The 1:1 inclusion complex of 5,10,15,20-tetrakis(4-sulfonatophenyl) porphinato iron(II) (FeIITPPS) and an O-methylated β-cyclodextrin dimer having a pyridine linker (1) binds dioxygen reversibly in aqueous solution. The O2 adduct was very stable (t1/2 = 30.1 h) at pH 7.0 and 25°C. ESI-MS and NMR spectroscopic measurements and molecular mechanics (MM) calculations indicated the inclusion of the sulfonatophenyl groups at the 5- and 15-positions of FeIIITPPS or FeIITPPS into two cyclodextrin moieties of 1, to form a supramolecular 1:1 complex (hemoCD1 for the FeIITPPS complex), whose iron center is completely covered by two cyclodextrin moieties. Equilibrium measurements and laser flash photolysis provided the affinities (P 1/2O2 and P1/2CO) and rate constants for O2 and CO binding of hemoCD1 (konO2, koffO2, konCO, and k offCO). The CO affinity relative to the O2 affinity of hemoCD1 was abnormally high. Although resonance Raman spectra suggested weak back-bonding of dπ(Fe) → π*(CO) and hence a weak CO-Fe bond, the CO adduct of hemoCD1 was very stable. The hydrophobic CO molecule dissociated from CO-hemoCD1 hardly breaks free from a shallow cleft in hemoCD1 surrounded by an aqueous bulk phase leading to fast rebinding of CO to hemoCD1. Isothermal titration calorimetry furnished the association constant (KO2), ΔH°, and ΔS° for O2 association to be (2.71 ± 0.51) × 104 M-1, -65.2 ± 4.4 kJ mol-1, and -133.9 ± 16.1 J mol-1 K-1, respectively. The autoxidation of oxy-hemoCD1 was accelerated by H+ and OH-. The inorganic anions also accelerated the autoxidation of oxy-hemoCD1. The O 2-FeII bond is equivalent to the O2 •--FeIII bond, which is attacked by the inorganic anions or the water molecule to produce met-hemoCD1 and a superoxide anion.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Inorganic Chemistry