Synthesis and structural characterization of tricarbomethoxymethanate complexes of copper(II) and barium(II) and evaluation of their suitability for MOCVD applications

Abstract

The reaction between Cu(OCH₃)₂ and HL [L = ⁻C(CO₂CH₃)₃] in THF gives a mixture of two products, CuL(OCH₃) and CuL₂(THF)₂. CuL(OCH₃) was characterized by elemental analysis, infrared spectroscopy and thermal decomposition studies, and its insolubility in THF, at room temperature, suggests that it may be polymeric. The solid state structure of CuL₂(THF)₂ was established ia a single-crystal X-ray diffraction analysis. This complex has a tetragonally distorted trans-disubstituted octahedral structure with L binding like a conventional bidentate β-diketonate ligand; the central CO₂CH₃ group of each L is not coordinated. CuL₂(THF)₂ readily loses the two THF molecules bound to the Cu(II) center upon drying in a stream of dinitrogen to produce THF-free CuL₂. Alternatively, THF-free CuL₂ can be prepared by the reaction of CuL(OCH₃) with HL in refluxing toluene. X-ray crystallography shows that CuL₂ is a polymeric solid composed of a planar Cu(η²-L)₂ repeat unit in which the central CO₂CH₃ group of each L binds weakly to the Cu(II) center of a neighboring unit along a “stepped” (displaced) stack of CuO₄ units. The reaction between elemental barium and HL in THF requires activation with NH_3(g). The product is a white solid of empirical formula BaL₂·0.36THF. It was characterized by elemental analysis, ¹H NMR, ¹³C{¹H} NMR, and infrared spectroscopies, and thermal decomposition studies. It is insoluble in common laboratory solvents but soluble in strong Lewis bases such as pyridine, dimethylsulfoxide, and hexamethylphosphoramide (HMPA). The product resulting from the depolymerization of BaL₂ ·0.36THF with HMPA was characterized ia a single-crystal X-ray diffraction analysis. It is a dimer of empirical formula Ba₂L₄(HMPA)₄. It consists of two BaL₂(HMPA)₂ fragments linked together, in a centrosymmetric fashion, by two μ²,η³-[⁻C(CO₂CH₃)₃] ligands; this is made possible by the utilization of the donor power of the pendant C(O)OCH₃ group not used in CuL₂(THF)₂. The remaining two ⁻C(CO₂CH₃)₃ ligands are also bidentate but are non-bridging, and the four HMPA molecules are bonded through oxygen, resulting in a coordination number of seven for each barium. CuL₂ is volatile and sublimes upon heating the amorphous material obtained from spontaneous desolvation of CuL₂(THF)₂, or polymeric CuL₂, or CuL(OCH₃) under high vacuum. The barium complexes are not volatile. Metallic copper is obtained upon heating the Cu(II) complexes to 1000 °C in a stream of argon, and formation of CuO results when these complexes are decomposed in the presence of oxygen. Thermal decomposition of BaL₂·0.36THF and Ba₂L₄(HMPA)₄ has been carried out under argon and O₂–Ar atmospheres; residues containing varying amounts of BaO, BaO₂ and BaCO₃ were obtained depending on the experimental conditions.