Origin of d‐π Interaction in Cobalt(II) Porphyrins under Synergistic Effects of Core Contraction and Axial Ligation: Implications for a Ligand Effect of Natural Distorted Tetrapyrrole

Abstract

The reactivity of the metalloporphyrins was closely related to their ligand effect at axial position. The electronic properties of six model Co(II) porphyrins are investigated by spectral and electrochemical methods. Structural parameters of the Co(II) complexes are directly obtained from their crystal structures. We demonstrate that the unpaired 3d electron of low‐spin Co(II) ions in nonplanar Co(II) porphyrin complexes activated by core contraction of porphyrin macrocycles can be further activated by the axial ligation of imidazole. The activated electron can combine with a π orbital of the porphyrin ring to form a new d‐π orbital, which can induce the Q‐band of Co(II) porphyrins to visibly split. Addition of imidazole causes the Co(II)/Co(III) and Co(II)/Co(I) reactions to shift to more negative potential. Our results indicate that strong axial ligation and core contraction both play important roles in electron transfer in redox catalysis involving Co(II) complexes.

Optimal Size Matching and Minimal Distortion Energy: Implications for Natural Selection by the Macrocycle of the Iron Species in Heme

“…An inspection of the plot shows that the structures of the three metal ions in the model porphyrins can be changed and their diameters are mutually independent of each other. It has been shown that changing the macrocycle core size can directly change the electronic structures of iron(III), and cobalt(II) , . A similar effect is seen for nickel(II); the conversion point is between n C = 4 and 5 (Figure , labeled ☆), which is in agreement with the position derived from the spectral deviation (Figure ).…”

“…Figure shows that only the curve for the iron series crosses that of the metal‐free one, and the other two slightly cross or drift away from it; even the largest core, containing nickel(II), is just close to the lowest limiting free‐metal core value. The results also imply that porphyrin distortion can be affected by the size of the central ion as well as stretching of alkyl bridges and crowding of peripheral groups in synthetic nonplanar porphyrins. In the current model series, the small nickel(II) [or cobalt(II)] ion slightly increases the degree of distortion, as shown by the fact that the L N1N3 values are shorter in the metal‐containing series than in the metal‐free one and the A mC values are larger in the former than in the latter (Table ).…”

“…Selected structural parameters for the 20 compounds are shown in Table . The ionic states of iron,, , cobalt,, and nickel, were taken from previous reports.…”

“…For the cobalt(II) ion, an E dist value of 64 kJ mol –1 can only fine‐tune its electronic structure to another LS form , . A much higher E dist value is required to profoundly change its structure or remove electrons to form high‐valent cobalt(III) (or its cation radical), which is found in vitamin B 12 , .…”

“…In recent studies, a relationship between macrocyclic distortion and the electronic activities of metals was established for several series of ruffle‐, saddle‐, and dome‐type porphyrins (Figure , a–d) . We found that macrocyclic distortion ( A mC ) directly affects the core size (Figure , a–b), which induces changes in the electronic configuration of the central metal ions (Figure , b–c) such as iron(III),, cobalt(II),, copper(II), and zinc(II) . In the conversion process, an electron (or electron pair) in a 3d orbital is activated (Figure , c–d), which guarantees the potential catalytic abilities of these metal complexes (Figure , d–e).…”