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 [{M-4(bptz)(4)(CH3CN)(8})subset of X][X](7), (M = Zn(II), Ni(II); X = [BF4](-), [ClO4](-)), whereas the larger anion [SbF6](-) favors the molecular pentagon [{Ni-5(bPtz)(5-)(CH3CN)(10})subset of 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 [{Ni-4(bPtz)(4)(CH3CN)(8)}subset of ClO4][subset of ClO4](7) reveal that a larger anion such as [104]- cannot replace [ClO4](-) inside the cavity, but that the linear [Br-3](-) 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.