Exploiting host–guest chemistry to manipulate magnetic interactions in metallosupramolecular M(4)L(6) 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(II)X(4)⊂Ni(II)(4)L(6)](OTf)(6), where M(II)X(4)(2−) = MnCl(4)(2−) (2), CoCl(4)(2−) (5), CoBr(4)(2−) (6), NiCl(4)(2−) (7), and CuBr(4)(2−) (8) or [M(III)X(4)⊂Ni(II)(4)L(6)](OTf)(7), where M(III)X(4)(−) = FeCl(4)(−) (3) and FeBr(4)(−) (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 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 MX(4)(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 MX(4)(n−) guest molecules lead to stronger host–guest magnetic exchange interactions.