Self-assembly of cobalt(II) and zinc(II) tetranitrooctaethylporphyrin via bidentate axial ligands: synthesis, structure, surface morphology and effect of axial coordination

摘要

A series of remarkably stable supramolecular architectures of cobalt(II) and zinc(II) tetranitrooctaethylporphyrins (tn-OEP) mediated by long and conjugated bipyridyl axial ligands (L) have been synthesized in one pot in excellent yields and structurally characterized. Linear ID polymeric chains were observed in the X-ray structures of the six-coordinated Co(II) complexes in which all the porphyrin units are aligned parallel to each other to have so-called "shish kebab" like architectures in order to maintain offset-stacked overlap. In contrast, Zn"(fn-OEP) forms five-coordinate porphyrin dimers [Zn~(II)(fn-OEP)]2-L with "wheel-and-axle" like architectures which are then self-aggregated in a perpendicular manner to fill space more effectively. The surface patterns of the polymeric molecules on HOPG surface are found similar as observed in the X-ray structures. Large downfield shift and the peak broadening of the axial ligand resonances in solution are discerned in the ~1H NMR of [Co~(II)(tn-OEP)]-L which is due to the contiguity to the paramagnetic Co(II) center. In contrast, upfield shift of the ~1H NMR resonances of the axial ligand L are observed for [Zn~(II)(tn-OEP)]2-L due to the shielding effect of porphyrin ring current which confirms the retention of the solid state structures in solution. However, the axial ligation with monodentate pyridine to Co~(II)(tn-OEP) produces six-coordinate complex [Co~(III)(tn-OEP)(py)2]~+ in which Co(II) spontaneously oxidizes to Co(III) in air. The first le-reductive response in cyclic voltammetry of [Co~(III)(tn-OEP)(py)2]+, which is assigned as Co~(III)/Co~(II), has been shifted to-0.42 V which explains why addition of pyridine to Co"(tn-OEP) in air results in spontaneous oxidation of Co(II) to Co(III). Weaker ligand field strength of bidentate bipyridyl axial ligand L compared to its smaller monodentate counterpart pyridine has also been demonstrated here.