One-step versus multistep equilibrium of carbazole-bridged dinuclear zinc(II) complex formation: metal-assisted π-association and -dissociation processes.

This work demonstrates a selection criteria that determines whether molecular assembly occurs through a one-step or stepwise manner in ligand-bridged dinuclear zinc(II) (Zn(2+)) complex formation, which is associated with the π stacking of building blocks. The building blocks of carbazole ligands (L(1) and L(4)) that contain two imidazole moieties at the 3,6-positions form 4:2 complexes (i.e., [L]4-(Zn(2+))2) at a molar ratio of 0.50 ([Zn(2+)]/[L]0 = 0.50), thereby providing π stacking between the carbazole ligands. At the molar ratio of 0.67 ([Zn(2+)]/[L]0 = 0.67), the 4:2 complexes change to 3:2 complexes (i.e., [L]3-(Zn(2+))2) with no π-stacked carbazole unit. In contrast, when the imidazole groups in L(1) are replaced with benzoimidazole groups (L(3)), L(3) also yields the 4:2 complex [(L(3))4-(Zn(2+))2] at a molar ratio of 0.50. However, there is no structural transition from (L(3))4-(Zn(2+))2 to other complex species above a molar ratio of 0.50. Similarly, when two imidazole groups are introduced into the carbazole ring at 2,7-positions (L(5)), L(5) also gives the 4:2 complex [(L(5))4-(Zn(2+))2] that shows no structural transition to other complex species at a higher molar ratio.