Synthesis, fluorescence properties and structural characterization of two heteronuclear Zn(II)-Pr(III) clusters with a chain-shaped ether Schiff base ligand

Abstract Two new Zn(II)-Pr(III) heteronuclear clusters were synthesized by interfacial diffusion, [ZnPr(L)(NO3)2(OAc)(CH3OH)]·CH3CN (1) and [Zn4Pr2L4(terephthalate)2]·(NO3)2 (2) (H2L = bis(3-methoxysalicylaldehyde-3-oxopene-1,5-diamine). They have been characterized by elemental analysis, UV-Vis and IR spectroscopy, and single-crystal X-ray diffraction. Structural analysis revealed that 1 has a heterodinuclear structure, with Zn(II) in the inner cavity generated by deprotonated ligands L2- and Pr(III) in the larger outer cavity. Cluster 2 is a heterohexanuclear structure, in which the Pr(III) bridges two [ZnL] to form a heterotrinuclear secondary unit [Zn2PrL2], and then two terephthalates connect two secondary units to form a heterohexanuclear cluster structure. The solid-state fluorescence properties of clusters and ligand show that there are not only slightly blue-shifted fluorescence peak of ligand, but also characteristic fluorescence peaks of Pr(III) ions in the two clusters: 615 nm (3P0→3H6) for 1, 609 nm (1D2→3H4) and 646 nm (3P0→3F2) for 2, respectively. In the two clusters, the difference of ion fluorescence peaks is attributed to the symmetry difference of coordination geometric environment of Pr(III).


Introduction
Rare earth fluorescent complexes have attracted attention due to their narrow emission bands [1], long excited state lifetimes [2], and large Stokes shifts [3,4].They are used in applications such as electroluminescent devices [5], laser technology [6], photoluminescent devices [7], display applications [8], fluorescent immunoassays [9,10], and sensors [11,12].However, the lanthanide ions are not directly sensitized for luminescence owing to the f-f transition; it is necessary to coordinate lanthanide metal ions with organic functional groups that can absorb energy and effectively sensitize the luminescence of lanthanide metal ions through energy transfer, the "antenna effect" [13][14][15][16].Recently, sensitization of rare earth ion luminescence through transition metal complexes has become a research hotspot in rare earth fluorescence materials [17][18][19][20][21].For example, in 2022, the photophysical properties of Cr-Ln heteronuclear complexes synthesized in the laboratory of L€ u exhibited high-purity NIR-emission centered on Ln(III) [22].In 2021, the Nikolaveskii group obtained the heterogeneous Zn 2 Eu and Zn 2 Tb compounds and studied the optical properties; the results show that the presence of transition metal ions leads to a significant increase in the efficiency of energy transfer to Eu 3þ and increases the overall quantum yield (62%) [23].In 2021, Ge's group studied the fluorescence properties of Zn-Ln complexes and found that the complexes exhibited strong fluorescence emission [24].Therefore, construction of d-f clusters with novel structures and excellent luminescent properties has become a challenge to be solved.
Generally, the challenge of obtaining heterometallic clusters still exists.Usually transition metal ions prefer O, N coordination sites, while rare earth ions prefer O coordination sites, caused by the difference in affinity between different ions [25][26][27].Therefore, selection of ligands is a critical factor for construction of d-f clusters.We chose to design a new ligand, H 2 L (Scheme 1), which has the following advantages: it contains both oxygen and nitrogen and can coordinate with rare earth and transition metal ions, respectively.On the other hand, the inner and outer cavities formed by ligand are conducive to formation of stable d-f complexes [28][29][30][31][32][33][34][35].Hence, H 2 L is an optimum ligand to synthesize d-f heteronuclear complexes.
We report the synthetic details, spectral characterizations, along with single crystal X-ray structures and solid-state fluorescence of two Zn(II)-Pr(III) heterometallic clusters containing the chain-shaped ether Schiff base ligand.The relationship between the Scheme 1.The structure of the ether Schiff base H2L and two distinct coordination cavities.
structure and the fluorescence properties of rare earth ions was also discussed (Scheme 2).

Materials and instruments
All chemicals and solvents were purchased commercially and used without purification unless otherwise stated.C, H, and N elemental analyses were determined using a Carlo Erba 1106 elemental analyzer.IR spectra were recorded from 4,000 to 400 cm À1 with a Nicolet FT-VERTEX 70 spectrometer using KBr pellets.Electronic spectra were recorded on a LabTech UV Bluestar spectrophotometer. 1 H-NMR spectra were obtained with a Varian VR400 MHz spectrometer with TMS as internal standard.Powder X-ray diffraction (PXRD) patterns were recorded using a Siemens D5005 diffractometer with Cu-Ka (k ¼ 1.5418 Å) radiation.Photoluminescence (PL) spectra were recorded on a HITACHI F-7000 fluorescence spectrometer.Absolute emission quantum yields were recorded on an Edinburgh FLS980 spectrometer equipped with an integrating sphere.2.2.Synthesis of bis(3-methoxysalicylidene)-3-oxapentane-1,5-diamine (H 2 L) H 2 L was obtained according to the literature [36,37].Yield: 1.59 g (68.0%). 1

Synthesis of [ZnPr
To a stirred solution of H 2 L (37.22 mg, 0.1 mmol) in MeOH (2 mL) was added Zn(OAc) 2 Á2H 2 O (21.9 mg, 0.1 mmol) in CH 3 CN (4 mL).After completion of addition, the solution was stirred for an additional 1 h at 78 C. Yellow precipitate was generated.Then, Pr(NO 3 ) 3 Á6H 2 O (45.3 mg, 0.1 mmol) was added and allowed to continue to react, and the yellow precipitate disappeared.After cooling to room temperature, the filtrate was obtained.Yellow block crystals suitable for X-ray diffraction studies were obtained by vapor diffusion of diethyl ether into the solution for

Synthesis of [Zn
A mixture of H 2 L (37.2 mg, 0.1 mmol), Zn(OAc) 2 Á2H 2 O (21.9 mg, 0.1 mmol) and Pr(NO 3 ) 3 Á6H 2 O (45.3 mg, 0.1 mmol) in dichloromethane and methanol (1:1 V/V, 4 mL) was stirred for 30 min at room temperature.Then, a solution of terephthalic acid (8 mg, 0.05 mmol) in DMF (10 mL) was added dropwise, and the mixture was stirred for another 30 min at room temperature.Then the mixture was filtered, and the filtrate was obtained.Crystals suitable for X-ray diffraction studies were obtained by vapor diffusion of diethyl ether into the filtrate for 5 days at room temperature.Yield: 72

X-ray structure determination
Appropriate single crystals of 1 and 2 were mounted on a glass fiber, and the intensity data were collected on a Bruker APEX II area detector with graphite-monochromated Mo-Ka radiation (k ¼ 0.71073 Å) at 296.15 K. Data reduction and cell refinement were performed using the SMART and SAINT programs [38,39].Absorption corrections are carried out by the empirical method [40].The crystal structures of 1 and 2 were solved by direct methods and refined by full-matrix least-squares against F 2 of data using OLEX 2 program [40][41][42].All hydrogen atoms were located geometrically and were subsequently refined in a riding-model approximation with C-H distances from 0.97 to 0.99 Å.The crystal data and experimental parameters relevant to the structure determination are listed in Table 1.Selected bond distances and angles are presented in Table S1.

Results and discussion
The chain-shaped ether Schiff base bis(3-methoxysalicylidene)-3-oxapentane-1,5diamine (H 2 L) was synthesized from condensation of o-vanillin with 3-oxapentane-1,5diamine in a molar ratio of 2:1 with a high yield.The d-f clusters 1 and 2 were prepared with interfacial diffusion methods.The elemental analysis of 1 and 2 are in good agreement with theoretical compositions.The clusters are soluble in polar aprotic solvents such as DMF and DMSO, slightly soluble in ethanol, methanol and ethylacetate, and insoluble in diethyl ether and petroleum ether.

IR, electronic spectral studies, and powder X-ray diffraction
In the 4000-500 cm À1 region, IR spectra of two Zn(II)-Pr(III) clusters are closely related to that of the free H 2 L (Figure S1).Free H 2 L shows characteristic bands at 1629 and 1259 cm À1 , respectively, which are attributed to the vibrations of m(C ¼ N) and m(Ar-O) [43][44][45].The characteristic absorption peaks at the corresponding positions of 1 and 2 show a red-shift by 14-20 and 64-69 cm À1 , indicating the ligand is coordinated to the metal ion.This is consistent with the results of the single crystal analysis [46][47][48].
Electronic spectra of H 2 L, 1 and 2 were recorded in DMF solution at 298 K (Figure 1).The electronic spectral values for the maximum absorption wavelength (k max ) and the molar absorption coefficient (E) are listed in Table 2.The absorption bands of H 2 L were at 269 and 327 nm, assigned to p!p Ã (benzene) and p!p Ã (C ¼ N) transitions [49][50][51][52].The UV-Vis spectra of two Zn(II)-Pr(III) clusters are similar to those of H 2 L. The two bands mentioned above red-shift by 9-14 and 41-47 nm in the cluster, respectively.The peak shift in the low-energy band is much greater than that in the high-energy band, which is evidence of the coordination between H 2 L and metal ions through Schiff base (C ¼ N) [53][54][55].
The bulk phase purities of the clusters were examined by powder X-ray diffraction measurements.Figure S2 shows the observed powder diffraction patterns acquired from the as prepared compounds together with the calculated patterns generated from the single-crystal X-ray diffraction data.Most peak positions of the simulated and experimental patterns are in agreement with each other, confirming that a single phase (more than $95% purity) is formed for each cluster.The differences in intensity may be due to the preferred orientation of the microcrystalline powder samples.
Compared with the raw materials for the synthesis of 1, terephthalic acid was additionally introduced and heterohexanuclear 2 was synthesized.The X-ray diffraction studies reveal that 2 crystallizes in the triclinic space group Pī with its thermal ellipsoid plot depicted in Figure 4(a).The asymmetric unit of 2 consists of two Pr(III) ions and  four Zn(II) ions, four fully deprotonated L 2-ligands, two quadridentate bridged terephthalate ions and two free nitrates.The Pr(III) bridges two [ZnL] to form a heterotrinuclear secondary unit [Zn 2 PrL 2 ], and then two terephthalates connect two secondary units to form a heterohexanuclear structure [60][61][62].In 2, the geometric structure of the Zn(II) ions (s 5 ¼ 0.68 and 0.62) and Pr(III) ions, as well as the coordination mode of the fully deprotonated L 2-ligands, are similar to those in 1 (Figure 4b,c).The terephthalate anion exhibits a l 4 -(j 1 o, j 2 o, j 2 o, j 1 o) bridging mode (Figure 5).

Fluorescence studies
The fluorescence data and spectra of H 2 L, 1 and 2 in the solid state at room temperature are shown in Table 3 and Figure 6.The emission spectra of H 2 L, 1 and 2 were obtained under excitation at 362 (H 2 L), 357 (1) and 385 (2) nm.The solid-state fluorescence properties of the clusters and ligand show that there are not only slightly blueshifted fluorescence peak of ligand, but also characteristic fluorescence peaks of Pr(III) ions in the two clusters: 615 nm ( 3 P 0 ! 3 H 6 , / ¼ 0.1%) for 1, 609 nm ( 1 D 2 ! 3 H 4 , / ¼ 0.08%) and 646 nm ( 3 P 0 ! 3 F 2 ) for 2, respectively [63][64][65].The blue shift of the emission peaks can be attributed to the electron-absorbing effect of the metal ion coordinated to H 2 L, which increases the delocalization of electrons between p-p Ã molecular orbitals [66][67][68].In 1 and 2, although all Pr(III) ions have a ten-coordinate bicapped square antiprism geometry, the degree of distortion varies, resulting in different symmetries [69][70][71][72].Therefore, the difference in ion fluorescence peaks is attributed to the symmetry difference of coordination environment of Pr(III).

Conclusion
Two new Zn(II)-Pr(III) clusters with a chain-shaped ether Schiff base ligand have been synthesized and characterized.Cluster 1 is heterodinuclear, while 2 is a heterohexanuclear structure.Solid fluorescence studies have shown that the complex unit [ZnL] is  suitable for sensitization of Pr(III) in both clusters.Moreover, the sensitization results are related to the symmetry of the coordination geometry of Pr(III) ions.The fluorescence properties of the Zn(II)-Pr(III) clusters imply that they may be good candidates for optical materials.

Disclosure statement
No potential conflict of interest was reported by the authors.

Table 1 .
Crystallographic data and data Collection parameters for 1 and 2.

Table 3 .
Fluorescence spectra data of H 2 L, 1 and 2 in the solid state.
a Absolute photoluminescence quantum yield with relative error ¼ ± 10%.