Anion Template Effect on the Self-Assembly and Interconversion of Metallacyclophanes

Reactions of 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine (bptz) with solvated first-row transition metals M(II) (M(II) = Ni, Zn, Mn, Fe, Cu) have been explored with emphasis on the factors that influence the identity of the resulting cyclic products for Ni(II) and Zn(II). The relatively small anions, namely [ClO<sub>4</sub>]<sup>-</sup> and [BF<sub>4</sub>]<sup>-</sup>, lead to the formation of molecular squares [{M<sub>4</sub>(bptz)<sub>4</sub>(CH<sub>3</sub>CN)<sub>8</sub>}⊂X][X]<sub>7</sub>, (M = Zn(II), Ni(II); X = [BF<sub>4</sub>]<sup>-</sup>, [ClO<sub>4</sub>]<sup>-</sup>), whereas the larger anion [SbF<sub>6</sub>]<sup>-</sup> favors the molecular pentagon [{Ni<sub>5</sub>(bptz)<sub>5</sub>-(CH<sub>3</sub>CN)<sub>10</sub>}⊂SbF<sub>6</sub>][SbF<sub>6</sub>]<sub>9</sub>. The molecular pentagon easily converts to the square in the presence of excess [BF<sub>4</sub>]<sup>-</sup>, [ClO<sub>4</sub>]<sup>-</sup>, and [I]<sup>-</sup> anions, whereas the Ni(II) square can be partially converted to the less stable pentagon under more forcing conditions in the presence of excess [SbF<sub>6</sub>]<sup>-</sup> ions. No evidence for the molecular square being in equilibrium with the pentagon was observed in the ESI-MS spectra of the individual square and pentagon samples. Anion-exchange reactions of the encapsulated ion in [{Ni<sub>4</sub>(bptz)<sub>4</sub>(CH<sub>3</sub>CN)<sub>8</sub>}⊂ClO<sub>4</sub>][ClO<sub>4</sub>]<sub>7</sub> reveal that a larger anion such as [IO<sub>4</sub>]<sup>-</sup> cannot replace [ClO<sub>4</sub>]<sup>-</sup> inside the cavity, but that the linear [Br<sub>3</sub>]<sup>-</sup> anion is capable of doing so. ESI-MS studies of the reaction between [Ni(CH<sub>3</sub>CN)<sub>6</sub>][NO<sub>3</sub>]<sub>2</sub> and bptz indicate that the product is trinuclear. Mass spectral studies of the bptz reactions with Mn(II), Fe(II), and Cu(II), in the presence of [ClO<sub>4</sub>]<sup>-</sup> anions, support the presence of molecular squares. The formation of the various metallacyclophanes is discussed in light of the factors that influence these self-assembly reactions, such as choice of metal ion, anion, and solvent.