三例同构稀土/钨氧簇化合物二维红外相关光谱研究

Two-Dimensional Infrared Spectroscopic Study of Three Isomorphic Rare Earth/Polyoxotungstates

多酸化合物(POMs)是一类重要的多金属氧簇,由过渡金属在其高氧化态(V^Ⅴ, Nb^Ⅴ, Ta^Ⅴ,Mo^Ⅵ,Mo^Ⅴ及W^Ⅵ)与氧桥联的簇阴离子组成。在前人研究基础(H2en)2{SiW11O39Sm(H2O)2}·(H3O)·6H2O之上,改变稀土盐,成功合成了与其同构的三例晶体(H2en)2{SiW11O39Ln(H2O)2}·(H3O)·6H2O[Ln=Ce(1), Pr(2), Nd(3)], X-射线单晶衍射实验测得四者属于三斜晶系,P1空间群,晶胞参数一致,表明它们晶体结构相同;X-射线粉末衍射实验显示四者峰位相同,表明物相一致。此类同构的晶体,由于簇阴离子相同,仅取代的稀土离子不同,在许多表征方式上,显示出类似的现象,例如一维红外光谱吸收曲线相似,在1 039, 949, 889和787 cm^-1均出现了归属于Keggin簇阴离子骨架的振动吸收,在3 600~3 300和1 600~1 630 cm^-1附近均出现νas(O—H)及δ(O—H)的吸收峰,在3 277, 2 927和2 855 cm^-1内均出现了乙二胺配体N—H和C—H伸缩振动峰。但是磁微扰下的二维红外相关光谱,磁性粒子对磁场响应很敏感,热微扰下的二维红外相关光谱,易于捕捉氢键振动模式的细微变化。因此,二维红外光谱可以用于精细测定分子结构,并且此类同构钨氧簇合物的二维红外光谱对比分析还未见报道。磁微扰下的二维红外相关光谱,化合物1在468, 560和810 cm^-1出现响应峰,分别归属于νas(Ce—O),骨架ν(W—O),νas(W—Ob)。化合物2在450,464和475 cm^-1出现νas(Pr—O), 570和675 cm^-1处响应峰归属于骨架ν(W—O)。化合物3在452, 468和472 cm^-1处响应峰归属于νas(Nd—O), 518, 533, 545, 565和695 cm^-1响应峰为骨架ν(W—O)。化合物1, 2, 3归属于骨架ν(W—O)的响应峰数目增多,这是由于磁微扰下的二维红外光谱, Ce^3+, Pr^3+, Nd^3+价电子组态分别为4f^1, 4f^2, 4f^3,价电子数增加,磁性粒子Ln^3+对邻近W—O键影响变大。热微扰下的二维红外相关光谱,化合物1, 2, 3均在400 cm^-1左右出现了νas(Ln—O)响应峰, 810, 860和940 cm^-1左右均出现了νas(W—Ob)和νas(W—Od)的响应峰,这是由于三个化合物的簇骨架相同,氢键相同。但是化合物1, 2, 3的νas(W—O)振动响应最强峰分别出现在810, 850和855 cm^-1,这可能由于取代在簇骨架上的Ln离子极性不同,对邻近W—O键偶极矩产生不同影响。因此通过二维红外光谱,可以很好地对此类同构的稀土取代钨氧簇的异同点进行分析。

Polyoxometalates(POMs)are an important class of metal-oxygen clusters,which are composed of cluster anions bridged by pre-transition metals(V^Ⅴ,Nb^Ⅴ,Ta^Ⅴ,MoⅥ,Mo^Ⅴand W^Ⅵ)and oxygen.On the basis of previous studies(H2en)2{SiW11O39Sm(H2O)2}·(H3O)·6 H2O,we changed rare earth salts and successfully synthesized three isomorphic crystals(H2en)2{SiW11O39Ln(H2O)2}·(H3O)·6 H2O[Ln=Ce(1),Pr(2),Nd(3)],X-ray single crystal diffraction experiment measured that the four belong to the triclinic system,the P1 space group,and the unit cell parameters are consistent,indicating that they have the same crystal structure.XRD experiments show that they have the same peaks,indicating that the substances are identical.Due to the same cluster anions and only the substituted rare earth ions are different,these isomorphic crystals exhibit similar phenomena in many characterization methods,for example the similar absorption curves in 1 D infrared spectroscopy:the vibrational absorption of anion skeleton belonging to Keggin cluster appears at 1039,949,889 and 787 cm^-1,and the absorption peaks ofνas(O—H)andδ(O—H)occur in the vicinity of 3600~3300 and 1600~1630 cm^-1.The stretching vibration peaks of N—H and C—H ligands of ethylenediamine were observed in 3277,2927 and 2855 cm^-1.However,the two-dimensional infrared correlation spectra under magnetic perturbation are sensitive to the magnetic field response.The two-dimensional infrared correlation spectra under thermal perturbation are easy to capture the subtle changes of hydrogen bond vibration modes.Therefore,two-dimensional infrared spectroscopy can be used for fine determination of molecular structure,and the comparative analysis of two-dimensional infrared spectroscopy of such isomorphic polyoxotungstate has not been reported.Two-dimensional infrared correlation spectra under magnetic perturbation show that compound 1 has response peaks at 468,560 and 810 cm^-1,which belong toνas(Ln—O),skeletonν(W—O)andνas(W—Ob),respectively.Compound 2 exhibits as(Ln—O)at 450,464,475 cm^-1,and the response peak at 570,675 cm^-1 belongs to skeletonν(W—O).The response peaks of compound 3 at 452,468,472 cm^-1 belong toνas(Ln—O),and 518,533,545,565,695 cm^-1 belong to skeletonν(W—O).The number of response peaks of compound 1,2,3 belongs to skeletonν(W—O)increases.This is due to the valence electron configurations of Ce^3+,Pr^3+,Nd^3+are 4f^1,4f^2,4f^3.The valence electron number increases,so the influence of magnetic particle Ln^3+on adjacent W—O bonds increases.Two-dimensional infrared correlation spectra under thermal perturbation show that compounds 1,2 and 3 have as(Ln—O)response peaks at about 400 cm^-1,and the response peaks ofνas(W—Ob)andνas(W—Od)appear at 810,860 and 940 cm^-1,which are due to the same cluster skeleton and the same hydrogen bond.However,the most strong peak positions ofνas(W—O)belong to compounds 1,2,3 appear at 810,850,855 cm^-1,which may because the different polarities of Ln ions substituted on cluster skeletons have different effects on the dipole moments of adjacent W—O bonds.The similarities and differences of these isomorphic rare earth substituted polyoxotungstate can be well analyzed by two-dimensional infrared spectroscopy.