Formation of exo–exo, exo–endo and tweezer conformation induced by axial ligand in a Zn(II) bisporphyrin: Synthesis, structure and properties

Abstract

A detailed structural and spectroscopic investigation of axial ligand coordination on Zinc(II)pyrrole-bridged bisporphyrin (Zn2DEP), have been reported here. Our findings demonstrate that the DEP ligand system provides a Pacman pocket with very high vertical and horizontal flexibility that leads to the formation of exo–endo, exo–exo and tweezer complexes of ZnII(bisporphyrinato) depending upon the size and type of axial ligands used and all the complexes are isolated in solid and structurally characterized. While addition of excess 1-Me imidazole to Zn2DEP produces complex in which the axial ligand binds in the exo–endo fashion, addition of 3-Cl pyridine binds in exo–exo fashion. Here, six-membered aromatic ring occupy more space than that of five-membered ring and thus 3-Cl pyridine binds in an exo–exo fashion rather than exo–endo fashion which require even larger vertical flexibility of two porphyrin rings. However, addition of 1,2-diaminobenzene and pyrazine to Zn2DEP produce only tweezer complexes. The Zn···Zn non-bonding distances are 7.69 and 7.08 Å in the exo–endo and exo–exo form, respectively. In the tweezer complexes, however, the values of said distances are 5.61 and 7.11 Å with ODAB and pyrazine ligand, respectively. The Zn–Np distances are relatively shorter while Zn–Nax(L) distances are longer in tweezer complexes compared to exo–endo/exo–exo forms. Also, the displacements of Zn from the mean porphyrin planes are much less in tweezers. 1H NMR of the complexes in solution show large upfield shift of the axial ligand protons while protons of porphyrin subunits are shifted downfield.