Insight into the synthesis, structure affirmation and catalytic efficiency of divalent and trivalent metal chelates of mandelic acid hydrazone derivative

Abstract

The current work reports the synthesis of Cr(III), Mn(II), Co(II), Ni(II) and Cu(II) chelates of the Schiff base ligand named hydroxy-phenyl-acetic acid (2-hydroxy-naphthalen-1-ylmethylene)-hydrazide with multi-chelation centre toward metal ions. The spectral tools, ¹H-NMR, FTIR, mass, UV-vis spectra, and the analytical elemental and thermal analysis, in addition to magnetic moment and conductivity measurements all combined have been applied to conclude the structure and geometry of the synthesized metal complexes. The formed metal chelates have been assured to be formed with the molar compositions of 1 L : 1 M for PANH-Cr, PANH-Mn, PANH-Co, PANH-Ni and 2 L : 1 M for PANH-Cu. All the complexes have been confirmed to be non-electrolytic except the PANH-Mn and PANH-Ni which are 1 : 1 electrolytes. FTIR spectral analysis assured the ligand to act as mono basic bi or tridentate ligand leading to the formation of octahedral complexes with all metals except Cu(II) complex which assured to has square planar structure. Except PANH-Cr, all the synthesized metal chelates exhibited phenoxazinone synthase like efficacy with varying activity with dramatically high activity for PANH-Mn complex with TOF number of 169.89 h⁻¹.

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1. Introduction

Hydrazones are considered flexible ligands that are classified as azomethine compounds, having the structure R₂C=NNR₂. They are distinguished from other members of this class, such as oximes, imines, and others, by the presence of linked nitrogen atoms.¹

Hydrazone-based ligands have gained interest due to their numerous coordination characteristics and capacity to coordinate in both protonated and deprotonated forms.^2,3 A dominant contribution in the advancement of coordination chemistry was achieved by hydrazone ligands formed by the condensation reaction of o-hydroxyaldehydes and ketones with hydrazides due to the formed conceivable poly-cleating ligands that pass amide, azomethine and phenol functionalities and provide a variety of bonding options to metal centres in metal chelates.⁴ Benzoylhydrazones of salicylaldehyde,⁵ 2-hydroxy-1-naphthaldehyde⁶ and their analogies compounds^7,8 are considered the most important hydrazone ligands since this structure building provide a class of ligands with bi-or tridentate cavity coordinating to the metal centres through the carbonyl oxygen, azomethine nitrogen and hydroxyl oxygen resulting in the formation of five and/or six membered chelate ring surrounding the metal centre. Condensation of 2-hydroxy-1-naphthaldehyde with mandelic acid hydrazide resulting in the formation of analogies ligand structure.

The great importance of hydrazone compounds and their metal chelates steam from their wide range of applications since they are applied in biological system as antimicrobial, antibiotic, antimalarial and antitumor^9–12 beside their uses in analytical,¹³ catalytic^14–16 applications and also as molecular sensors¹⁷ and luminescent probes.¹⁸

Among the transition elements, Mn element is considered a significant element for human healthiness, metabolism, antioxidant system and fundamental in photosystem for O₂ evolution.^19–22 Recently, great attention has been allocated for the design and synthesis of Mn coordination compounds due to their efficiently catalytic performance^23–26 specifically with Schiff base ligands incorporating N and O atoms. The reason is that in such complexes the Mn(II) centers are subjected to aerial oxidation to form complexes in which Mn ion is oxidized to higher oxidation states (i.e. Mn(III) and/or Mn(IV)).²⁷ Among the important catalytic reaction for Mn complexes is bio-mimicking oxidase (e.g. catecholase and phenoxazinone synthase) and oxygenase (e.g. epoxidation) reactions.^27–30

Phenoxazinone synthase is one of the enzymes whose behavior and effects have been experimentally and widely mimicked experimentally. This enzyme is called poly nuclear copper oxidase. This enzyme acts as a catalyst for the oxidative condensation of o-aminophenol and its derivatives to phenoxazinone chromophore and is found naturally in the bacteria Streptomyces antibioticus. This phenoxazinone chromophore is used therapeutically to treat Wilm's tumor curing, gestational choriocarcinoma, and other tumor types. Its function is to stop DNA intercalation from synthesizing RNA based on DNA in these tumor kinds.³¹ The major condition for a metal chelate to be as promising as phenoxazinone synthase is the availability of labile positions on the metal centers. Metal complexes incorporating other metals such as Cu(II), Co(II) and Ru(III) are also reported to catalyse such reactions.^31–35

All the previous facts motivated us to design and prepare a number of metal complexes of the hydrazone ligand named hydroxy-phenyl-acetic acid (2-hydroxy-naphthalen-1-ylmethylene)-hydrazide. Structure assertion of the synthesized metal ion chelates (Cr(III), Mn(II), Co(II), Ni(II) and Cu(II) chelates) were performed using the analytical and spectral methods. The interested complexes have been tested and evaluated for their catalytic activity as phenoxazinone synthase.

2. Experimental

2.1. Preparation of the Schiff base ligand (PANH)

The target Schiff base has been synthesised as previously reported.⁸ 0.01 mol of 2-hydroxy-1-naphthaldehyde and mandelic acid hydrazide (1.72 g and 1.66 g, respectively) were separately solubilized in the least sufficient amount of hot MeOH followed by slow addition of one solution to the other. The final solution was allowed for reflux at 60 °C for 4 h within which yellowish crystalline needles have been appeared. The hot reaction solution was filtered to separate the formed compound which rinsed several times using hot methanol and eventually with ether. The product was kept drying in vacuum desiccator (Fig. 1). Purity of the ligand has been assured by ¹H-NMR, FTIR, mass and elemental analysis.

Fig. 1 Structure of the ligand PANH with the IUPAC name.

¹H-NMR (600 MHz, DMSO-d₆, δ, ppm): δ: 3.81 (s, 1H, CH–OH), 5.17 (s, 1H, CH–OH), 6.65 (s, br, 1H, NH), 7.16–7.9 (d, m, 11H, Ar–H), 8.17 (s, 1H, CH=N), 9.51 (s, 1H, phenolic OH).

2.2. Synthesis of metal chelates

The above mentioned multidentate ligand (i.e. PANH) has been used for the synthesis of the Cr(III), Mn(II), Co(II), Ni(II) and Cu(II) chelates through dissolving 0.001 mol of each of Cr(CH₃COO)₃ (0.232 g) MnCl₂·4H₂O (0.197 g), CoCl₂·6H₂O (0.237 g), NiCl₂·6H₂O (0.237 g) and Cu(CH₃COO)·H₂O (0.199 g) in the least sufficient quantity of hot MeOH and added separately to a heated methanolic solution that included 0.32 g (0.001 mol of PANH). The resultant mixtures were refluxed for eight hours at boiling temperature, during which time colorful compounds developed. After being filtered out, the solid products were repeatedly washed with hot methanol and then with ether. The product was maintained in a vacuum desiccator for drying out.

2.3. Measurements instrumentation and OAP catalytic oxidation

The Sigma-Aldrich company is the supplier for all of the analytical grade reagents used in the current study, which were used exactly as they were given. Information regarding the tools and techniques used for structure assertion and implementation is provided from the previously reported study.³¹ Using o-aminophenol as the substrate, the methods and strategies employed to assign the complexes under investigation's phenoxazinone synthase-like activity are generally as previously mentioned³⁴ and as described in details in the ESI file.†

3. Results and discussion

3.1. ¹H NMR spectra of PANH ligand

Fig. 2 represents the ¹H-NMR spectrum of PANH ligand measured in DMSO-d₆ using the internal standard tetramethylsilane (TMS). The signal appearing at 9.51 ppm has been appointed to the phenolic proton whereas the one appearing at 8.17 ppm is assigned to the azomethine proton. The broad signal that appeared at 6.6 ppm has been attributed to the NH proton.³⁶ Aromatic protons of the phenyl and naphthyl rings featured as doublet and multiple signals inside 7.9–7.16 ppm range. The aliphatic CH and OH protons appeared at 5.17 and 3.81 ppm, respectively.^37,38 The OH proton signal at 3.81 ppm is overlapped with the solvent signal as it is appearing in the same range.

Fig. 2 ¹H-NMR spectrum of the ligand PANH in DMSO-d₆ solvent.

3.2. Complexes composition and conductivity

As concluded from the results of the contained elements percent, the isolated metal chelates have been formed in the molar composition 1 L : 1 M for PANH-Cr, PANH-Mn, PANH-Co, PANH-Ni. For the Cu(II) complex, PANH-Cu, although it has been synthesized in the molar ratio 1 L : 1 M, the isolated complex formed in 2 L : 1 M ratio. The molecular formulas of all complexes were concluded to be [Cr(PANH)(AOc)₂H₂O]·H₂O, [Mn(PANH)(H₂O)₃]·Cl·2H₂O, [Co(PANH)Cl(H₂O)₂]·3H₂O, [Ni(PANH)(H₂O)₃]·Cl·2H₂O and [Cu(PANH)₂]; Table 1. The solubility test of the highly stable complexes indicated that all complexes have no solubility in all non-polar solvents, partially soluble in MeOH and EtOH and extremely soluble in MeCN, DMSO and DMF. The molar conductivity values as measured from 10⁻³ M solution of each compound dissolved in DMSO were found to be 17.2, 69.5, 18.6, 71.4 and 15.7 Ω⁻¹ cm² mol⁻¹ for PANH-Cr, PANH-Mn, PANH-Co, PANH-Ni and PANH-Cu, successively, which is in good matching with the non-electrolytic features of PANH-Cr, PANH-Co, and PANH-Cu complexes and 1 : 1 electrolyte for and PANH-Mn and PANH-Ni as compared by reported compounds.^39,40

Table 1 Elemental analysis and physical properties of the complexes under investigation

Comp. symbol	Ligand/complex (molecular formula; M. wt)	Colour (m.p. °C)	Elemental analysis^b				Λm
			% C	% H	% N	% M
a Decomposition temperature.b Calculated (Found).
PANH	(C19H16N2O3; 320.34)	Yellow (242)	71.24 (71.32)	5.03 (5.14)	8.47 (8.53)	—	—
PANH-Cr	[Cr(PANH)(AOc)2H2O]·H2O (C23H25CrN2O9; 525.45)	Yellowish green (294)	52.57 (52.69)	4.80 (4.68)	5.33 (5.21)	9.90 (9.74)	17.2
PANH-Mn	[Mn(PANH)(H2O)3]·Cl·2H2O (C19H25ClMnN2O8, 499.80)	Brown (275)^a	45.66 (45.76)	5.04 (5.24)	5.60 (5.73)	10.99 (11.77)	69.5
PANH-Co	[Co(PANH)Cl(H2O)2]·3H2O (C19H23ClCoN2O7; 485.78)	Brown (252)^a	46.98 (47.22)	4.77 (4.81)	5.77 (5.92)	12.13 (12.93)	18.6
PANH-Ni	[Ni(PANH)(H2O)3]·Cl·2H2O (C19H25ClNiN2O8; 503.56)	Dark yellow (>300)	45.32 (45.45)	5.00 (5.23)	5.56 (5.71)	11.66 (11.40)	71.4
PANH-Cu	[Cu(PANH)2] (C38H30CuN4O6; 702.21)	Olive green (265)^a	65.00 (65.17)	4.31 (4.39)	7.98 (8.26)	9.05 (9.49)	15.7

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3.3 EI-mass spectra

The prospected molecular weights of the ligand, PANH, and its chelates, PANH-Cr, PANH-Mn, PANH-Co, PANH-Ni and PANH-Cu, have been assured through comparison of the mass spectral results. The molecular weight of the ligand and PANH-Mn complex were found to be 320.52 and 464.65 matching with the theoretical molecular weight of the ligand (i.e. 320.34) and M − 1 for PANH-Mn (without hydration water molecules); Fig. 3. The mass spectra of complexes PANH-Cr, PANH-Co, PANH-Ni and PANH-Cu (Fig. S1–S4†) gave the molecular ion peaks at 509.25 (M + 2 − 2H₂O), 449.92 (M − 2H₂O), 466.58 (M − 1 − 2H₂O) and 705.62 (M + 3), respectively, which is in large agreement with the concluded molecular weights.

Fig. 3 Mass spectra of PANH ligand and PANH-Mn chelate.

3.4. IR spectra and mode of bonding

The coordinating function groups in the ligand to the metal centers can be identified by comparing the FTIR spectra of the metal complexes with those of the free Schiff base, PANH. Changes in position and/or intensity of spectral bands are common in coordinating function groups. It's also possible for some bands to disappear during chelation. The most important infrared bands of PANH and its complexes are shown in Table 2, along with their respective assignments, the spectra are shown in Fig. 4, 5 & S5.†

Table 2 Assignment of the IR bands of PANH and its complexes

Comp.	νOH	νNH	νC=O	νC=N	νC–O^a^,^b	νC–O	νM–N
a Phenolic.b 2° alcohol.
PANH	3353	3052	1675	1622	1285	—	—
					1221
PANH-Cr	3415	3054	1649	1613	1250	536	448
					1220
PANH-Mn	3422	3056	1649	1617	1248	542	443
					1220
PANH-Co	3418	3052	1652(sh)	1617	1249	545	463
					1220
PANH-Ni	3414	3060	1656	1617	1247	529	460
					1192
PANH-Cu	3416	3073	1643(sh)	1619	1284	528	—
					1193

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Fig. 4 IR spectra of PANH in comparison & PANH-Mn complex.

Fig. 5 IR spectra of PANH-Co &PANH-Cu complexes.

The bands in the spectra of PANH that appeared at 3353 and 3052 cm⁻¹ were assigned to the stretching vibrations of the OH and NH groups, respectively, according to an analysis of the results shown in Table 2. While the band at 1622 cm⁻¹ was linked to v(C=N) groups, the band at 1675 cm⁻¹ was related to the stretching vibration of C=O. Two bands have been assigned to phenolic and secondary alcoholic C–O stretching at 1285 and 1221 cm⁻¹, respectively.^41,42

In the spectra of metal complexes PANH-Cr, PANH-Mn, PANH-Co and PANH-Ni, the three bands corresponding to the ν(C=O), ν(C=N) and ν(C–O_phenolic) appeared within the ranges 1649–1656, 1613–1617 and 1247–1250 cm⁻¹, respectively, affording a shift in their position in the ligand spectrum by 19–26, 5–9 and 35–38 cm⁻¹ which are large enough to confirm the involvement of C=O, C=N and C–O (phenolic) in ligation to the metal centre. Nearly no shift in the position of secondary alcoholic C–O band and hence no probable involvement of such band in coordination to the metal centre.⁴²

In the spectrum of PANH-Cu complex, although the band corresponding to ν(C=O) afforded a shift in its position by 32 cm⁻¹ relative to its position in the ligand spectrum, the band corresponding to C=N and C–O (phenolic) bonds appeared almost at the same position relative to the ligand spectrum and hence these two bands are not involved in coordination to the Cu centre. On the other hand, the secondary alcoholic C–O band that appeared in ligand spectrum at 1221 cm⁻¹ underwent a shift in its position appearing at 1193 cm⁻¹ in PANH-Cu complex's spectrum confirming its involvement in ligation to Cu centre through proton removal. The appearance of new bands within the ranges 443–463 cm⁻¹ (ν_M–N) and 528–545 cm⁻¹ (ν_M–O) confirms the participation of azomethine nitrogen and phenolic, carbonyl and/or alcoholic O atoms in formation of the complexes.

The two non-ligand bands appearing in the spectrum of PANH-Cr at 1582 and 1353 cm⁻¹ have been assigned to ν_asy(COO⁻) and ν_sym(COO⁻), respectively, with Δν separation value of 229 cm⁻¹ confirming the coordination of the carboxylate group to the Cr ion in a monodentate fashion.⁴³

All complexes' spectra have bands in the ranges 3414–3422 cm⁻¹ that are attributed to coordinated and lattice water molecules, whereas the bands in the range 3054–3073 cm⁻¹ are attributed to (NH). Although the NH group is not involved in the formation of complexes, H-bond formation is likely responsible for shifting the position of the NH band in the complexes' spectra relative to the free ligand.⁴⁴

3.5. Thermal gravimetric analytical results and kinetic parameters

One of the most helpful methods for predicting the molecular structure and stability of compounds is thermal analysis, which provides vital information on the thermal properties of compounds, the phases of thermal degradation, the kinds of intermediates, and the residual products of thermal degradation. Through such examination, the amount and kind of water and/or organic solvent molecules, as well as the other anions attached to the metal center, can be allocated. Thermogravimetric thermograms of the metal complexes are given in Fig. 6 and their analysis are given in details in Table 3. Investigation of the obtained results featured that PANH-Cr, PANH-Co, PANH-Ni and PANH-Cu complexes afforded 3-stages thermograms whereas PANH-Mn complex has 4-stages thermogram. Analysis of these thermograms showed that, except PANH-Cu complex, all the studied compounds started their decomposition from room temperature and the first decomposition stage extended up to 113 °C in which lattice water molecules included in complexes' structures got lost. The following decomposition step started at 91 °C and ended at 341 °C inside which the coordinated water and anion molecules (chloride or acetate) are lost. The successive 1 or 2 stages which started 255 °C and extended to 800 °C are analysed to degradation of the organic ligand molecule incorporated in the complexes' structure and lead to the formation of metallic oxide in addition to small percent of remaining carbon content. For PANH-Cu complex with 3-stages thermogram, this complex exhibited thermal stability up to 184 °C and the first stage of decomposition occurred in the range 184–298 °C within which the organic ligand partially decomposed with the loss of 2 molecules of hydroxy naphthyl groups. The second and third stages of decomposition occurred in 298–483 °C and 483–678 °C range and assigned to the loss of C₂H₄N₄ and C₁₆H₁₂O₃ fragments, respectively, leading to the formation of CuO residue with percent of 11.89% which is in accordance the calculated percent that is 11.33%.

Fig. 6 TGA thermograms of the synthesized metal chelates.

Table 3 Interpretation of TG results of metal chelates under interest

Symbol molecular formula (M. wt)	Temp. Range (°C)	Mass loss%		Assignment
		Found	Calc.
PANH-Cr [[CrL(AOc)2H2O]]·H2O 525.45	25–91	3.65	3.43	Loss of hydration H2O
	91–330	25.54	25.88	Loss of coordination H2O and 2 acetate molecules
	330–430	54.32	53.86	Loss of (C18H15N2O1.5)
	>430	16.49	16.75	Formation of 0.5 Cr2O3 + 1C as residue
PANH-Mn [MnL(H2O)3]·Cl·2H2O 499.80	26–105	7.13	7.20	Loss of 2 hydration H2O
	105–255	13.85	14.20	Loss of 3 coordination H2O and OH group
	255–399	22.66	22.51	Loss of Cl^− anion and dissociation of the organic ligand (C6H5)
	399–568	38.60	39.41	Further dissociation of the organic ligand (C12H9N2O)
	>568	17.76	16.60	Formation of MnO + 1C as residue
PANH-Co [CoLCl(H2O)2]·3H2O 485.78	29–113	7.11	7.41	Loss of 2 hydration H2O
	113–324	30.00	30.57	Loss of coordination H2O, Cl^− anion and C6H5.
	324–535	43.42	44.05	Further dissociation of the organic ligand (C12H10N2O2)
	>535	19.07	17.89	Formation of CoO + 1C as residue
PANH-Ni [NiL(H2O)3]·Cl·2H2O 503.56	29–107	7.01	7.15	Loss of 2 hydration H2O
	107–341	21.44	21.15	Loss of 3 coordination H2O, Cl^− anion and OH group
	341–426	52.37	52.03	Decomposition of the organic ligand (C17H14N2O) formation of NiO + 2C as residue
	>426	19.18	19.62
PANH-Cu [Cu(L)2] 702.21	29–184	—	—	Thermal stability
	184–298	40.70	40.77	Degradation of the ligand moiety and loss of (C20H14O2)
	298–483	11.69	11.97	Loss of (C2H4N4)
	483–678	35.72	35.92	Loss of (C16H12O3)
	>678	11.89	11.33	Formation of CuO as residue

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Application of Coats–Redfern equations for the successive decomposition stages enables the computation of the kinetic parameters “order of reaction (n), pre-exponential factor (A), and energy of activation (E)”⁴⁵ which have been obtained from the plots as shown in Fig. 7 and S6–S8† (Coats–Redfern plot for PANH-Mn and PANH-Ni complex, as illustrative examples) and the results are depicted in Table 4. The thermodynamic parameters, ΔH, ΔS, and ΔG, were also calculated applying the previous reported equations:

ΔH = ΔE − RT

ΔG = ΔH − TΔS

h: Planck constant; k: Boltzmann constant.

Fig. 7 Coats–Redfern plot for PANH-Mn (up) and PANH-Ni (down). (a) first step (b) second step (c) third step where, Y = [−ln(1 − α)]/T² for n = 1.

Table 4 Temperature of decomposition and activation parameters (ΔH*, ΔS*, ΔG*) for decomposition of complexes using Coats–Redfern equations

Complex	n	Step	r	T (K)		ΔH*	A	−ΔS*	ΔG*
PANH-Cr	1	1st	0.96834	327.25	65.904	63.6355	0.358 840 874	0.2640	149.95
	1	2nd	0.99910	440	27.254	23.5305	10 392 423.9	0.1222	77.34
	1	3rd	0.98761	647.2	194.344	188.946	2.30388 × 10−5	0.3484	414.47
PANH-Mn	1	1st	0.99581	331.625	41.269	38.5123	3365.103 219	0.1865	100.38
	1	2nd	0.99512	460.8	30.451	26.6200	6 337 456.085	0.1266	84.97
	1	3rd	0.99669	602.8	46.489	41.4031	2 461 953.223	0.1368	125.11
	1	4th	0.99617	745.8	79.119	72.8698	169 515.6068	0.1608	193.75
PANH-Co	1	1st	0.99352	329.125	40.103	37.3675	4752.113 441	0.1836	97.81
	1	2nd	0.98847	469.7	23.166	19.1924	75 716 377.5	0.1063	70.01
	0.5	3rd	0.99849	705	34.466	28.5238	76 813 121.51	0.1095	106.81
PANH-Ni	1	1st	0.98937	331	52.182	49.4307	78.27 469 713	0.2178	121.53
	1	2nd	0.98788	511.7	21.0727	16.7442	124 782 239.6	0.1028	70.29
	1	3rd	0.99575	640.5	128.334	122.970	3.584 854 628	0.2490	283.62
PANH-Cu	—	1st	—	—	—	—	—	—	—
	0.66	2nd	0.99266	529.9	73.858	69.4294	1507.203 925	0.1972	174.49
	0.66	3rd	0.99829	693.6	31.448	25.5912	138 476 513.3	0.1045	99.22
	1	4th	0.99763	894.1	152.693	145.201	183.5 464 653	0.2190	342.62

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Table 4 findings demonstrate the −ve ΔS* implication that either the activated complex is more ordered than its reactants or that the complexes' normal breakdown process is taking longer than usual.⁴⁶ Positive ΔH values signify endothermic breakdown processes.⁴⁷ Additionally, it has been found that the vast majority of the time, the heat decomposition stages of all complexes follow zero- or first-order kinetics.

3.6. UV-vis spectra and magnetic moment

DMSO solutions of PANH and its metal chelates were synthesized for recording UV-vis spectra which done in the range of 200–800 nm (Fig. S9 & S10†). The free ligand PANH showed the absorption band at 276 nm that assigned to π → π* of C=N group. The bands that appeared at and 310 and 428 nm assigned to n → π* transitions of C=N and C=O groups, successively.^43,48 The appearance of such bands in complexes' spectra shifted from these values is evidence for the attachment of both azomethine nitrogen and carbonyl oxygen to the metal ion in complex formation.

For PANH-Cr complex, the three observed bands centred at 390, 431 and 523 nm allocated to ⁴A_2g → ⁴T_1g (P) (ν₃), ⁴A_2g → ⁴T_1g (F) (ν₂), and ⁴A_2g → ⁴T_2g (F) (ν₁) transitions, respectively, and confirmed octahedral architecture of the complex.^49,50 The room temperature magnetic moment value of this complex was calculated and found to be of the complex was measured to be 3.82 B. M. that is approaching to the spin only value of three unpaired electrons (i.e. 3.88 B. M.) in octahedral Cr(III) chelates and hence confirming the predicted geometry.⁵⁰

In the spectrum of PANH-Mn chelate, the three transitions appeared in the spectrum at 586, 464 and 433 nm, respectively, were assigned to ⁶A_1g → ⁴T_1g(G), ⁶A_1g → ⁴T_2g(G) and ⁶A_1g → ⁴E_g(G) reported for Mn(II) chelates.^51,52 Magnetic moment value of 6.12 B. M. was calculated for this complex which assigned to high spin Mn(II) chelates.

The Co(II) chelate, PANH-Co, afforded its spectral bands at 572 and 475 nm that correspond to the transitions ⁴T_1g(F) → ⁴A_2g(F) and ⁴T_1g(F) → ⁴T_1g(p), successively, familiar to octahedral Co(II) complexes.⁵² Octahedral geometry for this complex is further supported by the magnetic moment value which calculated to be 4.63 B. M.

Octahedral arrangement around the Ni(II) centre in PANH-Ni complex was concluded by the appearance of the spectral peaks centred at 520 and 438 nm that attributed to the transitions of the type ³A_2g(F) → ³T_1g(F) (ν₃) and ³A_2g(F) → ³T_1g(P) (ν₂) specified for octahedral Ni(II) complexes.⁵³ Magnetic moment value of 3.32 B. M. was found to PANH-Ni complex.

The spectrum of PANH-Cu complex represented a broad low-intensity absorption band cantered at 728 nm that assigned to the transition of the type ²B_1g → ²A_1g reported for square planar Cu(II) complexes in addition to the peak appearing to 417 nm characteristic for LMCT.⁵⁴ Magnetic moment value of 1.84 B. M. was observed for this complex.

Collection of the results obtained above led to the prospected complexes' structures shown in Scheme 1.

Scheme 1 The structures prospected for the interested metal chelates.

3.7. Phenoxazinone synthase effectiveness and kinetic investigation

The existence of the metal complexes interested in this study in a solution containing OAP (o-aminophenol) led to its oxidation and generation of APX (2-aminophenoxazine-3-one). Such behaviour was assured by the generation and then increase in the absorbance of APX peak with time. Such band emerged in complexes' spectra at 430, 435, 433 and 428 nm for PANH-Mn, PANH-Co, PANH-Ni and PANH-Cu complexes, successively. Thus, the appearance of such band is evidence for the complexes phenoxazinone synthase catalytic efficacy. For PANH-Cr, the growing up of such band was not observed with time ensuring that it is not active as a catalyst for such catalytic reaction. The time-dependent spectra of complexes PANH-Mn, PANH-Co, PANH-Ni and PANH-Cu with complexes concentration of 3 × 10⁻⁵ M and 10⁻² M concentration of OAP in DMF medium and aerobic conditions are given in Fig. 8–10. For these experiments, time began when OAP was added to the complexes' solutions, and the compounds' spectra were taken every five minutes up to seventy minutes after that. There was no peak observed for (blank experiment). The observed rate constant (k_obs) and the initial rate of response (V_o) were determined by using the initial rate method upon using ε_(APX) equals 18 300 M⁻¹ cm⁻¹⁵⁵ (Table 5).

Fig. 8 Variation of UV-vis spectra of PANH-Mn chelate with time illustrating the increase of APX peak at 430 nm. The measurement solution contains 10⁻² M of OAP and 3 × 10⁻⁵ M of PANH-Mn chelate; DMF is the solvent and aerobic condition.

Fig. 9 Variation of UV-vis spectra of PANH-Cr and PANH-Co chelates with time illustrating the increase of APX peak at 435 nm. The measurement solution contains 10⁻² M of OAP and 3 × 10⁻⁵ M of PANH-Cr and PANH-Co chelates; DMF is the solvent and aerobic condition.

Fig. 10 Variation of UV-vis spectra of PANH-Ni and PANH-Cu chelates with time illustrating the increase of APX peak at 433, 428 nm, respectively. The measurement solution contains 10⁻² M of OAP and 3 × 10⁻⁵ M of PANH-Ni and PANH-Cu chelates; DMF is the solvent and aerobic condition.

Table 5 Kinetic parameters for phenoxazine synthase-like activity of the metal complexes under interest

Comp. code	Kobs (min^−1)	V (M min^−1)	Vmax (M min^−1)	KM (M)	kcat (h^−1)
PANH-Mn	0.02401	1.31 × 10^−6	8.49 × 10^−5	0.0754	169.89
PANH-Co	0.0038	2.07 × 10^−7	4.74 × 10^−7	1.31 × 10^−2	0.948
PANH-Ni	4.241 × 10^−4	2.32 × 10^−8	2.398 × 10^−8	6.73 × 10^−4	0.048
PANH-Cu	0.0066	3.61 × 10^−7	4.461 × 10^−7	2.704 × 10^−3	0.892

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Kinetics studies were implemented in order to investigate the extent of activity of the metal chelates under study with respect to each other and with respect to reported compounds. This study was done using a concentration of 3 × 10⁻⁵ M of the catalyst. To this concentration of the catalyst, alternative substrate concentrations in the range of (1–8) × 10⁻³ M were separately added. The generates mixtures were subjected to UV-vis spectral recording once the catalyst and the substrate were mixed and after each 3 minutes until the time reached 21 minutes. The absorbances of the generated APX were recorded to calculate the initial rate for the catalytic mixtures. Saturation kinetics were deduced from the shape of the plot of substrate concentrations ([S]) against initial rate (V_o) as obvious in Fig. 11 & 12 demonstrating the pre-equilibrium formation of the complex-substrate intermediate. The rate-determining stage for the catalytic cycle is the irreversible substrate oxidation that developed APX as the product. Such mechanism is well described by the equation:

Fig. 11 Plots of Michaelis–Menten (left) and Lineweaver–Burk (right) of PANH-Mn.

Fig. 12 Plots of Michaelis–Menten (left) and Lineweaver–Burk (right) of PANH-Co.

Since the tested catalysts led to saturation kinetics, so, the most suitable equation for treating these reactions is the Michaelis–Menten equation.³⁴ Application of this equation lead to the calculation of the kinetic parameters V_max, K_M and the turnover number (k_cat, h⁻¹); Table 5.

Comparison of the results collected in Table 5 demonstrated that the Mn(II) complex PANH-Mn exhibited dramatically high activity in comparison with the rest of examined complexes.

Comparison of the activity of the applied complexes based on the obtained results demonstrated that PANH-Cr chelate did not show any activity for catalytic the interested reaction (i.e. oxidation dimerization of OAP). On the other hand, PANH-Mn complex afforded dramatically high activity in comparison with the rest of examined complexes as observed form its k_cat value of 169.89 h⁻¹. The other investigated complexes afforded relatively low activity with k_cat in the range 0.048–0.948 h⁻¹.

It is noted that for APX to be produced by the oxidation of OAP by the catalytic action of metal complexes, molecular dioxygen must be present because there almost ever arises a peak of APX in a similar pattern of behavior in the published work.^56,57 So, the prospected mechanism is shown in Scheme 2, taking the structure of the most efficient PANH-Mn complex as a model.²⁸

Scheme 2 proposed route for the conversion of OAP to APX by oxidative dimerization catalysed by PANH-Mn.

Since all the investigated complexes are formed with the same ligand, so the key factors affecting their activity are related to the metal centres incorporated in the complexes' structure, their type and oxidation state. Likening the efficacy of the complexes with respect to each other revealed that Cr(III) complex afforded no activity toward oxidation of OAP while the highest activity was exhibited by Mn(II) chelate. For the rest of complexes, the order of activity was PANH-Co > PANH-Cu > PANH-Ni. Precise realization of the complexes' structures also led to notice that all the active complexes contained chloro ligand in their structure while the inactive chromium complex contained acetate ligand in its coordination sphere leading to conclude that the coordinated anion type is also among the factors with dominant the catalysts' efficiency. This behavior can be explained by the accessibility of the first coordination sphere's labile sites, which is preferable for chloro anion over acetate group.³⁴ Also, the steric influence caused by the CH₃COO⁻ group's bulkiness relative to Cl⁻ affect the complexes' reactivity.

4. Conclusion

The trivalent Cr and the divalent Mn, Co, Ni and Cu chelates of the designed Schiff base ligand hydroxy-phenyl-acetic acid (2-hydroxy-naphthalen-1-ylmethylene)-hydrazide have been synthesized and investigated. Successful structure confirmation of the ligand and its metal chelates was achieved by the aid of ¹H-NMR, molar conductivity, FTIR, mass, UV-vis spectra, elemental analysis and magnetic moment in addition to analytical results. The empirical formulas of the target metal complexes have been assured to be: [Cr(PANH)(AOc)₂H₂O]·H₂O, [Mn(PANH)(H₂O)₃]·Cl·2H₂O, [Co(PANH)Cl(H₂O)₂]·3H₂O, [Ni(PANH)(H₂O)₃]·Cl·2H₂O and [Cu(PANH)] in which the complexes PANH-Cr, PANH-Mn, PANH-Co, PANH-Ni have been formed with the molar ratio 1 L : 1 M whereas PANH-Cu complex was formed in the ratio 2 L : 1 M. Molar conductivity values showed that all the complexes have non-electrolytic nature except PANH-Mn and PANH-Ni complexes are 1 : 1 electrolyte. From the results of FTIR spectra, the ligand has been confirmed to act as monobasic bidentate in PANH-Cu complex and monobasic tridentate in the structure of the rest complexes. Octahedral geometry was confirmed to all the complexes except PANH-Cu complex which assured to has square planar geometry as concluded from UV-vis spectral results and magnetic moment. The complexes were applied to estimate their activity in oxidation coupling of o-aminophenol (OAP) to 2-aminophenoxazine-3-one (APX) revealing an extremely high activity for the Mn(II) complex PANH-Mn with k_cat value of 169.89 h⁻¹. The other complexes afforded low efficiency with TOF numbers within 0.048–0.948 h⁻¹ range while PANH-Cr complex afforded no catalytic activity. The other examined complexes afforded relatively low activity in comparison to Mn(II) complex. The uneven catalytic behavior of the metal complex can be explained on the basis of different metal center included in the complexes' structure (i.e., Cr, Mn, Co, Ni and Cu), different oxidation states of metal centers (i.e. +2 or +3) and the different surrounding environments concerned with different coordinating anions (i.e. chloro or acetate).

Conflicts of interest

There are no conflicts to declare.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ra05769d

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