Sandwich－type polyoxomolybdate constructed from tetra－nuclear Fe（Ⅱ）cluster and trivacant Keggin tungstophosphate fragment

（ ， ，475004，，）

Sandwich－type polyoxometalates（POMs）based on two［α－B－XW9O34］n－（X ＝ PV／AsV／SiIV／GeIV）or Wells－Dawson［α－X2W15O56］12－（X ＝ PV／AsV）fragments and four transition－metal（TM）centers constitute a well－known class of compounds［1－10］.The first tetra－CoIIsubstituted inorganic sandwich－type POM［Co4（H2O）2（α－B－PW9O34）2］10－was reported by WEAKLEY et al in 1973［1］.Later，FINKE and DROGE reported an analogous structural type based on two lacunary Wells－Dawson ions，［M4（H2O）2（P2W15O56）2］16－（M ＝ Co2＋，Cu2＋，Zn2＋）［3］.To date，the search for and discovery of sandwich－type POMs are of intensive interest owing to their widespread applications in optics，electronics，magnetism，and catalysis［2，4－6］.However，the synthesis and exploration of high－dimensional［（two－dimensional（2D）／three－dimensional（3D）］inorganic－organic composite structures constructed from sandwich－type TM substituted POMs（TMSPs）remain less developed hitherto［11－12］，which offers us great interest and opportunities.To obtain the above－mentioned high－dimensional sandwich－type TMSPs with benign physical－chemical properties，we exploit the TMSPs system under hydrothermal conditions while the following two aspects are considered.On the one hand，available surface oxygen atoms on inorganic sandwich－type TMSPs provide a significant likelihood for derivation of high－dimensional architectures.On the other hand，introduction of organic components provides possibility to alter and tune inorganic TMSP microstructure，which makes it feasible to manipulate the structure of the products.

Bearing those perspectives in mind，in this article we report the hydrothermal synthesis and structure characterization as well as composition analysis and thermal stability evaluation of a 2Dsandwich－type TMSP that remarkably differs from those known examples built only by inorganic sandwich－type TMSPs，（H2dien）4H2［Fe4（H2O）2（α－B－PW9O34）2］·8H2O （1）（dien＝ diethylenetriamine）.

1 Experiment

1.1 Materials and physical measurements

All reagents were purchased commercially and used without further purification.The infrared（IR）spectrum of the sample powder palletized with KBr was recorded with a Nicolet FT－IR 360spectrometer over the range of 4 000－400cm－1.C，H，N elemental analyses were performed with a Perkin－Elmer 2400－II elemental analyzer.

1.2 Synthesis of compound 1

A mixture of Na2WO4·2H2O （1.429g，4.34mmol），85%H3PO4（0.146mL），1.14mL 37%HCl，FeCl3·6H2O （0.20g，0.75mmol），dien（0.5mL）and H2O （10mL，556mmol）was sealed in a Teflonlined stainless－steel reactor and heated at 160℃for 6d.Black block crystals were isolated.Yield：18%；elemental analysis（%）calculated for C16H80Fe4N12O78P2W18：C 3.64，H 1.53，N 3.18；found：C 3.48，H 1.85，N 3.02.

1.3 Single－crystal X－ray diffraction

A good quality single crystal（dimensions 0.33mm×0.24mm×0.19mm）was used for the X－ray diffraction（XRD）intensity data collection with a Bruker APEX－II CCD detector（Mo Kαradiation；λ＝0.071 073nm）.The structure was solved by direct methods and refined by the full－matrix least－squares method onF2using the SHELXTL－97package［13］.Intensity data were corrected for Lorentz and polarization effects，as well as for multi－scan absorption.All non－hydrogen atoms were refined anisotropically.The crystallographic data are listed in Table 1.The atomic coordinates and other parameters of structure have been deposited at the Cambridge Crystallographic Data Centre（No.942184；deposit＠ccdc.cam.ac.uk）.

2 Results and discussion

2.1 Crystal structure

Compound1is a 2Dnetwork framework，constructed from a sandwich－type polyanion［Fe4（H2O）2（α－B－PW9O34）2］10－，four diprotonized H2dien2＋cations，and eight isolated water molecules（Fig.1）.Twoα－B－［PW9O34］9－anions are linked by a rhomb－like Fe4O14（H2O）2cluster in a centrosymmetric arrangement.Additionally，in the Fe4O14（H2O）2groups，two symmetric Fe（1）ions are each coordinated with one water molecule O（1W）.The four Fe centers are all in distorted octahedral geometry，with bond lengths of Fe（1）－O 0.207 9（17）－0.228 6（17）nm，and Fe（2）－O 0.200 6（14）－0.224 0（14）nm.The Fe…Fe separations are as follows：Fe（1）…Fe（2）＝ Fe（＃1）…Fe（＃2）＝0.324 17（67）nm，Fe（1）…Fe（＃2）＝ Fe（＃1）…Fe（2）＝0.323 33（58）nm.To the best of our knowledge，in previously reported similar compounds，most of the organic ligands are en，1，3－diaminopropane（dap），N，N－dimethylformamide（DMF），2，2′－bipyridine（2，2′－bpy），4，4′－bipyridine（4，4′－bpy），and so on.In other words，the sandwich－type polyoxometalate based on dien ligand，compound1in this research，is the first example of the system under investigation.The reason may lie in that the distance between the two adjacent N atoms of dien is too large，which makes it difficult for N atoms to chelate to metal atom.

Table 1 Crystal data and structural refinements for compound 1

Bond valence sum （BVS）calculations［14］indicate that，in as－synthesized compound1，the P site is in the＋5oxidation state，Fe site is in the＋2oxidation state，and all W sites are in the＋6oxidation state.

In addition，supramolecular interactions are present in compound1，due to hydrogen－bonding interactions between N atoms of organoamine ligands and surface O atoms of polyoxoanions（Table 2）.Specifically，N atoms from organoamine ligands act as the proton donors，and O atoms from the surface oxygen of sandwiched［Fe4（H2O）2（α－B－PW9O34）2］10－units serve as the proton acceptors.The donors and acceptors are hydrogen－bonded to generate the infinite 2Dsupramolecular framework（Fig.2）.Such supramolecular interactions may be favorable for the chemical stability of compound1.

Fig.1 Polyhedral／ball－and－stick representation of the molecular structural unit of compound 1

Fig.2 View of the three－dimensional framework along the baxis

Table 2 Hydrogen bond lengths（nm）and bond angles（°）

2.2 IR spectrum

Fig.3 IR spectrum of compound 1

The IR spectrum of1（Fig.3）was recorded between 4 000cm－1and 400cm－1，and it is very useful for the identification of characteristic vibration bands of trilacunary Keggin polyoxoanions and organic components in the products.The characteristic vibration patterns derived fromα－Keggin－type polyoxoanions are observed in 1 100－700cm－1，which is in good agreement with what is reported on trilacunary Keggin－type polyoxoanion.The obvious characteristic bands at 942，864 and 742cm－1are attributed toν（W－Ot），ν（W－Ob），andν（W－Oc），respectively.The strong absorption peaks at 1 499and 1 599cm－1are associated with the dien ligands.The vibration resonances at 1 043 cm－1are assigned toν（P－Oa）.These IR data are in agreement with those in the literature［10］.

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