Syn–anti conformational switching in an ethane-bridged Co(ii)bisporphyrin induced by external stimuli: effects of inter-macrocyclic interactions, axial ligation and chemical and electrochemical oxidations

Abstract

The syn–anti conformational switching has been demonstrated in the ethane-bridged dicobalt(II)bisporphyrin which is present in the syn-form only. The addition of either perylene or axial ligands to Co^II(bisporphyrin) completely transforms the syn form into the anti because of strong π–π interaction and axial coordination, respectively. The complex undergoes four 1e-oxidations in CH₂Cl₂ which are indicative of strong through space interactions between the two cofacial Co-porphyrins at 295 K. The first oxidation is a metal centered one and occurs at a potential much lower than that of the monomeric analog. However, the second oxidation, which is again metal centered, was at a significantly higher potential. The large difference between the first two oxidations, as observed here, is due to much stronger inter-porphyrin interactions. The step-wise oxidations have been performed both chemically and electro-chemically while the progress of the reactions was monitored by UV-visible and ¹H NMR spectroscopy. After 1e-oxidation, a very broad ¹H NMR signal results with increased difference between two meso resonances, which indicates that the two macrocycles are in the syn-form with lesser interplanar separation as also observed by DFT. However, 2e-oxidation results in the stabilization of the anti form. The addition of axial ligands to Co^II(bisporphyrin) also completely transforms the syn form into the anti form. While additions of THF and I₂/I⁻ both result in the formation of five-coordinate complexes, Co(II) is oxidized to Co(III) in the case of the latter. However, additions of 1-methylimidazole, pyridine and pyrazine as axial ligands result in the formation of a six-coordinate complex in which Co(II) is spontaneously oxidized to Co(III) in air.