Reaction of a Superoxochromium(III) Ion with Nitrogen Monoxide:  Kinetics and Mechanism

The important roles that nitric oxide (NO) plays in biological environments, and the need for precise and targeted delivery of NO for medicinal and other purposes have led to intense research in the area of metal nitrosyl complexes as thermal and photochemical sources of NO. Complexes with a good combination of chemical stability and high quantum yield for photochemical release of NO upon irradiation with visible light in aqueous solutions are rare. Here we report that a simple macrocyclic nitrosylrhodium complex [L(2)(H(2)O)Rh(NO)](2+) (L(2)=Me(6)[14]aneN(4)) exhibits unique chemical and photochemical properties that make it an excellent photochemical precursor of NO. The complex is highly soluble in water, thermally stable, and resistant toward O(2). Irradiation in the 648 nm band generates NO and [L(2)(H(2)O)Rh](2+) in aqueous solutions with a quantum yield of 1.00±0.07, the highest ever reported for a nitrosyl complex under any conditions. In the absence of O(2), the two fragments combine to regenerate [L(2)(H(2)O)Rh- (NO)](2+), but in O(2)-containing solutions, [L(2)(H(2)O)RhOO](2+) is formed as determined in spectral and kinetic measurements. The kinetics of the reaction of this superoxo complex with NO were measured by laser flash photolysis, k=(3.9±0.4)×10(7) M(-1) s(-1). Steady-state photolysis of [L(2)(H(2)O)Rh(NO)](2+) under O(2) yielded [L(2)(H(2)O)Rh(ONO(2))](2+), a long-lived nitrato intermediate that can also be generated in a direct reaction between NO and genuine [L(2)(H(2)O)RhOO](2+). Thus, visible-light photolysis of the [L(2)(H(2)O)Rh(NO)](2+)/O(2) system converts it to the [L(2)(H(2)O)RhOO](2+)/NO combination.

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“…2 + has now made it possible to determine this rate constant in a laser flash photolysis study, [25] as follows.…”

Section: Introductionmentioning

confidence: 99%

Visible Light‐Induced Release of Nitrogen Monoxide from a Nitrosylrhodium Complex

Song

Kristian

Bakač

2011

Chemistry A European J

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“…[3,[10][11][12] Among metal complexes, iron-porphyrins have received most attention, [11,13,14] but non-porphyrin complexes of a number of metal ions have also provided important mechanistic data on the formation and chemistry of metal nitrosyls. [10,12,[15][16][17] Thermal and photochemical cleavage of a metal À NO bond, and the low pH-induced dissociation of NO from NONOates are among the most common ways of generating NO for chemical and biological purposes. Several other mechanisms for NO release exist, and it is quite likely that some of them may be advantageous under some circumstances.…”

Section: Introductionmentioning

confidence: 99%

Oxidative Homolysis of a Nitrosylchromium Complex

Song

Bakač

2008

Chemistry A European J

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Metal(III)-polypyridine complexes [M(NN)(3)](3+) (M = Ru or Fe; NN = bipyridine (bpy), phenanthroline (phen), or 4,7-dimethylphenanthroline (Me(2)-phen)) oxidize the nitrosylpentaaquachromium(III) ion, [Cr(aq)NO](2+), with an overall 4:1 stoichiometry, 4 [Ru(bpy)(3)](3+) + [Cr(aq)NO](2+) + 2 H(2)O --> 4 [Ru(bpy)(3)](2+) + [Cr(aq)](3+) + NO(3)(-) + 4 H(+). The kinetics follow a mixed second-order rate law, -d[[M(NN)(3)](3+)]/dt = nk[[M(NN)(3)](3+)][[Cr(aq)NO](2+)], in which k represents the rate constant for the initial one-electron transfer step, and n = 2-4 depending on reaction conditions and relative rates of the first and subsequent steps. With [Cr(aq)NO](2+) in excess, the values of nk are 283 M(-1) s(-1) ([Ru(bpy)(3)](3+)), 7.4 ([Ru(Me(2)-phen)(3)](3+)), and 5.8 ([Fe(phen)(3)](3+)). In the proposed mechanism, the one-electron oxidation of [Cr(aq)NO](2+) releases NO, which is further oxidized to nitrite, k = 1.04x10(6) M(-1) s(-1), 6.17x10(4), and 1.12x10(4) with the three respective oxidants. Further oxidation yields the observed nitrate. The kinetics of the first step show a strong correlation with thermodynamic driving force. Parallels were drawn with oxidative homolysis of a superoxochromium(III) ion, [Cr(aq)OO](2+), to gain insight into relative oxidizability of coordinated NO and O(2), and to address the question of the "oxidation state" of coordinated NO in [Cr(aq)NO](2+).

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“…204 In other examples, NO reacts with superoxo complexes of Rh(III) and Cr(III) to produce peroxonitrito intermediates that decompose in complicated ways. 205,206 NO adds to oxo complexes, including CrO 2 þ and MbFe IV O, to form nitrito complexes. 203 Similar reactions of the ferryl(V) moiety in various metalloproteins are likely to become an important physiological consideration.…”

Section: Ligand Reactionsmentioning

confidence: 99%

Reactivity of Inorganic Radicals in Aqueous Solution

Stanbury

2010

Physical Inorganic Chemistry

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Reaction of a Superoxochromium(III) Ion with Nitrogen Monoxide: Kinetics and Mechanism

Cited by 45 publications

References 47 publications

Visible Light‐Induced Release of Nitrogen Monoxide from a Nitrosylrhodium Complex

Visible Light‐Induced Release of Nitrogen Monoxide from a Nitrosylrhodium Complex

Oxidative Homolysis of a Nitrosylchromium Complex

Reactivity of Inorganic Radicals in Aqueous Solution

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