The 1:1 inclusion complex of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato iron(II) (Fe
IITPPS) and an O-methylated
-cyclodextrin dimer having a pyridine linker (
1) binds dioxygen reversibly in aqueous solution. The O
2 adduct wasvery stable (
t1/2 = 30.1 h) at pH 7.0 and 25
C. ESI-MS and NMR spectroscopic measurements and molecularmechanics (MM) calculations indicated the inclusion of the sulfonatophenyl groups at the 5- and 15-positions ofFe
IIITPPS or Fe
IITPPS into two cyclodextrin moieties of
1 to form a supramolecular 1:1 complex (hemoCD1 for theFe
IITPPS complex), whose iron center is completely covered by two cyclodextrin moieties. Equilibrium measurementsand laser flash photolysis provided the affinities (
and
) and rate constants for O
2 and CO binding ofhemoCD1 (
,
,
, and
k). The CO affinity relative to the O
2 affinity of hemoCD1 was abnormally high.Although resonance Raman spectra suggested weak back-bonding of d
(Fe)
*(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 tofast rebinding of CO to hemoCD1. Isothermal titration calorimetry furnished the association constant (
KO2),
H,and
S for O
2 association to be (2.71 ± 0.51) × 10
4 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 alsoaccelerated the autoxidation of oxy-hemoCD1. The O
2-Fe
II bond is equivalent to the O
2--Fe
III bond, which isattacked by the inorganic anions or the water molecule to produce met-hemoCD1 and a superoxide anion.