Direct Synthesis of Two-Dimensional SnSe and SnSe₂ through Molecular Scale Preorganization

Two-dimensional tin monoselenide (SnSe) and tin diselenide (SnSe₂) materials were efficiently produced by the thermolysis of molecular compounds based on a new class of seleno-ligands. Main group metal chalcogenides are of fundamental interest due to their layered structures, thickness-dependent modulation in electronic structure, and small effective mass, which make them attractive candidates for optoelectronic applications. We demonstrate here the synthesis of stable tin selenide precursors by in situ reductive bond cleavage in the dimeric diselenide ligand (SeC₂H₄N(Me)C₂H₄Se)₂ in the presence of SnCl₄. New molecular precursors [Sn^IV(SeC₂H₄N(Me)C₂H₄Se)₂], [Sn^IVCl₂(SeC₂H₄N(Me)C₂H₄Se)], and [Sn^IV(SC₂H₄N(Me)C₂H₄S)(SeC₂H₄N(Me)C₂H₄Se)] were thoroughly characterized by multinuclear magnetic resonance studies and single-crystal X-ray diffraction analysis that revealed the Sn(IV) center to be octahedrally coordinated by two tridentate dianionic chelating ligands or trigonally pyramidally coordinated by one chelating ligand and two chlorido ligands. Preorganization of metal-selenium bonds in both compounds offered direct and reproducible synthetic access to two-dimensional tin chalcogenides (SnSe and SnSe₂) via simple adjustment of the pyrolysis temperature. Additionally, SnSe₂ and SnSSe_2- particles could be successfully synthesized by microwave-assisted decomposition of the molecular precursors, which was unambiguously corroborated by both experimental and computational analyses that explained the formation of a selenium rich SnSSe_2- phase from a single molecular precursor containing both Sn-Se and Sn-S bonds.