Open Access Article
This Open Access Article is licensed under a
Creative Commons Attribution 3.0 Unported Licence

Unexpected neutral aza-macrocycle complexes of sodium

Matthew Everett , Andrew Jolleys , William Levason , David Pugh and Gillian Reid *
School of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK. E-mail: G.Reid@soton.ac.uk; Tel: +44 (0)23 8059 3609

Received 24th February 2014 , Accepted 15th April 2014

First published on 15th April 2014


Abstract

Highly unusual Na+ complexes with neutral tri- and tetra-amines are isolable in good yield from the reaction of NaBArF with the amine in organic media. Structural characterisation reveals primary Na–N bonding, including an unusual sandwich cation [Na(Me3tacn)2]+, derived from homoleptic N6-coordination via two Me3-tacn ligands, and the distorted 5-coordinate [Na(thf)(Me4cyclam)]+.


Group 1 metals form hard metal cations with a very strong affinity for both anionic and neutral O-donor ligands. This includes the crown ether macrocycles, with the size of the binding cavity in the macrocycle dictating which alkali metal cation is preferentially coordinated.1 These complexes are much more soluble in organic solvents than typical Group 1 salts, hence crown ethers often find application as phase transfer catalysts.1c Additionally, by encapsulating the Group 1 cation inside a hydrophobic shell and eliminating cation–anion interactions, the nucleophilicity of the associated anion can be increased, leading to increased utility in organic transformations.1d

N-donor macrocycles can impart extra stability to alkali metal complexes when compared to the analogous crown ethers. Dye et al. used an alkylated N-donor cryptand to encapsulate sodium cations as the key part of a system of room temperature stable electrides.2a It is notable that cryptands containing O-donors form electrides which are unstable above −60 °C owing to cleavage of the C–O functionalities.2b Despite this, compounds of neutral N-donor macrocyclic ligands with alkali metal cations other than Li+ are not well studied. Several examples exist where N-donor macrocycles have been functionalised with anionic or neutral donor pendant arms and coordinated to alkali metal cations,3 but there are no structurally authenticated examples of neutral N-donor macrocycles coordinating to K+, Rb+ and Cs+. Further, only two structurally authenticated examples with Na+ have been reported: Nöth and co-workers coordinated 1,5,9-trimethyl-1,5,9-trizacyclododecane to NaBH4, and Okuda et al. obtained an unexpected complex of NaI with 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane (Me3cyclen).4

As part of our recent work on coordination complexes of silicon5 and germanium6 as potential precursors for supercritical fluid electrodeposition (SCFED) processes,7 we have been utilising the sodium salt of tetrakis(3,5-bis{trifluoromethyl}phenyl)borate (hereafter referred to as [BArF]) both as a supporting electrolyte for use in supercritical fluids7b,c and as a halide abstractor in the presence of neutral N-donor ligands (Scheme 1). Unexpectedly, we found that these N-donor ligands have a high affinity for Na+ under certain conditions. This resulted in coordination of the polyamine to Na+ rather than halide abstraction from SiCl4 or [GeCl2(1,4-dioxane)]. From the reaction of SiCl4, NaBArF·2(thf) and N,N,N′,N′′,N′′-pentamethyldiethylenetriamine (pmdta) we were able to isolate compound 1, [Na(pmdta)(thf)(κ1-BArF)], as colourless crystals suitable for X-ray diffraction (Fig. 1).


image file: c4cc01407c-s1.tif
Scheme 1 Unexpected formation of 1 from the attempted use of NaBArF as a halide abstractor. B = remainder of [BArF] anion (omitted for clarity).

image file: c4cc01407c-f1.tif
Fig. 1 ORTEP representation of 1 with ellipsoids at 50% probability. H atoms and positional disorder of three CF3 groups and the thf ring are omitted for clarity. The CF3 group interacting with Na1 is disordered and only one refined position for the CF3 group (containing F15a) is shown. Selected bond lengths (Å): N1–Na1 2.473(5), N2–Na1 2.452(5), N3–Na1 2.477(5), O1–Na1 2.262(5), F15a–Na1 2.60(1), F15b–Na1 2.349(6) (not shown).

It was also possible to synthesise compound 1 directly, by adding a solution of the ligand in CH2Cl2 to a suspension of NaBArF·2(thf) in CH2Cl2.8 Complete dissolution occurred upon addition of the ligand and 1 was isolated as a white solid via precipitation with hexane.

Compound 1 crystallises in the monoclinic space group Cc with evidence for positional disorder present in the coordinated thf molecule as well as in some of the CF3 groups of the anion. The latter is not unusual in weakly coordinating anions which contain CF3 groups, but the [BArF] anion in particular is notorious for being disordered in its solid state structures.9 Nonetheless, the structure of the molecule is unambiguous with one tridentate pmdta ligand, one thf molecule and one κ1 Na⋯F interaction from a CF3 group completing a 5-coordinate, distorted square-based pyramidal geometry at Na+. The Na–N bond distances are significantly longer than the Na–O bond distance, reflecting the strong oxophilicity of Na+, but they are among the shorter range of known Na–N bond lengths in the CSD.10 The κ1 interaction between a [BArF] CF3 group and Na+ is only the second example of [BArF] interacting with any metal,11 although the positional disorder of the CF3 group in compound 1 precludes an accurate comparison of Na⋯F bond distances. However, the observed Na⋯F distances in 1 are only slightly longer than the sum of the ionic radii for Na+ and F (1.02 and 1.33 Å respectively), and well within the sum of van der Waals radii for Na and F (3.96 Å).12

Analysis of the product (1H and 13C{1H} NMR spectroscopy, elemental analysis) supported the formulation observed crystallographically, although it is noteworthy that the signals for the triamine ligand in the 1H NMR spectrum were very broad at room temperature as a result of the amine-coordinated cations undergoing fast ligand exchange in solution, as is typical of labile alkali metal complexes. Solution 23Na NMR spectroscopy showed a signal at +2.1 ppm (relative to a 0.1 mol dm−3 solution of NaCl in D2O), which is ∼6 ppm to high frequency of NaBArF·2(thf) (−4.8 ppm in CH2Cl2) and reflects the difference between a homoleptic O-donor environment at Na and one with significant N-donation.

Following the new procedure,8 two complexes of NaBArF with the macrocycle 1,4,7-trimethyl-1,4,7-triazacyclononane (Me3tacn) were also synthesised using a 1[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio of macrocycle to Na (2a) and a 2[thin space (1/6-em)]:[thin space (1/6-em)]1 ratio (2b). Whilst crystals of compound 2a did not give a viable X-ray data set, spectroscopic analysis indicated that a product with similar composition to compound 1 was obtained and elemental analysis supported the formulation [Na(Me3tacn)(thf)][BArF]. 1H NMR spectroscopy at room temperature showed broad resonances for the macrocycle protons. The 23Na NMR shift of +3.7 ppm is similar to that observed from compound 1, consistent with N-donor atom coordination to Na+.

Compound 2b, [Na(Me3tacn)2][BArF], crystallises in the monoclinic space group C2/c with two symmetry-independent half cations and one disordered anion comprising the asymmetric unit. One half-cation also contained positional disorder of the ethylene linkers within the macrocyclic ring, but the nitrogen atoms were not disordered and the coordination environment around both half cations is clearly that of a sandwich complex with two macrocycles surrounding one Na+ cation. Despite the oxophilicity of the alkali metal cations, no thf is retained in the structure, presumably due to the strong preference for the Me3tacn ligand for tridentate coordination (Fig. 2).


image file: c4cc01407c-f2.tif
Fig. 2 ORTEP representation showing one of two crystallographically distinct cations in the asymmetric unit of compound 2b. A picture of the major disorder component from the other cation is shown in the ESI (Fig. S1). Thermal ellipsoids at 50% probability. The BArF anion, which does not interact with the Na+ centre, and the H atoms are omitted for clarity. Selected bond lengths (Å): N1–Na1 2.554(5), N2–Na1 2.527(5), N3–Na1 2.545(5) (this cation not shown), N4–Na2 2.507(4), N5–Na2 2.600(5), N6–Na2 2.567(5). Range of cis N–Na–N angles within the same macrocyclic ring (°): 70.5(2)–72.0(2), between neighbouring rings: 108.1(2)–109.5(2). Symmetry code: −x + 1/2, −y + 1/2, −z.

The Na–N bond distances for 2b are slightly longer (by ∼0.1 Å) than those in 1. This may simply be due to the increased coordination number at Na+. The solution 23Na NMR spectrum shows a singlet at +6.2 ppm.

It is notable that 2b is only the second structurally characterised example of a sandwich complex of any metal with Me3tacn as ligand. The other was reported by Wieghardt et al. with Ag(I), which has a larger ionic radius than Na(I) (1.15 Å and 1.02 Å, respectively, for 6-coordination).13 The cis N–Na–N angles around Na are significantly distorted away from the ideal 90°; those within a ring are about 40° smaller than those between neighbouring macrocycles and this is almost certainly due to steric repulsion between the methyl groups on alternating macrocyclic rings causing the Na–N bond distance to increase, thus reducing the intra-macrocycle bond angles. This is consistent with the Ag(I) structure reported by Wieghardt which also showed a large disparity between the inter-ring and intra-ring N–M–N angles.

We observed that Me3tacn was readily protonated in the presence of even trace water; one attempted synthesis of 2a was inadvertently exposed to the atmosphere forming the hydrolysis product [Me3tacnH][BArF] (see ESI). This suggests that judicious choice of conditions (solvent and anion) is required in order to access the potentially very rich coordination chemistry of alkali metal cations with a wider range of Lewis bases.

Reaction of NaBArF·2(thf) with the tetradentate N-donor macrocycle, Me4cyclam (1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) yielded colourless crystals of 3. Structural analysis (Fig. 3) revealed a tetradentate coordination mode for the macrocycle, with the Me groups in the ‘all-up’ configuration, directed towards the same side of the N4 plane as the metal, with one thf molecule completing a distorted 5-coordinate geometry at sodium. The sodium cation lies above the N4-plane by 0.889(1) Å, reflecting a mismatch between the Na+ cation size and the macrocyclic binding cavity. The τ value14 of 0.44 indicates that the geometry at Na+ is almost half-way between ideal square based pyramidal and ideal trigonal bipyramidal. The Na–O bond length is about 0.05 Å longer than the corresponding distance in compound 1, while the Na–N bond lengths are identical to 1 (within experimental error) and they are also in accord with the Na–N bond lengths found in [Na(Me3cyclen)I], the only other neutral tetradentate macrocyclic ligand coordinated to sodium.4b


image file: c4cc01407c-f3.tif
Fig. 3 ORTEP representation of the cation of compound 3. Thermal ellipsoids at 50% probability. The BArF anion, which does not interact with the Na+ centre, and the H atoms are omitted for clarity. Selected bond lengths (Å) and angles (°): N1–Na1 2.438(2), N2–Na1 2.454(2), N3–Na1 2.451(2), N4–Na1 2.473(2), O1–Na1 2.313(2). N1–Na1–N3 123.91(7), N2–Na1–N4 150.33(7), N1–Na1–N2 77.94(7), N1–Na1–N4 89.49(7), N2–Na1–N3 88.00(6), N3–Na1–N4 76.82(6).

Although the amine-coordinated cations are very likely to be undergoing fast ligand exchange in solution, as is typical of labile alkali metal complexes, 23Na NMR studies show that the Na–N coordination is clearly detectable in solution. 23Na NMR studies of homoleptic O-donor complexes (e.g. NaBPh4 with dibenzo-18-crown-6 and dibenzo-24-crown-8) show δ(23Na) to low frequency of the zero reference, typically ca. 0 to −16 ppm.15 However, for complexes 1–3, δ(23Na) is significantly to high frequency (to ca. +10 ppm). This effect of N-coordination on δ(23Na) was also seen by Lin and Popov, who measured δ(23Na) of NaBPh4 with 15-crown-5 in a variety of solvents.16 With O-donor solvents, δ(23Na) was always to low frequency of the reference irrespective of the concentration of crown ether, but when pyridine was used as the solvent with a low concentration of crown ether (i.e. significant N-coordination to Na is present), a shift to high frequency relative to the reference was observed. Thus it seems that the 23Na chemical shifts are sensitive to both the nature and number of the donor atoms coordinated.

We have shown that neutral alkylated multidentate N-donor ligands have a surprisingly high affinity for Na+ cations. The reagent NaBArF readily forms complexes with N-donor ligands in preference to abstracting a halide from Lewis acids such as SiCl4 and [GeCl2(1,4-dioxane)], and the complexes have also been synthesised without the Lewis acid. The complexes exhibit a rich range of structural interactions, including a very rare example of two Me3tacn macrocycles forming a sandwich complex and only the second example of a [BArF] anion interacting with a metal. The successful formation of these new species may be a result of the low lattice energy of NaBArF when compared to other Group 1 ionic salts, coupled with the high solubility of the [BArF] anion in organic media which do not compete with the N-donor ligand, and suggests that judicious choice of solvent and anion may lead to the unveiling of a rich area of coordination chemistry of softer donor ligands with hard Group 1 cations. Further investigations into extending this study to other alkali metal cations and ligands are underway.

This work was funded by EPSRC (through a Programme Grant EP/I033394/1 and also through EP/I010890/1). The SCFED Project (http://www.scfed.net) is a multidisciplinary collaboration of British universities investigating the fundamental and applied aspects of supercritical fluids.

Notes and references

  1. (a) J. W. Steed and J. L. Atwood, Supramolecular Chemistry, Wiley, West Sussex, 2nd edn, 2009, p. 116 Search PubMed; (b) J. S. Bradshaw, R. M. Izatt, A. V. Bordunov, C. Y. Zhu and J. K. Hathaway, in Comprehensive Supramolecular Chemistry, ed. J. L. Atwood, J. E. D. Davies, D. D. MacNicol and F. Vögtle, Elsevier, Oxford, 1996, p. 35 Search PubMed; (c) T. Hashimoto and K. Maruoka, in Asymmetric Phase Transfer Catalysis, ed. K. Maruoka, Wiley, Weinheim, 2008, p. 1 Search PubMed; (d) J. McMurry, Organic Chemistry, Brooks/Cole, California, 8th edn, 2010, p. 690 Search PubMed.
  2. (a) M. Y. Redko, J. E. Jackson, R. H. Huang and J. L. Dye, J. Am. Chem. Soc., 2005, 127, 12416 CrossRef CAS PubMed; (b) P. M. Cauliez, J. E. Jackson and J. L. Dye, Tetrahedron Lett., 1991, 32, 5039 CrossRef CAS.
  3. See, for example: (a) G. R. Giesbrecht, A. Gebauer, A. Shafir and J. Arnold, J. Chem. Soc., Dalton Trans., 2000, 4018 RSC; (b) H. Misake, H. Miyake, S. Shinoda and H. Tsukube, Inorg. Chem., 2009, 48, 11921 CrossRef PubMed.
  4. (a) H.-H. Giese, T. Habereder, H. Nöth, W. Ponikwar, S. Thomas and M. Warchhold, Inorg. Chem., 1999, 38, 4188 CrossRef CAS; (b) S. Standfuss, T. P. Spaniol and J. Okuda, Eur. J. Inorg. Chem., 2010, 2987 CrossRef CAS.
  5. (a) K. George, A. L. Hector, W. Levason, G. Reid, G. Sanderson, M. Webster and W. Zhang, Dalton Trans., 2011, 40, 1584 RSC; (b) W. Levason, D. Pugh and G. Reid, Inorg. Chem., 2013, 52, 5185 CrossRef CAS PubMed.
  6. (a) M. F. Davis, W. Levason, G. Reid and M. Webster, Dalton Trans., 2008, 2261 RSC; (b) F. Cheng, A. L. Hector, W. Levason, G. Reid, M. Webster and W. Zhang, Inorg. Chem., 2010, 49, 752 CrossRef CAS PubMed; (c) P. N. Bartlett, C. Y. Cummings, W. Levason, D. Pugh and G. Reid, Chem. – Eur. J., 2014, 20, 5019–5027 CrossRef CAS PubMed.
  7. (a) J. Ke, W. Su, S. M. Howdle, M. W. George, D. Cook, M. Perdjon-Abel, P. N. Bartlett, W. Zhang, F. Cheng, W. Levason, G. Reid, J. Hyde, J. Wilson, D. C. Smith, K. Mallik and P. Sazio, Proc. Natl. Acad. Sci. U. S. A., 2009, 106, 14768 CrossRef CAS PubMed; (b) P. N. Bartlett, D. C. Cook, M. W. George, J. Ke, W. Levason, G. Reid, W. Su and W. Zhang, Phys. Chem. Chem. Phys., 2011, 13, 190 RSC; (c) J. Ke, P. N. Bartlett, D. Cook, T. L. Easun, M. W. George, W. Levason, G. Reid, D. Smith, W. Su and W. Zhang, Phys. Chem. Chem. Phys., 2014 10.1039/C3CP54955K.
  8. A typical procedure is as follows: to a suspension of NaBArF·2(thf) (240 mg, 0.23 mmol) in anhydrous CH2Cl2 (5 mL) was added a solution of the ligand (1 mol eq.) in CH2Cl2 (2 mL). The solution was stirred under N2 for 4 h, then layered with anhydrous hexane (30 mL) to produce colourless crystals. Full experimental details, crystallographic and analytical data for all products are contained in the ESI.
  9. A. B. Chaplin and A. S. Weller, Eur. J. Inorg. Chem., 2010, 5124 CAS.
  10. F. H. Allen, Acta Crystallogr., Sect. B: Struct. Sci., 2002, 58, 380 CrossRef PubMed.
  11. S. I. Pascu, T. Jarrosson, C. Naumann, S. Otto, G. Kaiser and J. K. M. Sanders, New J. Chem., 2005, 29, 80 RSC.
  12. S. Alvarez, Dalton Trans., 2013, 42, 8617 RSC.
  13. C. Stockheim, K. Wieghardt, B. Nuber, J. Weiss, U. Flörke and H.-J. Haupt, J. Chem. Soc., Dalton Trans., 1991, 1487 RSC.
  14. A. W. Addison, T. N. Rao, J. Reedijk, J. van Rijn and G. C. Verschoor, J. Chem. Soc., Dalton Trans., 1984, 1349 RSC.
  15. (a) H. D. H. Stöver, A. Deville and C. Detellier, J. Am. Chem. Soc., 1985, 107, 4167 CrossRef; (b) A. Deville, H. D. H. Stöver and C. Detellier, J. Am. Chem. Soc., 1987, 109, 7293 CrossRef.
  16. J. D. Lin and A. I. Popov, J. Am. Chem. Soc., 1981, 103, 3773 CrossRef CAS.

Footnote

Electronic supplementary information (ESI) available: Full experimental details, plus all the crystallographic and analytical data for all products. CCDC 987718–987721. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc01407c

This journal is © The Royal Society of Chemistry 2014