Volatility, High Thermal Stability, and Low Melting Points in Heavier Alkaline Earth Metal Complexes Containing Tris(pyrazolyl)borate Ligands

Treatment of MI₂ (M = Ca, Sr) or BaI₂(THF)₃ with 2 equiv of potassium tris(3,5-diethylpyrazolyl)borate (KTp^Et2) or potassium tris(3,5-di-n-propylpyrazolyl)borate (KTp^nPr2) in hexane at ambient temperature afforded CaTp^Et2₂ (64%), SrTp^Et2₂ (64%), BaTp^Et2₂ (67%), CaTp^nPr2₂ (51%), SrTp^nPr2₂ (75%), and BaTp^nPr2₂ (39%). Crystal structure determinations of CaTp^Et2₂, SrTp^Et2₂, and BaTp^Et2₂ revealed monomeric structures. X-ray structural determinations for strontium tris(pyrazolyl)borate (SrTp₂) and barium tris(pyrazolyl)borate ([BaTp₂]₂) show that SrTp₂ exists as a monomer and [BaTp₂]₂ exists as a dimer containing two bridging Tp ligands. The thermogravimetric analysis traces, preparative sublimations, and melting point/decomposition determinations demonstrate generally very high thermal stabilities and reasonable volatilities. SrTp₂ has the highest volatility with a sublimation temperature of 200 °C/0.05 Torr. [BaTp₂]₂ is the least thermally stable with a decomposition temperature of 330 °C and a percent residue of 46.5% at 450 °C in the thermogravimetric analysis trace. SrTp^Et2₂, BaTp^Et2₂, CaTp^nPr2₂, SrTp^nPr2₂, and BaTp^nPr2₂ vaporize as liquids between 210 and 240 °C at 0.05 Torr. BaTp^Et2₂ and BaTp^nPr2₂ decompose at about 375 °C, whereas MTp^Et2₂ and MTp^nPr2₂ (M = Ca, Sr) are stable to >400 °C. Several of these new complexes represent promising precursors for chemical vapor deposition and atomic layer deposition film growth techniques.