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

Abstract

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.