[GaF3(BzMe2-tacn)] – a neutral ‘metalloligand’ towards alkali metal and ammonium cations in water†

Metal fluoride complexes often display quite different properties and reactivities compared to the corresponding chlorides and bromides, due to the small size and high electronegativity of the hard fluoride ligand. As part of our work investigating the potential of metal coordination complexes towards new classes of PET imaging agents we have reported the radio-fluorination of group 13 trichloride complexes based upon the triaza-macrocyclic ligand scaffold, showing that F incorporation into [GaCl3(L)] (L = 1-benzyl-4,7dimethyl-1,4,7-triazacyclononane, BzMe2-tacn) occurs readily in aqueous MeCN at room temperature via halide exchange. The formation of the [GaF3(L)] is driven by the high Ga–F bond energy, and the trifluoride complex is extremely stable both in water and in phosphate buffered saline. Crystallographic studies showed that [MF3(L)] and [MF3(Me3-tacn)] are heavily hydrated and form extended supramolecular networks through strong F H–OH hydrogen bonding. Earlier work by Wieghardt and co-workers had reported that first observation of the S6-symmetric methanol hexamer, crystallised within the hydrophobic cavity present in the complex [GaF3(L)] (L0 = 1,4,7-tris(2-amino-3,5-ditert-butylbenzyl)-1,4,7-triazacyclononane). Further, we found from ESI mass spectrometry studies on the HPLC purified radio-product [GaF3(L)] that [GaF3(L) + NH4] + was the major ionic species observed, and proposed that association of [NH4] + (from the NH4OAc(aq) HPLC mobile phase) with the highly electronegative region created by the three facial fluorides of [GaF3(L)]. A very recent report of [Gd3M III 2] based molecular magnets derived from Gd(NO3)3 5H2O with [MF3(Me3-tacn)] xH2O (M = Cr, Fe, Ga) further fuelled our interest in exploiting the wider scope of the [GaF3(L)] complex to act as a robust ‘metalloligand’ towards other cations, building novel complexes with (functional) supramolecular architectures, adding a new type of building block for this type of application. Binary p-block fluorides such as XeF2 and AsF3 can coordinate to Ln and alkaline earth dications in extreme solvents, i.e. anhydrous HF or liquid AsF3. 6

Metal fluoride complexes often display quite different properties and reactivities compared to the corresponding chlorides and bromides, due to the small size and high electronegativity of the hard fluoride ligand. 1 As part of our work investigating the potential of metal coordination complexes towards new classes of PET imaging agents we have reported the radio-fluorination of group 13 trichloride complexes based upon the triaza-macrocyclic ligand scaffold, 2 showing that 18 F incorporation into [GaCl 3 (L)] (L = 1-benzyl-4,7dimethyl-1,4,7-triazacyclononane, BzMe 2 -tacn) occurs readily in aqueous MeCN at room temperature via halide exchange. The formation of the [Ga 18/19 F 3 (L)] is driven by the high Ga-F bond energy, and the trifluoride complex is extremely stable both in water and in phosphate buffered saline. Crystallographic studies showed that [MF 3 (L)] and [MF 3 (Me 3 -tacn)] are heavily hydrated and form extended supramolecular networks through strong FÁ Á ÁH-OH hydrogen bonding. Earlier work by Wieghardt and co-workers had reported that first observation of the S 6 -symmetric methanol hexamer, crystallised within the hydrophobic cavity present in the complex [GaF 3 (L 0 )] (L 0 = 1,4,7-tris(2-amino-3,5-ditert-butylbenzyl)-1,4,7-triazacyclononane). 3 4 further fuelled our interest in exploiting the wider scope of the [GaF 3 (L)] complex to act as a robust 'metalloligand' towards other cations, building novel complexes with (functional) supramolecular architectures, adding a new type of building block for this type of application. 5 Binary p-block fluorides such as XeF 2 and AsF 3 can coordinate to Ln 3+ and alkaline earth dications in extreme solvents, i.e. anhydrous HF or liquid AsF 3 . 6 In order to investigate this behaviour further, we used ESI + MS to probe the speciation from 1 : 1 molar ratios of preformed [GaF 3 (L)] with various alkali metal cations (via the salts LiBF 4 , NaBF 4 , KPF 6 and Cs 2 CO 3 ) in 5 : 1 MeCN : H 2 O. In each case peaks due to [GaF 3 (L) + M] + , with the correct isotopic distribution, were observed, as well as [{GaF 3 (L)} 2 + M] + in some cases. Similarly, addition of NH 4 PF 6 gave [GaF 3 (L) + NH 4 ] + -see ESI. † The high affinity of the alkali metal cations for water is well-known, 7 and, with the exceptions of the ubiquitous crown ether and cryptand derivatives, and group 1 cation-p(arene) complexes 8 which have attracted interest due to their relevance in biological systems (such as potassium-selective channels 9 and sodium-dependent allosteric regulation of serine proteases 10 ), few coordination complexes of the group 1 cations formed in aqueous solution with neutral ligands are known. 7 We were able to prepare directly and isolate [{GaF 3 (L)} 2 Na 2 (BF 4 ) 2 ] (1) and [{GaF 3 (L)} 2 K 2 (OH 2 ) 4 (PF 6 ) 2 ]ÁH 2 O (2) by combining equimolar solutions of [GaF 3 (L)] and NaBF 4 or KPF 6 , respectively, in water and allowing the products to form as colourless crystals over several days. A few (poorly diffracting) crystals of the mixed Na + /NH 4 + compound [{GaF 3 (L)}Na(NH 4 )(PF 6 ) 2 ] (3) were also obtained from a 1 : 1 molar ratio of [GaF 3 (L)] and NH 4 PF 6 in water; the adventitious Na + most likely originating from the glassware; further evidence, however, for the high affinity of the gallium fluoride complex for the group 1 cations.
Compound 1 crystallises in the monoclinic space group P2 1 /c with two GaF 3 (L) moieties and two NaBF 4 units in the asymmetric unit. The structure shows (Fig. 1) two [GaF 3 (L)] moieties bridged by two five-coordinate, distorted square based pyramidal (t = 0.21 (Na1), 0.23 (Na2)) sodium cations. Each sodium ion is coordinated through two m 2 -bridging fluoride ligands from one [GaF 3 (L)] unit (k 2 ), one fluoride from a (k 1 ) BF 4 À ion, and a single m 3 -bridging fluoride from each of two further distinct (symmetry-related) gallium moieties. This leads to an extended 1-D zig-zag chain structure. The m 3 -F atoms form a Na 2 F 2 rhombus at the core. The Na-F bond distances involving the GaF 3 unit slightly longer for the m 3 -F atoms (F3 and F6) than for the m 2 -F atoms. The latter are little different from those observed in [GaF 3 (Me 3 -tacn)]Á4H 2 O, 2 where the F atoms are involved in significant H-bonding with the H 2 O solvate. The Na-F distances lie in the ranges 2.210(3)-2.426(3) Å (Na1) and 2.200(3)-2.421(3) Å (Na2). These mostly lie within the sum of the ionic radii for Na + and F À (1.16 and 1.19 Å respectively) 11 derived from crystalline NaF. Despite the hydrophilicity of the Na + cations, no water is retained in the crystal structure of 1.
IR spectroscopy and ESI + mass spectrometry of compound 1 supported the formulation observed crystallographically, although in D 2 O solution the 1 H, 19 F{ 1 H}, 23 Na and 71 Ga NMR resonances are not significantly different from those of the constituents in water. This indicates that 1 is extensively dissociated in water, typical of very labile alkali metal complexes.
Compound 2 crystallises in the triclinic space group P% 1 with one half of a centrosymmetric tetranuclear entity (2) in the asymmetric unit. The structure also confirms coordination of the K + to [GaF 3 (L)] through the fluorides (Fig. 2a). The structure is based upon eight-coordinate K + , coordinated to two F atoms from one GaF 3 (L) moiety (one of which is m 2 , and the other m 3 ), one m 3 -F from the second GaF 3 (BzMe 2 -tacn) unit, one terminal and two bridging OH 2 ligands. The coordination environment  (5). Symmetry codes: a = 2 À x, 1 À y, Àz; b = 1 À x, 1 À y, Àz. (b) View down the a-axis of 1. Colour key: turquoise = Ga, teal = Na, pink = P, green = F, blue = N, black = C. at each K + ion is completed by a k 2 -coordinated PF 6 À ion. This tetranuclear (Ga 2 K 2 ) species shows H-bonding interactions between the bridging OH 2 ligands and two fluorides ligand of the gallium species (O1Á Á ÁF2 2.893(3), O1Á Á ÁF3 2.762(3) Å). In addition, further H-bonding is evident between the terminal OH 2 ligands and a fluoride ligand from an adjacent 'Ga 2 K 2 ' unit (O2Á Á ÁF1 2.740(4) Å), resulting in a 1D chain polymer motif (Fig. 2b). This results in a strongly H-bonded supramolecular assembly. The solvent water molecule also forms a Ga-FÁ Á ÁH-OH hydrogen bond (O3Á Á ÁF3 2.729(4) Å). The K-F distances lie in the range 2.578(3) to 2.928(3) Å, comparable with the sum of the ionic radii of K + (1.65 Å for eight coordination) and F À (1.19 Å). 11 As expected, the Ga-N bond lengths are not significantly affected by the alkali metal cation coordination in compounds 1 and 2.
Microanalytical, IR spectroscopic and ESI + MS data from an isolated sample of 2 are consistent with the formula identified crystallographically. The IR spectra of 1 and 2 show significant broadening and splitting of the n(BF 4 À ) and n(PF 6 À ) stretching vibrations compared to the parent tetrahedral and octahedral anions, respectively, probably resulting from their coordination to the alkali metal ions.
Although the crystal data quality for compound 3 was much inferior compared to 1 and 2, analysis of the structure confirms the composition and reveals the key features of the coordination environment. The structure shows (Fig. 3) 3 is a chain polymer with two alternating types of six coordinate Na + ions, both with F 6 coordination; one type involving two k 2 -GaF 3 (L) units and two k 1 -[PF 6 ] À anions, the second involving two k 1 -GaF 3 (L) units and two k 2 -[PF 6 ] À anions. Interestingly, the [NH 4 ] + cations also form significant FÁ Á ÁH-N hydrogen bonding interactions with adjacent Fs both from GaF 3 (L) and from [PF 6 ] À anions. The presence of both Na + and [NH 4 ] + ions in 3 is also supported by ESI + mass spectrometry data on the isolated product.
We have shown that [GaF 3 (L)] can function as a very effective F-donor 'metalloligand' towards alkali metal cations in water, leading to highly unusual and distinct structural types. The results suggest that rational development of new multimetallic frameworks and assemblies based upon metal fluoride coordination complexes as metalloligands towards other inorganic and organic cations should be possible. This work was funded by EPSRC and GE Healthcare through a CASE studentship (G.S.). The authors thank Dr M. E. Light for help with the crystallographic analyses.
Notes and references Fig. 3 (a) View of the structure of a portion of the polymeric structure formed by 3. Ellipsoids are drawn at 50% probability level. H-atoms on the BzMe 2 -tacn ligand are omitted for clarity; those of the NH 4 + cation were not located (see ESI †). Colour key: turquoise = Ga, teal = Na, pink = P, green = F, blue = N, black = C. (b) Line drawing illustrating the coordination environment at Na + and the FÁ Á ÁHNH 3 + hydrogen bonding interactions in 3.