Group 3 metal trihalide complexes with neutral N-donor ligands – exploring their affinity towards fluoride

Abstract

Fluorination of [ScCl₃(Me₃-tacn)] (Me₃-tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane) and [ScCl₃(BnMe₂-tacn)] (BnMe₂-tacn = 1,4-dimethyl-7-benzyl-1,4,7-triazacyclononane) by Cl/F exchange with 3 mol. equiv. of anhydrous [NMe₄]F in CH₃CN solution yields the corresponding [ScF₃(R₃-tacn)] (R₃ = Me₃ or BnMe₂). These are the first examples of scandium fluoride complexes containing neutral co-ligands. The fluorination occurs stepwise, and using a deficit of [NMe₄]F produced [ScF₂Cl(Me₃-tacn)]. Attempts to fluorinate [YCl₃(Me₃-tacn)], [YI₃(Me₃-tacn)], [LaCl₃(Me₃-tacn)(OH₂)] or [MCl₃(terpy)] (M = Sc, Y or La; terpy = 2,2′:6′2′′-terpyridyl) using a similar method were unsuccessful, due to the Cl/F exchange being accompanied by loss of the neutral ligand from the metal centre. Fluorination of [ScCl₃(Me₃-tacn)] or [ScCl₃(terpy)] with Me₃SnF was also successful. The products were identified as the very unusual heterobimetallic [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] and [Sc(terpy)F(μ-F)₂(SnMe₃Cl)₂], in which the Me₃SnCl formed in the reaction behaves as a weak Lewis acid towards the scandium fluoride complex, linked by Sc–F–Sn bridges. [Sc(terpy)F(μ-F)₂(SnMe₃Cl)₂] decomposes irreversibly in solution but, whilst multinuclear NMR data show that [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] is dissociated into the [ScF₃(Me₃-tacn)] and Me₃SnCl in CH₃CN solution, the bimetallic complex reforms upon evaporation of the solvent. The new scandium fluoride complexes and the chloride precursors have been characterised by microanalysis, IR and multinuclear NMR (¹H, ¹⁹F, ⁴⁵Sc) spectroscopy as appropriate. X-ray crystal structures provide unambiguous evidence for the identities of [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl], [ScF₂Cl(Me₃-tacn)], [YI₃(Me₃-tacn)], [{YI₂(Me₃-tacn)}₂(μ-O)], [ScCl₃(terpy)], [YCl₃(terpy)(OH₂)], and [{La(terpy)(OH₂)Cl₂}₂(μ-Cl)₂]. Once formed, the [ScF₃(R₃-tacn)] complexes are stable in water and unaffected by a ten-fold excess of Cl⁻ or MeCO₂⁻, although they are immediately decomposed by excess F⁻. The potential use of [ScF₃(R₃-tacn)] type complexes as platforms for ¹⁸F PET (positron emission tomography) radiopharmaceuticals is briefly discussed. Attempts to use the Group 3 fluoride “hydrates”, MF₃·xH₂O, as precursors were unsuccessful; no reaction with R₃-tacn or terpy occurred either on reflux in CH₃CN or under hydrothermal conditions (H₂O, 180° C, 15 h). PXRD data showed that these “hydrates” actually contain the anhydrous metal trifluorides with small amounts of surface or interstitial water.