Oxidative Homolysis of a Nitrosylchromium Complex

Abstract

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+).

Nitric Oxide Release from a Nickel Nitrosyl Complex Induced by One-Electron Oxidation

“…The reactivity of nitric oxide (NO) remains a fundamentally important area of research. This is due in part to the extensive roles of NO in biology and in the environment. − Nitric oxide is an integral part of immune response, vasodilatation, and platelet aggregation, ,, and compounds that release NO have been sought for their potential use as therapeutics. − Several organic reagents, including alkyl nitrates (glyceryl trinitrate), alkyl nitrites (iso-amyl nitrite), and NONOates − have been utilized as NO-releasing compounds. , Metal nitrosyl complexes have also been used as NO delivery agents, the classic example being sodium nitroprusside (Na 2 [FeNO(CN) 5 ], SNP). , Photodissociation, ,,− reduction, and oxidation − have all been employed to effect NO release from a metal nitrosyl complex. The latter two approaches, in particular, require a clear understanding of the redox chemistry of metal nitrosyl complexes.…”

“…There are only a few well-defined examples of NO dissociation upon oxidation of a metal nitrosyl complex. − For instance, Bakac and co-workers reported that the one-electron (1e – ) oxidation of [CrNO(H 2 O) 5 ] 2+ or [L 1 (H 2 O)CrNO] 2+ (L 1 = 1,4,8,11-tetraazacyclotetradecane) resulted in rapid denitrosylation. − The putative 1e – oxidation products ([CrNOL 5 ] 3+ ) were not detected, suggesting that denitrosylation occurred immediately upon electron transfer. Similarly, Ledgzins and co-workers investigated the reactivity of [CpCr(NO)X 2 ] − (X = Cl, Br, I, OTf) with a variety of oxidants .…”

Preparation, Crystal Structure, and Unusually Facile Redox Chemistry of a Macrocyclic Nitrosylrhodium Complex

“…The reverse of reaction sometimes provides a useful route to NO for chemical and medicinal purposes. The kinetics of this step are a sensitive function of the ligand system, solvent, pH, and the metal. , As one might expect, the oxidation state of the metal strongly influences the NO dissociation rates, so that the reduction or oxidation , of inert nitrosyl complexes may provide a mechanism for NO release. In view of the complex electronic structure of nitrosyl complexes, with three limiting (M n - • NO, M n +1 -NO − , and M n −1 -NO + ) and countless intermediate electronic states, the entire MNO moiety should be considered the site of electron transfer, as opposed to purely metal-centered or NO-centered processes.…”

“…In our earlier work, , we have examined the oxidation of nitrosylchromium complexes Cr(H 2 O) 5 NO 2+ and L(H 2 O)CrNO 2+ (L = L 1 = 1,4,8,11-tetraazacyclotetradecane, and L 2 = meso -Me 6 -1,4,8,11-tetraazacyclotetradecane) with polypyridine complexes of iron(III) and ruthenium(III), M(NN) 3 3+ . Four equivalents of M(NN) 3 3+ were required for complete oxidation which yielded nitrate and Cr(III).…”

Kinetics and thermodynamics of nitric oxide binding to transition metal complexes. Relationship to dioxygen binding

“…The rates of _NO dissociation vary with the M-NO bond strength, nature and oxidation state of the metal, ancillary ligands, etc. [1][2][3][4][5][6] Our past work in this area focused on thermally stable nitrosyl complexes of chromium 7 and rhodium, 8,9 and on mechanistic studies of their redox chemistry and photochemistry. 9,10 Among the rhodium complexes, the macrocyclic L 2 (H 2 O)Rh(NO) 2+ (L 2 ¼ meso-Me 6 -cyclam) was studied most extensively.…”