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^II ₄ L ₆]⁸⁺. By selectively exchanging the bound triflate from [OTf⊂Ni^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^IIX₄⊂Ni^II ₄ L ₆](OTf)₆, where M^IIX₄ ^2- = MnCl₄ ^2- (2), CoCl₄ ^2- (5), CoBr₄ ^2- (6), NiCl₄ ^2- (7), and CuBr₄ ^2- (8) or [M^IIIX₄⊂Ni^II ₄ L ₆](OTf)₇, where M^IIIX₄ ^- = 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 [Ni^II ₄] complex, and between the host and the MX₄ 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 guest molecules lead to stronger host-guest magnetic exchange interactions.