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 (KTpEt2) or potassium tris(3,5-di-n-propylpyrazolyl)borate (KTpnPr2) in hexane at ambient temperature afforded CaTpEt22 (64%), SrTpEt22 (64%), BaTpEt22 (67%), CaTpnPr22 (51%), SrTpnPr22 (75%), and BaTpnPr22 (39%). Crystal structure determinations of CaTpEt22, SrTpEt22, and BaTpEt22 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. SrTpEt22, BaTpEt22, CaTpnPr22, SrTpnPr22, and BaTpnPr22 vaporize as liquids between 210 and 240 °C at 0.05 Torr. BaTpEt22 and BaTpnPr22 decompose at about 375 °C, whereas MTpEt22 and MTpnPr22 (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.