Volatility Enhancement in Calcium, Strontium, and Barium Complexes Containing β-Diketiminate Ligands with Dimethylamino Groups on the Ligand Core Nitrogen Atoms

Treatment of M(N(SiMe3)2)2(THF)2 with 2 equiv of 4-(2,2-dimethylhydrazino)dimethylhydrazone-3-penten-2-one (LNMe2H) in toluene at ambient temperature afforded Ca(η2-LNMe2)2 (88%), [Sr(η5-LNMe2)(μ-η1:η5-LNMe2)]2 (85%), and [Ba(η5-LNMe2)(μ-η2:η3-LNMe2)]2 (83%) as colorless or pale yellow crystalline solids. The formulations of the new complexes were assigned from spectral and analytical data and by X-ray crystal structure determinations. In the solid state, Ca(η2-LNMe2)2 exists as a tetrahedral monomer. [Sr(η5-LNMe2)(μ-η1:η5-LNMe2)]2 is a dimer that contains a terminal η5-LNMe2 ligand on each strontium ion. The dimer is held together by two μ-η1:η5-LNMe2 ligands, in which one dimethylamino moiety per LNMe2 ligand acts as a donor ligand to the adjacent strontium ion. [Ba(η5-LNMe2)(μ-η2:η3-LNMe2)]2 crystallizes as a dimer that contains a terminal η5-LNMe2 ligand on each barium ion. The two bridging LNMe2 ligands adopt a μ-η2:η3-coordination mode, where each LNMe2 ligand is bonded to one barium ion through the in-plane nitrogen atom lone pairs and to the other barium ion through the out-of-plane π-system of one nitrogen and two carbon atoms. Ca(η2-LNMe2)2, [Sr(η5-LNMe2)(μ-η1:η5-LNMe2)]2, and [Ba(η5-LNMe2)(μ-η2:η3-LNMe2)]2 sublime at 90, 115, and 145 °C, respectively, at 0.05 Torr. These sublimation temperatures are 25 to 55 °C lower than those of previously reported analogous calcium, strontium, and barium complexes that contain isopropyl groups on the β-diketiminate ligand core nitrogen atoms. The increased volatility of complexes containing LNMe2 ligands is attributed to the lattice energy-lowering effects of the dimethylamino group lone pairs of electrons, much like what is observed in group 2 β-diketonate complexes containing fluorinated alkyl groups. The relevance to film growth precursor design is discussed.