Synthesis, structure and properties of a Pr-containing organophosphonate-functionalized polyoxotungstate

Abstract

Polyoxometalates (POMs), a unique class of metal-oxygen clusters with an oxygen-rich surface and high negative charge, have attracted great attention due to their unique structural variety and excellent physicochemical properties. In addition, organophosphonates, as an important kind of organic ligands with wide various and rich coordination modes, have lots of potential applications in materials, medicine and catalysis. Therefore, the synthesis of organophosphonates-based POMs with novel structure and good property is of contemporary interest. Polyoxotungstates (POTs) are viewed as one of the most important and vibrant subfamilies in POM chemistry and have been increasingly used as building blocks for the construction of systems with inorganic-organic hybrid compounds. Nevertheless, compared to the abundant derivatives of polyoxomolybdates and polyoxovanadates, organophosphonate-based POTs have been rarely studied. It is commonly that most of the structures just anchor some common transition metals (Co, Ni, Cu) or even do not anchor any metal ions. In contrast, POTs incorporated with lanthanide (Ln) ions and organophosphonate ligands have been rarely explored. Up to now, only a series of Ln-containing organophosphonatebased POTs have been reported. Moreover, the Ln ions possess the 4f electrons, leading to interesting properties such as magnetic and photoluminescent properties. It appeals us to devote in investigating the interesting Ln-containing organophosphonate-functionalized POTs. Herein, a new organic-inorganic hybrid CsH[Pr(H2O)4(AsW9O33)(W2O3)(O3PCOHCH3PO3)Pr(H2O)7]2∙19H2O ( 1 ) has been successfully synthesized by reaction of K14[As2W19O67(H2O)], HEDP and PrCl3∙6H2O in a conventional aqueous method. This structure was structurally characterized by single-crystal X-ray diffraction, TG analysis, PXRD and IR spectrum. The results show that compound 1 exhibits a three-dimensional (3D) framework structure which is consist of the S-shaped inorganic building block {[(AsW9O33)(W2O3)]2}, four Pr ions and two organophosphonate groups {O3PCOHCH3PO3}. Variable temperature magnetic susceptibility of the solid state 1 was also measured at 1.8−300 K with an applied field of 1 kOe. At 300 K, the χ m T product is 6.42 cm3 K mol-1, close to the value of 6.40 cm3 K mol-1 expected for four uncoupled PrIII ions ( L =5, 3H4, g =4/5, S =1). With cooling, the χ m T value decreases continuously down to a value of 5.16 cm3 K mol-1 at 101 K. As the temperature further decreases, the χ m T value starts to decrease rapidly and reaches a minimum of 0.43 cm3 mol-1 K at 1.8 K. This behavior indicates the antiferromagnetic exchange interactions and which is further confirmed by a negative Weiss constant θ = -39.02 K derived by fitting the Curie-Weiss law to the magnetic data between 68 and 300 K The near-infrared (NIR) emission spectra of 1 was also investigated at room temperature. Under the excitation at 448 nm, the NIR emission spectrum of 1 consists of three characteristic peaks of the PrIII ions at 871 nm (ID2→3F2), 1017 nm (ID2→3F4) and 1515 nm (1D2→1G4). Moreover, the NIR decay curve of 1 is fitted into a second-order exponential function with lifetimes of τ 1=4.76 μs (35.53%), τ 2=9.71 μs (64.47%). In the following work, other bridging ligand with different functional groups such as amino or carboxyl-functionalized organophosphonate will be introduced into this system to explore much more Ln-containing organophosphonate-based POTs with intriguing structures and properties.