Structure and Bonding of High‐Spin Nitrosyl–Iron(II) Compounds with Mixed N,O‐Chelators and Aqua Ligands

Abstract

High‐spin nitrosyl–iron centres of the Enemark–Feltham {FeNO}7 type exist in aqueous solution. Examples include the tentative “brown‐ring” species [Fe(H2O)5(NO)]2+ and a tentative FeII/edta/NO species formed in the processes for scrubbing NO from flue‐gas streams. Inert‐gas bubbling through the solutions subdivides the ferrous nitrosyl complexes in a less stable subclass – a prominent member being the brown‐ring complex – and a more stable subclass to which the edta species belongs. The structural chemistry of the less stable subclass of {FeNO}7‐type complexes from aqueous media is presented here. They contain aminecarboxylato co‐ligands of limited denticity and aqua ligands that complete an OC‐6 environment of the Fe atom. Crystalline compounds for single‐crystal structure analysis were obtained for various co‐ligands: [Fe(H2O)2(ida)(NO)] (2a), [Fe(H2O)(heida)(NO)] (2b), [Fe(H2O)2(NO)(oda)] (2c), [Fe(H2O)2(NO)(phida)]·H2O (2d), [Fe(bnida)(H2O)2(NO)] (2e), [Fe(brbnida)(H2O)2(NO)] (2f) and [Fe(dipic)(H2O)2(NO)] (2g) (ida = iminodiacetate, heida = hydroxyethyliminodiacetate, oda = oxodiacetate, phida = N‐phenyliminodiacetate, bnida = N‐benzyliminodiacetate, brbnida = N‐4‐bromobenzyliminodiacetate, dipic = dipicolinate). The Fe–NO interaction was studied by DFT and CASSCF methods. Due to mostly covalent Fe–NO π bonds, the charge distribution in the less stable subclass is close to FeII(NO) with a small FeIII(NO–) contribution.

The Structural Chemistry of Stable High‐Spin Nitrosyl–Iron(II) Compounds with Aminecarboxylato Co‐Ligands in Aqueous Solution

“…In compound 3 , a peculiarity of the structures in this work becomes distinct, namely the normal shape of the thermal ellipsoid of the nitrosyl O atom. However, a markedly anisotropic shape as well as disorder was usually found among the less stable analogues – in line with the DFT result that the Fe–N–O bending potential is flat . It was thus expected that the typical bending curve would show a more pronounced minimum close to the experimentally determined value.…”

“…This publication is devoted to this stable group, which contains aminecarboxylates of denticity four and higher. Moreover, the typical bonding situation is analysed in some depth in the accompanying publication on the less stable iron–nitrosyl compounds . Remarkably, the bonding analysis presented there turns out to be essentially the same here for the aminecarboxylate co‐ligands.…”

“…The early investigations on the resulting mononitrosyl–iron compounds, which, according to the Enemark–Feltham notation, are quartet‐{FeNO} 7 species, were motivated by the attempt to elucidate the chromophore of the brown‐ring test for nitrate detection and the same chromophore in nitrite analysis. The pioneering work of Kohlschütter and Manchot before World War I was thus devoted to simple aqua and halogenido species; their work has been included in more detail in the accompanying article on less stable centres of this kind …”

Mass Transfer in Reactive Bubbly Flows – A Single‐Bubble Study

“…However, the stability of the Fe II (Ligand)-NO system is not fully understood since the exact mechanism of the product formation and the rules of the relationship of structure and feature are, to our knowledge, still unknown. With two publications, the research group of Klüfers tries to fill this gap [24,25].…”

Reactive Mass Transfer of Single NO Bubbles and Bubble Bouncing in Aqueous Ferric Solutions – A Feasibility Study

“…In two forthcoming publications [41,42], the research group of Klüfers tries to fill this gap based on knowledge of structural characterization of Fe II (L)(NO) complexes with amincarboxylato coligands. …”