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

Abstract

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 1D 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^II(tn-OEP) forms five-coordinate porphyrin dimers [Zn^II(tn-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 1e-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^II(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.