Unprecedented encapsulation of a [Fe^IIICl₄]⁻ anion in a cationic [Fe^II₄L₆]⁸⁺ tetrahedral cage derived from 5,5′′′-dimethyl-2,2′:5′,5′′:2′′,2′′′-quaterpyridine

Abstract

A unique example of incorporation of a tetrahalometalate anion in a small supramolecular cage is described in which a tetrahedral cage of type [Fe₄L₆]⁸⁺ selectively encapsulates a [Fe^IIICl₄]⁻ anion over a [Fe^IICl₄]²⁻ anion in its central cavity to yield a discrete, mixed oxidation state, Fe(II)/Fe(III) supramolecular assembly. This unusual outcome has been achieved using two alternative synthetic strategies.

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In recent years there has been a great deal of attention focused on the design and synthesis of new supramolecular polyhedra.^1–4 In part, this interest may be attributed to the presence of the central cavities in such systems which give rise to the possibility of achieving interesting host–guest chemistry.^5–8 Potential applications for such systems include their use as guest-selective sequestration agents,^9–12 as delivery systems for drugs and unstable molecular species^13,14 and as nano-scale reaction vessels.^5,15 Among the polyhedral host systems reported recently, tetrahedral M₄L₆ host systems^4,7,11–19 have received considerable attention due to their accessibility via predictable metal-directed assembly procedures. Thus, a M₄L₆ host may often be generated from the 2 : 3 interaction of a relatively labile octahedral metal ion with an appropriately designed bis-bidentate bridging ligand. Depending on the choice of ligand and the oxidation state of the metal ion employed, the resulting M₄L₆ host may bear a net positive,^7,12,18 neutral¹⁹ or negative charge.^4,7,16,17 This aspect allows for an additional capacity for tuning M₄L₆ hosts for specific host–guest chemistry applications. For example, Raymond et al.^2,9,14,17,20 have extensively investigated negatively charged M₄L₆ hosts generated from the interaction of tetra-anionic bis-catecholate ligands with selected trivalent octahedral metal ions; their systems have been demonstrated to encapsulate a range of positively charged guest species. Alternatively, Ward et al.,^4,18 amongst others, have investigated positively charged [M₄L₆]⁸⁺ cages generated from the interaction of octahedral metal ions with bis-pyrazolylpyridine ligands and a number of such systems have been demonstrated to readily encapsulate counter-anions.

Studies within our group have also seen the development of both neutral^12,19 and positively charged¹²M₄L₆ hosts. For example, the [Fe₄L₆]⁸⁺ cation (1) was generated from the interaction of Fe^II with 5,5′′′-dimethyl-2,2′:5′,5′′:2′′,2′′′-quaterpyridine (L) (Fig. 1). This system was shown to encapsulate both BF₄⁻ and PF₆⁻ guest species, with uptake selectivity for PF₆⁻ over BF₄⁻ being evident.

Fig. 1 Schematic representation of the assembly of the tetrahedral [Fe₄L₆]⁸⁺host (1) incorporating a guest anion.

We now report the encapsulation of a [Fe^IIICl₄]⁻ anion by 1, which to the best of our knowledge represents the first reported example of such inclusion of a tetrahalometallate anion in a small supramolecular cage. This outcome has been achieved using two (alternative) synthetic strategies.

In our previous report¹² we described the formation of [Fe₄L₆ ⊃ BF₄](BF₄)₇ from the reaction of Fe(BF₄)₂·6H₂O with L in acetonitrile and showed that the BF₄⁻ anion spontaneously occupies the central cavity of the tetrahedral cage. A related synthesis was undertaken as part of the present study in which a solution of L was heated with FeCl₂·5H₂O in acetonitrile at reflux over 24 h to yield a fine suspension of a purple precipitate. On removal of the acetonitrile the resulting solid was dissolved in water and excess KPF₆ was added to yield a deep red precipitate which was then purified by chromatography on Sephadex LH-20 using acetonitrile as eluent. Even though the ¹H NMR spectrum of the purified material showed significant line broadening, it was evident from the spectrum that the product was symmetrical. Microanalysis of the red product was in accordance with the stoichiometry Fe₄L₆·(FeCl₄)·(PF₆)₇·CH₃OH.

Crystals of X-ray quality of the above complex were obtained by diffusion of methanol into an acetonitrile solution of this product. The crystal structure determination showed that the product was a unique solvated tetranuclear complex of type [Fe^II₄L₆ ⊃ Fe^IIICl₄](PF₆)₇ (Fig. 2), in which a tetrahedral [Fe^IIICl₄]⁻ anion (see later) occupies the central cavity (see Section S1 in the ESI†). The above product crystallised in the cubic space groupP4̄3n with each Fe^II centre existing on a 3-fold special position. The asymmetric unit contains 1/12th of the complex; the crystal corresponds to a racemic mixture in which the four chiral pseudo-octahedral Fe^II centres at the apices either have all Δ or all Λ configurations. The internuclear distance between apical Fe^II centres is 9.43 Å, equating to a cavity volume of ∼100 ų.

Fig. 2 (a) Space filling representation of the crystal structure of [Fe₄L₆ ⊃ FeCl₄]⁷⁺ viewed down the C₃-axis of the ΔΔΔΔ enantiomer and (b) schematic representation of the encapsulated tetrahedral [FeCl₄]⁻ anion.

The encapsulated [FeCl₄]⁻ anion has perfect tetrahedral symmetry with Cl–Fe–Cl bond angles of 109.5° and Fe–Cl bond lengths of 2.1994(17) Å that lie within the expected range for a [Fe^IIICl₄]⁻ anion.²¹ For comparison, the corresponding average for [Fe^IICl₄]²⁻ is ∼2.32 Å (although some overlap of the respective Fe^II and Fe^III ranges is evident in the crystallographic literature).^22,23 With respect to the above, it is noted that several examples of the formation of Fe^II metal complexes incorporating associated [Fe^IIICl₄]⁻ counterions derived from FeCl₂ precursor solutions in the presence of air under a variety of conditions have been documented previously.²⁴

Clearly the PF₆⁻ anion, present in solutions of the initial product during its purification, does not successfully compete with [FeCl₄]⁻ for encapsulation. Interestingly, as observed in our previous study, the tetrahedral host [Fe₄L₆]⁸⁺ does, however, exhibit clear encapsulation selectivity for PF₆⁻ over BF₄⁻.¹² This selectivity for [FeCl₄]⁻, taken together with the X-ray structural results, strongly suggest that the size, shape (and symmetry) of the [FeCl₄]⁻ guest is quite complementary to the cavity of 1. Nevertheless, it is noted that the encapsulation of the similarly sized [ZnCl₄]²⁻ was not observed when [Fe₄L₆]([ZnCl₄])₄ was prepared previously in the presence of this anion.¹² Both this result and our current observation are consistent with the observations of Raymond et al.,²⁵ that the larger enthalpy of desolvation associated with doubly charged species tends to make their encapsulation significantly less favourable.

The high resolution mass spectrum of the present product revealed +2 to +7 ions corresponding to the successive losses of PF₆⁻ from a formula corresponding to [Fe₄L₆ ⊃ FeCl₄](PF₆)₇ in agreement with our observation that the encapsulated anionic guest species is indeed [Fe^IIICl₄]⁻ (Fig. 3a; see Section S2 in the ESI†). However, as an aside it is noted that the mass spectrum also contained two additional ion cluster series: a series of ions corresponding to successive losses of PF₆⁻ from a parent species of formula [Fe₄L₆](PF₆)₈ and a further series consistent with the successive losses of PF₆⁻ from a parent species of formula [Fe₄L₆ ⊃ Fe^IICl₄](PF₆)₆; the observed and calculated isotopic distributions of the +3 ions from each series are illustrated in Fig. 3 (b)–(d) The former of these latter two series may be rationalised by the loss of the [FeCl₄]⁻ guest in the electrospray while the latter presumably reflects the occurrence of a redox reaction²⁶ under the ionisation conditions present in the positive ion ESI measurement; consistent with this is the observation that the peak ratio for the peaks arising from [Fe₄L₆ ⊃ Fe^IICl₄](PF₆)₆ and [Fe₄L₆ ⊃ Fe^IIICl₄](PF₆)₇ is varied on changing the applied potential of the capillary exit in the electrospray process.

Fig. 3 (a) The mass spectrum of [Fe₄L₆ ⊃ Fe^IIICl₄](PF₆)₇ illustrating the +2 to +4 ion clusters; (b), (c) and (d) illustrate the observed (top) and calculated (bottom) isotopic distributions consistent with the respective ions:{[Fe₄L₆ ⊃ Fe^IICl₄](PF₆)₃}³⁺, {[Fe₄L₆](PF₆)₅}³⁺ and {[Fe₄L₆ ⊃ Fe^IIICl₄](PF₆)₄}³⁺.

A bulk electrolysis (oxidation) experiment involving [Fe^II₄L₆ ⊃ Fe^IIICl₄](PF₆)₇ in acetonitrile was also undertaken. In an initial study, the cyclic voltammogram showed a single broad wave (E_1/2 = 1.08 V; ΔE_p = 111 mV; 4 e⁻) under the conditions employed (see Section S3 in the ESI†). Based on this result, an oxidation potential of 1.4 V was chosen for the bulk electrolysis experiment. During the progress of the electrolysis the characteristic deep red colour of the tris-bipyridyl Fe^IIchromophore was discharged, with the final colour of the electrolysis solution being light green. The experimentally measured charge (Q) for the oxidation of [Fe^II₄L₆ ⊃ Fe^IIICl₄](PF₆)₇ was 1.692 C (101%) versus the theoretical value for a 4e⁻ (4Fe^II → 4Fe^III) process of 1.673 C. Thus, the results from the bulk electrolysis investigation are in full agreement with the mixed Fe^II/Fe^III formulation for the [Fe^II₄L₆ ⊃ Fe^IIICl₄]⁷⁺ cation discussed above.

An alternative synthetic strategy involving a microwave driven procedure analogous to that described previously by us¹² for obtaining [Fe₄L₆]⁸⁺ free of its included polyatomic anion was repeated (but with methanol substituted for water as the reaction solvent) and the product from this reaction then precipitated by the addition of an ether solution of H[FeCl₄]^27,28 to the reaction solution. Microanalysis of the resulting red product was consistent with the formula [Fe₄L₆ ⊃ FeCl₄](FeCl₄)₇·2CH₃OH·CH₃CN. X-ray quality crystals of this material were obtained by diffusion of Et₂O into an acetonitrile solution of the above product. The product crystallises in the cubic space groupF23 and the structure once again revealed that the product was the M₄L₆ complex of type [Fe₄L₆ ⊃ FeCl₄](FeCl₄)₇ with a tetrahedral [FeCl₄]⁻ anion occupying the central cavity. Importantly, the encapsulated [FeCl₄]⁻ anion again had perfect tetrahedral symmetry with Cl–Fe–Cl bond angles of 109.5° and Fe–Cl bond lengths of 2.2059(8) Å, which are almost identical to those in the ‘external' [FeCl₄]⁻ counterions whose mean Fe–Cl bond length is 2.20 Å. As anticipated, this X-ray result is in complete accordance with the formulation of the corresponding PF₆⁻ salt as [Fe₄L₆ ⊃ Fe^IIICl₄](PF₆)₇ (for which the mean of the Fe–Cl bond lengths, at 2.1997(17) Å, is identical within experimental error to both the above values).

[Fe₄L₆ ⊃ Fe^IIICl₄](FeCl₄)₇·2CH₃OH·CH₃CN is water soluble and the addition of KPF₆ to an aqueous solution of this product led to the isolation of the corresponding PF₆⁻ salt whose microanalysis, mass spectrum and unit cell analysis were in agreement with the results obtained for the earlier-prepared PF₆⁻ salt discussed above.

Finally, in view of the unusual mixed Fe^II/Fe^III oxidation states present in the above unique inclusion products, we embarked on variable temperature investigations (4.2 to 300 K) of the magnetic susceptibility behaviour of both [Fe₄L₆ ⊃ Fe^IIICl₄](PF₆)·CH₃OH and [Fe₄L₆ ⊃ Fe^IIICl₄](FeCl₄)·2CH₃OH·CH₃CN, together with Mössbauer spectral measurements obtained at 78 K. In both cases the results accord well with the oxidation states assigned for the iron centres in each of the above compounds (full details of these studies are presented in Section S5 of the ESI†).

In this report we describe the synthesis and characterisation of a new cationic inclusion complex of type [Fe₄L₆ ⊃ FeCl₄]⁷⁺. This system is especially noteworthy on at least three counts. First, the product is a rare example of a supramolecular mixed-valent Fe(II)/Fe(III) inclusion assembly. Secondly, it also appears unprecedented in that it selectively extracts a [Fe^IIICl₄]⁻ anion from a mix of Fe^II and/or Fe^III chloro species undoubtedly present in the respective reaction solutions under the conditions employed. Finally, to the best of our knowledge it represents a unique example of the incorporation of a tetrahalometallate anion in a small supramolecular cage.

We (GVM, LFL, KSM) thank the Australian Research Council for support. JKC acknowledges a Marie Curie International Incoming Fellowship within the 7th European Community Framework Programme.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The ESI contains experimental details of the synthesis of the inclusion products together with their X-ray structure determinations. Further details of the mass spectral measurements, electrochemistry, magnetic susceptibility and Mössbauer studies are also presented. CCDC 783500 ([Fe₄L₆ ⊃ FeCl₄](FeCl₄)₇·3MeOH·3MeCN·9H₂O)) and 783501 ([Fe₄L₆ ⊃ FeCl₄](PF₆)₇·12H₂O) contain the supplementary crystallographic data for this paper. These can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.]. CCDC reference numbers 783500, 783501. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0sc00523a

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