Synthesis and Ligand-Exchange Reactions of a Tri-Tungsten Cluster with Applications in Biomedical Imaging

In this experiment students are exposed to concepts in inorganic synthesis and various spectroscopies as applied to a tri-tungsten cluster with applications in biomedical imaging. The tungsten−acetate cluster, NaW3(μ-O)2(CH3COO)9, 1, was synthesized and characterized by 1H-NMR, UV-vis, FT-IR and Raman spectroscopy. 1H-NMR shows three characteristic chemical shifts of the bridging (δ 2.30 ppm), terminal (δ 2.15 ppm), and free (δ 2.07 ppm) acetate groups in methanol-d4. The UV-vis spectrum shows two distinct absorption peaks at 376 and 455 nm, and the IR and Raman spectra exhibit a number of coincident peaks that verify the non-centrosymmetric D3h or C3h symmetry of the structure. From a 1H-NMR experiment in which 1 is heated in neat acetic-d3 acid-d, students are able to monitor the sequential exchange of the terminal acetate groups followed by the bridging acetate groups with deuterated acetate. Extrapolating from this experiment, students are in a position to adapt the ligand-exchange reaction and replace the acetate groups with acrylate to form a new, functionalized tungsten acrylate cluster 2. UV-vis spectroscopy shows the conversion from 1 to 2 with a distinctive red shift of the absorption peak from 455 nm to 470 nm and the disappearance of the peak at 376 nm. Undergraduate and graduate teaching laboratories can easily adapt this experiment into their curriculum, allowing students to gain deeper experience with both synthetic and analytical aspects of inorganic chemistry.