Synthesis, structure, spectra and metal redox of imidazolium tris(arylazo oximato)iron(II): the association of imidazole with a tris chelate oximato face and analogy with distal iron-imidazole disposition

Abstract

The reaction of Na[Fe(RL)₃]·H₂O with imidazole (im) in acidified methanol affords the green-coloured low-spin (t₂⁶) complexes Him[Fe(RL)₃], where [RL^-] is [RC(NO)NNPh-] and R is Me, Ph, p-MeC₆H₄. The X-ray structure of the solvate Him [Fe(p-MeC₆H₄L)₃]·2MeOH has revealed that the iron(II) atom is tris-chelated in facial geometry, the FeN₆ coordination sphere being distorted octahedral. Hydrogen bonding among the oximato oxygen, imidazole nitrogen and methanol oxygen atoms affords a centrosymmetric dimeric arrangement. The imidazole ring lies near the oximato periphery of the tris chelate. The different oximato functions are differently hydrogen bonded and the stronger the hydrogen bonding the longer the Fe---N(oxime) length. The shortest non-bonded FeN(im) length is 4.260(6) Å [motif Fe---NO-N(im)], which can be compared with the FeN(distal) length in oxyhaemoglobin, 4.34 Å, and oxymyoglobin, 4.53 Å [motif Fe---OO..N(im)]. The Him[Fe(RL)₃] salts have low electrical conductivity in solution, showing that strong association persists in the dissolved state. An allowed band near 600 nm is assigned to the MLCT excitation dπ(Fe)→π^∗(azo) on the basis of an extended Hückel examination of a model system. A quasi-reversible stereoretentive one-electron couple, [Fe^III(RL)₃]/[Fe^II(RL)₃^-], is electrochemically observable, the EPR spectrum of electrogenerated [Fe(RL)₃] being axial (g_|, ~2.1).