Polyoxometalates (POMs),as an excellent family of inorganic metal-oxide clusters and inorganic ligands,are currently of great interest in the design and construction of inorganic-organic hybrid metal-organic compounds (MOCs),owing to not only their various architectures and coordination capability,but also their promising applications in photochemistry,electrochemistry and catalysis, etc. ^{[1, 2, 3, 4]}. At present,the reported POM-based MOCs almost rest on classical polyanions,such as Keggin-,Wells-Dawson- and isopolymolybdate- type polyanions. The reduced molybdenum phosphate [P4Mo₆O₂₈(OH)₃]^9- (P₄Mo₆),as an important member of POM family,owns some merits as inorganic building blocks: (i) it possesses multiple terminal and bridging oxygen atoms to coordinate with transition metal (TM) ions; (ii) The exposed O atoms of theMo6-fragment can be fused byTM ions to obtain a TMlinked dimer P4Mo₆-TM-P₄Mo₆ ^{[5, 6, 7, 8]}. Thus,the P4Mo6 anions can be used to construct versatile POM-based complexes. For example, Wang’s group has obtained a series of P4Mo6-based cadmiumorganic complexes with pyridine-based ligands ^[9]. However,the reports based on P₄Mo₆ anions are relatively limited,to the best of our knowledge. Therefore,exploring new P₄Mo₆-based MOCs has become an attractive branch of the POMs field.

Up to now,the organic ligands used to modify P4Mo6 anions are mostly based on rigid organonitrogen ligands,such as 2,2'-,4,4'- bipyridine and 1,1'-phenanthroline ^{[10, 11, 12]}. In this work,we try to modify this synthetic strategy and introduce a flexible N-donor ligand 1,3-bis-(4-pyridyl)propane (bpp) in the P₄Mo₆/TM system, basing on the following considerations: (i) The -(CH₂)₃-spacer can enhance its flexibility without affecting the strong coordination ability of the pyridyl group,which enables the ligands to accommodate several coordination modes of polyanions and TM ions; (ii) The most appealing character of bpp is that it may transform in situ into a completely new ligand. For example,Li’s group reported two Keggin-type POM-based metal-organic frameworks based on two tetradentate ligands in situ transformed from the bpp ligands ^[13]. Though the in situ transformation of bpp is not the concern of the target syntheses,this changeable factor may increase the opportunity to synthesize novel structures, which also can further provide more informative examples of in situ ligand transformation. Furthermore,according to the related reports,the Fe³⁺ ions are usually used as a catalyst in the oxidative carbon-carbon coupling of bpp ^[14]. Thus,in this work,we use Fe³⁺ ions as a catalyst in the P₄Mo₆/TM/bpp system to realize the in situ transformation of bpp. Thus,to some extent,the favorable conditions for the in situ transformation of bpp should also be viewed as a deliberate synthetic strategy.

In this work,we select the P₄Mo₆/Cd/bpp system and a new reduced molybdenum phosphate-based metal-organic complex, [Cd^II₃(HPO₄)(Hbpp)(H₂O)₂]{Cd^II[P₄Mo₆O₂₈-H_3.5(OH)₃]₂}(H₄tpb)·7H2O (1) (tpb = 1,2,4,5-tetra(4-pyridyl)-benzene) has been obtained under hydrothermal conditions. The in situ transformation of bpp to tpb is realized as expected. To our knowledge,there is no report on the P₄Mo₆ combining with in situ ligand transformation. In this work,the mixture of Cd²⁺ ions, H₃PO₄,MoO₃ and D,L-α-alanine are selected as the raw materials for the synthesis of [Cd(P₄Mo₆)₂] dimer ^[9]. The complex exhibits a 2D network,which is constructed from the sandwich-type [Cd(P₄Mo₆)₂] dimers and the trinuclear Cd^II subunits. Moreover, the electrochemical and fluorescent properties of the complex have been investigated in this paper.

All reagents and solvents for syntheses were purchased from commercial sources and used without further purification. Elemental analyses (C,H and N) were performed on a Perkin- Elmer 240C elemental analyzer. Fluorescence spectra were recorded at room temperature on a Hitachi F-4500 fluorescence/ phosphorescence spectrophotometer. The FT-IR spectrum with KBr pellet was taken on a Varian FT-IR 640 spectrometer. A CHI 440 Electrochemical Quartz Crystal Microbalance was used for the electrochemical experiments. The complex 1 bulk-modified carbon paste electrode (1-CPE) was used as the working electrode.

Complex 1 was synthesized from a mixture of MoO₃ (0.3 mmol),Cd(OAc)₂·2H₂O (0.6 mmol),FeCl₃ (0.6 mmol),D,L-α-alanine (0.9 mmol),bpp (0.6 mmol),H₃PO₄ (0.3 mL) and 5 mL of H₂O. The pH value was adjusted to 3.8 by 1 mol/L NH₃·H₂O,and then the resulting suspension was transferred to a Teflon-lined stainless autoclave and kept under autogenous pressure at 160 ℃ for 4 days. After slow cooling to room temperature at a rate of 10 ℃/h,orange block crystals suitable for X-ray diffraction of 1 were isolated by mechanical separation from a yellow amorphous solid in 18% yield (based on Mo). Elemental analysis (%) calcd. for C₃₉H₆₉Cd₄Mo₁₂N₆O₇₅P₉: C 12.65,H 1.88,N 2.27. Found: C 12.61,H 1.77,N 2.25. IR (KBr,cm^-1): v 3271.4 (s),1637.1 (s),1505.2 (m), 1458.0 (w),1115.4 (w),1109.1 (s),962.9 (s),795.2 (w),739.9 (m), 708.3 (w),617.6 (w),560.6 (m).

Single crystal X-ray diffraction data for complex 1 were collected on a Bruker APEX II diffractometer with Mo Kα (graphite monochromator,λ = 0.71073Å ) by θ and u scan modes at 293 K. The structure was solved by direct methods and refined on F² by full-matrix least squares using the SHELXL package ^[15]. A summary of the crystallographic data and structural refinements for complex 1 is given in Table 1. Selected bond distances (Å ) and angles (°) of the complex are listed in Table S1 (Supporting information). All non-H atoms were refined anisotropically and H atoms on the C atoms were fixed in calculated positions. H atoms on coordinated water and crystal water were not found from the difference Fourier maps and directly included in the final molecular formula. The H atoms on the μ₂-O atoms between the non-bonding Mo atoms and part of the O atoms (O₁₀,O₁₃,O₁₆, O₂₀,O₂₉,O₄₂ and O₅₁) from the PO₄ groups,as well as the H atoms on the N atoms on the N-donor ligands are also added according to the charge balance and these H atoms are attached to the final molecular structure. Crystallographic data for the structure have been deposited at the Cambridge Crystallographic Data Centre as CCDC 914939 for 1.

Table 1

Table 1 Crystal data and structure refinement for complex 1.

Table 1 Crystal data and structure refinement for complex 1.

The complex 1 is a 2D network,which consists of sandwich-type [Cd(P₄Mo₆)₂] subunits and trinuclear cadmium-phosphates-Hbpp fragments. Valence bond calculations ^[16] confirm that the Mo,P and Cd atoms are in +5,+5 and +2 oxidation state,respectively (Table S2,Supporting information). There are four crystallographically independent Cd^II ions in complex 1 (Fig. 1,Table S3, Supporting information): Cd(1) is located in the [Cd(P₄Mo₆)₂] dimer and bridges two [P₄Mo₆] units through six μ₃-O atoms [O(1),O(3),O(4)] to generate a centrosymmetrical [Cd(P₄Mo₆)₂] dimer [with Cd(1)-O bond lengths of 2.302(6) Å ,2.254(7) Å , 2.264(6) Å]. Cd(2) adopts a distorted {CdNO₅} octahedral configuration to coordinate with one N atom [N(1)] in a bpp ligand [Cd(2)-N(1) distance of 2.336Å],and five O atoms [O(6), O(22),O(27),O(33) and O(43)] from four PO₄ tetrahedrons of different [P₄Mo₆] units [with Cd(2)-O(6),Cd(2)-O(22),Cd(2)- O(27),Cd(2)-O(33) and Cd(2)-O(43) bond distances of 2.248, 2.246,2.816,2.192 and 2.254Å ,respectively]. The coordination mode of Cd(3) exhibits a distorted {CdO₆} octahedral geometry, which is defined by four phosphate O atoms [O(14),O(39),O(48) andO(67)]with Cd-O distances of 2.323,2.247,2.277 and 2.262Å , respectively,one O atom[O(26)] from the MoO₆ octahedron of the [P4Mo6] cluster [Cd(3)-O(26) = 2.304Å )] and one coordinated water molecule O(2W) [Cd(3)-O(2W) = 2.302Å]. Cd(4) is sixcoordinated by six O atoms,one [O(8)] from aMoO₆ octahedron of the [P₄Mo₆] cluster,four phosphate O atoms [O(23),O(39),O(53) and O(67)],and one coordinated water molecule O(1W),[Cd(4)- O(8) = 2.493Å ,Cd(4)-O(23) = 2.232Å ,Cd(4)-O(39) = 2.304Å , Cd(4)-O(53) = 2.277Å ,Cd(4)-O(67) = 2.349Å ,and Cd(4)-O (1W) = 2.293Å].

Fig. 1

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Fig. 1. Ball-stick view of the molecule structure of 1. All H atoms are omitted for clarity.

Interestingly,there are nine crystallographically independent PO₄ groups with various coordination modes in the complex, which is unprecedented in the P₄Mo₆-system (shown in Table S4, Supporting information). Eight of the PO₄ groups are ordinary, while the last phosphate group (P(9)) is a special one,which is isolated from the polyoxometalate [Cd(P₄Mo₆)₂] dimer,linking three crystallographically independent Cd^II ions [Cd(2),Cd(3) and Cd(4)] with the μ₂-O atoms [O(14),O(22) and O(23)] of the PO₄ to generate a trinuclear cadmium subunit. The trinuclear cadmium subunits are further connected with four different [Cd(P4Mo6)2] dimers through PO₄ groups to form a 2D network,which is unusual in the P₄Mo₆-system (Fig. 2,Figs. S1-2 in Supporting information). The bpp ligands only exhibit monodentate coordination mode and are suspended at both sides of the 2D layer (Fig. S3,Supporting information).

Fig. 2

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Fig. 2. Polyhedral and ball/stick representation of the 2D network in 1.

It is worth mentioning that,in the process of the reaction,an organic compound tpb is obtained in situ from bpp molecules through oxidative carbon-carbon coupling. The tpb molecules are protonated and reside between the 2D layers and connect the adjacent2Dlayers to a3Dsupramolecular framework via H-bonding interactions between the nitrogen atoms in the tpb molecules and the oxygen atoms from P4Mo6 units [N(3)-H(3)A···O(11),2.6657Å , 165° and N(4)-H(4)A···O(55),2.6138Å ,1718] (Fig. S3,Supporting information).

In the field of POMs,it is rare to synthesize the organic molecules in situ under hydrothermal conditions without presyntheses. To the best of our knowledge,there is only one example on the transformation of bpp ligands to tpb molecule by oxidative carbon-carbon coupling in POM-based complexes ^[13]. In this work,the transformation occurred in the presence of Fe³⁺,which acts as an effective catalyst. When the experiment was performed without the FeCl₃,the bpp molecules do not transform into the tpb (considering the limited length of this article,the complex obtained without the FeCl₃ is not described here). Therefore,it can be concluded that Fe³⁺ plays a key role in the transformation of the bpp to the tpb.

The IR spectrum of complex 1 exhibits a broad band at 3271.4 cm^-1,which is associated with the coordinated and lattice water molecules (Fig. S4,Supporting information). The characteristic bands at 1115.5 and 1109.1 cm^-1 are attributed to v(P-O) and the features at 962.9,795.2 and 739.9 cm^-1 are associated with v(Mo=O) and v(Mo-O-Mo),respectively ^[10]. Bands in the regions of 1637.1,1505.2 and 1458.0 cm^-1 are assigned to the bpp and tpb, respectively ^[13].

The electrochemical properties of complex 1 bulk-modified carbon paste electrode (1-CPE) are investigated in the aqueous solution of 0.1 mol/L H₂SO₄ + 0.5mol/LNa₂SO₄ at room temperature. The cyclic voltammetric behaviors of 1-CPE in the aqueous solution at different scan rates are presented in Fig. 3a. In the potential range of 500 mV to -250 mV,there are three pairs of reversible redox peaks,I-I',II-II' and III-III',with the mean peak potentials E_1/2 = (E_pc + E_pa)/2 of 281,113 and -139 mV,respectively, which should be ascribed to the redox of [P₄Mo₆] units ^[12]. The peak potentials change gradually following the scan rates from 40 mV/s to 450 mV/s for 1-CPE: the cathodic peak potentials shift to the negative direction and the corresponding anodic peak potentials shift toward the positive direction with increased scan rates. As can be seen from Fig.S5 (Supporting information),the peak currents are proportional to the scan rates,indicating that the redox process of 1-CPE is surface-controlled ^[17]. As is well known,nitrite is a dangerous pollutant and a health hazard,but its direct electroreduction requires a large overpotential at most electrode surfaces and no obvious response can be observed in the potential range of 500 mV to -250 mV at a bare CPE.However,the 1-CPE displays good electrocatalytic activity toward the reduction of nitrite. At the 1-CPE,with the addition of nitrite,all the three reduction peak currents increase noticeably,while the corresponding oxidation peak currents decrease gradually (Fig. 3b),which indicate that the reduction of nitrite is mediated by the reduced species of [P₄Mo₆] in complex 1.

Fig. 3

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Fig. 3. (a) Cyclic voltammograms of 1-CPE in a 0.1 mol/L H2SO4 + 0.5 mol/L Na2SO4 aqueous solution at different scan rates (from inner to outer: 40,80,120,160,200,250,350 and 450 mV/s). (b) Cyclic voltammograms of 1-CPE in a 0.1 mol/LH2SO4 and 0.5 mol/L Na2SO4 aqueous solution containing 0.0-8.0 mmol/L KNO2 and a bare CPE in a 0.1 mol/L H2SO4 and 0.5 mol/L Na2SO4 mixed solution. Scan rate: 20 mV/s.

Photoluminescent measurement of the title complex was performed in the solid state at room temperature. The solidstate photoluminescent spectra of complex 1 and the free bpp ligand are shown in Fig. S6 (Supporting information). It can be seen that the free bpp ligand displays fluorescence with an emission at 406 nm (λ_ex = 275 nm),which can be assigned to the π*-π charge transition. While tpb exhibits an emission in the range of 500-550 nm ^[14]. The emission peak at about 400 nm is found in the fluorescence spectrum of 1 when it is excited at 250 nm,which may be attributed to the intramolecular charge transition of bpp.

In conclusion,we present a new POM-based metal-organic complex with the tpb being transformed in situ from the precursor bpp. The classic sandwich-type [Cd(P₄Mo₆)₂] dimers are connected by the trinuclear CdII subunits to construct 2D networks,which are further extended by the protonated tpb to a 3D supramolecular framework through H-bonding interactions. Moreover,the excellent electrocatalytic activity of the title complex may make it an excellent candidate for potential catalytic material.

Financial supports of this research by the National Natural Science Foundation of China (Nos. 21171025 and 21101015),New Century Excellent Talents in University (No. NCET-09-0853), Natural Science Foundation of Liaoning Province (No. 201102003) and Program of Innovative Research Team in University of Liaoning Province (No. LT2012020).

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2013.06.022.