– synthesis, properties and prospects towards potential PET probes.

The coordination chemistry of the ﬁ rst row transition metal tri ﬂ uorides with terpy (2,2 ’ :6 ’ ,2 ’’ -terpyridine) and Me 3 -tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) was explored to identify potential systems for 18 F radiolabelling. The complexes [MF 3 (L)] (M = Cr, Mn, Fe, Co; L = Me 3 -tacn, terpy) were synthesised and fully characterised by UV-vis and IR spectroscopy, microanalysis, and, for the diamagnetic [CoF 3 (L)], using 1 H, 19 F{ 1 H} and 59 Co NMR spectroscopy. Single crystal X-ray analyses are reported for [MF 3 (Me 3 -tacn)] (M = Mn, Co), [FeF 3 (terpy)] and [FeF 3 (BnMe 2 -tacn)]. Stability tests on [MF 3 (Me 3 -tacn)] (M = Cr, Mn, Fe) and [M ’ F 3 (terpy)] (M ’ = Cr, Fe) were performed and Cl/ 19 F halide exchange reactions on [CrCl 3 (Me 3 -tacn)] using [Me 4 N]F in anhydrous MeCN solution, and [FeCl 3 (Me 3 -tacn)] using [Me 4 N]F in anhydrous MeCN or KF in aqueous MeCN solution were also carried out. Halide exchange reactions proved to be successful in forming [FeF 3 (Me 3 -tacn)] in aqueous MeCN solution within 30 minutes. Based upon the clean Cl/F exchange and the good stability observed for [FeF 3 (Me 3 -tacn)] in a range of competitive media, this was identi ﬁ ed as a possible candidate for radiolabelling. 18 F/ 19 F isotopic exchange was achieved by addition of [ 18 F]F − in the cyclotron target water to a MeCN solution of the benzyl-substituted analogue, [FeF 3 (BnMe 2 -tacn)], at a range of concentrations down to 24 nM with heating to 80 °C for 10 min.; the resulting [Fe 18 F 19 F 2 (BnMe 2 -tacn)] shows radiochemical purity (RCP) ≥ 90% after 2 h in a range of formulations, including 10% EtOH/phosphate bu ﬀ ered saline (PBS) and 10% EtOH/human serum albumin (HSA). This is the ﬁ rst reported complex with a transition metal directly bonded to [ 18 F]F − .


Introduction
C-18 F based radiotracers are extensively used in positron emission tomography (PET) for imaging purposes in oncology, cardiology and neurology. 1 Thousands of PET scans are performed daily on cancer patients worldwide, providing important diagnostic and clinical information. Peptides that target receptors overexpressed on the surface of the diseased cells are becoming increasingly important in diagnostic PET imaging agents. 2 However, the reaction conditions for the formation of the C-18 F bond are often incompatible with biomolecules (e.g. peptides). 3 Organic solvents that degrade the peptide, high temperatures and competing sites for the 18 F-labelling reaction present in the biomolecule are the major problems encountered. As a result, often the [ 18 F]F − is first incorporated into an organic molecule ( prosthetic group) and then conjugated to a peptide, resulting in an increase of the total reaction time and the number of steps of the process. 3 The need to improve these aspects has led to the investigation of alternative non-C- 18 F moieties. To date, several main group elements attached to [ 18 F]F − have been studied, including boron-, 4 aluminium-, 5,6 gallium-, 7,8 sulfur-, 9 and silicon-fluoride 10 systems. These are discussed in recent reviews. 11 The two Group 13 metals are often coordinated to macrocyclic ligands (triazacyclononane derivatives) which confer thermodynamic and kinetic stability to the chelates and, in the case of the systems with 1,4dimethyl-7-benzyl-1,4,7-triazacyclononane (BnMe 2 -tacn), the benzyl group provides a site for further functionalisation and bioconjugation. 6,7 In this approach to PET radiotracers based on metal coordination complexes, the stability of the complex will be strongly dependant on the properties of the metal centre. The M-F bond dissociation energy has an important role since it should be stronger compared to M-Cl, hence allowing fluorination through halide exchange reactions and, ultimately, it should be resistant in physiological conditions. Other aspects related to the metal to be considered are its size (dictating the coordination number), its redox chemistry and oxophilicity (water or anions such as phosphate should not disrupt the coordination around the metal), its Lewis acidity and its lability, allowing sufficiently rapid substitution of fluoride into the coordination sphere (when the 18 F half-life of 110 min is considered).
We previously developed the chemistry of the Group 13 metal fluorides (Al, Ga) towards neutral nitrogen ligands and reported that radioactive 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. Their properties are discussed in order to identify the best prospects for fluorination for future possible applications in PET. Finally, we demonstrate successful 18 F/ 19 F isotopic exchange using [FeF 3 (BnMe 2 -tacn)], and confirm its stability in ethanolic phosphate buffered saline (PBS) or human serum albumin (HSA) over at least 2 h.
In setting out the scope for this study, Ti 3+ and V 3+ were not considered as the former is very readily oxidised, while the latter is also likely to form V(IV) in aqueous solution. Previous work has shown that [VOF 2 (Me 3 -tacn)] is readily obtained by adding a few drops of water to a methanol solution of [VF 3 (Me 3 -tacn)] in air. 14 Nickel was also excluded as no trifluoride complexes are known and Ni(III) is reduced in water. 15 The ligand substitution kinetics for the majority of second and third row transition metal complexes are expected to be too slow to allow sufficiently fast halide substitution, given the short half-life of the 18 F radionuclide.
The electronic configurations of the 3d metal ions influence their reactivity and the kinetic robustness of the complexes formed. Neutral trifluoride complexes of these metals are scarce or unknown, in contrast to the heavier halides. Octahedral d 3 Cr(III) mono-, di-and tri-fluoride complexes with N-donor ligands have been reported. 16 In particular, complexes of ammonia and amines have been studied extensively for their absorption and emission properties, contributing to the development of inorganic electronic spectroscopy. 17 The neutral species [CrF 3 (bipy)(OH 2 )], 18 fac-[CrF 3 (Me 3 -tacn)] 19,20 and mer-[CrF 3 (terpy)] 20 have also been reported and structurally characterised. Complexes with tetradentate N-donor ligands can also be found in the literature. 21 3 ] has been characterised spectroscopically. 29 The coordination chemistry of transition metal fluoride complexes has been reviewed recently, 30 including their complexes with neutral ligands. 15 Experimental Synthetic procedures and characterisation details are presented in the ESI † MBq) and the vial was heated to 80°C for 10 min. The crude reaction solution was diluted with water (20 mL) so that approximately 10% of the solvent composition was organic. A small sample (∼100 µL) of the diluted crude reaction solution was removed for analysis by analytical HPLC, which confirmed the percentage incorporation of [ 18 F]F − into the metal complex (based upon integration of the radio peaks). Approximately 6% [ 18 F]F − incorporation was observed when the radiolabelling experiment was performed using 1 mg of the iron complex in MeCN/H 2 O (75 : 25) at room temperature. The product was purified by either a SPE process or by HPLC.

SPE purification protocol
The diluted reaction mixture was trapped on a HLB cartridge, washed with water (5 mL × 3) to remove the residual [ 18 F]F − and MeCN and then the product was eluted from the cartridge with ethanol (1 mL) into either (i) water to result in a formulated product in 80 : 20 water : EtOH; (ii) PBS to result in a formulated product in 90 : 10 PBS : EtOH or (iii) HSA to result in a formulated product in 90 : 10 HSA : EtOH. The formulated product was analysed by HPLC at t = 0 and various time intervals up to 120 min.
Experiments were analysed on an Agilent 1290 HPLC system with an Agilent 1260 DAD UV detector (G4212B) and a Bioscan FC3200 sodium iodide PMT with rate meter. Dionex Chromeleon 6.8 Chromatography data recording software was used to integrate the peak areas.

Results and discussion
The syntheses of the complexes [MF 3 (L)] (M = Cr, Mn, Fe, Co; L = terpy, Me 3 -tacn) were carried out in alcoholic (n-BuOH or MeOH) or dmf solutions at room temperature or under reflux (Scheme 1) and the products were characterised by IR spectroscopy, microanalysis and UV-vis spectroscopy (diffuse reflectance and solution).
The trifluoro complexes have a strong tendency to form H-bonding interactions between the fluorides and water molecules in the lattice; 8 this has often led to discrepancy in the number of water molecules co-crystallised in the lattice compared to the literature 19,20,24 and in some cases in this work leading to differences in the number of associated water molecules between the bulk materials and the crystal structures (for example [MF 3 (Me 3 -tacn)], M = Mn, Co, have two water molecules in the bulk, but four in the crystal structure). This might be due to differences in the crystallisation methods employed and the length of time the bulk materials were dried in vacuo. The products are air stable and can be stored outside the glovebox for several weeks. The stability of the complexes was challenged in aqueous solution by the presence of up to 10-fold excess of competitive ions (NaCl, NaF, Na 2 CO 3 , Na 3 PO 4 , NaOAc), increased temperature and changes in pH at two different time points (t = 0 and t = 4 h). UV-vis spectroscopy was used to monitor the solutions, changes in the position of the relevant d-d transitions and or appearance/disappearance of bands were taken as an indication of the instability of the complex during the experiments.  3 ] with the ligands in n-BuOH or dmf, respectively, following litera-ture methods. 20 The IR spectra of the solids confirm the presence of water and show two ν(Cr-F) bands for [CrF 3 (Me 3 -tacn)]·3.5H 2 O, as expected for a fac octahedral configuration in C 3v symmetry, whereas one very broad band is seen for [CrF 3 (terpy)]·4H 2 O (three bands are expected in a mer C 2v symmetry, but not resolved). The diffuse reflectance spectra of the complexes are shown in Fig. S8 and S12, † and resemble those reported previously. 20

Manganese
The reaction of MnF 3 with Me 3 -tacn or terpy in anhydrous MeOH at room temperature produces the species [MnF 3 (Me 3 )). The molecular composition of [MnF 3 (Me 3 -tacn)] was confirmed by a single crystal X-ray structure analysis (Fig. 1).

Scheme 1 Reaction conditions for the preparation of the complexes.
The structure shows a distorted octahedral environment with the fluorides facially coordinated to the metal. The Mn-F3 and Mn-N3 bond lengths are elongated by ∼0.17 Å and ∼0.18 Å compared to the other Mn-F and Mn-N distances, respectively. This significant difference in the bond lengths is consistent with a Jahn-Teller distortion in the high spin d 4 configuration. 25 As confirmation of this, the complex has a magnetic moment of 4.94 BM. 31 The tetragonal elongation in the related complex [{MnF 2 (Me 3 -tacn)} 2 (μ-F)][PF 6 ] 25 is observed along the axis on which Mn-F bridging and Mn-N trans to it lie. These bond lengths are ∼0.18 and ∼0.23 Å longer than the other Mn-N and Mn-F bonds, respectively. 25 The Extensive H-bonding involving the water molecules and the fluorides is also present (Fig. 2). The same H-bonding pattern was observed in [GaF 3 (Me 3 -tacn)]·4H 2 O. 7 The diffuse reflectance UV-vis spectrum of [MnF 3 (Me 3tacn)]·4H 2 O (Fig. S19 †) shows intense bands in the UV region due to ligand to metal charge transfer transitions (σN → Mn) and a single d-d transition at ∼520 nm, generically assigned to 5 E g → 5 T 2g . Although in an octahedral d 4 high spin configuration only one spin-allowed transition is predicted ( 5 E g → 5 T 2g ), Jahn-Teller distortions can often lower the symmetry, resulting in splitting of the single transition. However, in this case splitting is not resolved. These data resemble those of the dimer [{MnF 2 (Me 3 -tacn)} 2 (μ-F)][PF 6 ]. 25 Splitting is more resolved in [MnF 3 (terpy)]·MeOH·3H 2 O (Fig. S15 †) and the three bands shown in the spectra are tentatively assigned to 5 B 1g → 5 A 1g , 5 B 1g → 5 B 2g , 5 B 1g → 5 E g . The metal centre symmetry is C 2v and it is possible that both this and the Jahn-Teller effect results in greater splitting of the bands compared to [MnF 3 (Me 3 -tacn)]·2H 2 O. Strong absorptions at high energy, due to the LMCT transitions, σN → Mn, and π-π* transitions within the aromatic rings, are also present.

Iron
[FeF 3 (Me 3 -tacn)]·H 2 O was obtained as a pale yellow solid (in contrast to the green colour reported previously in the literature 19 ) after reaction of FeF 3 ·3H 2 O with Me 3 -tacn in refluxing n-BuOH. Its IR spectrum shows the expected two ν(Fe-F) bands at 512 and 529 cm −1 . [FeF 3 (terpy)]·2H 2 O, made using the same method, was obtained as a light purple powder and characterised similarly. The diffuse reflectance UV-vis spectra of the complexes are shown in Fig. S26 and S30. † The electronic transitions in a d 5 high spin system with a ground state 6 A 1g are all spin-forbidden and weak bands in the visible region are therefore seen.
Crystals suitable for single crystal X-ray analysis of [FeF 3 (terpy)]·3H 2 O were obtained by slow evaporation of a concentrated solution of the complex in water (Fig. 3).   The structure shows a meridional configuration around the metal. The Fe-F bond lengths are slightly longer than those in [FeF 3 (Me 3 -tacn)]·H 2 O 19 (1.878-1.907 Å vs. 1.850-1.866 Å) whereas the opposite trend is seen in the Fe-N bonds with the terpy complex having the shortest Fe-N distance (2.142-2.156 vs. 2.223-2.228). The rigidity of the terpy ligand forces some of the angles to deviate from the 180/90°expected for an octahedron. Extensive H-bonding and π-stacking interactions are also present in the lattice (Fig. S1 †). The UV-vis diffuse reflectance spectra of the complexes are shown in Fig. S34 and S40. † The two spin-allowed transitions predicted for a low spin d 6 system are present in the spectrum of fac-[CoF 3 (Me 3 -tacn)] (Fig. S40 †) at ∼570 and ∼375 nm and are assigned to 1 A 1g → 1 T 1g and 1 A 1g → 1 T 2g , respectively. The mer-[CoF 3 (terpy)]·MeOH·H 2 O has C 2v symmetry and splitting is greater than in the Me 3 -tacn complex (C 3v ). The 1 T 1g level split into three components and the transitions observed are tentatively assigned to 1 A 1g → 1 B 1g , 1 A 1g → 1 B 2g and 1 A 1g → 1 B 3g (Fig. S34, † from low to high energy). 17,34 In this case, the third (higher energy) spin-allowed transition 1 A 1g → 1 T 2g is masked by the ligand to metal charge transfer and/or π-π* bands involving the terpy ligand. Crystals of [CoF 3 (Me 3 -tacn)]·4H 2 O suitable for single crystal X-ray analysis were obtained by slow evaporation of a concentrated solution of the complex in water (Fig. 4). The complex is isostructural to [MnF 3 (Me 3 -tacn)]·4H 2 O and shows the same H-bonding pattern in the lattice (Fig. S2 †).

Stability tests
The stability of a 10  Fig. S33. † The complex is stable to the presence of all the anions studied at t = 0. However, the spectra acquired after 4 h show that the presence of carbonate anions causes decomposition of the complex. Similar behaviour was observed in the pH 7 experiment (stable at t = 0 and unstable at t = 4 h), whereas the complex is unstable at pH 11 from t = 0. The complex is stable after 2 h at 80°C in water and is unchanged after one week in aqueous solution. The stability tests on [FeF 3 (terpy)]·2H 2 O in the same conditions (Fig. S29 †) showed that the positions of the peaks were unchanged, however a change in their intensity and in the colour of the  (Fig. S3 †), showing that the CrF 3 -complex is formed during the reaction (ν Cr-F 539, 507 cm −1 ) but it appears that some CrCl 3 -complex and/or mixed chloride/ fluoride species are still present (ν Cr-Cl 343, 333 cm −1 ). This is not surprising given the slow substitution kinetics in the d 3 systems. However, much more promisingly,  These studies suggest that the [FeF 3 (R 3 -tacn)] system may be worth further investigation as a possible platform for PET applications. In order to test this, the Bn-substituted analogue, [FeF 3 (BnMe 2 -tacn)] was prepared; the presence of the Bn group aids identification of the final radio-product via UV-vis spectroscopy. The crystal structure (Fig. 7) confirms the formulation [FeF 3 (BnMe 2 -tacn)]·2H 2 O, with two co-crystallised water molecules in the lattice. Two crystallographically independent molecules are present in the asymmetric unit, although the bond distances are not significantly different.      Fig. 8 and 9, Fig. S53 and S54 †), showing good stability over at least 2 hours with RCP = 99% for the EtOH/H 2 O and EtOH/PBS, and RCP = 90% for EtOH/HSA. The target radioproduct could also be purified through a prep. HPLC system, giving the same RCP at t = 0 as in the SPE purification protocol. Stability tests performed on [MF 3 (Me 3 -tacn)] and [MF 3 (terpy)] (M = Cr, Fe) have shown that the terpy complexes do not have the stability required to be a contender for future PET applications. This was also observed in the Group 13 (Al, Ga) and ScF 3 terpy complexes. 8,13 Among the complexes with Me 3 -tacn as ligand, [FeF 3 (Me 3 -tacn)] and [CrF 3 (Me 3 -tacn)] showed good stability in most conditions; however, the reaction kinetics of the Cl/F exchange on [CrCl 3 (Me 3 -tacn)] using 4 mol. equiv. of [Me 4 N]F in MeCN under reflux proved to be slow, with a mixture of the chloride and fluoride complexes present after 24 h. The d 5 system, [FeF 3 (Me 3 -tacn)], proved to be more successful. Fluorination was achieved within 30 min in aqueous MeCN at room temperature using KF as the fluoride source, causing complete conversion to [FeF 3 (Me 3 -tacn)].

Conclusions
Stability tests indicated that tacn derivatives bearing the FeF 3 fragment may be suitable for radiofluorination, and this was borne out by 18

Conflicts of interest
There are no conflicts to declare.