexes with a metal-ligand ratio of 1:2, which are provided by two tridentate chelate ligands designated as HL, and coordinated by two nitrogen atoms from azomethine (N5 and N6), and two oxygen atoms from carbohydrazide (O2 and O3), as well as two oxygen atoms from ketones (O1 and O4), and these atoms are arranged in an octahedral structure. The bond lengths within (M-O1) of (2.74-1.81), (M-O2) of (2.19-1.84), (M-O3) of (2.72-1.13), (M-O4) of (2.27-1.84), (M-N5) of (2.34-1.87) and (M-N6) of (2.22-1.84).The (N5-N7), (N6-N8), (O1-C) and (O3-C) bond length   are enlarged owing to  coordination of azomethane nitrogen carbohydrazide oxygen, and carbonyl oxygen atoms, and formation of  M-N and M-O bond as shown in Table 6. These observed bond lengths are in agreement with the bond distances found in similar six-coordinated structures reported in the literature 28,29,30.

Upon coordination, the angles formed by the bonds within the hydrazone moiety of the ligands undergo some changes. Notably, the bond angles (-O=C-N-N=C-) are either reduced or increased during complex formation, depending on the bonding sequence. However, the detected bond angles in the complexes lie within the normal range for octahedral geometry. This suggests that the bonding of these complexes likely involves sp3d2 or d2sp3 hybridization30.

Table 6: Computation of geometric bond lengths in synthesized compounds using the B3LYP/6-311++G(d,p) technique. Bond length are given in Å

No. Compounds

M-O1

M-O3

M-O2

M-O4

M-N5

M-N6

N5-N7

N6-N8

O1-C

O3-C

HL1

—-

—-

—-

—-

—-

—-

1.29

—-

1.21

1.22

1

1.81

1.85

1.84

1.84

1.87

1.84

1.39

1.39

1.24

1.43

2

2.15

2.72

2.09

1.87

2.02

1.99

1.38

1.39

1.23

1.27

3

2.74

1.88

1.87

2.27

1.91

1.91

1.40

1.40

1.21

1.30

4

2.13

1.59

1.94

1.94

2.03

2.03

1.39

1.39

1.21

1.30

5

2.13

1.74

2.00

1.98

2.17

2.11

1.43

1.42

1.25

1.32

HL2

—-

—-

—-

—-

—-

—-

1.29

—-

1.21

1.22

6

2.07

2.67

1.90

1.90

2.02

2.02

1.44

1.44

1.24

1.36

7

1.96

2.09

1.94

1.92

2.04

1.94

1.39

1.39

1.26

1.28

8

2.11

1.90

1.90

1.58

1.98

1.98

1.41

1.41

1.24

1.32

9

2.23

1.13

1.95

1.95

2.03

2.03

1.43

1.42

1.24

1.34

10

2.38

2.01

1.97

1.96

2.03

2.04

1.43

1.42

1.21

1.30

HL3

—-

—-

—-

—-

—-

—-

1.37

—-

1.22

1.24

11

2.28

2.31

2.15

2.20

2.34

2.22

1.38

1.37

1.22

1.26

12

2.10

2.45

1.95

1.99

2.24