Two hybrid polyoxometalates constructed from Preyssler P5W30 clusters and Schiff base exhibiting interesting third-order NLO properties

College of Chemistry and Chemical E Materials-Oriented Chemical Engineering, N PR China. E-mail: yanxu@njtech.edu.cn Coordination Chemistry Institute, State Ke Nanjing University, Nanjing 210093, PR Ch † Electronic supplementary information bond lengths and angles; table of hydrog XRD, TG and coordination modes of 1562173 (1) and 1562174 (2). For ESI and electronic format see DOI: 10.1039/c7ra12 Cite this: RSC Adv., 2017, 7, 55427


Introduction
Polyoxometalates (POMs), a peculiar subunit of discrete transition metal-oxo clusters with oxygen-rich surfaces and strong coordinating ability, are intriguing secondary building blocks for constructing various inorganic-organic hybrid materials with potential applications in catalytic reactions, 1 magnetic materials, 2 nonlinear optics, 3 materials science 4 and electrochemistry. 5 Since the distinguished Keggin-type polyoxoanion [a-PMo 12 O 40 ] 3À was isolated, 6 POM chemistry has been rapidly developed in recent decades. In the above-mentioned area, transition metal complexes (TMCs) are commonly employed to decorate well-known POMs, for instance,  In the documented larger POM clusters, the well-known Preyssler [P 5 W 30 O 110 ] 15À (abbreviating P 5 W 30 ) anionic clusters attracted our considerable attention, which were discovered in 1970 (ref. 8) and structurally characterized through X-ray diffraction 15 years later. 9 The P 5 W 30 polyoxoanion possesses many merits: (i) the internal cavity can accommodate distinct metal cations with appropriate scale; (ii) abundant oxygen-rich surface and high stability over a large pH range of 1-10; (iii) high negative charges can capture TM cations. However, examples of selecting P 5 W 30 anion as precursor to design and synthesize of POMs with decorated or expanding conguration have seldom been documented. 10 The headmost expanding structures built on Preyssler anions and rare earth cations were successfully prepared by Wang et al. 11 Therefore, rational design and synthesis of charming Preysslertype POMs modied by TMCs have been a hot area of research in recent years. The Wang 12 and Sun 13 groups reported some inorganic-organic hybrid materials based on P 5 W 30 anions and TM cations, they show moderate electrocatalytic abilities toward the reduction of NO 2 À and H 2 O 2 .
Schiff bases are a signicant class of organic ligands and have been studied widely. Polydentate Schiff base ligands are capable of combining various metals through imine nitrogen, hydroxyl and carbonyl groups, it will alter the electron structure and improve the catalytic performance. The documented on POMs decorated by Schiff base are relatively rare, 14 while the hybrid polyoxometalate constructed P 5 W 30 and Schiff base ligands has not been prepared. On account of the solubility and steric-hindrance effects, we selected DAPSC as conjugated organic ligand. Owing to DAPSC possesses more coordination sites and a weaker conjugated effect, it was good candidate for decorating POMs to establish inorganic-organic hybrid materials. Herein, we have successfully obtained two unprecedented Preyssler P 5  (2), respectively. Additionally, the electrochemical properties and electrocatalytic activities of both compounds were investigated. The molecular 2PA cross section s of compounds 1-2 were 888 GM and 707 GM, which manifests that the Schiff base in the modied Preyssler polyoxoanions will strengthen the third-order NLO responses.

Experimental section
Materials and physical methods (NH 4 ) 14 NaP 5 W 30 O 110 $31H 2 O was synthesized according to the literature. 15 DAPSC was isolated applying a previous approach. 16 All other chemical reagents and solvents were purchased commerically and used without further purication. Elemental analysis (C, H, and N) was carried out on a PerkinElmer 2400 CHN elemental analyzer. FTIR spectra were measured in the range of 400-4000 cm À1 on a FTIR-8900 IR spectrometer using KBr pellets. TG analyses were recorded on a Pekin-Elmer Pyris Diamond TG analyzer. PXRD data were measured on a Bruker D8X diffractometer with graphite monochromatized Cu Ka radiation (l ¼ 0.154 nm) at room temperature. Two-photon absorption (2PA) cross-sections were gained by using a Chameleon II femtosecond laser pulse and a Ti:95 sapphire system (680-1080 nm) with 80 MHz repetition rate and a 140 fs pulse duration. Electrochemical measurements were conducted on a CHI760 electrochemical workstation. A conventional threeelectrode system was utilized. A Ag/AgCl (3 mol L À1 , KCl) and Pt foil were used as the counter electrode and reference electrode. The working electrode was a modied glassy carbon electrode (GCE). A mixture of (NH 4 ) 14 NaP 5 W 30 O 110 $31H 2 O (0.2 g, 0.024 mmol), DAPSC (0.02 g, 0.073 mmol), MnCl 2 $4H 2 O (0.099 g, 0.5 mmol) and 10 mL distilled water and stirred for 30 min at ambient temperature. The resulting suspension was transformed into 25 mL Teon-lined stainless-steel autoclave and heated at 120 C for 72 h. Aer the autoclave cooled over 12 h to room temperature, the orange strip crystals were isolated and washed with distilled water in 40% yield (based on Mn A mixture of (NH 4 ) 14 NaP 5 W 30 O 110 $31H 2 O (0.2 g, 0.024 mmol), DAPSC (0.02 g, 0.073 mmol) and CoCl 2 $6H 2 O (0.1189 g, 0.5 mmol) was dissolved in sodium acetate buffer (10 mL, 0.5 mol L À1 , pH ¼ 4.0) and stirred for 100 min at room temperature. The resulting suspension was sealed in 25 mL Teon-lined stainless-steel autoclave and kept at 120 C for 72 h. Aer being cooled to room temperature, the red block crystals were obtained and washed with distilled water in 45% X-ray crystallography X-ray analyses data for 1-2 were performed on a Bruker Apex II CCD diffractometer at 296 K, with graphite monochromatized Mo-Ka radiation (l ¼ 0.71073Å). Structures was solved by direct methods and rened by full-matrix least-squares using SHELXL-2014 crystallographic soware package. The non-hydrogen atoms were rened anisotropically. CCDC 1562173 and 1562174. † All the crystallographic information for 1-2 are listed in Table 1. Selected bond lengths and angles for 1-2 are listed in Tables S2-S5. †

Synthesis
Over the past few years, hydrothermal method has been proved to be a cogent approach in preparation of metal and Schiff base decorated Preyssler P 5 W 30 polyoxoanion. During a conventional hydrothermal method, numerous elements can impact on the nucleation and crystal development of nal products, for example, initial reactant, reactant concentration, solvents,  (4) 18.672 (6) 19.013 (4) 19.068 (6) 28.923 (6) 28.995 (9) reaction time, pH values, and reaction temperature. It is a remarkable fact that the selection appropriate precursors and solvent is vital to the success of this kind of reactions. Namely, the chosen solvent as well as organic and inorganic subunits should be favored to each other. Thus we use buffer solution, which offers a moderate pH environment for TM, Schiff base and POMs. The experimental PXRD patterns of the bulk products of compounds 1-2 are consistent with the simulated ones calculated from X-ray single-crystal diffraction ( Fig. S1 and S2 †), which indicates the phase purity of the two compounds. The intensity difference between experimental and simulation PXRD patterns may be ascribed to the variation in the preferred orientation of the powder sample during measurements. The bond valence calculations 17 suggest that the valence of Mn and Co atoms in compounds 1 and 2 are both in +2.

Description of the structures
Crystal structure of 1. X-ray single-crystal structural analysis shows that complex 1 crystallizes in triclinic P 1 space group. As shown in Fig. 1 TMC cations (Section B), seven and a half lattice water molecules. The Preyssler-type P 5 W 30 polyoxoanion comprises ve PW 6 units, 30 m 1 -O and 60 m 2 -O atoms spread on the shell. The P 5 W 30 anion owns a perfect 5-fold symmetry axis. In a mirror plane includes ve P atoms perpendicular to this axis. All of 30 W atoms in the P 5 W 30 shell are located on four parallel planes perpendicular to the center axis: each of the external and internal planes includes 5 and 10 W atoms (Fig. S11 †). All W centers are connected with 6 O atoms, exhibiting a WO 6 octahedron geometry. 9 Two [Mn(H 2 O) 2 (DAPSC)] 2+ fragments are bonded to the Preyssler P 5 W 30 anion by Mn-O bonds to form an interesting framework in Section A. Both Mn1 and Mn3 are heptacoordinated and display distorted pentagonal bipyramid geometry {MnN 3 O 4 } coordination modes (Fig. S9 †). The coordination geometries of Mn1 and Mn3 are dened by three N atoms and two oxygen atoms from a DAPSC ligand. The other two oxygen atoms, one from coordinated water molecule and the other from the Preyssler P 5 W 30 anion, respectively. It is noteworthy that all nonhydrogen atoms of each [Mn(H 2 O) 2 -(DAPSC)] 2+ unit are almost planar in Section A.
Compared with other polyoxometalates modied by Schiff bases, this is the rst Preyssler P 5 W 30 polyoxoanion decorated by Schiff base ligand. In Section A, the occupancies of Na2 and Na4 are both 75%, Na3 at half occupancy. The polyoxoanion connect to another P 5 W 30 cluster by means of Mn2, Na3 and O atoms, which forms a amusing dimer conguration (Fig. 2). Additionally, the two free [Mn(H 2 O) 2 (DAPSC)] 2+ coordination cations (Section B) are regarded as counter ions to sustain balance of charge. Mn4 and Mn5 are in hepta-coordinated MnN 3 O 4 pentagonal bipyramid environment, the axial two oxygen atoms are both from two coordinated water molecules. The Mn-N bond and the Mn-O bond lengths are in the range of 2.25(2)-2.33(2) and 2.129(16)-2.27(2)Å. The aforementioned values are in agreement with those observed in other known Mn-contained complexes. 18 It is notable that compound 1 possesses hydrogen bonds between the neighboring clusters, which extremely reinforces the whole structural stability. By means of hydrogen bonds and electrostatic interaction, the adjacent Preyssler-type {P 5 W 30 } polyoxoanion and the [Mn(H 2 O)(DAPSC)] 2+ cations alternately connect with each other to give a 3D supramolecular network (Fig. 3).
Crystal structure of 2. The crystallographic analysis reveals that compounds 1 and 2 are isomorphic. Their structure, bond lengths and angles, and water molecules are slightly distinct. ]} 3.5+ (Section C), and six lattice water molecules (Fig. 4). In Section C, Na1 atom has a tetracoordinated geometry, which is dened by two water molecules and two oxygen atoms from one DAPSC ligand. Na2 and Na3 atoms are at half occupancy. The coordination mode of Co ions are shown in Fig. S10. † The Co-N bond and the Co-O Fig. 1 The asymmetric unit of compound 1. Color codes: PO 4 , yellow tetrahedra WO 6 , green octahedra; W, green; Mn, bright green; Na, turquoise; P, pink; O, red; N, bule; C, black. (All the hydrogen atoms and free water molecules have been omitted for clarity). Fig. 2 The dimer of compound 1. frameworks. Comparing with above-mentioned two complexes, compounds 1-2 are greatly different. The Co ions (except for Co4) are in seven-coordinated CoN 3 O 4 pentagonal bipyramid environment. The P 5 W 30 anion is linked to one neighbor through a single O-Na-O-Co-O-Na-O bridge form a amusing dimer (Fig. 5). Similar to compound 1, compound 2 exhibits 3D supramolecular framework by strong hydrogen-bonding interactions (Fig. S12 †).

IR spectra
The IR spectra of 1-2 were recorded varies from 400 to 4000 cm À1 with KBr pellets (Fig. S3 and S4 †). In IR spectra, characteristic vibration patterns of the P 5 W 30 anions, namely,

Thermogravimetric (TG) analysis
In the cause of examine the thermostabilities of compounds 1-2, thermogravimetric analyses were investigated in N 2 atmosphere. The TG curves of both compounds show some similarities in the range of 25-800 C ( Fig. S5 and S6 †). Both compounds illustrate a consecutive weight-loss process, and the observed entire weight losses are 16.68% and 16.18% for compounds 1 and 2, respectively, which correspond to the loss of all water molecules and the decomposition of the DAPSC ligands.

Voltammetric behavior of 1 and 2-GCEs
A naphthol-modied glassy carbon electrode (GCE) was prepared as working electrode to study the electrochemical behaviors of compounds 1-2 in 1 mol L À1 sulfuric acid aqueous solution at different scanning speed. As depicted in Fig. 6a and b, from 0 to À800 mV, three pairs of reversible redox peaks related to redox reactions are explored for 1 and 2-GCE. The average peak potentials E 1/2 ¼ 0.5(E pa + E pc ) of I/I 0 , II/II 0 , and III/ III 0 are À0.300, À0.496, and À0.687 V for 1-GCE as well as À0.314, À0.504, and À0.658 V for 2-GCE. Two pairs of reversible  redox peaks I/I 0 and II/II 0 correspond to two successive double electron processes of the modied P 5 W 30 polyoxoanion. 10b, 13,21 The cyclic voltammetry behavior of 1 and 2-GCEs were recorded at changing scan rates. The cathodic peak potentials move to the minus orientation and the homologous anodic peak potentials move toward the positive orientation with enhancing scanning rates, which indicates that the redox process of 1 and 2-GCEs are surface-controlled. 22 Electrocatalytic properties of 1 and 2-GCEs for the reduction of H 2 O 2 POMs have been widely employed as electrocatalysts for the reduction of H 2 O 2 . Fig. 6c and d reveal 1 and 2-GCEs show good electrocatalytic activities on the reduction of H 2 O 2 in 1 M H 2 SO 4 aqueous solution with the potential changing from 0 mV to À800 mV. With the increase of the concentration of H 2 O 2 from 0.0 to 15.0 mmol L À1 , the reduction peak currents increase clearly while the corresponding oxidation peak currents remarkably reduced, which indicates that the 1 and 2-GCEs have good electrocatalytic ability for the reduction of H 2 O 2 .

Nonlinear optical properties
DAPSC is an outstanding conjugated polydentate ligand, and the incorporation of POMs with Schiff base may be able to enhance the NLO response. Herein, we explored the electronic spectra of both compounds in DMF solution with concentration of 1.0 Â 10 À4 mol L À1 at ambient temperature. The 2PA coefficient b and 2PA cross section of s were calculated by the openaperture Z-scan method with a femtosecond laser pulse and Ti:95 sapphire system. 23 Fig. 7 and 8 reveal the open-aperture Zscan data of 1-2. The red empty circles are the test data and the black solid line on behalf of the theoretical tting line modied by the equations: 24 here, z 0 ¼ pu 0 2 /l is the diffraction distance of the beam, In which u 0 is the spot size at focus, l is wave length of the beam, z is sample position. I 0 is the input intensity at the focus z ¼ 0, L eff ¼ (1 À e ÀaL )/a is the effective length, where a is the linear absorption coefficient, L is the sample length. By using the above equations, we inference that the 2PA absorption coefficient b is calculated as 0.002099 cm GW À1 and 0.001715 cm GW À1 for 1-2, respectively. Furthermore, the molecular 2PA cross-section s could be determined by the subjacent formula: where h, n, N A , and d are the Planck constant, frequency of input intensity, the Avogadro's constant, and is the concentration of   This journal is © The Royal Society of Chemistry 2017 the compound, respectively. According to formula (3), the molecular 2PA cross-section s of both compounds were computed as 888 GM and 707 GM (1 GM ¼ 10 À50 cm 4 per photon). It is well known that the modied p-electron delocalization in the framework leaded to the third-order nonlinear optics response. But so far the investigation of POMs in the area of nonlinear optical materials basically concentrate on the wellknown Keggin-and Dawson-type polyoxoanions. 25 As far as we know, two compounds have bigger molecular TPA cross section s comparing with other reported NLO materials. 26 The thirdorder NLO properties of two compounds manifest that POMs modied by Schiff base lead to a powerful NLO response, and could be promising candidates for making NLO materials.

Conclusions
In conclusion, two novel based on Preyssler P 5 W 30 compounds modied by Schiff base and TM ions were synthesized under hydrothermal conditions. The cyclic voltammograms of both compounds have been studied. Both compounds show better electrocatalytic activities toward the reduction of H 2 O 2 . Moreover, the both compounds have moderate molecular 2PA cross section s, hence they own potential applications in NLO materials. The successful isolation of compounds 1-2 can provide new synthetic strategy for the construct of distinct POMs-based inorganic-organic hybrid materials by means of combining diverse exible organic molecules. Further work in this area will be focused on preparing Preyssler-based compounds with fascinating structure and rich physicochemical properties.

Conflicts of interest
There are no conicts to declare.