title: Dynamics and equilibrium of heme axial ligation in mesoporous nanocrystalline TiO(2) thin films.
abstract: 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.
authors.affiliation: Department of Chemistry, Johns Hopkins University, 3400 North Charles Street, Baltimore, Maryland 21218, USA.
authors.forename: Amanda J
authors.initials: AJ
authors.lastname: Morris
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Stromberg
authors.forename: Gerald J
authors.initials: GJ
authors.lastname: Meyer