Reactivity of iron(II)-bound nitrosyl hydride (HNO, nitroxyl) in aqueous solution

Three Redox States of Metallonitrosyls in Aqueous Solution

“…The main absorption band of , >80%), with a half-life time of $50 min. However, and rather surprisingly, a rapid shift to pH $6 immediately after completing the 2-electron reduction allowed attaining a great stability of the 445 nm band intensity, which decayed with a pseudo-first-order rate constant of $10 À6 s À1 (14). The metric data ( [9]aneN 3 )(bpy)(HNO)] 2+ ion could be generated in aqueous solution by electrochemical reduction of [Ru(Me 3 [9] aneN 3 )(bpy)(NO)] 2+ at pH 2.5 (15).…”

“…Reaction (15) usually involves a bond-reorganization within the adduct, with oxidation of B and reduction of NO + , formation of the M(II) aqua-complex, and, in most instances, the evolution of gases (N 2 and/or N 2 O), or of some other nitrosated product, depending on the 49 Three Redox States of Metallonitrosyls in Aqueous Solution nucleophile and the initial nitrosyl complex (91). A simple nonredox process is operative in reaction (14) with B ¼ OH À , yielding a nitrous acid intermediate that upon rapid deprotonation becomes a bound nitrite. The latter species may remain stable for some time and be easily detectable, or undergo aquation (releasing free nitrite) with a rate that depends on the M II L 5 fragment (92).…”

“…A recent revision of the aqueous solution behavior of [Fe(CN) 5 HNO] 3À shows that its main absorption band at 445 nm decays very slowly at pH 6, through an oxidation process of HNO leading mainly to [Fe(CN) 5 NO] 2À , with k decomp 77 Three Redox States of Metallonitrosyls in Aqueous Solution $5 Â 10 À7 s À1 at 25 C (negligible N 2 O-formation) (14). The latter value is a low limit to k -HNO , the specific dissociation rate constant for HNO.…”

“…By comparing with related [Fe II (CN) 5 L] nÀ complexes, the order of k -L s is: NO + ( CO < CN À < HNO < NOBz < NO < dmso < pz < py < NH 3 , with the latter k ÀNH3 at $10 À2 s À1 . HNO fits in an intermediate position, reflecting its ubiquity in the so-called spectrochemical series, which reflects the capability of the L ligands toward σ-π bonding to iron (14).…”

“…[Fe(CN) 5 (NO)] 2À is an n ¼ 6 species that allowed us exploring the chemistry of the 1-electron reduced [Fe(CN) 5 (NO)] 3À (n ¼ 7) (10)(11)(12) and of [Fe(CN) 5 (L)] nÀ (L ¼ NO À /HNO), the 2-electron reduced products (n ¼ 8) (12)(13)(14). For ruthenium, we have accessed to three {RuNO} 6,7,8 analogs containing the [Ru(Me 3 [9]aneN 3 )(bpy)] 2+ fragment, thus characterizing not only NO + , NO • , NO À but also HNO and NO 2 À as ligands (15).…”