Theoretical Investigation on the Electronic Structure of Pentacyano(L)ferrate(II) Complexes with NO⁺, NO, and NO^-Ligands. Redox Interconversion, Protonation, and Cyanide-Releasing Reactions

Abstract

Reaction pathways for the one- and two-electron reductions of [Fe(CN)(5)NO](2)(-) have been investigated by means of a density functional theory (DFT) approach combined with the polarized continuum model (PCM) of solvation. In addition, UV-vis spectroscopic data were obtained using ZINDO/S calculations including a point-charge model simulation of solvent effects. DFT methodologies have been used to assess the thermodynamical feasibility of protonation and cyanide-release processes for the reduced species. We conclude that [Fe(CN)(5)NO](3)(-) is a stable species in aqueous solution but may release cyanide yielding [Fe(CN)(4)NO](2)(-), consistent with experimental results. On the other hand, the [Fe(CN)(5)NO](4)(-) complex turns out to be unstable in solution, yielding the product of cyanide release, [Fe(CN)(4)NO](3)(-), and/or the protonated HNO complex. All the structural and spectroscopic (IR, UV-vis) predictions for the [Fe(CN)(5)HNO](3)(-) ion are consistent with the scarce but significant experimental evidence of its presence as an intermediate in nitrogen redox interconversion chemistry. Our computed data support an Fe(II)(LS) + NO(+) assignment for [Fe(CN)(5)NO](2)(-), an Fe(II)(LS) + NO assignment for the one-electron reduction product, but an Fe(I)(LS) + NO(+) for the one-electron product after dissociation of an axial cianide, and an Fe(II) + singlet NO(-) for the two-electron reduction species.

Three Redox States of Metallonitrosyls in Aqueous Solution

“…Eventually, a calculated electronic spectrum of the latter showed weak absorption bands in the 500-600 nm range. Interestingly, the theoretical work predicts bipyramidal geometry for the latter anion, with the nitroxyl in the equatorial position (12).…”

“…At the time we reported the pK a for [Fe(CN) 5 HNO] 3À , we had payed no much attention to our own previous computational DFT result predicting that [Fe(CN) 5 NO] 4À was an unstable species that should lose CN À from the coordination sphere, in contrast with the successful predictions on the stability of other 5C and 6C cyano-nitrosyl complexes (12 …”

“…In a related way, the [Fe(3,5-Me-BAFP)(NO)] À ion has been characterized as a result of spectroelectrochemical reduction of the n ¼ 7 compound (Table 2) (116). Table 3 also includes the calculated DFT structural parameters for the [Fe(CN) 4 (NO)] 3À complex, which is presumably generated as a moderately stable species upon cyanide-labilization from the unstable [Fe(CN) 5 (NO)] 4À ion (see below) (12).…”

“…[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).…”

Computational investigations of HNO in biology

“…Calculations on non-heme HNO metal complexes were also reported [58,67,68]. For [Fe(CN) 5 (HNO)] 3− , a reduction product of pentacyanoferrate, DFT calculations predicted two frequencies of 1394 and 1338 cm −1 for ν NO modes with some hydrogen participation [67].…”