Fine-Tuning of Electronic Structure of Cobalt(II) Ion in Nonplanar Porphyrins and Tracking of a Cross-Hybrid Stage: Implications for the Distortion of Natural Tetrapyrrole Macrocycles

The core size of the porphyrin macrocycles was closely related to their stability of the different electron structure in the central metal ion. Cobalt­(II) ions can undergo a conversion in electron configurations upon N₄ core contraction of 0.05 Å in nonplanar porphyrins, and these ions still maintain low spin forms after and before conversion. The structural fine-tuning can induce the appearance of a cross-hybrid stage [d­(_{<i>x</i>²−<i>y</i>}^₂)­sp² ↔ d­(_<i>z</i>²)­sp²] based on quadrilateral coordination of the planar core. The results indicate that the configuration conversion plays a key role in electron transfer in redox catalysis involving cobalt complexes. The electronic properties of six monostrapped cobalt­(II) porphyrins were investigated by spectral, paramagnetic, and electrochemical methods. The macrocyclic deformations and size parameters of Co-containing model compounds were directly obtained from their crystal structures