A comparative mechanistic study of the reversible binding of NO to a water-soluble octa-cationic FeIII porphyrin complex

The water-soluble, non-mu-oxo dimer-forming porphyrin, [5,10,15,20-tetrakis-4'-t-butylphenyl-2',6'-bis-(N-methylene(4"-t-butylpyridinium))porphyrinato]iron(III) octabromide, (P8+)Fe-III, with eight positively charged substituents in the ortho positions of the phenyl rings, was characterized by UV-vis and H-1 NMR spectroscopy and O-17 NMR water-exchange studies in aqueous solution. Spectrophotometric titrations of (P8+)Fe-III indicated a pK(a1) value of 5.0 for coordinated water in (P8+) Fe-III (H2O)(2). The monohydroxo-ligated (P8+)Fe-III(OH)(H2O) formed at 5 < pH < 12 has a weakly bound water molecule that undergoes an exchange reaction, k(ex) = 2.4 x 10(6) s(-1), significantly faster than water exchange on (P8+)Fe-III(H2O)(2), viz. k(ex) = 5.5 x 10(4) s(-1) at 25 degrees C. The porphyrin complex reacts with nitric oxide to yield the nitrosyl adduct, (P8+)Fe-II(NO+)(L) (L = H2O or OH-). The diaqua-ligated (P8+)Fe-III(H2O)(2) binds and releases NO according to a dissociatively activated mechanism, analogous to that reported earlier for other (P)Fe-III(H2O)(2) complexes. Coordination of NO to (P8+)Fe-III(OH)(H2O) at high pH follows an associative mode, as evidenced by negative Delta S not subset of(on) and Delta V-on(not subset of), values measured for this reaction. The observed ca. 10-fold decrease in the NO binding rate on going from six-coordinate (P8+)Fe-III(H2O)(2) (k(on) = 15.1 X 10(3) M-1 s(-1)) to (P8+)Fe-III(OH)(H2O) (k(on) = 1.56 x 10(3) M-1 s(-1) at 25 degrees C) is ascribed to the different nature of the rate-limiting step for NO binding at low and high pH, respectively. The results are compared with data reported for other water-soluble iron(III) porphyrins with positively and negatively charged meso substituents. Influence of the porphyrin periphery on the dynamics of reversible NO binding to these (P)Fe-III complexes as a function of pH is discussed on the basis of available experimental data.