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} subsetX]X]7, (M = Zn(II), Ni(II); X = BF4]-, ClO4]-), whereas the larger anion SbF6]- favors the molecular pentagon {Ni5(bptz)5-(CH3CN)10} subsetSbF6]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} subsetClO4]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.