Preparation, Characterization of Some Lanthanide(III) Complexes of N' -(phenyl(pyridin-2-yl)methylene)nicotinohydrazide

The present investigation describes the synthesis and structural study of the ligand N' - (phenyl(pyridin-2-yl)methylene)nicotinohydrazide (H L ), which was used to generate two type complexes formulated as [Ln(HL) 2 (NO 3 ) 2 (H 2 O)].(NO 3 ) (Ln = La ( 1 ) and Pr ( 2 )) and [Ln(HL) 2 (NO 3 )(H


Introduction
The Schiff bases resulting from the condensation reaction of nicotinic hydrazide and a ketoprecursor such as aldehydes and ketones are widely used in the synthesis of organic ligands in order to prepare coordination compounds with lanthanide ions and transition metals ions [1][2][3][4][5].These complexes are particularly interesting in various fields such as magnetism [6], catalysis [7], in medicine [8], and luminescence [9,10].The presence of phenolic moiety suggests that these complexes can be antioxidants as reported in the literature [11] but also as antibacterial agents [12,13].In recent studies, nicotinic hydrazide complexes were prepared and showed enzyme-like activities [14,15].It is in this context that we have undertaken to synthesize complexes at room temperature using nicotinic hydrazide and 2-benzoylpyridine in the presence of lanthanide salts.Several complexes have been isolated in powder form and are characterized by different techniques.

Materials and procedures
Nicotinic acid hydrazide, 2-hydroxy-3-methoxybenzaldehyde as well as Ln(NO 3 ) 3 .nH 2 O (Ln = La(III), Pr(III), Nd(III), Sm(III), Gd(III) or Tb (III)) were commercial products (from Alfa and Aldrich) and were used without further purification.Solvents were of reagent grade and were purified by the usual methods.Elemental analyzes were performed in a Carlo-Erba EA microanalyzer.Infrared spectra were recorded as KBr discs on a Bruker IFS-66V spectrophotometer.LSI-MS were recorded using a MicromassAutospec spectrometer with 3-nitrobenzyl alcohol as the matrix.The 133 mmol) in 20 mL of absolute ethanol.After dissolution, add nicotinic hydrazide (2.00g; 14.58 mmol) previously dissolved in 20 mL of ethanol.The mixture is refluxed for four hours.After filtration, a limpid brown solution is obtained.Left for slow evaporation, the solution gives crystals suitable for X-ray diffraction.IR ν(cm

General procedure for the synthesis of lanthanide complexes with the ligand HL
In a 100 mL round bottomed flask containing 10 mL of methanol, dissolve the ligand (HL) (0.100 g; 0.30 mmol).A brown solution is obtained.To this solution, add 0.24 mmol of Ln(NO 3 ) 3 .xH 2 O already dissolved in 10 mL of ethanol.The resulting solution was stirred at room temperature for one hour, then filtered and left for slow evaporation.After one week, the precipitate recovered was washed with 2 x 5 mL methanol followed by 2 x 5 mL ether before dried in the open air.The solids obtained are then recrystallized from the solvent mixture DMF/EtOH (50/50).After three weeks, yellow crystals which can be analyzed by X-ray diffraction are recovered for the La(III) complex.

Crystal structure determination
Crystals suitable for X-diffraction, of the reported compound, were grown by slow evaporation of MeOH solution of the complex.Details of the X-rays crystal structure solution and refinement are given in Table 1.Diffraction data were collected using a Preparation, Characterization of Some Lanthanide(III) Complexes … Earthline J. Chem.Sci.Vol. 10 No. 1 (2023), 131-146 135 Rigaku XtaLAB PRO diffractometer with graphite monochromatized Mo Kα radiation (λ = 0.71073 Å).All data were corrected for Lorentz and polarization effects.No absorption correction was applied.Using Olex2 [16] the structures were solved by intrinsic phasing methods with SHELXT [17] and SHELXL [18] was used for full matrix least squares refinement.The hydrogen atoms of water molecules and NH groups were located in the Fourier difference maps and refined.Others H atoms were geometrically optimized and refined as riding model by AFIX instructions.Molecular graphics were generated using ORTEP-3 [19].

General study
The infrared spectrum of the ligand presents towards the high frequencies two bands pointed at 3062 and 2924 cm -1 attributed respectively to the vibration of valence ν(N-H) and that of ν(C-H) of the aromatic rings.The vibration of the carbonyl group C=O is identified at 1684 cm -1 and that of the imine function C=N at 1630 cm -1 [20].Between 1541 and 1422 cm -1 the ν(C=C) vibrations of the aromatic rings show up, while the ν(C=N) band of the pyridine ring is located at 1583 cm -1 .The band pointed at 1282 cm -1   is attributed to the valence vibration ν(C-N).The vibration of the N-N bonds is located at 1142 cm -1 .The deformation vibrations of the aromatic C-H bonds are located between 804 and 614 cm -1 .Spectral analyzes of Nuclear Magnetic Resonance of the proton 1 H, of carbon 13 C are carried out using dmso-d 6 solutions.The 1 H NMR spectrum indicates a set of signals in the form of multiplets between 7.39 and 8.20 ppm and integrating eight protons attributed to those of the aromatic nuclei of this ligand.In addition to these signals, two singlets are identified at 8.73 and 9.01 ppm and each integrating a proton attributed respectively to the proton H15 and that of NH [21].The 13 C NMR spectrum indicates the signal of the carbon of the carbonyl function (C=O) at 160.10 ppm and that of the carbon of the imine function (C=N) at 148.91 ppm [9,11].The signal in the range 123.10-148.34 are assigned to the aromatic carbon atoms.The mass spectrum confirms the formation of the HL ligand with the presence of the peak at m/z=303.12 corresponding to the mass of the protonated molecular fragment [HL+H + ] (M+1).It should be recalled that we have determined the X-ray structure of this organic ligand examined its antioxidant activities [22].The infrared spectrum of the HL ligand shows bands at 1684, 1635 and 1583 cm -1 attributed, respectively, to the vibrations ν(C=O) of the carbonyl, ν(C=N) of the azomethine function and ν(C=N) of the pyridine ring.On the spectra of the complexes, these bands are observed with a shift towards low frequencies, respectively, in the regions (1633-1620 cm -1 ), (1597-1588 cm -1 ) and (1547-1532 cm -1 ).This fact indicates the coordination to the metal of the oxygen atom of the carbonyl, the nitrogen atom of the azomethine function and the nitrogen atom of the pyridine [23].In the high-frequencies region of the spectra of all the complexes, the broad bands pointed between 3210 and 3415 cm -1 are attributed to the ν(OH) vibration of the coordinated water molecule [24].On all the spectra, the three bands observed in the regions (1479-1459 cm -1 ), (1305-1282 cm -1 ) and (1099-1056 cm -1 ) are respectively due to the ν as (NO 2 ) (ν 1 ), ν s (NO 2 ) (ν 5 ) and ν(N=O) (ν 2 ) vibrations of the coordinated nitrate group (NO 3 -).The Preparation, Characterization of Some Lanthanide(III) Complexes … Earthline J. Chem.Sci.Vol. 10 No. 1 (2023), 131-146 137 magnitude value (Δν = ν 1 -ν 5 ) between 171 and 176 cm -1 shows that the nitrate group acts as a bidentate chelating ligand [25].The ν 3 vibration band of free nitrate (NO 3 -) of the lanthanum complex 1 is observed on the FTIR spectrum as a sharp band at ca.1379 cm -1 .The infrared spectroscopic data of the complexes are collected in Table 2.The conductometric measurements are taken in a millimolar solution of DMF.In fresh solution, the values obtained for the lanthanum complex (1) (93 Ω -1 .cm 2 .mol -1 ) and praseodymium complex) (95 Ω -1 .cm 2 .mol -1 ) (2) are indicative of a 1:1 electrolyte [26], while the other DMF solution of complexes of neodymium (3), samarium (4), gadolinium (5) and terbium (6) display Λvalues in the range 120 and 138 Ω -1 .cm 2 .mol -1 .These conductivity values are consistent with 2:1 electrolyte.After two weeks of storage, the values increase significantly for all complexes except for complex 4. Complex 1 becomes 2:1 electrolyte, complexes 2, 3, 5 and 6 become 3:1, while complex 4 remains 2:1 electrolyte.According to these conductometric data, only the complex 4 is stable in DMF solution.The conductivity measurement data of the complexes are grouped together in Table 3.
Based on spectroscopic, magnetic and conductimetric analyzes we concluded that the complexes are mononuclear.This indication is confirmed by both infrared spectroscopy and conductometric data.In the structure of these complexes, the ligand is tridentate and is coordinated to the lanthanide cations through the oxygen atom of the carbonyl function, the nitrogen atom of the azomethine function and the nitrogen atom of the pyridine ring.It is also noted that in the solid state, the complexes are in salt form.
The UV-visible spectra of the complexes recorded in DMF are similar and the absorption bands observed in (257-284) and (313-389) nm regions, which are also present in the spectrum of the free ligand are attributed to the π → π* and n → π* transitions.The additional absorption bands observed between 402 and 483 nm are assigned to the charge transfers of the C=N chromophore of the coordinated ligand.On all the spectra of the complexes, we observe the presence of bands of low intensity in the region (505-516 nm), which can be attributed to the 4f →4f transitions of Ln 3+ ions.These forbidden electronic transitions can become allowed after elimination of the degeneracy of the 4f orbitals by an external crystal field.For this purpose, we observe weak bands of f-f transitions which are often obscured by the intense bands of charge transfers [27,28] .
Apart from the diamagnetic lanthanum complex (1), magnetic measurements at room temperature show that the complexes are paramagnetic.The values of magnetic moments 138 found are very close to those of free ions [29].This result shows that the complexes are mononuclear.The UV-visible spectrophotometric data and magnetic measurements of the complexes are recorded in Table 4. Structures are proposed in Figure 1.
Table 2. Infrared data of the ligand HL and the complexes.

Compounds
Fresh DMF solution 15days (after)  Once isolated, complex 1 was found to be air-stable and soluble in common organic solvents such as methanol, DMF and DMSO.The mononuclear complex [La(η 3 -HL) 2 (η 2 -NO 3 ) 2 (η-H 2 O)] .(NO 3 ) (1) crystallizes in the orthorhombic system Pbcn.Crystal and structure refinement data are consigned in Table 1.The selected bond lengths and angles are summarized in Table 5.The ORTEP plots is shown in Figure 2. The asymmetric unit contains one lanthanum ion, two neutral ligand molecules, one coordinated water molecule, two coordinated nitrate anions and one uncoordinated nitrate anion.Each of the two ligand molecules is coordinated to the La 3+ in η 3 -mode, through one carbonyl nitrogen atom, one azomethine nitrogen atom and one nitrogen pyridine atom.Each of the two coordinated nitrate anions acts in bidentate chelating η 2 -mode.Thus, the central metal atom is eleven coordinated (Figure 3).The environment around the La(III) is best 140 described as a distorted pentacapped trigonal prism.The interatomic distance La-O1(carbonyl) which is equal to 2.506(4) Å, is shorter than those of La-ONO 2 of bidentate nitrate anions, which are respectively 2.733( 5) Å [La-O3] and 2.741 ( 6) Å [La-O4].These values are in the range expected for La-ONO 2 [30][31][32].The La-O bond involving coordinated water molecule has a distance of 2.560 ( 7) Å [La-O5] which is shorter than the bond length La-OH 2 (2.659(3) Å) reported for a similar complex [33].The La-N involving the azomethine nitrogen atom and the pyridine nitrogen atom are the longest distance with respective values of 2.780( 5)Å [La-N2] and 2.760( 5) Å [La-N1] (Table 5).The bond lengths of the La-O carbonyl and La-N azomethine are similar to those reported by Tamboura et al. [34]  The hydrogen atom from each NH group is involved in an intramolecular N-H⋯ONO 2 hydrogen bond with an oxygen atom of uncoordinated nitrate group.An oxygen of a coordinated nitrated groups acts as donor in a C-H⋯O hydrogen bond thus forming a S(5) ring.Intramolecular hydrogen bonding in which the coordinated water molecule acts as donor and the nitrogen atom and the oxygen atom of the same nitrate group act as acceptor [O5-H5•••N5 ii ; O5-H5•••O3 ii ii = −x+1, y+1, −z+1/2] (Table 6) interconnect the molecules (Figure 4).

Conclusion
This present work describes the results of the preparation of coordination complexes of lanthanide (III) ions with the ligand hydrazide (HL).The ligand and the complexes are characte spectrometric methods to confirm their structures in solid state and in solution.FTIR spectroscopy shows that the ligand remains in its neutral form upon complexation.The coordinate nitrate cations act as bidentate chelating while the free.UV-Vis spectrophotometry indicates internal transition in the ligand and metal ligand charge transfer.Conductimetric studies shows that complex electrolyte while complexes of the complex 1 was elucidated by X   ).The ligand and the complexes are characterized by different spectrometric methods to confirm their structures in solid state and in solution.FTIR spectroscopy shows that the ligand remains in its neutral form upon complexation.The coordinate nitrate cations act as bidentate chelating while the others nitrate cations remain Vis spectrophotometry indicates internal transition in the ligand and metal ligand charge transfer.Conductimetric studies shows that complex 1 electrolyte while complexes 3-6 are 2:1 electrolyte in fresh DMF solutions.The structure was elucidated by X-ray diffraction study and formulated as (NO 3 ).

Figure 4 .
Figure 4. Crystal packing of 1 as seen along c axis.

Table 3 .
Conductimetric data of complexes in DMF.

Table 4 .
UV-Visible and Magnetic moments of the complexes.