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 [Ni^II₄L₆]⁸⁺. By selectively exchanging the bound triflate from [OTfÌNi^II₄L₆](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^IIX₄ÌNi^II₄L₆](OTf)₆, where M^IIX₄²⁻ = MnCl₄²⁻ (2), CoCl₄²⁻ (5), CoBr₄²⁻ (6), NiCl₄²⁻ (7), CuBr₄²⁻ (8) or [M^IIIX₄ÌNi^II₄L₆](OTf)₇, where M^IIIX₄⁻ = FeCl₄⁻ (3), 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} vertex 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 [Ni^II₄] complex, and between the host and the MX₄^n- 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 {MX₄}ⁿ⁻ guest molecules leads to stronger host-guest magnetic exchange interactions.