Polyoxometalates (POMs) have been applied in many fields such as catalysis, medicine, biology, magnetism, and material science, because of their wide variety of compositions and structural versatility. The POM based covalently linked frameworks has been focused on. Keggin, Dawson, Anderson, and Strandberg–type POMs have been widely used as the basic building units to construct novel extended structures by means of different linkers such as transition-metal cations, organic groups and metal-organic frameworks.

Prof. PAN Shilie and his Ph.D. students from Xinjiang Technical Institute of Physics & Chemistry, Chinese Academy of Sciences (XTIPC), have synthesized a novel Strandberg-type POM based compound, Cs₄Mo₅P₂O₂₂. Colorless block crystals of Cs₄Mo₅P₂O₂₂were synthesized by spontaneous crystallization from stoichiometric composition melt of Cs₂CO₃, MoO₃, and NH₄H₂PO₄.The structure of Cs₄Mo₅P₂O₂₂ (orthorhombic, C222₁) exhibits 1D _∞[Mo₅P₂O₂₂]^4- anion chainswhich are separated by Cs⁺ cations. The fundamental structure of the _∞[Mo₅P₂O₂₂]^4- chain is diphosphopentamolybdate [Mo₅P₂O₂₃]^6- unit, which could be described as a ring formed by five distorted edge- or corner-shared MoO₆ octahedra, capped by two PO₄ tetrahedra from either side (Strandberg-type POM unit).

Many POMs contain octahedrally coordinated d0 transition metal cations (V^5+, Nb⁵⁺, Ta⁵⁺, Mo⁶⁺or W⁶⁺)susceptible to second-order Jahn-Teller (SOJT) distortion, which benefit for second harmonic generation (SHG) in nonlinear optical (NLO) materials. It is expected that POMs with multiple highly distorted octahedra polymerized into clusters tend to exhibit larger polarizations and strong SHG efficiencies.

A noncentrosymmetric chiral Strandberg-type POM

The most important feature of this compound is its unusual structure: the unusually condensated POM units linked by corner - shared PO₄ polyhedra in Cs₄Mo₅P₂O₂₂ violate the fourth rule of Pauling. There is no report on such an anion topology inStrandberg-type POMs.Besides, Cs₄Mo₅P₂O₂₂ displays a moderateSHG response of about 2 times of that of KH₂PO₄ (KDP), and is phase-matchable, which is potentially useful in nonlinear optical (NLO) materials.

 The results have been published in Chem. Commun., 2013,49, 306-308.The work was supported by the National Natural Science Foundation of China and “One Hundred Talents” program  of Chinese Academy of Sciences.