Highly Distorted κ³-N,N,H Bonding of Bis(3,5-di-<i>tert</i>-butylpyrazolyl)borate Ligands to the Heavier Group 2 Elements

Treatment of MI2 (M = Ca, Sr, Ba) with two equivalents of thallium bis(3,5-di-tert-butylpyrazolyl)borate (TlBptBu2) in tetrahydrofuran at ambient temperature afforded CaBptBu22 (67%), SrBptBu22 (79%), and BaBptBu22(THF) (63%). Sublimation of BaBptBu22(THF) at 205 °C/0.05 Torr afforded BaBptBu22 (37%) along with loss of tetrahydrofuran. Crystal structure determinations of SrBptBu22, BaBptBu22(THF), and BaBptBu22 revealed monomeric structures containing highly distorted κ3-N,N,H-BptBu2 ligands. The M−N−N−B torsion angles in SrBptBu22, BaBptBu22(THF), and BaBptBu22 range from 20.00(8)° to 60.90(1)°, which indicate significant deformation of the 3,5-di-tert-butylpyrazolyl groups in order to avoid intraligand and interligand tert-butyl group steric repulsions. BH2(tBu2pz)(tBu2pzH) was prepared in 78% yield by treatment of Li(BptBu2)(THF) with pivalic acid, and its X-ray crystal structure was determined. To assess the viability of MBptBu22 (M = Ca, Sr, Ba) as potential thin-film growth precursors, solid-state decomposition studies, thermogravimetric analyses, and preparative sublimations were performed. SrBptBu22 is the most thermally stable among the series, with a solid-state decomposition temperature of 325 °C, a sublimation temperature of 190 °C/0.05 Torr, and a nonvolatile residue of 3.6% in a preparative sublimation. The TGA traces of CaBptBu22 and SrBptBu22 show weight loss regimes from 150 to 325 °C, with final percent residues of 20% and 25%, respectively. Several of the new complexes exhibit much higher thermal stability than existing group 2 chemical vapor deposition precursors and, thus, may serve as film growth precursors.