Heterometallic ZnII–LnIII–ZnII Schiff Base Complexes with Linear or Bent Conformation—Synthesis, Crystal Structures, Luminescent and Magnetic Characterization

A series of racemic, heteronuclear complexes [Zn2Nd(ac)2(HL)2]NO3·3H2O (1), [Zn2Sm(ac)2(HL)2]NO3·3CH3OH·0.3H2O (2), [Zn2Ln(ac)2(HL)2]NO3·5.33H2O (3–5) (where HL is the dideprotonated form of N,N′-bis(5-bromo-3-methoxysalicylidene)-1,3-diamino-2-propanol, ac = acetate ion, and Ln = Eu (3), Tb (4), Dy (5), respectively) with an achiral multisite coordination Schiff base ligand (H3L) were synthesized and characterized. The X-ray crystallography revealed that the chirality in complexes is centered at lanthanide(III) ions due to two vicinally located μ-acetato-bridging ligands. The presented crystals have isoskeletal coordination units but they crystallize in monoclinic (1, 2) or trigonal crystal systems (3–5) with slightly different conformation. In 1 and 2 the ZnII–LnIII–ZnII coordination core is linear, whereas in isostructural crystals 3–5 the chiral coordination cores are bent and lie on a two-fold axis. The complexes 1, 3–5 show a blue emission attributed to the emission of the ligand. For ZnII2SmIII complex (2) the characteristic emission bands of f-f* transitions were observed. The magnetic properties for compounds 1, 4 and 5 are characteristic for the paramagnetism of the corresponding lanthanide(III) ions.


Introduction
The synthesis and characterization of 3d-4f polynuclear coordination compounds are current and intensively developing field of science. The studies cover diverse and fascinating structural characteristics as well as potential application such as molecular magnets, catalysts, optical devices, luminescent probes in biology or bioactive agents [1][2][3][4][5][6][7][8][9]. Schiff base ligands constitute a convenient template for designing and synthesis of new heteronuclear 3d-4f coordination compounds. But there is still a need to study the factors that influence the coordination architecture starting from coordination modes of the ligand, through the preferential coordination number of the metal ions, ending with the presence of solvent molecules in the inner coordination sphere, which is crucial for preventing from quenching of luminescent properties due to the nonradiative energy loss processes.
We report here the heterotrinuclear coordination compounds Zn II -Ln III -Zn II of N,N -bis(5-bromo-3-methoxysalicylidene)-1,3-diamino-2-propanol (H 3 L) (Scheme 1). There is only one report in the Cambridge Structural Database (ver. 5.39 with updates) [10] of a crystal structure of coordination compounds of this ligand. These are two homotetranuclear coordination compounds of Co II and Ni II and a homotrinuclear complex of Cd II [11]. The trinuclear complex of Cd II is reported to have photoluminescent properties with luminescence decay of the order of 10 2 ns. The presence of Br atom in the ligand structure (heavy halogen effect) often causes a decrease of luminescence intensity and a shift in the ligand emission position [11,12], but sometimes it may be beneficial for the luminescent properties as well as for the intermolecular interactions, which can stabilize the crystal structure [13][14][15]. The ligand molecules with 1,3-diamino-2-propanol bridge may undergo double or triple deprotonation forming N 2 O 2 or N 2 O 3 coordination pockets (Table S1). The doubly deprotonated ligands form classic mono-up to tetranuclear coordination compounds [11,[16][17][18][19][20][21][22][23][24][25][26][27][28][29]. But only the triple deprotonation enables the higher nuclearity of complexes: to hexa- [22,30] and heptanuclear units [31] with the use of additional anions such as nitrate or acetate.
The control over the diversity of coordination compounds is not a trivial problem. In the presented here complexes 1-5 an issue of chirality arises. Around the lanthanide(III) cation is formed a stereogenic center because of the vicinal arrangement of two µ-acetato-bridged ligands. The metal centered chirality may provide compounds with very interesting features applicable in modern technology. We discuss here the aspect of the linearity of the Zn II -Ln III -Zn II metal core in the molecular and crystal structure as well as the luminescent properties registered in a solution and in a solid state of compounds 1-5 and the magnetic properties of 1, 4 and 5.

Crystal Structure
The crystallographic details of structures 1-5 and the selected molecular geometric data are given in Table 1 and  (5)). The coordination units in complexes 1-5 are isoskeletal but have different conformations and different intermolecular interactions. The syntheses gave as a result cationic trinuclear Zn 2 Ln coordination cores with a propeller like arrangement of the ligands (Figure 1) but different from the one observed in the homotrinuclear complex of Cd II [11]. The Zn II ion occupies the N 2 O 2 cavity of the doubly deprotonated Schiff base ligand HL 2− from which the metal cation is deviated by ca. 0.5 Å (parameter ∆ in Table 1). The square pyramidal geometry of Zn II is supplemented by the acetate O atom at the apical position. The intermetallic distances Zn II -Ln III within the core are in the ranges of 3.366(2)-3.556(1) Å ( Table 1). The Ln III ion is ten-coordinated and has deformed bicapped anticube polyhedron (Figure 1). The dihedral angle σ between ZnO (phenoxo)2 and LnO (phenoxo)2 planes and the valence angles Zn-O-Ln do not change much from monoclinic to trigonal structures (Table 1). Although, the ligand molecules are achiral, 1-5 form chiral coordination units crystallizing as racemic compounds in centrosymmetric space groups. The two acetate ions form bridges between Zn II and Ln III ions. Their cisoid spatial arrangement results in the formation of complexes with the stereogenic center located at the lanthanide(III) ion. The torsion angle ω between the vectors along the attitudes of the tetragonal pyramids O11-Zn1-Zn2-O14 in 1 and 2 is ca. 35 • and the analogous torsion O6-Zn1-Zn1 i -O6 i (symmetry code: i 1/3+x-y, 2/3-y, 1/6-z) in structures 3-5 is ca. 62 • (Table 1, Figure 2). The torsion between two N 2 O 2 mean planes (ε in Table 1) is of ca. 42 • for 1, 2 and 62 • for 3-5.    (3), Tb (4), Dy (5), respectively; b ∆-deviation of Zn II ion from the N 2 O 2 mean plane; c σ-dihedral angle between ZnO (phenoxo)2 and LnO (phenoxo)2 planes; d ω-torsion angle between two Zn-O vectors within the coordination unit O11-Zn1-Zn2-O14 in 1 and 2 and the analogous torsion O6-Zn1-Zn1 i -O6 i (symmetry code: I 1/3+x-y, 2/3-y, 1/6-z) in 3-5; e ε-torsion angle between two N 2 O 2 mean planes in the coordination core; f φ-valence angle along the Zn II -Ln III -Zn II line.
Usually, the analogous angle in 3d-4f heterotrinuclear salen type Schiff base complexes bridged by µ-acetato anions is in the range of 14-49 • [32-40] which means they have chiral cores, or 180 • when the structures are not chiral [41,42]. The coordination units in trigonal structures 3-5 have a unique structural conformation for such voluminous molecules. The complexes viewed along the Zn II -Ln III -Zn II axis resembles a pseudohexagonal star ( Figure 2) with planes formed by 4-bromo-2-(iminomethyl)-6-methoxyphenol molecular fragments and lying opposite to them acetate ions repeated by ca. 60 • . Very important difference in geometry of monoclinic and trigonal structures is the arrangement of metal ions Zn II -Ln III -Zn II in the coordination cores, which is linear in complexes 1 and 2 and bent one in structures of 3-5 with the valence angle φ being of about 173 • ( Table 1). The hydrophilic pockets near the protonated hydroxyl groups at the propyl bridges are occupied: one by NO 3 − anion and the other one by a solvent molecule (water in 1 or by disordered water and ethanol molecules in 2) (Figures S1 and S2). In structures 3-5 the both symmetrically equivalent pockets are filled with substitutional disordered nitrate ions and water molecules (coordination units lie on a twofold axis) ( Figure S3). Additionally, in trigonal crystals the nitrate ions are located at the three-fold rotoinversion axis in the tunnels running along the c crystallographic direction ( Figure 3). There are no π stacks in the analyzed crystal structures.

Luminescent Properties
The UV-VIS absorption spectra of the Schiff-base ligand (H 3 L) and its corresponding Zn II -Ln III -Zn II heteronuclear complexes 1-5 were obtained in a MeOH solution at 298 K ( Table 2). The electronic spectra of ligand and complexes are shown in Figure 4. The ligand shows four absorption bands with maxima ( Table 2). The band with the greatest intensity at 229.0 nm is attributed to π-π* transition in the benzene ring. The weak peaks at 292.5 and 341.5 nm correspond to the π-π* transition of the phenol and C = N chromophore groups, whereas weak peak at 425.5 nm is ascribed to the n-π* electron transition in the ligand [43][44][45]. Table 2. Photophysical data of H 3 L and complexes 1-5.  The spectral profiles of the absorption spectra for the heterotrinuclear complexes are similar ( Figure 4 and Figure S4). A weak absorption band of the ligand observed at λ = 292.5 nm, in the spectra of all complexes disappears. All complexes displayed three bands at ca. 234, 267 and 359 nm due to ligand based or ligand to metal charge transfer (LMCT) [46]. The absorption peaks maxima of complexes are red-shifted in the Zn II -Ln III -Zn II complexes which could be evidence of coordination between the ligand H 3 L and metal atoms. The luminescence spectra of the free ligand and 1-5 complexes in a MeOH solution (2 × 10 −5 M) were recorded in the UV-VIS region at room temperature. As can be seen from Figure 4 and Figure S4, each of the Zn II -Ln III -Zn II complexes displays an essentially identical absorption spectrum, with a broad peak at ca. 359 nm. The emission of Ln III ions in complexes is observed, when energy absorbed by the ligand is transferred from a triplet state of the ligand to a resonance state of the Ln III by an intramolecular energy transfer process.
Excitation of the Schiff-base ligand at λ ex = 337 nm produced luminescence with an emission maximum at λ em = 414 nm which can be assigned to the π-π* electronic transition in the ligand. The corresponding emission spectra upon excitation at λ ex = 359 nm are shown in Figure 5 and Figure S5. The emission spectra of 1 and 3-5 complexes present a broad emission band with an emission maximum at 421, 458, 424 and 430 nm respectively. The H 3 L ligand plays an important role in the emission spectra of these complexes ( Figure 5 and Figure S5, Table 2). The emission spectra of all complexes are red shifted compared to the emission spectrum of the free ligand H 3 L. This is presumably attributed to the coordination effect of the ligand to the metal ions [42,46,47].
The emission spectra of complexes 3, 4 and 5 don't exhibit characteristic emission bands of f-f transition, implying that energy transfer from the ligand to Eu III , Tb III and Dy III ions was inefficient. Moreover, emission intensity of the complex 3 decreased obviously under the same conditions, as it was observed earlier [48]. It may be due to the thermal quenching of the 5 D 0 level of Eu III by an intramolecular LMCT process [49]. Excited at λ ex = 359 nm ( Figure 6, Table 2), complex 2 shows three characteristic emission bands of Sm III ion at 561, 598 and 644 nm. The strongest emission was observed at 598 nm for a 4 G 5/2 -6 H 7/2 transition [50,51]. Also, this spectrum shows a weak, broad band at 458 nm, which has been assigned to the π-π* transitions of the ligand H 3 L. It indicates that the intramolecular energy transfer process from the triplet state of the Schiff base ligand H 3 L to the emission levels of Ln III ions was efficient in the Sm III complex (2) only [52][53][54]. The luminescence properties of compounds 1-5 and the ligand H 3 L have been studied in the solid state. The intense broad emission bands at 541 nm for H 3 L ligand is observed with excitation wavelength of 363 nm (Figure 7 and Figure S6, Table 2). Upon excitation at 357 nm, compounds 3-5 show intense, broad emission bands, with the maximum peaks at 461, 507 and 519 nm for 3-5, respectively, whereas for 1 and 2, low emission at 460 and 457 nm was observed. Compared with the free H 3 L ligand, their emissions are blue shifted (similar as in Ref. [11]). These bands can be tentatively assigned to intra ligand luminescence. A much more shifts of emission in 3 (in solution and solid phase) may originate from the ligand-to-metal-charge transfer emission [48,49,52]. The luminescence behavior of the complex 2 with a linear arrangement of the Zn II -Sm III -Zn II core in solid state is similar to that of 2 in methanol solution. This may arise from the less evident changes in the conformations between a solution and a solid state of 2 than in the compounds 3-5 crystallizing in trigonal space group with strained conformation of pseudo hexagonal symmetry and a bent layout of the trinuclear metal core. Excitation of complex 2 results in the luminescence of Sm III at 561, 598 and 644 nm originating from the 4 G 5/2 -level ( Figure 7).

Magnetic Properties
Temperature-dependent of molar susceptibility measurements of samples were carried out in an applied magnetic field of 0.1 T over the temperature range 1.8-300 K. The data are presented as plots of χ M T and χ M −1 versus T in Figure 8, Figures S7 and S8, where χ M is the molar magnetic susceptibility and T is the absolute temperature. The experimentally determined the χ M T values of compounds 1, 4 and 5 at room temperature and χ M T values theoretically calculated by the following equation [55]: where N is Avogadro constant, β is the Bohr magneton, k is Boltzman's constant, g Ln is the g factor of the ground J terms of Ln III and is expressed as: g Ln = 3/2 + [S(S + 1) − L(L + 1)]/2J(J + 1) are summarized in the Table S3. At 300 K the determined χ M T values of 1.62 cm 3 Kmol −1 (1), 12.10 cm 3 Kmol −1 (4), 14.39 cm 3 Kmol −1 (5), respectively are compatible with those expected for Nd III ( 4 I 9/2 , S = 3/2, L = 6, J = 9/2), Tb III ( 7 F 6 , S = 3, L = 3, J = 6), Dy III ( 6 H 15/2 , S = 5/2, L = 5, J = 15/2) magnetically isolated metal ions. As shown in Figure 8 there is a smooth decrease of the χ M T product in the studied compounds from about 50 K to about 2 K for 2, from 60 K to about 20 K for 4 and from 60 K to about 10 K for 5, respectively. Finally, these values drop at 2.0 K down to 11.08 cm 3 Kmol −1 (4), and 10.28 cm 3 Kmol −1 (5), respectively. The reduction of χ M T values at the low temperature could mainly arise from the crystal field splitting of Ln III ions and/or a weak interaction between magnetic centers of neighboring molecules [55][56][57]. In lanthanides the 4f orbitals are efficiently shielded and the influence of the neighboring groups on the magnetic properties is less evident than in 3d paramagnetic compounds. The magnetic properties of these ions are governed by strong orbital angular momentum contribution and the spin-orbit coupling that splits the 2+1 L multiplets of the 4f n ions into 2S+1 L J J-states. Usually, for most of the trivalent rare-earths ions, the 2+1 L J free ion ground state is well separated from the excited ones, thus at room and at lower temperatures the ground state is the only thermally populated one. As lanthanide ions are placed in a crystal field, the 2+1 L J states are split into Stark components (up to 2J + 1, n = even; J + 1/2, n = odd). The magnetic behavior will then be governed by the thermal population of these sets of levels, preventing the application of a spin-only Hamiltonian for quantitative investigations of exchange interactions between lanthanide ions (only the Gd III ion exhibits a quenched orbital momentum which can be therefore treated as a pure spin ground state) [55,56].

Methods
The contents of carbon, hydrogen and nitrogen in the complexes were determined by elemental analysis using a CHN 2400 Perkin Elmer analyser. The contents of zinc and lanthanides were established using ED XRF spectrophotometer (Canberra-Packard, Schwadorf, Austria). The infrared spectra were acquired using a Thermo Scientific Nicolet 6700 FTIR with a Smart iTR diamond ATR accessory. Data was collected between 4000 cm −1 and 520 cm −1 , with a resolution of 4 cm −1 for 16 scans. The NMR spectra were recorded on Bruker Avance 500 MHz in CDCl 3 and DMSO-d 6 solutions. Magnetic susceptibility measurements were performed on finely ground crystalline samples over the temperature range 1.8-300 K at magnetic field 0.1 T using a Quantum Design SQUID-VSM magnetometer. Corrections are based on subtracting the sample-holder signal and contribution χ D estimated from the Pascal's constants [55,59].
The electronic absorption spectra of the ligand H 3 L and complexes 1-5 were recorded with a Shimadzu UV-2401 PC 2001 spectrophotometer, in 10 × 10 mm quartz cells, in methanol at concentration about 2·10 −5 mol/L. Excitation and emission spectra of the studied systems, in methanol solution and in solid state, were measured at ambient temperature and UV-VIS range used a Hitachi F-7000 fluorescence spectrophotometer. For accuracy of data, the solid state samples were measured within the same sample holder to ensure the consistent amount of luminescent materials in all samples. Excitation and emission spectra were corrected for the instrumental response. All measurements were carried out under the same experimental conditions. Diffraction data were collected at room temperature (1, 4) on Oxford Diffraction Xcalibur CCD diffractometer with graphite-monochromated MoK α radiation (λ = 0.71073 Å) using the ω scan technique and at 120(1) K (2, 3, 5) on SuperNova X-ray diffractometer equipped with Atlas S2 CCD detector using the mirror-monochromatized CuKα radiation (λ = 1.54184 Å). The diffraction data resolution was 0.79 Å for 2 and 0.83 Å for 1,3-5. Absorption effects were corrected with CrysAlisPro 1.171.38.46 [60] by empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm for 1 and 4, and by analytical numeric absorption correction using a multifaceted crystal model based on expressions derived by R.C. Clark & J.S. Reid for 2, 3, 5 [61]. The structures were solved by direct methods using the ShelXT [62] structure solution program using intrinsic phasing and refined with the Olex2.refine refinement package using Gauss-Newton minimisation [63]. The lanthanide ions in 3-5 lie on a 2-fold axis with sof = 0.5. In 3-5 the nitrate ion occupies a special position at a 3-fold rotoinversion axis with sof 1/3 and a position near the coordination unit which is shared with a solvent molecule with sof's 1/6. Therefore, for nitrate ions in 3-5 the bond length restraints were applied by DFIX instructions to 1.22(1) and DANG 2.10(2) Å for N-O and O-O distances, respectively. Non-hydrogen atoms with except of nitrate N and O atoms and solvent molecules were refined with anisotropic displacement parameters. The C-bound H atoms were positioned geometrically and the 'riding' model for the C-H bonds was used in the refinement. The summary of experimental details and the crystal structure refinement parameters are given in Table 1 and Table S2. The experimental details and final atomic parameters have been deposited with the Cambridge Crystallographic Data Centre as supplementary material (CCDC: 1849665-1849669).
Copies of the data can be obtained free of charge on request via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk.

Conclusions
A new family of trinuclear Zn II -Ln III -Zn II complexes have been synthesized and characterized by single-crystal X-ray diffraction, IR and elemental analyses. The luminescent and magnetic properties of the complexes were studied. The two µ-acetato anions arranged in a cisoid manner are responsible for the chirality of the coordination units. All the studied crystals 1-5 are racemic compounds. The chirality of the coordination units influences the structure of the outer coordination spheres. The hydroxyl group at the propyl linkage does not participate in coordination in 1-5 but it forms hydrogen bonds with anions or solvent molecules blocking the direct access to the metal ions. In this way there are formed two hydrophilic pockets accessible for hydrogen bonded small molecules. There are no inner-sphere-coordinated solvent molecules or nitrate anions, which is beneficial for luminescent properties. Different conformation of coordination units in crystals 1, 2 and 3-5 influences the linearity of the metal arrangement Zn II -Ln III -Zn II and the mutual orientation of Zn-O apical vectors, which may be important for the physicochemical properties.
The complexes 1, 3-5 containing Nd III , Eu III , Tb III and Dy III ions show a blue emission attributed to the emission of the ligand H 3 L. The characteristic emission bands of f-f* transitions were observed for the Sm III complex 2 with a linear arrangement of Zn II -Sm III -Zn II metal ions.
The magnetic properties for Zn II -Ln III -Zn II compounds 1, 4, 5 are characteristic for the paramagnetism of the corresponding lanthanide(III) ions.
The results will be employed in the further investigation as a reference for the future studies concerning structural versus luminescent and magnetic properties of homo-and heteronuclear coordination compounds.

Conflicts of Interest:
The authors declare no conflict of interest.