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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 (1)H NMR spectroscopy. After 1e-oxidation, a very broad (1)H 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.

Two new supramolecular complexes consisting of an achiral bisporphyrin host and a chiral diamine guest are reported. One shows a remarkably high amplitude bisignate CD signal while the other one shows a very low value. X-ray structure and other spectroscopic investigations of the tweezer complexes clearly rationalize the origin of the optical activity that has so far remained an unresolved issue., (This journal is © The Royal Society of Chemistry 2012)

Selective reaction of 2,6-isophthaloyl chloride at the methine positions of two dibenzotetramethyltetraaza[14]annulene (tmtaa) units results in tethering of the macrocycles by either one or two 2,6-isophthaloyl bridging groups. The single tethered dibenzotetramethyltetraaza[14]annulene derivative (SIT-tmtaa) was isolated and characterized by X-ray crystallography, ESI-MS, 1H and 13C NMR. The syn conformation of the two tmtaa units observed in the crystal structure, has tetraaza[14]annulene macrocycles that are more distorted from planarity than in the structure of the parent tmtaa. The di-nickel(ii) complex was prepared and characterized by ESI-HRMS, 1H and 13C NMR. DFT computations place the LUMO on the 2,6-isophthaloyl bridge rather than on the tethered tmtaa units and suggest that the 2,6-isophthaloyl tether provides a level of flexibility in the inter tmtaa distances that is comparable to the Pacman Effect observed for tethered porphyrins. [ABSTRACT FROM AUTHOR]

A highly oxidized cobalt porphyrin dimer is reported. Each cobalt(II) ion and porphyrin ring underwent 1e oxidation with iodine as the oxidant to give a 4e-oxidized cobalt(III) porphyrin π-cation radical dimer The bridging ethylene group allows for substantial conjugation of the porphyrin macrocycles, thus leading to a strong antiferromagnetic coupling between the π-cation radicals and to stabilization of the singlet state. X-ray crystallography clearly showed that the complex may be considered as a real supramolecule rather than two cobalt(III) porphyrin p-cation radicals that interact through space. [ABSTRACT FROM AUTHOR]

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, ZnII(tn-OEP) forms five-coordinate porphyrin dimers [ZnII(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 [CoII(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 [ZnII(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 CoII(tn-OEP) produces six-coordinate complex [CoIII(tn-OEP)(py)2]+ in which Co(ii) spontaneously oxidizes to Co(iii) in air. The first 1e-reductive response in cyclic voltammetry of [CoIII(tn-OEP)(py)2]+, which is assigned as CoIII/CoII, has been shifted to −0.42 V which explains why addition of pyridine to CoII(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. [ABSTRACT FROM AUTHOR]

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 CoII(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 CoII(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. [ABSTRACT FROM AUTHOR]

The binding of a series of substituted phenols as axial ligands onto a diiron(III)bisporphyrin framework have been investigated. Spectroscopic characterization revealed high-spin states of the iron centers in all of the phenolate complexes, with one exception in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin, which only stabilized the pure intermediate-spin ( S=3/2) state of the iron centers. The average FeN (porphyrin) and FeO (phenol) distances that were observed with the 2,4,6-trinitrophenolate complex were 1.972(3) Å and 2.000(2) Å, respectively, which are the shortest and longest distances reported so far for any FeIIIporphyrin with phenoxide coordination. The alternating shift pattern, which shows opposite signs of the chemical shifts for the meta versus ortho/para protons, is attributed to negative and positive spin densities on the phenolate carbon atoms, respectively, and is indicative of π-spin delocalization onto the bound phenolate. Electrochemical data reveals that the E1/2 value for the FeIII/FeII couple is positively shifted with increasing acidity of the phenol. However, a plot of the E1/2 values for the FeIII/FeII couple versus the p Ka values of the phenols shows a linear relationship for all of the complexes, except for the 2,4,6-trinitrophenolate complex. The large deviation from linearity is probably due to the change of spin for the complex. Although 2,4,6-trinitrophenol is the weakest axial ligand in the series, its similar binding with the corresponding FeIIImonoporphyrin only results in stabilization of the high-spin state. The porphyrin macrocycle in the 2,4,6-trinitrophenolate complex of diiron(III)bisporphyrin is the most distorted, whilst the 'ruffling' deformation affects the energy levels of the iron d orbitals. The larger size and weaker binding of 2,4,6-trinitrophenol, along with hemeheme interactions in the diiron(III)bisporphyrin, are responsible for the larger ring deformations and eventual stabilization of the pure intermediate-spin states of the iron centers in the complex. [ABSTRACT FROM AUTHOR]