“Brown‐Ring”‐Related Coordination Polymers of the Quartet‐{FeNO}7 Chromophore

A conspicuous detail of the so‐called brown‐ring test (the analytical test on nitrate) is the reddish color of the bottom layer of concentrated sulfuric acid, which develops upon the bleeding of the brown layer into the acid. Crystals of the same color form from a solution of ferrous sulfate in concentrated sulfuric acid on saturation with gaseous nitric oxide. The structure of this H3O[{Fe(NO)(μ4‐SO4)(μ2‐SO4)0.5}n/n] (1a) is made up from infinite chessboard‐type layers with sulfur on the field junctions and Fe(NO) moieties below the black and above the white fields. An Fe–N–O angle of about 160° causes disorder in the tetragonal space group I4/mmm. A similar crystal pathology was found in the related [{Fe(MeOH)(NO)(μ4‐SO4)}n/n] (1b) in the same crystal class. A one‐dimensional coordination polymer is formed in crystals of a third compound that comprises the Fe(NO)O5 coordination pattern, namely the brown oxalato species [{Fe(H2O)(NO)(μ2‐ox)}n/n·H2O] (2). A still larger NO tilt of about 156° is not obscured by disorder in the triclinic crystals of 2

{FeNO} 7 -Type Halogenido Nitrosyl Ferrates: Syntheses, Bonding, and Photoinduced Linkage Isomerism

International audienceMononitrosyliron compounds (MNICs) of the Enemark-Feltham {FeNO} 7 type divide into a dublet (S = 1/2) and a quartet (S = 3/2) spin variant. The latter relies on weak-field coligands such as aminecarboxylates. Aqua-only co-ligation appears to exist in the long-known 'brown-ring' [Fe(H 2 O) 5 (NO)] 2+ cation which was prepared originally from ferrous salts and NO in sulfuric acid. A chloride variant of this species, the green [FeCl 3 (NO)] − ion, was first prepared analoguosly by using hydrochloric instead of sulfuric acid. As a tetrahedral species, it is the simple prototype of sulfur-bonded {FeNO} 7 (S = 3/2) MNICs of biological significance. Though being investigated for more than a century, neither clean preparative routes nor reliable structural parameters were available for the [FeCl 3 (NO)] − ion and related species such as the [FeCl 2 (NO) 2 ] − ion, a prototypical dinitrosyliron species (a 'DNIC'). In this work, both issues have been resolved. In addition, we report on a computational study on the ground-and excited-state properties including an assignment of the chromophoric transitions. Photoinduced metastable isomers were characterised in a combined experimental and computational approach which resulted in the confirmation of a single photoinduced linkage isomer of the paramagnetic nitrosyl-metal coordination entity