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The ongoing goal of this project is to synthesize new hybrid inorganic-organic materials based on the tetrathiometallate anions, MoS₄^2- and WS₄^2-.  Compared with the large number of network compounds based on group VI oxides, relatively few hybrid compounds have been prepared with tetrathiometallates as structural components, partly due to the challenges of crystallizing sulfide-based networks.  We are using solvothermal synthesis in non-aqueous solvents to crystallize new materials. 

Using ethylenediamine as a solvent, we have prepared a new polymorph of Ni(en)₃MoS₄ (en = ethylenediamine) and determined its orthorhombic Pcab structure through single-crystal X-ray diffraction.  In order to allow rigorous comparison with the new polymorph, we did a redetermination at 100K of the structure of a polymorph that had been found previously by Bensch and coworkers, confirming the orthorhombic Pna2₁ structure.  The polymorphs differ through the packing of the Ni(en)₃²⁺ and MoS₄^2- ions in the unit cell as well as in the detailed conformation of the en ligand backbones.  We are able to control formation of the polymorphs through variation of solvothermal reaction condtions.  The new polymorph is produced in reactions carried out between ambient temperature and 80^oC, and the earlier polymorph is produced at higher temperatures.  Water content in the reaction mixture and the anion (Br^- or NO₃^-) provided by the nickel(II) starting material also affect the phase composition of the product.  Through solvent-mediated conversion studies, we have characterized the system as enantiotropic, with the new polymorph favored at ambient temperature and the Bensch polymorph favored at 120 ^oC.  However, kinetic factors are influential in the intermediate temperature range, and conversion is kinetically hindered under certain conditions.  

We have also crystallized Co(en)₃MoS₄ and Mn(en)₃MoS₄ and found through single-crystal X-ray diffraction that these compounds are isostructural to the new polymorph of Ni(en)₃MoS₄.  While a Zn analogue is obtained under certain reaction conditions, we have also found a new double salt, Zn(en)₃MoS₄×Zn(en)₃Cl₂, that forms a modulated structure.  We have determined the average structure and are now working toward growing the larger crystals needed for full refinement of the modulated structure.  We are also expanding our work to include other organic ligands, including those capable of forming bridges between metal centers.

Because an important application of tetrathiomolybdate-based hybrid materials is as precursors to hydrodesulfurization (HDS) catalysts, study of the thermal decomposition of the new materials is important.  Each of the M(en)₃MoS₄ (M = Mn, Co, Ni) compounds decomposes through ligand loss between 200 and 300 ^oC, forming amorphous products that become poorly crystalline MS and MoS₂ upon heating to 500 ^oC.  We have carried out differential scanning calorimetry on the two polymorphs of Ni(en)₃MoS₄.  Both polymorphs decompose directly through ligand loss without melting, and no solid-solid interconversion takes place.

Support from an ACS-PRF grant has been instrumental in helping me to set up laboratory infrastructure, recruit a summer student, and report on our work at conferences.  I have developed a fruitful collaboration with Dr. Matthias Zeller at the STaRBURSTT CyberInstrumentation Consortium at Youngstown State University through which I send single crystals for diffraction experiments and receive electronic data files that allow me to carry out structure solution and refinement at Oberlin.  Two Oberlin undergraduates have worked on research related to this grant – one full-time during summer 2008 and one for a senior honors project during the 2008-09 year.  Both students gave oral presentations about their work at the Meeting-in-Miniature of the Cleveland Section of the ACS, and one won an award for an outstanding undergraduate talk.  They have also been my co-authors on three presentations that I have given at regional and national conferences and on a manuscript that I recently submitted to Crystal Growth and Design.