Exploiting Host-Guest Chemistry to Manipulate Magnetic Interactions in Metallosupramolecular M4L6 Tetrahedral Cages
<p>Reaction of Ni(OTf)<sub>2</sub> with the bisbidentate quaterpyridine ligand L results in the self-assembly of a tetrahedral, paramagnetic cage [Ni<sup>II</sup><sub>4</sub>L<sub>6</sub>]<sup>8+</sup>. By selectively exchanging the bound triflate from [OTfÌNi<sup>II</sup><sub>4</sub>L<sub>6</sub>](OTf)<sub>7</sub> (<b>1</b>), 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<sup>II</sup>X<sub>4</sub>ÌNi<sup>II</sup><sub>4</sub>L<sub>6</sub>](OTf)<sub>6</sub>, where M<sup>II</sup>X<sub>4</sub><sup>2− </sup>= MnCl<sub>4</sub><sup>2−</sup> (<b>2</b>), CoCl<sub>4</sub><sup>2−</sup> (<b>5</b>), CoBr<sub>4</sub><sup>2−</sup> (<b>6</b>), NiCl<sub>4</sub><sup>2−</sup> (<b>7</b>), CuBr<sub>4</sub><sup>2−</sup> (<b>8</b>) or [M<sup>III</sup>X<sub>4</sub>ÌNi<sup>II</sup><sub>4</sub>L<sub>6</sub>](OTf)<sub>7</sub>, where M<sup>III</sup>X<sub>4</sub><sup>−</sup> = FeCl<sub>4</sub><sup>−</sup> (<b>3</b>), FeBr<sub>4</sub><sup>−</sup> (<b>4</b>). Triflate-to-tetrahalometallate exchange occurs in solution and can also be accomplished through single-crystal-to-single-crystal transformations. Host-guest complexes <b>1</b>-<b>8</b> all crystallise as homochiral racemates in monoclinic space groups, wherein the four {NiN<sub>6</sub>} vertex within a single Ni<sub>4</sub>L<sub>6</sub> 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<sup>II</sup><sub>4</sub>] complex, and between the host and the MX<sub>4</sub><sup>n-</sup> 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<sub>4</sub>}<sup>n−</sup> guest molecules leads to stronger host-guest magnetic exchange interactions. </p>