Exploiting host–guest chemistry to manipulate magnetic interactions in metallosupramolecular M<sub>4</sub>L<sub>6</sub> tetrahedral cages
Reaction of Ni(OTf) 2 with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [Ni II 4 L 6 ] 8+ . By selectively exchanging the bound triflate from [OTf3Ni II 4 L 6 ](OTf) 7 (1), we have been able to prepare a series of host-guest complexes that feature an encapsulated paramagnetic tetrahalometallate ion inside this paramagnetic host giving [M II X 4 3Ni II 4 L 6 ](OTf) 6 , where M II X 4 2À ¼ MnCl 4 2À (2), CoCl 4 2À (5), CoBr 4 2À (6), NiCl 4 2À (7), and CuBr 4 2À (8) or [M III X 4 3Ni II 4 L 6 ](OTf) 7 , where M III X 4 À ¼ FeCl 4 À (3) and FeBr 4 À(4). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host-guest complexes 1-8 all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN 6 } vertexes within a single Ni 4 L 6 unit possess the same D or L stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [Ni II 4 ] complex, and between the host and the MX 4 nÀ guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the magnetic exchange are in close agreement with experiment, revealing that large spin densities on the electronegative X-atoms of particular MX 4 nÀ guest molecules lead to stronger host-guest magnetic exchange interactions.
Introduction<!>Results and discussion<!>SQUID magnetometry<!>Theoretical studies<!>Electronic structure of the empty Ni 4 L 6 cage<!>Conclusions
<p>The inherent ability of metallosupramolecular cages to encapsulate different chemical species within their cavity can be exploited for a myriad of applications, including the stabilisation of reactive species, 1 catalysis, 2,3 and drug-delivery. 4,5 In all but a few cases, these cages are constructed from diamagnetic metal ions (most commonly Pd II , Pt II , Fe II , Ru II , Ga III ), 6 and even when paramagnetic ions (e.g. Co II ) are employed, characterising magnetic properties has not been a key focus. 7,8 However, the exploitation of (reversible) guest encapsulation to induce magnetic exchange interactions with the host could be used in a variety of potential applications including magnetic sensing and switching, 9 the construction of single-molecule magnets, 10 the encapsulation and stabilisation of highly anisotropic single ion magnets with specic geometries, 11,12 dilution of magnetic molecules in the solid-state, 13 and the organisation of electron spin based qubits within ordered structural frameworks and/or on surfaces. 14,15 The latter has proven to be extremely difficult since the magnetic properties of molecules are oen changed upon deposition. 15 Introducing a magnetic guest into the cavity of a magnetic host could result in a number of potential outcomes. (1) Guest encapsulation has no effect, i.e. there is no magnetic interaction between host and guest and/or there is no geometrical change in either component. (2) There is no magnetic interaction between host and guest, but binding induces structural changes, altering the geometries of the metal ions in the cage and/or the encapsulated guest, modifying magnetic anisotropy.</p><p>(3) There is a magnetic interaction between host and guest, which may or may not also change the magnetic exchange between metals ions in the host. (4) There is a combination of points (2) and (3).</p><p>In the chemistry of porous coordination polymers, or metalorganic frameworks (MOFs), the ingress of (non-magnetic) guest molecules into the pores of 3D frameworks built from paramagnetic metal ions, such as Co II , has shown that even simple solvent molecules can modify the magnetic properties of the metal ions via geometry changes induced by intermolecular interactions. The resulting changes in metal anisotropies can lead to signicant changes in magnetisation relaxation dynamics. 16 In spin crossover (SCO) MOFs the high spinlow spin transition temperature is well known to be highly guestdependent, proffering potential application in molecular recognition. 17,18 Studies of coordination cages and capsules incorporating a paramagnetic component are limited to the examination of magnetic exchange interactions between metal ions in the cage, 19 SCO (of the cage and guest), [20][21][22][23] and the interaction of organic-radicals in the cavity (with themselves or the cage) [24][25][26] or in the host framework. 27,28 The ability to understand, and ultimately control, host-guest magnetic exchange interactions and single ion magnetoanisotropies in such molecular species would represent an important step toward making coordination cages with tuneable, and potentially useful, magnetic properties. Herein, we discuss the construction of a tetrahedral cage [Ni II 4 L 6 ] 8+ (L ¼ quaterpyridine) that can (reversibly) bind a range of tetrahedral, paramagnetic MX 4 1/2À guests, inducing magnetic exchange interactions between host and guest.</p><!><p>The tetrahedral Ni II 4 L 6 cage was synthesised by combining Ni(OTf) 2 (4 equivalents) with quaterpyridine (L, 6 equivalents) in acetonitrile, followed by heating for 24 h (see ESI, † Section 4). The ESI-MS of the isolated complex conrmed the presence of the Ni II 4 L 6 cage, with the +2 to +7 m/z cations being present. The absence of the +8-state strongly suggests that in solution a single triate anion is bound within the cavity of the cage (Fig. S2-S3 †). X-ray crystallography conrms that the structure of 1 is [OTf3Ni ). 29 All eight complexes crystallise in monoclinic cells, with 1-4, 7 and 8 being in the space group C2/c, and 5 and 6 in the P2 1 /n and P2 1 /c space groups, respectively (Tables S1 and S2 †).</p><p>The structures of compounds 1-8 are very similar, and so for the sake of brevity, a generic description is provided. Pertinent bond lengths and angles given in Table S3. † The cationic cage describes a [Ni II 4 L 6 ] 8+ tetrahedron, which, akin to other M 4 L 6 assemblies, has the Ni II ions occupying the four vertices linked by bisbidentate L ligands lying on each of the six edges (Fig. 1a). The approximate dimensions of the tetrahedron in 1-8 are Ni II /Ni II ¼ 9.2-9.6 Å, with the internal cage volumes ranging from 62-81 Å3 . The volumes, pore diameters and average window diameters for each compound are given in Table S4. † 30 The Ni II ions are six-coordinate and in distorted NiN 6 octahedral geometries, with Ni-N distances between 2.03(3)-2.17 the tetrahalometallate halide atoms and the ortho-pyridyl positions of the cage's ligand. For example, in 7, the distances between the host and guest are: Cl/HÀAr z 2.86 Å (Fig. 2b). As with 1, the external triate anions and solvent of crystallisation connect neighbouring cages through a network of interactions with the host framework. In the extended structure this results in alternating layers of cages/anions and solvent molecules of crystallisation (Fig. S18-S34, Table S5 †).</p><!><p>The direct-current (d.c.) molar magnetic susceptibility, c M , of polycrystalline samples of 1-8, were measured in an applied magnetic eld, B, of 0.1 T, over the 2-300 K temperature, T,   To better dene the low temperature magnetic properties of 1-8, we performed variable-temperature-variable-eld (VTVB) dc magnetisation measurements on polycrystalline samples in the temperature range 2-7 K and in applied magnetic elds up to 7 T. The results of these VTVB measurements are given in the insets of Fig. 3 as the eld dependent magnetisation, and as the magnetisation dependence against the reduced quantity m B B/kT with m B and k the Bohr magneton and Boltzman constant, respectively, (Fig. S35-S42 †) that expresses the ratio between Zeeman and thermal energies. Inspection of these reduced magnetisation traces reveals that the ground states of 1-8 are weakly anisotropic, as evidenced by the limited nesting of the curves. Thus, the magnetic anisotropy of the constitutive single ions is either very small, as expected for Cu II , Fe III and Mn II , or very large (Ni II ) with respect to the experimental conditions (B, T). For the quantitative interpretation of the magnetic properties of 1-8, we used spin-Hamiltonian (1):</p><p>with</p><p>and</p><p>where Ĥhost is the spin-Hamiltonian relative to 1, Ĥguest is the Hamiltonian relative to the guests in 2-8 and their interaction with the host 1, i, j are indices that run over the constitutive centres, g Ni the g-value of Ni II , Ŝi the spin operator of the i th paramagnetic centre, D Ni the single-ion axial anisotropy parameter of Ni II , S Ni ¼ 1 the total spin of Ni II , J ij the pairwise isotropic magnetic exchange interaction parameter between centres i and j, with the equivalent quantities for the guests. The c M T product and the VTVB data for 1-8 were simultaneously tted to spin-Hamiltonian (1) by full matrix numerical diagonalisation of its matrix representation and by use of the Simplex algorithm. 31 For 1, tting of the c M T product and the VTVB data resulted in the best-t parameters: g Ni ¼ 2.13, jD Ni j ¼ 1.575 cm À1 and J Ni-Ni ¼ À0.078 cm À1 (Fig. 3). The relatively small uniaxial anisotropy parameter, D Ni , of the Ni II centres of the host is in agreement with their approximate octahedral symmetry. These parameters were subsequently xed for the quantitative interpretation of the magnetic properties of 2-8. For simplicity, the g-values of all guests were xed to 2, except for 7 where we set the g-value of the guest Ni II ion equal to g Ni ¼ 2.13, as determined for 1. Thus for 2 the model contained only one free parameter, namely J Ni-Mn . Simultaneous tting of the c M T and VTVB data of 2 resulted in the best-t parameters: J Ni-Mn ¼ À0.041 cm À1 . Analogously, the best t parameters for 3 were: J Ni-Fe ¼ À0.068 cm À1 (D Fe was neglected for Fe III ); for 4: J Ni-Fe ¼ À0.084 cm À1 (D Fe was neglected for Fe III ); for 5: jD Co j ¼ 2.37 cm À1 and J Ni-Co ¼ À0.005 cm À1 ; for 6: jD Co j ¼ 6.30 cm À1 and J Ni-Co ¼ À0.001 cm À1 ; for 7: jD 0 Ni j ¼ 85.5 cm À1 and J 0</p><p>Ni -Ni ¼ À0.476 cm À1 ; and for 8: J Ni-Cu ¼ À0.062 cm À1 (D Cu was neglected for Cu II ). The values are tabulated in Table 1 for convenience. Note that the large D 0</p><p>Ni value for the guest in 7 is of the magnitude expected for a tetrahedral Ni II ion, and in agreement with the faster drop of the c M T product with decreasing temperature, absent for all other compounds. For these other compounds, the guest magnetic anisotropy is approximately two orders of magnitude smaller, or entirely negligible.</p><!><p>The magnetic properties of all eight complexes have been investigated using both DFT and ab initio methods. DFT calculations have been employed to estimate the isotropic exchange coupling constants (J) using the Hamiltonian Ĥ ¼ À2J ij P isj Ŝi $ Ŝj . On-site spin-orbit coupling/zero-eld splitting (zfs) calculations require accurate estimation of excited state energies, and a singledeterminant description of the wavefunction is not sufficient for systems having orbital degeneracies or those possessing low-lying excited states. For this reason, ab initio CASSCF/NEVPT2 calculations have been performed (see the Computational Details in the ESI for more information).</p><!><p>Calculations have been performed on the full X-ray structure without any geometry relaxation. The J Ni-Ni interaction is estimated to be À0.062 cm À1 , very close to the experimental value of À0.078 cm À1 (Table 2). The magnitude of the antiferromagnetic exchange suggests that the SOMOs of neighbouring Ni II ions are weakly interacting. The distance between metal ions is not particularly long ($9.5 Å), but signicant twisting between the bipyridine units (Fig. 1 and 4; dihedral angle ¼ 45-60 ) hinders the spin polarisation mechanism. Spin density values on the Ni II ions in 1 are found to be $1.64, which is as expected for octahedral Ni II centres possessing strong spin delocalisation (Fig. S43 †). The spin ground state is found to be an S ¼ 0 state with two "spin-up" and two "spin-down" Ni II ions. The axial zfs of the octahedral Ni II ions in the cage is found to be D ¼ À2.25 cm À1 , in agreement with the experimental data. Such a value would be expected given the high symmetry and close-toperfect octahedral geometry around the Ni II centres. 32,33 For the CASSCF/NEVPT2 calculations, the other Ni II centres were substituted by diamagnetic Zn II ions.  Ni exchange interactions are estimated to be À0.09 cm À1 and À0.01 cm À1 , respectively. Here the spin ground state is S ¼ 5/2 (Fig. 4, S44 and Table S6 † (BS3)). The weaker J Mn-Ni exchange originates from the dipolar Cl/HÀAr interaction which mediates the coupling. The zfs of the Mn II ion is estimated to be very small, D ¼ À0.002 cm À1 , in accordance with the isotropic nature of a tetrahedral d 5 centre. 34 Similar analyses were performed on the remaining host-guest complexes. For complex 3, the J Fe-Ni and J Ni-Ni interactions are found to be À0.073 cm À1 and À0.074 cm À1 , respectively (Table 2). Note that J Fe-Ni is stronger than J Mn-Ni : a closer examination of the spin densities computed on the Cl atoms of 2 and 3 reveals stronger delocalisation of the spin density in 3 compared to that in 2 facilitating stronger exchange interactions. This is correlated to the shorter Fe-Cl distance (2.20 Å) in 3 compared to the Mn-Cl distance (2.38 Å) in 2. The CASSCF/NEVPT2 computed D values for the guest ions in complexes 2-4 are small. Indeed, they are smaller than the energy separation between the spin state energies arising from the exchange interaction.</p><p>A similar situation is observed for complex 4 with [FeBr 4 ] À as the guest. In this case the host-guest antiferromagnetic exchange is larger (À0.085 cm À1 ) than for 2 and 3. The spin density on the Fe III ion is signicantly reduced due to strong spin delocalisation on to the electronegative Cl À and Br À ions in 3 and 4 (Table S6 †). Interestingly, in complexes 5 and 6, where [CoCl 4 ] 2À and [CoBr 4 ] 2À are the guest molecules, J Co-Ni was found to be weakly ferromagnetic from DFT calculations (+0.012 and +0.025 cm À1 , respectively; Table 2). The sign of J Co-Ni is contrary to experiment, albeit both the magnitude of the exchange and the absolute difference in the exchange is extremely small. Thus, both experiment and theory point to the presence of extremely weak exchange in this instance, and we note that these particular J values are at the limit of what DFT can accurately reproduce. More importantly, the D Co values Comparison of experimental and computational spin-Hamiltonian parameters for complexes 1-8</p><p>Neglected for Cu II -(Cu) 2.00</p><p>Cu: g iso ¼ 2.43</p><p>Fig. 4 Lowest energy broken symmetry spin density plot for complex 2. Iso-surface value, 0.005 e À /Bohr. 3 The red and yellow colours represent "spin-up" and "spin-down", respectively.</p><p>determined from ab initio methods are À6.64 cm À1 (5) and À3.96 cm À1 (6), three orders of magnitude higher than the energy separation between the exchange-coupled spin states, i.e., jDj > J.</p><p>The M S level separations, i.e., the gap between the AE3/2 and AE1/2 microstates for Co II are much larger ($2jDj) and close to 13 cm À1 and 8 cm À1 , respectively. Deviation from ideal T d symmetry is well-known to result in a signicant D value for tetrahedral Co II ions. [35][36][37][38] For complex 7, the orbital degeneracy of the tetrahedral Ni II ion precluded convergence of the DFT calculations and thus no hostguest J Ni-Ni exchange coupling could be estimated. NEVPT2 calculations yield a D value of +214 cm À1 for [NiCl 4 ] 2À , a value much larger than that estimated from experimental susceptibility and magnetisation data. 39 The origin of this very large anisotropy can be explained from the NEVPT2-LFT orbital splitting pattern of the pseudo-T d [NiCl 4 ] 2À complex shown in Fig. 5 S7 †). CASSCF/NEVPT2 calculations carried out on the DFT optimised [NiCl 4 ] 2À geometry yield a similar value, D ¼ +210 cm À1 . We have also computed the deformation energy of [NiCl 4 ] 2À inside the cage. This is calculated as the difference in energy between the DFT optimised structure and the single-point energy calculated on the guest [NiCl 4 ] 2À . This is estimated to be 18.6 kJ mol À1 , which indicates a small structural distortion upon encapsulation.</p><p>In complex 8, the exchange interactions between Ni-Ni and Ni-Cu ions are rather similar, J Ni-Ni ¼ À0.064 cm À1 and J Ni-Cu ¼ À0.066 cm À1 . The spin ground state is S ¼ 1/2 (BS3, Fig. S45 †). Due to strong spin delocalisation from the Cu II ion onto its four Br À ions the spin density value on the metal ion reduces to just 0.39.</p><p>The theoretically determined spin-Hamiltonian parameters have been used to simulate the experimental susceptibility and magnetisation data (Fig. S46-S47 †). During simulation, we adopted three different spin-Hamiltonians to describe the magnetic properties for the eight complexes.</p><p>For complexes 1-4 and 8 both the metal ions in the host cage and in the guest anions are found to be completely isotropicthe zfs/D values of the Mn II , Fe III and Ni II (cage) centres are very small and comparable to the J values (i.e. D z J). For these ve species, spin-Hamiltonian (4) was employed for simulation. All give very good agreement with the experimental data (Fig. S46 †).</p><p>For complex 8, the g-factors for the Cu II ion from the NEVPT2 level of theory, g x ¼ 1.633, g y ¼ 2.126 and g z ¼ 4.078 are overestimated. This is a well-known problem in the literature for Cu II . Higher electron correlation, higher reference space and/or ligand orbital inclusion, should be taken into consideration. 40 We have therefore performed multi-reference CI (MRCI) calculations in combination with the CASSCF wavefunction to obtain the nal gfactors, g x ¼ 1.945, g y ¼ 2.470 and g z ¼ 2.879 (g iso ¼ 2.43), which remain anisotropic due to mixing with the bromide orbitals. Simulation of the susceptibility and magnetisation data shows excellent agreement with the experimental data (Fig. S47 †).</p><p>For complexes 5 and 6, the D parameters of Co II are small but still much larger than the spin state energies/J values (i.e. jDj > J). In these two cases, we have used spin-Hamiltonian (5) to simulate the experimental susceptibility and magnetisation data. Instead of using D values for the Co II ion, ground state effective g-factors for individual Kramers pairs are used as S¼ 1/ 2 pseudo-spins (Ising Hamiltonian; Table 2). 41 This produces a very nice simulation of the experimental data, given the simplicity of the model (Fig. S46-S47 †). Note that the simulations are unaffected by the sign of the J Ni-Co exchange.</p><p>For complex 7, which contains the highly anisotropic [NiCl 4 ] 2À guest anion, we have used spin-Hamiltonian (6) in which all exchange interactions are neglected, since D Ni \ J [the inclusion of any reasonable J host-guest value does not affect the simulation]. Note that the g-factors obtained from the NEVPT2 method for [NiCl 4 ] 2À are overestimated, as expected for the highly anisotropic Ni II ion. 42 Simulation of susceptibility and magnetisation data is given in Fig. S47 † and shows good agreement with the experimental data, albeit of a slightly larger magnitude.</p><!><p>The tetrahedral cage [OTf3Ni  Magnetic susceptibility and magnetisation data show that the magnetic exchange interactions between metal ions in the host complex, and between host and guest, are of comparable magnitude and antiferromagnetic in nature. Theoretically derived values for the exchange are in close agreement with experiment and reveal that large spin densities on the electronegative X-atoms of certain MX 4 nÀ guest molecules leads to stronger host-guest magnetic exchange interactions. For the tetrahedral Co II guests, the anisotropy is small but still much larger than the magnitude of exchange coupling between hosthost and host-guest. The orbital degeneracy of the tetrahedral Ni II ion and the very large zfs that results makes accurate estimation of J Ni-Ni and D Ni(tet) in (7) rather difficult, as reected in the large differences in the results obtained between experiment and theory. What is clear, however, is that the encapsulation of paramagnetic guests inside dia/paramagnetic cages can be very useful in an array of potential applications. These include sensing and switching, the encapsulation and stabilisation of highly anisotropic (and/or air-and moisture-sensitive) magnetic molecules and the organisation (and/or dilution) of magnetic molecules within ordered, solution-stable structural matrices. To date, surface deposition of magnetic molecules has proved problematic, since in the vast majority of cases structural/ magnetic integrity is compromised hindering application. Encapsulation of metal complexes such as spin crossover species, single-ion magnets (SIMs) or electron spin based qubits within a dia/paramagnetic cage whose exohedral organic skeleton is easily derivatised may prove to be an interesting option. The cage acting both as a surface anchor and a protective coating for the magnetic molecule. 43,44 Although only relatively small changes to the geometries of the MX 4 nÀ guests were observed here, this work also suggests that the deliberate distortion/construction of magnetic molecules through encapsulation within the connes of a sterically restricted cavity of a coordination cage may offer an alternative route to producing highly unusual/anisotropic SIMs with specic geometries, tailored ligand elds, and targeted symmetries. In turn, such species may display a breadth of fascinating structures and magnetisation relaxation dynamics that may not exist outwith the cage.</p>