Structural Characterization of Heterodinuclear ZnII-LnIII Complexes (Ln = Pr, Nd) with a Ring-Contracted H2valdien-Derived Schiff Base Ligand

Synthesis and structural characterization of two heterodinuclear ZnII-LnIII complexes with the formula [ZnLn(HL)(µ-OAc)(NO3)2(H2O)x(MeOH)1-x]NO3 · n H2O · n MeOH [Ln = Pr (1), Nd (2)] and the crystal and molecular structure of [ZnNd(HL)(µ-OAc)(NO3)2(H2O)] [ZnNd(HL)(OAc)(NO3)2(H2O)](NO3)2 · n H2O · n MeOH (3) are reported. The asymmetrical compartmental ligand (E)-2-(1-(2-((2-hydroxy-3-methoxybenzylidene)amino)-ethyl)imidazolidin-2-yl)-6-methoxyphenol (H2L) is formed from N1,N3-bis(3-methoxysalicylidene)diethylenetriamine (H2valdien) through intramolecular aminal formation, resulting in a peripheral imidazoline ring. The structures of 1–3 were revealed by X-ray crystallography. The smaller ZnII ion occupies the inner N2O2 compartment of the ligand, whereas the larger and more oxophilic LnIII ions are found in the outer O2O2’ site. Synthesis and structural characterization of two heterodinuclear ZnII-LnIII complexes (Ln = Pr, Nd) bearing an asymmetrical compartmental ligand formed in situ from N1,N3-bis(3-methoxysalicylidene)diethylenetriamine (H2valdien) through intramolecular aminal formation are reported.


Introduction
Acyclic and macrocyclic Schiff base ligands are among the most extensively used ligands in coordination chemistry [1]. In general, Schiff bases can be readily prepared in good yields through condensation of primary amines with aldehyde or ketones. Owing to the ease of their synthesis, their versatility and ability to form stable complexes with almost all transition metals, Schiff base ligands have enormously contributed to the development of coordination chemistry and their transition metal complexes have been particularly important in bioinorganic chemistry, magnetochemistry, catalysis [2][3][4] and biomedical and related applications [5].

Synthesis of 1 and 2
Zn(OAc) 2 · 2 H 2 O (0.220 g, 1.0 mmol) dissolved in 10 mL of methanol was added to H 2 valdien (0.371 g, 1.0 mmol) dissolved in 10 mL of acetonitrile and the mixture was stirred under reflux at 40 °C for 1 h. Subsequently, the yellow precipitate so formed was added to Pr(NO 3 ) 3 · 6 H 2 O (0.435 g, 1.0 mmol) for 1 or Nd(NO 3 ) 3 · 6 H 2 O (0.438 g, 1.0 mmol) for 2 in 40 mL of methanol and the reaction mixture was refluxed for a further 3 h. The solution was then filtered and the filtrate was set aside undisturbed at ambient temperature. Yellow-brown crystals of 1 and yellow crystals of 2 suitable for single-crystal X-ray diffraction were obtained after several days. Analytical data for the compounds are given below. [

Physical Methods
Energy-dispersive X-ray spectroscopy (EDX) was undertaken on a Hitachi S3500N scanning electron microscope using a Si(Li) Pentafet Plus detector from Oxford Instruments GmbH with a 25 kV excitation voltage, 600 s measuring time and 100 × magnification from a fine powder sample sprinkled on a self-adhesive carbon guide tap. IR spectra were measured in the range 4000-400 cm -1 with a Bruker ALPHA Platinum-ATR FT-IR spectrometer. ESI mass spectra were recorded on a Q ExactiveTM Plus Orbitrap mass spectrometer (Thermo Scientific, Bremen, Germany).

X-ray Crystallography
The X-ray intensity data were collected on a Bruker AXS Kappa Mach3 APEXII diffractometer at T = 100(2) K, using Mo-K α radiation (λ = 0.71073 Å) from an Incoatec IµS microfocus X-ray source with Helios mirrors. The data were processed with SAINT [32] and absorption corrections were carried out with SADABS [33]. The crystal structures were solved with SHELXT [34] and refined with SHELXL-2018/3 [35]. Disordered parts of the structures were refined with appropriate geometrical restraints and using free variables for the occupancies (see supplementary crystallographic data). Carbon-bound hydrogen atoms were placed in geometrically calculated positions and refined using the appropriate riding model. Hydrogen atoms attached to nitrogen and oxygen were treated by semifree refinement using appropriate distance restraints. Some solvate methanol and water hydrogen atoms could not be located in the final difference Fourier map and were therefore excluded from the structure refinement. The structure of 3 was refined as an inversion twin, resulting in a Flack x parameter of 0.440 (19) [36].

Results and Discussion
The H 2 valdien ligand was prepared through Schiff base condensation of o-vanillin and diethylenetriamine in a 2:1 molar ratio [23]. Reaction with Zn(OAc) 2 · 2 H 2 O and, subsequently, with Ln(NO) 3  isomerization through an intramolecular aminal formation during the complexation reaction, resulting in an imidazolidine ring in the periphery. This phenomenon has been observed previously for the H 2 valdien ligand and it was suggested that ring contraction optimizes binding of the Zn II ion [27]. Some minor discrepancies between the sum formulae derived from elemental analysis of the bulk material as synthesized and those obtained from X-ray crystallography are ascribed to partial loss of co-crystallized solvents on drying before analysis. The presence of Zn and respectively, Pr and Nd in 1 and 2 was confirmed by EDX analysis (Fig. S1  in the supplementary material) Figure 1 depicts the molecular structures of the cationic complexes in 1 and 2 in the solid-state, as determined by X-ray crystallography. The structures of 1 and 2 were found to be isostructural. The Zn II ion occupies the inner N 2 O 2 compartment of the rearranged ligand, whereas the Ln III ion is situated in the outer O 2 O 2 compartment. The intramolecular distance between the two metal ions is ca. 3.5 Å. The coordination sphere of the five-coordinate Zn II ion is best described as square-pyramidal with the imine (N1) and aminal nitrogen (N2) atoms and the bridging phenolate oxygen atoms of the chelate ligand in the basal plane and an acetate oxygen atom in the apical position. The geometry index τ 5 is 0.31 for 1 and 0.32 for 2 [38], indicating that the coordination geometry lies between square-pyramidal and trigonal-bipyramidal but closer to square-pyramidal (C 4v symmetry). The Ln III ion is ten-coordinate with the two bridging phenolate and the two methoxy oxygen atoms of the chelate ligand occupying four coordination sites. The remaining positions are filled by two nitrate ions in a symmetrically bidentate coordination mode [39,40], a water or alternatively a methanol oxygen atom (site of O13), and an oxygen atom of the µ-acetato-κO,O' ligand. The coordination geometry of the Ln III ion can be best described as an approximate sphenocorona (C 2v symmetry), as determined by comparison with ideal polyhedra using continuous shape measures [41,42]. As structural consequence of the intramolecular aminal formation, the H 2 L compartmental ligand adopts a bent conformation with the mean planes of the two aromatic rings being almost perpendicular (dihedral angle ca. 80°). A similar conformation of the ligand was found for XODFOM. In the crystal, the aminal nitrogen atom N3 is protonated, making the complex cationic, and forms N − H···O hydrogen bonds to a methanol molecule of crystallisation and a nitrate counter ion, which balances the charge.
Serendipitously, we found a crystal in one crystallization batch of 2, representing an unknown methanol solvate hydrate of a co-crystal (3) (Fig. 2). In the latter, a water molecule occupies the apical position at Zn II and the acetato ligand binds solely to Nd III in a symmetrical bidentate fashion. The coordination geometries of Nd III and Zn II are retained in the two isomers. In 3, the geometry index τ 5 is 0. 14  The structures of 1 and 2 appear to be isostructural with the above-mentioned XODFOM, which has the reported molecular composition "[ZnLa(HL)(NO 3 ) 3 (S)](NO 3 )" (S = H 2 O or C 2 H 5 OH) [27], in which water and ethanol alternatively occupy one coordination site at La III . However, whereas a µ-acetato-κO,O' ligand bridges the Zn II and Ln III in 1 and 2, in XODFOM a bridging nitrate ion the Zn II and La III ions is reported. The reported N−O bond of the non-coordinating oxygen atom of the bridging nitrate ion in XODFOM is unusually long at 1.429(13) Å [39,40], and the corresponding atomic displacement parameters are rather large, which may be a warning sign for incorrect atom type assignment [43][44][45]. Taking the reported synthetic route into account, the presence of a bridging acetate ligand in XOD-FOM is a possibility, since the precursor complex described as "[Zn(valdien)] · 1.5 CH 3 OH", which was not structurally characterized by X-ray crystallography, was prepared from H 2 valdien and Zn(OAc) 2 · 2 H 2 O. Considering previous work by Naskar et al. [29], the constitution of the precursor complex might have been rather [Zn 2 (H 2 valdien) 2 (OAc) 2 ]. Elemental analysis calcd. for [Zn 2 (H 2 valdien) 2 (OAc) 2 ] (C 53.40, H 5.50, N 8.49%) differs little from that reported for "[Zn(valdien)] · 1.5 CH 3 OH" (C 53.10, H 5.80, N 8.30%) by Benetollo et al. [27]. We should note that the crystal structure of [Zn(valdien)] · CH 3 OH was published very recently [46], but an additional base (LiOH) was used in the synthesis contrary to the synthesis of "[Zn(valdien)] · 1.5 CH 3 OH" reported by Benetollo et al. The syn-syn bidentate bridging mode of the nitrate ion is known for inorganic nitrates [39,47], but is rather unusual for organic or organometallic nitrato complexes [40]. In this connection, we note that the external N−O distances in the crystal structure with the CSD refcode ADURAV, reportedly containing two syn-syn bridging bidentate nitrato ligands between La III and Zn II , at 1.506(18) and 1.52(2) Å are also suspiciously long [48]. This coordination mode is, in contrast, well known for carboxylate ions [49]. The comparable C21−C22 bond lengths of 1.499(3) and 1.495(6) Å in 1 and 2, respectively, and the corresponding atomic displacement parameters clearly support the presence of acetate ions at this site in XODFOM.

Conclusions
We have synthesized the heterodinuclear Zn II -Ln III complexes 1 and 2 by successive treatment of the H 2 valdien compartmental ligand with Zn(OAc) 2 · 2 H 2 O and, respectively, Pr(NO 3 ) 3 · 6 H 2 O and Nd(NO) 3 · 6 H 2 O, affording the asymmetrical, ring-contracted isomerized compartmental ligand H 2 L from H 2 valdien in situ. Such a rearrangement of the H 2 valdien ligand, which has been described the literature, is enabled through the reversibility of the Schiff base condensation. Its occurrence in the formation of 1 and 2 can be ascribed to a better accommodation of the smaller Zn II ion in the inner N 2 O 2 binding site instead of the inner N 3 O 2 site of the parent H 2 valdien. As anticipated, the Ln III ions are found in the outer O 2 O 2 ' compartment with counter ions and solvent molecules filling the remaining coordination sites of the ten-fold coordinated ions. Bond lengths, atomic displacement parameters and electron density maps resulting from the X-ray structural analysis provide clear evidence that the Zn II and Ln III ions in 1 and 2 are additionally linked by acetate anions in a synsyn bridging mode rather than by nitrate anions, as has been proposed for similar structures. Detection of the [Zn(HL)] + ion but no Ln III adducts by ESI mass spectrometry suggests that the binding of Zn II to the inner pocket of the ligand is, as expected, more stable than that of the Ln III ions in the outer compartment. The crystal structure of 3 reveals that structural isomers of 2 occur.

Acknowledgements
The authors would like to thank Heike Schucht for technical assistance with the X-ray intensity data collections, Sylvia Palm for the EDX measurements, Dirk Kampen for recording the ESI mass spectra and Professor Christian W. Lehmann for providing access to the X-ray diffraction and electron microscopy facilities at the Max-Planck-Institut für Kohlenforschung, Mülheim an der Ruhr, Germany. S.N. would like to thank the Head of the Department of Applied Sciences & Humanities, Faculty of Engineering & Technology, Jamia Millia Islamia, New Delhi, India, who facilitated this research.

Conflict of interest
There are no conflicts of interest/competing interests to declare.
Ethical Approval Not applicable.

Consent for Publication
All authors have seen the manuscript and agree to its publication.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.