Assembly of a calix[4]arene-supported Mn^IIIMn^II cluster mediated by halogen interactions

Abstract

A new calix[4]arene-supported Mn^IIIMn^II cluster, formed by the introduction of 3,5-dichlorobenzoate to a system known to afford Mn^III₂Mn^II₂ Single-Molecule Magnets (SMMs), assembles in a layered manner through halogen interactions; structural and magnetic properties of this new cluster are presented.

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Calix[4]arenes (C[4]s) have been used extensively in the formation of supramolecular structures, as well as in various aspects of coordination chemistry due to their polyphenolic nature.¹p-^tBu-calix[4]arene (TBC[4], Fig. 1A) is readily accessible^2a and is a typical starting point for the alteration of the general C[4] framework.^2b The C[4] polyphenolic pocket (at what is termed the lower-rim) is an attractive feature for metal complexation,³ and in this regard we (amongst others) have used TBC4 and related C[4]s for the construction of polynuclear transition metal (TM), lanthanide metal (LnM) and 3d–4f clusters that possess interesting magnetic properties.⁴ In our studies we have discovered a range of structural/cluster motifs, and those most frequently encountered include a) [Mn^III₂Mn^II₂(TBC[4])₂] Single-Molecule Magnets (SMMs),^4b,c b) [Mn^III₄Ln^III₄(C[4])₄] clusters that are magnetic refrigerants or SMMs depending on the lanthanide employed^4f,g and [Cu^II₉(TBC[4])₃] clusters^4d that are versatile anion binding materials.

Fig. 1 A) p-^tBu-calix[4]arene, TBC[4]. B) TBC[4]-supported [Mn^III₂Mn^II₂] SMM motif.^4b,c C) Phenylphosphinate-bridged dimer of TBC[4]-supported [Mn^IIIMn^II] dimers.^4e D) TBC[4]-supported Mn^III₃Mn^II₂ cluster formed with hmp as an ancillary ligand.^4j Colour code: C – grey, O – red, N – royal blue, Mn – purple, P – green. H atoms are omitted for clarity. Figures not to scale.

The synthesis of the TBC[4]-supported Mn^III₂Mn^II₂ cluster shown in Fig. 1B is high yielding and we recently began to investigate the role of ancillary ligands in hybrid TBC[4]-supported cluster formation with a view to disrupting formation of this favourable structural motif. Addition of either sodium phenylphosphinate or 2-(hydroxymethyl)pyridine (hmpH) to the reaction used to form the [Mn^III₂Mn^II₂(TBC[4])₂] SMM affords very different results.^4e,j The former results in a modulated TBC[4]-supported Mn^III₂Mn^II₂ cluster in which two Mn^IIIMn^II dimers are linked by two bridging phenylphosphinates (Fig. 1C).^4e The latter results in a Mn^III₃Mn^II₂ cluster in which both ligands display characteristic metal complexation properties (Fig. 1D).^4j In both examples the TBC[4]s house Mn^III ions within the cavity formed by the four lower-rim oxygen atoms, which also bridge to Mn^II ions located at the centre of the cage; behaviour entirely consistent with other systems involving additional metal ions such as lanthanides.^4f,g

Thia-, sulfinyl- and sulfonyl-bridged C[4]s have also been used in polynuclear metal cluster formation,^5,6 but the resulting materials differ markedly to those formed with methylene-bridged C[4]s due to the influence of the bridging atoms on the resulting coordination chemistry. The Liao^6a–c and Hong^6d,e groups recently used these alternative thia-C[4]-supported TM₄ sub-units as building blocks in a series of studies, where they showed that polybenzoates can been used as ancillary ligands to sequentially direct the formation/assembly of novel metal–organic nanocapsules. Given this, as well as our successful use of ancillary ligands in hybrid TBC[4]-supported cluster formation, we began to investigate the effect of aryl monocarboxylates on the structure of the [Mn^III₂Mn^II₂(TBC[4])₂] SMM motif, prior to investigating topologically directing analogues. Here we report the first result of these studies, obtained by the introduction of sodium 3,5-dichlorobenzoate (Nadcb) to the reaction used to form the [Mn^III₂Mn^II₂(TBC[4])₂] SMM. The benzoate was found to have a marked effect on cluster formation, affording a new [Mn^IIIMn^II(TBC[4])(dcb)(μ-dmso)(dmso)₃(H₂O)] species (1)⁷ that assembles though halogen⋯halogen interactions into bi-layers that are reminiscent of the packing observed in TBC4 solvates.

Reaction of MnCl₂·4H₂O with TBC[4] and Nadcb in a basic dmso solution afforded single crystals of 1 that were suitable for X-ray diffraction studies.‡ The crystals were found to be in a monoclinic cell and structure solution was performed in the space group C2/c. The structure of 1 (Fig. 2) is best described as a mixed valence Mn^IIIMn^II dimer in which Mn1 is in the third oxidation state and bonded centrally within the plane of all four phenoxide oxygens of a tetraanionic TBC[4] (Mn–O range 1.910(7)–1.958(6) Å). The formation of this [Mn^III–TBC[4]]⁻ moiety is expected, and is an extraordinarily common motif observed in our coordination chemistry experiments performed with the C[4] framework. The Jahn–Teller axis deviates from linearity (∠O5–Mn1–O12 = 172.19°) and along this vector Mn1 is bonded to a bridging dmso (Mn1–O5 2.282(6) Å) as well as the oxygen of a disordered ligated solvent (Mn1–O12 2.312(6) Å).⁷ Distorted octahedral Mn2 is in the second oxidation state and is connected to Mn1 by a μ-phenoxide and a μ-dmso (Mn2–O2 2.200(7) Å and Mn2–O5 2.266(6) Å respectively). One aqua ligand (Mn2–O28 2.204(8) Å), two ligated dmso molecules (one of which is disordered over two positions, Mn–O range of 2.12(1)–2.24(1) Å) and a monodentate dcb ligand (Mn2–O10 2.169(8) Å) complete the coordination sphere of Mn2, as shown in Fig. 2. A hydrogen-bonding interaction is observed between the aqua ligand and the non-bonded C=O of the appended dcb, occurring with an O28⋯O11 distance of 2.650 Å.

Fig. 2 Partially labelled structure of 1 showing the 3,5-dichlorobenzoate appended to the Mn^IIIMn^II cluster core, ligation of DMSO in the TBC[4] cavity and both bridging and terminal ligated solvent. Colour code: C – grey, O – red, S – yellow, Mn – purple, Cl – green. H atoms omitted for clarity.

By comparing Figures 1B and 2 it is clear that 1 resembles one half of the Mn^III₂Mn^II₂ butterfly. Thus, incorporation of 3,5-dichlorobenzoate has a profound impact on cluster formation. We recently reported cases of ancillary ligands “interrupting” the assembly process of the [Mn^III₂Mn^II₂(TBC[4])₂] SMM; for example addition of sodium phenylphosphinate resulted in an ‘elongated’ or ‘expanded’ form of the archetypal [Mn^III₂Mn^II₂(TBC[4])₂] SMM.^4e This dimer of dimers (Fig. 1C) is important in the context of the present study as each (ferromagnetically coupled) Mn^IIIMn^II subunit resembles the metallic skeleton in 1. The μ-O atom of dmso in 1 is replaced with a μ-O atom from a μ₃-phosphinate that in turn connects to the symmetry equivalent (s.e.) dimer. The use of 2-(hydroxymethyl)pyridine (hmpH) afforded a ferromagnetic Mn^III₃Mn^II₂ cluster (Fig. 1D) in which the present motif is also found, with hmp occupying the equivalent positions of coordinated water molecule and benzoate anion in 1.^4j These closely related structures all show the versatility of TBC[4] towards cluster formation with ancillary ligands, and demonstrate the retention of common coordination motifs for individual ligand types.

Analysis of the extended structure of 1 reveals that molecules assemble in a head-to-head bilayer that is akin to the packing found in TBC[4] solvates (Fig. 3A and B).^4b,8 Each appended benzoate moiety interdigitates between the cavities of TBC[4] moieties of s.e. molecules of 1 within each layer, thus forming alternating up–down chain type assemblies along the c axis (Fig. 3B). Closer inspection reveals that individual layers are bridged by halogen⋯halogen interactions between s.e. molecules of 1, with two crystallographically unique Cl⋯Cl distances of 3.278(6) and 4.224(7) Å (Fig. 3C); these s.e. molecules are related by a twofold axis such that x, y, z becomes 1 − x, y, 1/2 − z. The halogen bonding is type I and a survey of the literature shows that these distances are similar to those previously reported for aryl chlorides.⁹ The individual layers in 1 are well separated and accordingly display a closest interlayer Mn⋯Mn contact of 14.636 Å. Within the layers adjacent clusters are less well isolated and display closest Mn⋯Mn contacts of 7.669 Å.

Fig. 3 A) Common anti-parallel bi-layer arrangement found for TBC[4] in the solid state.^4b,8 B) Extended structure in 1 showing modulated anti-parallel bilayer packing and intercalation of appended dcb. C) Type I halogen interactions⁹ between symmetry equivalents of 1 across the bi-layer arrangement.

Dc magnetic susceptibility studies on powdered microcrystalline samples of 1 were performed in the 275–5 K temperature range in an applied field of 0.1 T, and are plotted as the χ_MT product versus T in Fig. 4. The room temperature value of 7.195 cm³ K mol⁻¹ is close to the expected 7.375 cm³ K mol⁻¹ for non-interacting Mn^III and Mn^II ions with g = 2.00. As the temperature is decreased the χ_MT product increases, reaching a maximum value of ~10.7 cm³ K mol⁻¹ at 5 K. This behaviour is suggestive of weak ferromagnetic exchange between the metal ions. For the interpretation of the magnetic properties of 1 we employed the following spin-Hamiltonian:

where J is the isotropic exchange interaction parameter, Ŝ is a spin-operator, i runs from 1 to 2, μ_B is the Bohr magneton, B is the applied magnetic field, g = 2 is the g-factor of the Mn ions, D is the uniaxial anisotropy parameter of Mn^III and S = 2 and S = 5/2 are the electronic spins of Mn^III and Mn^II, respectively. The χ_MT product and the magnetisation versus field data at temperatures between 2 and 7 K in magnetic fields ranging from 2 to 7 T (inset of Fig. 4), were simultaneously fitted to spin-Hamiltonian (1) by use of a simplex algorithm,¹⁰ affording the best-fit parameters J = +0.38 cm⁻¹ and D_Mn(III) = −3.34 cm⁻¹. These values are entirely consistent with other Mn–TBC[4] cages.^4b,c,e Alternating current magnetic susceptibility measurements were performed on a polycrystalline sample of 1 in the 1.8–10 K range in zero applied dc field and a 3.5 G ac field oscillating in the 50–1000 Hz frequency range (Fig. S1†). 1 does display frequency-dependent out-of-phase (χ_M′′) signals suggestive of SMM behaviour but no peaks are observed down to 2 K, precluding a more detailed analysis.

Fig. 4 Plot of χ_MT vs. T for complex 1 in an applied field of 0.1 T. Inset: magnetisation versus field in the 2–7 K and 2–7 T temperature and field ranges. The solid red lines are a fit of the experimental data to spin-Hamiltonian (1). See text for details.

Conclusions

We have synthesised and characterised a [Mn^IIIMn^II] cluster that results from addition of sodium 3,5-dichlorobenzoate to the reaction used to form C[4]-supported [Mn^III₂Mn^II₂] SMMs. The appended benzoate has disrupted formation of the SMM motif and the resulting cluster can be regarded as a half of this common structural type. The new [Mn^IIIMn^II] cluster reported here, despite having an unusual shape, packs so as to mimic the solid state behaviour of both TBC[4] and the [Mn^III₂Mn^II₂] SMMs, with assembly occurring through halogen bonding interactions which bridge bi-layers. The magnetic behaviour of the dimer is also analogous to the parent butterfly structures, the exchange between the Mn^III and Mn^II ions being weakly ferromagnetic. This study has used ancillary ligands containing one carboxylate on the aromatic ring with a view to establishing structure/cluster altering viability of this ligand type. We will now focus on using polytopic benzoates in order to form a new family of molecules with tailored geometries and magnetic properties. Given the importance of halogen bonding in driving assembly¹¹ we will also investigate the structurally directing role of halogens placed at the C[4] upper-rim; this will be undertaken with a view to controlling the arrangement of [Mn^III₂Mn^II₂] SMMs in the solid state relative to TBC[4]-supported analogues. The results of these new avenues of investigation will be reported in due course.

Acknowledgements

We thank EPSRC for financial support of this work (EP/I03255X/1 & EP/I031421/1).

Notes and references

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3. A significant amount of work has been reported in which various groups have used the C[n] lower-rim as a platform to synthesise novel transition, lanthanide and alkali metal complexes (and in some cases clusters) using air sensitive rather than ambient techniques/conditions. For representative papers please see: A. J. Petrella and C. L. Raston, J. Organomet. Chem., 2004, 689, 4125; D. M. Homden and C. Redshaw, Chem. Rev., 2008, 108, 5086; E. D. Gueneau, K. M. Fromm and H. Goesmann, Chem. Eur. – J., 2003, 9, 509; G. Guillemot, B. Castellano, T. Prangé, E. Solari and C. Floriani, Inorg. Chem., 2007, 46, 5152; G. Guillemot, E. Solari, C. Rizzoli and C. Floriani, Chem. – Eur. J., 2002, 8, 2072; U. Radius and J. Attner, Inorg. Chem., 2004, 43, 8587 , and references therein.
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7. There is disorder present within the TBC[4] cavity with respect to the solvent coordinated to Mn1. In the interests of brevity, discussion has been limited to one species..
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Footnotes

† Electronic supplementary information (ESI) available. CCDC 995697. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ce00729h

‡ TBC[4] was synthesised according to literature procedure,^2a while 3,5-dichlorobenzoic acid was purchased from Sigma-Aldrich. Synthesis of 1: MnCl₂·4H₂O (0.15 g, 0.75 mmol), TBC[4] (0.1 g, 0.15 mmol), Nadcb (0.213 g, 1.0 mmol) and NEt₃ (0.1 g, 1.0 mmol) were dissolved in a mixture of DMSO (10 ml) and EtOH (10 ml). After 2 hours of stirring the solution was filtered an allowed to stand; crystals grew upon evaporation of the mother liquor over several days (23 mg, 12%). Elemental analysis (%) calculated for 1, C₆₀H_84.6Cl₂O_12.3S_3.3Mn₂: C, 55.89%; H, 6.61%. Found: C, 55.97%; H, 6.52%. General crystallographic details: data were collected on a Bruker X8 Apex II CCD Diffractometer operating at 100(2) K with Mo-Kα radiation (λ = 0.71073 Å). Crystal data (CCDC 995697): C₆₀H_84.60Cl₂O_12.30S_3.30Mn₂, M = 1289.25, black block, 0.20 × 0.15 × 0.15 mm³, monoclinic, space group C2/c, a = 38.315(3), b = 12.5251(10), c = 29.193(2) Å, β = 109.401(2)°, V = 13214.2(18) ų, Z = 8, 2θ_max = 41.74°, 22 773 reflections collected, 6899 unique (R_int = 0.0679). Final GooF = 1.575, R₁ = 0.0930, wR₂ = 0.2546, R indices based on 4417 reflections with I > 2sigma(I) (refinement on F²). Unit cells of several single crystals and further analysis by powder X-ray diffraction (Fig. S2†) confirmed the bulk composition to be 1.

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