Three copper dibenzoporphyrin(2.1.2.1) complexes having two dipyrromethene units connected through o-phenylen bridges and 4-MePh, Ph, or F5Ph substituents at the meso positions of the dipyrrins were synthesized and characterized according to their spectral, electrochemical, and structural properties. As indicated by the single-crystal X-ray structures, all three derivatives have highly bent molecular structures, with angles between each planar dipyrrin unit ranging from 89° to 85°, indicative of a nonaromatic molecule. The insertion of copper­(II) into dibenzoporphyrins(2.1.2.1) induced a change in the macrocyclic cavity shape from rectangular in the case of the free-base precursors to approximately square for the metalated copper derivatives. Solution electron paramagnetic resonance (EPR) spectra at 100 K showed hyperfine coupling of the Cu­(II) central metal ion and the N nucleus in the highly bent molecular structures. Electrochemical measurements in CH2Cl2 or N,N-dimethylformamide (DMF) containing 0.1 M tetrabutylammonium perchlorate (TBAP) were consistent with ring-centered electron transfers and, in the case of reduction, were assigned to electron additions involving two equivalent π centers on the bent nonaromatic molecule. The potential separation between the two reversible one-electron reductions ranged from 230 to 400 mV in DMF, indicating a moderate-to-strong interaction between the equivalent redox-active dipyrrin units of the dibenzoporphyrins(2.1.2.1). The experimentally measured highest occupied molecular orbital (HOMO)–lowest unoccupied molecular orbital (LUMO) gaps ranged from 2.14 to 2.04 eV and were smaller than those seen for the planar copper tetraarylporphyrins(1.1.1.1), (Ar)4PCu.

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The molecular structure, electrochemistry, spectroelectrochemistry and electrocatalytic oxygen reduction reaction (ORR) features of two Co II porphyrin-(2.1.2.1) complexes bearing Ph or F 5 Ph groups at the two meso-positions of the macrocycle are examined. Single crystal X-ray analysis reveal a highly bent, nonplanar macrocyclic conformation of the complex resulting in clamp-shaped molecular structures. Cyclic voltammetry paired with UV/Vis spectroelectrochemistry in PhCN/0.1 M TBAP suggest that the first electron addition corresponds to a macrocyclic-centered reduction while spectral changes observed during the first oxidation are consistent with a metal-centered Co II /Co III process. The activity of the clamp-shaped complexes towards heterogeneous ORR in 0.1 M KOH show selectivity towards the 4e À ORR pathway giving H 2 O. DFT first-principle calculations on the porphyrin catalyst indicates a lower overpotential for 4e À ORR as compared to the 2e À pathway, consistent with experimental data.

Planar Ni(II) porphyrinoid complexes have been widely used in electrochemical carbon dioxide reduction reaction and oxygen reduction reaction as well as hydrogen evolution reaction (HER). However, nonplanar Ni(II) tetra-pyrrolic complexes have not been thoroughly investigated thus far. In this study, three highly bent bis(dipyrrin) Ni(II) complexes have been synthesized to investigate their structure, electronic property, and electrocatalytic HER activities. Cyclic voltammetry and thin-layer UV−visible spectroelectrochemistry studies revealed four redox processes, yielding two reduced species as the final products. The i c /i p values of phenyl-and pentafluorophenyl-bearing bis(dipyrrin) Ni(II) complexes were >30 when trifluoroacetic acid was used as the proton source, and their Faradaic efficiencies for H 2 generation were >93%. Density functional theory calculations of the HERs revealed low endothermic energies of bent bis(dipyrrin) Ni(II) complexes.

Novel 24π antiaromatic and 26π aromatic meso-aryl rosarins were successfully obtained for the first time from β-free bipyrrole through a one-pot synthesis. Because of the absence of substituents at the β-positions of the pyrrole units, the β-free pristine rosarin backbones were highly planar, as confirmed using X-ray crystallography. Optical measurements indicated interconversion between 24π antiaromatic and 26π aromatic β-free pristine rosarin via redox reactions, which was not observed in distorted β-dodecamethyl rosarin.