Abstract
The coordination chemistry of the first row transition metal trifluorides with terpy (2,2':6',2''-terpyridine) and Me3-tacn (1,4,7-trimethyl-1,4,7-triazacyclononane) was explored to identify potential systems for 18F radiolabelling. The complexes [MF3(L)] (M = Cr, Mn, Fe, Co; L = Me3-tacn, terpy) were synthesised and fully characterised by UV-vis and IR spectroscopy, microanalysis, and, for the diamagnetic [CoF3(L)], using 1H, 19F{1H} and 59Co NMR spectroscopy. Single crystal X-ray analyses are reported for [MF3(Me3-tacn)] (M = Mn, Co), [FeF3(terpy)] and [FeF3(BnMe2-tacn)]. Stability tests on [MF3(Me3-tacn)] (M = Cr, Mn, Fe) and [M'F3(terpy)] (M' = Cr, Fe) were performed and Cl/19F halide exchange reactions on [CrCl3(Me3-tacn)] using [Me4N]F in anhydrous MeCN solution, and [FeCl3(Me3-tacn)] using [Me4N]F in anhydrous MeCN or KF in aqueous MeCN solution were also carried out. Halide exchange reactions proved to be successful in forming [FeF3(Me3-tacn)] in aqueous MeCN solution within 30 minutes. Based upon the clean Cl/F exchange and the good stability observed for [FeF3(Me3-tacn)] in a range of competitive media, this was identified as a possible candidate for radiolabelling. 18F/19F isotopic exchange was achieved by addition of [18F]F- in the cyclotron target water to a MeCN solution of the benzyl-substituted analogue, [FeF3(BnMe2-tacn)], at a range of concentrations down to 24 nM with heating to 80 °C for 10 min.; the resulting [Fe18F19F2(BnMe2-tacn)] shows radiochemical purity (RCP) ≥90% after 2 h in a range of formulations, including 10% EtOH/phosphate buffered saline (PBS) and 10% EtOH/human serum albumin (HSA). This is the first reported complex with a transition metal directly bonded to [18F]F-.
Highlights
C–18F based radiotracers are extensively used in positron emission tomography (PET) for imaging purposes in oncology, cardiology and neurology.[1]
We previously developed the chemistry of the Group 13 metal fluorides (Al, Ga) towards neutral nitrogen ligands and reported that radioactive 18F can be introduced into [MCl3(BnMe2-tacn)] (M = Al, Ga) through Cl/18F halide exchange reactions and that [GaF3(BnMe2-tacn)] can be 18F-radiolabelled through 18F/19F isotopic exchange using a very small amount of material (27 nmol); both methods were successful in mild conditions, i.e. aqueous solution, room temperature or 80 °C.6,7,12
We present an evaluation of the 3d transition metal (Cr3+, Mn3+, Fe3+ and Co3+) trifluoride complexes for possible PET applications
Summary
C–18F based radiotracers are extensively used in positron emission tomography (PET) for imaging purposes in oncology, cardiology and neurology.[1]. Halide exchange reactions proved to be successful in forming [FeF3(Me3-tacn)] in aqueous MeCN solution within 30 minutes. Splitting is more resolved in [MnF3(terpy)]·MeOH·3H2O (Fig. S15†) and the three bands shown in the spectra are tentatively assigned to 5B1g → 5A1g, 5B1g → 5B2g, 5B1g → 5Eg. The metal centre symmetry is C2v and it is possible that both this and the Jahn–Teller effect results in greater splitting of the bands compared to [MnF3(Me3-tacn)]·2H2O.
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