Formation and characterization of carbon monoxide adducts of iron "twin coronet" porphyrins. Extremely low CO affinity and a strong negative polar effect on bound CO.

The carbon monoxide (CO) adducts of iron “twin coronet” porphyrins (TCPs) are characterized by UV−vis, resonance Raman (RR), IR, and 13C NMR spectroscopies. A superstructured porphyrin, designated as TCP, was used as a common framework for the four different types of iron complexes. TCP bears two binaphthalene bridges on each side and creates two hydrophobic pockets surrounded by the bulky aromatic rings. In the CO-binding cavities, the hydroxyl groups are oriented toward the center above the heme. The iron complexes investigated are as follows:  TCP (which is without a covalently linked axial ligand), TCP−PY (which has a linked pyridine ligand), and TCP−TB and TCP−TG (both of which have a linked thiolate ligand). These complexes were synthesized as ferric forms and identified by the various spectroscopic methods. The UV−vis spectra of TCP−CO and TCP−PY−CO exhibit λmax at 432, 546 and 428, 541 nm, respectively. On the other hand, the CO adducts of TCP−TB and TCP−TG show typical hyperporphyrin spectra for ...