Trinucleation of Arylazo Oxime Ensembles: Structure and Reactions of Novel Linear Fe < sup > II < /sup > Fe < sup > III < /sup > Fe < sup > II < /sup > Species

Abstract

The reaction of phenylazo aldoximes, RC(=NOH)N=NPh (HRL; R = Me, Ph), with iron(II) perchlorate hexahydrate in ethanol affords [Fe3(RL)6]C1O4 (1) crystallizing with or without solvent molecules. The structure of one solvate, viz., [Fe3(PhL)6]-ClO4•2CHC13, has been determined X-ray crystallographically. The compound forms monoclinic crystals in the space group P21/n with Z = 2 and unit cell dimensions a = 15.521 (4) Å, b = 12.743 (5) Å, c = 21.788 (5) Å, β = 104.35 (2)°, and V= 4175 (2) Å3. The structure was refined to R = 0.0881 and Rw= 0.1281. The trinuclear cation Fe3(PhL)6+ sits on a crystallographic center of symmetry. The central iron(III) atom is surrounded by an octahedron of six oximato oxygen atoms. Each of the two terminal iron(II) atoms is held in the facial tris chelate environment of azo and oxime nitrogen atoms. The FeN6 coordination sphere is trigonal antiprismatic. Each oximato function acts as a bridge between iron(III) and iron(II) atoms. Both the ClO4-anion and the CHCl3 molecules are disordered in the crystal. The reaction of iron(III) chloride with HRL furnishes [Fe3(RL)6]FeCl4(2). The iron (III) and iron(II) atoms of Fe3(RL)6+ are high spin (S = 5/2) and low spin (S = 0), respectively. Subtle rhombic distortion of the FeIIIO6sphere in Fe3(RL)6+ is revealed by strong EPR lines near g = 4 in both 1 and 2. The observation of intervalence band(s) in 1 and 2 has been vitiated by the strong visible and near-UV bands localized on the FeN6 chromophore. Hydroxide ions selectively and quantitatively extrude iron(III) from both 1 and 2, liberating the green anion Fe(RL)3-(isolated as Na+ and Ph4As+ salts), which retains the facial stereochemistry characteristic of the parent aggregate. Reaction with iron(III) salts causes reaggregation into Fe3(RL)6+. The Fe(RL)3-anion is electrochemically oxidized to axially symmetric (EPR data) low-spin Fe(RL)3, the FeIIIN6/FeIIN6formal potential being in the range 0.2-0.4 V vs. SCE. In Fe3(RL)6+, the couple FeIIIO6/FeIIO6due to the central iron atom occurs near 0.0 V but the terminal FeIIIN6/FeIIN6couple is shifted to potentials above 1.5 V and is not directly observable. In the case of 2 the FeCl4-/FeCl42-couple is exactly superposed on the FeIIIO6/FeIIO6couple of the cation. These redox patterns have been used to rationalize the formation of 1 and 2 by the chelative redox reactions of iron salts with HRL. The possible role of Fe(RL)3-as a translocator of metal ions via the reaggregation-extrusion sequence and its similarity to ferroverdin, the green pigment of a Streptomyces species, are noted. © 1985, American Chemical Society. All rights reserved.