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.

Exploring transition metal fluoride chelates – synthesis, properties and prospects towards potential PET probes

“…This was also observed in the Group 13 (Al, Ga) and ScF 3 terpy complexes. 8,13 Among the complexes with Stability tests indicated that tacn derivatives bearing the FeF 3 fragment may be suitable for radiofluorination, and this was borne out by 18…”

“…6,7,12 We also investigated the transition metal fluorides using Sc(III), Y(III), La(III) and Lu(III). 13 Among these d-and f-block metal systems, only the [ScF 3 (RMe 2 -tacn)] (R = Me, Bn) were successfully synthesised through Cl/F halide exchange reactions. [ScF 3 (BnMe 2 -tacn)] was identified as a promising system for future 18 F-radiolabelling, although, in contrast to the Group 13 systems, it could only be obtained from the trichloride analogue under anhydrous conditions using [NMe 4 ]F or Me 3 SnF as the fluoride source.…”

“…However, the trifluoride complex is very stable in water. 13 In this work, we present an evaluation of the 3d transition metal (Cr 3+ , Mn 3+ , Fe 3+ and Co 3+ ) trifluoride complexes for possible PET applications. The distorted octahedral MF 3 complexes bearing tridentate Me 3 -tacn and terpy ligands, are described and their stabilities in water probed by UV-vis spectroscopy.…”

Phosphorescence Afterglow and Thermal Properties of [ScCl3(ptpy)] (ptpy: 4'‐phenyl‐2,2',6',2''‐terpyridine)

“…The molecular structure of [ScCl 3 (ptpy)] is quite similar to trichlorido(2,2′:6′,2′′‐terpyridine)scandium (crystallizing in space group P 2 1 / n ), the main difference between two complexes being the additional phenyl ring in ptpy [32] . Due to the small ionic radius of trivalent scandium, no additional molecules, such as solvent molecules, are coordinated to the metal center in the resulting [ScCl 3 (ptpy)] complex, unlike with trivalent lanthanides, which form complexes of general formula [LnCl 3 (ptpy)(py)], in which the metal ion coordination sphere forms a distorted pentagonal bipyramid [28] .…”

“…Recently, we have presented two series of lanthanide coordination polymers and complexes including organic ligand sensitized 4f‐4f emission of NIR and visible emitters [28] . Despite the wide usage of terpyridine and other tridentate N‐ligands in the coordination chemistry of lanthanides, only a few examples of coordination compounds obtained with scandium have been published, so far: with 4‐amino‐bis(2,6‐(2‐pyridyl))‐1,3,5‐triazine [29] and 2,6‐bis(4‐isopropyl‐4,5‐dihydro‐1,3‐oxazol‐2‐yl)pyridine, [30] which have an N N N coordination site similar to terpyridine, and two with 2,2′:6′,2′′‐terpyridine itself [31,32] . Other examples are in situ generated trivalent scandium complexes with terpyridine or related molecules for catalysis, [33,34] or monitoring the changes of organic‐based luminescence of 2,3,5,6‐tetrakis(2‐pyridyl)pyrazine in presence of Sc 3+ and other metal ions [35,36] .…”