Synthesis and crystal structure of a polyoxometalate dimmer containing rare earth cation

MA Peng－tao ，QING Wei－xia，ZHANG Shao－wei

（1.College of Chemistry and Chemical Engineering，Henan University，Kaifeng475004，Henan，China；2.College of Medicine，Henan University，Kaifeng475004，Henan，China）

Currently，polyoxometalates（POMs）have been attracting considerable attention because of their diverse structural and electronic characteristics combined with a multitude of potential applications in diverse fields such as catalysis，medicine，analytical chemistry，materials science and molecular electronics［1－5］.POMs can be versatile inorganic building blocks for the construction of molecule－based materials and can also bind most lanthanide（Ln）cations，resulting in a family of lanthanide substituted polyoxometalates（LSPs）that exhibit interesting luminescence and magnetic properties［6－7］.Compared with 3d－transitionmetal cations，Ln cations can offer stronger oxophilicity，higher coordination numbers，more flexible coordination geometry and more functionality when they are incorporated into lacunary POMs.This makes it feasible to readily link different lacunary polyoxoanions together thereby affording some novel LSP oligomers or larger aggregates with unexpected structure and properties.Also，it is well－studied and well understood that Ln complexes exhibit excellent luminescent behavior，since the 4felectrons of Ln are shielded by the fully occupied 5sand 5p orbitals thereby allowing f－f transitions with sharp emission feature and negligible environmental influence［6－7］.In this respect，it is imperative to design and synthesize LSPs with unique magnetism or luminescence，with which monovacant polyoxoanions containing Ln cations are of particular significance.In 1971，PEACOCK and WEAKLEY first put forward that the combination of monovacant Keggin polyoxoanions［XW11O39］n－（X ＝ SiIV，PV）with Ln cations leads to both 1∶1－type and 1∶2－type derivatives in solution［8］.In 2000，POPE et al employed a simple strategy to isolate and characterize infinite one－dimensional 1∶1－type POM－based Ln derivatives［Ln（RSiW11O39）（H2O）3］5－（Ln＝ LaIIIand CeIII）which represent two types of polymeric chain structures in the solid state［9］.Since 2004，we have been exploiting the reactions of Ln cations with lacunary polyoxoanion precursors in conventional aqueous solutions and obtained a series of LSPs derivatives［10－16］.Herein，we report the synthesis and structural characterization of another new 2∶2－type dimeric LSP［（CH3）4N］6［（α－PW11O39H）Sm（H2O）3］2·9H2O.

1 Experiment

1.1 Materials and physical measurements

Na7［α－PW11O39］·nH2O was prepared according to previous references［17］and identified by infrared spectrometry（IR）.Other reagents were purchased commercially and used without further purification.A Nicolet FT－IR 360spectrometer was performed to record the IR spectrum of as－synthesized product palletized with KBr（wavenumber range：4 000－400cm－1）.C，H，N elemental analyses were performed with a Perkin－Elmer 2400－II elemental analyzer.The ultraviolet（UV）spectrum was recorded with a U－4100spectrometer（distilled water as solvent）.

1.2 Synthesis of the title compound

0.70 g（ca.0.24mmol）of Na7［α－PW11O39］·nH2O was dissolved in 15mL of water at 80℃，followed by dropwise addition of 0.58g（1.38mmol）Sm（NO3）3·6H2O in 15mL of water.Resultant mixed solution was adjusted to the desired pH of 5.4with 4mol·L－1KOH under stirring and allowed to react for 1 h also under stirring.Upon completion of the reaction，the solution was cooled to room temperature and filtered.Then 0.15g（1.38mmol）of tetramethylammonium chloride was added into the filtrate under stirring，followed by additional 0.5hof reaction.The resulting solution was filtered and evaporated at room temperature affording colorless rhombic crystals of the title compound in two weeks（0.36g，yield ca.47%based on W）.For C24H104N6O93P2Sm2W22аnal.calcd（%）：C，4.52，H，1.65，N，1.32.Found（%）：C，4.46，H，1.67，N，1.29.

1.3 Single－crystal X－ray diffraction

A good quality single crystal（dimensions 0.37×0.21×0.16mm3）was used for the intensity data collection with 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－97 package［18］.Intensity data were corrected for Lorentz and polarization effects，as well as for multi－scan absorption.All of the non－h(huán)ydrogen atoms were refined anisotropically.The crystallographic data are listed in Table 1.The atomic coordinates and other structural parameters are deposited at the Cambridge Crystallographic Data Centre（923285，deposit＠ccdc.cam.ac.uk）.

Table 1 Crystal data and structural refinements for the title compound

2 Results and discussion

2.1 Crystal structure

Fig.1 （a）Ball－and－stick representation of the molecular structure of the title compound，（b）The square antiprismatic geometry of the Sm1cation in the title compound

Single－crystal structural analysis reveals that the title compound crystallizes in the monoclinic space groupP2（1）／c（14），and its molecular structural unit consists of one dimeric Keggin－type polyoxoanion［｛（α－PW11O39H）Sm（H2O）4｝2］6－，six dissociative cations［（CH3）4N］＋，and nine lattice water molecules（Fig.1a）.The dimeric Keggin－type polyoxoanion ［｛（α－PW11O39H）Sm（H2O）4｝2］6－consists of two ［α－PW11O39H］6－subunits linked together by two［Sm（H2O）4］3＋cations.The Sm1cation，incorporated to the vacant site of the［α－PW11O39H］6－subunit in the“cap”region，is eight－coordinated and it adopts a distorted square antiprismatic coordination geometry（pseudo－D4d）（Fig.1b）.Namely，Sm1cation is bonded to four oxygen atoms from the defect site of the［α－PW11O39H］6－framework ［Sm－O：0.234 5（13）－0.234 9（14）nm］as well as three water molecules［Sm－O：0.249 0（19）－0.258 2（19）nm］and one terminal oxygen atom from another［（α－PW11O39H）Sm（H2O）4］3－moiety［Sm－O：0.254 0（13）nm］.Besides，the Sm1cation is displaced outward and away from the normal twelfth position in theα－Keggin framework.In the coordination polyhedron around the Sm1cation，the O20，O23，O26and O27atoms and the O31A（A＝ －x，1－y，－z），O42，O47and O51atoms constitute two bottom planes of the square antiprism，and their average deviations from their least－squares planes are 0.001 41nm and 0.011 98nm，respectively.The dihedral angle for the two bottom surfaces is 3.7°.The distances between the Sm1cation and the two bottom planes are 0.114 60（12）nm and 0.154 65（12）nm，respectively，and the O－Sm－O bond angles are in the range of 67.2（5）°－144.0（5）°.The above－mentioned data indicate that the square antiprism is severely distorted，which may be related to the coordination environments of different coordination atoms.In addition，the molecules of1are stacked along thec－axis to form circular channels with dimensions of ca.0.92×0.74nm2，in which the discrete［（CH3）4N］＋cations and solvent water molecules are filled（Fig.2）.

Fig.2 The packing arrangement of the title compound along the c axis

2.2 IR spectrum

The IR spectrum of the title compound（Fig.3）recorded in the wavenumber range of 4 000～400cm－1is very useful for the identification of characteristic vibration bands of monolacunary Keggin polyoxoanions and organic components.The characteristic vibration patterns derived fromα－Keggin－type polyoxoanions are observed in 1 100－700cm－1，which is in good agreement with that of monolacunary Keggin－type polyoxoanion.The obvious characteristic bands at 942，897，and 789cm－1are attributed toν（W－Ot），ν（WOb），andν（W－Oc），respectively.The vibration resonances at 1 093cm－1and 1 049cm－1are assigned toν（P－Oa）.In addition，theν（C－H）stretching band emerges at about 3 036cm－1，and theν（C－N）stretching band appears at 1 383cm－1.The vibration bands at 3 468cm－1is assigned to theν（O－H）stretching vibration of water molecules，and the characteristic peaks centered at 1 627cm－1and 1 477cm－1are attributed to the bending vibration ofν（O－H）andν（C－H），respectively.The appearance of these characteristic signals confirms the presence of［（CH3）4N］＋cation and lattice water molecules，being in good agreement with relevant single－crystal structural analysis results.

Fig.3 IR spectrum of the title compound

Fig.4 UV spectrum of the title compound

2.3 UV spectrum

The UV absorbance spectrum of the title compound measured in the range of 190－400nm at room temperature is shown in Fig.4.The UV spectrum shows a strong and sharp absorption band centered at 193nm and a wide shoulder band around 256nm，which can be attributed to the charge－transfer bands of pπ（Od）→dπ＊（W）and pπ（Ob，c）→dπ＊（W），respectively.

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