Metal based synthetic routes to heavy alkaline earth aryloxo complexes involving ligands of moderate steric bulk

Treatment of an alkaline earth metal (Ca, Sr, Ba) with 2,4,6- trimethylphenol (HOmes) at elevated temperatures in the presence of mercury under solvent-free conditions, followed by extraction of the reaction mixture with 1,2-dimethoxyethane (dme), afforded dinuclear alkaline earth aryloxo complexes [Ae₂(Omes)₄(dme)₄] (Ae = Ca 1, Sr 3, Ba 6). Extraction of the Ca metal and HOmes reaction mixture with thf afforded [Ca₃(Omes)₆(thf)] 2. In contrast, redox transmetallation ligand exchange reactions between an alkaline earth metal, diphenylmercury and HOmes in dme yielded solely 1 for Ca metal, a mixture of 3 and the methoxide bridged cage [Sr₅(Omes)₅(OMe)₅(dme) ₄]·2dme 4 for Sr metal, and solely [Ba₅(Omes) ₅(OMe)₅(dme)₄]·dme 7 for Ba metal. The methoxide ligands originate from the C-O activation of the dme solvent. Treatment of liquid ammonia activated Sr or Ba metal with HOmes in thf afforded the linear species [Ae₃(Omes)₆(thf)₆] (Ae = Sr 5, Ba 8), and 8 was also obtained from barium metal and HOmes in refluxing thf. The structures of 1 and 3, determined by X-ray crystallography, consist of two six coordinate Ae metal atoms, to each of which is bound a terminal aryloxide ligand, two bridging aryloxide ligands, and chelating and unidentate dme ligands. The structures of 4 and 7 contain five Ae metal atoms arranged on the vertices of a distorted square based pyramid. The Ae atoms are linked by four μ₃-OMe ligands and a μ₄-OMe ligand. Four bridging aryloxide ligands and four chelating dme ligands complete the coordination spheres of the four seven coordinate Ae atoms at the base of the pyramid, and a terminal aryloxide ligand is bound to the five coordinate apical Ae atom. The structures of 5 and 8 consist of a trinuclear linear array of Ae metal atoms, and contain solely bridging aryloxide ligands. Three thf ligands are bound to each terminal Ae atom, giving all Ae atoms a coordination number of six.