Tin(II) Ureide Complexes: Synthesis, Structural Chemistry, and Evaluation as SnO Precursors

In an attempt to tailor precursors for application in the deposition of phase pure SnO, we have evaluated a series of tin (1-6) ureide complexes. The complexes were successfully synthesized by employing N,N'-trialkyl-functionalized ureide ligands, in which features such as stability, volatility, and decomposition could be modified with variation of the substituents on the ureide ligand in an attempt to find the complex with the ideal electronic, steric, or coordinative properties, which determine the fate of the final products. The tin(II) ureide complexes 1-6 were synthesized by direct reaction [Sn{NMe₂}₂] with aryl and alkyl isocyanates in a 1:2 molar ratio. All the complexes were characterized by NMR spectroscopy as well as elemental analysis and, where applicable, thermogravimetric (TG) analysis. The single-crystal X-ray diffraction studies of 2, 3, 4, and 6 revealed that the complexes crystallize in the monoclinic space group P2(1)/n (2 and 4) or in the triclinic space group P-1 (3 and 6) as monomers. Reaction with phenyl isocyanate results in the formation of the bimetallic species 5, which crystallizes in the triclinic space group P-1, a consequence of incomplete insertion into the Sn-NMe₂ bonds, versus mesityl isocyanate, which produces a monomeric double insertion product, 6, under the same conditions, indicating a difference in reactivity between phenyl isocyanate and mesityl isocyanate with respect to insertion into Sn-NMe₂ bonds. The metal centers in these complexes are all four-coordinate, displaying either distorted trigonal bipyramidal or trigonal bipyramidal geometries. The steric influence of the imido-ligand substituent has a clear effect on the coordination mode of the ureide ligands, with complexes 2 and 6, which contain the cyclohexyl and mesityl ligands, displaying κ²-O,N coordination modes, whereas κ²-N,N' coordination modes are observed for the sterically bulkier tert-butyl and adamantyl derivatives, 3 and 4. The thermogravimetric analysis of the complexes 3 and 4 exhibited excellent physicochemical properties with clean single-step curves and low residual masses in their TG analyses suggesting their potential utility of these systems as MOCVD and ALD precursors.