Synthesis and structural characterization of mixed halide–N,N-diethylcarbamates of group 4 metals, including a case of unusual tetrahydrofuran activation

Abstract

Solid-state, thermally stable tetramethylguanidinium N,N-diethylcarbamate [TMGH][O₂CNEt₂] was prepared from tetramethylguanidine (TMG), N,N-diethylamine and carbon dioxide (1 atm). The reaction of [TMGH][O₂CNEt₂] with TiCl₄ in toluene afforded [TMGH]₂[TiCl₆] and TiCl₂(O₂CNEt₂)₂. The tetrachlorides of titanium, zirconium and hafnium were effectively converted into the respective mixed dichloride-dicarbamates, MCl₂(O₂CNEt₂)₂, by the reaction with either silver diethylcarbamate or the corresponding homoleptic metal N,N-diethylcarbamate, M(O₂CNEt₂)₄. The titanium and hafnium derivatives are trinuclear in the solid state according to X-ray diffraction analysis (M = Ti) and DFT calculations (M = Hf). Instead, ZrCl₂(O₂CNEt₂)₄ was obtained as a mixture of two isomers presumably holding dinuclear and trinuclear structures, according to DFT calculations. Difluoride-dicarbamate TiF₂(O₂CNEt₂)₄ was prepared in a similar way as the homologous Cl-compound. All the products were characterized by analytical and spectroscopic methods, and by X-ray diffraction in the cases of [TMGH]₂[TiCl₆] and Ti₃Cl₆(μ-O₂CNEt₂)₆. The complex Hf₄Cl₄(O)₂(O₂CNEt₂)₆[O(CH₂)₃CHO]₂ was isolated in low yield in crystalline form from a thf solution of HfCl₂(O₂CNEt₂)₂, and then X-ray characterized.