Monomeric Group 2 metal β-diketonates stabilised by protonated amine ligands as tight cation–anion pairs; crystal structure of [Htmen]2[Sr(tfpd)4] (tfpd = 1,1,1-trifluoropentane-2,4-dionate, tmen = N,N, N′,N′ -tetramethylethane-1,2-diamine)

Abstract

The in situ reactions of the tetraethanol-solvated alkaline-earth metal ethoxides [{M(OEt) ₂ (EtOH) ₄ } _n ] (M = Ca, Sr or Ba) with selected β-diketones and multidentate amines, resulted in the formation of tight cation–anion pairs, [H(L–L)] ₂ [M(β-diket) ₄ ] [β-diket = 1,1,1-trifluoropentane-2,4-dionate (tfpd) or 1,1,1,5,5,5-hexafluoropentane-2,4-dionate (hfpd); L–L = N,N,N′, N′-tetramethylethane-1,2-diamine (tmen) or N,N,N′,N′,N″-pent amethyldiethylene-triamine (pmdien)] in relatively low yields. The products have been characterised by microanalysis, IR, ¹ H and ¹³ C NMR and mass spectrometry. The complex [Htmen] ₂ [Sr(tfpd) ₄ ] has been further characterised by X-ray crystallography. The structure contains a monomeric eight-co-ordinate [Sr(tfpd) ₄ ] ^2- dianion in which the four chelating tfpd ligands are in a square-prismatic arrangement, while the protonated tmen ligands are strongly hydrogen bonded to the tfpd chelates. The Sr–O distances lie in the range 2.530(2)–2.593(2) Å, average 2.571 Å.