PREPARATION AND PHYSICO-CHEMICAL PROPERTIES OF LANTHANIDE(III) COMPLEXES WITH 2-(( E )-( TERT -BUTYLIMINO)METHYL)- 6-METHOXYPHENOL

. The reactions of lanthanide nitrates with 2-(( E )-( tert -butylimino)methyl)-6-methoxyphenol (HL 1 ) have yielded five complexes that are described by the formulae [Ce(HL 1 ) 2 (NO 3 ) 3 ]∙MeOH and [Ln(HL 1 ) 2 (NO 3 ) 3 ] (Ln = Nd(III), Gd(III), Ho(III) and Er(III)) and were characterized using physico-chemical techniques including single-crystal X-ray diffraction spectroscopy. The cerium complex crystallized in a triclinic P-1 space group, while the rest of the complexes crystallized in the monoclinic P21/c space group. All the complexes are ten-coordinate, adopting a tetradecahedron geometry with two HL 1 molecules coordinated through the phenolic and methoxy oxygen atoms. The coordination sphere is completed by six oxygen atoms from three bidentately coordinated nitrate ligands. Electronic data reveals that only the neodymium, holmium and erbium complexes exhibit weak f - f transitions in the visible


INTRODUCTION
Schiff base ligands with oxygen and nitrogen donor atoms in their structures are good chelating agents for transition and non-transition metals, as well as in the preparation of heterometallic 3d-4f complexes [1][2][3][4].The nitrogen and oxygen donor atoms are suitable for the complexation of transition metal ions, while the oxygen donor atoms prefer oxophilic ions, such as lanthanide ions [4].

Materials and physical measurements
The following organic solvents were used in the study: methanol, ethanol, dimethyl formamide, pyridine and diethyl ether.Dimethyl formamide was purchased from Sigma-Aldrich, while all the other solvents were sourced from Merck and used without further purification.The deuterated solvents utilized in NMR spectroscopy, CDCl3 and DMSO-d6 were obtained from Merck.The lanthanide nitrates, Ln(NO3)3•xH2O (Ln = Ce, Nd, Gd, Ho and Er; x = 6 for Ce, Nd and Gd; x = 5 for Ho and Er) with 99.9% purity were sourced from Sigma-Aldrich.o-Vanillin (99.0%) and tert-butylamine (99.0%) used in the preparation of the organic ligand were purchased from Sigma-Aldrich and Riedel-de Haën, respectively, and used as supplied.
A Bruker Tensor 27 FT-IR spectrophotometer (equipped with the Platinum ATR attachment) was utilized to obtain the infrared spectra of all the compounds synthesized in this study.The samples were run neat on ATR and the data recorded and analysed with OPUS 6.5 software.The UV-Vis spectra were carried out on a PerkinElmer Lambda 35 UV-Vis spectrophotometer and processing done using UV WinLab software.Nuclear Magnetic Resonance (NMR) spectra were recorded at 295K using a Bruker AvanceIII 400 NMR Spectrometer and acquisition done using TopSpin 3.0 software.Analysis of the NMR spectra was performed using ACD/Labs software.A Bruker APEX II CCD diffractometer was used to record X-ray crystallographic data.The data reduction and cell refinement were done using SAINT software [11].The crystal structures were solved and refined using SHELXS97 and SHELX97 software, respectively [12,13].Molecular graphics were obtained using ORTEP-3 [14] and Mercury [15].The coordination geometries of the lanthanide complexes were analysed using SHAPE 2.1 software.A summary of the crystal data and parameters for data collection and refinement for complexes 1-5 is shown in Table 1.

RESULTS AND DISCUSSION
Complexes 1-5 were synthesized by the reaction of three equivalents of HL1 with lanthanide nitrate salts in methanol (Scheme 1).The resulting mixtures were kept at room temperature and single crystals formed after 24 hours.All the complexes were found to be stable in air for months, are non-hygroscopic and readily dissolved in DMF, DMSO and pyridine.However, they were partially soluble in both hot and cold methanol and ethanol.The complexes were found to be insoluble in acetonitrile, chloroform, chlorobenzene, diethyl ether, dichloromethane, isopropyl alcohol and water.
The molar conductivity of 10 -3 M solutions of the rare-earth complexes in DMF were measured at ambient temperature.The molar conductivity of 1 and 4 is 160.7 and 130.6 ohm -1 cm 2 mol -1 , respectively, which fall within the 2:1 electrolyte type range [16].The conductivity of complexes 2, 3 and 5 are 96.0,120.6 and 212.0 ohm -1 cm 2 mol -1 , respectively.These values fall outside both the 1:1 and 2:1 electrolyte type ranges.

IR and NMR spectroscopy
The IR spectra of HL1 exhibited a series of weak bands from 2967 to 2907 cm -1 , which are due to the C-H stretching vibrations.The weak intensity band at 2837 cm -1 is attributed to the v(-OCH3), while the strong intensity band at 1626 cm -1 is attributed to the azomethine absorption.The stretching frequency at 1251 cm -1 arises from the C-O vibration of the phenol group.
Upon coordination to the metal, most of the ligand's C-H band intensities are reduced significantly except for the stretching vibration at about 2978 cm -1 which has shifted to a higher frequency compared to the free ligand.The C=N stretching frequency, which was observed at 1641 cm -1 in the free ligand, has shifted to a higher wavenumber by approximately 15 cm -1 upon coordination.The three bands near 1453 cm -1 (v4), and those at 1294 cm -1 (v1), 1037 cm -1 (v2) and 817 cm -1 (v6) can be attributed to the coordinated nitrate ions [17].The difference between the peak positions of v4 and v1 is 209 cm -1 which is indicative of bidentately coordinated nitrates [4,18].Coordination of the phenolic oxygen was confirmed by the blue shift of the C-O vibration frequency from 1251 to about 1299 cm -1 .The v(Ln-O) band in the complexes ranges from 411 to 420 cm -1 [19].
The 1 H and 13 C NMR spectra of HL1 were recorded in CDCl3 and DMSO-d6, respectively.The peak at 8.26 ppm is assigned to proton 8 of the azomethine (C=N) group.The doublet at 6.84 ppm integrating to two protons is attributed to protons 2 and 4 of the benzene ring.The triplet between 6.72-6.68ppm is due to the resonance of proton 3. The phenolic proton resonates as a singlet at 3.88 ppm.The peak at 3.86 ppm is assigned to the three o-vanillin protons.The singlet peak at 1.33 ppm is attributed to the tert-butyl protons.
Complexes 1-5 exhibit similar 1 H NMR spectra, each containing a broad peak at approximately 14.67 ppm which was attributed to the N-H resonance of the zwitterion proton.The resonance of proton H8 is observed at about 8.58-8.54ppm which is a downfield shift compared to the free ligand (8.26 ppm).The chemical shift of H2 and H4 in 2, 4 and 5 appear at 7.02, 7.04 and 7.04 ppm, respectively, as a broad singlet.In complexes 1 and 3, the resonance of protons H2 and H4 appears as triplets between 7.05-6.96and 7.06-6.97ppm, respectively.In complexes 1 and 3, the aromatic proton H3 appears as doublet at about 6.73 ppm, and in complexes 2, 4 and 5, proton H3 appears as a singlet at about 6.74 ppm.It was also noted that the aromatic protons H2, H4 and H3 shifted down-field upon coordination of the ligand.The absence of the phenolic proton signal at ~3.89 ppm in 1-5 confirms the zwitterionic nature, and subsequent coordination of the ligand.In all the complexes, the singlet observed at about 3.75 ppm is assigned to the resonance of the methoxy protons; the up-field shift of these protons is attributed to the coordination to the paramagnetic Ln(III) ions.This is caused by the pseudocontact shift as the result of a through-space dipolar interaction between the magnetic moments of the metal ion and the methoxy protons [20].The chemical shift of the tert-butyl protons is identical to that of the free ligand.
The NMR analysis of the compound reveals several key chemical shifts for different carbon atoms.The imine group carbon atom (-C=N-) shows a chemical shift of 161.75 ppm.The aromatic carbons (C1-C6) exhibit chemical shifts ranging from 117.08 to 154.44 ppm.The carbon atom of the methoxy group is observed at a chemical shift of 56.25 ppm, while C9 is found at a slightly higher chemical shift of 56.83 ppm, likely due to its proximity to the imine group.The methyl group carbons (C10-C12) belonging to the butyl moiety display a chemical shift of 29.75 ppm.

Electronic spectroscopy
The electronic spectra of a 0.05 mM DMF solution of the HL1 ligand and complexes were recorded, with the UV-Vis spectrum of the free ligand showing three main maxima at 268, 332 and 416 nm, which are attributed to π → π* transitions of the aromatic ring and the imine group (Figure 3a).The spectrum of 1 reveals no net change to the peak at 268 nm, while at 335 and 402 nm a red and blue shift is observed, respectively.The electronic spectra of 2-5 are similar, with the average absorption maxima being 268, 332 and 414 nm.The slight shifts in the maxima could be attributed to the changes in the energy levels of the ligand orbitals upon complexation [4].
The spectra of 1 mM DMF solutions of complexes 2, 4 and 5 display weak absorption maxima in the visible region, which are characteristic of Laporte forbidden f-f transitions, as depicted in Figure 3b.The six bands in the visible region of 2 were shifted to longer wavelengths compared to the free aqua complex (see Table 2) -this shift is due to the nephelauxetic effect [21][22][23].The red shift is accepted as an indication of a higher degree of covalency in the compound of interest compared to its aqua compound [24].The f-f transitions in complex 4 shifted to shorter wavelength relative to the literature values.The following transitions in complex 5 experienced no significant shifts: 4 I15/2 → 4 H9/2 and 4 I15/2 → 4 S3/2.However, the transition 4 I15/2 → 2 H11/2 showed a blue shift compared to the free Ln 3+ ions in water.

X-ray crystallography
Complex 1 crystallized in a triclinic (P-1) space group, while the isostructural complexes 2-5 crystallized in the monoclinic (P21/c) space group.Single-crystal X-ray diffraction data reveals that all the ten-coordinate complexes have identical primary coordination spheres composed of two bidentately coordinated HL1 molecules which are bonded through the phenolic and methoxide oxygens, and also three bidentately coordinated nitrates (Figures 4a and 4b).The Ce(III) complex is distinguished from 2-5 by possessing one crystallographic methanol molecule.The geometry around the metal centers were analysed using SHAPE 2.1 [25].The preferred geometry for all the complexes is distorted tetradecahedron (TD-10) which is characterized by 123+24 faces, 22 edges and D2d symmetry (Figure 4c) [26].Figure 4d illustrates the relationship between the continuous shape measurement (CShM) values of TD-10 geometry and the ionic radii of the lanthanides.It can be seen that there is a decrease in CShM values as the ionic radii decreases from Ce(III) to Er(III) due to decreased room for angular distortions and stronger metal-ligand binding as the metal ions get smaller [27].This trend reveals that the heavier lanthanide ions prefer the tetradecahedron geometry compared to the earlier ions.
Figure 4c.The distorted polyhedral representation of the tetradecahedron geometry (complex 2), generated by VESTA.
Bond parameters of the metal complexes are gathered in Table 3a.The average bond lengths of the Ln-O(phenolate) and Ln-O(methoxide) are 2.329 and 2.648 Å, respectively.The Ln-O(nitrate) bond distances lie between 2.4190(2)-2.6233(15)Å in 1-5.It was observed that the Ln-O(phenolate) bond length is relatively shorter than the Ln-O(methoxide).This can be attributed to the fact that the phenolate oxygen has a higher electronegativity compared to the methoxide oxygen, and thus a greater attraction exists between the phenolate oxygen and the metal ion, which consequentially leads to a shorter bond length.All the observed Ln-O bond lengths lie within the range of the literature values reported for similar complexes containing bidentate O,O-donor ligands (Ln-O(phenolate), Ln-O(methoxide) and Ln-O(nitrate) varies from 2.2257-2.726Å) [28,29].Upon coordination of the ligand, keto-enol tautomerism occurred in which the phenolic proton migrated to the imino nitrogen.The positively charged C=N-H + group is countered by the negative charge on the phenolate ring.The double bond nature of the azomethine (C=N) and phenolate (O=C) groups are confirmed by bond lengths of 1.288 and 1.305 Å, respectively.These bond lengths compare well with similar lanthanide complexes containing ligands displaying keto-enol tautomerism [30].The N1-C12-C111 bond angle of 123.47(1) o supports the sp 2 hybridization of C12.The C3-C4 and C5-C6 bond distances are 1.374 and 1.369 Å, respectively, suggesting the presence of a double bond.The latter is comparable to the literature value (average C-C bond length = 1.364Å) [31].Thus, the crystallographic data supports the ketonic form present in the complexes.Figure 4e illustrates the effect of the lanthanide contraction on the average Ln-O(phenolate), Ln-O(methoxide) and Ln-O(nitrate) bond distances when traversing from Ce to Er.It is expected that the bond distance will gradually decrease as the ionic radius decreases due to the lanthanide.This contraction is attributed to the ineffective screening of the nucleus by the 4f electrons, leading to a stronger attraction of the nucleus to the electrons in the outermost shell as the nuclear charge increases.The net result is contraction in the atomic radius [32]

1 Scheme 1 .
Scheme 1.General synthetic protocol for complexes 1-5, with atom numbering scheme for the ligand.

Figure 3b .
Figure 3b.Visible spectra of 10 -3 M solutions of HL 1 and complexes 2, 4 and 5 (complexes 1 and 3 exhibit no f-f transitions and are excluded from the plot).
Figure4eillustrates the effect of the lanthanide contraction on the average Ln-O(phenolate), Ln-O(methoxide) and Ln-O(nitrate) bond distances when traversing from Ce to Er.It is expected that the bond distance will gradually decrease as the ionic radius decreases due to the lanthanide.This contraction is attributed to the ineffective screening of the nucleus by the 4f electrons, leading to a stronger attraction of the nucleus to the electrons in the outermost shell as the nuclear charge increases.The net result is contraction in the atomic radius[32].The average Ln-O(nitrate) bond distance steadily decreases and, except for the slight deviations in the Ho-O bond length, both the Ln-O(phenoxide) and the Ln-O(methoxide) distances gradually decrease from Ce to Er.The graph also reveals that, on average, the Ln-O(phenoxide) bond length is shorter than both the Ln-O(methoxide) and the Ln-O(nitrate) lengths.It is worth noting that the (O-Ln-O)HL1 and (O-Ln-O)nitrate bond angles lie in the ranges 60.89(4)-65.43(6)o and 48.72-52.10o , respectively, and these bond angles gradually increase as the ionic radius decrease.The observed (O-Ln-O)HL1 and (O-Ln-O)nitrate angles are similar to those found in the literature (55.71-64.51o and 47.6-51.07o , respectively)[28, 29, 32,

Figure 4e .
Figure 4e.The plot of the average Ln-O bond lengths in complexes 1-5 versus the lanthanide ionic radius.

Table 1 .
Crystal and structure refinement data for complexes 1