The coordination behaviour of 2-(( E )-( tert -butylimino)methyl) phenol towards lanthanide nitrate and chloride salts

Five novel complexes were prepared by reacting 2-(( E )-( tert -butylimino)methyl)phenol (HL 2 ) with Ln(NO 3 ) 3 ∙ x H 2 O (Ln = Gd and Dy ; x = 6 and 5, respectively) and LnCl 3 ∙6H 2 O (Ln = Nd, Gd and Dy). The crystal structures of the former two Ln(III) nitrate complexes are isostructural and the coordination sphere is composed of three monodentate HL 2 ligands bonded to the metal centre through the phenolic oxygen atom and three nitrate ions coordinated in a bidentate fashion. Both complexes are nine-coordinate and SHAPE analysis reveals that they adopted a muffin polyhedra geometric type. The average Ln-O nitrate and Ln-O phenolate bond lengths are 2.5059 and 2.2816 Å, respectively. The complexes derived from the chloride salts exhibited an octahedral geometry with four monodentate Schiff base ligands [Ln-O phenolate distances range from 2.229(4) to 2.2797(18) Å] coordinating in the equatorial positions and two chloride ions [average Ln-Cl bond length is 2.6530 Å, and average Cl-Ln-Cl angles is 180 o ] in axial positions. The ligand coordinated through the phenolate oxygen with the phenolic proton migrating to the imino nitrogen to give a zwitterionic form of the ligand. There are weak C-H ∙∙∙ Cl interactions present and O-H ∙∙∙ N hydrogen bonds are also observed in the crystal packing.


INTRODUCTION
Schiff bases and their complexes have been investigated for their interesting and important properties, such as their ability to reversibly bind oxygen and their catalytic activity in the hydrogenation of olefins. 1,2 Schiff bases also display biological activity such as antibacterial, antifungal, antitumour and antioxidant properties. 3 On an industrial scale, these versatile ligands are used as pigments and dyes. They are also able to stabilize oxidation states of different metals, therefore controlling the performance of metals in numerous catalytic transformations. 4 The d-and f-block metal coordination compounds containing phenolate ligands have been studied due to their interesting properties, such as luminescence and catalytic activities. 5,6 Complexation studies of 2-(2′-hydroxyphenyl)benzimidazole (HPBI) with Nd(NO 3 3 OH indicated monodentate binding of HPBI. 5 The unit cell of the neodymium complex consisted of two [Nd(HPBI) 2 (NO 3 ) 4 ] − units with each Nd 3+ ion being ten-coordinate, with two phenolate oxygen atoms of the ligand and eight oxygen donor atoms from four bidentate nitrate ions surrounding the metal. Structural analysis of the ytterbium complex indicated that the Yb 3+ ion is lying on an inverse centre and is connected to two phenolate oxygen atoms donated from two ligands and four chloride ions. The resulting six-coordinate complex adopts an octahedral geometry. One protonated ligand is also present to balance the charge of the [Yb(HPBI) 2 Cl 4 ] − unit. It was also reported that both complexes displayed near-infrared luminescence which is characteristic of Nd 3+ and Yb 3+ ions. 5 Khorshidifard et al. reported transition metal complexes with the potentially bidentate N,O-donor ligand 2-tert-butyliminomethylphenol (HL 2 , Figure 1a), as depicted in Figure 1b. 6 Crystallographic data revealed that two Schiff base ligands coordinated to the metal centre via the two phenolate oxygen and the two imino nitrogen atoms. The geometries around the metal centres in the CoL 2 , CuL 2 and ZnL 2 complexes were distorted tetrahedron, while the palladium complex adopted a square-planar geometry. It was found that these complexes promoted the oxidation of thionisole to the corresponding sulfone and sulfoxide. Several factors were found to influence the reaction, viz. the reaction temperature, the geometrical structure of the catalyst and the oxidation of different sulphides. The ZnL 2 complex was found to display the best catalyst activity. It was proposed that the low catalytic activity of the CoL 2 complex was due to the geometry of the ligands, which increased steric hindrance, and thus lowered the accessibility of the metal centre in the catalyst. 6 Lanthanide chloride complexes would be expected to be better catalysts compared to their nitrate counterparts. This could be attributed to the fact that the coordination sphere of lanthanide nitrate complexes tends to be more saturated compared to the chloride complexes, because the oxygen donor-atoms of the nitrate ions are more electronegative than the chloride ions, making them harder to substitute during catalytic processes. The catalytic performance of a complex has been proposed to be dependent on the steric congestion around the metal centre. 7,8 Herein, we report the reactions of the Schiff base ligand HL 2 derived from tert-butylamine with Ln(NO 3 ) 3 •xH 2 O (Ln = Gd and Dy) and LnCl 3 •xH 2 O (Ln = Nd, Gd and Dy) to yield [Ln(NO 3 ) 3 (HL 2 ) 3 ] and  A Bruker Tensor 27 FT-IR spectrophotometer, equipped with the Platinum ATR attachment was used to obtain the infrared spectra of the compounds. The samples were run neat on ATR and the recorded data analysed with OPUS 6.5 software. The UV-Vis spectra were done on a PerkinElmer Lambda 35 UV-Vis spectrophotometer and processing performed using UV WinLab software. Nuclear Magnetic Resonance (NMR) spectra were recorded using a Bruker AvanceIII 400 NMR Spectrometer at 295K and acquisition of data done using TopSpin 3.0 software. The ACD/Labs software was used in the analysis of the NMR spectra.
A Bruker APEX II X-ray Crystallography System was utilized in single-crystal diffractometry at 200 K (λ = 0.71073 Å). The SAINT software was used for data reduction and cell refinement. 9 The structures were solved and refined using SHELXS97 and SHELX97 software, respectively. 10,11 Molecular graphics were obtained using ORTEPIII for Windows. 12 The coordination geometries of the Ln(III) complexes were evaluated using SHAPE 2.1 software and polyhedral representations created using VESTA software. Table 1 gives a summary of the crystal data and parameters for data collection and refinement for complexes 1-5.

Synthesis and spectral characterization
The synthesis of complexes 1 and 2 was carried out by reacting lanthanide nitrate salts and HL 2 in a 1:3 molar ratio in methanol as illustrated in Scheme 1. The reaction of Nd(NO 3 ) 3 •6H 2 O with HL 2 yielded thin flexible fibers which could not be analyzed crystallographically. Complexes 3-5 were prepared from the reactions of lanthanide chloride salts and HL 2 (Scheme 1). X-ray quality crystals of 1-5 were obtained by vapour diffusion using diethyl ether at 0 o C. The complexes are non-hygroscopic, crystalline solids which are air stable for months. All the compounds are partially soluble in methanol and ethanol, insoluble in acetonitrile, iso-propanol, dichloromethane and water, and soluble in dimethyl formamide and dimethyl sulfoxide.  16 In the IR spectrum of HL 2 ( Figure S1a; in supplementary information), the sharp band observed at 1628 cm −1 is characteristic of the >C=N-functional group stretch, and in 1 and 2 ( Figure S1b), this band appears at ca 1636 cm −1 , a shift of about +8 cm −1 . The spectrum of the free ligand also displays a band at 1281 cm −1 , which is assigned to the phenolic C-O group vibration. The band corresponding to the phenolic v(C-O) shifted to a higher wavenumber (~1286 cm −1 ), which is attributed to the formation of the metal-oxygen bond. The four bands attributed to the vibrations of the coordinated nitrate groups were observed at approximately 1481(v 4 ), 1286(v 1 ), 1032(v 2 ) and 818(v 6 ) cm −1 . The difference between v 4 and v 1 (195 cm −1 ) confirms the bidentate nature of the NO 3 − groups. 14,15 The infrared spectra of 3, 4 and 5 ( Figure S1c) presents the azomethine vibrational bands at about 1621-1639 cm −1 . The C-O stretches exhibited a blue shift in the range 1299-1303 cm −1 in the spectra of the complexes; this confirms the coordination of phenolic oxygens to the metal ions. The new bands appearing at 476-478 cm −1 in 1-5 are assigned to v(Ln-O). 8,15,17 The 1 H NMR spectrum of HL 2 in CDCl 3 ( Figure S2a; see Figure S2b for 13 C spectrum) shows a singlet at 8.52 ppm, which is attributed to the azomethine proton H7. The doublet at 7.45-7.43 ppm, the triplet at 7.31-7.30 ppm, the doublet at 6.95-6.93 ppm and the triplet at 6.89-6.87 ppm are assigned to protons H5, H3, H2 and H4, respectively. The phenolic proton appeared as a singlet at 2.00 ppm. The resonance at 1.31 ppm integrating to nine protons is assigned to the tert-butyl protons of HL 2 . In the 1 H spectrum of 1 ( Figure S2c The paramagnetic nature of complexes 1, 3 and 4 creates lanthanide induced shifts (LIS), which causes the proton resonances of the ligand to shift upon coordination. The LIS depends on the paramagnetic anisotropy of the Ln(III) ion, the geometrical position of the nucleus within the complex and the distance of the ligand's proton from the paramagnetic ion. 18 The NMR data reveals that the tert-butyl protons

RESEARCH ARTICLE
Kwakhanya Mkwakwi, Eric C. Hosten, Richard Betz, Abubak'r Abrahams and Tatenda Madanhire 39 S. Afr. J. Chem., 2023, 77, 36-41 https://journals.co.za/content/journal/chem/ experience a lesser shift, because of their special proximity to the nucleus. The NMR spectra of 1 and 4 also display signal broadening, which is attributed to the increased nuclear relaxation induced by the electronic magnetic moment. 19 However, the proton spectrum of 3 exhibited no line-broadening.
The overlay UV-Vis spectra of the free ligand and complexes 1 and 2 were recorded in DMF (Figure 2a). The intense band at 265 nm in the free ligand is due to π→π* transitions of the benzene ring. The bands at 315 nm and 400 nm are attributed to the π→π* and n→π* transitions of the phenolic -OH and the azomethine moieties, respectively. 20 The electronic spectra of 1 and 2 exhibited absorption bands similar to those of the free ligand; these bands are observed at about 265, 315 and 383 nm. The first two bands remained unaffected by coordination, whilst the last absorption band shifted to a lower wavelength. The spectra of complexes 3 and 5 (Figure 2b) Table 2). The coordination geometries of the nine-coordinate complexes 1 and 2 are classified as distorted muffin (MFF-9), with CShM values of 2.472 and 2.379, respectively (Figure 4). The relative path-deviation functions for complexes 1 and 2 are 19.3 and 18.7%, respectively. The imino nitrogen remained uncoordinated, and the Schiff base ligands in the complexes exist in their zwitterionic forms (upon complexation the phenolic proton was transferred to the imino nitrogen atom). This is similar to the monomeric ninecoordinate neodymium salicylaldimine coordination polymer described by the formula [Nd 2 (H 2 salen) 3 (NO 3 ) 6 ] n . 22 The bond parameters of complexes 1-5 are listed in Table 3. The average Ln-O nitrate bond lengths in 1 = 2.5166 and 2 = 2.4952 Å. These values compare well with the similar complex (H 2 PBI) 2 [Nd(HPBI) 2 (NO 3 ) 4 ] 2 ⋅2CH 3 OH⋅2H 2 O (average Ln-O nitrate = 2.5675 Å). 5 The average Ln-O phenolate bond lengths in 1 and 2 are 2.2986 and 2.2646 Å, respectively. In the Nd salicylaldimine complex, the average Nd-O phenolate bond length is 2.3443 Å. 22 The shorter Ln-O phenolate bond lengths compared to the Ln-O nitrate bond distances are due to more resonance delocalization of electrons (more stability) in the phenolate moiety compared to the nitrate ion. The shorter Ln-O bond distances in 2 compared to 1 is attributed to the lanthanide contraction. The Ln-O distances are all within and similar to those     23,24 In both complexes, the average phenolic C-O bond length is 1.305 Å, which is notably shorter than those of related free ligands, while the average C=N bond distance of 1.285 Å is slightly longer compared to similar free ligands. 22 Table 2 reveal that complexes 3-5 adopt the octahedron (OC-6) geometry, with four oxygen atoms occupying the equatorial positions and the chloride ions occupying the axial positions ( Figure 4). An increase in ChSM values as the ionic radius decreases can be attributed to steric crowding around the ion.
In complexes 3-5, the Ln-O distances range from 2.229(4) to 2.2797(18) Å ( Table 3) Table 4). In the crystal packing of 5, the hydrogen bonds are observed between the phenolic proton and the azomethine nitrogen. It is worth noting that the weak C-H•••Cl interactions in all the complexes play an important role in stabilising their structures.

CONCLUSION
Five novel complexes described by the general formulae [Ln(HL 2 ) 3 (NO 3 ) 3 ] (1 and 2) and [Ln(HL 2 ) 4 Cl 2 ]Cl (3-5) were prepared and characterized by the usual physico-chemical techniques including single-crystal X-ray diffractometry. The crystallographic data reveals that the potentially bidentate N,O-donor ligand HL 2 coordinated in a monodentate manner to the metal ions. The molecular structures of 1 and 2 reveals that their coordination spheres consist of three HL 2 ligands bonded to the metal centres via the phenolate oxygen and three nitrate ions; the latter coordinated in a bidentate manner and the complexes adopted a muffin (MFF-9) geometry.
Complexes 3-5 adopted a distorted octahedral geometry with the coordination environment around each metal centre consisting of four monodentate neutral ligands. In complexes 1-4 the phenolic proton migrated to the azomethine nitrogen atom to form a zwitterion; however, in 5 phenolic proton migration did not occur.

SUPPLEMENTARY DATA
Supplementary information for this article is provided in the online supplement. Crystallographic data for the reported complexes 1-5