Syn–anti conformational switching: Synthesis and X-ray structures of tweezer and anti form in a zinc porphyrin dimer induced by axial ligands

Stepwise addition of 1,2-diaminobenzene to a Zn–1,2-bis(meso-octaethylporphyrinyl)ethane produces both tweezer and anti-form of the complex depending on the concentration of the axial ligand which exhibit two major equilibrium steps (with two step-wise binding constants): first, guest ligation leading to the formation of 1:1 host–guest tweezer structure (K1) and, second, guest molecule ligation (K2) forming 1:2 host–guest anti species and the corresponding binding constants are 1.82 × 103 M−1 and 1.34 × 102 M−1, respectively. However, when guest like 1,4-diaminobenzene and 4-CN-pyridine are used, the ligand geometry prevents its entry into interporphyrin cavity to form a tweezer structure, thus producing only the 1:2 anti complex. Single crystal X-ray structures of both tweezer and anti form produced in a single Zn-bisporphyrin are reported here for the first time. The nonbonding Zn···Zn distance within a molecule is 5.55 and 10.01 Å in tweezer and anti form, respectively. Although the average Zn–N (por) distances are comparable for both the forms, the Zn–N (1,2-diaminobenzene/4-CN-pyridine) distances and the displacement of Zn from the mean porphyrin planes are larger in tweezer compared to anti conformation.