Mechanistic information on the reversible binding of NO to selected iron(II) chelates from activation parameters

Mechanistic insight on the reversible binding of NO to Fe II chelate complexes as potential catalysts for the removal of NO from effluent gas streams has been obtained from the temperature and pressure parameters for the “on” and “off” reactions determined using a combination of flash photolysis and stopped-flow techniques. These parameters are correlated with those for water exchange reactions on the corresponding Fe II and Fe III chelate complexes, from which mechanistic conclusions are drawn. Small and positive ¢ V q values are found for NO binding to and release from all the selected complexes, consistent with a dissociative interchange (I d ) mechanism. The only exception in the series of studied complexes is the binding of NO to [Fe II (nta)(H 2 O) 2 ] - . The negative volume of activation observed for this reaction supports the operation of an I a ligand substitution mechanism. The apparent mechanistic differences can be accounted for in terms of the electronic and structural features of the studied complexes. The results indicate that the aminocarboxylate chelates affect the rate and overall equilibrium constants, as well as the nature of the substitution mechanism by which NO coordinates to the selected complexes. There is, however, no simple correlation between the rate and activation parameters and the selected donor groups or overall charge on the iron(II) complexe

Kinetics and mechanism of the reversible binding of nitric oxide to reduced cobalamin B12rB_{12r} (Cob(II)alamin)

The reduced form of aquacobalamin binds nitric oxide very effectively to yield a nitrosyl adduct, Cbl(II) - NO. UV - vis, 1 H-, 31 P-, and 15 N NMR data suggest that the reaction product under physiological conditions is a six-coordinate, “base-on” form of the vitamin with a weakly bound R -dimethylbenzimidazole base and a bent nitrosyl coordinated to cobalt at the ‚ -site of the corrin ring. The nitrosyl adduct can formally be described as Cbl(III) - NO - . The kinetics of the binding and dissociation reactions was investigated by laser flash photolysis and stopped-flow techniques, respectively. The activation parameters, ¢ H q , ¢ S q , and ¢ V q , for the forward and reverse reactions were estimated from the effect of temperature and pressure on the kinetics of these reactions. For the “on” reaction of Cbl(II) with NO, the small positive ¢ S q and ¢ V q values suggest the operation of a dissociative interchange ( I d ) substitution mechanism at the Co(II) center. Detailed laser flash photolysis and 17 O NMR studies provide evidence for the formation of water-bound intermediates in the laser flash experiments and strongly support the proposed I d mechanism. The kinetics of the “off” reaction was studied using an NO-trapping technique. The respective activation parameters are also consistent with a dissociative interchange mechanism

Thermodynamics and kinetics of RuIII(edta)Ru^{III}(edta) as an efficient scavenger for nitric oxide in aqueous solution

The edta complex of Ru III reacts very rapidly with NO in aqueous solution at pH = 5 to form a stable nitrosyl complex. The results from FT-IR (ATR) and 15 N-NMR studies clearly support the NO character of coordinated NO, such that the nitrosyl product can be formulated as [Ru II (edta)NO] . A combination of UV-Vis spectroscopy and electrochemical detection of NO was used to determine the overall equilibrium constant K NO as (9.1 ± 1.2) × 10 7 M 1 at 25 C and pH = 5.0. Stopped- fl ow kinetic studies on the reaction of acetate-bu ff ered solutions of [Ru III (edta)H 2 O] with NO gave k on values two orders of magnitude lower than that reported in the literature as a result of bu ff er e ff ects. The values of k on determined at low and high pH, viz . 3.8 × 10 4 and 1.2 × 10 5 M 1 s 1 , respectively, are signi fi cantly smaller than that found at pH = 5.0, and in agreement with that observed for the substitution reactions of Ru III (edta) with other entering nucleophiles. Attempts to determine k on for the binding of NO to [Ru III (edta)H 2 O] using laser fl ash photolysis failed due to the occurrence of side reactions. Under speci fi c conditions (NO in excess and NO 2 as impurity), the formation of the disubstituted [Ru II (edta)(NO )(NO 2 )] 2 species was detected using 15 N-NMR spectroscopy. Laser fl ash photolysis of this complex leads to multiple chemical reaction steps as a result of the formation of two primary photoproducts, which decay with di ff erent rate constants to the starting complex. Possible mechanisms for these photoinduced reactions are proposed and compared to related systems reported in the literature

Kinetics, mechanism, and spectroscopy of the reversible binding of nitric oxide to aquated iron(II). An undergraduate text book reaction revisited

A detailed kinetic and mechanistic analysis of the classical "brown-ring" reaction of [Fe(H2O)(6)](2+) with NO was performed using stopped-flow and laser flash photolysis techniques at ambient and high pressure. The kinetic parameters for the "on" and "off" reactions at 25 degreesC were found to be k(on) = 1.42 x 10(6) M-1 s(-1), DeltaH(on)(double dagger) = 37.1 +/- 0.5 kJ mol (-1), DeltaS(on)(double dagger) = -3 +/- 2 J K-1 mol(-1), DeltaV(on)(double dagger) = + 6.1 +/- 0.4 cm(3) mol(- 1), and k(off) = 3240 +/- 750 s(-1), Delta(off)(double dagger) = 48.4 +/- 1.4 kJ mol(-1), DeltaS(off)(double dagger) = -15 +/- 5 J K-1 mol-1, DeltaV(off)(double dagger) = +1.3 +/- 0.2 cm(3) mol(-1). These parameters suggest that both reactions follow an interchange dissociative (I-d) ligand substitution mechanism, which correlates well with the suggested mechanism for the water exchange reaction on [Fe(H2O)(6)](2+). In addition, Mossbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H2O)(5)(NO)](2+). The Mossbauer and EPR parameters closely resemble those of the {FeNO}(7) units in any of the other well-characterized nitrosyl complexes. It is concluded that its electronic structure is best described by the presence of high-spin Fe-III antiferromagnetically coupled to NO- (S = 1) yielding the observed spin quartet ground state (S= 3/2), i.e., [Fe-III(H2O)(5)(NO-)](2+), and not [Fe- I(H(2)0)(5)(N0(-))](2+) as usually quoted in undergraduate text books

Ligand effects on the kinetics of the reversible binding of NO to selected aminocarboxylato complexes of iron(II) in aqueous solution

Rate constants for the formation and dissociation of FeII(L)NO (L = aminocarboxylato) have been determined using stopped-flow, temperature jump, flash photolysis, and pulse radiolysis techniques. For a series of ligands the formation rate constants vary between 1.6 × 106 and 2.4 × 108M−1 s−1, whereas the dissociation rate constants vary between 0.11 and 3.2 × 103 s−1 at 25 °C. These rate constants result in stability constants (KNO = kf/kd) ranging from 5.0 × 102 to 1.1 × 107M−1, which are in good agreement with values of KNO determined by a combined spectrophotometric and potentiometric technique. The results are discussed with reference to available literature data, and interpreted in terms of a generalized reaction mechanism