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].

Polydentate Schiff base chelate ligands derived from condensation of 2-hydroxy-3-methoxybenzaldehyde (o-vanillin) or derivatives as aldehyde component with polyamines have been exploited for the synthesis of homo- and heterodinuclear complexes. These have attracted research attention because of interesting properties such as magnetism and luminescence [6,7,8,9,10,11,12,13,14,15,16,17,18]. N1,N3-bis(3-methoxysalicylidene)diethylenetriamine (H2valdien), which is obtained via Schiff base condensation of two equivalents of o-vanillin with diethylenetriamine, belongs to this class of ligands. After deprotonation of the phenol moieties, the resulting valdien2− anion represents a compartmental ligand [19, 20], providing an inner binding site with an N3O2 donor set and an outer binding site with an outer O2O2’ donor set. Whereas the inner compartment is expected to accommodate metal ions from the 3d row, the outer site may bind the more oxophilic lanthanide ions [6, 7, 21, 22]. Nevertheless, the inner compartment can also host lanthanide ions [8, 23,24,25,26]. Moreover, the H2valdien ligand has also been encountered in a chelate-spacer-chelate bridging mode in dinuclear complexes with the aliphatic secondary amine linkages remaining unbound to the metal ions [27,28,29,30].

Herein, we report on the synthesis and structural characterization of two heterodinuclear ZnII-LnIII complexes with the general formula [ZnLn(HL)(µ-OAc)(NO3)2(H2O)x(MeOH)1-x]NO3 · n H2O · n MeOH [Ln = Pr (1), Nd (2)], containing bridging acetate ions and the asymmetrical compartmental ligand (E)-2-(1-(2-((2-hydroxy-3-methoxybenzylidene)amino)-ethyl)imidazolidin-2-yl)-6-methoxy-phenol (H2L) formed in situ from H2valdien through isomerization by intramolecular aminal formation, resulting in a peripheral imidazoline ring [27]. Such rearrangements are due to the reversibility of the Schiff base (imine) formation [31]. The rearranged ligand H2L thus formed likewise represents a compartmental ligand but exhibits lower symmetry and a smaller inner compartment than the parent H2valdien ligand. In addition, the structure of the serendipitously discovered compound [ZnNd(HL)(µ-OAc)(NO3)2(H2O)] [ZnNd(HL)(OAc)(NO3)2(H2O)](NO3)2 · n H2O · n MeOH (3) is described.

Experimental Section

General

The H2valdien ligand was synthesised as described in the literature [23]. Zn(OAc)2 · 2 H2O (Fischer Scientific), Pr(NO3)3 · 6 H2O and Nd(NO3)3 · 6 H2O (Sigma Aldrich) were purchased and used as received. Methanol was of reagent grade. CHN microanalysis was carried out by Mikroanalytisches Labor Kolbe (Mülheim, Germany).

Synthesis of 1 and 2

Zn(OAc)2 · 2 H2O (0.220 g, 1.0 mmol) dissolved in 10 mL of methanol was added to H2valdien (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(NO3)3 · 6 H2O (0.435 g, 1.0 mmol) for 1 or Nd(NO3)3 · 6 H2O (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.

[ZnPr(HL)(µ-OAc)(NO3)2(H2O)0.35(MeOH)0.65]NO3 · 2 MeOH · H2O (1; L2−  = C20H23N3O42−): Yield: 0.580 g (0.62 mmol, 62%). Anal. calcd. for C24.65H40.30N6O19PrZn (M = 931.00 g mol−1): C 31.8, H 4.4, N 9.0%; found: C 31.9, H 4.3, N 9.2%. MS(ESI+): m/z [H3L]+ calcd. for C20H26N3O4+ 372.2, found 372.2, [Zn(HL)]+ calcd. for C20H24N3O4Zn+ 434.1, found 434.1; IR(ATR): 1640 cm–1 (C = N stretch).

[ZnNd(HL)(µ-OAc)(NO3)2(H2O)0.75(MeOH)0.25]NO3 · 3 MeOH (2; L2−  = C20H23N3O42−): Yield: 0.570 g (0.60 mmol, 60%). Anal. calcd. for C25.24H41.50N6NdO19Zn (M = 942.64 g mol−1): C 32.2, H 4.4, N 8.9%; found: C 32.1, H 4.3 N 9.2%. MS(ESI+): m/z [H3L]+ calcd. for C20H26N3O4+ 372.2, found 372.2, [Zn(HL)]+ calcd. for C20H24N3O4Zn+ 434.1, found 434.1; IR(ATR): 1640 cm–1 (C = N stretch).

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-Kα 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 semi-free 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].

Crystal data and refinement details for 1: C25H39.68N6O18.34PrZn, Mr = 924.03, monoclinic, P21/n, Z = 4, a = 11.3498(15) Å, b = 14.8170(19) Å, c = 21.488(3) Å, β = 101.864(2)°, V = 3536.5(8) Å3, F(000) = 1870, crystal size 0.223 × 0.066 × 0.031 mm, ρcalcd = 1.735 g cm−3, μ = 2.125 mm−1, 2θmax = 62.22°, reflections collected/unique 104,376 / 11,339 (Rint = 0.0522), parameters/restraints 512/16, R1 [I > 2σ(I)] = 0.0246, wR2 (all data) = 0.0592, S = 1.031, Δρmax/Δρmin = 0.65/ − 0.46 e Å−3.

Crystal data and refinement details for 2: C23.94H35.26N6NdO18.90Zn, Mr = 919.13, monoclinic, P21/n, Z = 4, a = 11.3439(13) Å, b = 14.7501(18) Å, c = 21.454(3) Å, β = 101.914(2)°, V = 3512.4(7) Å3, F(000) = 1848, crystal size 0.086 × 0.044 × 0.024 mm, ρcalcd = 1.736 g cm−3, μ = 2.231 mm−1, 2θmax = 62.14°, reflections collected/unique 153,810/11,217 (Rint = 0.1000), parameters/restraints 529/85, R1 [I > 2σ(I)] = 0.0384, wR2 (all data) = 0.0942, S = 1.048, Δρmax/Δρmin = 1.48/ − 1.30 e Å−3.

Crystal data and refinement details for 3: C22H27N6NdO18.05Zn, Mr = 873.87, monoclinic, Pn, Z = 4, a = 11.2800(2) Å, b = 15.749(3) Å, c = 18.5316(8) Å, β = 102.908(2)°, V = 3209.0(6) Å3, F(000) = 1742, crystal size 0.130 × 0.060 × 0.030 mm, ρcalcd = 1.809 g cm−3, μ = 2.435 mm−1, 2θmax = 66.18°, reflections collected/unique 88,652/24,240 (Rint = 0.1126), parameters/restraints 869/2, R1 [I > 2σ(I)] = 0.0705, wR2 (all data) = 0.1700, S = 1.025, Δρmax/Δρmin = 2.08/ − 1.48 e Å−3.

Results and Discussion

The H2valdien 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 H2O and, subsequently, with Ln(NO)3 · 6 H2O (Ln = Pr or Nd) in methanol under reflux conditions afforded the heterodinuclear ZnII-LnIII complexes 1 (Ln = Pr) and 2 (Ln = Nd), as depicted in Scheme 1. X-ray crystallography revealed that the H2valdien compartmental ligand underwent an 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 H2valdien ligand and it was suggested that ring contraction optimizes binding of the ZnII 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). The IR band at 1640 cm−1 observed for both 1 and 2 (Figs. S2 and S3 in the supplementary material) is assigned to the imine C = N stretching vibration and agrees well with that reported for a heterodinuclear ZnII-LaIII complex having the reported formula “[ZnLa(HL)(NO3)(S)](NO3)”, where S = H2O or C2H5OH, (CSD refcode: XODFOM) [27] and [Zn4Dy2(L)2(L’)2(N3)2]Cl2 · 2 H2O (BIRXEI), [Zn4Tb2(L)2(L’)2(Cl)2][ZnN3Cl3] · 2 H2O (BIRXOS), and [Zn4Gd2(L)2(L’)2(Cl)2][ZnN3Cl3] · 2 H2O (BIRZAG) [37], containing the same rearranged compartmental ligand (herein abbreviated H2L) and in the case of the latter three complexes the carbamate ligand L’, derived from L by the absorption of CO2 from the air in the presence of ZnII. In the ESI+ mass spectra of 1 and 2 (Figs. S4-S7 in the supplementary material), the peaks at m/z 372.2 and 434.1 can be assigned to the fragment ions [H3L]+ and [Zn(HL)]+, respectively.

Scheme 1
scheme 1

Synthesis of 1 and 2. The coordination site on Ln occupied by water or methanol in the solid-state is represented by water only in the diagram for the sake of clarity. Co-crystallized solvent molecules are not shown

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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 ZnII ion occupies the inner N2O2 compartment of the rearranged ligand, whereas the LnIII ion is situated in the outer O2O2 compartment. The intramolecular distance between the two metal ions is ca. 3.5 Å. The coordination sphere of the five-coordinate ZnII 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 τ5 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 (C4v symmetry). The LnIII 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 LnIII ion can be best described as an approximate sphenocorona (C2v symmetry), as determined by comparison with ideal polyhedra using continuous shape measures [41, 42]. As structural consequence of the intramolecular aminal formation, the H2L 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.

Fig. 1
figure 1

Molecular structures of 1 and 2 in the crystal. Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for those attached to nitrogen), counter ions and solvent molecules are omitted for clarity. The site of O13 is occupied by water or methanol (not shown here for the sake of clarity) in the crystal

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Serendipitously, we found a crystal in one crystallization batch of 2, representing an unknown methanol solvate hydrate of a co-crystal (3) of [ZnNd(HL)(µ-OAc)(NO3)2(H2O)]NO3 and a structural isomer [ZnNd(HL)(OAc)(NO3)2(H2O)]NO3 (Fig. 2). In the latter, a water molecule occupies the apical position at ZnII and the acetato ligand binds solely to NdIII in a symmetrical bidentate fashion. The coordination geometries of NdIII and ZnII are retained in the two isomers. In 3, the geometry index τ5 is 0.14 for Zn1 in [ZnNd(HL)(µ-OAc)(NO3)2(H2O)]+ and 0.43 for Zn1 in [ZnNd(HL)(OAc)(NO3)2(H2O)]+.

Fig. 2
figure 2

Molecular structures of the two isomeric cations in the crystal structure of [ZnNd(HL)(µ-OAc)(NO3)2(H2O)] [ZnNd(HL)(OAc)(NO3)2(H2O)](NO3)2 ⋅ n H2O ⋅ n MeOH (3). Displacement ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for those attached to nitrogen), counter ions and solvent molecules are omitted for clarity

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The structures of 1 and 2 appear to be isostructural with the above-mentioned XODFOM, which has the reported molecular composition “[ZnLa(HL)(NO3)3(S)](NO3)” (S = H2O or C2H5OH) [27], in which water and ethanol alternatively occupy one coordination site at LaIII. However, whereas a µ-acetato-κO,O’ ligand bridges the ZnII and LnIII in 1 and 2, in XODFOM a bridging nitrate ion the ZnII and LaIII 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 XODFOM is a possibility, since the precursor complex described as “[Zn(valdien)] · 1.5 CH3OH”, which was not structurally characterized by X-ray crystallography, was prepared from H2valdien and Zn(OAc)2 · 2 H2O. Considering previous work by Naskar et al. [29], the constitution of the precursor complex might have been rather [Zn2(H2valdien)2(OAc)2]. Elemental analysis calcd. for [Zn2(H2valdien)2(OAc)2] (C 53.40, H 5.50, N 8.49%) differs little from that reported for “[Zn(valdien)] · 1.5 CH3OH” (C 53.10, H 5.80, N 8.30%) by Benetollo et al. [27]. We should note that the crystal structure of [Zn(valdien)] · CH3OH was published very recently [46], but an additional base (LiOH) was used in the synthesis contrary to the synthesis of “[Zn(valdien)] · 1.5 CH3OH” 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 LaIII and ZnII, 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 ZnII-LnIII complexes 1 and 2 by successive treatment of the H2valdien compartmental ligand with Zn(OAc)2 · 2 H2O and, respectively, Pr(NO3)3 · 6 H2O and Nd(NO)3 · 6 H2O, affording the asymmetrical, ring-contracted isomerized compartmental ligand H2L from H2valdien in situ. Such a rearrangement of the H2valdien 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 ZnII ion in the inner N2O2 binding site instead of the inner N3O2 site of the parent H2valdien. As anticipated, the LnIII ions are found in the outer O2O2’ 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 ZnII and LnIII ions in 1 and 2 are additionally linked by acetate anions in a syn-syn bridging mode rather than by nitrate anions, as has been proposed for similar structures. Detection of the [Zn(HL)]+ ion but no LnIII adducts by ESI mass spectrometry suggests that the binding of ZnII to the inner pocket of the ligand is, as expected, more stable than that of the LnIII ions in the outer compartment. The crystal structure of 3 reveals that structural isomers of 2 occur.