A detailed kinetic and mechanistic analysis of the classical "brown-ring" reaction of [Fe(H(2)O)(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 degrees C were found to be k(on) = 1.42 x 10(6) M(-1) s(-1), DeltaH(++)(on) = 37.1 +/- 0.5 kJ mol(-1), DeltaS(++)(on) = -3 +/- 2 J K(-1) mol(-1), DeltaV(++)(on) = +6.1 +/- 0.4 cm(3) mol(-1), and k(off) = 3240 +/- 750 s(-1), DeltaH(++)(off) = 48.4 +/- 1.4 kJ mol(-1), DeltaS(++)(off) = -15 +/- 5 J K(-1) mol(-1), DeltaV(++)(off) = +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(H(2)O)(6)](2+). In addition, Mössbauer spectroscopy and EPR measurements were performed on the reaction product [Fe(H(2)O)(5)(NO)](2+). The Mössbauer 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)(H(2)O)(5)(NO(-))](2+), and not [Fe(I)(H(2)O)(5)(NO(+))](2+) as usually quoted in undergraduate text books.
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 alpha-dimethylbenzimidazole base and a bent nitrosyl coordinated to cobalt at the beta-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, DeltaH, DeltaS, and DeltaV, 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 DeltaS and DeltaV 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.
Chelate complexes of FeII were investigated with respect to their reactivity against nitric oxide and dioxygen. Through a systematic variation of the structure of the polyaminocarboxylate EDTA, a series of 38 potential chelate ligands were selected for FeII. The nitrosyl complexes were prepared from the FeII chelates with NO gas and examined spectroscopically by UV/Vis and ATR‐IR techniques, and themodynamically by determining the overall binding constants for NO. In addition, the reversibility of NO binding to these FeII chelates and the rate of the competing oxidation by dioxygen were studied qualitatively. Whereas the studied complexes all form more or less stable nitrosyl complexes with a characteristic band pattern in the UV/Vis spectra and only slightly diverging frequencies for the NO stretching vibration in the IR spectra, they differ considerably in the reversibility of NO binding, the overall NO binding constants and the sensitivity towards dioxygen. It was found that an increasing number of donor groups on the chelate ligand causes a stronger coordination to FeII, and increases the tendency of the FeII chelates to transfer electron density from iron to substrates like dioxygen or nitric oxide. This results in an accelerated oxidation of FeII to FeIII by dioxygen and a more pronounced tendency of the corresponding nitrosyl complexes to slowly decompose to FeIII and N2O. In addition, the overall binding constant for NO (KNO), which spans a range from 1·103 to 2·107 M−1, increases in the same direction as a result of the inductive effect of the chelate ligand.
The effect of temperature and pressure on the water-exchange reactions of complexes of the type [FeIII(L)(H2O)x]n-, where L = edta4- (ethylenediaminetetraacetate), Hedta3- (monoprotonated form of edta), cdta4- (trans-1,2-diaminocyclohexanetetraacetate), edds4- (s,s-ethylenediaminedisuccinate), 1,3-pdta4- (1,3-propylenediaminetetraacetate), and alpha,beta-eddadp4-(alpha,beta-ethylenediaminediaceatedipropionate), was studied by employing 17O NMR techniques. The effect of potentially hexadentate ligands, covering a systematic variation of the size, substituents, and overall coordination geometry, on iron(III) complexes was investigated in terms of the lability of the coordinated water and the underlying exchange mechanism. For most of the systems studied, the results are in agreement with a dissociatively activated water-exchange mechanism for the seven-coordinate complexes. The absolute magnitudes of the volumes of activation are small and fit an I(d) mechanism. The results contribute to a better understanding of the nature, reactivity, and substitution mechanism of the selected complexes in solution.
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