Exploiting host–guest chemistry to manipulate magnetic interactions in metallosupramolecular M4L6 tetrahedral cages†

Reaction of Ni(OTf)2 with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [NiII4L6]8+. By selectively exchanging the bound triflate from [OTf⊂NiII4L6](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 [MIIX4⊂NiII4L6](OTf)6, where MIIX42− = MnCl42− (2), CoCl42− (5), CoBr42− (6), NiCl42− (7), and CuBr42− (8) or [MIIIX4⊂NiII4L6](OTf)7, where MIIIX4− = FeCl4− (3) and FeBr4− (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 {NiN6} vertexes within a single Ni4L6 unit possess the same Δ or Λ stereochemistry. Magnetic susceptibility and magnetisation data show that the magnetic exchange between metal ions in the host [NiII4] complex, and between the host and the MX4n− 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 MX4n− guest molecules lead to stronger host–guest magnetic exchange interactions.


Materials and Methods
Unless stated otherwise, all reagents and solvents were purchased from Alfa Aesar, VWR, Fluorochem or Sigma Aldrich and used without further purification. Where the use of anhydrous solvent is stated, drying was carried out using a solvent purification system manufactured by Glass Contour. Column chromatography was carried out using Geduran Si60 (40-63 μm) as the stationary phase and TLC was performed on precoated Kieselgel 60 plates (0.20 mm thick, 60F254. Merck, Germany) and observed under UV light at 254 nm or 365 nm. All reactions were carried out under air, unless stated otherwise.
All 1 H and 13 C NMR spectra were recorded on either a 500 MHz Bruker AV III equipped with a DCH cryo-probe (Ava500), a 500 MHz Bruker AV IIIHD equipped with a Prodigy cryo-probe (Pro500), a 600 MHz Bruker AV IIIHD equipped with a TCI cryo-probe (Ava600) or a 400 MHz Bruker AV III equipped with BBFO+ probe (Ava400) at a constant temperature of 300 K. Chemical shifts are reported in parts per million (ppm).
MS of the compounds was performed on a Synapt G2 (Waters, Manchester, UK) mass spectrometer or a Q-ToF (Micromass UK Ltd), using a nano-electrospray ionization source (ESI), controlled using Masslynx v4.1 software. All the scans in the experimental are for positive ions. Crystals of the samples were dissolved in acetonitrile at 50 µM. Prior to analysis, instruments were calibrated using a solution of sodium iodide (2 mg/mL) in 50:50 water:isopropanol. Capillary voltages were adjusted between 1.5 and 2.5 kV to optimize spray quality, while the sampling cone and the extraction cone voltage were minimised to reduce breakdown of the assemblies.
Source temperature was set at 80 °C. The data was analysed using the MassLynx v4.1 software.

S4
Magnetisation measurements were carried out on a Quantum Design SQUID MPMS-XL magnetometer at The University of Edinburgh, operating between 1.8 and 300 K for DC applied magnetic fields ranging from 0 to 5 T. Some measurements were made on the MPMS3 magnetometer at The University of Glasgow, operating between 1.8 and 300 K for DC applied magnetic fields ranging from 0 to 7 T.

Synthesis
Tetrahalometallates All tetrahalometallates were prepared based on previously published methods. 1 M = Mn, Fe, Co, Ni and Cu, X = Cl and Br. Anion X matched in the synthesis i.e. MX2 and Et4NX = CoCl2 and Et4NCl. MX2 (3 mmol) was dissolved in EtOH (30 mL) and stirred, Et4NX (9 mmol) was then added and stirred at room temperature for 1 hour. The precipitate was then filtered and washed with cold EtOH (3 × 10 mL) and Et2O (3 × 10 mL). The product was then dried under vacuum to yield the product. Yields in excess of 80%.
The structures were solved using ShelXT 4 employing the Intrinsic Phasing solution method through Olex2 5 as the graphical interface. The model was refined with ShelXL 6 using Least Squares minimisation. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atom positions were calculated geometrically and refined using the riding model. The RIGU restraint was applied to all triflate anions to appropriately model atomic displacement parameters.
All crystal structures contain large accessible voids that are filled with diffuse electron density belonging to disordered solvent, whose electron contribution was masked using the SQUEEZE 7 routine of PLATON 8 . This missing solvent is included in the total formula, triggering checkCIF alerts which should be ignored.
Single crystal X-ray diffraction data for 2, 7 and 8 were collected remotely 9 at Diamond Light Source, beamline I19-1, 10 under beam time award CY22240. An Oxford Cryosystems Cryostream 700+ low temperature device was used to maintain a crystal temperature of 100.0 K (2 and 8) and 120.0 K (7). The diffraction patterns were indexed with Xia2 11-13 . The structures were solved, refined, and disordered solvent masked as mentioned for 1 and 3-5.

S19
[Co II Br4Ni II 4L6](OTf)6 (6) Single crystal X-ray diffraction data for 6 were collected using a Rigaku FRE+ diffractometer with MoKα radiation. An Oxford Cryosystems Cryostream 700+ low temperature device was used to maintain a crystal temperature of 100.0 K.
The structures were solved, refined, and disordered solvent masked as mentioned for 1 and 3-5.                performed on the X-ray structures using the Gaussian 09 program. 17 Broken symmetry methodology was employed using the fragmentation method to obtain the magnetic coupling constants. 18 The unrestricted B3LYP functional was used with Ahlrich's all electron triple zeta valence (TZV) basis set for all atoms. 19,20 Wavefunction reoptimisation was performed after the SCF convergence to check the stability of the wavefunction. Geometry optimisation was also carried out for the anionic [NiCl4] 2-guest with the B3LYP/TZVP level of theory and basis set to compare the change in S46 geometry and zero-field splitting before and after the insertion to the cage. The isotropic coupling constant J was computed from the following pairwise interaction formula. 21 = − 2(2 1 2 + 2 ) S47 Table S6 -Spin density values obtained from the uB3LYP/TZV level of theory on the metal ions in complexes 1-8 (excluding complex 7).