Dynamics and equilibrium of heme axial ligation in mesoporous nanocrystalline TiO(2) thin films

Comparative studies of axial CO and solvent coordination to iron(II) protoporphyrin IX (FeIIPPIX) anchored to the surface of mesoporous nanocrystalline (anatase) TiO2 thin films (FeIIPPIX/TiO2) immersed in dimethyl sulfoxide (DMSO), pyridine (py), and methanol (MeOH) and to FeIIPPIX in fluid DMSO and py solution are reported. The equilibrium constants, KeqCOS, for CO coordination to FeIIPPIX/TiO2 immersed in py (2.4 x 10(3) M-1) < DMSO (6 x 10(4) M-1) < MeOH (2.3 x 10(5) M-1) were quantified. The corresponding values in fluid py or DMSO solution were 2 times larger (4.5 x 10(3) and 1 x 10(5) M-1, respectively). The observed ligand exchange rates (kobs) measured after pulsed 532 nm laser excitation (5-6 ns fwhm, 1-3 mJ/pulse) of (S)(CO)FeIIPPIX/TiO2, where S is solvent, in saturated CO solutions were measured: py (2.2 s-1), DMSO (460 s-1), MeOH (2.09 x 10(5) s-1). The corresponding values in fluid solution were 2.0 s-1 (py) and 230 s-1 (DMSO). The observed ligand exchange rate varied linearly with [CO], and second-order rate constants were determined for FeIIPPIX/TiO2 immersed in DMSO (3.1 x 10(5) M-1 s-1) and MeOH (1.5 x 10(7) M-1 s-1). The observed rate for CO addition to (py)2FeIIPPIX/TiO2 immersed in py did not vary linearly with [CO]. The relevance of the measured kinetics and thermodynamics to a dissociative mechanism for ligand exchange is discussed.