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 λ_max 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 a thiolate-ligated iron(II) porphyrin-CO complex. In the RR spectra, the ν(Fe-CO) bands were observed at 506, 489 cm^-1 (TCP), 465 cm^-1 (TCP-PY), 458, 437 cm^-1 (TCP-TG) and 429 cm^-1 (TCP-TB). Compared with the reported ν(Fe-CO) frequencies of hemoproteins and their model systems, these observed values are unusually low. Further, abnormally high ν(C-O) bands are observed at 1990 cm^-1 (TCP-CO) and 2008 cm^-1 (TCP-PY-CO) in IR spectra. The lower ν(Fe-CO) and the higher ν(C-O) frequencies can be ascribed to the strong negative polar effect caused by the vicinal hydroxyl groups in the cavity. This prediction is further supported by the observation of significant ¹³C shieldings exhibited by TCP-CO (δ = 202.6 ppm) and TCP-PY-CO (δ = 202.3 ppm), in comparison to hemoproteins and other heme models. The CO affinity of TCP-PY (P_1/2CO = 0.017 Torr at 25°C) is unusually lower than other heme models. The unique behavior of these CO adducts is discussed in context of the TCP structures.