Guest-induced magnetic exchange in paramagnetic [M 2 L 4 ] 4+ coordination cages †

Paramagnetic complexes that possess magnetically switchable properties show promise in a number of applications. A signi ﬁ cantly underdeveloped approach is the use of metallocages, whose magnetic properties can be modulated through host – guest chemistry. Here we show such an example that utilises a simple [Cu II2 L 4 ] 4+ lantern complex. Magnetic susceptibility and magnetisation data shows an absence of exchange in the presence of the diamagnetic guest tri ﬂ ate. However, replacement of the bound tri ﬂ ate by ReBr 62 − switches on antiferomagnetic exchange between the Cu and Re ions, leading to an S = 1/ 2 ground state for the non-covalent complex [ReBr 62 − ⊂ Cu II2 L 4 ] 2+ . Comparison of this complex to a “ control ” palladium-cage host – guest complex, [ReBr 62 − ⊂ Pd II2 L 4 ] 2+ , shows that the encapsulated ReBr 62 − anions retain the same magnetic anisotropy as in the free salt. Theoretically calculated spin-Hamiltonian parameters are in close agreement with experiment. Spin density analysis shows the mode of interaction between the Cu II and Re IV centres is through the Re-Br ⋯ Cu pathway, primarily mediated through the Cu(d x 2 − y 2 )|Br sp |Re(d yz ) interaction. This is further supported by overlap integral calculations between singly occupied molecular orbitals (SOMOs) of the paramagnetic ions and natural bonding orbitals analysis where considerable donor-to-acceptor interactions are observed between hybrid 4s4p orbitals of the Br ions and the empty 4s and 4p orbitals of the Cu ions.


Introduction
3][4][5][6][7] It has been investigated in multiple contexts, as it's host-guest chemistry can be tuned to bind anions, 8 as well as neutral species. 9,10This versatility has allowed it to be exploited for a number of applications that involve the binding of drug 5 and imaging molecules, 11 as well as substrates for catalysis. 12,13Paramagnetic M 2 L 4 lantern cages are much less well explored, 14 indeed investigations of the magnetic behaviour of any supramolecular cages remains virtually unexplored. 15However, the host-guest chemistry of these systems offers a range of potential advantages for the exploitation of magnetic materials properties.These include, for example, the reversible inducement of magnetic exchange interactions, the encapsulation of unstable/reactive molecules or those with unusual geometries/coordination numbers, solid-state dilution, and the tuning of magnetic anisotropy.Such properties are sought-after for the construction of singleion 16 and single-molecule magnets, 17 electron-spin based qubits, 18 and may find application in magnetic sensing, switching and molecular recognition. 19Functionalisation of the organic framework would also aid surface deposition and the transformation of 0D molecular cages to 2D sheets and/or 3D MOFs imbued with the same physical properties.
Successful ingress of a magnetic guest into a magnetic host may have little effect on magnetic properties if there are no significant interactions between the two, nor any geometrical change in either component.However, this is unlikely if size/ symmetry/electrostatic matching is efficient.Encapsulation may solely induce structural changes to the host/guest and this has previously been shown to have significant impact upon, for example, the magnetic anisotropy of 3d transition metal ions in magnetic MOFs, 20 and the high spin-low spin transition temperature in spin crossover materials. 21Indeed, recent studies of single-ion magnets have shown how crucial geometry is in determining magnetisation relaxation dynamics, 22 and thus metallosupramolecular cages could play a key role here if their internal cavity can be designed to suit a specific d/f metal ion geometry.Covalent bonding through an intervening organic/inorganic ligand or a short dipolar interaction between the metal ions in the host and guest will mediate a magnetic exchange interaction, the sign and strength of which can be controlled by the nature of the linker and the identity of the metal ions.This then allows for control over the magnetic ground/excited states of the cage, the manipulation of which underpins application in a breadth of technologies. 23nteresting potential guest molecules include the rhenium(IV) hexahalides, [ReX 6 ] 2− , that possess very large spin-orbit coupling constants (λ ∼ 1000 cm −1 in the free ion) that results in significant magnetic anisotropy.In addition, spin delocalisation of the electron density from the metal to the halide imparts significant Re-X⋯X-Re intermolecular exchange interactions which can be strong enough to induce magnetic order in the salts of these anions at relatively high temperatures. 24,25or example, K 2 [ReBr 6 ] shows antiferromagnetic order below 14 K. 26 Here we show that the [ReBr 6 ] 2− anion can be encapsulated inside a paramagnetic [Cu

Magnetic properties
Direct current magnetic susceptibility (χ) and magnetisation (M) data for 1-3 were measured in the T = 270-2 K, B = 0.1 T and T = 2-7 K, B = 0.5-5.0T temperature and field ranges, respectively.These are plotted as the χT product versus T and M versus B in Fig. 2. For the quantitative interpretation of the magnetic properties of 1-3 we used spin-Hamiltonian (1): where the first term corresponds to the single ion axial anisotropy of the Re IV ion, the second term is the Zeeman effect of the applied magnetic field, and the third term the exchange interaction between the constituent (Cu II , Re IV ) metal centres.
The χT product and the variable temperature variable field magnetisation data for 1-3 were simultaneously fitted to spin-Hamiltonian (1), affording the best fit parameters collected in Table 1.
In complex 1, the two Cu II ions sit at a distance of ∼11.5 Å and even though they are connected by a conjugated organic ligand (L) one would not expect to see any significant magnetic interaction between them.This is reflected in the χT data which is invariant with temperature, and both the susceptibility and magnetisation data can be simultaneously fitted with J Cu-Cu = 0.0 cm −1 with g Cu = 2.095.The susceptibility and magnetisation data for 2 are near identical to those reported in the literature for [ReBr 6 ] 2− salts. 24This is to be expected since there has been little distortion to its geometry upon encapsulation, as shown in the overlay plots in Fig. S10 † and shape analysis (Table S5 †). 28hus a simultaneous fit of the susceptibility and magnetisation data affords g Re = 1.832,D Re = +21.9cm −1 , in agreement with both previously published experimental 24 and theoretical values. 29In order to the fit the data for 3, the g Cu , g Re and D Re from the fits of compounds 1 and 2 were fixed, and only the J Cu-Re interaction allowed to vary.A simultaneous fit of the susceptibility and magnetisation data to spin-Hamiltonian (1) afforded J Cu-Re = −0.45cm −1 .The data cannot be fitted without including exchange between the Cu and Re metal ions.Thus, the experimental magnetic data suggests: (a) there is no magnetic interaction between the Cu II centres (at least for the sensitivity of a SQUID magnetometer).(b) There is a small but finite antiferromagnetic interaction between the Cu II -Re IV ions.(c) Analogous D Re values to previously published Re IV metal salts are observed for the encapsulated ReBr 6 2− ions, due to retention of analogous/non-distorted structures.In order to probe these details further, we now turn to theory. .See the main text and Table 1 for the best fit parameters.
Table 1 Experimental and calculated best fit parameters for the magnetic exchange ( J) and axial zero-field splitting (D Re ) parameters in complexes 1-3  S6 and Fig. S11, S12 †). 30 In order to estimate the J Cu-Cu and J Cu-Re exchange interactions DFT calculations have been performed on complexes 1 and 3.

Fig. 2
Fig. 2 (a) Magnetic susceptibility data for 1-3 measured in an applied field, B = 0.1 T. (b-d) Magnetisation data for 1-3, respectively, in the T = 2-7 K range in fields up to, B = 5 T. The solid lines represent the simultaneous fits of the susceptibility and magnetisation data to spin-Hamiltonian (1).See the main text andTable 1 for the best fit parameters.

3
suggests that the major contribution to D Re comes from the spin-flip transition |d yz/xz → d xy |.Due to the relatively weak π-donor nature of the Br ion, the splitting between the d yz/xz and d xy orbitals is found to be rather small, facilitating strong in-plane anisotropy (Table II centres (Cu(d x 2 −y 2)|Br3|Re(d yz ); Fig. 3, S14, Table S7 †) leading to an antiferromagnetic interaction.In the other two possible interactions (Cu(d x 2 −y 2)||Re(d xz ) and Cu(d x 2 −y 2)||Re(d xy )), the Re IV magnetic orbitals (d xz/xy ) are not directly interacting with the Cu II (d x 2 −y 2) orbital, rather they interact through the long, extended π/π* orbitals of L (Fig. S14 and Table ).ConclusionsLantern-like guest⊂[M 2 L 4 ] 4+ coordination cages, normally associated with diamagnetic metals ions such as Pd II , can also be made with paramagnetic M II ions, here Cu II , through simple self-assembly of four molar equivalents of the ligand molecule (L = 1,3-bis(3-ethynylpyridyl)benzene) with two molar equivalents of the corresponding metal salt, followed by one molar equivalent of guest, forming [Cu II 2 L 4 (H 2 O)(OTf ) 3 ] (OTf )•MeCN (1), ReBr 6 ⊂[Pd II 2 L 4 ](BF 4 ) 2 (2) and ReBr 6 ⊂[CuII 2 L 4 (OTf ) 2 ] (3), respectively.Complex formation was confirmed by ESI-MS and 1 H NMR, both revealing solution stability.Magnetic measurements combined with theoretical calculations show that: (a) there is no magnetic interaction between the Cu II ions in the "empty" [Cu 2 L 4 ] 4+ cage, 1.(b) The geometry of the encapsulated [ReBr 6 ] 2− ion remains essentially unchanged with respect to its metal salt and thus the axial zero-field splitting parameter, D Re , also remains the same.(c) The ingress of the ReBr 6 2− ion induces a magnetic exchange interaction between the Re IV guest and the Cu II host, mediated primarily by the Cu(d x 2 −y 2)||Re(d yz ) orbitals in the Re-Br⋯Cu pathway leading to dominant antiferromagnetic exchange and an S = 1 2 ground state for the complex.This was confirmed by overlap integral calculations and NBOs analysis where significant donor-to-acceptor interactions are observed between hybrid |4s 0.37 4p x 0.63 | orbitals of the Br ions and the empty 4s, 4p orbitals of the Cu ions.The ability of paramagnetic host complexes to encapsulate paramagnetic guest complexes highlights some interesting possibilities for future work.These include: (1) the ability to switch on/off magnetic interactions through simple solution-based/redox chemistry, or through external perturbation, e.g.light, pressure, magnetic/electric fields.(2) To specifically design host frameworks able to isolate/stabilise unstable/reactive magnetic molecules or those with unusual geometries/coordination numbers.The structural and physical characterisation of such species will have potential application across a breadth of electron-spin based quantum technologies.