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

Abstract

The neutral complex, [GaF₃(L)] (L = 1-benzyl-4,7-dimethyl-1,4,7-triazacyclononane, BzMe₂-tacn), acts as a ‘metalloligand’ to Na⁺, K⁺ and [NH₄]⁺ cations in aqueous solution, forming supramolecular assemblies containing significant Na/K–F and H₃N⁺H⋯F coordination. κ¹-[BF₄]⁻ and κ²-[PF₆]⁻ coordination is also evident to Na⁺ and K⁺, respectively.

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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 ¹⁸F incorporation into [GaCl₃(L)] (L = 1-benzyl-4,7-dimethyl-1,4,7-triazacyclononane, BzMe₂-tacn) occurs readily in aqueous MeCN at room temperature via halide exchange. The formation of the [Ga^18/19F₃(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′)] (L′ = 1,4,7-tris(2-amino-3,5-di-tert-butylbenzyl)-1,4,7-triazacyclononane).³ Further, we found from ESI⁺ mass spectrometry studies on the HPLC purified radio-product [Ga^18/19F₃(L)] that [GaF₃(L) + NH₄]⁺ was the major ionic species observed, and proposed² 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 [Gd^III₃M^III₂] based molecular magnets derived from Gd(NO₃)₃·5H₂O with [M^IIIF₃(Me₃-tacn)]·xH₂O (M = Cr, Fe, Ga)⁴ 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.⁵ Binary p-block fluorides such as XeF₂ and AsF₃ can coordinate to Ln³⁺ and alkaline earth dications in extreme solvents, i.e. anhydrous HF or liquid AsF₃.⁶

In order to investigate this behaviour further, we used ESI⁺ MS to probe the speciation from 1 : 1 molar ratios of preformed [GaF₃(L)] with various alkali metal cations (via the salts LiBF₄, NaBF₄, KPF₆ and Cs₂CO₃) in 5 : 1 MeCN : H₂O. In each case peaks due to [GaF₃(L) + M]⁺, with the correct isotopic distribution, were observed, as well as [{GaF₃(L)}₂ + M]⁺ in some cases. Similarly, addition of NH₄PF₆ gave [GaF₃(L) + NH₄]⁺ – 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–π(arene) complexes⁸ which have attracted interest due to their relevance in biological systems (such as potassium-selective channels⁹ and sodium-dependent allosteric regulation of serine proteases¹⁰), 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 [{GaF₃(L)}₂Na₂(BF₄)₂] (1) and [{GaF₃(L)}₂K₂(OH₂)₄(PF₆)₂]·H₂O (2) by combining equimolar solutions of [GaF₃(L)] and NaBF₄ or KPF₆, respectively, in water and allowing the products to form as colourless crystals over several days. A few (poorly diffracting) crystals of the mixed Na⁺/NH₄⁺ compound [{GaF₃(L)}Na(NH₄)(PF₆)₂] (3) were also obtained from a 1 : 1 molar ratio of [GaF₃(L)] and NH₄PF₆ 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 P2₁/c with two GaF₃(L) moieties and two NaBF₄ units in the asymmetric unit. The structure shows (Fig. 1) two [GaF₃(L)] moieties bridged by two five-coordinate, distorted square based pyramidal (τ = 0.21 (Na1), 0.23 (Na2)) sodium cations. Each sodium ion is coordinated through two μ²-bridging fluoride ligands from one [GaF₃(L)] unit (κ²), one fluoride from a (κ¹) BF₄⁻ ion, and a single μ³-bridging fluoride from each of two further distinct (symmetry-related) gallium moieties. This leads to an extended 1-D zig-zag chain structure. The μ³-F atoms form a Na₂F₂ rhombus at the core.

Fig. 1 (a) View of the asymmetric unit for compound 1, including the symmetry related atoms (F2a and F4b) that complete the coordination environment at Na1 and Na2 with associated numbering scheme. H atoms are omitted for clarity. Selected bond lengths (Å): Ga1–F1 1.842(2), Ga1–F2 1.848(2), Ga1–F3 1.902(2), Ga2–F5 1.834(2), Ga2–F4 1.849(2), Ga2–F6 1.896(2), Na1–F4b 2.210(3), Na1–F5 2.241(3), Na1–F7 2.254(3), Na1–F3 2.377(2), Na1–F6 2.426(3), Na2–F2a 2.200(3), Na2–F1 2.229(2), Na2–F11 2.264(3), Na2–F6 2.355(3), Na2–F3 2.421(3), F7–B1 1.399(5), F8–B1 1.378(5), F9–B1 1.385(5), F10–B1 1.403(5), F11–B2 1.412(5), F12–B2 1.394(5), F13–B2 1.381(6), F14–B2 1.386(5). Symmetry codes: a = 2 − x, 1 − y, −z; b = 1 − x, 1 − y, −z. (b) View down the a-axis of 1. Colour key: turquoise = Ga, teal = Na, pink = P, green = F, blue = N, black = C.

The Na–F bond distances involving the GaF₃ unit slightly longer for the μ³-F atoms (F3 and F6) than for the μ²-F atoms. The latter are little different from those observed in [GaF₃(Me₃-tacn)]·4H₂O,² where the F atoms are involved in significant H-bonding with the H₂O 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⁻ (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 D₂O solution the ¹H, ¹⁹F{¹H}, ²³Na and ⁷¹Ga 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 P1̄ with one half of a centrosymmetric tetranuclear entity (2) in the asymmetric unit. The structure also confirms coordination of the K⁺ to [GaF₃(L)] through the fluorides (Fig. 2a). The structure is based upon eight-coordinate K⁺, coordinated to two F atoms from one GaF₃(L) moiety (one of which is μ², and the other μ³), one μ³-F from the second GaF₃(BzMe₂-tacn) unit, one terminal and two bridging OH₂ ligands. The coordination environment at each K⁺ ion is completed by a κ²-coordinated PF₆⁻ ion. This tetranuclear (Ga₂K₂) species shows H-bonding interactions between the bridging OH₂ ligands and two fluorides ligand of the gallium species (O1⋯F2 2.893(3), O1⋯F3 2.762(3) Å). In addition, further H-bonding is evident between the terminal OH₂ ligands and a fluoride ligand from an adjacent ‘Ga₂K₂’ unit (O2⋯F1 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–F⋯H–OH hydrogen bond (O3⋯F3 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⁻ (1.19 Å).¹¹

Fig. 2 (a) View of the structure of the centrosymmetric ‘Ga₂K₂’ species present in compound 2 with associated numbering scheme. Dashed bonds indicate H-bonding contacts. H atoms associated with the macrocycle are omitted for clarity (H atoms on the water were not located in the difference map – see Experimental). Selected bond lengths (Å): Ga1–F2 1.838(2), Ga1–F1 1.872(3), Ga1–F3 1.872(3), K1a–F2 2.578(3), K1–F1 2.649(3), K1–F5 2.866(3), K1–O2 2.887(4), K1–F2 2.882(3), K1–O1 2.914(4), K1–F6 2.928(3), K1–O1a 2.959(4), P1–F7 1.588(3), P1–F4 1.590(3), P1–F6 1.597(3), P1–F9 1.600(3), P1–F8 1.600(3), P1–F5 1.609(3). Symmetry operation: a = 2 − x, 1 − y, −z. (b) View of the extended structure formed by compound 2 (excluding the H₂O solvent molecules). Colour key: turquoise = Ga, teal = K, pink = P, green = F, blue = N, black = C.

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 ν(BF₄⁻) and ν(PF₆⁻) 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 F₆ coordination; one type involving two κ²-GaF₃(L) units and two κ¹-[PF₆]⁻ anions, the second involving two κ¹-GaF₃(L) units and two κ²-[PF₆]⁻ anions. Interestingly, the [NH₄]⁺ cations also form significant F⋯H–N hydrogen bonding interactions with adjacent Fs both from GaF₃(L) and from [PF₆]⁻ anions. The presence of both Na⁺ and [NH₄]⁺ ions in 3 is also supported by ESI⁺ mass spectrometry data on the isolated product.

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 BzMe₂-tacn ligand are omitted for clarity; those of the NH₄⁺ 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 F⋯HNH₃⁺ hydrogen bonding interactions in 3.

We have shown that [GaF₃(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

1. S. L. Benjamin, W. Levason and G. Reid, Chem. Soc. Rev., 2013, 42, 1460; H. C. S. Clark and J. H. Holloway, in Advanced Inorganic Fluorides, ed. T. Nakajima, B. Žemva and A. Tressaud, Elsevier, Oxford, 2000, ch. 3; N. M. Doherty and N. W. Hoffman, Chem. Rev., 1991, 91, 553; E. F. Murphy, R. Murugavel and H. W. Roesky, Chem. Rev., 1997, 97, 3425.
2. R. Bhalla, C. Darby, W. Levason, S. K. Luthra, G. McRobbie, G. Reid, G. Sanderson and W. Zhang, Chem. Sci., 2014, 5, 381.
3. F. N. Penkert, T. Weyhermüller and K. Wieghardt, Chem. Commun., 1998, 557.
4. K. S. Pedersen, G. Lorusso, J. J. Morales, T. Weyhermüller, S. Piligkos, S. K. Singh, D. Larsen, M. Schau-Magnussen, G. Rajaraman, M. Evangelisti and J. Bendix, Angew. Chem., Int. Ed., 2014, 53, 2394.
5. G. Kumar and R. Gupta, Chem. Soc. Rev., 2013, 42, 9403.
6. M. Tramšek and B. Žemva, J. Fluorine Chem., 2006, 127, 1275 and references therein.
7. T. P. Hanusa, in Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, Oxford, 2004, ch. 1, vol. 3.
8. See for example: B. Werner, T. Kräuter and B. Neumüller, Organometallics, 1996, 15, 3746; G. B. Deacon, T. C. Feng, P. C. Junk, B. W. Skelton and A. H. White, J. Chem. Soc., Dalton Trans., 1997, 1181.
9. R. L. Nakamura, J. A. Anderson and R. F. Gaber, J. Biol. Chem., 1997, 272, 1011.
10. Q. D. Dang, E. R. Guinto and E. Di Cera, Nat. Biotechnol., 1997, 15, 146.
11. R. D. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751.

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Footnote

† Electronic supplementary information (ESI) available: Full experimental details for the compounds reported, in situ ESI⁺ MS data from 1 : 1 molar ratios of [GaF₃(L)] with alkali metal salts and NH₄PF₆, plus all the crystallographic and spectroscopic data for all products. CCDC 1008581–1008583. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc05603e

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