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(x2−y2)­sp2 ↔ d­(z2)­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.