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 [ClO₄]^- and [BF₄]^-, lead to the formation of molecular squares [{M₄(bptz)₄(CH₃CN)₈}⊂X][X]₇, (M = Zn(II), Ni(II); X = [BF₄]^-, [ClO₄]^-), whereas the larger anion [SbF₆]^- favors the molecular pentagon [{Ni₅(bptz)₅-(CH₃CN)₁₀}⊂SbF₆][SbF₆]₉. The molecular pentagon easily converts to the square in the presence of excess [BF₄]^-, [ClO₄]^-, and [I]^- anions, whereas the Ni(II) square can be partially converted to the less stable pentagon under more forcing conditions in the presence of excess [SbF₆]^- 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 [{Ni₄(bptz)₄(CH₃CN)₈}⊂ClO₄][ClO₄]₇ reveal that a larger anion such as [IO₄]^- cannot replace [ClO₄]^- inside the cavity, but that the linear [Br₃]^- anion is capable of doing so. ESI-MS studies of the reaction between [Ni(CH₃CN)₆][NO₃]₂ 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 [ClO₄]^- 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.