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.

---

Introduction

The coordination chemistries of scandium and yttrium have been explored much less than those of the other 3d and 4d metals. The presence of colourless metal ions, in a single (3+) oxidation state and with closed shell configurations (hence no magnetic or d–d spectroscopic fingerprints), coupled with their limited availability, low purity and high cost, restricted early work.^1,2 Often their chemistry was included in studies of the lanthanide elements, which tended to see similarities rather than explore differences.² More recent work³ has shown that there are significant differences, especially for scandium, and the structural chemistry of scandium is surprisingly diverse.⁴ A rich, but synthetically challenging, organometallic chemistry of both metals has been explored in recent years,⁵ and C–H bond activation, ethene, styrene and α-olefin polymerisation, and aromatic C–F bond activation have all been observed in appropriate systems.^5–9 Scandium fluoride has attracted considerable interest due to its negative thermal expansion,¹⁰ and nanocrystals of ScF₃ doped with lanthanide ions exhibit tuneable luminescent properties.¹¹ As expected for oxophilic metal ions, the coordination chemistry with neutral ligands has been dominated by oxoanion salts – nitrate, triflate, carboxylates, etc., with much less work on the metal halides.³ Apart from a range of fluoro-anions mostly made by solid state syntheses,^2,12 complexes containing Sc–F or Y–F bonds are very rare and contain charged N- or C-donor co-ligands,⁴e.g. [ScL(μ-F)₂(SnMe₃Br)₂] (L⁻ = N,N′′-(1,3-dimethyl-1,3-propanediylidine)bis(N′,N′-diethyl-1,2-ethanediamine)) made from [ScLBr₂] and Me₃SnF,¹³ [Sc{MeC(N(2,6-ⁱPrC₆H₃))CHC(Me)(NCH₂CH₂NMe)NH(2,6-ⁱPrC₆H₃)}F],⁶ [Sc₂(μ-F)₂{Fe(C₅H₄NSi^tBuMe₂)₂}],⁹ [{Cp₂Y(μ-F)(thf)₂}₂],⁸ and [{Cp₂ScF}₃].¹⁴ Attempts to prepare complexes of ScF₃ or YF₃ with neutral donor ligands,¹⁵ such as R₃PO,¹⁶ have failed, whilst treatment of [ScI₃{o-C₆H₄(PMe₂)₂}₂] with [NMe₄]F in CH₂Cl₂ resulted in liberation of the diphosphine and precipitation of ScF₃.¹⁷

In an effort to develop new PET (positron emission tomography) imaging agents, the ability of Group 13 metal (Al, Ga or In) aza-macrocycle complexes to undergo Cl/F exchange, including in some cases with radiofluorine, ¹⁸F, under mild conditions and in aqueous solution has been demonstrated.¹⁸ These include both the neutral trifluoride complexes, [MF₃(R₃-tacn)] (M = Al, Ga; R₃-tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane, 1,4-dimethyl-7-benzyl-1,4,7-triazacyclononane)¹⁹ and anionic complexes, [M(nota)F] (H₂-R-nota = 1-R-1,4,7-triazacyclononane-4,7-dicarboxylic acid; R = benzyl or a peptide conjugate).^20,21 The ideal requirements for PET imaging agents are that the metal complex precursor can be rapidly radiofluorinated in a single step, with minimal subsequent purification needed, and that the resulting complex is stable in water, to reaction with other competitive anions and at near neutral pH. The present work sought to establish whether Group 3 metal centres (Sc, Y or La) could offer prospects as alternative reagents for radiofluorination as potential future PET platforms and therefore explores the synthesis, properties and stability of the metal trifluoride complexes with tridentate neutral N₃-donor ligands, R₃-tacn and 2,2′:6′2′′-terpyridyl.

Experimental

All complex syntheses were carried out using standard Schlenk and vacuum line techniques. Samples were handled and stored in a glove box under a dry dinitrogen atmosphere to exclude moisture, which decomposes many of the samples. [ScCl₃(thf)₃] and [YCl₂(thf)₅][YCl₄(thf)₂] were prepared by the literature methods.^22,23 2,2′:6′2′′-terpyridyl was obtained from Sigma-Aldrich and dried in vacuo prior to use. 1,4,7-Trimethyl-1,4,7-triazacyclononane and 1,4-dimethyl-7-benzyl-1,4,7-triazacyclononane were prepared as described previously.²⁴ Anhydrous [NMe₄]F was obtained by recrystallising the commercial sample (Aldrich) from ⁱPrOH as described.²⁵ Infrared spectra were recorded as Nujol mulls between CsI plates using a PerkinElmer Spectrum 100 spectrometer over the range 4000–200 cm⁻¹. ¹H, ¹⁹F{¹H} and ⁴⁵Sc NMR spectra were recorded from CH₂Cl₂/CD₂Cl₂ or CH₃CN/CD₃CN solutions using a Bruker AV400 spectrometer and referenced to TMS via the residual solvent resonance, CFCl₃ and [Sc(H₂O)₇]³⁺ in water at pH = 1. Microanalyses were undertaken by London Metropolitan University. n-Hexane was dried by distillation from sodium and CH₂Cl₂ and CH₃CN from CaH₂.

Metal trifluoride “hydrates”

ScF₃·xH₂O. Sc₂O₃ (2.9 g, 0.021 mol), and a 6 M solution of HCl (43 mL) were heated to reflux for 3 h, during which period the mixture changed from a cloudy white suspension to a clear yellow solution. The solvent was removed in vacuo whilst heating at 65 °C. ScCl₃·6H₂O was obtained as a white solid. This was dissolved in water in a plastic beaker and 6 mL of 40% HF_(aq) (CARE) were added causing the precipitation of a white solid. The mixture was heated to boiling and the solvent evaporated, giving a white gel-like solid. A portion of the gel was suspended in water, causing the formation of the solid, which was isolated by evaporation of the solvent. The same procedure was repeated portion by portion and the solid combined (3.94 g, 93%).

YF₃·xH₂O. Method 1: Y₂(SO₄)₃·8H₂O (3.0 g, 4.92 mmol) was dissolved in water. 5 mL of a solution of 40% HF_(aq) was added and a white precipitate formed. The precipitate was left to settle overnight. The solution was filtered and the solid washed with water and dried in vacuo (1.07 g, 75%).

Method 2: Y₂(SO₄)₃·8H₂O (3.0 g, 4.92 mmol) was suspended in hot water (80 °C) until most of the solid dissolved. The liquid was decanted off from any residue and a solution of 40% HF_(aq) (3 mL) was added to the solution. A white solid precipitated immediately. The reaction was left stirring for 1.5 h and then the solid was left to settle overnight. The solution was decanted off and the solid dried overnight in a desiccator (1.24 g, 86%).

LaF₃·xH₂O. LaCl₃·7H₂O (5.0 g, 13.5 mmol) was dissolved in water (30 mL). 40% HF_(aq) (1.5 mL) was diluted in water (10 mL) and added dropwise to the solution, giving a white gelatinous material which was stirred for 1 h. This solid was collected by evaporating the solvent off at 110 °C, leaving a fine white solid. Yield: 2.50 g, 95%.

Metal complexes

[ScCl₃(terpy)]. A solution of terpy (0.055 g, 0.24 mmol) in 3 mL of CH₃CN was added to a solution of [ScCl₃(thf)₃] (0.076 g, 0.24 mmol) in 5 mL of CH₃CN, causing the immediate precipitation of a white solid. After a few minutes, the solid was filtered off, washed with n-hexane and dried in vacuo. Yield: 0.054 g, 60%. Required for C₁₅H₁₁Cl₃N₃Sc: C, 46.8; H, 2.9; N, 10.9. Found: C, 46.7; H, 3.1; N, 11.1%. ¹H NMR (CD₂Cl₂, 298 K): δ = 9.25 (m, [2H], Ar), 8.30 (m, [7H], Ar), 7.80 (s, [2H], Ar). ⁴⁵Sc NMR (CD₂Cl₂, 298 K): δ = 254 (s, br). IR (Nujol, ν/cm⁻¹): 292, 339, 337 (Sc–Cl). Colourless crystals were obtained from slow diffusion of Et₂O into a concentrated solution of the complex in CH₃CN.

[YCl₃(terpy)(OH₂)]. A solution of terpy (0.046 g, 0.20 mmol) was added to a solution of [YCl₂(thf)₅][YCl₄(thf)₂] (0.08 g, 0.09 mmol) in anhydrous CH₃CN, causing the immediate precipitation of a white solid. After 30 minutes, the white solid was filtered, washed with n-hexane and dried in vacuo. Yield: 0.051 g, 61%. Required for C₁₅H₁₃Cl₃N₃OY: C, 40.3; H, 2.9; N, 9.4. Found: C, 40.2; H, 3.0; N, 9.5%. ¹H NMR (CD₃CN, 298 K): δ = 9.84 (m, [2H], Ar), 8.48 (m, [2H], Ar), 8.39 (m, [3H], Ar), 8.17 (td, [2H], Ar), 7.69 (t, [2H], Ar), 2.15 (s, H₂O). IR (Nujol, ν/cm⁻¹): 3338, 1641 (H₂O), 272, 262(sh) (Y–Cl). Colourless crystals were grown by placing the Schlenk tube containing the filtrate in the freezer (−18 °C) for a few days.

[LaCl₃(terpy)(OH₂)]·4H₂O. LaCl₃·7H₂O (0.108 g, 0.44 mmol) was dissolved in ethanol (10 mL). Terpy (0.098 g, 0.42 mmol) was suspended in ethanol (10 mL) and the reagents combined. A white precipitate formed, which was stirred for 45 min. The solvent was then removed by filtration and the solid washed with ethanol, then diethyl ether and dried in vacuo. Yield: 0.09 g, 40%. Required for C₁₅H₂₁Cl₃LaN₃O₅: C, 31.7; H, 3.7; N, 7.4. Found: C, 31.8; H, 4.0; N, 6.9%. ¹H NMR (CD₃OD, 298 K): δ = 9.37 (m, br, [2H], Ar), 8.59 (m, br, [3H], Ar), 8.40 (m, br, [2H], Ar), 8.27 (m, br, [2H], Ar), 7.77 (m, br, [2H], Ar), 4.85 (H₂O). IR (Nujol, ν/cm⁻¹): 3369, 1633 (H₂O), 209, 205 (La-Cl). Colourless crystals of [{La(terpy)(OH₂)Cl₂}₂(μ-Cl)₂] were obtained by layering an ethanol solution of LaCl₃·7H₂O with an equimolar solution of terpy in ethanol, and leaving undisturbed for 48 h.

[LuCl₃(terpy)(OH₂)]. LuCl₃·6H₂O (0.113 g, 0.29 mmol) was dissolved in ethanol (10 mL). Terpy (0.069 g, 0.30 mmol) was dissolved in ethanol (5 mL) and added dropwise. After stirring for 2 h, a white precipitate had formed. The precipitate was collected via filtration, washed with diethyl ether (2 mL) and then dried in a desiccator for one hour, leaving a white powder. Yield: 0.120 g, 77%. Required for C₁₅H₁₃Cl₃LuN₃O: C, 33.8; H, 2.5; N, 7.9. Found: C, 33.7; H, 2.5; N, 7.7%. ¹H NMR (CD₃OD, 298 K): δ = 9.41 (d, [2H], Ar), 8.72 (d, [2H], Ar), 8.67 (m, [3H], Ar), 8.51 (t, [2H], Ar), 8.35 (t, [2H], Ar). IR (Nujol, ν/cm⁻¹): 3424, 1657 (H₂O) 205, 201 (Lu–Cl). Colourless crystals of [LuCl₃(terpy)(OH₂)] were grown by layering an ethanol solution of LuCl₃·6H₂O with an equimolar solution of terpy in ethanol, and leaving undisturbed for one week.

[ScCl₃(Me₃-tacn)]. [ScCl₃(Me₃-tacn)] was prepared by the literature method.²⁶ Required for C₉H₂₁Cl₃N₃Sc: C, 33.5; H, 6.6; N, 13.0. Found: C, 33.5; H, 6.7; N, 13.2%. ¹H NMR (CD₃CN, 298 K): δ = 3.22 (m, [6H], CH₂), 2.93 (m, [6H], CH₂), 2.87 (s, [9H], CH₃). ⁴⁵Sc NMR (CD₃CN, 298 K): δ = 300 (s). IR (Nujol, ν/cm⁻¹): 353, 330 (Sc–Cl).

[ScCl₃(BnMe₂-tacn)]. [ScCl₃(thf)₃] (0.066 g, 0.2 mmol) was dissolved in acetonitrile (10 mL). A solution of BnMe₂-tacn (0.051 g, 0.2 mmol) in acetonitrile (5 mL) was then added. The reaction was left stirring for 3 h and the solvent was removed in vacuo giving an off-white solid, which was washed with hexane and dried in vacuo. Yield: 0.079 g, 57%. Required for C₁₅H₃₁Cl₃N₃Sc: C, 45.2; H, 6.3; N, 10.5. Found: C, 45.3; H, 6.5; N, 10.7%. ¹H NMR (CD₃CN, 298 K): δ = 7.35 (m, [5H], Ar), 3.82 (s, [2H], N–CH₂–Ar), 2.98 (s, [4H], CH₂), 2.77 (m, [8H], CH₂), 2.50 (s, [6H], CH₃). ⁴⁵Sc NMR (CD₃CN, 298 K) δ = 302 (s). IR (Nujol, ν/cm⁻¹): 333, 301 (br, Sc–Cl).

[YCl₃(Me₃-tacn)]. [YCl₃(Me₃-tacn)] was prepared by the literature method.²⁶ Required for C₉H₂₁Cl₃N₃Y: C, 29.5; H, 5.8; N, 11.5. Found: C, 29.8; H, 5.9; N, 11.3%. ¹H NMR (CD₃CN, 298 K): δ = 3.12 (m, [6H], CH₂), 2.91 (m, [6H], CH₂), 2.82 (s, [9H], CH₃). IR (Nujol, ν/cm⁻¹): 323, 289 (Y–Cl).

[YI₃(Me₃-tacn)]·1.5CH₃CN. YI₃ (0.122 g, 0.26 mmol) was suspended in acetonitrile (10 mL). A solution of Me₃-tacn (0.044 g, 0.26 mmol) in acetonitrile (10 mL) was added. The reaction mixture was heated to 40 °C until a clear solution was obtained. After cooling, the volatiles were reduced in vacuo to 5 mL causing the precipitation of a white solid, which was filtered and dried in vacuo (0.078 g, 47%). Required for C₉H₂₁I₃N₃Y·1.5CH₃CN: C, 20.5; H, 3.7; N, 9.0. Found: C, 21.2; H, 3.9; N, 8.4%. ¹H NMR (CD₃CN, 298 K): δ = 3.46 (m, [6H], CH₂), 3.12 (s, [9H], CH₃), 2.92 (m, [6H], CH₂), 1.98 (s, CH₃CN). IR (Nujol, ν/cm⁻¹): 2253, 2187 (CH₃CN). Crystals suitable for single crystal X-ray analysis were obtained from a separate reaction by placing the Schlenk flask in the freezer (−18 °C). After a few days, two different crystal morphologies were found, which were identified by single crystal X-ray diffraction to be [YI₃(Me₃-tacn)]·CH₃CN and [{YI₂(Me₃-tacn)}₂(μ-O)], the latter formed via hydrolysis from trace water ingress into the flask during crystallisation in the freezer.

[LaCl₃(Me₃-tacn)(OH₂)]. LaCl₃·7H₂O (0.101 g, 0.41 mmol) was dissolved in ethanol (10 mL). Me₃-tacn (0.06 mL, 0.41 mmol) in ethanol (5 mL) was added to form a white precipitate. After stirring for 45 min, the solvent was removed in vacuo leaving a white sticky solid which was dried in a desiccator for two hours. The solid was then washed with diethyl ether (3 mL) and dried again in vacuo, leaving a white powder. Yield: 0.09 g, 54%. Required for C₉H₂₃Cl₃LaN₃O: C, 24.9; H, 5.3; N, 9.7. Found: C, 25.6; H, 5.2; N, 10.0%. ¹H NMR (CD₃CN, 298 K): δ = 2.83 (m, [12H], CH₂), 2.57 (s, [9H], CH₃), 2.10 (H₂O). ¹H NMR (CD₃OD, 178 K): δ = 3.30 (H₂O), 2.92 (m, [6H], CH₂), 2.77 (m, [6H], CH₂), 2.62 (s, [9H], CH₃). IR (Nujol, ν/cm⁻¹): 3376, 1635 (H₂O), 207 (La–Cl).

[Sc(terpy)F(μ-F)₂(SnMe₃Cl)₂]. [ScCl₃(terpy)] (0.05 g, 0.13 mmol) was suspended in CH₃CN (8 mL) and a suspension of Me₃SnF (0.083 g, 0.45 mmol) in CH₃CN (15 mL) was added. After one hour most of the solid had dissolved. The mixture was left stirring for 6 hours. The liquid was decanted via cannula and the solvent removed in vacuo, giving a slightly pink solid. The solid was washed with hexane and dried in vacuo (0.031 g, 33%). Required for C₂₁H₂₉Cl₂F₃N₃ScSn₂: C, 34.4; H, 4.0; N, 5.7. Found: C, 34.8; H, 3.9; N, 6.9%. ¹H NMR (CD₃CN, 298 K): δ = 8.68 (br, [2H], Ar–CH₂), 8.48 (br, [3H], Ar–CH₂), 8.05 (br, [2H], Ar–CH₂), 7.95 (br, [2H], Ar–CH₂), 7.45 (br, [2H], Ar–CH₂), 0.60 (s. ²J_SnH = 64 Hz, Me₃SnCl). ¹⁹F{¹H} NMR (CD₃CN, 298 K): δ = −37.7 (br s, [2F]), −53.3 (br s, [F]). ⁴⁵Sc NMR (CD₃CN, 298 K): δ = 64 (s, br). IR (Nujol, ν/cm⁻¹): 489, 509, 544 (Sc–F), 270 (Sn–Cl).

[ScF₃(Me₃-tacn)]. [ScCl₃(Me₃-tacn)] (0.05 g, 0.15 mmol) was dissolved in acetonitrile (10 mL). [NMe₄]F (0.046 g, 0.46 mmol) was suspended in CH₃CN (10 mL) and added dropwise to the solution of the complex. The reaction was left stirring for 90 minutes. Some white solid was evident throughout the reaction. The volatiles were removed in vacuo and the resulting white solid was washed with CH₂Cl₂, filtered and dried in vacuo (0.041 g, 97%). Required for C₉H₂₁F₃N₃Sc·NMe₄Cl: C, 40.8; H, 8.7; N, 14.6. Found: C, 41.7; H, 9.2; N, 14.6%. ¹H NMR (CD₃CN, 298 K): δ = 3.16 (s, [12H], NMe₄Cl), 2.85 (s, [9H], CH₃), 2.72–2.87 (m, [12H], CH₂). ¹⁹F{¹H} NMR (CD₃CN, 298 K): δ = 8.2 (m, [ScF₃(Me₃-tacn)]). ⁴⁵Sc NMR (CD₃CN, 298 K): δ = 104 (quartet, [ScF₃(Me₃-tacn)], ¹J_ScF = 215 Hz). IR (Nujol, ν/cm⁻¹): 579, 546 (Sc–F).

[Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl]. Me₃SnF was ground and dried under vacuum for 30 min. [ScCl₃(Me₃-tacn)] (0.066 g, 0.20 mmol) dissolved in CH₃CN (10 mL) was added to a solution of Me₃SnF (0.133 g, 0.72 mmol), giving a clear solution in 20 min. The reaction mixture was stirred for 4 h. The solvent was then removed in vacuo and the white solid was dissolved in CH₂Cl₂ (5 mL). Hexane was added causing the precipitation of a white solid, which was filtered and dried in vacuo (0.078 g, 81%). Required for C₁₂H₃₀ClF₃N₃O₃ScSn: C, 30.5; H, 6.4; N, 8.9. Found: C, 30.3; H, 6.5; N, 8.8%. ¹H NMR (CD₃CN, 298 K): δ = 2.72–2.87 (m, [12H], CH₂), 2.61 (s, [9H], CH₃), 0.60 (s, [9H], Me₃SnCl). ¹⁹F{¹H} NMR (CD₃CN, 298 K): δ = 7.7 (m). ⁴⁵Sc NMR (CD₃CN, 298 K): δ = 104 (q, ¹J_ScF = 227 Hz). IR (Nujol, ν/cm⁻¹): 556, 547 (Sc–F), 269 (Sn–Cl). Crystals of [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] were obtained from slow evaporation of a concentrated solution of the product in CH₃CN.

[ScF₃(BnMe₂-tacn)]. A suspension of [NMe₄]F (0.027 g, 0.38 mmol) in CH₃CN (5 mL) was added to a suspension of [ScCl₃(BnMe₂-tacn)] (0.038 g, 0.13 mmol) in CH₃CN (10 mL). The white precipitate present in solution was removed by filtration. The filtrate was taken to dryness in vacuo giving a light yellow solid (0.010 g, 57%). ¹H NMR (CD₃CN, 298 K): δ = 7.35 (m, [5H], Ar) 3.82 (s, [2H], N–CH₂–Ar), 3.16 (s, [12H], NMe₄Cl), 2.98 (s, [4H], CH₂), 2.77 (m, [8H], CH₂), 2.50 (s, [6H], CH₃). ¹⁹F{¹H} NMR (CD₃CN, 298 K): δ = 10.1 (br). ⁴⁵Sc NMR (CD₃CN, 298 K): δ = 104 (br).

X-ray experimental

Crystals of the complexes were grown as reported in the Experimental section. Data collections used a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724+ detector mounted at the window of an FR-E+ SuperBright molybdenum (λ = 0.71073 Å) rotating anode generator with VHF Varimax optics (70 micron focus) with the crystal held at 100 K. Structure solution and refinement were performed using SHELX(S/L)97, SHELX-2013 or SHELX-2014/7.²⁷ H atoms bonded to C were placed in calculated positions using the default C–H distance, and refined using a riding model. Details of the crystallographic parameters are given in Table S1.† CCDC reference numbers in cif format are [ScCl₃(terpy)]: 1590538, [YCl₃(terpy)(OH₂)]: 1590539, [ScF₂Cl(Me₃-tacn)]: 1590540, [YI₃(Me₃-tacn)]·CH₃CN: 1590541, [{YI₂(Me₃-tacn)}₂(μ-O)]·CH₃CN: 1590542, [{La(terpy)(OH₂)Cl₂}₂ (μ-Cl)₂]: 1590543, [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl]: 1590544, [LuCl₃(terpy)(OH₂)]: 1590545.† Powder X-ray diffraction data were collected on a Bruker D2 diffractometer using Cu K_α X-rays and refined using the GSAS software.²⁸

Results and discussion

Three possible routes to Group 3 metal trifluoride complexes of R₃-tacn and terpy (L) were considered:

(1) Direct reaction with the “hydrated” Group 3 metal trifluorides with the neutral ligands;

(2) Chloride(iodide)/fluoride exchange by reaction of the pre-formed Group 3 metal trichloride or triiodide complexes, [MX₃(L)] (X = Cl or I), with anhydrous [NMe₄]F;

(3) Chloride(iodide)/fluoride exchange by reaction of the pre-formed Group 3 metal trichloride or triiodide complexes, [MX₃(L)] (X = Cl or I), with Me₃SnF.

Attempted synthesis from MF₃·xH₂O (M = Sc, Y, La)

The anhydrous Group 3 metal fluorides, MF₃, are inert, polymeric solids and unreactive towards neutral ligands.^1,15 Whilst the anhydrous Group 13 metal fluorides, MF₃ (M = Al, Ga, In) are similarly inert, the hydrates MF₃·3H₂O are more reactive; they can be reacted with neutral ligands to form metal trifluoride ligand complexes, [MF₃(L)]·xH₂O, most easily under hydrothermal conditions (180 °C).^19,29,30 Therefore, we postulated that it may be possible to use the Group 3 metal fluoride hydrates [MF₃·xH₂O] (M = Sc, Y, La) to form analogous metal trifluoride ligand complexes. Literature data on the hydrates is sparse and the amount of water present apparently variable, although “x” typically appears to be less than one.

The addition of a fluoride source, either KF or aqueous HF, to aqueous solutions of yttrium or lanthanum salts, gave immediate white precipitates, MF₃·xH₂O. Using scandium salts and aqueous HF also gave ScF₃·xH₂O, but using alkali metal fluorides gave mixtures, and in one case pure KSc₂F₇, which was identified by its PXRD pattern (see ESI†). The PXRD patterns obtained from MF₃·xH₂O sometimes showed rather broad reflections, but corresponded to the patterns reported for anhydrous MF₃ (Fig. 1 and ESI†).

Fig. 1 PXRD patterns obtained from the attempted synthesis of yttrium fluoride hydrate. (A) PXRD of anhydrous YF₃, A. K. Cheetham and N. Norman, Acta Chem. Scand., Ser. A, 1974, 28, 55; (B) YF₃·xH₂O (this work); (C) YF₃·xH₂O after heating under hydrothermal conditions.

Thus, we conclude that the MF₃·xH₂O actually comprise of the “anhydrous” MF₃ polymer, with water of crystallisation on the surface or occupying voids in the crystal lattice, rather than coordinated to the metal ion. This also explains the range of values of x in MF₃·xH₂O reported in the limited literature available.^1,3 Attempts to react the MF₃·xH₂O with terpy or Me₃-tacn under hydrothermal conditions (180 °C, 15 h) were unsuccessful, with the MF₃·xH₂O being recovered, although with noticeably increased crystallinity (sharper PXRD patterns).

These results explain why ‘hydrated’ Group 3 fluorides are not a viable entry into the coordination chemistry of these fluorides with neutral ligands. This contrasts sharply with the Group 13 fluoride hydrates,^19,29,30 and the Group 4 compounds, [MF₄(OH₂)₂] (M = Zr, Hf),³¹ whose crystal structures show the water is coordinated to the metal, from which it can be displaced by neutral ligands. These are effective synthons for wider coordination chemistry. We reported previously that the f-block tetrafluorides, [MF₄·xH₂O] (M = Ce, Th) have very limited coordination chemistry (CeF₄·xH₂O dissolves only very slowly in refluxing dmso to form [CeF₄(dmso)₂]). They too are likely to contain only lattice/surface water.³² The wider implications of these results in metal fluoride coordination chemistry suggest that for other metals, only those ‘hydrated’ fluorides that contain water within the metal coordination sphere, are likely to be viable synthons for neutral ligand complexes.

Chloride/iodide precursor complexes

Many trichloride complexes of Sc(III), Y(III) and La(III) with neutral ligands are highly moisture sensitive and must be synthesised and handled in anhydrous systems. In work with other early d-block systems, we noted that the corresponding metal iodides and iodo-complexes were often more soluble in weakly coordinating solvents, probably due to lower lattice energy, although the gain in solubility comes at the cost of even greater moisture sensitivity.^17,33,34 In the present study we synthesised complexes with Me₃-tacn and terpy, which give examples of pseudo-octahedral complexes with fac and mer geometries for scandium, respectively, although for Y and La higher coordination numbers were often produced. The known²⁶ [MCl₃(Me₃-tacn)] (M = Y, Sc) were made from [ScCl₃(thf)₃] or [YCl₂(thf)₅][YCl₄(thf)₂] and the triaza macrocycle in anhydrous acetonitrile. The corresponding [ScCl₃(BnMe₂-tacn)] was prepared similarly, whilst reaction of LaCl₃·7H₂O with Me₃-tacn in EtOH yielded [LaCl₃(Me₃-tacn)(OH₂)] as a white powdered solid. All complexes have been characterised spectroscopically (IR, ¹H, ⁴⁵Sc NMR, Table 1, as appropriate) and by microanalysis.

Table 1 ⁴⁵Sc and ¹⁹F{¹H} NMR data^a

Complex	δ(^45Sc)/ppm	δ(^19F{^1H})/ppm
a CH3CN/CD3CN except. b CH2Cl2/CD2Cl2.
[ScCl3(Me3-tacn)	+300	—
[ScCl3(BnMe2-tacn)	+302	—
[ScFCl2(Me3-tacn)]	+219	77.2
[ScF2Cl(Me3-tacn)]	+155	40.1
[ScF3(Me3-tacn)]	104 (q, ^1JSc–F = 219 Hz)	7.7
[ScF3(BnMe2-tacn)	104	10.1
[ScF3(terpy)]	+64	−37.7 [2F], −53.3 [F]
[ScCl3(terpy)]^b	+254	—

---

Treatment of YI₃ with one molar equivalent of Me₃-tacn in anhydrous acetonitrile under a dinitrogen atmosphere formed a light yellow, extremely moisture sensitive solid, identified as [YI₃(Me₃-tacn)]·1.5CH₃CN. The ¹H NMR spectrum of the product displayed multiplets at 2.81 and 2.73 ppm, corresponding to the ligand CH₂ groups, as well as a singlet at 2.58 ppm, due to the ligand methyl groups, consistent with facial complexation of the Me₃-tacn. Crystallisation from CH₃CN solution yielded two different crystal morphologies, both of which were analysed by single crystal X-ray diffraction and shown to be [YI₃(Me₃-tacn)]·CH₃CN (Fig. 2), with the expected tridentate Me₃-tacn and three mutually facial iodides completing the distorted octahedral environment, and the partial hydrolysis product, [{YI₂(Me₃tacn)}₂(μ-O)] (Fig. 3), formed via reaction with trace water. The Y–N bond distances in [YI₃(Me₃-tacn)] are similar to those reported in functionalised tacn complexes, e.g. [{ⁱPr₂-tacn(CH₂)₂N^tBu}Y(CH₂SiMe₃)₂].³⁵

Fig. 2 Structure of [YI₃(Me₃-tacn)]·CH₃CN with atom numbering scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Select bond lengths (Å) and angles (°): Y1–I1 = 2.9671(8), Y2–I2 = 2.9509(9), Y1–I3 = 2.9460(8), Y1–N1 = 2.468(6), Y1–N2 = 2.480(5), Y1–N3 = 2.467(7), I3–Y1–N1 = 160.9(1), I2–Y1–N3 = 162.3(2), I1–Y1–N2 = 161.6(1), I1–Y1–I2 = 99.59(2), I3–Y1–I2 = 98.87(2), I3–Y1–N2 = 94.5(1), N1–Y1–N2 = 72.0(2).

Fig. 3 The structure of [{YI₂(Me₃tacn)}₂(μ-O)]·CH₃CN with atom numbering scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Y1–I1 = 3.0213(6), Y1–I2 = 2.9969(7), Y1–O1 = 2.0332(5), Y1–N2 = 2.580(5), Y1–N1 = 2.473(5), Y1–N3 = 2.495(5), Y1–O1–Y1 = 180.00(3), I2–Y1–I1 = 93.991(19), O1–Y1–I1 = 104.69(2), O1–Y1–I2 = 104.12(2), O1–Y1–N2 = 156.16(11), O1–Y1–N1 = 90.72(11), O1–Y1–N3 = 93.79(11).

The reaction of [ScCl₃(thf)₃] with terpy in anhydrous CH₃CN gave mer-[ScCl₃(terpy)] (Fig. 4).

Fig. 4 Structure of mer-[ScCl₃(terpy)] with ellipsoids drawn at 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Sc1–Cl1 = 2.414(2), Sc1–Cl2 = 2.396(2), Sc1–Cl3 = 2.451(2), Sc1–N1 = 2.229(4), Sc1–N2 = 2.231(5), Sc1–N3 = 2.248(4), Cl1–Sc1–Cl3 = 174.51(7), Cl2–Sc1–N2 = 174.2(1), N1–Sc1–N3 = 142.3(2), N1–Sc1–N2 = 71.3(2), N2–Sc1–N3 = 71.1(2), N2–Sc1–Cl3 83.7(1), Cl3–Sc1–Cl2 91.22(6), Cl2–Sc1–N1 = 105.9(1).

The complex has a distorted octahedral coordination around the metal centre conferred by the rigid terpy ligand, the angles involving the ligand are significantly less than the 180/90° expected for a regular octahedron, with N1–Sc1–N3 = 142.3°. The extended crystal structure of [ScCl₃(terpy)] shows π-stacking interactions (3.82 Å) between the aromatic ring of the terpy ligand of the adjacent molecule, connecting them into 1D zig-zag chains (see ESI Fig. S4†).

Reaction of [YCl₂(thf)₅][YCl₄(thf)₂] with terpy in CH₃CN yields the colourless complex [YCl₃(terpy)(OH₂)] incorporating adventitious water and characterised spectroscopically and by microanalysis. Confirmation of the seven-coordinate geometry follows from a crystal structure determination (Fig. 5).

Fig. 5 Crystal structure of [YCl₃(terpy)(OH₂)] with ellipsoid drawn at 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Y1–Cl1 = 2.572(2), Y1–Cl2 = 2.611(2), Y1–Cl3 = 2.629(1), Y1–N1 = 2.507(5), Y1–N2 = 2.480(4), Y1–N3 = 2.514(5), Y1–O1 2.348(3), Cl2–Y1–Cl1 176.49(5), Cl3–Y1–Cl1 90.77(5), Cl1–Y1–O1 96.6(1), Cl1–Y1–N2 96.3(1), Cl2–Y1–N2 80.8(1), N2–Y1–N1 64.9(2).

The crystal structure (Fig. 5) shows a pentagonal-bipyramidal coordination around the metal centre with the Y–Cl bond lengths in the axial positions shorter than that in the equatorial plane. The angle between the yttrium centre and the nitrogen atoms are less than the 72° value expected for the perfect pentagonal–bipyramidal conformation, due to the rigid terpy ligand and the equatorial plane is puckered. Furthermore, the packing in the crystal structure shows both H-bonding (Cl⋯HOH) between adjacent molecules to form associated dimers, and weak π-stacking (4.04 Å) linking the dimers into zig-zag chains (Fig. S5†). The bond lengths are generally shorter than in the eight-coordinate [YCl(terpy)(OH₂)₄]Cl₂·2H₂O.³⁶

Using the heavier f-block ions, La(III) and Lu(III), [LaCl₃(terpy)(OH₂)]·4H₂O and [LuCl₃(terpy)(OH₂)] were obtained via reaction of LaCl₃·7H₂O or LuCl₃·6H₂O, respectively, with one mol. equiv. of terpy in ethanol. X-ray crystallographic analyses show that the La(III) complex exists as a chloro-bridged dimer, [{La(terpy)(OH₂)Cl₂}₂(μ-Cl)₂] (Fig. 6) involving eight-coordinate La(III) with adjacent molecules linked into chains via H-bonding interactions between the coordinated water molecule on one La(III) centre and the Cl ligands on adjacent molecules (Fig. S6†). On the other hand, [LuCl₃(terpy)(OH₂)] (Fig. S7†) is a seven-coordinate monomer, isostructural with the Y(III) analogue above (and hence also displaying the same H-bonding and π-stacking interactions in the solid state).

Fig. 6 Crystal structure of [{La(terpy)(OH₂)Cl₂}₂(μ-Cl)₂] with ellipsoids drawn at 50% probability level. H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): La1–Cl1 = 2.8385(9), La1–Cl2 = 2.8014(8), La1–Cl3 = 2.8895(9), La1–N1 = 2.638(3), La1–N2 = 2.678(3), La1–N3 = 2.658(3), La1–O1 = 2.576(3), La1–Cl3ⁱ = 2.9209(9), Cl1–La1–Cl3 = 144.65(2), Cl1–La1–Cl2 = 86.82(2), Cl1–La1–O1 = 72.14(7), Cl2–La1–O1 = 78.77(7), Cl3–La1–O1 = 140.69(7), N3–La1–N1 123.52(9), N3–La1–O1 145.03(9), N3–La1–N2 = 62.21(9).

Chloride(iodide)/fluoride exchange reactions using [NMe₄]F

Addition of three mol. equiv. of anhydrous [NMe₄]F to a CH₃CN solution of [ScCl₃(Me₃-tacn)] gave a colourless solution whose ¹⁹F{¹H} and ⁴⁵Sc NMR spectra³⁷ each showed three broadened resonances at 77.2, 40.1 and 7.7 ppm (¹⁹F{¹H}) and 219, 155 and 104 ppm (⁴⁵Sc) (Fig. 7, Table 1), corresponding to [ScFCl₂(Me₃-tacn)] (very minor), [ScF₂Cl(Me₃-tacn)] and [ScF₃(Me₃-tacn)], respectively. The modest quadrupole moment of ⁴⁵Sc (I = 7/2) means that resonances are observed in many systems, but couplings to other nuclei are often lost in the line broadening, unless the scandium is in a high symmetry environment.^17,34,38,39 Since these complexes are the first examples of scandium fluoride species with neutral ligands, there are no comparable literature data, but the chemical shifts of the chloro-species are reasonable compared to data on other ScCl₃ adducts.^34,38,39

Fig. 7 Fluorination of [ScCl₃(Me₃tacn)]: A: ⁴⁵Sc NMR showing the species [ScFCl₂(Me₃tacn)] (219 ppm), [ScF₂Cl(Me₃tacn)] (155 ppm) and [ScF₃(Me₃tacn)] (104 ppm) when a deficit of [NMe₄]F is used; B: ⁴⁵Sc NMR of [ScF₃(Me₃tacn)] showing the quartet at 104 ppm (¹J_Sc–F = 219 Hz).

Adding further small aliquots of [NMe₄]F in CH₃CN initially led to depletion of the resonances assigned to the mixed chloro/fluoro complexes, and enhancement of the broadened quartet at δ = 104 ppm, attributed to [ScF₃(Me₃-tacn)] (Fig. 7). The broad quartet shows coupling to three equivalent fluorides with ¹J_ScF = 219 Hz (since the efg is small). However, excess fluoride caused complete loss of all the ⁴⁵Sc and ¹⁹F{¹H} resonances from the tacn complexes (see below).

The ¹⁹F{¹H} and ⁴⁵Sc NMR data for the fluorination reaction are also strongly indicative of the fluorination of [ScCl₃(Me₃-tacn)] occurring in a stepwise manner. Further confirmation of this follows from a single crystal X-ray structure determination on [ScF₂Cl(Me₃-tacn)], a few crystals of which were grown by concentrating the NMR solution in acetonitrile (Fig. 8).

Fig. 8 Crystal structure of [ScF₂Cl(Me₃-tacn)] with ellipsoids drawn at the 50% probability level. H atoms are omitted for clarity. Select bond lengths (Å) and angles (°): Sc1–F1 = 1.947(3), Sc1–F2 = 1.940(3), Sc1–Cl1 = 2.444(2), Sc1–N1 = 2.339(5), Sc1–N2 = 2.364(4), Sc–N3 = 2.334(4), Cl1–Sc1–F1 = 101.7(1), Cl1–Sc1–N1 = 162.4(1), Cl1–Sc1–N2 = 89.9(1), F1–Sc1–F2 = 103.3(1), N2–Sc1–N3 = 74.6(1), F1–Sc1–N3 = 158.8(1).

The complex is six-coordinate with the three nitrogen atoms of the ligand coordinated to the metal in a facial conformation, while the three other positions are occupied by two fluorine atoms and one chlorine, with no evidence for disorder being present. As expected, the Sc–F bonds are significantly shorter than the Sc–Cl bond (1.94 against 2.44 Å). The analogous [ScCl₃(BnMe₂-tacn)] shows similar behaviour with [NMe₄]F, with [ScF₃(BnMe₂-tacn)] showing a broad ⁴⁵Sc NMR resonance at 104 ppm, with δ(¹⁹F{¹H}) = 10.1 ppm.

Attempts to react [ScCl₃(terpy)] with dry [NMe₄]F in CH₃CN on an NMR scale caused complete loss of the resonances from the trichloro complex, without the appearance of any new resonances in either the ⁴⁵Sc or ¹⁹F spectra.

The reaction of [YCl₃(Me₃-tacn)] with three equivalents of [NMe₄]F in CH₃CN resulted in a white precipitate that was insoluble in CH₃CN or CH₂Cl₂ and the ¹H NMR spectrum of the solution showed liberation of Me₃-tacn, whilst the ¹⁹F NMR spectrum showed only small amounts of free fluoride. Similar results were obtained using [YI₃(Me₃-tacn)], and it was concluded that Cl(I)/F exchange resulted in decomposition and loss of the Me₃-tacn from the yttrium. Similar decomposition occurred using [LaCl₃(Me₃-tacn)(OH₂)].

Chloride/fluoride exchange reactions using Me₃SnF

Me₃SnF is a useful fluorinating agent, its polymeric structure makes it insoluble in most solvents,^40,41 but it dissolves as the Cl/F exchange reaction proceeds, and usually, the Me₃SnCl (which contains tbp tin centres weakly chlorine-bridged into polymeric chains)⁴² formed is easily removed from the products by washing with hexane. The reagent does not provide free fluoride ions so an excess can be used without the risk of the decomposition observed using [NMe₄]F. The fac-octahedral trifluoro complexes [ScF₃(R₃-tacn)] were readily obtained by treatment of the trichloro species with three mol. equiv. of Me₃SnF (and adding excess Me₃SnF has no further effect) (Fig. 9). The broad “doublets” observed in the ¹⁹F{¹H} NMR spectra result from partial collapse of the couplings to ⁴⁵Sc (I = 7/2) in the low symmetry environments.

Fig. 9 ¹⁹F{¹H} NMR spectra of [ScFCl₂(Me₃tacn)] (77.2 ppm), [ScF₂Cl(Me₃tacn)] (40.1 ppm) and [ScF₃(Me₃tacn)] (7.7 ppm). A: ∼ 2.5 mol. equiv. of Me₃SnF were added to a CD₃CN solution of [ScCl₃(Me₃tacn)]; B: <3 mol. equiv. of Me₃SnF were added to a CD₃CN solution of [ScCl₃(Me₃tacn)]; C: >3 mol. equiv. of Me₃SnF were added to a CD₃CN solution of [ScCl₃(Me₃tacn)].

However, the product obtained had a microanalysis corresponding to [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl]; note that whilst three equivalents of Me₃SnCl are produced in the reaction, only one is retained in the scandium complex.

Crystals of [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] were obtained by slow evaporation from a solution of the product in CH₃CN. The X-ray crystal structure confirms the presence of three mutually facial fluoride ligands, with the tridentate triaza macrocycle completing the distorted octahedral geometry at Sc with Sc–F = 1.980(6), 1.92(2) and Sc–N = 2.334(6), 2.351(4) Å. The [ScF₃(Me₃-tacn)] complex acts as a neutral Lewis base towards the Me₃SnCl Lewis acid through one bridging fluoride ligand, Sc–F1 = 1.980(6), Å, Sn1–F1 = 2.307(6) Å (Fig. 10). This results in a trigonal bipyramidal geometry at Sn, with the Cl and F ligands in the axial positions and a near linear F–Sn–Cl unit (178.7(2)°). The Sc–N and Sc–F_terminal bonds are similar to those in [ScF₂Cl(Me₃tacn)], but the Sc–F(Sn) bond is significantly longer. The Sn–F bond is longer than that in Me₃SnF (2.1620(7) Å),⁴¹ whilst the Sn–Cl is also longer than that in Me₃SnCl polymer (2.430(2) Å).⁴² The Lewis basic properties of fac-[MF₃(R₃-tacn)] (M = Al, Ga, Fe, Cr, etc.) complexes has been noted previously, and exploited to form mixed metal species with alkali metal⁴³ and lanthanide cations, the latter with applications as molecular magnets.⁴⁴

Fig. 10 Crystal structure of [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] with atom numbering scheme. Ellipsoids are shown at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): Sc1–N1 = 2.334(6), Sc1–N2 = 2.351(4), Sc1–F1 = 1.980(6), Sc1–F2^a = 1.92(2), Sn1–F1 = 2.307(6), Sn1–Cl1 = 2.502(2), Sn1–C1 = 2.124(7), Sn1 = C2^a = 2.153(3), N2–Sc1–N2 = 75.36(18), N1–Sc1–N2 = 74.92(14), F2^a–Sc1–N1 = 94.6(10), F1–Sc1–N1 159.7(3), F1–Sn–Cl1 = 178.7(2).

The only similar complex to have been reported is [ScL(μ-F)₂(SnMe₃Br)₂] (L⁻ = N,N′′-(1,3-dimethyl-1,3-propanediylidine)bis(N′,N′-diethyl-1,2-ethanediamine)), made from [ScLBr₂] and Me₃SnF.¹³

The ⁴⁵Sc and ¹⁹F{¹H} NMR spectra of [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] and [ScF₃(Me₃-tacn)] in CH₃CN are identical, as are the ¹H resonances of the Me₃-tacn moieties, whilst the ¹H NMR resonance of the Me₃SnCl in the former complex is consistent with the free organotin, indicating the adduct is dissociated in solution. Me₃SnCl is a weak Lewis acid and complexes such as [Me₃SnCl(Ph₃PO)] also appear largely dissociated in solution.⁴⁵ However, attempts to crystallise [ScF₃(Me₃-tacn)] from solutions of [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] failed with the bimetallic species reformed in the isolated solid. This contrasts with [ScL(μ-F)₂(Me₃SnBr)₂] which decomposes when the mother liquor is removed.¹³ Gently heating a finely ground sample of [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] (40 °C) under vacuum leads to partial removal of the Me₃SnCl, but complete removal of the tin species could not be achieved without decomposition of the scandium moiety.

The reaction of [ScCl₃(terpy)] with three equivalents of Me₃SnF gave a white solid identified by microanalysis and multinuclear NMR as [Sc(terpy)F(μ-F)₂(SnMe₃Cl)₂]. The ¹⁹F{¹H} NMR (CD₃CN) δ = −37.7 (br s, [2F]), −53.3 (br s, [F]) and ⁴⁵Sc NMR δ = 64 (s, br) are consistent with the formulation. However, in contrast to [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl], [Sc(terpy)F(μ-F)₂(SnMe₃Cl)₂] is unstable in solution and slowly deposits a white insoluble solid.

The reaction of [YCl₃(Me₃-tacn)] with Me₃SnF in CH₃CN resulted in decomposition, and no YF₃ complex was identified.

Unlike the trichloro analogues, which are very hydrolytically sensitive, solutions of [ScF₃(Me₃-tacn)] and [ScF₃(BnMe₂-tacn)] in water at neutral pH are stable for many weeks, monitored by ¹⁹F and ⁴⁵Sc NMR spectroscopy, and to heating aqueous solutions at 80 °C for several hours. The complexes are decomposed by excess F⁻, and by Na₃PO₄ and Na₂CO₃ (which have a pH ∼ 9), but are stable to a 10-fold excess of Cl⁻ or MeCO₂⁻.

Conclusions

The [ScF₃(Me₃-tacn)] and [ScF₃(BnMe₂-tacn)], which represent the first reported examples of scandium fluoride complexes with neutral co-ligands, have been prepared by Cl/F exchange reactions from the corresponding chlorides, using [NMe₄]F. Attempts to isolate similar complexes of Y or La, or [ScF₃(terpy)] have been unsuccessful. In these cases fluorination is accompanied by dissociation of the neutral ligands and formation of insoluble MF₃. Highly unusual [Sc(terpy)F(μ-F)₂(SnMe₃Cl)₂] and [Sc(Me₃-tacn)F₂(μ-F)SnMe₃Cl] have been isolated using Me₃SnF as the fluoride source. The high stability of [ScF₃(R₃-tacn)] in water over a range of temperatures and pHs, as well as towards common ions such as Cl⁻ or MeCO₂⁻, indicates that are worth further examination to determine their potential as possible carriers for ¹⁸F in PET imaging radio-tracers.

The decomposition of analogous Y and La trichloride complexes upon attempted fluorination, indicates that the R₃-tacn or terpy ligands are insufficiently strongly bound to these metal ions to prevent their dissociation and formation of MF₃ polymer. Since Y and La are larger than Sc, tetra-azamacrocycles (cyclen or cyclam) may be better suited. Alternatively, anionic pendant arm ligands such as the H₂-R-NOTA type, which will bind more strongly, may be better for these oxophilic metal centres. These possibilities will be investigated in future work.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the EPSRC and GE Healthcare for funding through a CASE studentship to F. M. M., and we thank EPSRC for support through EP/L505651/1.

References

1. Scandium, ed. C. T. Horovitz, Academic Press, London, 1976.
2. F. A. Hart, Comprehensive Coordination Chemistry I, ed. G. Wilkinson, R. D. Gillard and J. A. McCleverty, Pergamon, Oxford, 1987, vol. 3, p. 1060.
3. S. A. Cotton, Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, Oxford, 2004, vol. 3, p. 93; K. J. Fisher, Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, Oxford, 2004, vol. 4, p. 1.
4. J. M. Sears and T. J. Boyle, Coord. Chem. Rev., 2017, 340, 154.
5. P. M. Zeimentz, S. Arndt, B. R. Elvidge and J. Okuda, Chem. Rev., 2006, 106, 2404.
6. J. Chu, X. Han, C. E. Kefalidis, J. Zhou, L. Maron, X. Leng and Y. Chen, J. Am. Chem. Soc., 2014, 136, 10894.
7. S. A. Bartlett, G. Cibin, A. J. Dent, J. Evans, M. J. Hanton, G. Reid, R. P. Tooze and M. Tromp, Dalton Trans., 2013, 42, 2213 and refs therein.
8. Z. Xie, K. Chui, Q. Yang, T. C. W. Mak and J. Sun, Organometallics, 1998, 17, 3937.
9. W. Huang and P. L. Diaconescu, Organometallics, 2017, 36, 89.
10. J. Chen, Q. Gao, A. Sanson, X. Jiang, Q. Huang, A. Carnera, C. G. Rodriguez, L. Olivi, L. Wang, L. Hu, K. Lin, Y. Ren, Z. Lin, C. Wang, L. Gu, J. Deng, J. P. Attfield and X. Xing, Nat. Commun., 2017, 8, 14441; L. Hu, J. Chen, A. Sanson, H. Wu, C. G. Rodriguez, L. Olivi, Y. Ren, L. Fan, J. Deng and X. Xing, J. Am. Chem. Soc., 2016, 138, 8320.
11. L. Han, Y. Wang, L. Guo, L. Zhao and Y. Tao, Nanoscale, 2014, 6, 5907; J. Xie, X. Xie, C. Mi, Z. Gao, Y. Pan, Q. Fan, H. Su, D. Jin, L. Huang and W. Huang, Chem. Mater., 2017, 29, 9758.
12. D. Brown, Halides of the lanthanides and actinides, Wiley, NY, 1968; M. Leblanc, V. Maisonneuve and A. Tressaud, Chem. Rev., 2015, 115, 1191.
13. A. M. Neculai, D. Neculai, G. B. Nikiforov, H. W. Roesky, C. Schlicker, R. Herbst-Irmer, J. Magull and M. Noltemeyer, Eur. J. Inorg. Chem., 2003, 3120.
14. F. Bottomley, D. E. Paez and P. S. White, J. Organomet. Chem., 1985, 291, 35.
15. S. L. Benjamin, W. Levason and G. Reid, Chem. Soc. Rev., 2013, 42, 1460.
16. N. J. Hill, W. Levason, M. C. Popham, G. Reid and M. Webster, Polyhedron, 2002, 21, 1579; N. J. Hill, W. Levason, M. C. Popham, G. Reid and M. Webster, Polyhedron, 2002, 21, 445.
17. M. Carravetta, M. Concistre, W. Levason, G. Reid and W. Zhang, Inorg. Chem., 2016, 55, 12890.
18. K. Chansaenpak, B. Vabre and F. P. Gabbai, Chem. Soc. Rev., 2016, 45, 954.
19. R. Bhalla, C. Darby, W. Levason, S. K. Luthra, G. McRobbie, G. Reid, G. Sanderson and W. Zhang, Chem. Sci., 2014, 5, 381; W. Levason, S. K. Luthra, G. McRobbie, F. M. Monzittu and G. Reid, Dalton Trans., 2017, 46, 14519.
20. R. Bhalla, W. Levason, S. K. Luthra, G. McRobbie, G. Reid and G. Sanderson, Chem. – Eur. J., 2015, 21, 4688.
21. P. Laverman, W. McBride, R. Sharkey, A. Eek, L. Joosten, W. Oyen, D. Goldberg and O. Boerman, J. Nucl. Med., 2010, 51, 454; W. McBride, C. D'Souza, R. Sharkey, H. Karacay, E. Rossi, C. Chang and D. Goldberg, Bioconjugate Chem., 2010, 21, 1331; W. McBride, R. Sharkey, H. Karacay, C. D'Souza, E. Rosso, P. Laverman, C. Chang, O. Boerman and D. Goldberg, J. Nucl. Med., 2009, 50, 991; S. Lutje, G. M. Franssen, R. M. Sharkey, P. Laverman, E. A. Rossi, D. M. Goldberg, W. J. G. Oyen, O. C. Boerman and W. J. McBride, Bioconjugate Chem., 2014, 25, 335; W. J. McBride, D. M. Goldberg and R. M. Sharkey, J. Nucl. Med., 2014, 55, 1043.
22. L. E. Manzer, Inorg. Synth., 1982, 21, 1579.
23. P. Sobota, J. Utko and S. Szafert, Inorg. Chem., 1994, 33, 5203.
24. K. Wieghardt, P. Chaudhuri, B. Nuber and J. Weiss, Inorg. Chem., 1982, 21, 3086; M. J. Belousoff, M. B. Duriska, B. Graham, S. R. Batten, B. Moubaraki, K. S. Murray and L. Spiccia, Inorg. Chem., 2006, 45, 3746.
25. K. O. Christe, W. W. Wilson, R. D. Wilson, R. Bau and J. A. Feng, J. Am. Chem. Soc., 1990, 112, 7619.
26. S. Hajela, W. P. Schaefer and J. E. Bercaw, J. Organomet. Chem., 1997, 532, 45.
27. G. M. Sheldrick, Acta Crystallogr., Sect. A: Fundam. Crystallogr., 2008, 64, 112.
28. A. C. Larson and R. B. Von Dreele, Generalized Structure Analysis System (GSAS), Los Alamos National Laboratory Report LAUR, 2004, p. 86; B. H. Toby, J. Appl. Crystallogr., 2001, 34, 210.
29. R. Bhalla, W. Levason, S. K. Luthra, G. McRobbie, F. M. Monzittu, J. Palmer, G. Reid, G. Sanderson and W. Zhang, Dalton Trans., 2015, 44, 9569.
30. R. Bhalla, J. Burt, A. L. Hector, W. Levason, S. K. Luthra, G. McRobbie, F. M. Monzittu and G. Reid, Polyhedron, 2016, 106, 65.
31. S. L. Benjamin, W. Levason, D. Pugh, G. Reid and W. Zhang, Dalton Trans., 2012, 41, 12548.
32. M. J. D. Champion, W. Levason and G. Reid, J. Fluorine Chem., 2014, 57, 19.
33. W. Levason, M. L. Matthews, B. Patel, G. Reid and M. Webster, Dalton Trans., 2004, 3305.
34. M. J. D. Champion, P. Farina, W. Levason and G. Reid, Dalton Trans., 2013, 42, 13179.
35. S. Bambirra, A. Meetsma, B. Hessen and A. P. Bruins, Organometallics, 2006, 25, 3486.
36. C. J. Kepert, L. Weimin, B. W. Skelton and A. H. White, Aust. J. Chem., 1994, 47, 365.
37. ⁴⁵Sc is 100% abundant and has I = 7/2, Ξ = 29.24 MHz, Dc = 1710, Q = −0.22 × 10⁻²⁸ m²; J. Emsley, The Elements, Oxford, 1989.
38. Multinuclear NMR, ed. J. Mason, Plenum, NY, 1987; Transition metal nuclear magnetic resonance, ed. P. S. Pregosin, Elsevier, Amsterdam, 1991.
39. N. J. Hill, W. Levason, M. C. Popham, G. Reid and M. Webster, Polyhedron, 2002, 21, 1579; L. Deakin, W. Levason, M. C. Popham, G. Reid and M. Webster, J. Chem. Soc., Dalton Trans., 2002, 2439.
40. E. F. Murphy, R. Murugavel and H. W. Roesky, Chem. Rev., 1997, 97, 3425.
41. P. Chaudhary, M. Bieringer, P. Hazendonk and M. Gerken, Dalton Trans., 2015, 44, 19651.
42. J. L. Lefferts, K. C. Molloy, M. R. Hossain, D. Van der Helm and J. J. Zuckerman, J. Organomet. Chem., 1982, 240, 349.
43. R. Bhalla, W. Levason, S. K. Luthra, G. McRobbie, G. Reid, G. Sanderson and W. Zhang, Chem. Commun., 2014, 50, 12675.
44. K. S. Pedersen, M. A. Sørensen and J. Bendix, Coord. Chem. Rev., 2015, 299, 1.
45. M. F. Davis, W. Levason, R. Ratnani, G. Reid, T. Rose and M. Webster, Eur. J. Inorg. Chem., 2007, 306 and refs therein.

---

Footnote

† Electronic supplementary information (ESI) available: X-ray crystallographic parameters are given in Table S1. The ESI also contains IR and NMR spectra for the complexes and figures showing the π-stacking and H-bonding in the terpy complexes. CCDC 1590538–1590545. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8dt00480c

---