Self-assembly of [Sn(OPMe₃)₃(CF₃SO₃)₂]₆ metallocyclic Sn(II) hexamer stacks with CF₃-lined channel interiors

Abstract

[Sn(OPMe₃)₃(CF₃SO₃)₂]₆ (1) self-assembles to form a highly unusual hexagonal metallocyclic array with bridging CF₃SO₃⁻ groups. The discrete hexamers stack along the c-axis to form hydrophobic CF₃-lined channels with an F⋯F distance across the channel diameter of 0.8 nm. This Sn(II) complex is different from its Pb(II) analogue, which instead forms a discrete, doubly CF₃SO₃-bridged dimer of formula, [{Pb(OPMe₃)₃(CF₃SO₃)}₂(μ-CF₃SO₃)₂] (3).

---

Main group chemistry has attracted considerable research effort in the last two decades in response to the desire for tailored precursors for p-block materials deposition, metal-free catalysts, and porous coordination polymers, in addition to exploring new classes of compounds and reaction chemistry.¹

While many of the reported p-block complexes are derived from main group halide precursors with secondary bonding interactions to neutral group 15 and 16 donor ligands, replacing the halide with more weakly coordinating (less Lewis basic) anions, such as CF₃SO₃⁻, and the bulky fluorous tetra-arylborates and fluorous aluminates, etc., can provide an entry to highly Lewis acidic p-block cations that are of interest in organic/organometallic chemistry,^2–5 while delocalised fluorous anions including CF₃SO₃⁻, SiF₆²⁻, PF₆⁻, etc., can be used to lower the dimensionality of porous coordination polymers and metal–organic frameworks (MOFs).^6–9 Weakly coordinating anions, in which the negative charge is delocalised and diffuse, including triflate, fluorous tetra-arylborates and fluorous aluminates, amongst others, have attracted considerable research effort in the field of organometallic chemistry.^2–5 They aid solubility in low polarity solvents and can facilitate stabilisation of highly reactive species such as the univalent [Ga(PPh₃)₃]⁺,¹⁰ and [Ag(P₄)₂]⁺ cations,¹¹ as well as unusual homoleptic phosphine¹² or thioether complexes with alkali metal cations.¹³

The CF₃SO₃⁻ group has been used extensively as a weakly coordinating anion in work developing group 14 chemistry and cations thereof. Notable examples include the stabilisation of ‘naked’ Ge(II) dications encapsulated within 2.2.2-cryptand,¹⁴ Ge(II) dications bearing N-heterocyclic carbenes,¹⁵ and pnictines,¹⁶ as well as tetravalent group 14 fluoride species with soft pnictine ligands.¹⁷ We have also observed that the CF₃SO₃⁻ anion can engage in a range of different secondary interactions that play an important role in determining the speciation and structures adopted. For example, in [In(OTf)₂(Ph₃PO)₄]–[In{(OH₂)₄(OTf)₄}(Ph₃PO)₂], which forms a weakly associated 24-membered pseudo-macrocyclic ring via H-bonding between the coordinated water molecules and the CF₃SO₃⁻ anions.¹⁸ In view of these observations, we became interested to investigate the coordination chemistry of the divalent group 14 ions (Ge, Sn, Pb) with the hard and sterically compact OPMe₃ ligand.

Reaction of Sn(CF₃SO₃)₂ with three mol equiv. of OPMe₃ in a weakly coordinating solvent such as CH₂Cl₂ or MeCN readily leads to the formation of a colourless crystalline solid, Sn(OPMe₃)₃(CF₃SO₃)₂, in high yield. Spectroscopic data for this species are presented in Fig. S3–S5 (ESI†) and discussed below. However, to probe the role of the CF₃SO₃⁻, single crystals suitable for X-ray analysis were grown either by diffusion of hexane into a CH₂Cl₂ solution or by vapour diffusion of Et₂O into an MeCN or MeNO₂, solution with several crystals grown from different batches and solvents all producing the same crystalline form.

The material crystallises in the trigonal space group (P3̄) and the structure determination shows that it self-assembles into discrete hexamers, [Sn(OPMe₃)₃(CF₃SO₃)₂]₆ (1) (Fig. 1(a)) comprising six pseudo-octahedrally coordinated Sn(II) centres with three mutually fac OPMe₃ ligands (d(Sn–OPMe₃) = 2.094(1), 2.156(2), 2.167(1) Å) and three weakly bound, CF₃SO₃⁻ groups (Fig. 1(b)). Two of the triflates bridge between adjacent Sn centres to form the cyclic arrangement (d(Sn⋯OO₂SCF₃) = 2.858(1), 3.028(2) Å), while the third is bonded terminally (d(Sn⋯OO₂SCF₃) = 3.082(1) Å).

Fig. 1 (a) View of the hexameric assembly adopted by [Sn(OPMe₃)₃(CF₃SO₃)₂]₆ (1) via the bridging CF₃SO₃⁻ groups; (b) the coordination environment at Sn(II) with the atom labelling scheme. Ellipsoids are drawn at the 50% probability level and H atoms are omitted for clarity. Selected bond lengths (Å): Sn1–O1 = 2.1557(11), Sn1–O2 = 2.1670(10), Sn1–O3 = 2.0943(11), Sn1⋯O5 = 2.8578(12) (bridging), Sn1⋯O6 = 3.0277(13) (bridging), Sn1⋯O7 = 3.0822(12) (terminal); selected bond angles (°): O1–Sn1–O2 = 82.15(4), O1–Sn1–O3 = 85.97(4), O2–Sn1–O3 = 87.49(4); (c) space-filling diagram of a discrete hexamer – top view; (d) space-filling diagram of a discrete hexamer – edge view.

Notably, the CF₃ groups from the six bridging CF₃SO₃⁻ anions all point towards the central void of the metallocyclic hexamer, with the F⋯F distance across the void diameter being 8.019(2) Å (ESI† Fig. S1). These hexamers stack along the crystallographic c-axis, resulting in a structure with hydrophobic CF₃-lined nanochannels. There are no significant interactions between the hexamers. The structure determination showed the void to contain ∼1 disordered hexane molecule per hexamer unit. The crystal has a void volume of 6.1% determined by PLATON.¹⁹ Brunauer–Emmett–Teller (BET) surface area analysis performed on compound (1) after heating the powdered samples in vacuo at 80 °C overnight revealed no appreciable N₂ sorption.

Although the cyclic hexamer is very unusual, it is readily reproduced from different product batches and via crystallisation from different solvent systems. There is no evidence for other polymorphs and powder X-ray diffraction from the bulk product gives a good phase match to the pattern simulated from the single crystal structure (ESI† Fig. S2).

The corresponding Ge(II) and Pb(II) complexes, M(OPMe₃)₃(CF₃SO₃)₂, were also prepared; the former from GeCl₂(dioxane), Me₃SiO₃SCF₃ and OPMe₃ in a 1 : 2 : 3 molar ratio, and the latter directly from Pb(CF₃SO₃)₂ and 3 mol equiv. of OPMe₃. The Ge(OPMe₃)₃(CF₃SO₃)₂ (2) is a low-melting, waxy solid, therefore it was not possible to obtain diffraction data for this compound. Its spectroscopic and microanalytical data are consistent with the formulation shown (Fig. S4†), although it is not possible to confirm from the spectra whether the CF₃SO₃⁻ groups are coordinated. In the case of Pb(II), a white powdered solid was isolated and assigned as [{Pb(OPMe₃)₃(CF₃SO₃)}₂(μ-CF₃SO₃)₂] (3) based on its X-ray crystal structure (below) and elemental analysis, with solution NMR spectroscopic data also consistent with this (Fig. S5†). Despite the stoichiometry of the product being the same as for the Sn(II) analogue, the X-ray crystal determination reveals that it adopts a very different structure from the Sn(II) hexamer.

In this case the Pb(II) complex is a discrete dimer with two CF₃SO₃⁻ bridges (Fig. 2(a)), and with distorted octahedral coordination at each Pb centre via three OPMe₃ ligands (d(Pb–OPMe₃) = 2.3218(17), 2.3295(17), 2.3628(18) Å), and three longer O-bound CF₃SO₃⁻ groups (two μ²-bridging and one terminal) located around 2.8–3.0 Å from the Pb atoms. Fig. 2(b) shows the packing arrangement in the bc-plane, but illustrating that the triflate CF₃ groups decorate the upper and lower surfaces at the interfaces between the layers.

Fig. 2 (a) View of the structure of the [{Pb(OPMe₃)₃(CF₃SO₃)}₂(μ-CF₃SO₃)₂] dimer (3) with the atom labelling scheme. Selected bond lengths (Å): Pb1–O1 = 2.3628(18), Pb1–O2 = 2.3295(17), Pb1–O3 = 2.3218(17), Pb1–O4 = 2.7325(19), Pb1⋯O6 = 2.889(2) Pb1⋯O7 = 2.888(2). Selected bond angles (°): O1–Pb1–O2 = 97.35(6), O1–Pb1–O3 = 76.97(6), O2–Pb1–O3 = 80.09(6); (b) view of the bc plane showing the CF₃ groups aligned on the top and bottom surfaces.

Solution ¹H, ¹³C{¹H} and ³¹P{¹H} NMR spectra (Fig. S3–S5†) for the complexes dissolved in CD₂Cl₂ (for 1 and 2) or CD₃CN (for 3) each show only one OPMe₃ environment, with the ³¹P{¹H} chemical shifts showing a marginal low frequency shift upon going down group 14 from (2; Ge) → (1; Sn) → (3; Pb). The ¹⁹F{¹H} NMR shifts are sharp singlets at ca. −79 ppm in all three cases, consistent with ionic CF₃SO₃⁻,¹⁶ suggesting triflate dissociation in solution. The ¹¹⁹Sn NMR resonance for (1) occurs at −793 ppm, similar to the chemical shifts reported for Sn(II) phosphines and phosphine chalcogenides with SbF₆⁻ anions.²⁰

The formation of the [Sn(OPMe₃)₃(CF₃SO₃)₂]₆ metallocycle identified in this work was very unexpected and the weakly coordinating CF₃SO₃⁻ linkers clearly play an important role in its assembly. Very recently Itoh and co-workers described a series of fluorous oligoamide nanorings with diameters ranging from 0.9–1.9 nm, which undergo supramolecular polymerisation in bilayer membranes forming nanochannels with a similar dense F-atom coverage of the interior walls as observed in our hexamer assemblies. Notably, Itoh et al. showed that the 0.9 nm channels exhibit a water permeation flux two orders of magnitude higher than those of aquaporins and carbon nanotubes.²¹ It should be noted that the covalent oligoamide nanorings are rather more robust compared to the weakly coordinated hexameric assemblies reported in our work. Nevertheless, these results indicate that the incorporation of CF₃ groups can promote highly hydrophobic molecular and supramolecular nanochannels with fluorine-rich interior walls. There is also a growing interest in the development of both fluorous porous materials, as well as Sn-based MOFs for a range of applications.^{4–6,22–24}

Our current work aims to understand the different behaviours observed for (1)–(3) in this work and to explore how modifying the pnictogen and/or the organic substituents influences the speciation with divalent group 14 triflates, and whether altering the denticity and architecture of the pnictine oxide increases the scope for developing other novel assemblies.

We thank the EPSRC for support through the ADEPT Programme Grant (EP/N035437/1) and EP/R513325/1. We also thank Sidrah Hussain for assistance recording the PXRD data and Alice Oakley for the BET measurements.

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. For examples see: Coordination Chemistry of the s, p, and f metals, Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, 2003, vol. 3; S. Aldridge and C. Jones, Modern Main Group Chemistry (special issue), Chem. Soc. Rev., 2016, 45, 763; J. Parr, Germanium, tin and lead, Comprehensive Coordination Chemistry II, ed. J. A. McCleverty and T. J. Meyer, Elsevier, 2003, vol. 3, p. 545, and references therein.
2. G. A. Lawrance, Chem. Rev., 1986, 86, 17.
3. S. H. Strauss, Chem. Rev., 1993, 93, 927.
4. I. Krossing, Angew. Chem., Int. Ed., 2004, 43, 2066.
5. T. A. Engesser, M. R. Lichtenhaler, M. Schleep and I. Krossing, Chem. Soc. Rev., 2016, 45, 789.
6. S. Noro and T. Nakamura, NPG Asia Mater., 2017, 9, e433.
7. A. Kondo, H. Noguchi, L. Carlucci, D. M. Proserpio, G. Ciani, H. Kajiro, T. Ohba, H. Kanoh and K. Kaneko, J. Am. Chem. Soc., 2007, 129, 12362.
8. S. Noro, Y. Hijikata, M. Inukai, T. Fukushima, S. Horike, M. Higuchi, S. Kitagawa, T. Akutagawa and T. Nakamura, Inorg. Chem., 2013, 52, 280.
9. A. Kondo, A. Chinen, H. Kajiro, T. Nakagawa, K. Kato, M. Takata, Y. Hattori, F. Okino, T. Ohba, K. Kaneko and H. Kanoh, Chem. – Eur. J., 2009, 15, 7549.
10. J. M. Slattery, A. Higelin, T. Bayer and I. Krossing, Angew. Chem., Int. Ed., 2010, 49, 3228.
11. I. Krossing, J. Am. Chem. Soc., 2001, 123, 4603.
12. M. Carravetta, M. Concistre, W. Levason, G. Reid and W. Zhang, Chem. Commun., 2015, 51, 9555.
13. M. J. D. Champion, J. M. Dyke, W. Levason, M. E. Light, D. Pugh, H. Bhakhoa, L. Rhyman, P. Ramasami and G. Reid, Inorg. Chem., 2015, 54, 2497.
14. P. A. Rupar, V. N. Staroverov and K. M. Baines, Science, 2008, 322, 1360.
15. P. A. Rupar, V. N. Staroverov, P. J. Ragogna and K. M. Baines, J. Am. Chem. Soc., 2007, 129, 15138.
16. R. P. King, V. K. Greenacre, W. Levason, J. M. Dyke and G. Reid, Inorg. Chem., 2021, 60, 12100.
17. R. P. King, M. S. Woodward, G. McRobbie, J. Grigg, W. Levason and G. Reid, Dalton Trans., 2021, 50, 14400; R. P. King, W. Levason and G. Reid, Dalton Trans., 2021, 50, 17751.
18. K. C. Cairns, W. Levason, G. Reid and W. Zhang, Polyhedron, 2021, 207, 115367; K. C. Cairns, W. Levason, G. Reid and W. Zhang, Polyhedron, 2021, 210, 115529.
19. A. L. Spek, Acta Crystallogr., Sect. D: Biol. Crystallogr., 2009, 65, 148.
20. P. A. W. Dean, Can. J. Chem., 1983, 61, 1795; P. A. W. Dean, D. D. Phillips and L. Polensek, Can. J. Chem., 1981, 59, 50; P. A. W. Dean, Can. J. Chem., 1981, 59, 50.
21. Y. Itoh, S. Chem, R. Hirahara, T. Konda, T. Aoki, T. Ueda, I. Shimada, J. J. Cannon, C. Shao, J. Shiomi, K. V. Tabata, H. Noji, K. Sato and T. Aida, Science, 2022, 376, 738–743.
22. B. Y. Mingbo, S. Renliang, H. Wei, Q. Rongxin and S. He, Colloids Surf., A, 2021, 616, 126322; P. M. Bhatt, Y. Belmabkhout, A. Cadiau, K. Adil, O. Shekhah, A. Shkurenko, L. J. Barbour and M. Eddaoudi, J. Am. Chem. Soc., 2016, 138, 9301; M. R. Tchalala, P. M. Bhatt, K. N. Chappanda, S. R. Tavares, K. Adil, Y. Belmabkhout, A. Shkurenko, A. Cadiau, N. Heymans, G. De Weireld, G. Maurin, K. N. Salama and M. Eddaoudi, Nat. Commun., 2019, 10, 1328.
23. Y. Kamakura, S. Fujisawa, K. Takahashi, H. Toshima, Y. Nakatani, H. Yoshikawa, A. Saeki, K. Ogasawara and D. Tanaka, Inorg. Chem., 2021, 60, 12691.
24. K. Liu, C. Li, L. Yan, M. Fan, Y. Wu, X. Meng and T. Ma, J. Mater. Chem. A, 2021, 9, 27234; X. Zhou, Y. Yu, J. Yang, H. Wang, M. Jia and J. Tang, ChemElectroChem, 2019, 6, 205; X. Jiang, M. Shao, K. Li, L. Ding and M. Zeng, J. Electroanal. Chem., 2022, 912, 116268.

---

Footnote

† Electronic supplementary information (ESI) available: Full experimental, spectroscopic and microanalytical data for the new complexes, together with the X-ray crystallographic parameters and simulated and experimental powder X-ray patterns for the Sn(II) hexamer. Experimental methods and characterisation data are provided in the ESI, along with the crystallographic parameters (Table S1), alternative views of the structure of [Sn(OPMe₃)₃(CF₃SO₃)₂]₆ (Fig. S1), simulated and experimental PXRD patterns for [Sn(OPMe₃)₃(CF₃SO₃)₂]₆ (Fig. S2). Cif files for the crystal structures are available from the Cambridge Crystallographic Data Centre with CCDC numbers 2169639 ([Sn(OPMe₃)₃(CF₃SO₃)₂]₆, 1) and 2169640 ([{Pb(OPMe₃)₃}₂(CF₃SO₃)(μ-CF₃SO₃)₂], 3). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2ce01029a

---