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.